February 6, 2020 • 42 mins
The speed of light is constant but is it possible to slow it down?
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Speaker 1 (00:08):
Hey, jorgey, do you think there's anything good about getting older?
Why are you asking me? I'm only twenty three. How
long you've been twenty three for a while? Yeah? All right,
But do you think like there's any advantage to getting
old and slowing down? I guess fewer speeding tickets. Maybe
(00:29):
like new kinds of joint pain to feel so new experiences. Yeah,
the early bird specials at restaurants. Sometimes I wonder if
it's good to forget some of those painful memories of youth.
So there are advantages. Maybe physics should try slowing down.
You want me to tell these photons to take it easy,
you know. I know they're billion years old and they've
(00:51):
been going at the speed of light for a long time,
but maybe it's time for them to slow down. They
can get the early Photon special. Hi'm Jorge Mae, cartoonist
(01:16):
and the creator of PhD comments. Hi. I'm Daniel. I'm
a particle physicist and I hope to be aging gracefully
and welcome to our podcast. Daniel and Jorge age and
explain the universe. At the same time, Daniel and Jorge
explained the aging of the universe, which is mostly my
knees at this point, which is technically happening right now,
(01:37):
right now, as you're listening to us. We are getting older, yes,
and the universe is getting older. You will finish this
podcast in a different, older universe than the when you
started it in, right, A little bit bigger, a little
bit older and wiser, a little bit cooler, a little
bit darker. The universe is getting older and colder and slower.
Um though dark energy, of course is accelerating. But those things,
(02:00):
when they get older, so to slow down, just like us,
it gets a little bit more boring. No, we are
getting funnier as we get older. Jorge, Oh right, right, No,
I mean like entropy, Like entropy is increasing in the universe,
and so is the entropy of our knees and jokes. Well,
maybe as we get older we don't get funnier, but
(02:20):
we think we're funnier, So I laugh more at your jokes,
even as they decay in quality. It's all about perspective.
It's all about perspective. Just like in relativity, there is
no universal time or distance in comedy, right, there is
no universal humor. There's no universal law for jokes. Is
there no theory of special comedy. Um, well, there are
(02:41):
a lot of jokes about relatives, but everybody jokes about
their family, right, Yeah. Yeah, this idea that everything is
slowing down is kind of inevitable. But there are some
things in the universe and maybe we'll never slow down.
That's right. Even though photons travel through billions of light
(03:01):
years of space, when they get here, they're still moving
at the speed of light. You you fired that photon
from your flashlight at Alpha Centauri, It's gonna get there,
and it's going to be going the same speed when
it does. Yeah, I kind of thought about it. I
guess photons are going at the same speed wherever they go,
no matter how long they've been going. Yeah, photons don't
get tired, right. You push a photon and it goes,
(03:23):
and it just goes and goes and goes and goes
and goes, and it doesn't like run out of energy,
you know, unless it gets absorbed by something or bounces
off something. It will just keep going forever. It's sort
of amazing at the maximum speed of the universe, not
just at any speed. At the maximum speed of the universe. Yeah,
and photons. People have been asking me on Twitter whether
photons experienced time, Like do photons even get old? You know,
(03:46):
because photon has taken a billion years to get here
from somewhere else, And they wonder do photons experienced time?
And it's a tricky question because you have to imagine,
like a photon having a clock. What is that clock
made out of? If it has mass than the photons can,
how does it look at the clock? Photons to look
at the clock. What is it like to be a photon? Man?
(04:06):
It seems like maybe more of a philosophical question than
a physics question. Yeah, I don't think photons even have
knees to feel their joint. Paine, I'm not asking whether
we have anything in common with photons. There are weird
quantum mechanical little particles. But where we ended up with
that question was that photons are moving at the speed
of light, and so they see most of the universe contracted.
(04:28):
So a photon leaves here and it feels like it
hasn't gone very far because the universe has been sort
of shortened. To them, the universe is just it's it's
it's basically tiny, right, It's the size of a point. Maybe, Yeah,
that's right, And it's bizarre to imagine what it might
be like to travel as a photon. But again, I
don't think we want to get into the question of
what is it like to be a photon? But it
(04:49):
is sort of a good life example to not slow down,
you know, take some inspiration. Photons are my new heroes.
But other folks have been writing in and asking about
whether photons always do at the speed of light? Can
they not go at the speed of light? Can they
throttle down? Yeah? When photons get old, is there any
way they can sort of chill out a little bit
(05:09):
and relax? So to the on the podcast, we'll be
askering this question dead several, he says. Several readers wrote
in about there are a bunch of people who've been
reading articles online about physicists managing to slow down light,
and they thought, what we better get Daniel and Jorge
to explain that to us, So to be on the program,
we'll be asking the question can light be slowed down?
(05:35):
It's fascinating because people think that, of course, photons travel
at the speed of light, right, that's why we call
it the speed of light, And we make a big
deal about how that's constant. How if you shoot a
photon out you measure traveling at the speed of light,
somebody else measures it traveling at the speed of light.
This sort of this like fundamental constancy, This like stubbornness
of photons to always go with the same speed. Other
(05:57):
things can vary their speed, right, Electrons orcs sure, pretty
much all the other trinos. Right, they can go slow,
they can sit in the palm of your hand, or
they can go at nearly the speed of light. Yeah,
anything that's massive that has any mass to it, electrons,
even newtrinos, anything like that has a rest mass, Like
you can catch up to it and have no relative velocity.
(06:19):
You can exist next to it, hold it, throw it around, etcetera.
But things that have no mass, what would happen if
you caught up to them? Right, If there's a photon
and you caught up to it, a photon is just motion, right,
There's nothing to it other than it's motion. So if
you catch up to it and there's no motion, there's
no velocity relative to the photon, it's not there anymore.
(06:41):
So that's why you can never catch up to a photon.
It just doesn't make sense. It's like what happens if
you catch a wave. It's no longer a wave. Yeah,
that's right. If you're surfing right and you look down,
then it's not really a wave anymore. It's just sort
of like a stationary shape of the water, and so
there's no relative motion there. Yeah, So don't surf on
photons people. Not a good idea, especially as you get older. Yeah,
(07:05):
and so this is an interesting question and so that
we're going to tackle it today. We're gonna talk about
whether or not you can slow light down and whether
it seems that people some people have. Yeah, there's been
some pretty exciting experiments with click baby headlines about slowing
down light and stopping light, and so we're gonna dig
into that today. And see doesn't make any sense. What
are they actually doing and is it cool or what?
(07:27):
Oh stopping You're gonna slow it down to the point
of maybe stopping light. Yeah, yeah, wow, these faces are
trying to break the rules of the universe. Well, as usual,
I was curious, do people think it's possible to slow
down light or is that just sound like crazy talk?
So I walked around campus at you see Irvine. Yeah,
so before you listen to these answers, think about it
for a second, do you think that light can be
(07:50):
slowed down? Here's what people have to say. I don't
think so. I don't know. I would say no, theoretically right, yes, yes,
how would you do it? I'm no clue. I don't
know that that photons within the sun, they it takes
very long time until they actually reach the surface of
the sun and are released into space. I don't know.
(08:10):
I mean it's not actually it's it's learned on the photons.
I mean, there's there's relativity obviously, so I closed down time.
But that's that's time. But that's light is supposed to
be the constant nation. Because it is a constant. I
think if you put light through different media, that should
be slow. But I don't think that it is like
I would expect that if you put light through water,
(08:31):
I would expect there would be some sort of effect.
But but we have a constant for light, So how
can it be how can it change through different media?
And if we have a constant for it, that's like,
but we that's constants for an error? Yes, isn't it okay?
So maybe it is okay? So I think there might
be slight changes possibly, but I think it's inherently supposed
(08:52):
to be constant. No, because it isn't like a constant number.
I think somehow through velocity or something of that, changing
velocity at a very high rate might be able to
I'm not too sure. Maybe like a black hole type
gravity type of situation. Yeah, No, cool, some pretty cool
answers here. A lot of a lot of yeses and
a lot of maybes. I don't know how how did
(09:14):
people react to the question itself, because it's not a
question that I had heard much about before you send
me the email this morning. Uh, you know it's you
don't think about the slowing down light down much. Well.
I think that feeling is backed up by these people's experience,
because when I asked them, do you think it's possible
to slow down light? I got a lot of quizzical
looks and a lot of long pauses as people thought
(09:36):
about it, And you can hear people reaching for answers.
They're like, I don't know, is a black hole involved? Maybe?
Or you kind of have to stop and parse the
words a little bit, like slow down light, slow down
something that is not quite there, yep, And parsing the
words is going to be a solution to today's question
because the answer is sort of technical and legalistic. It's
(09:59):
sort of one of these pole loopholes in the laws
of physics, like have you really slowed it down? Some
people would say they have. Some people would, you know,
quibble with that. All right, So we have to put
on our physics lawyer heads today. Whenever you're reading the
laws of physics, you've got to be very careful, you know,
to understand exactly what they mean and what they don't mean.
Are we sort of the shark type of physics lawyer?
(10:19):
Are we been nice type of physics lawyers today? Are
we out to get the universe? Are we up to?
Are we out for justice? Danny? We are not the
physics police, right, we are on this. We're on the
defense side here. We are trying to make sure physicists
can do whatever they like. We're trying to allow physics
to do weird stuff with the universe because in the end,
that's how we learn what the rules really are. We
(10:41):
find the cracks and we exploit them, try to figure
out what's actually possible given the rules that we know.
It seems like people had a sense that, you know,
the universe is interesting enough that maybe there are situations
in which you can slow light down. Like that didn't
seem impossible to a lot of people. Yeah, that's an
optimistic way to look at it. Like, no matter how
constant you think that law is, there must be some way,
(11:03):
someplace in the universe where we could break and we
could find something crazy happening. I like thinking that people
are open to the fact that the universe is filled
with crazy bunker stuff. Okay, so let's break it down
for folks here, Daniel, and let's talk about light and
how facet travels and what we know about light. And
it's the first thing to understand, which I think most
(11:25):
people already do, is that light does always travel at
the same speed in a vacuum. And remember what light
is like. You can think about light as a photon
is a little particle, and it is quantized. But if
fundamentally think about light is a sort of a ripple.
Space is filled with quantum fields. Even empty space has
the possibility to have these particles in it because it
(11:47):
contains within it these quantum fields, like the electromagnetic field
that can ripple. So I think of space is sort
of like having these different possible sort of sheets rubber
sheets in it, and mostly they're just sort of spread.
It's smooth, but occasionally you can get a ripple in one,
and that ripple is like a particle. I see. So
I think the main idea is that light is a
photon is not a thing. It's not like a little
(12:09):
spherical thing. It's more like like a little divid in
the kind of the fabric of the universe. Yeah, it's
got no stuff to it, if that's what you mean
by it's not a thing. It's a it's a transient property, right,
It's that's why it's always moving. It's a it's a ripple.
It's like a change in information. It's saying, oh, the
electromag than the field was this, so now it's that,
And it's that change that is the photon. That's why
(12:31):
we say that photons carry information because they're like they're
carrying information about how these fields are changing, the fields
going up, and then it's going down, the fields going
up and then it's going down. That is, the photon
doesn't actually move like a like a thing. We can
think of a coffee mug moving from one side of
my desk the other side of the desk, but a particle.
A quantum particle is more like um like an effect.
(12:53):
This part tells the other part, and then that part
there's the next part, and then it just propagates. Yeah,
Like if you and I hold it ump rope between
us and we spind the jump rope, no part of
the jump rope is moving sideways, like closer to me
or to you. That's just moving up and down. But
we can send pulses down that jump rope. Right, if
I wiggled it really fast, I can send what looks
(13:13):
like a pulse, But no part of the rope is
moving sideways. But the pulse is moving sideways that the
wiggle is moving, but the rope isn't moving. Yeah, and
even the coffee cup, and the coffee cup is made
of particles, and those particles are ripples in their fields,
electron fields, cork fields, whatever, So when the coffee cup moves,
it's actually the same thing. It's just like a bunch
(13:34):
of little tiny ripples are moving sideways through the universe.
That's such a hard warming image, you know, the two
of us jumping rope together. I feel like I feel
like I imagine we're in some you know, like city
block and we're jumping rope and there's children singing, and
we're having the time of our lives and there's leaves
blowing everywhere. And afterwards we're sipping hot cocoa and sort
(13:55):
of a warm, fuzzy camera glow, and we're being interviewed
by Oprah Like, that's what you're imagining. Yeah, you're sure
that's going to happen for us. I feel so much
lighter there. Well, that's good because we're talking about light.
And so that's what photons are, right, their ripples in
this electromagnetic field. And the reason that they move at
the speed of light in a vacuum is that then
(14:16):
it's just pure electromagnetic field, and they move at the
maximum speed of information, which is the speed of light
in a vacuum. So things that are not photons, like
electrons and corks, they're also you're saying, they're also a
little perturbations in fields as well. They're just like photons
in that they're They also don't move in the universe.
(14:38):
They just propagate. Yes, they propagate through the electron field
or the cork field, for example. And so an electron
moves through the universe and it moves at whatever speed
it's moving, and it's a wiggle in that field. And
you might ask, well, how come the electron field wiggles
and it doesn't wiggle the same speed as the photon field? Right, Yeah,
how come an electron wiggle can variate its speed? Boom?
(15:00):
We have a gray answer for that. It's the Higgs
boson because the electron field is not free, it's tied
to this other field, the Higgs field, that changes how
it wiggles. And that's what it means for a particle
to have mass, is that it interacts with this Higgs
Boson field that changes how ripples move through it, and
it changes it exactly the way you would expect if
(15:20):
something had mass. It gives those fields inertia. So mass
is more like whether or not it interacts with the
Higgs field, which would slow it down as it moves
through the universe. Yeah, it doesn't actually slow it down.
It gives it inertia. It makes it harder to speed
up and harder to slow down. Right, It's a tiny
bit more complicated than just slowing down. The Higgs field
is not like molasses that tries to get everything to
(15:41):
go to zero speed. It just keeps things at the
same speed. It makes it hard to change velocity. Oh,
I see, but light light doesn't interact with the Higgs field,
so it just always kind of ignores the Higgs field
and just goes as fast as it can. Yeah, and
it's actually fascinating. We should do an entire podcast on
this electroweak symmetry. Like the photon doesn't interact with the
(16:03):
Higgs field at all, but the W and the Z particles,
which are very similar to the photon, do interact with
the Higgs field and become really heavy, and that's why
the weak field is weak. All right. So there's a
lot of details in there, but the main idea is
that a light is a wiggle and it always travels
at this speed of light in a vacuum, in a
vacuum precisely. All right, Let's get into this idea of
(16:25):
maybe whether or not you can slow this wiggle of
light and how you would do that and why you
would want to do that. First, let's take a quick
break up. Okay, Daniel, So you're telling me that light
(16:49):
always moves at the speed of light. Yeah, deep inside
here from did you know horge also always travels at
the speed of Horge that's gonna to a hundred dollars
physics consulting feed speed of X moves at the speed
of X. Yeah, but I guess on like light, my
the horhead, the speed of horhead slows down, is going
(17:11):
to be slowing down, is slowing down, is going to
slow down as I get older. But the speed of
light doesn't seem to change at all. Yeah, And so
we should separate because again, physics has been terrible about naming,
because when we talk about the speed of light, often
what we mean is maximum speed of information in the universe.
Light in a vacuum happens to travel at that speed,
(17:32):
and because of historical reasons, we called that speed the
speed of light. Oh man, you mean physicists name things wrong, Yes, badly,
and they could have really named it that really the
speed of light, We should just call it the maximum
speed of the universe. Yes, the maximum speed the universe
is separate from the speed of light, because it turns
(17:54):
out there are lots of different ways to make light
move slower than the maximum speed of the universe. So
it's really like a definitions thing, like we just have
to wrap our shift that little lever in our heads
and not call it the speed of light anymore. That's right,
Welcome to physics legalities. Right, But officer, I was not
traveling faster than the maximum speed of the universe. It
(18:16):
technically didn't break the speed limit of this highway. It's
just that the speed limit of this highway is not
the maximum speed of the universe. I'm sure that that's
going to get you off that ticket yet. Well, well, well,
and in the ideal scenario, the officer is a fan
of our show and just pulled you over to get
(18:37):
your autograph. That's right on this speeding ticket. And we
talked in the podcast UM recently actually about drink off
radiation and what happens when life gets slowed down as
it passes through a material and other particles zip passed it.
That's a whole fun, complicated topic. Okay, so we can't
decrease the maximum speed of the universe, but you can
(18:59):
make the light go slower than it would in a vacuum,
or slower than the maximum pet Yeah, you can make
photons move slower than the maximum speed. And the way
you do it is you get them to go through
some material. Because we said that photons move at the
maximum speed when it's just a pure electromagnetic field and
a ripple. But if you put other stuff in there,
(19:21):
like an electron field. Electrons interact with electromagnetic fields, right,
They have charge and so they will interact with it.
And like a photon can get absorbed by an electron,
you have an electron that's zipping around a nucleus, it
can absorb that photon, hang onto it for a minute,
and then re emit it. So that effectively slows down
the photon because it gets from A to B in
(19:42):
more time than it would if it hadn't been absorbed. Right,
Like if you want to slow down Hussain Bolt, you
would have him run through a crowd of people, not
on an empty track, That's right. Or Jorge driving to
work takes less time than Jorge driving to work past
the banana shop because he stops, he takes some bananas.
It takes some ten minutes. Everybody wonders why he's late.
You know, I'm not sure anyone wonders anymore why I'm late.
(20:05):
Daniel put, I'm just glad that you didn't say that
I drove through a crowd of people. That's where where
you were going. I'm like, oh, it just just got
really dark though this is a family friendly podcast, that's right, right, Yeah,
I guess what you mean is if you want to
slow light down, you sort of keep it busy. Yeah,
you interact with it. You you know, Jorge passing through
(20:27):
a crowd of his admirers is going to have to
stop and sign autographs more often, and so he's not
going to get to the other side of the room
as quickly. Yeah, although these days people don't ask for
autograph date just asked for selfies mostly. Well, that saves
your risk, I hope. And to listeners that might feel
like a technicality, like is the light really traveling less
than the speed of light? Because it's not really. It's
(20:49):
between interactions. It's still moving at the maximum speed limit.
It's just that those interactions take some time. And and
that's that's totally fair. And again it just depends on
how you define it. Like you shoot a beam of
light into glass, when does it come out the other
side more time? It would take more time than if
you shot a beam into vacuum. So that's what we
mean when we say have we slowed down the light?
(21:11):
I guess you know the image I have in my
head is sort of like a pinball machine or like
a pachinko machine. Have you seen those Japanese pinball games
where the little ball wants to go in a straight line,
but it keeps bumping into things in between? Now, is
that sort of what's happening where it's like bumping between
things or is it really more like the interaction slow
(21:33):
the light down? Both. It can't move in a straight
line through material. It's getting deflected. But all deflections are
also interactions, and sort of a philosophical question here, right,
If the photon is deflected, is it the same photon
as the one that came in? It seems like a
clear question if it gets like fully absorbed and then
re emitted. But if it gets deflected, it's still there's
(21:55):
an interaction there, and so it's a different quantum state.
Oh it actually does get deflected. Well, yeah, Photons moving
through material don't move in a straight line. They get
bounced around like a pachinko machine between particles. Sometimes they
get absorbed and re emitted, sometimes just deflected um. And
you know, some could argue that those are very different
kind of experiences for the photons. Some could argue they're
all interactions so it's all the same deal. But yeah,
(22:17):
they interact and they get deflected, so it takes longer
to get from one side of the material to the other.
I see. So is what slows the light down the
detours that's taking or is there actually kind of time
wasted in getting absorbed and re emitted by something in
the material. It's both both. Those are two different kinds
of interactions, but they both contribute to making light go
(22:38):
most more slowly through the material. And it's sort of
similar to like, you know, making waves move more slowly
through some material, Like if you if you make waves
in water, it's different than if you make waves in
honey or in molasses. Right, there's just a different sort
of speed of information traveling through that material. And again
it's because the interactions, like honey holds itself together with
(22:59):
more wrongly than water does. It's really the same deal.
So then it sort of depends on what you mean
by light, like it's light natural photon or is light
kind of the beam of light? Because the photon itself
doesn't slow down, does it? It just it just gets busier.
That's right. The photon itself doesn't slow down unless you're
averaging right, if you're averaging over it's time through the material,
(23:21):
then its effective speed is slower. So again it's a
bit technical. But what these folks have done with these
experiments that we've been hearing about slowing down light and
stopping light is even something different than that. It's even
more of a technicality than just slowing down light. Overall,
there are different ways to slow light down. One as
you can slow the photon as it goes across the material.
(23:43):
And then there's another way, which is what these physicists
have done. Yeah, because slowing things down as it goes
through material, that's old news. Like Isaac Newton did that, right,
he used prisms. Everybody is known for years that you
can slow things down as they move through a material. Right,
it's old news. That's from the time of Yeah, this
it's old news. What these folks have done is not
just slow things down by moving through ice or water
(24:04):
or blast. That's last centuries physics news. What they've done
is something different. Or they claim that it's a big
step forward, all right, well, um, step us store it.
What have they done? What did these physicists due to
slow down light? So what they're doing is not slowing
down like a beam of light by slowing down individual photons.
What they're doing is they're slowing down sort of a pulse.
And when you send a pulse of light, it's not
(24:27):
just one photon, it's like a collection of photons, the
way like if you send wiggles down a jump rope,
it's not a single like sign wave doesn't have a
single frequency. It has a bunch of different frequencies all
added up inside of it to make that one shape.
So are we still talking about individual photons? Are we
(24:47):
just talking about kind of the the shape of the
light pulse. We're talking about the pulse, but the pulse
is made up of a bunch of different photons. It's
like if you have a group of bike riders and
you send them all out in a pack. You've got
a hundred of them or something, and they're all biking together.
You've got some fast ones and some slow ones and
(25:07):
sort of like the you know, the shape of the
pack is changing as it travels, and so the whole
How old are these cyclists. They're twenty three, just like
you are. So they're excellent shape. Okay, I just want
to paint a complete picture here, and any of you
out there who know anything about like Fourier analysis, that
you can break down wiggles into sort of their component
(25:28):
frequencies or so you know, if you're like listening to
audio and you're looking at equalizer, it shows you, like,
am I hearing the high pitches or the low pitches
and the medium pitches? All sound is just a bunch
of combinations of different wiggles at different frequencies. The same
thing is true for a pulse of light. If you
want a pulse of light, you have to make some
high wiggles and some slower wiggles and patch them all
(25:49):
together to make that pulse. Right. And and is it
that different photons of different these different frequencies or is
it okay? Yes? And so what these researchers have done
is not slow down individual photons, because that's old news,
is to try to slow down this whole pulse. To say,
if we're gonna like send information through fiber optics or whatever,
(26:11):
you do that by sending pulses of information, you flick
the switch on and off and that sense whole. But
a grouping of photons that's a that's a pulse. That's
a pulse. And so what these experiments are focused on
is taking one of those pulses and trying to make
it go more slowly through a material the entire pulse.
And the key thing there is that, as you said,
the pulse is made of photons at different frequencies, So
(26:33):
you've got some that wiggle fast and some the wiggle slow.
And that's the key idea, because what they do is
they construct some crazy material, some weird quantum material or
something where the speed of light through the material is
different for different frequencies. So the light is slowed down
differently based on how much it wiggles, Like the really
blue light is slowed down more than the really red light.
(26:56):
So it's not that they slowed light down, is that
they made a material that selectively slows different frequencies. Yes,
And the consequence is that you like spread out the pulse,
like you have that group of riders and you said,
all right, I'm gonna make all the old people ride
more slowly. Now, then they're gonna start to fall behind,
and the pulse of riders, this group of bikers is
(27:17):
going to get more and more spread out, and so
when the trailing edge of the pulse arrives is going
to be later, and so sort of the whole pulse
takes longer to get there. Uh, you stretched it out.
When it gets there later, well you stretched it out
and sort of so if you look at like where
the peak of the pulse is, it moves back because
part of the pulse got stretched backwards. But doesn't that
(27:41):
happen normally when you shoot light through glass or ice?
Doesn't it normally get spread out? It does normally get
spread out because there's this dispersion relationship, that's what we
call it, where the slowing downiness depends on the frequency.
And that's how a prism works, right. A prism works
because you shoot light into it and the mount that
it bends depends on this frequency and that's the same
(28:03):
thing that down so it spreads out exactly, And so
is what have they done? Then? That's different? Well, these
materials have like a very strong dispersion. It's like extremely
extra extra so you can Yeah, so you can shoot
like a laser, like let's say you want to slow
it do on a laser pulse. A laser pulse has
a bunch of frequencies really close together. Because you know,
(28:24):
a laser is usually like a single color, so it's
all a bunch of really tightly packed frequencies, it's harder
to spread out. So what they've done is developed these
materials where the dispersion is really strong. So even a
tightly packed laser pulse, which is a bunch of frequencies
really similar to each other, gets spread out and effectively
slowed down. It gets blurry. Yeah, yeah, so it gets blurry.
(28:45):
You smush it. You just gonna smooche the laser. But
then you know, But then, but like the leading edge
of the poles, like the first electrons that go in
there are the fastest, still come out the other side
as fast as they can. Yeah, the first photons get
there um at a very high speed. It's not exactly
the speed of light, because every photon is slowed down
as it goes through a material for the reasons we
(29:06):
talked about before. But you're right there. The difference now
in the speed between the leading edge and the trailing edge.
And this is where it gets kind of technical. These
folks talk not about phase velocity, which is a velocity
of any individual photon in the pulse, a group velocity,
which is like where is the location of the peak
half ast is the peak moving. It's like slowing light
down in the sense that in the sense of what
(29:30):
happens when you slow down a song or the audio
of of somebody speaking, it's just goods sort of you know,
more on the lower it gets blurrier, and it gets
more in the lower tones. You know, goods slow. But
they're not technically slowing down the sound pulled the wave.
(29:53):
They're just kind of spreading it out. And so that's
what they mean by slowing light down. Yeah, that's what
they mean by slowing down light. And so you're gonna
be like clever lawyers, clever lawyers, you gotta sort of
dig and several layers there before you figure out, like
what do you really mean by slowing down? Like it
is sort of more like slowing down in the audio sense, right,
like you're slowing down a song or speech. Yes, in
(30:16):
the sense of audio, like the analogy would be that
you're extending the wavelength of every part of that sound.
But then you know, as I say to you, hey, Jorge, um,
you're slowing down like the lower frequencies even more so,
like you not not only would it sound deeper, would
I sound more like you know, al green? But also
the length of the Jorge phrase would take longer to
(30:38):
get there because the trailing edge of it would take
longer because you're slowing down the even lower bits. Folks.
And that was not just Jorge ad living. That was
actual scientific experimentation, right. That wasn't me burping a banana.
That was an actual physics. Yeah. And there's one group
that claims to have even stopped light. Okay, let's get
(31:01):
into this idea, and maybe you can stop light. That's
pretty crazy. But first let's take a quick break. Okay,
so my lawyer says I can slow light down if
(31:23):
by slowing down I mean like spreading out the wavelengths
and making it just sort of smooshier. But you're saying
some scientists can actually or have claim to have actually
stopped light. Yes, and this is a physics topic, which
is sort of amazing and then also like sort of
disappointing and nonsensical because I feel like I stopped light
(31:44):
all the time. Just you know, I've always been able
to do that. Half of that power. I just go
outside and I seem to stop all the light from
the sun. You do make a better door than a window.
It's true. I cast a long shadow that you. Well,
what these folks did when they say they stop light
is they shoot a laser into some kind of material
and then the laser gets absorbed by that material, and
(32:06):
then they can flick a switch and the beam shoots
back out the other side and an arbitrary later moment. Okay,
that does sound pretty cool. It sounds pretty cool. They're
like shoot a beam in, it gets like absorbed, it
gets he stored, and you can wait like you know,
a second, an hour, a day, and then you flip
another laser switch and then the beam shoots it back
(32:26):
out the other side. Wow, Like, did they trapped it
inside of this material and then they can let it
go later? Yes, they've trapped inside the material. All right,
I mean I'm intrigued. You might ask like, well, did
they stop the photons? Like and no, of course they
can't stop the photons. It's not like if they zoomed
in with a microscope they could see these photons just
like sitting there. They didn't freeze them, right, they didn't
(32:47):
freeze the photons. What's that comic book character with the
with the freezing powers? Oh? Mr Freeze did you just
make that up? And over see? I tell you they
right if out of I what's that guy with the
cartoon powers for particle physics? Is it? Mr particle Physics
booms exactly? That's how clever that idea sounds. Anyway, Mr
(33:08):
Freeze didn't like Mr Freeze. These photons, we can like
look at them covered in ice or anything. What do
they do? They keep them busy, or they record them
or what they get. I feel like that comes comes
from parenting. You're like, all right, I'm gonna run all
these crazy kids into this room, keep them busy, and
they'll shoot out the other side. Here's my phone if
if you want to see a room full of children,
(33:30):
slow down and maybe Freeze just hand them your phone. Yeah.
They just have a little phone for these for these photons. Yeah,
there you go, and these phones admit photons. Now now
it is getting too deep. Yeah. Essentially, what they do
is they build some system that can absorb the light,
store the information from that light, and then re emit
(33:51):
it at an arbitrary later time based on some external input.
Isn't that called a camera? Yes, exactly exactly. So I'm
one sense, it's like, wow, that's awesome. This cloud of
atoms absorbed this laser and can re emit it. On
the other hand, like, well, every photograph is basically stopping
light and later re emitting it. Um. So in one
(34:12):
sense it's awesome because it's something that nobody's ever done before,
and another sense, it is accomplishing something we've been doing forever. Well,
I mean, a photograph and the TV is pretty complicated, right,
because you need all these different things. But it might
get in the sense and maybe these guys did it
in a much more fundamental sense, like they actually build
a material that takes a photographs and and admits it. Well,
(34:34):
I wouldn't say it's less complicated, Like I think a
camera and a screen is less complicated than having like
you know, um Millie kelvin refrigerator and high intensity lasers.
But this thing does it sort of all at once,
like it both absorbs the photons and then later re
emits it. It's not like it's you know, stored electronically
and then regenerated. But again you might say it's still
(34:56):
it's not the same photons, like they were absorbed by
the material and then re emitted. Is it the same photons?
Not really stem me through it? What did they do?
How does this material work. Well. They used a Bose
Einstein condensate, which is a very cold collection of atoms
in a really weird quantum state. We'll do a whole
podcast on what a Bose Einstein condensate is. But what
(35:18):
they did is they shoot a laser into it from
the side and then that's sort of like holds it.
What do you mean, hold it? Like it the light
is absorbed by the atoms and the condensate, and then
they just sort of hold it. This first laser is
like the switch laser, just like gets the atoms in
the right situation and puts them in the right sort
of quantum state to do this trick. Then they have
the second laser, the one they actually want to absorb
(35:40):
and re emit. They fire that into this pile of
atoms and then it gets absorbed and then when they
want to release it, they just turn off the first
laser and then it emits the pulse from the second
laser in the same direction and with the same shape
and intensity everything. Yeah, precisely, and you know, there's some
loss of information there, but mostly it captures the you know, light.
(36:00):
That is pretty cool. I think that it's pretty awesome.
Otherwise it's just a superpower that only a comic book
character Mr. Light Freezer would have. Mr. Laser Freeze. Okay,
so oh wow, so um, I mean that that sounds
pretty impressive that you shoot it and then the atoms themselves,
not some like memory or some electronic it's like the
(36:22):
atoms themselves somehow remember this laser and then admit it
when you want to wanted to in the same way
that it came in. Yeah, that is pretty cool, And
I think it's awesome that people just try to make
materials do new stuff, Like does this whole field of
you know, atomic physics and condensed matter physics where people
are like, hey, can we make some new kind of
(36:44):
goo that has this weird property? Can make some new
kind of good that has that weird property. And it's
sort of just like an exploration of the way things
can be in the universe. And you know, we're used
to like different kinds of stuff water, metals, rocks, whatever,
but it's possible that there's all different kinds of stuff
in the universe that we've never experienced just because doesn't
occur naturally here on Earth. So these folks are like
(37:06):
pushing the boundaries right, they're trying to make things do
new things. Yes, exactly, they're trying to bring the rules.
They're like, well, nobody thought things could do this, Let's
see if we can make it work. It's kind of like,
you know, nobody ever thought you could walk on water,
but hey, it turns out if you freeze it, you
can walk on water. Yes, And in some sense it's
as unsatisfying as that. It's like, are you really walking
(37:28):
on water if you're walking on a frozen lake? Technically yes,
what have you shoot a lasers through it? Is it
still technically a miracle? Well, you know, if you're ice skating,
then you're walking water because you're making this very thin
layer of water between your blades and the ice. So
you know, there you go. More Jesus lawyer. And people
(37:50):
try to say, maybe there's some application to this, like
if you could slow down light, you could make circuits
that use less power or something interesting. Yeah, rank Frankly,
I don't find any of that convincing. I think that
the motivation for this is like, hey, can we make
something in the universe that's weird and different? And I
think there's there's a there's a lot of value to
that well, there's a lot of applications that they say
(38:12):
so in solid state physics, where if you get these
materials to do these weird things, that they could maybe
be the basis for like a quantum computer. Yeah, precisely,
there are ways and you could find applications for materials
that do weird stuff. And you know, people were building
atom traps and bose Einstein particles for years before they
(38:33):
had ideas for how to apply them. So it's a
good idea to develop new kinds of materials because they
can inspire new applications. And sometimes you work on something
for ten years and then people realize, hey, that's exactly
what we needed for this other totally unrelated problem. So
you know, we want to make progress as a species,
we've got to like explore broadly and try all sorts
of weird stuff. And you never know, like it could
(38:54):
have been that they didn't make light stop, but something
else totally weird and unexpected happened and discovered something crazy.
You never know. When you do research right or just
that improving these things, you learn something new about the
materials themselves, or how atoms are like behave, or if
in order to make this happen. You have to invent
something new and weird and that turns out to be
(39:15):
really useful for something else, you know, like the whole
semiconductor industry and computers exist because of basic research into
stuff that wasn't motivated at building computers. Like, for example,
I just got this great idea to shoot myself with
a laser to stop aging or at least on my knees.
There is laser therapy for everything, Like you can get
(39:35):
laser therapy to like stop smoking. I wonder, like what
part do you are they sting with the laser? Probably
your wallet and your Yeah, I don't know. I wouldn't
recommend any of those laser treatments. Okay, well it is.
It does sound like they maybe have technically slowed down
(39:55):
light and maybe sort of in a ways stop light.
So that's pretty pretty interesting, pretty cool. It's pretty cool.
They've definitely done something nobody has done before. And today
we only talked about a couple of groups doing a
couple of experiments, but it's a big field and people
are doing it will all sorts of new kinds of
materials and in new ways, and applying it to different
kinds of light and higher intensity, lower intensity, and bigger
(40:17):
packets and smaller packets. We couldn't possibly review all the
recent work on it. It's a whole burgeoning industry. All right. Well,
I think that answers that question that a lot of
people wrote in about you can slow down light and
you can stop light. Have you defined things in the
right way and use both speakers? Was that the main
lesson here? Yes? And so if you find a bottle
(40:38):
and the genie comes out and he's a physicist, and
you ask for some new special power, make sure you
are very clear about what you're asking for, because these
physicists will find a way to squirrel out of it
and not give you what you were hoping for. Right,
You might just hand you a six pack of light
beer and then you say your way. This light will
make you slow down. That's one way to slow down. There,
(41:02):
you go do that a laser and you can sell
it for a lot more. Yeah, you don't want us
as your physics genies may that that's what would make
hell a little bit more tolerable. Sin, you can freeze
some beer down and that you can drink it with
the physicists. All right, Well, thank you very much to
everybody who wrote in to ask us about this weird headline.
And if you read something online about weird discoveries and
(41:23):
physics or something you're not quite sure about, send it
to us. We will break it down for you. You
hope you enjoyed that. Thanks for listening. Let's see you
next time. Before you still have a question after listening
to all these explanations, please drop us a line. We'd
love to hear from you. You can find us on Facebook, Twitter,
(41:46):
and Instagram at Daniel and Jorge That's one word, or
email us at Feedback at Daniel and Jorge dot com.
Thanks for listening, and remember that Daniel and Jorge explained.
The Universe is a production of I Heart Radio More
podcast from my Heart Radio. Visit the I heart Radio
Apple podcasts or wherever you listen to your favorite shows.
(42:12):
Yeah
Hey, jorgey, do you think there's anything good about getting older?
Why are you asking me? I'm only twenty three. How
long you've been twenty three for a while? Yeah? All right,
But do you think like there's any advantage to getting
old and slowing down? I guess fewer speeding tickets. Maybe
(00:29):
like new kinds of joint pain to feel so new experiences. Yeah,
the early bird specials at restaurants. Sometimes I wonder if
it's good to forget some of those painful memories of youth.
So there are advantages. Maybe physics should try slowing down.
You want me to tell these photons to take it easy,
you know. I know they're billion years old and they've
(00:51):
been going at the speed of light for a long time,
but maybe it's time for them to slow down. They
can get the early Photon special. Hi'm Jorge Mae, cartoonist
(01:16):
and the creator of PhD comments. Hi. I'm Daniel. I'm
a particle physicist and I hope to be aging gracefully
and welcome to our podcast. Daniel and Jorge age and
explain the universe. At the same time, Daniel and Jorge
explained the aging of the universe, which is mostly my
knees at this point, which is technically happening right now,
(01:37):
right now, as you're listening to us. We are getting older, yes,
and the universe is getting older. You will finish this
podcast in a different, older universe than the when you
started it in, right, A little bit bigger, a little
bit older and wiser, a little bit cooler, a little
bit darker. The universe is getting older and colder and slower.
Um though dark energy, of course is accelerating. But those things,
(02:00):
when they get older, so to slow down, just like us,
it gets a little bit more boring. No, we are
getting funnier as we get older. Jorge, Oh right, right, No,
I mean like entropy, Like entropy is increasing in the universe,
and so is the entropy of our knees and jokes. Well,
maybe as we get older we don't get funnier, but
(02:20):
we think we're funnier, So I laugh more at your jokes,
even as they decay in quality. It's all about perspective.
It's all about perspective. Just like in relativity, there is
no universal time or distance in comedy, right, there is
no universal humor. There's no universal law for jokes. Is
there no theory of special comedy. Um, well, there are
(02:41):
a lot of jokes about relatives, but everybody jokes about
their family, right, Yeah. Yeah, this idea that everything is
slowing down is kind of inevitable. But there are some
things in the universe and maybe we'll never slow down.
That's right. Even though photons travel through billions of light
(03:01):
years of space, when they get here, they're still moving
at the speed of light. You you fired that photon
from your flashlight at Alpha Centauri, It's gonna get there,
and it's going to be going the same speed when
it does. Yeah, I kind of thought about it. I
guess photons are going at the same speed wherever they go,
no matter how long they've been going. Yeah, photons don't
get tired, right. You push a photon and it goes,
(03:23):
and it just goes and goes and goes and goes
and goes, and it doesn't like run out of energy,
you know, unless it gets absorbed by something or bounces
off something. It will just keep going forever. It's sort
of amazing at the maximum speed of the universe, not
just at any speed. At the maximum speed of the universe. Yeah,
and photons. People have been asking me on Twitter whether
photons experienced time, Like do photons even get old? You know,
(03:46):
because photon has taken a billion years to get here
from somewhere else, And they wonder do photons experienced time?
And it's a tricky question because you have to imagine,
like a photon having a clock. What is that clock
made out of? If it has mass than the photons can,
how does it look at the clock? Photons to look
at the clock. What is it like to be a photon? Man?
(04:06):
It seems like maybe more of a philosophical question than
a physics question. Yeah, I don't think photons even have
knees to feel their joint. Paine, I'm not asking whether
we have anything in common with photons. There are weird
quantum mechanical little particles. But where we ended up with
that question was that photons are moving at the speed
of light, and so they see most of the universe contracted.
(04:28):
So a photon leaves here and it feels like it
hasn't gone very far because the universe has been sort
of shortened. To them, the universe is just it's it's
it's basically tiny, right, It's the size of a point. Maybe, Yeah,
that's right, And it's bizarre to imagine what it might
be like to travel as a photon. But again, I
don't think we want to get into the question of
what is it like to be a photon? But it
(04:49):
is sort of a good life example to not slow down,
you know, take some inspiration. Photons are my new heroes.
But other folks have been writing in and asking about
whether photons always do at the speed of light? Can
they not go at the speed of light? Can they
throttle down? Yeah? When photons get old, is there any
way they can sort of chill out a little bit
(05:09):
and relax? So to the on the podcast, we'll be
askering this question dead several, he says. Several readers wrote
in about there are a bunch of people who've been
reading articles online about physicists managing to slow down light,
and they thought, what we better get Daniel and Jorge
to explain that to us, So to be on the program,
we'll be asking the question can light be slowed down?
(05:35):
It's fascinating because people think that, of course, photons travel
at the speed of light, right, that's why we call
it the speed of light, And we make a big
deal about how that's constant. How if you shoot a
photon out you measure traveling at the speed of light,
somebody else measures it traveling at the speed of light.
This sort of this like fundamental constancy, This like stubbornness
of photons to always go with the same speed. Other
(05:57):
things can vary their speed, right, Electrons orcs sure, pretty
much all the other trinos. Right, they can go slow,
they can sit in the palm of your hand, or
they can go at nearly the speed of light. Yeah,
anything that's massive that has any mass to it, electrons,
even newtrinos, anything like that has a rest mass, Like
you can catch up to it and have no relative velocity.
(06:19):
You can exist next to it, hold it, throw it around, etcetera.
But things that have no mass, what would happen if
you caught up to them? Right, If there's a photon
and you caught up to it, a photon is just motion, right,
There's nothing to it other than it's motion. So if
you catch up to it and there's no motion, there's
no velocity relative to the photon, it's not there anymore.
(06:41):
So that's why you can never catch up to a photon.
It just doesn't make sense. It's like what happens if
you catch a wave. It's no longer a wave. Yeah,
that's right. If you're surfing right and you look down,
then it's not really a wave anymore. It's just sort
of like a stationary shape of the water, and so
there's no relative motion there. Yeah, So don't surf on
photons people. Not a good idea, especially as you get older. Yeah,
(07:05):
and so this is an interesting question and so that
we're going to tackle it today. We're gonna talk about
whether or not you can slow light down and whether
it seems that people some people have. Yeah, there's been
some pretty exciting experiments with click baby headlines about slowing
down light and stopping light, and so we're gonna dig
into that today. And see doesn't make any sense. What
are they actually doing and is it cool or what?
(07:27):
Oh stopping You're gonna slow it down to the point
of maybe stopping light. Yeah, yeah, wow, these faces are
trying to break the rules of the universe. Well, as usual,
I was curious, do people think it's possible to slow
down light or is that just sound like crazy talk?
So I walked around campus at you see Irvine. Yeah,
so before you listen to these answers, think about it
for a second, do you think that light can be
(07:50):
slowed down? Here's what people have to say. I don't
think so. I don't know. I would say no, theoretically right, yes, yes,
how would you do it? I'm no clue. I don't
know that that photons within the sun, they it takes
very long time until they actually reach the surface of
the sun and are released into space. I don't know.
(08:10):
I mean it's not actually it's it's learned on the photons.
I mean, there's there's relativity obviously, so I closed down time.
But that's that's time. But that's light is supposed to
be the constant nation. Because it is a constant. I
think if you put light through different media, that should
be slow. But I don't think that it is like
I would expect that if you put light through water,
(08:31):
I would expect there would be some sort of effect.
But but we have a constant for light, So how
can it be how can it change through different media?
And if we have a constant for it, that's like,
but we that's constants for an error? Yes, isn't it okay?
So maybe it is okay? So I think there might
be slight changes possibly, but I think it's inherently supposed
(08:52):
to be constant. No, because it isn't like a constant number.
I think somehow through velocity or something of that, changing
velocity at a very high rate might be able to
I'm not too sure. Maybe like a black hole type
gravity type of situation. Yeah, No, cool, some pretty cool
answers here. A lot of a lot of yeses and
a lot of maybes. I don't know how how did
(09:14):
people react to the question itself, because it's not a
question that I had heard much about before you send
me the email this morning. Uh, you know it's you
don't think about the slowing down light down much. Well.
I think that feeling is backed up by these people's experience,
because when I asked them, do you think it's possible
to slow down light? I got a lot of quizzical
looks and a lot of long pauses as people thought
(09:36):
about it, And you can hear people reaching for answers.
They're like, I don't know, is a black hole involved? Maybe?
Or you kind of have to stop and parse the
words a little bit, like slow down light, slow down
something that is not quite there, yep, And parsing the
words is going to be a solution to today's question
because the answer is sort of technical and legalistic. It's
(09:59):
sort of one of these pole loopholes in the laws
of physics, like have you really slowed it down? Some
people would say they have. Some people would, you know,
quibble with that. All right, So we have to put
on our physics lawyer heads today. Whenever you're reading the
laws of physics, you've got to be very careful, you know,
to understand exactly what they mean and what they don't mean.
Are we sort of the shark type of physics lawyer?
(10:19):
Are we been nice type of physics lawyers today? Are
we out to get the universe? Are we up to?
Are we out for justice? Danny? We are not the
physics police, right, we are on this. We're on the
defense side here. We are trying to make sure physicists
can do whatever they like. We're trying to allow physics
to do weird stuff with the universe because in the end,
that's how we learn what the rules really are. We
(10:41):
find the cracks and we exploit them, try to figure
out what's actually possible given the rules that we know.
It seems like people had a sense that, you know,
the universe is interesting enough that maybe there are situations
in which you can slow light down. Like that didn't
seem impossible to a lot of people. Yeah, that's an
optimistic way to look at it. Like, no matter how
constant you think that law is, there must be some way,
(11:03):
someplace in the universe where we could break and we
could find something crazy happening. I like thinking that people
are open to the fact that the universe is filled
with crazy bunker stuff. Okay, so let's break it down
for folks here, Daniel, and let's talk about light and
how facet travels and what we know about light. And
it's the first thing to understand, which I think most
(11:25):
people already do, is that light does always travel at
the same speed in a vacuum. And remember what light
is like. You can think about light as a photon
is a little particle, and it is quantized. But if
fundamentally think about light is a sort of a ripple.
Space is filled with quantum fields. Even empty space has
the possibility to have these particles in it because it
(11:47):
contains within it these quantum fields, like the electromagnetic field
that can ripple. So I think of space is sort
of like having these different possible sort of sheets rubber
sheets in it, and mostly they're just sort of spread.
It's smooth, but occasionally you can get a ripple in one,
and that ripple is like a particle. I see. So
I think the main idea is that light is a
photon is not a thing. It's not like a little
(12:09):
spherical thing. It's more like like a little divid in
the kind of the fabric of the universe. Yeah, it's
got no stuff to it, if that's what you mean
by it's not a thing. It's a it's a transient property, right,
It's that's why it's always moving. It's a it's a ripple.
It's like a change in information. It's saying, oh, the
electromag than the field was this, so now it's that,
And it's that change that is the photon. That's why
(12:31):
we say that photons carry information because they're like they're
carrying information about how these fields are changing, the fields
going up, and then it's going down, the fields going
up and then it's going down. That is, the photon
doesn't actually move like a like a thing. We can
think of a coffee mug moving from one side of
my desk the other side of the desk, but a particle.
A quantum particle is more like um like an effect.
(12:53):
This part tells the other part, and then that part
there's the next part, and then it just propagates. Yeah,
Like if you and I hold it ump rope between
us and we spind the jump rope, no part of
the jump rope is moving sideways, like closer to me
or to you. That's just moving up and down. But
we can send pulses down that jump rope. Right, if
I wiggled it really fast, I can send what looks
(13:13):
like a pulse, But no part of the rope is
moving sideways. But the pulse is moving sideways that the
wiggle is moving, but the rope isn't moving. Yeah, and
even the coffee cup, and the coffee cup is made
of particles, and those particles are ripples in their fields,
electron fields, cork fields, whatever, So when the coffee cup moves,
it's actually the same thing. It's just like a bunch
(13:34):
of little tiny ripples are moving sideways through the universe.
That's such a hard warming image, you know, the two
of us jumping rope together. I feel like I feel
like I imagine we're in some you know, like city
block and we're jumping rope and there's children singing, and
we're having the time of our lives and there's leaves
blowing everywhere. And afterwards we're sipping hot cocoa and sort
(13:55):
of a warm, fuzzy camera glow, and we're being interviewed
by Oprah Like, that's what you're imagining. Yeah, you're sure
that's going to happen for us. I feel so much
lighter there. Well, that's good because we're talking about light.
And so that's what photons are, right, their ripples in
this electromagnetic field. And the reason that they move at
the speed of light in a vacuum is that then
(14:16):
it's just pure electromagnetic field, and they move at the
maximum speed of information, which is the speed of light
in a vacuum. So things that are not photons, like
electrons and corks, they're also you're saying, they're also a
little perturbations in fields as well. They're just like photons
in that they're They also don't move in the universe.
(14:38):
They just propagate. Yes, they propagate through the electron field
or the cork field, for example. And so an electron
moves through the universe and it moves at whatever speed
it's moving, and it's a wiggle in that field. And
you might ask, well, how come the electron field wiggles
and it doesn't wiggle the same speed as the photon field? Right, Yeah,
how come an electron wiggle can variate its speed? Boom?
(15:00):
We have a gray answer for that. It's the Higgs
boson because the electron field is not free, it's tied
to this other field, the Higgs field, that changes how
it wiggles. And that's what it means for a particle
to have mass, is that it interacts with this Higgs
Boson field that changes how ripples move through it, and
it changes it exactly the way you would expect if
(15:20):
something had mass. It gives those fields inertia. So mass
is more like whether or not it interacts with the
Higgs field, which would slow it down as it moves
through the universe. Yeah, it doesn't actually slow it down.
It gives it inertia. It makes it harder to speed
up and harder to slow down. Right, It's a tiny
bit more complicated than just slowing down. The Higgs field
is not like molasses that tries to get everything to
(15:41):
go to zero speed. It just keeps things at the
same speed. It makes it hard to change velocity. Oh,
I see, but light light doesn't interact with the Higgs field,
so it just always kind of ignores the Higgs field
and just goes as fast as it can. Yeah, and
it's actually fascinating. We should do an entire podcast on
this electroweak symmetry. Like the photon doesn't interact with the
(16:03):
Higgs field at all, but the W and the Z particles,
which are very similar to the photon, do interact with
the Higgs field and become really heavy, and that's why
the weak field is weak. All right. So there's a
lot of details in there, but the main idea is
that a light is a wiggle and it always travels
at this speed of light in a vacuum, in a
vacuum precisely. All right, Let's get into this idea of
(16:25):
maybe whether or not you can slow this wiggle of
light and how you would do that and why you
would want to do that. First, let's take a quick
break up. Okay, Daniel, So you're telling me that light
(16:49):
always moves at the speed of light. Yeah, deep inside
here from did you know horge also always travels at
the speed of Horge that's gonna to a hundred dollars
physics consulting feed speed of X moves at the speed
of X. Yeah, but I guess on like light, my
the horhead, the speed of horhead slows down, is going
(17:11):
to be slowing down, is slowing down, is going to
slow down as I get older. But the speed of
light doesn't seem to change at all. Yeah, And so
we should separate because again, physics has been terrible about naming,
because when we talk about the speed of light, often
what we mean is maximum speed of information in the universe.
Light in a vacuum happens to travel at that speed,
(17:32):
and because of historical reasons, we called that speed the
speed of light. Oh man, you mean physicists name things wrong, Yes, badly,
and they could have really named it that really the
speed of light, We should just call it the maximum
speed of the universe. Yes, the maximum speed the universe
is separate from the speed of light, because it turns
(17:54):
out there are lots of different ways to make light
move slower than the maximum speed of the universe. So
it's really like a definitions thing, like we just have
to wrap our shift that little lever in our heads
and not call it the speed of light anymore. That's right,
Welcome to physics legalities. Right, But officer, I was not
traveling faster than the maximum speed of the universe. It
(18:16):
technically didn't break the speed limit of this highway. It's
just that the speed limit of this highway is not
the maximum speed of the universe. I'm sure that that's
going to get you off that ticket yet. Well, well, well,
and in the ideal scenario, the officer is a fan
of our show and just pulled you over to get
(18:37):
your autograph. That's right on this speeding ticket. And we
talked in the podcast UM recently actually about drink off
radiation and what happens when life gets slowed down as
it passes through a material and other particles zip passed it.
That's a whole fun, complicated topic. Okay, so we can't
decrease the maximum speed of the universe, but you can
(18:59):
make the light go slower than it would in a vacuum,
or slower than the maximum pet Yeah, you can make
photons move slower than the maximum speed. And the way
you do it is you get them to go through
some material. Because we said that photons move at the
maximum speed when it's just a pure electromagnetic field and
a ripple. But if you put other stuff in there,
(19:21):
like an electron field. Electrons interact with electromagnetic fields, right,
They have charge and so they will interact with it.
And like a photon can get absorbed by an electron,
you have an electron that's zipping around a nucleus, it
can absorb that photon, hang onto it for a minute,
and then re emit it. So that effectively slows down
the photon because it gets from A to B in
(19:42):
more time than it would if it hadn't been absorbed. Right,
Like if you want to slow down Hussain Bolt, you
would have him run through a crowd of people, not
on an empty track, That's right. Or Jorge driving to
work takes less time than Jorge driving to work past
the banana shop because he stops, he takes some bananas.
It takes some ten minutes. Everybody wonders why he's late.
You know, I'm not sure anyone wonders anymore why I'm late.
(20:05):
Daniel put, I'm just glad that you didn't say that
I drove through a crowd of people. That's where where
you were going. I'm like, oh, it just just got
really dark though this is a family friendly podcast, that's right, right, Yeah,
I guess what you mean is if you want to
slow light down, you sort of keep it busy. Yeah,
you interact with it. You you know, Jorge passing through
(20:27):
a crowd of his admirers is going to have to
stop and sign autographs more often, and so he's not
going to get to the other side of the room
as quickly. Yeah, although these days people don't ask for
autograph date just asked for selfies mostly. Well, that saves
your risk, I hope. And to listeners that might feel
like a technicality, like is the light really traveling less
than the speed of light? Because it's not really. It's
(20:49):
between interactions. It's still moving at the maximum speed limit.
It's just that those interactions take some time. And and
that's that's totally fair. And again it just depends on
how you define it. Like you shoot a beam of
light into glass, when does it come out the other
side more time? It would take more time than if
you shot a beam into vacuum. So that's what we
mean when we say have we slowed down the light?
(21:11):
I guess you know the image I have in my
head is sort of like a pinball machine or like
a pachinko machine. Have you seen those Japanese pinball games
where the little ball wants to go in a straight line,
but it keeps bumping into things in between? Now, is
that sort of what's happening where it's like bumping between
things or is it really more like the interaction slow
(21:33):
the light down? Both. It can't move in a straight
line through material. It's getting deflected. But all deflections are
also interactions, and sort of a philosophical question here, right,
If the photon is deflected, is it the same photon
as the one that came in? It seems like a
clear question if it gets like fully absorbed and then
re emitted. But if it gets deflected, it's still there's
(21:55):
an interaction there, and so it's a different quantum state.
Oh it actually does get deflected. Well, yeah, Photons moving
through material don't move in a straight line. They get
bounced around like a pachinko machine between particles. Sometimes they
get absorbed and re emitted, sometimes just deflected um. And
you know, some could argue that those are very different
kind of experiences for the photons. Some could argue they're
all interactions so it's all the same deal. But yeah,
(22:17):
they interact and they get deflected, so it takes longer
to get from one side of the material to the other.
I see. So is what slows the light down the
detours that's taking or is there actually kind of time
wasted in getting absorbed and re emitted by something in
the material. It's both both. Those are two different kinds
of interactions, but they both contribute to making light go
(22:38):
most more slowly through the material. And it's sort of
similar to like, you know, making waves move more slowly
through some material, Like if you if you make waves
in water, it's different than if you make waves in
honey or in molasses. Right, there's just a different sort
of speed of information traveling through that material. And again
it's because the interactions, like honey holds itself together with
(22:59):
more wrongly than water does. It's really the same deal.
So then it sort of depends on what you mean
by light, like it's light natural photon or is light
kind of the beam of light? Because the photon itself
doesn't slow down, does it? It just it just gets busier.
That's right. The photon itself doesn't slow down unless you're
averaging right, if you're averaging over it's time through the material,
(23:21):
then its effective speed is slower. So again it's a
bit technical. But what these folks have done with these
experiments that we've been hearing about slowing down light and
stopping light is even something different than that. It's even
more of a technicality than just slowing down light. Overall,
there are different ways to slow light down. One as
you can slow the photon as it goes across the material.
(23:43):
And then there's another way, which is what these physicists
have done. Yeah, because slowing things down as it goes
through material, that's old news. Like Isaac Newton did that, right,
he used prisms. Everybody is known for years that you
can slow things down as they move through a material. Right,
it's old news. That's from the time of Yeah, this
it's old news. What these folks have done is not
just slow things down by moving through ice or water
(24:04):
or blast. That's last centuries physics news. What they've done
is something different. Or they claim that it's a big
step forward, all right, well, um, step us store it.
What have they done? What did these physicists due to
slow down light? So what they're doing is not slowing
down like a beam of light by slowing down individual photons.
What they're doing is they're slowing down sort of a pulse.
And when you send a pulse of light, it's not
(24:27):
just one photon, it's like a collection of photons, the
way like if you send wiggles down a jump rope,
it's not a single like sign wave doesn't have a
single frequency. It has a bunch of different frequencies all
added up inside of it to make that one shape.
So are we still talking about individual photons? Are we
(24:47):
just talking about kind of the the shape of the
light pulse. We're talking about the pulse, but the pulse
is made up of a bunch of different photons. It's
like if you have a group of bike riders and
you send them all out in a pack. You've got
a hundred of them or something, and they're all biking together.
You've got some fast ones and some slow ones and
(25:07):
sort of like the you know, the shape of the
pack is changing as it travels, and so the whole
How old are these cyclists. They're twenty three, just like
you are. So they're excellent shape. Okay, I just want
to paint a complete picture here, and any of you
out there who know anything about like Fourier analysis, that
you can break down wiggles into sort of their component
(25:28):
frequencies or so you know, if you're like listening to
audio and you're looking at equalizer, it shows you, like,
am I hearing the high pitches or the low pitches
and the medium pitches? All sound is just a bunch
of combinations of different wiggles at different frequencies. The same
thing is true for a pulse of light. If you
want a pulse of light, you have to make some
high wiggles and some slower wiggles and patch them all
(25:49):
together to make that pulse. Right. And and is it
that different photons of different these different frequencies or is
it okay? Yes? And so what these researchers have done
is not slow down individual photons, because that's old news,
is to try to slow down this whole pulse. To say,
if we're gonna like send information through fiber optics or whatever,
(26:11):
you do that by sending pulses of information, you flick
the switch on and off and that sense whole. But
a grouping of photons that's a that's a pulse. That's
a pulse. And so what these experiments are focused on
is taking one of those pulses and trying to make
it go more slowly through a material the entire pulse.
And the key thing there is that, as you said,
the pulse is made of photons at different frequencies, So
(26:33):
you've got some that wiggle fast and some the wiggle slow.
And that's the key idea, because what they do is
they construct some crazy material, some weird quantum material or
something where the speed of light through the material is
different for different frequencies. So the light is slowed down
differently based on how much it wiggles, Like the really
blue light is slowed down more than the really red light.
(26:56):
So it's not that they slowed light down, is that
they made a material that selectively slows different frequencies. Yes,
And the consequence is that you like spread out the pulse,
like you have that group of riders and you said,
all right, I'm gonna make all the old people ride
more slowly. Now, then they're gonna start to fall behind,
and the pulse of riders, this group of bikers is
(27:17):
going to get more and more spread out, and so
when the trailing edge of the pulse arrives is going
to be later, and so sort of the whole pulse
takes longer to get there. Uh, you stretched it out.
When it gets there later, well you stretched it out
and sort of so if you look at like where
the peak of the pulse is, it moves back because
part of the pulse got stretched backwards. But doesn't that
(27:41):
happen normally when you shoot light through glass or ice?
Doesn't it normally get spread out? It does normally get
spread out because there's this dispersion relationship, that's what we
call it, where the slowing downiness depends on the frequency.
And that's how a prism works, right. A prism works
because you shoot light into it and the mount that
it bends depends on this frequency and that's the same
(28:03):
thing that down so it spreads out exactly, And so
is what have they done? Then? That's different? Well, these
materials have like a very strong dispersion. It's like extremely
extra extra so you can Yeah, so you can shoot
like a laser, like let's say you want to slow
it do on a laser pulse. A laser pulse has
a bunch of frequencies really close together. Because you know,
(28:24):
a laser is usually like a single color, so it's
all a bunch of really tightly packed frequencies, it's harder
to spread out. So what they've done is developed these
materials where the dispersion is really strong. So even a
tightly packed laser pulse, which is a bunch of frequencies
really similar to each other, gets spread out and effectively
slowed down. It gets blurry. Yeah, yeah, so it gets blurry.
(28:45):
You smush it. You just gonna smooche the laser. But
then you know, But then, but like the leading edge
of the poles, like the first electrons that go in
there are the fastest, still come out the other side
as fast as they can. Yeah, the first photons get
there um at a very high speed. It's not exactly
the speed of light, because every photon is slowed down
as it goes through a material for the reasons we
(29:06):
talked about before. But you're right there. The difference now
in the speed between the leading edge and the trailing edge.
And this is where it gets kind of technical. These
folks talk not about phase velocity, which is a velocity
of any individual photon in the pulse, a group velocity,
which is like where is the location of the peak
half ast is the peak moving. It's like slowing light
down in the sense that in the sense of what
(29:30):
happens when you slow down a song or the audio
of of somebody speaking, it's just goods sort of you know,
more on the lower it gets blurrier, and it gets
more in the lower tones. You know, goods slow. But
they're not technically slowing down the sound pulled the wave.
(29:53):
They're just kind of spreading it out. And so that's
what they mean by slowing light down. Yeah, that's what
they mean by slowing down light. And so you're gonna
be like clever lawyers, clever lawyers, you gotta sort of
dig and several layers there before you figure out, like
what do you really mean by slowing down? Like it
is sort of more like slowing down in the audio sense, right,
like you're slowing down a song or speech. Yes, in
(30:16):
the sense of audio, like the analogy would be that
you're extending the wavelength of every part of that sound.
But then you know, as I say to you, hey, Jorge, um,
you're slowing down like the lower frequencies even more so,
like you not not only would it sound deeper, would
I sound more like you know, al green? But also
the length of the Jorge phrase would take longer to
(30:38):
get there because the trailing edge of it would take
longer because you're slowing down the even lower bits. Folks.
And that was not just Jorge ad living. That was
actual scientific experimentation, right. That wasn't me burping a banana.
That was an actual physics. Yeah. And there's one group
that claims to have even stopped light. Okay, let's get
(31:01):
into this idea, and maybe you can stop light. That's
pretty crazy. But first let's take a quick break. Okay,
so my lawyer says I can slow light down if
(31:23):
by slowing down I mean like spreading out the wavelengths
and making it just sort of smooshier. But you're saying
some scientists can actually or have claim to have actually
stopped light. Yes, and this is a physics topic, which
is sort of amazing and then also like sort of
disappointing and nonsensical because I feel like I stopped light
(31:44):
all the time. Just you know, I've always been able
to do that. Half of that power. I just go
outside and I seem to stop all the light from
the sun. You do make a better door than a window.
It's true. I cast a long shadow that you. Well,
what these folks did when they say they stop light
is they shoot a laser into some kind of material
and then the laser gets absorbed by that material, and
(32:06):
then they can flick a switch and the beam shoots
back out the other side and an arbitrary later moment. Okay,
that does sound pretty cool. It sounds pretty cool. They're
like shoot a beam in, it gets like absorbed, it
gets he stored, and you can wait like you know,
a second, an hour, a day, and then you flip
another laser switch and then the beam shoots it back
(32:26):
out the other side. Wow, Like, did they trapped it
inside of this material and then they can let it
go later? Yes, they've trapped inside the material. All right,
I mean I'm intrigued. You might ask like, well, did
they stop the photons? Like and no, of course they
can't stop the photons. It's not like if they zoomed
in with a microscope they could see these photons just
like sitting there. They didn't freeze them, right, they didn't
(32:47):
freeze the photons. What's that comic book character with the
with the freezing powers? Oh? Mr Freeze did you just
make that up? And over see? I tell you they
right if out of I what's that guy with the
cartoon powers for particle physics? Is it? Mr particle Physics
booms exactly? That's how clever that idea sounds. Anyway, Mr
(33:08):
Freeze didn't like Mr Freeze. These photons, we can like
look at them covered in ice or anything. What do
they do? They keep them busy, or they record them
or what they get. I feel like that comes comes
from parenting. You're like, all right, I'm gonna run all
these crazy kids into this room, keep them busy, and
they'll shoot out the other side. Here's my phone if
if you want to see a room full of children,
(33:30):
slow down and maybe Freeze just hand them your phone. Yeah.
They just have a little phone for these for these photons. Yeah,
there you go, and these phones admit photons. Now now
it is getting too deep. Yeah. Essentially, what they do
is they build some system that can absorb the light,
store the information from that light, and then re emit
(33:51):
it at an arbitrary later time based on some external input.
Isn't that called a camera? Yes, exactly exactly. So I'm
one sense, it's like, wow, that's awesome. This cloud of
atoms absorbed this laser and can re emit it. On
the other hand, like, well, every photograph is basically stopping
light and later re emitting it. Um. So in one
(34:12):
sense it's awesome because it's something that nobody's ever done before,
and another sense, it is accomplishing something we've been doing forever. Well,
I mean, a photograph and the TV is pretty complicated, right,
because you need all these different things. But it might
get in the sense and maybe these guys did it
in a much more fundamental sense, like they actually build
a material that takes a photographs and and admits it. Well,
(34:34):
I wouldn't say it's less complicated, Like I think a
camera and a screen is less complicated than having like
you know, um Millie kelvin refrigerator and high intensity lasers.
But this thing does it sort of all at once,
like it both absorbs the photons and then later re
emits it. It's not like it's you know, stored electronically
and then regenerated. But again you might say it's still
(34:56):
it's not the same photons, like they were absorbed by
the material and then re emitted. Is it the same photons?
Not really stem me through it? What did they do?
How does this material work. Well. They used a Bose
Einstein condensate, which is a very cold collection of atoms
in a really weird quantum state. We'll do a whole
podcast on what a Bose Einstein condensate is. But what
(35:18):
they did is they shoot a laser into it from
the side and then that's sort of like holds it.
What do you mean, hold it? Like it the light
is absorbed by the atoms and the condensate, and then
they just sort of hold it. This first laser is
like the switch laser, just like gets the atoms in
the right situation and puts them in the right sort
of quantum state to do this trick. Then they have
the second laser, the one they actually want to absorb
(35:40):
and re emit. They fire that into this pile of
atoms and then it gets absorbed and then when they
want to release it, they just turn off the first
laser and then it emits the pulse from the second
laser in the same direction and with the same shape
and intensity everything. Yeah, precisely, and you know, there's some
loss of information there, but mostly it captures the you know, light.
(36:00):
That is pretty cool. I think that it's pretty awesome.
Otherwise it's just a superpower that only a comic book
character Mr. Light Freezer would have. Mr. Laser Freeze. Okay,
so oh wow, so um, I mean that that sounds
pretty impressive that you shoot it and then the atoms themselves,
not some like memory or some electronic it's like the
(36:22):
atoms themselves somehow remember this laser and then admit it
when you want to wanted to in the same way
that it came in. Yeah, that is pretty cool, And
I think it's awesome that people just try to make
materials do new stuff, Like does this whole field of
you know, atomic physics and condensed matter physics where people
are like, hey, can we make some new kind of
(36:44):
goo that has this weird property? Can make some new
kind of good that has that weird property. And it's
sort of just like an exploration of the way things
can be in the universe. And you know, we're used
to like different kinds of stuff water, metals, rocks, whatever,
but it's possible that there's all different kinds of stuff
in the universe that we've never experienced just because doesn't
occur naturally here on Earth. So these folks are like
(37:06):
pushing the boundaries right, they're trying to make things do
new things. Yes, exactly, they're trying to bring the rules.
They're like, well, nobody thought things could do this, Let's
see if we can make it work. It's kind of like,
you know, nobody ever thought you could walk on water,
but hey, it turns out if you freeze it, you
can walk on water. Yes, And in some sense it's
as unsatisfying as that. It's like, are you really walking
(37:28):
on water if you're walking on a frozen lake? Technically yes,
what have you shoot a lasers through it? Is it
still technically a miracle? Well, you know, if you're ice skating,
then you're walking water because you're making this very thin
layer of water between your blades and the ice. So
you know, there you go. More Jesus lawyer. And people
(37:50):
try to say, maybe there's some application to this, like
if you could slow down light, you could make circuits
that use less power or something interesting. Yeah, rank Frankly,
I don't find any of that convincing. I think that
the motivation for this is like, hey, can we make
something in the universe that's weird and different? And I
think there's there's a there's a lot of value to
that well, there's a lot of applications that they say
(38:12):
so in solid state physics, where if you get these
materials to do these weird things, that they could maybe
be the basis for like a quantum computer. Yeah, precisely,
there are ways and you could find applications for materials
that do weird stuff. And you know, people were building
atom traps and bose Einstein particles for years before they
(38:33):
had ideas for how to apply them. So it's a
good idea to develop new kinds of materials because they
can inspire new applications. And sometimes you work on something
for ten years and then people realize, hey, that's exactly
what we needed for this other totally unrelated problem. So
you know, we want to make progress as a species,
we've got to like explore broadly and try all sorts
of weird stuff. And you never know, like it could
(38:54):
have been that they didn't make light stop, but something
else totally weird and unexpected happened and discovered something crazy.
You never know. When you do research right or just
that improving these things, you learn something new about the
materials themselves, or how atoms are like behave, or if
in order to make this happen. You have to invent
something new and weird and that turns out to be
(39:15):
really useful for something else, you know, like the whole
semiconductor industry and computers exist because of basic research into
stuff that wasn't motivated at building computers. Like, for example,
I just got this great idea to shoot myself with
a laser to stop aging or at least on my knees.
There is laser therapy for everything, Like you can get
(39:35):
laser therapy to like stop smoking. I wonder, like what
part do you are they sting with the laser? Probably
your wallet and your Yeah, I don't know. I wouldn't
recommend any of those laser treatments. Okay, well it is.
It does sound like they maybe have technically slowed down
(39:55):
light and maybe sort of in a ways stop light.
So that's pretty pretty interesting, pretty cool. It's pretty cool.
They've definitely done something nobody has done before. And today
we only talked about a couple of groups doing a
couple of experiments, but it's a big field and people
are doing it will all sorts of new kinds of
materials and in new ways, and applying it to different
kinds of light and higher intensity, lower intensity, and bigger
(40:17):
packets and smaller packets. We couldn't possibly review all the
recent work on it. It's a whole burgeoning industry. All right. Well,
I think that answers that question that a lot of
people wrote in about you can slow down light and
you can stop light. Have you defined things in the
right way and use both speakers? Was that the main
lesson here? Yes? And so if you find a bottle
(40:38):
and the genie comes out and he's a physicist, and
you ask for some new special power, make sure you
are very clear about what you're asking for, because these
physicists will find a way to squirrel out of it
and not give you what you were hoping for. Right,
You might just hand you a six pack of light
beer and then you say your way. This light will
make you slow down. That's one way to slow down. There,
(41:02):
you go do that a laser and you can sell
it for a lot more. Yeah, you don't want us
as your physics genies may that that's what would make
hell a little bit more tolerable. Sin, you can freeze
some beer down and that you can drink it with
the physicists. All right, Well, thank you very much to
everybody who wrote in to ask us about this weird headline.
And if you read something online about weird discoveries and
(41:23):
physics or something you're not quite sure about, send it
to us. We will break it down for you. You
hope you enjoyed that. Thanks for listening. Let's see you
next time. Before you still have a question after listening
to all these explanations, please drop us a line. We'd
love to hear from you. You can find us on Facebook, Twitter,
(41:46):
and Instagram at Daniel and Jorge That's one word, or
email us at Feedback at Daniel and Jorge dot com.
Thanks for listening, and remember that Daniel and Jorge explained.
The Universe is a production of I Heart Radio More
podcast from my Heart Radio. Visit the I heart Radio
Apple podcasts or wherever you listen to your favorite shows.
(42:12):
Yeah
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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