October 12, 2021 • 49 mins
Daniel and Jorge step through the details of a mind-boggling version of the double slit experiment. Check out their new book: universefaq.com
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Speaker 1 (00:01):
Hey, Jorhan, Daniel here, and we want to tell you
about our new book. It's called Frequently Asked Questions about
the Universe because you have questions about the universe, and
so we decided to write a book all about them.
We talk about your questions, we give some answers, we
make a bunch of silly jokes as usual, and we
tackle all kinds of questions, including what happens if I
fall into a black hole? Or is there another version
(00:22):
of you out there that's right? Like usual, we tackle
the deepest, darkest, biggest, craziest questions about this incredible cosmos.
If you want to support the podcast, please get the
book and get a copy, not just for yourself, but
you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters,
and for the aliens. So get your copy of Frequently
Asked Questions about the Universe is available for pre order now,
(00:46):
coming out November two. You can find more details at
the book's website, Universe f a Q dot com. Thanks
for your support, and if you have a hamster that
can read, please let us know. We'd love to have
them on the podcast. Daniel, do you ever feel like
(01:08):
you really understand quantum mechanics. No, you know, I think
it's probably just too alien for us to really ever
feel comfortable with. I guess it's too bad there aren't
any macroscopic big quantum optics we can really like poke
and play with, I know, like a big fat electron.
But actually I'm working on a theory that children are
governed by the rules of quantum mechanics. Oh really, like
(01:29):
there's uncertainty about where they are. Have you lost your kids? No,
but I've noticed that you can't like observe your children
without sort of perturbing the system, right, I think I
know what you mean. Like they won't do their homework
unless you're there watching exactly. And when I walk into
the room somehow all their conversations collapse suddenly into silence. Yeah,
it's like sure the anger as children, they're both excited
(01:51):
and sad to see you. Hi am for Handmad cartoonists
and the creator of PhD comics. Hi. I'm Daniel. I'm
(02:11):
a particle physicist and a professor UC Irvine, and I'm
always in many quantum states at once. Really, so that
does that mean you're not real? It means I don't
even know if I'm real man. Well, I hope you are,
because that would mean that I'm talking to myself right now,
and that would be a little concerning. Maybe you are
the only brain in the universe and the rest of
the universe is just part of your mind. That would
(02:32):
make a lot of sense why I'm so successful and
good looking. But anyways, welcome to our podcast. Daniel and
Jorge explain the university production of I Heart Radio, in
which we do try to explode our minds out to
capture the entire universe. We want to take this vast, glittering, crazy, violent,
wild and white cosmos and wrap it all up inside
(02:53):
our brains. It's not enough for us to just live
in this universe to experience it and to see it.
We want to understand it. We want download the whole
thing into our minds, and that means understanding the basic
rules about how it works, what's really going on with
tiny little particles or whatever is happening at the smallest scale.
(03:13):
On this podcast, we dive the deepest you can into
the hardest and trickiest of questions and we try to
explain all of them to you. Yeah, because it is
a pretty tricky universe. It's full of interesting rules and
interesting phenomena that happens at the smallest of levels and
at the largest of scales, and a lot of it
is understandable, even if it's not very um intuitive to
(03:34):
think about. That's right. And I look at the universe
like a big puzzle. It's like a detective novel or
a murder mystery, and I want to figure out who
did it. I want to understand how it works. It's
amazing to me, sort of philosophically, that the universe is
presented to us that way, like this big puzzle that
isn't obvious to figure out, but yet can somehow be
understood if you push hard enough. Do you think the
universe is understandable? Daniel? That's a big question in physics,
(03:57):
isn't it. It's a big question in the philosophy of
physics x and And my answer is it doesn't make
sense for it to be understandable, Like how could it
be possible that the complexities of this maybe infinite universe
could be stored in the minds of a human. On
the other hand, we have all these theories that work
really well, like surprisingly well, and so I don't know
how to hold those two ideas. In my mind, they're
(04:19):
in quantum conflict. You're both confused and feeling smart at
the same time. That's what this podcast is all about.
I'm the confused quantum state and you are the feeling smart.
That's right. And all scientists sort of hold those two
feelings in their mind at once, like, look at all
we have understood, and yet look at all that we
do not, And that's both exciting and terrifying. Yeah, and
(04:39):
we have a lot of questions about the universe. And
by the way, speaking of questions, we have a new
book coming out pretty soon in on November two. That's right.
Jorge and I celebrate asking questions about the universe, and
we love thinking about these questions. We love hearing about
your questions about the universe. So we wrote a book
that wraps up the most frequently asked questions that we
(05:00):
get about the universe. Yeah. So, if you like this
podcast and you want to support us, please check it out.
It's called Frequently Ask Questions about the Universe and it's
on a pre order right now. You can order it
now and get it as soon as it comes out.
And I think you know what's important is that we
kind of didn't quite write it for our listeners, right, Daniel.
We kind of wrote it for the people that know
our listeners. You know, if you ever have like a
(05:21):
nephew or a cousin or an uncle who you want
to share this amazing information about the universe, I think
this is the book for you or for them, that's right.
So every one of you out there, you should buy
five copies and give them to all your friends and
family so that they can understand the answers to questions
like where did the universe come from? Or how can
we travel to the stars. Yeah, we tackle all kinds
(05:41):
of pretty cool questions in it, and we try to
answer it for people like your relatives and friends with
all kinds of interesting and clear answers. And also cartoons,
which is I think something you don't see in every
day in physics books. That's right. All these awesome fun
drawings that help clarify the topic and amuse you along
the way. That or added to this book. So if
(06:02):
you like this blend of physics and silly jokes, then
I think you'll enjoy this book. So go out and
get your copy. You can find it at universe f
a Q dot com all right, Well, speaking of questions,
we are tackling a question today and it has something
to do with quantum mechanics, which is, I guess, for
lack of a better technical term, bonkers. It is my
(06:24):
favorite kind of bonkers. It's the kind of bonkers that
doesn't make sense to your mind. But the math works
perfectly and it keeps predicting absurd experimental conclusions that experimentalists
keep verifying. Yeah, because I guess you started with a
little nugget of an experiment, and then you worked out
some math, and then you find out that the crazy
(06:44):
math that it suggests is actually also true. Yeah, we
had to change the basic concept of what we thought
was going on at the very heart of the universe,
at the core of everything that's around me and you.
Even though it seems intuitive and like it follows rules
that we're familiar with from growing up, it turn turns
out that the tiniest little parts inside are following totally
different rules, which mean that the nature of reality is
(07:06):
quite different from the one that we thought it was.
And people work that out and they thought that's crazy.
And if it's true, it would mean you could do
this bonkers experiment, which would have this nonsense result, so
obviously it can't be true. And then business went out
and did the experiment and got the nonsense results, which
turns out to be the truth of our reality. Yeah,
(07:26):
because I guess at the core of it, it's kind
of weird for us humans to think of things that
are like two things at the same time, right, I mean,
not just in a conceptual ot but like actually in reality,
in quantum objects, things can be multiple things at the
same time. That's right, And that phrase in reality is
the key there, because we imagine at the very basic
level that there is a reality out there, that there's
(07:47):
a truth that even if we're not looking, the universe
is there and it's operating and it's following some rules
and it doesn't really matter if we are looking or not.
But the reality suggests that the universe is quite different
from that, that it does matter if you interact with it,
and that what is happening in the universe is not
exactly well determined until you interact with it. Yeah, you
(08:09):
might say that in reality, the way the world works
and the universe works, it's kind of fuzzy, kind of
not quite as solid as we might think it is.
From our everyday lives. That's right. We have a weird
and particular view of this quantum universe. We are only
used to interacting with enormous quantum objects like baseballs and
rocks and trees, which are quantum objects. But they contain
(08:29):
like ten to the twenty six quantum objects. And when
you have that many, they do things differently than when
you have one or two of them isolated to reveal
their sort of true fundamental nature. So today we're gonna
be talking about some really crazy experiments that try to
reveal exactly what the rules are of how these particles
work at the smallest scale when they're left alone. Yeah,
(08:50):
and so it turns out that also reality is not
just a little bit fuzzy, but it may not even
be as permanent as we think it is. Quantum in
for meation and quantum things. We think they're there for real,
but it turns out that maybe things can be taken
away from the universe. That's right. This fuzzy question of
what things are doing when you're not looking at them,
(09:11):
and if you look at them and then look away,
and it doesn't matter who's looking at them and how
they look at them, and whether they store the information
and look at it later. All these fun thought experiments
can help us try to understand what's really going on
at the smallest scales. So to be on the podcast,
we'll be talking about what is a quantum eraser? Now, Daniel,
(09:36):
is this a rubber eraser or what is it made
out of? It's something which will erase your mind if
you think about it too much. It's kind of stretch
it out like a piece of rubber exactly, and push
it too hard and it might just snap. No. Quantum
eraser refers to the concept of quantum information and what
happens if you create information and then erase that information
(09:57):
from the universe. So it's an extension of some really
on experiments that listeners on this podcast have heard us
talk about, the double slit experiment, which reveals how particles
can interfere with themselves and have the chance to be
in multiple places at once. We dog into that experiment
with a fun conversation with Adam Becker, the author of
What Is Real, and today we're gonna go double down
(10:17):
on those experiments and think about even crazier versions. Yeah, so,
what is a quantum erasor now, Daniels this a thing
or like a concept. Yeah, it's both. It was first
a concept and then people made it a thing. It's
like that in quantum mechanics, a lot of people think, well,
if the universe really is that way, here's a ridiculous
scenario that should lead to a silly result. And then
physicists go out and they do the experiment. They make
(10:40):
it real. They figure out a way to like build
it in their lab to test that crazy property universe,
and they get these ridiculous results, which in the end
you have to accept because that's what the experiment says.
They say, the universe really works that way. All right,
Well we'll dig into it, we'll rub that quantum erazor
all of our brains and see there is anything left
(11:00):
at the end. But first we were wondering if how
many people out there had thought about this question or
even heard the term quantum eraser. So Daniel went out
there and ask people on the internet what they thought
a quantum eraser is. That's right. So if you'd like
to be a participant in our virtual person on the
street interviews and love to speculate about physics without looking
(11:21):
anything up, then please write to us two questions at
Daniel and Jorge dot com. All right, so think about
it for a second. If a random physicist came up
to you on the street and you didn't run away first,
and I actually listened to them, and they asked you,
what is a quantum eraser? What would you say? You
are people's answers something an angry physics grad student uses.
(11:41):
I have no idea. Maybe something that erases things at
random and like gets rid of things, because that's what
quantum generally is, is the randomness. Quantum araser could be
something that we invent in the future to erase quantum
mechanics and quantum physics just because pretty hard to understand.
If we don't have it around anymore, we don't have
(12:02):
to deal with it. We can stick with general relativity
and regular gravity. So yeah, just get rid of that stuff. Really,
I don't know either if I were to compare a
quantum or race or to a regular racer, which essentially
just kind of distorts the little graphite particles and absorbs
(12:23):
them in in a way that you know, it gets
rid of the material on a piece of paper or something,
so that you know you can reuse that material to
write on something. Maybe a quantum race, or some sort
of force or phenomenon that causes sometime of particles to
break up from a given area or concentration, so that
the information is very very difficult to understand, or to
(12:47):
receive or to observe. Maybe well, I'm a teacher, and
I know that my students use a razors to hide
their mistakes, So I think a quantum e razor is
something that quantum physicists used to hide their errors from
everyone else. All I can come up with for that is,
sometimes you make a mistake and it's for the better,
(13:08):
it's a better idea than what you originally planned. And
sometimes it's a big catastrophe. But since you don't know
ahead of time, you use your quantum eraser two either
undo your mistake or make it permanent, and you just
roll the dice and let fate decide. No idea. Never
heard of a quantum eracer, but the image of a really,
(13:29):
really kindy eraser pops into my mind. So let's say
a tool that allows you to change the sub potomic
structure of stuff. I would guess that a quantum eracer
is having to do with erasing particles that have certain
(13:51):
quantum states. Thereby leaving behind particles that are in the
state you want, uh that, or it's the weapon that
they used in the movie Racer with Arnold Schwarzenegger. All right,
a lot of fun answers here. Everyone's a comedian on
the internet. Well, especially if I have no idea what
we're talking about, then they got to go to that
joking place. I like the joke about the really tiny eraser, right, Yeah,
(14:14):
like if you have a quantum pencil, I guess it
would have a quantum eraser on the one end of it. Yeah,
or e raises quantum particles or something like that. I'm
imagining a super tiny little vacuum cleaner that like slurps
up electrons. Yeah, and and those one with them passes
them through a wormhole into another universe. I guess, tantalizing.
All right, well, let's get into this concept of a
quantum erasor Daniel, you're saying it has something to do
(14:36):
with the double slit experiment. Now, this is going to
be kind of hard because I feel like this is
a podcast and it's an audio only and this is
a very kind of visual experiment. But I guess we
can try our best to describe what it is. Yeah,
maybe we can add a dance element to it. Do
you think that will help? Yeah? Or I could draw
our tunes. I'm doing watercolor painting at the same time
as we do our podcast. By the way, Oh really,
(14:57):
you're in a quantum artistic s am. Know. This quantum
er racer is an experiment that's like a permutation or
an add on to the basic double slit experiment. So
to understand why the quantum racer is so weird and crazy,
you definitely have to understand what's going on in the
double slit experiment. And so I think we're gonna have
to use our words to describe the wiggly crazy nature
(15:18):
of that experiment, and then we can build on it
to get to the quantumer racer. All right, So I
guess the double slicks experiment starts with a single slit first.
I guess we'll explain that one, and then we'll multiply
by two. So the basic experiment is like you have
a wall, like a barrier, like a plate of metal,
and you cut a little slit on it, like a
little opening that's long in one direction, and then you
(15:40):
shoot like a laser or like just a regular beam
of light through it and then onto a wall behind
the first wall. Yeah, exactly. So imagine your mind some
source of light. A laser is good because then you
have photons of all the same wavelength in the same direction,
and then a screen on the other side with the
laser hits. What do you get. You get a laser spot. Now,
as you say, put something in between, like a barrier
(16:01):
that has a very thin slit in it. Then what
do you see on the screen Instead of the full
laser spot. Now you see like a slice of that spot.
You get a smooth pattern on the screen, but it's
sort of cut by the slit in the barrier, and
it has smooth edges, not a sharp edge, because that's
what happens when light goes through a slit, intends to
like spread out a little bit and smooth out. So
the thing you start with is this single slit where
(16:23):
the light goes through and hits the barrier on the
other side, right, and you get a smooth light pattern
on the other side. Now, the weird thing is in
what happens if you put a second slit next to
the first slip, right, that's right, So now you put
two slits really close together so that the beam could
pass through one slit or the other slit, and once
you get on the other side, instead of having like
(16:44):
two smooth patterns or you know, the simple addition of
two patterns like you saw before, now you get an
interference pattern, which means that you get these patterns of
light and dark and light and dark and light and dark.
And what's happening there is interference. Just like if you
have waves when you add them, if one wave is
going up while the other wave is going down, then
(17:04):
they cancel each other out. Whereas if one wave is
going up and the other one is going up at
the same time, then they add up on top of
each other. They get twice as strong. So the interference
pattern has these slices that are twice as bright as
the previous pattern, and these dark slices as well. And
that's because you have two sources of light. Now each
of the slits is giving you photons and they can
either constructively or destructively interfere on the screen. Right because
(17:28):
I guess you're shooting a laser at both slits at
the same time, or you're like shooting a laser and
it and the beam of the laser kind of goes
through both slits at the same time, right, Yeah, the
slits are very narrow and very very close together. This
only works if the scale we're talking about here is
sort of related to the wavelength of light that we're
shooting at it. So this needs to be very microscopic. Right,
So if you have one slid, you get a fuzzy,
(17:49):
like a plain fuzzy image on the other side. But
if you have two slids, then suddenly it's not a smooth,
fuzzy image. It's like a weird Rippley kind of image,
which means at somehow light is interacting with itself. Yeah,
in this case, we don't know if the light is
interacting with itself. You could say, hey, look, light is
a wave, and waves interfere. This happens with waves in
(18:10):
the bathtub, it happens with waves in the air. Like
noise canceling headphones, right, they generate a second pattern of
noise to cancel out the noise that's coming into your ear.
So interference in waves is not necessarily a quantum mechanical thing.
It's just a wave thing. So in this version of
the experiment so far, you could just say, look, light's
a wave. It's interfering, No big whoop. It gets quantum
(18:31):
when you remember that the beam is actually made not
of waves but of photons, little individual packets, and so
you can take it to the next step by slowing
down the experiment and dimming the laser. So that's shooting
like one photon through the experiment at a time. Yeah,
you shoot one photon at a time, and then you
would think that just throwing like one ft doon at
(18:52):
a time, this photon would pick like the right or
the left slid and then end up on the other
side of the wall and you would get the same
fuzzy smooth pattern. But the weird part I guess is
that you're shooting one ft at a time, but you
still get the ripley kind of interference pattern on the
other side. Exactly. You expect that if you shoot one
photon at a time that it can't interfere because you
were thinking, well, the interference comes from two photons going
(19:14):
through both slits at the same time. Now you have
just one photon in the experiment, so what's it interfering with?
Because you still see the interference pattern on the other
side of the screen and you shoot one photon through
at a time. It's just that it takes longer to
build up if you watch it for an hour or so.
As those photons go through, one lands here, one lands there,
one lands this other spot, it gradually builds up that
(19:36):
interference pattern. So what's it interfering with. It's interfering with itself.
It has the probability to go through both slits, and
that wave function, which controls where a quantum particle goes,
interferes with itself and creates this probability distribution on the
screen for where it might land, and that probability distribution
has the interference effects inside of it, and that's why
(19:58):
you get this interference pattern. Every photon that goes through
like randomly pulls a number from the probability distribution on
the screen which has the interference pattern built in, and
lands there, and gradually it builds up that distribution. It's
almost like, you know, if you were to shoot a
photon as a little ball, it would go through one
of the slids, but because it's quantum, it's almost like
it's going through both slits at the same time. Right.
(20:20):
That's that's kind of the quantum thing. It's going through
both slits. At the same time, and then it's sort
of going through both slits and then interacting with itself
in a quantum way so that when it gets to
the screen it's not a smooth pattern. Yeah, it's tempting
to say that it's in two places at once, or
that goes through both slits at the same time, and
that's our tendency to try to like tell a story
(20:40):
for what happens. But I'm not sure that's the right
way to think about it. The way I think about
it is that it has a probability to go through
both at once. What it actually does is not determined,
you know, until it gets to the other side. So
what happened when it went through the slits we don't know.
We might never know. There isn't necessarily a story there.
So it's a all change in wording, but an important
(21:01):
change in meaning for me to say that it had
probability to go through both slits rather than it actually
went through both. Right, it's like saying that it's not
that the cat is alive and dead. It's just said
it has the same probability, where it has a certain
probability of being alive and a certain probability of being dead.
All right, Well, then now the weird part here. Now
it's going to come when we try observing this photon,
(21:21):
and that's when we get into this idea of quantum razors.
So let's talk about that. But first let's take a
quick break. Alright, we're asking the question what is a
(21:42):
quantum erasor? And now Daniel feel like half of my
brain is already erased trying to talk about quantum objects.
And now we were explaining the double slit experiment, and
so I think we were done with that. Like if
you shoot a laser at a two small slits on
a screen, then on the other side you're gonna get
an interference parent because of the way the quantum probabilities
(22:02):
kind of affect each other. And so now the weird
thing happens when you try to like add a detector, right,
when you try to see which slid it actually went through.
That's right, because our tendency is to want to know, like, well,
what happened, Right, did it go through one slit or
did it go through the other. We feel like it
must have gone through one or the other, right, because
you know it was over here and then it's over there.
So it must go from here to there is our sense,
(22:25):
and so trying to get it like a more accurate
understanding of what happened. You can add a little detector
when that gives you a signal if a photon goes
through slit A, for example, instead of slit B. So
that way you can know, hey, did it go through
slit A or slip B. Because you know, photons are observable, right,
you can interact with them. They make splashes of light,
you can measure them. They are quantum objects, but they
are also physical. And so what happens when you do that,
(22:47):
when you ask, when you insist on knowing which slid
it went through, is that the interference pattern disappears. When
you add that detector that just tries to understand which
slid it went through, then the interference pattern is gone.
So if you try to like measure at the photon
as it goes through slid how would you even do that?
Don't you have to stop the photon to do that.
That's the crux of the matter right there. To measure
(23:08):
a photon, you have to interact with it somehow. You
can't just like observe a photon without interacting with it.
You know, a photon that like passes in front of you,
you can't see it. It's a piece of light, but
unless it hits your eyes, you can't see it, or
unless you put something in front of it to stop it,
measure it and then re emit it. Right, So, for example,
(23:30):
the reason you see something in front of you as
red is because the photons hit that object and then
emitted red photons. So you can't see things without interacting
with them, even light, right, you need to interact with
it somehow. So you can make up lots of different
physical systems that could do this, but the simplest one is,
you know, just like a simple photon detector, a photo
multiplier tube for example, or a scintilator screen that indicates
(23:53):
when a photon went through and then re emits it
on the other side. Oh, I see what you're saying,
Like if you catch it and then really sit back
on the other side, that's one way you can measure
the photon. And you'd like to think, oh, can't I
just take a peek? Can I just look and see
where it went without touching it, without interfering with it,
without messing with it in any way. But you can't
(24:13):
do that. Quantum mechanics tells us that the only way
to get information about an object is to interact with it.
You can like bounce a photon off of it, or
you can bounce an electron off of it, or you
can put another little screen, but somehow you have to
interact with it. You can't get information from that particle
without somehow interfering with its path. Yeah, because I guess
in our everyday lives were used to this idea of
(24:35):
being able to like see things but not touch them,
and so we think we can tell where something is
without actually like influencing it. But when you get down
to the smallest of levels, like all seeing is interacting
in a way, that's right, and it's also true with
the macroscopic scales, just that you don't notice it. Like
if you are walking outside at night and you want
to know, hey, is there a rock in my path?
(24:55):
You turn on your flashlight. You are shooting photons at
that rock. Those photons are hitting the rock, they're warming
up the rock. Then the rock is reflecting photons back
at you. So yeah, you're not touching the rock, but
you're definitely interacting with the rock and you're changing its
quantum state. It's just like it doesn't like really heat
up the rock or push the rock far away. When
these are quantum particles that they can have significant effects
(25:17):
if you interact with them by shooting beams of light
at them or other particles. All right, So then in
the double slit experiment, meant if you try to measure
these photons before they hit the wall in the bag,
then what you're doing is you're collapsing the quantum wave, right,
You're messing up the quantum information. Yeah, so here's where
the different interpretations of quantum mechanics. I'll tell you different
(25:37):
stories for what happens the experiments. Save you measure the
photon before or after the screen, that the interference pattern
goes away. The classical interpretation of quantum mechanics, the Copenhagen interpretation,
is what you just described. It says that the probability
to go through both slits only exists if you haven't
made a measurement. But then if you interact with it,
it collapses the wave function. Now it can only go
(25:58):
through one slit or the there so there's no interference
because the interference came from the ambiguity came from the
probabilities to go through both. The many worlds interpretation tells
a different story. It says collapse is nonsense, that doesn't happen,
it's ridiculous. It says that the universe splits into two,
one where the photon went through one slit and one
where the photon went through the other, and you're in
one of those universes and not the other. Right, So
(26:20):
that's those are the two ways in which you can
interpret what happens. So how does that relate to, like
the information and the quantum information. So the idea here
is that you have the information about whether it went
through slit A or slit B, and that's what destroys
the interference because you've made this measurement. Somehow it changes
the experiment. And you know, this is not something that
(26:40):
we understand very well, this whole concept of measurement and
quantum mechanics. And that was the topic of our episode
with Adam Becker, and then recently we did an episode
with Carlo Rovelli and he's got a whole new theory
for how to understand measurement and quantum mechanics. It's not
something that physics understands very well, how interacting with something
changes its way? Function, does it collapse it as it
split the universe? All of this kind of stuff. But
(27:02):
the key idea here is if you extract information about
which way the photon went, then there's no interference. Right,
you somehow get rid of the quantumness of it, Like
when when I poke something, it's no longer quantum if
you poke something with a classical object like a big detector, right,
that big detector can't be in two stays at once.
It can't be like well, yes I saw it and
(27:23):
no I didn't. It has to make a decision, and
so it decoheres, and you get this weird thing where
quantum object is interacting with the classical objects and so
now it has to like follow the classical rules. So
the quantum or racer is an attempt to get around that,
is to say, what if instead of poking it with
a big finger or a big classical detector, what if
we got this information but we somehow kept it quantum
(27:45):
at the same time. I see. So it's almost like
the cat in show Dingerous Box, Like before you open
the box, it's both alive and dead. The probability of
it being one or the other. And if you open
the box, that's the classical way of checking it out,
Like you open it and it's either alive or dead,
and and then you get it of the quantum probabilities.
You're saying, can I like somehow, you know, poke my
(28:05):
quantum finger into the box and measure but not kind
of destroy that superpocisition of being both alive and dead exactly?
The quantum or racer experiment tries to do that as well.
Let's try to get this information out, but not look
at it directly, not use like our classical objects, our eyeballs,
our brains, even our computers to access that information, so
(28:27):
we can stay in a quantum superposition, so we can
make a decision later about whether we want that information.
And here's where the mind bending stuff comes in. If
you can like extract that information about which way the
photon went, keep it in a quantum state by storing
it in some other entangled particles, then you can decide
after the photons I hit the screen whether or not
(28:48):
you want to know which way it went wait to
see it again. So the idea is you want to
know which way the photons went, right? Did they go
through slit air slip B. You know, if you add
a classical object like a big detector, you're going to
collapse the a function. So instead you add a quantum
detector one that can record this information, but maybe without
collapsing the state of the wave function. Somehow it gets
(29:09):
this information. But because it's a quantum object, it doesn't
trigger the collapse, right, So it can be like entangled
with the photon without like forcing the photon to deco
here completely. And so it's different from interacting with like
your big body or something. You interact with it like
with a single particle, and that stores the information about
which way the photon went, but you haven't looked at it.
You haven't collapsed that wave function yet. You let the
(29:30):
photon then go hit the screen, and then after the
photon has already hit the screen and decide where it's
going to land, then you access that quantum information. It's
called the delayed choice version, where you decide after the
photon has hit the screen whether or not you want
to know the information about which way it went. You're
saying that the photon did go through one of the
two slits, like once it hits the screen in the
(29:50):
bag then it sort of chooses a history of having
gone through the left or the right slit. That's right.
This is trying to like force the photon to make
its decision about whether or not to make an interfere
it's pattern before you decide whether you want to know
which that it went through. So it's sort of like,
you know, trying to play quantum bluff with the photon.
And that's when we get into these really funny questions
(30:11):
of like, how does the photon know whether it's going
to be measured? How does the photon know whether you're
going to have information about it. It's almost like you wanna,
you know, peek inside of the short Inger's box, but
not look at the answer, so that it's still alive
and dead inside the box. But you sort of have
the answer in your pocket, but you don't. You haven't
looked at the answer yet exactly. And so why is
(30:32):
that call it a quantum eraser? Right, So this is
not yet a quantum eraser. This is the delayed choice version,
the delayed measurement, Yes, exactly, delayed measurement choosing whether or
not you have the information, that's the delay. So you
might wonder, like, well, what happens on the screen. What
does the screen look like? What does the experiment look like?
If you do this, If you capture this information in
a quantum state, but you don't look at it yet, well,
(30:52):
what happens is you don't see interference on the screen
because by doing this, by slurping this information out of
the photons, you have stored the interference. But people think, well,
that's interesting. But I haven't yet looked at that information, right,
So what happens if I then erase that information? This
is where the quantum eracer comes in. If I take
that quantum information which is stored in these quantum objects,
(31:14):
but I haven't looked at it yet, if I erase
that information somehow, can I then recover the interference? Can
I make the interference pattern reappear on the screen by
adding this quantum eracer, which like deletes that information from
the universe because I never peeked at it. What are
you saying that this is an actual experiment, Like we've
sort of intercepted the photon before it goes into the slids,
(31:34):
and we've stored that information and we see that it
now it doesn't generate an interference pattern. A weekly pattern
on the screen, even though nobody really knows which slid
it went through. That's right, nobody knows which slid it
went through, though it is in principle stored in this
quantum object. Although that quantum object can be in a superposition,
it doesn't have to be in a definite state. Right.
(31:55):
You can say there's a probability of one and a
probability of the other, and we don't see that interference patterns.
And then people thought, well, what if we delete that information?
What would the universe do if we measure the photon
but don't look at it, keep in the quantum state,
and then erase that information. Can it somehow go back
and recover the interference pattern? I see you're saying that
maybe it's not the fact that it's interactive with your
(32:16):
little secret finger poking quantum poking that destroyed the interference pattern.
Maybe if I poke it with my quantum finger and
then I destroy my finger, will it go back to
being a quantum object? Does that what you mean? Like,
we know that if I poke it, even with a
quantum finger and not look at the answer, it destroys
the quantum information. But now what happens if I poke
(32:36):
it with a band of finger and then the story
to finger will go back to being a quantum object.
Is that kind of the idea, that's the quantum eraser
is destroying that quantum information you've extracted from the experiment
but haven't yet looked at, so it's still quantum. So
that's the crazy experiment. And I can hear you react
and say, what is that real experiment? Did we really
do that? And yes, we have really done this experiment.
We have done it with photons, and you can go
(32:58):
up and google and learn all about the details of
this experiment. I think there's a slightly simpler version that's
easier to talk about, where you use electrons. But the
principles are all the same. And so how can you
destroy quantum information? Well, for example, if when the photon
is passing through the slit, you have some detector, and
that detector takes an electron and puts it in like
a spin up state. If the photon went through one
(33:19):
slit and it's been downstate, if the photon went through
another slit. This is just a way to like store
that information about which way the photon went and keep
it quantum. Right, we don't want to mark it on
a piece of paper, or put it in a computer
or some big classical object. We want to keep it
as quantum information. So here the electron is just like
a single cube bit. It contains some quantum information. But
(33:40):
it can be in a super position. It can be
a little spin up and a little spin down. We
don't know yet, right, but I feel like you bumped
the photon right, Like the photon was going through the slip,
but you made a bump into this electron. And now
it feels like it's in now an impure experiment because
you bumped it right, not just in a quantum way,
but you did sort of. It's not maybe the same photon,
or it's not the same path as a photon who
(34:00):
didn't bump into an electron. That's right, And that's why
you no longer see the interference. Right, you add this
experiment where you're bumping it into the electron, it destroys
the interference pattern because that information is now stored in
the electron, so it can be extracted. The knowledge of
which way the photon went can be measured in the universe,
and so that destroys this interference pattern on the screen.
So you're right, it's a different experiment, right, Like the
(34:22):
quantumness went from being on the wall behind the screen
to now being in this electron you poked it with,
And now I guess the question is if I destroy
the information in that electron, do I get back my
weekly pattern on the screen. That's the idea. It's totally
mind bending and crazy what actually happens. I love it.
And so you take this electron and you might wonder like, well,
(34:43):
how do you destroy the information? How do you erase
the information? Well, it's actually not that hard to erase
quantum information. It happens all the time. Like if you,
for example, measure particles momentum, then that scrambles your knowledge
of the particle's position because the Heisberg uncertainty prints will
says you can't know both very very precisely. If you
have a particle, for example, you measure its position really precisely,
(35:05):
and then you measure its momentum, then you've erased the
quantum information about its position because you can't have both simultaneously.
So you can do certain things sort of similar to
this electron. You can't know an electron spin in one
direction and in another direction at the same time. So
if you want to erase the spin up and down information,
all you need to do is measure the spin of
(35:26):
the electron sort of left right, and that will scramble
the information about the electron spin up down. I plucked
the box with the cat with my quantum finger, and
now instead of running my finger through a filter, that
then kind of scrambles or filters out the information from
the cat. That's right. So now it's no longer possible
to know which state it was in. Was it's been
(35:48):
up or was it's been down. We don't know anymore.
And it's scrambled. It's not like the information existed and
we've overridden it. It was in a quantum superposition. It
was undetermined, and now the information about those probability is lost.
So that's the quantum eraser. It says, destroy the information
that you've extracted from this experiment. All right, So we
did the experiment, actually we poked it with something and
(36:09):
then we erased the information. And did they actually somebody
actually built this. Somebody actually built this, and they did it.
They did this experiment. And so there's a lot of
discussion of this kind of experiment online, and I find
a lot of these to be sort of misleading because
they suggest that what happens when you apply the quantum racer,
when you erase this experiment, is that the interference pattern
like reappears on the screen, which is impossible because you
(36:32):
could do this like quantum eraser experiment like years later,
after you've already done the original experiment. You know, you
could like store these electrons somehow and then five years
later decided to erase the information. You can't go back
in time and then change the interference pattern on the screen.
So that's not what happens. That would be crazy in
bonkers and awesome. But instead, what happens is that if
(36:52):
you do this, if you erase the quantum information, then
you are making a measurement of those electrons. You're measuring
them like left right instead of up down. If you
take those results and you look at only the ones
that have like electron that turned out to be left,
or only the ones that electron that turned out to
be right, then you see the interference pattern. So the
photons that had like a right spinning electron, you see
(37:13):
an interference pattern in those photons, And there's an interference
pattern in the photons that had a left spinning electron.
If you put them together, they add up to the
same smooth shape. So it's sort of like the interference
pattern was hiding inside that smooth shape. And if you
scramble the information that you knew about which photon went where,
you can recover the interference pattern from within the smooth
(37:35):
shape that you saw on the screen. All right, But
then that still requires an observation, right, because you're seeing
some of that information you thought you destroyed, but you
didn't really destroy it in a way, or like, did
you destroy the information in one direction and so the
quantum objects sort of adjusted into the other direction. Yeah,
you destroyed the original information, you can't know which way
the photon went right, and so that allows you to
(37:58):
have interference, and you can recover that interference if you
look at like some of the photons. And the reason
is that you know some of these photons are entangled
with some of these electrons in this way, and you
need to like know how to pull out the subset
of photons that have the interference pattern you're looking for.
You can only get that if you have erased the
quantum information you're looking for. If you access the quantum
(38:21):
information directly. If you measure spin up or down so
you know which photon went through with slit, then that
collapses the way functions essentially and means that you see
no interference. You cannot access any interference only if you
erase the information in those electrons by measuring left right
instead of up down. Can you then go back and
split the photons into two categories, each of which shows interference. Interesting?
(38:43):
All right, let's get into what this all means and
what it can mean about how we see reality. But
first let's take another quick break. All right, Daniel, I'm
(39:03):
still sort of stuck in the cat in the box
experiment because I feel like that's a little easier to grasp.
So we had a cat in the box, we puked
up with a plantum finger, and then we measured my
finger in one direction, and then we sort of destroyed
that information by measuring the finger in a different direction,
and then we see that the cat is still sort
of alive and that but only if we use the
(39:25):
information we got from the finger, right, Yeah, exactly. I
think if you want to talk about cats and boxes,
then you'll need like a hundred cats and a hundred boxes,
because in the end, This is a probabilistic effect. Just
like with the double slid experiment, these interference patterns are
only obvious if you do a lot of photons, so
you can see the patterns. Because a single photon could
hit the screen wherever, you can't tell if you're seeing
(39:47):
an interference pattern or not from one photon. So let's
say you have you know you're a cat person. Your
house is swarming with cats. Each one you put in
a box, right, and then you make this quantum measurement
of each one, but you don't look at the result.
You poke it with a quantum finger, you don't look
at the result, all right, So then you've figured out
I guess with this experiment that the objects sort of
(40:09):
goes back to being quantum, but not really because maybe
you're in a way, you're sort of cheating, right, You're
using some of the information you got from poking it
to make it look quantum again in a way. Right,
It's almost like the photon went back to being quantum,
but only half quantum because you were able to measure
some of it. Yeah, exactly. It's a bit mind bending
because we like to think about what happens to these
(40:31):
particles like what are they really doing? And we like
to think that things can't go back in time and
change their decision. And you know, the core fuzziness of
this experiment is that if you think about these things
in terms of particles and waves, you like to think
that it's a wave and then it gets collapsed into
a particle after it goes through the slit if there's
a detector there, and then needs to decide like am
(40:52):
I a particle or am I a wave before it
hits the screen, so that it can either make an
interference pattern or not. Right, it doesn't seem like it
would make sense for or that to depend on what
you do later on, because you can make this like
quantum information decision a year later, or ten years later,
or a thousand years later. So some people are attempted
to say that this means that there's retro causality that
(41:14):
based on what you do later, you can go back
in time and change the results of the experiment. I
think that's kind of nonsense. Really, what you're doing here
is just interpreting the experiment in a different way using
additional information you've extracted from the experiment. As you said,
you're sort of cheating but it's backwards right now. You're
making a measurement to know how to separate those photons
(41:35):
to see the interference pattern. You're not making a measurement
about which way the photon went about which slid it
went through. Instead, you're just separating the photons into the
ones that were entangled with electrons in one way versus
photons that were entangled with the electrons in the other way.
And those subsets do have the interference going on, it's
just it was masked because when you add up the
two kinds of interference, they add up to the smooth pattern.
(41:58):
It's like it's still the same sort of get to
a quantum that was going on before. Like the photon
looked like it had lost this quantum information, but really
like if you take that information that you got from
the poking with the finger, then you can sort of
find its quantumness in the direction that you didn't poke
it in. Yeah, exactly. And so it's a really fun
experiment to try to think about this nature of decoherence.
(42:21):
Like you were saying before, if you poke something with
a big classical object, it decoheres. It gets entangled with
the whole environment. Millions and millions of particles, and so
all of its quantum properties are essentially lost. Here. What
we're doing is we're like kind of cheating, we're deco
hearing it only Italian a little bit by interacting with
it with a quantum object. So we can play quantum
games with that decoherence later and in the end recover
(42:45):
some of that interference by erasing that information. And so
it's really sort of a great mental exercise to think
about whether you understand decoherence. And we had a whole
podcast episode about what quantum decoherence is. It's closely connected
to this question of what is a quantum measurement and
what happens when you measure something, but it's not quite
the same thing. It's more about whether quantum properties can
(43:07):
be observed because the different quantum states are still coherent,
whether they add up and cancel out in just the
right ways to make something have a quantum effect. I
think the main point is that you know everything is quantum,
but quantumness of something can exist kind of in different
directions in a way, or in like different aspects that
are part of the whole, but it's still you can
sort of take out half of the quantumness of an
(43:27):
object and still for serve sort the other half of
the quantumness that it has in the other direction. Yeah, exactly,
And so you know, the trickiness here relies in the
fact that, like by becoming entangled with a single electron
rather than the whole environment, these photons hit the screen
only become kind of decohered, right, and so it's just
a single particle to worry about. We're sort of able
(43:50):
to think about measuring in different ways, and that's really fun,
and it's easier to think about what this experiment means
in some interpretations of quantum mechanics than in others. Like
in many world it's not that big a deal because
the whole universe has a wave function and now we're
just talking about the quantum wave function of the photon
and the electron and they're kind of entangled and that's
no big deal, whereas in like a strict theory where
(44:11):
you have collapse, then you have to wonder like, well,
did the photon collapse or not, because if I don't
destroy the information and I measure it, then the photon
has to collapse because I knew which way it went.
But if I do destroy the information, then how am
I getting an interference pattern later on? Because for that
to happen, it has to stay a wave. And so
this is sort of troublesome for the collapse theories of
(44:34):
quantum mechanics, not so much trouble for other theories like
many worlds and relational quantum mechanics. Doesn't it just mean
that maybe like the wave collapse in one direction but
not the other direction, Like couldn't you still you know,
use the coping dating interpretation and just say that it
collapse in like one direction and not the other. Well,
the electron is the one that has these multiple directions
of information that spin up down versus spin left right.
(44:57):
The photon is either interfering or it's not, you know,
and it either collapses and it's just like a single
source which gives you the smooth pattern, or it doesn't
collapse and you have the wave function which does interfere.
There aren't multiple directions there, and so it's hard to
understand how it collapse theory can really work because that
does kind of require going back in time and like
(45:18):
uncollapsing the way of function. To me, collapsing the way
function makes no sense at all it's not even consistent
with quantum mechanics because it destroys quantum information in a
way that we know violates basic principles and it violates
the like time continuity of quantum mechanics that says you
should be able to run experiments forward and backwards. So
the collapse theory never made any sense to me, really,
and I think this experiment really highlights how it's sort
(45:41):
of nonsense. But then the only other interpretation that we
have is the multi world theory, right, which this multiverse
theory that every time a quantum object makes a decision,
that the two universes are created. Yeah, that's another interpretation,
and that one is pretty happy with this experiment. There
are other interpretations that you can use the are consistent
with this experiment, Like relational quantum mechanics works well with
(46:03):
this because it says that like, hey, everything in the
universe has its own measurement of these things, and so
it doesn't matter what you measure, there is no reality anyway.
And then there are also like other variants of collapse
theories that are not as strict. You know, let's say, well,
collapse happens in this way or in that way, so
there's a whole spectrum of them, but this is troublesome
for like the most hardcore collapse theories. All right, well,
(46:23):
then I guess to answer the question what is a
quantum eraser? I feel like the answer to that is
sort of straightforward, but it's sort of the implications of
what quantum eraser can do. That's really sort of what
we spend in our check on and that's really hard
to sort of get your head around. So a quantum
razer is just taking quantum information from something and erasing
it in a way. Right, Like, if I have quantum
(46:45):
information stored in one direction of an electron spin, by
measuring it in the other direction, I can destroy that
quantum information. Right, that's the idea of a quantum razor.
And if that electron happens to be entangled with photons
which may or may not be interfering, then whether or
not you erase that information or not can determine whether
or not you can see interference in those photons. Well,
(47:06):
it doesn't determine whether or not you can see it
the way it tells you how to look for that interference. Well,
if you measure the which way of the photons using
those electrons, then you cannot see interference. The only way
to see interference is to destroy that information and then
use the results of destroying that information to pick out
the interference patterns from the photons. You can't do that
if you measure which way the photon went right, right,
(47:28):
But you're sort of still measuring the electron, and then
that's telling you how to look for the interference in
the photon parent right. Yes, you're measuring the electron, but
you're not measuring which way the original photon went. You're
measuring something else about the electron, which destroys that information.
All right, it sounds like we erase people's brain and
hopefully not their time for the last hour. Thanks very
(47:50):
much for going on this journey into the weird quantum world.
I love these thought experiments, the ones people think of
and say, whoa, what would actually happen? Because that's the
fun thing about experien mental physics is confronting the universe
and saying, all right, universe, show us what you got.
We set up a situation that forced you to reveal
what's happening, and the quantum universe always responds with something crazy.
(48:11):
And that's why we're here. To uh talk about the
craziness and to hopefully get you to wrap your mind
around all the different and interesting implications about what it
means about the things around you that you see in
touch or maybe don't see your touch. And so, if
you're a person who likes questions and maybe even answers,
check out our book Frequently Asked Questions about the Universe,
available now and coming out in just a couple of weeks.
(48:32):
You can find the links at Universe f a Q
dot com. All right, well, thanks for joining us. We
hope you enjoyed that. See you next time. Thanks for listening,
and remember that Daniel and Jorge explained. The Universe is
a production of I Heart Radio or more podcast from
(48:55):
my Heart Radio. Visit the I Heart Radio Apple Apple
Podcasts or where every you listen to your favorite shows.
Hey, Jorhan, Daniel here, and we want to tell you
about our new book. It's called Frequently Asked Questions about
the Universe because you have questions about the universe, and
so we decided to write a book all about them.
We talk about your questions, we give some answers, we
make a bunch of silly jokes as usual, and we
tackle all kinds of questions, including what happens if I
fall into a black hole? Or is there another version
(00:22):
of you out there that's right? Like usual, we tackle
the deepest, darkest, biggest, craziest questions about this incredible cosmos.
If you want to support the podcast, please get the
book and get a copy, not just for yourself, but
you know, for your nieces and nephews, cousins, friends, parents, dogs, hamsters,
and for the aliens. So get your copy of Frequently
Asked Questions about the Universe is available for pre order now,
(00:46):
coming out November two. You can find more details at
the book's website, Universe f a Q dot com. Thanks
for your support, and if you have a hamster that
can read, please let us know. We'd love to have
them on the podcast. Daniel, do you ever feel like
(01:08):
you really understand quantum mechanics. No, you know, I think
it's probably just too alien for us to really ever
feel comfortable with. I guess it's too bad there aren't
any macroscopic big quantum optics we can really like poke
and play with, I know, like a big fat electron.
But actually I'm working on a theory that children are
governed by the rules of quantum mechanics. Oh really, like
(01:29):
there's uncertainty about where they are. Have you lost your kids? No,
but I've noticed that you can't like observe your children
without sort of perturbing the system, right, I think I
know what you mean. Like they won't do their homework
unless you're there watching exactly. And when I walk into
the room somehow all their conversations collapse suddenly into silence. Yeah,
it's like sure the anger as children, they're both excited
(01:51):
and sad to see you. Hi am for Handmad cartoonists
and the creator of PhD comics. Hi. I'm Daniel. I'm
(02:11):
a particle physicist and a professor UC Irvine, and I'm
always in many quantum states at once. Really, so that
does that mean you're not real? It means I don't
even know if I'm real man. Well, I hope you are,
because that would mean that I'm talking to myself right now,
and that would be a little concerning. Maybe you are
the only brain in the universe and the rest of
the universe is just part of your mind. That would
(02:32):
make a lot of sense why I'm so successful and
good looking. But anyways, welcome to our podcast. Daniel and
Jorge explain the university production of I Heart Radio, in
which we do try to explode our minds out to
capture the entire universe. We want to take this vast, glittering, crazy, violent,
wild and white cosmos and wrap it all up inside
(02:53):
our brains. It's not enough for us to just live
in this universe to experience it and to see it.
We want to understand it. We want download the whole
thing into our minds, and that means understanding the basic
rules about how it works, what's really going on with
tiny little particles or whatever is happening at the smallest scale.
(03:13):
On this podcast, we dive the deepest you can into
the hardest and trickiest of questions and we try to
explain all of them to you. Yeah, because it is
a pretty tricky universe. It's full of interesting rules and
interesting phenomena that happens at the smallest of levels and
at the largest of scales, and a lot of it
is understandable, even if it's not very um intuitive to
(03:34):
think about. That's right. And I look at the universe
like a big puzzle. It's like a detective novel or
a murder mystery, and I want to figure out who
did it. I want to understand how it works. It's
amazing to me, sort of philosophically, that the universe is
presented to us that way, like this big puzzle that
isn't obvious to figure out, but yet can somehow be
understood if you push hard enough. Do you think the
universe is understandable? Daniel? That's a big question in physics,
(03:57):
isn't it. It's a big question in the philosophy of
physics x and And my answer is it doesn't make
sense for it to be understandable, Like how could it
be possible that the complexities of this maybe infinite universe
could be stored in the minds of a human. On
the other hand, we have all these theories that work
really well, like surprisingly well, and so I don't know
how to hold those two ideas. In my mind, they're
(04:19):
in quantum conflict. You're both confused and feeling smart at
the same time. That's what this podcast is all about.
I'm the confused quantum state and you are the feeling smart.
That's right. And all scientists sort of hold those two
feelings in their mind at once, like, look at all
we have understood, and yet look at all that we
do not, And that's both exciting and terrifying. Yeah, and
(04:39):
we have a lot of questions about the universe. And
by the way, speaking of questions, we have a new
book coming out pretty soon in on November two. That's right.
Jorge and I celebrate asking questions about the universe, and
we love thinking about these questions. We love hearing about
your questions about the universe. So we wrote a book
that wraps up the most frequently asked questions that we
(05:00):
get about the universe. Yeah. So, if you like this
podcast and you want to support us, please check it out.
It's called Frequently Ask Questions about the Universe and it's
on a pre order right now. You can order it
now and get it as soon as it comes out.
And I think you know what's important is that we
kind of didn't quite write it for our listeners, right, Daniel.
We kind of wrote it for the people that know
our listeners. You know, if you ever have like a
(05:21):
nephew or a cousin or an uncle who you want
to share this amazing information about the universe, I think
this is the book for you or for them, that's right.
So every one of you out there, you should buy
five copies and give them to all your friends and
family so that they can understand the answers to questions
like where did the universe come from? Or how can
we travel to the stars. Yeah, we tackle all kinds
(05:41):
of pretty cool questions in it, and we try to
answer it for people like your relatives and friends with
all kinds of interesting and clear answers. And also cartoons,
which is I think something you don't see in every
day in physics books. That's right. All these awesome fun
drawings that help clarify the topic and amuse you along
the way. That or added to this book. So if
(06:02):
you like this blend of physics and silly jokes, then
I think you'll enjoy this book. So go out and
get your copy. You can find it at universe f
a Q dot com all right, Well, speaking of questions,
we are tackling a question today and it has something
to do with quantum mechanics, which is, I guess, for
lack of a better technical term, bonkers. It is my
(06:24):
favorite kind of bonkers. It's the kind of bonkers that
doesn't make sense to your mind. But the math works
perfectly and it keeps predicting absurd experimental conclusions that experimentalists
keep verifying. Yeah, because I guess you started with a
little nugget of an experiment, and then you worked out
some math, and then you find out that the crazy
(06:44):
math that it suggests is actually also true. Yeah, we
had to change the basic concept of what we thought
was going on at the very heart of the universe,
at the core of everything that's around me and you.
Even though it seems intuitive and like it follows rules
that we're familiar with from growing up, it turn turns
out that the tiniest little parts inside are following totally
different rules, which mean that the nature of reality is
(07:06):
quite different from the one that we thought it was.
And people work that out and they thought that's crazy.
And if it's true, it would mean you could do
this bonkers experiment, which would have this nonsense result, so
obviously it can't be true. And then business went out
and did the experiment and got the nonsense results, which
turns out to be the truth of our reality. Yeah,
(07:26):
because I guess at the core of it, it's kind
of weird for us humans to think of things that
are like two things at the same time, right, I mean,
not just in a conceptual ot but like actually in reality,
in quantum objects, things can be multiple things at the
same time. That's right, And that phrase in reality is
the key there, because we imagine at the very basic
level that there is a reality out there, that there's
(07:47):
a truth that even if we're not looking, the universe
is there and it's operating and it's following some rules
and it doesn't really matter if we are looking or not.
But the reality suggests that the universe is quite different
from that, that it does matter if you interact with it,
and that what is happening in the universe is not
exactly well determined until you interact with it. Yeah, you
(08:09):
might say that in reality, the way the world works
and the universe works, it's kind of fuzzy, kind of
not quite as solid as we might think it is.
From our everyday lives. That's right. We have a weird
and particular view of this quantum universe. We are only
used to interacting with enormous quantum objects like baseballs and
rocks and trees, which are quantum objects. But they contain
(08:29):
like ten to the twenty six quantum objects. And when
you have that many, they do things differently than when
you have one or two of them isolated to reveal
their sort of true fundamental nature. So today we're gonna
be talking about some really crazy experiments that try to
reveal exactly what the rules are of how these particles
work at the smallest scale when they're left alone. Yeah,
(08:50):
and so it turns out that also reality is not
just a little bit fuzzy, but it may not even
be as permanent as we think it is. Quantum in
for meation and quantum things. We think they're there for real,
but it turns out that maybe things can be taken
away from the universe. That's right. This fuzzy question of
what things are doing when you're not looking at them,
(09:11):
and if you look at them and then look away,
and it doesn't matter who's looking at them and how
they look at them, and whether they store the information
and look at it later. All these fun thought experiments
can help us try to understand what's really going on
at the smallest scales. So to be on the podcast,
we'll be talking about what is a quantum eraser? Now, Daniel,
(09:36):
is this a rubber eraser or what is it made
out of? It's something which will erase your mind if
you think about it too much. It's kind of stretch
it out like a piece of rubber exactly, and push
it too hard and it might just snap. No. Quantum
eraser refers to the concept of quantum information and what
happens if you create information and then erase that information
(09:57):
from the universe. So it's an extension of some really
on experiments that listeners on this podcast have heard us
talk about, the double slit experiment, which reveals how particles
can interfere with themselves and have the chance to be
in multiple places at once. We dog into that experiment
with a fun conversation with Adam Becker, the author of
What Is Real, and today we're gonna go double down
(10:17):
on those experiments and think about even crazier versions. Yeah, so,
what is a quantum erasor now, Daniels this a thing
or like a concept. Yeah, it's both. It was first
a concept and then people made it a thing. It's
like that in quantum mechanics, a lot of people think, well,
if the universe really is that way, here's a ridiculous
scenario that should lead to a silly result. And then
physicists go out and they do the experiment. They make
(10:40):
it real. They figure out a way to like build
it in their lab to test that crazy property universe,
and they get these ridiculous results, which in the end
you have to accept because that's what the experiment says.
They say, the universe really works that way. All right,
Well we'll dig into it, we'll rub that quantum erazor
all of our brains and see there is anything left
(11:00):
at the end. But first we were wondering if how
many people out there had thought about this question or
even heard the term quantum eraser. So Daniel went out
there and ask people on the internet what they thought
a quantum eraser is. That's right. So if you'd like
to be a participant in our virtual person on the
street interviews and love to speculate about physics without looking
(11:21):
anything up, then please write to us two questions at
Daniel and Jorge dot com. All right, so think about
it for a second. If a random physicist came up
to you on the street and you didn't run away first,
and I actually listened to them, and they asked you,
what is a quantum eraser? What would you say? You
are people's answers something an angry physics grad student uses.
(11:41):
I have no idea. Maybe something that erases things at
random and like gets rid of things, because that's what
quantum generally is, is the randomness. Quantum araser could be
something that we invent in the future to erase quantum
mechanics and quantum physics just because pretty hard to understand.
If we don't have it around anymore, we don't have
(12:02):
to deal with it. We can stick with general relativity
and regular gravity. So yeah, just get rid of that stuff. Really,
I don't know either if I were to compare a
quantum or race or to a regular racer, which essentially
just kind of distorts the little graphite particles and absorbs
(12:23):
them in in a way that you know, it gets
rid of the material on a piece of paper or something,
so that you know you can reuse that material to
write on something. Maybe a quantum race, or some sort
of force or phenomenon that causes sometime of particles to
break up from a given area or concentration, so that
the information is very very difficult to understand, or to
(12:47):
receive or to observe. Maybe well, I'm a teacher, and
I know that my students use a razors to hide
their mistakes, So I think a quantum e razor is
something that quantum physicists used to hide their errors from
everyone else. All I can come up with for that is,
sometimes you make a mistake and it's for the better,
(13:08):
it's a better idea than what you originally planned. And
sometimes it's a big catastrophe. But since you don't know
ahead of time, you use your quantum eraser two either
undo your mistake or make it permanent, and you just
roll the dice and let fate decide. No idea. Never
heard of a quantum eracer, but the image of a really,
(13:29):
really kindy eraser pops into my mind. So let's say
a tool that allows you to change the sub potomic
structure of stuff. I would guess that a quantum eracer
is having to do with erasing particles that have certain
(13:51):
quantum states. Thereby leaving behind particles that are in the
state you want, uh that, or it's the weapon that
they used in the movie Racer with Arnold Schwarzenegger. All right,
a lot of fun answers here. Everyone's a comedian on
the internet. Well, especially if I have no idea what
we're talking about, then they got to go to that
joking place. I like the joke about the really tiny eraser, right, Yeah,
(14:14):
like if you have a quantum pencil, I guess it
would have a quantum eraser on the one end of it. Yeah,
or e raises quantum particles or something like that. I'm
imagining a super tiny little vacuum cleaner that like slurps
up electrons. Yeah, and and those one with them passes
them through a wormhole into another universe. I guess, tantalizing.
All right, well, let's get into this concept of a
quantum erasor Daniel, you're saying it has something to do
(14:36):
with the double slit experiment. Now, this is going to
be kind of hard because I feel like this is
a podcast and it's an audio only and this is
a very kind of visual experiment. But I guess we
can try our best to describe what it is. Yeah,
maybe we can add a dance element to it. Do
you think that will help? Yeah? Or I could draw
our tunes. I'm doing watercolor painting at the same time
as we do our podcast. By the way, Oh really,
(14:57):
you're in a quantum artistic s am. Know. This quantum
er racer is an experiment that's like a permutation or
an add on to the basic double slit experiment. So
to understand why the quantum racer is so weird and crazy,
you definitely have to understand what's going on in the
double slit experiment. And so I think we're gonna have
to use our words to describe the wiggly crazy nature
(15:18):
of that experiment, and then we can build on it
to get to the quantumer racer. All right, So I
guess the double slicks experiment starts with a single slit first.
I guess we'll explain that one, and then we'll multiply
by two. So the basic experiment is like you have
a wall, like a barrier, like a plate of metal,
and you cut a little slit on it, like a
little opening that's long in one direction, and then you
(15:40):
shoot like a laser or like just a regular beam
of light through it and then onto a wall behind
the first wall. Yeah, exactly. So imagine your mind some
source of light. A laser is good because then you
have photons of all the same wavelength in the same direction,
and then a screen on the other side with the
laser hits. What do you get. You get a laser spot. Now,
as you say, put something in between, like a barrier
(16:01):
that has a very thin slit in it. Then what
do you see on the screen Instead of the full
laser spot. Now you see like a slice of that spot.
You get a smooth pattern on the screen, but it's
sort of cut by the slit in the barrier, and
it has smooth edges, not a sharp edge, because that's
what happens when light goes through a slit, intends to
like spread out a little bit and smooth out. So
the thing you start with is this single slit where
(16:23):
the light goes through and hits the barrier on the
other side, right, and you get a smooth light pattern
on the other side. Now, the weird thing is in
what happens if you put a second slit next to
the first slip, right, that's right, So now you put
two slits really close together so that the beam could
pass through one slit or the other slit, and once
you get on the other side, instead of having like
(16:44):
two smooth patterns or you know, the simple addition of
two patterns like you saw before, now you get an
interference pattern, which means that you get these patterns of
light and dark and light and dark and light and dark.
And what's happening there is interference. Just like if you
have waves when you add them, if one wave is
going up while the other wave is going down, then
(17:04):
they cancel each other out. Whereas if one wave is
going up and the other one is going up at
the same time, then they add up on top of
each other. They get twice as strong. So the interference
pattern has these slices that are twice as bright as
the previous pattern, and these dark slices as well. And
that's because you have two sources of light. Now each
of the slits is giving you photons and they can
either constructively or destructively interfere on the screen. Right because
(17:28):
I guess you're shooting a laser at both slits at
the same time, or you're like shooting a laser and
it and the beam of the laser kind of goes
through both slits at the same time, right, Yeah, the
slits are very narrow and very very close together. This
only works if the scale we're talking about here is
sort of related to the wavelength of light that we're
shooting at it. So this needs to be very microscopic. Right,
So if you have one slid, you get a fuzzy,
(17:49):
like a plain fuzzy image on the other side. But
if you have two slids, then suddenly it's not a smooth,
fuzzy image. It's like a weird Rippley kind of image,
which means at somehow light is interacting with itself. Yeah,
in this case, we don't know if the light is
interacting with itself. You could say, hey, look, light is
a wave, and waves interfere. This happens with waves in
(18:10):
the bathtub, it happens with waves in the air. Like
noise canceling headphones, right, they generate a second pattern of
noise to cancel out the noise that's coming into your ear.
So interference in waves is not necessarily a quantum mechanical thing.
It's just a wave thing. So in this version of
the experiment so far, you could just say, look, light's
a wave. It's interfering, No big whoop. It gets quantum
(18:31):
when you remember that the beam is actually made not
of waves but of photons, little individual packets, and so
you can take it to the next step by slowing
down the experiment and dimming the laser. So that's shooting
like one photon through the experiment at a time. Yeah,
you shoot one photon at a time, and then you
would think that just throwing like one ft doon at
(18:52):
a time, this photon would pick like the right or
the left slid and then end up on the other
side of the wall and you would get the same
fuzzy smooth pattern. But the weird part I guess is
that you're shooting one ft at a time, but you
still get the ripley kind of interference pattern on the
other side. Exactly. You expect that if you shoot one
photon at a time that it can't interfere because you
were thinking, well, the interference comes from two photons going
(19:14):
through both slits at the same time. Now you have
just one photon in the experiment, so what's it interfering with?
Because you still see the interference pattern on the other
side of the screen and you shoot one photon through
at a time. It's just that it takes longer to
build up if you watch it for an hour or so.
As those photons go through, one lands here, one lands there,
one lands this other spot, it gradually builds up that
(19:36):
interference pattern. So what's it interfering with. It's interfering with itself.
It has the probability to go through both slits, and
that wave function, which controls where a quantum particle goes,
interferes with itself and creates this probability distribution on the
screen for where it might land, and that probability distribution
has the interference effects inside of it, and that's why
(19:58):
you get this interference pattern. Every photon that goes through
like randomly pulls a number from the probability distribution on
the screen which has the interference pattern built in, and
lands there, and gradually it builds up that distribution. It's
almost like, you know, if you were to shoot a
photon as a little ball, it would go through one
of the slids, but because it's quantum, it's almost like
it's going through both slits at the same time. Right.
(20:20):
That's that's kind of the quantum thing. It's going through
both slits. At the same time, and then it's sort
of going through both slits and then interacting with itself
in a quantum way so that when it gets to
the screen it's not a smooth pattern. Yeah, it's tempting
to say that it's in two places at once, or
that goes through both slits at the same time, and
that's our tendency to try to like tell a story
(20:40):
for what happens. But I'm not sure that's the right
way to think about it. The way I think about
it is that it has a probability to go through
both at once. What it actually does is not determined,
you know, until it gets to the other side. So
what happened when it went through the slits we don't know.
We might never know. There isn't necessarily a story there.
So it's a all change in wording, but an important
(21:01):
change in meaning for me to say that it had
probability to go through both slits rather than it actually
went through both. Right, it's like saying that it's not
that the cat is alive and dead. It's just said
it has the same probability, where it has a certain
probability of being alive and a certain probability of being dead.
All right, Well, then now the weird part here. Now
it's going to come when we try observing this photon,
(21:21):
and that's when we get into this idea of quantum razors.
So let's talk about that. But first let's take a
quick break. Alright, we're asking the question what is a
(21:42):
quantum erasor? And now Daniel feel like half of my
brain is already erased trying to talk about quantum objects.
And now we were explaining the double slit experiment, and
so I think we were done with that. Like if
you shoot a laser at a two small slits on
a screen, then on the other side you're gonna get
an interference parent because of the way the quantum probabilities
(22:02):
kind of affect each other. And so now the weird
thing happens when you try to like add a detector, right,
when you try to see which slid it actually went through.
That's right, because our tendency is to want to know, like, well,
what happened, Right, did it go through one slit or
did it go through the other. We feel like it
must have gone through one or the other, right, because
you know it was over here and then it's over there.
So it must go from here to there is our sense,
(22:25):
and so trying to get it like a more accurate
understanding of what happened. You can add a little detector
when that gives you a signal if a photon goes
through slit A, for example, instead of slit B. So
that way you can know, hey, did it go through
slit A or slip B. Because you know, photons are observable, right,
you can interact with them. They make splashes of light,
you can measure them. They are quantum objects, but they
are also physical. And so what happens when you do that,
(22:47):
when you ask, when you insist on knowing which slid
it went through, is that the interference pattern disappears. When
you add that detector that just tries to understand which
slid it went through, then the interference pattern is gone.
So if you try to like measure at the photon
as it goes through slid how would you even do that?
Don't you have to stop the photon to do that.
That's the crux of the matter right there. To measure
(23:08):
a photon, you have to interact with it somehow. You
can't just like observe a photon without interacting with it.
You know, a photon that like passes in front of you,
you can't see it. It's a piece of light, but
unless it hits your eyes, you can't see it, or
unless you put something in front of it to stop it,
measure it and then re emit it. Right, So, for example,
(23:30):
the reason you see something in front of you as
red is because the photons hit that object and then
emitted red photons. So you can't see things without interacting
with them, even light, right, you need to interact with
it somehow. So you can make up lots of different
physical systems that could do this, but the simplest one is,
you know, just like a simple photon detector, a photo
multiplier tube for example, or a scintilator screen that indicates
(23:53):
when a photon went through and then re emits it
on the other side. Oh, I see what you're saying,
Like if you catch it and then really sit back
on the other side, that's one way you can measure
the photon. And you'd like to think, oh, can't I
just take a peek? Can I just look and see
where it went without touching it, without interfering with it,
without messing with it in any way. But you can't
(24:13):
do that. Quantum mechanics tells us that the only way
to get information about an object is to interact with it.
You can like bounce a photon off of it, or
you can bounce an electron off of it, or you
can put another little screen, but somehow you have to
interact with it. You can't get information from that particle
without somehow interfering with its path. Yeah, because I guess
in our everyday lives were used to this idea of
(24:35):
being able to like see things but not touch them,
and so we think we can tell where something is
without actually like influencing it. But when you get down
to the smallest of levels, like all seeing is interacting
in a way, that's right, and it's also true with
the macroscopic scales, just that you don't notice it. Like
if you are walking outside at night and you want
to know, hey, is there a rock in my path?
(24:55):
You turn on your flashlight. You are shooting photons at
that rock. Those photons are hitting the rock, they're warming
up the rock. Then the rock is reflecting photons back
at you. So yeah, you're not touching the rock, but
you're definitely interacting with the rock and you're changing its
quantum state. It's just like it doesn't like really heat
up the rock or push the rock far away. When
these are quantum particles that they can have significant effects
(25:17):
if you interact with them by shooting beams of light
at them or other particles. All right, So then in
the double slit experiment, meant if you try to measure
these photons before they hit the wall in the bag,
then what you're doing is you're collapsing the quantum wave, right,
You're messing up the quantum information. Yeah, so here's where
the different interpretations of quantum mechanics. I'll tell you different
(25:37):
stories for what happens the experiments. Save you measure the
photon before or after the screen, that the interference pattern
goes away. The classical interpretation of quantum mechanics, the Copenhagen interpretation,
is what you just described. It says that the probability
to go through both slits only exists if you haven't
made a measurement. But then if you interact with it,
it collapses the wave function. Now it can only go
(25:58):
through one slit or the there so there's no interference
because the interference came from the ambiguity came from the
probabilities to go through both. The many worlds interpretation tells
a different story. It says collapse is nonsense, that doesn't happen,
it's ridiculous. It says that the universe splits into two,
one where the photon went through one slit and one
where the photon went through the other, and you're in
one of those universes and not the other. Right, So
(26:20):
that's those are the two ways in which you can
interpret what happens. So how does that relate to, like
the information and the quantum information. So the idea here
is that you have the information about whether it went
through slit A or slit B, and that's what destroys
the interference because you've made this measurement. Somehow it changes
the experiment. And you know, this is not something that
(26:40):
we understand very well, this whole concept of measurement and
quantum mechanics. And that was the topic of our episode
with Adam Becker, and then recently we did an episode
with Carlo Rovelli and he's got a whole new theory
for how to understand measurement and quantum mechanics. It's not
something that physics understands very well, how interacting with something
changes its way? Function, does it collapse it as it
split the universe? All of this kind of stuff. But
(27:02):
the key idea here is if you extract information about
which way the photon went, then there's no interference. Right,
you somehow get rid of the quantumness of it, Like
when when I poke something, it's no longer quantum if
you poke something with a classical object like a big detector, right,
that big detector can't be in two stays at once.
It can't be like well, yes I saw it and
(27:23):
no I didn't. It has to make a decision, and
so it decoheres, and you get this weird thing where
quantum object is interacting with the classical objects and so
now it has to like follow the classical rules. So
the quantum or racer is an attempt to get around that,
is to say, what if instead of poking it with
a big finger or a big classical detector, what if
we got this information but we somehow kept it quantum
(27:45):
at the same time. I see. So it's almost like
the cat in show Dingerous Box, Like before you open
the box, it's both alive and dead. The probability of
it being one or the other. And if you open
the box, that's the classical way of checking it out,
Like you open it and it's either alive or dead,
and and then you get it of the quantum probabilities.
You're saying, can I like somehow, you know, poke my
(28:05):
quantum finger into the box and measure but not kind
of destroy that superpocisition of being both alive and dead exactly?
The quantum or racer experiment tries to do that as well.
Let's try to get this information out, but not look
at it directly, not use like our classical objects, our eyeballs,
our brains, even our computers to access that information, so
(28:27):
we can stay in a quantum superposition, so we can
make a decision later about whether we want that information.
And here's where the mind bending stuff comes in. If
you can like extract that information about which way the
photon went, keep it in a quantum state by storing
it in some other entangled particles, then you can decide
after the photons I hit the screen whether or not
(28:48):
you want to know which way it went wait to
see it again. So the idea is you want to
know which way the photons went, right? Did they go
through slit air slip B. You know, if you add
a classical object like a big detector, you're going to
collapse the a function. So instead you add a quantum
detector one that can record this information, but maybe without
collapsing the state of the wave function. Somehow it gets
(29:09):
this information. But because it's a quantum object, it doesn't
trigger the collapse, right, So it can be like entangled
with the photon without like forcing the photon to deco
here completely. And so it's different from interacting with like
your big body or something. You interact with it like
with a single particle, and that stores the information about
which way the photon went, but you haven't looked at it.
You haven't collapsed that wave function yet. You let the
(29:30):
photon then go hit the screen, and then after the
photon has already hit the screen and decide where it's
going to land, then you access that quantum information. It's
called the delayed choice version, where you decide after the
photon has hit the screen whether or not you want
to know the information about which way it went. You're
saying that the photon did go through one of the
two slits, like once it hits the screen in the
(29:50):
bag then it sort of chooses a history of having
gone through the left or the right slit. That's right.
This is trying to like force the photon to make
its decision about whether or not to make an interfere
it's pattern before you decide whether you want to know
which that it went through. So it's sort of like,
you know, trying to play quantum bluff with the photon.
And that's when we get into these really funny questions
(30:11):
of like, how does the photon know whether it's going
to be measured? How does the photon know whether you're
going to have information about it. It's almost like you wanna,
you know, peek inside of the short Inger's box, but
not look at the answer, so that it's still alive
and dead inside the box. But you sort of have
the answer in your pocket, but you don't. You haven't
looked at the answer yet exactly. And so why is
(30:32):
that call it a quantum eraser? Right, So this is
not yet a quantum eraser. This is the delayed choice version,
the delayed measurement, Yes, exactly, delayed measurement choosing whether or
not you have the information, that's the delay. So you
might wonder, like, well, what happens on the screen. What
does the screen look like? What does the experiment look like?
If you do this, If you capture this information in
a quantum state, but you don't look at it yet, well,
(30:52):
what happens is you don't see interference on the screen
because by doing this, by slurping this information out of
the photons, you have stored the interference. But people think, well,
that's interesting. But I haven't yet looked at that information, right,
So what happens if I then erase that information? This
is where the quantum eracer comes in. If I take
that quantum information which is stored in these quantum objects,
(31:14):
but I haven't looked at it yet, if I erase
that information somehow, can I then recover the interference? Can
I make the interference pattern reappear on the screen by
adding this quantum eracer, which like deletes that information from
the universe because I never peeked at it. What are
you saying that this is an actual experiment, Like we've
sort of intercepted the photon before it goes into the slids,
(31:34):
and we've stored that information and we see that it
now it doesn't generate an interference pattern. A weekly pattern
on the screen, even though nobody really knows which slid
it went through. That's right, nobody knows which slid it
went through, though it is in principle stored in this
quantum object. Although that quantum object can be in a superposition,
it doesn't have to be in a definite state. Right.
(31:55):
You can say there's a probability of one and a
probability of the other, and we don't see that interference patterns.
And then people thought, well, what if we delete that information?
What would the universe do if we measure the photon
but don't look at it, keep in the quantum state,
and then erase that information. Can it somehow go back
and recover the interference pattern? I see you're saying that
maybe it's not the fact that it's interactive with your
(32:16):
little secret finger poking quantum poking that destroyed the interference pattern.
Maybe if I poke it with my quantum finger and
then I destroy my finger, will it go back to
being a quantum object? Does that what you mean? Like,
we know that if I poke it, even with a
quantum finger and not look at the answer, it destroys
the quantum information. But now what happens if I poke
(32:36):
it with a band of finger and then the story
to finger will go back to being a quantum object.
Is that kind of the idea, that's the quantum eraser
is destroying that quantum information you've extracted from the experiment
but haven't yet looked at, so it's still quantum. So
that's the crazy experiment. And I can hear you react
and say, what is that real experiment? Did we really
do that? And yes, we have really done this experiment.
We have done it with photons, and you can go
(32:58):
up and google and learn all about the details of
this experiment. I think there's a slightly simpler version that's
easier to talk about, where you use electrons. But the
principles are all the same. And so how can you
destroy quantum information? Well, for example, if when the photon
is passing through the slit, you have some detector, and
that detector takes an electron and puts it in like
a spin up state. If the photon went through one
(33:19):
slit and it's been downstate, if the photon went through
another slit. This is just a way to like store
that information about which way the photon went and keep
it quantum. Right, we don't want to mark it on
a piece of paper, or put it in a computer
or some big classical object. We want to keep it
as quantum information. So here the electron is just like
a single cube bit. It contains some quantum information. But
(33:40):
it can be in a super position. It can be
a little spin up and a little spin down. We
don't know yet, right, but I feel like you bumped
the photon right, Like the photon was going through the slip,
but you made a bump into this electron. And now
it feels like it's in now an impure experiment because
you bumped it right, not just in a quantum way,
but you did sort of. It's not maybe the same photon,
or it's not the same path as a photon who
(34:00):
didn't bump into an electron. That's right, And that's why
you no longer see the interference. Right, you add this
experiment where you're bumping it into the electron, it destroys
the interference pattern because that information is now stored in
the electron, so it can be extracted. The knowledge of
which way the photon went can be measured in the universe,
and so that destroys this interference pattern on the screen.
So you're right, it's a different experiment, right, Like the
(34:22):
quantumness went from being on the wall behind the screen
to now being in this electron you poked it with,
And now I guess the question is if I destroy
the information in that electron, do I get back my
weekly pattern on the screen. That's the idea. It's totally
mind bending and crazy what actually happens. I love it.
And so you take this electron and you might wonder like, well,
(34:43):
how do you destroy the information? How do you erase
the information? Well, it's actually not that hard to erase
quantum information. It happens all the time. Like if you,
for example, measure particles momentum, then that scrambles your knowledge
of the particle's position because the Heisberg uncertainty prints will
says you can't know both very very precisely. If you
have a particle, for example, you measure its position really precisely,
(35:05):
and then you measure its momentum, then you've erased the
quantum information about its position because you can't have both simultaneously.
So you can do certain things sort of similar to
this electron. You can't know an electron spin in one
direction and in another direction at the same time. So
if you want to erase the spin up and down information,
all you need to do is measure the spin of
(35:26):
the electron sort of left right, and that will scramble
the information about the electron spin up down. I plucked
the box with the cat with my quantum finger, and
now instead of running my finger through a filter, that
then kind of scrambles or filters out the information from
the cat. That's right. So now it's no longer possible
to know which state it was in. Was it's been
(35:48):
up or was it's been down. We don't know anymore.
And it's scrambled. It's not like the information existed and
we've overridden it. It was in a quantum superposition. It
was undetermined, and now the information about those probability is lost.
So that's the quantum eraser. It says, destroy the information
that you've extracted from this experiment. All right, So we
did the experiment, actually we poked it with something and
(36:09):
then we erased the information. And did they actually somebody
actually built this. Somebody actually built this, and they did it.
They did this experiment. And so there's a lot of
discussion of this kind of experiment online, and I find
a lot of these to be sort of misleading because
they suggest that what happens when you apply the quantum racer,
when you erase this experiment, is that the interference pattern
like reappears on the screen, which is impossible because you
(36:32):
could do this like quantum eraser experiment like years later,
after you've already done the original experiment. You know, you
could like store these electrons somehow and then five years
later decided to erase the information. You can't go back
in time and then change the interference pattern on the screen.
So that's not what happens. That would be crazy in
bonkers and awesome. But instead, what happens is that if
(36:52):
you do this, if you erase the quantum information, then
you are making a measurement of those electrons. You're measuring
them like left right instead of up down. If you
take those results and you look at only the ones
that have like electron that turned out to be left,
or only the ones that electron that turned out to
be right, then you see the interference pattern. So the
photons that had like a right spinning electron, you see
(37:13):
an interference pattern in those photons, And there's an interference
pattern in the photons that had a left spinning electron.
If you put them together, they add up to the
same smooth shape. So it's sort of like the interference
pattern was hiding inside that smooth shape. And if you
scramble the information that you knew about which photon went where,
you can recover the interference pattern from within the smooth
(37:35):
shape that you saw on the screen. All right, But
then that still requires an observation, right, because you're seeing
some of that information you thought you destroyed, but you
didn't really destroy it in a way, or like, did
you destroy the information in one direction and so the
quantum objects sort of adjusted into the other direction. Yeah,
you destroyed the original information, you can't know which way
the photon went right, and so that allows you to
(37:58):
have interference, and you can recover that interference if you
look at like some of the photons. And the reason
is that you know some of these photons are entangled
with some of these electrons in this way, and you
need to like know how to pull out the subset
of photons that have the interference pattern you're looking for.
You can only get that if you have erased the
quantum information you're looking for. If you access the quantum
(38:21):
information directly. If you measure spin up or down so
you know which photon went through with slit, then that
collapses the way functions essentially and means that you see
no interference. You cannot access any interference only if you
erase the information in those electrons by measuring left right
instead of up down. Can you then go back and
split the photons into two categories, each of which shows interference. Interesting?
(38:43):
All right, let's get into what this all means and
what it can mean about how we see reality. But
first let's take another quick break. All right, Daniel, I'm
(39:03):
still sort of stuck in the cat in the box
experiment because I feel like that's a little easier to grasp.
So we had a cat in the box, we puked
up with a plantum finger, and then we measured my
finger in one direction, and then we sort of destroyed
that information by measuring the finger in a different direction,
and then we see that the cat is still sort
of alive and that but only if we use the
(39:25):
information we got from the finger, right, Yeah, exactly. I
think if you want to talk about cats and boxes,
then you'll need like a hundred cats and a hundred boxes,
because in the end, This is a probabilistic effect. Just
like with the double slid experiment, these interference patterns are
only obvious if you do a lot of photons, so
you can see the patterns. Because a single photon could
hit the screen wherever, you can't tell if you're seeing
(39:47):
an interference pattern or not from one photon. So let's
say you have you know you're a cat person. Your
house is swarming with cats. Each one you put in
a box, right, and then you make this quantum measurement
of each one, but you don't look at the result.
You poke it with a quantum finger, you don't look
at the result, all right, So then you've figured out
I guess with this experiment that the objects sort of
(40:09):
goes back to being quantum, but not really because maybe
you're in a way, you're sort of cheating, right, You're
using some of the information you got from poking it
to make it look quantum again in a way. Right,
It's almost like the photon went back to being quantum,
but only half quantum because you were able to measure
some of it. Yeah, exactly. It's a bit mind bending
because we like to think about what happens to these
(40:31):
particles like what are they really doing? And we like
to think that things can't go back in time and
change their decision. And you know, the core fuzziness of
this experiment is that if you think about these things
in terms of particles and waves, you like to think
that it's a wave and then it gets collapsed into
a particle after it goes through the slit if there's
a detector there, and then needs to decide like am
(40:52):
I a particle or am I a wave before it
hits the screen, so that it can either make an
interference pattern or not. Right, it doesn't seem like it
would make sense for or that to depend on what
you do later on, because you can make this like
quantum information decision a year later, or ten years later,
or a thousand years later. So some people are attempted
to say that this means that there's retro causality that
(41:14):
based on what you do later, you can go back
in time and change the results of the experiment. I
think that's kind of nonsense. Really, what you're doing here
is just interpreting the experiment in a different way using
additional information you've extracted from the experiment. As you said,
you're sort of cheating but it's backwards right now. You're
making a measurement to know how to separate those photons
(41:35):
to see the interference pattern. You're not making a measurement
about which way the photon went about which slid it
went through. Instead, you're just separating the photons into the
ones that were entangled with electrons in one way versus
photons that were entangled with the electrons in the other way.
And those subsets do have the interference going on, it's
just it was masked because when you add up the
two kinds of interference, they add up to the smooth pattern.
(41:58):
It's like it's still the same sort of get to
a quantum that was going on before. Like the photon
looked like it had lost this quantum information, but really
like if you take that information that you got from
the poking with the finger, then you can sort of
find its quantumness in the direction that you didn't poke
it in. Yeah, exactly. And so it's a really fun
experiment to try to think about this nature of decoherence.
(42:21):
Like you were saying before, if you poke something with
a big classical object, it decoheres. It gets entangled with
the whole environment. Millions and millions of particles, and so
all of its quantum properties are essentially lost. Here. What
we're doing is we're like kind of cheating, we're deco
hearing it only Italian a little bit by interacting with
it with a quantum object. So we can play quantum
games with that decoherence later and in the end recover
(42:45):
some of that interference by erasing that information. And so
it's really sort of a great mental exercise to think
about whether you understand decoherence. And we had a whole
podcast episode about what quantum decoherence is. It's closely connected
to this question of what is a quantum measurement and
what happens when you measure something, but it's not quite
the same thing. It's more about whether quantum properties can
(43:07):
be observed because the different quantum states are still coherent,
whether they add up and cancel out in just the
right ways to make something have a quantum effect. I
think the main point is that you know everything is quantum,
but quantumness of something can exist kind of in different
directions in a way, or in like different aspects that
are part of the whole, but it's still you can
sort of take out half of the quantumness of an
(43:27):
object and still for serve sort the other half of
the quantumness that it has in the other direction. Yeah, exactly,
And so you know, the trickiness here relies in the
fact that, like by becoming entangled with a single electron
rather than the whole environment, these photons hit the screen
only become kind of decohered, right, and so it's just
a single particle to worry about. We're sort of able
(43:50):
to think about measuring in different ways, and that's really fun,
and it's easier to think about what this experiment means
in some interpretations of quantum mechanics than in others. Like
in many world it's not that big a deal because
the whole universe has a wave function and now we're
just talking about the quantum wave function of the photon
and the electron and they're kind of entangled and that's
no big deal, whereas in like a strict theory where
(44:11):
you have collapse, then you have to wonder like, well,
did the photon collapse or not, because if I don't
destroy the information and I measure it, then the photon
has to collapse because I knew which way it went.
But if I do destroy the information, then how am
I getting an interference pattern later on? Because for that
to happen, it has to stay a wave. And so
this is sort of troublesome for the collapse theories of
(44:34):
quantum mechanics, not so much trouble for other theories like
many worlds and relational quantum mechanics. Doesn't it just mean
that maybe like the wave collapse in one direction but
not the other direction, Like couldn't you still you know,
use the coping dating interpretation and just say that it
collapse in like one direction and not the other. Well,
the electron is the one that has these multiple directions
of information that spin up down versus spin left right.
(44:57):
The photon is either interfering or it's not, you know,
and it either collapses and it's just like a single
source which gives you the smooth pattern, or it doesn't
collapse and you have the wave function which does interfere.
There aren't multiple directions there, and so it's hard to
understand how it collapse theory can really work because that
does kind of require going back in time and like
(45:18):
uncollapsing the way of function. To me, collapsing the way
function makes no sense at all it's not even consistent
with quantum mechanics because it destroys quantum information in a
way that we know violates basic principles and it violates
the like time continuity of quantum mechanics that says you
should be able to run experiments forward and backwards. So
the collapse theory never made any sense to me, really,
and I think this experiment really highlights how it's sort
(45:41):
of nonsense. But then the only other interpretation that we
have is the multi world theory, right, which this multiverse
theory that every time a quantum object makes a decision,
that the two universes are created. Yeah, that's another interpretation,
and that one is pretty happy with this experiment. There
are other interpretations that you can use the are consistent
with this experiment, Like relational quantum mechanics works well with
(46:03):
this because it says that like, hey, everything in the
universe has its own measurement of these things, and so
it doesn't matter what you measure, there is no reality anyway.
And then there are also like other variants of collapse
theories that are not as strict. You know, let's say, well,
collapse happens in this way or in that way, so
there's a whole spectrum of them, but this is troublesome
for like the most hardcore collapse theories. All right, well,
(46:23):
then I guess to answer the question what is a
quantum eraser? I feel like the answer to that is
sort of straightforward, but it's sort of the implications of
what quantum eraser can do. That's really sort of what
we spend in our check on and that's really hard
to sort of get your head around. So a quantum
razer is just taking quantum information from something and erasing
it in a way. Right, Like, if I have quantum
(46:45):
information stored in one direction of an electron spin, by
measuring it in the other direction, I can destroy that
quantum information. Right, that's the idea of a quantum razor.
And if that electron happens to be entangled with photons
which may or may not be interfering, then whether or
not you erase that information or not can determine whether
or not you can see interference in those photons. Well,
(47:06):
it doesn't determine whether or not you can see it
the way it tells you how to look for that interference. Well,
if you measure the which way of the photons using
those electrons, then you cannot see interference. The only way
to see interference is to destroy that information and then
use the results of destroying that information to pick out
the interference patterns from the photons. You can't do that
if you measure which way the photon went right, right,
(47:28):
But you're sort of still measuring the electron, and then
that's telling you how to look for the interference in
the photon parent right. Yes, you're measuring the electron, but
you're not measuring which way the original photon went. You're
measuring something else about the electron, which destroys that information.
All right, it sounds like we erase people's brain and
hopefully not their time for the last hour. Thanks very
(47:50):
much for going on this journey into the weird quantum world.
I love these thought experiments, the ones people think of
and say, whoa, what would actually happen? Because that's the
fun thing about experien mental physics is confronting the universe
and saying, all right, universe, show us what you got.
We set up a situation that forced you to reveal
what's happening, and the quantum universe always responds with something crazy.
(48:11):
And that's why we're here. To uh talk about the
craziness and to hopefully get you to wrap your mind
around all the different and interesting implications about what it
means about the things around you that you see in
touch or maybe don't see your touch. And so, if
you're a person who likes questions and maybe even answers,
check out our book Frequently Asked Questions about the Universe,
available now and coming out in just a couple of weeks.
(48:32):
You can find the links at Universe f a Q
dot com. All right, well, thanks for joining us. We
hope you enjoyed that. See you next time. Thanks for listening,
and remember that Daniel and Jorge explained. The Universe is
a production of I Heart Radio or more podcast from
(48:55):
my Heart Radio. Visit the I Heart Radio Apple Apple
Podcasts or where every you listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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