May 16, 2019 • 40 mins
Everybody talks about it, but what IS it?
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Speaker 1 (00:00):
Ythaniel. Do you ever get caught using a word that
you don't really understand? Do you actually always know what
you're talking about? I don't think I'm willing to devote
that here on the podcast, and maybe we can have
that conversation over a beer sometimes. But the thing that
(00:22):
I love is that there are some topics in physics,
some idea, some words that I'm pretty sure nobody actually
really understands. I feel like this is taking us to
another episode about quantum physics that's right and wrong at
the same time. No, I mean, yes, exactly what at
(00:47):
the same time. Hi'm Jorge. I'm a cartoonist and the
creator of PhD Comics. Hi I'm Daniel, I'm a particle physicist,
(01:11):
and Welcome to our podcast, Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we take
crazy and fascinating and amazing and hot and wet and
nasty things about the universe and try to explain them
to you. Today's episode is dedicated to a Lana, whose
boyfriend Nick is a local fan of the podcast here
in Irvine. Happy Birthday, Alana. Today's topic is a really
(01:38):
fun one, and in preparation for it. Not only did
I go and do the normal street interviews I usually
do with random people, but I actually went around the
halls of my physics department and I asked a bunch
of grad students if they could explain today's topic for me,
and they found it pretty tough. Wow, that's a pretty
interesting spin on the topic in our process. That's right,
(02:00):
that's right to be on the podcast. We're going to
be talking about quantum spin exactly. Quantum spin? What is it?
What does it mean? Are things actually spinning? Why is
it quantum um? We've got a lot of people sending
in requests to talk about this topic. And I think
when people are interested in physics and digging around in
(02:23):
videos on the internet and listening to podcast you hear
this come up a lot. It comes up in quantum entanglement,
It's in science fiction everywhere, and of course people want
to know what does it mean, what's going on? Why
is it so important? That's right? What is spin? How
fast is it spinning? What's the spin on it? Exactly?
Is it's spinning out of control? That's right? Is it
(02:43):
different from political spin? Um? It's definitely a thing, right,
It's something fascinating and more importantly, if you're an expert
in this field, does that make you a spin doctor
doctor of spin? Yes, exactly. You know, we have sort
of a bad track record in physics of naming things
um naming things using familiar words, like we give corks flavors, right,
(03:06):
the up cork and the charm cork and the top
cork a different flavors of corks, colors and colors, right
when they don't really have color, they don't really have color,
and they don't really taste different. I mean, I don't know,
I've never actually tasted a top cork, So there you go.
I'm speaking on something I don't really understand, but we
don't mean it in that way. So sometimes we're like
adopting existing words and just using them totally inappropriately. Other
(03:32):
times we're trying to use words that are similar to
some other concepts because specifically because we want to call
up structures and ideas from those concepts. Like color is
actually a pretty cool one because while the particles are
not colored, there's something about quantum color which is similar
to color, and so we want to express that. You're
(03:52):
sort of trying to grab onto an intuitive idea, even
if you're trying to like a reference, you're trying to
reference an intuitive idea in our everyday experience and and
kind of latch that onto a physics concept exactly. And
we do that all the time, right, That's basically what
physics is is try to explain the unknown in terms
of the known, right, Like when we try to talk
(04:14):
about particles, we're doing that all the time. You know,
we're saying, oh, it's a particle, No, it's a wave. Right,
It's really neither, and it's both and it's something else
more complicated, And we're just trying to patch together a
description of using ideas in your head, right, we weave
these together into some sort of understanding. Here's such a
suggestion for you guys. Maybe you should just instead of
calling being bold and calling it like colors or flavors,
(04:37):
just at the word, like at the beginning in your
official definition. So the announcement should be like, like we're
calling this color, I mean, like let's call flavor. No,
I just call it like a quantum light color. It's
light color. It has a color like property or a
spin like property, or you know, I mean, if you
(04:58):
want to get mathematically, just with a little Tilda in
the front right, and then nobody would be confused. Yeah,
I like that idea a lot good. You like like it? Yeah,
I totally like it, and I think, um, I hope
on Twitter it gets a lot of likes. There you go.
I think I want to something. I think I think
you should be nominated onto the secret International Physics Naming Committee.
(05:23):
I don't know where it meets or when it meets,
or who's in charge of it, but they have been
making some dubious decisions recently and they need some fresh blood. Well,
I think if I was in charge, I would just
put the tilt in front of physics itself. You know,
what are you study till the physics? Physics? I'm studying
something like physics, but it's not actually, I mean, isn't
(05:44):
everything like physics? Pretty next thing, you're gonna be calling
me like a physicist, not actually a physicist, but something
like approximating a physicist. You know, you're gonna spread this
Tilda Daniels sort of like a Daniel into it. Lee,
You're Daniel. You know I'm actually a Daniel. I am
the definition of me, right, You're Daniel sub zero exactly.
(06:09):
I'm not like me I am me, right, you are,
I think therefore I am Daniel. Well, anyways, before we
spin out of control here on this um side conversation, um,
we're talking about quantum spin, and so it's it. It
plays a big deal in quantum physics and quantum entanglement, right,
and atomic orbitals. I mean it's it's sort of it's
important in everybody's atoms, which um which are a kind
(06:32):
of important to people, that's right. I like my atoms,
you know, they're not like atoms. I just like them.
But exactly, spin is everywhere, and you hear come up,
and especially when you're hearing explanations of quantum entanglement or
quantum computing or or orbitals and this kind of stuff,
you hear about it. And when I was a student
and I was learning about quantum spin, I was like, okay,
(06:54):
but what is it like? Are these things actually spinning?
Why do we call it spin? Right? How could a
particle spin? And so let's dig into all that today. Well,
as usual, Daniel went out there and asked people in
the street if they knew what quantum spin was. And
you actually have kind of an interesting spin twid this time, right, Yeah,
I went out and I asked people on the U
C Irvine campus and then a few other folks at
(07:16):
the at an Irvine mall Um what they thought quantum
spin was, if they could describe it, and if they
did understand quantum spin. I also asked them, are these
particles actually spinning or is that just like spin? Is
that just like a word we use. Well, here's what
people had to say, um, so energy, I just think
of energy. Oh yes, what's the quantum spin? Uh? Like
(07:39):
quantum spin as an like spin up, spin down with
within molecular orbitals and electrons um. So are those particles
like actually spinning gesa Uh No, I haven't quantum spin? No?
(08:00):
Uh yeah, it's electron spin um that it can be
positive or negative um. And then how's it different from
normal spin? Like a tennis ball can spin? Is electron
spain the same thing? Are they physically spinning? Uh? I'm
not sure. I know some stuff about Polly's principle that
(08:20):
the spin should be in a specific way, and they're prepared,
so I know some way in distunt stuff, but I'm
not sure about the people. Right. Yeah, isn't because um
every electron has a different type of spin. It's like
either a plus one half a minus one half. I
forgot what it would Yeah, and are they like actually spinning.
Is it like the same thing as a tennis ball spinning?
(08:41):
Or is it's um? No, because it's not like because
you can't treat it like it's a particle, right, because
it's um like an electron is a mix of a
particle and a way. I've heard of it, but I'm
not familiar with the meaning. Okay, No, I don't know
what that is, all right. Yes, there's up and down spin, right,
and it's in me. Uh well, I don't know much
(09:03):
more than that, Okay, all right. It seemed very binary.
Some people had had no idea, never even heard of it,
and some people had a lot to say about this topic. Yeah,
and some people knew some stuff sort of in the
vicinity of the topic, but not actually like reluing to
the question the vicinity. Yeah, yeah, exactly. I like when
when I asked somebody question and I can tell they
(09:23):
haven't thought about this in a long time, and they
have sort of like a free association going off in
their mind that wait, this is connected to that idea,
to that idea of that idea, and then they go no, Actually,
I don't know. I don't know the answer. That's really fun,
but you can see their process. Yeah, because a lot
of people to be heard about it or read about
it a long time ago, maybe in high school physics
or something, and so you sort used to feel like
(09:45):
it's in there in your brain. You just need some
time to you know, boot up the hard drives. Yeah, exactly.
But I think my favorite part of this experience asking
people these questions was that one gentleman after I asked him, um,
if you knew about quantum spin, he then asked me,
he said, is this some thing you need for your dissertation?
Said yes, yes, sir, Yes, sir, I am a twenty
(10:08):
five year old graduate student. Absolutely, are you saying that
you can be a grad student at forty something? There
are fewer of them. Yes, there are a few or
forties something year old grad students. But I have to
say the ones that are forty years old, like folks
that went out into the real world and worked in
real estate or law or something and then came back
to do physics grad school, they're really good students. They
(10:29):
really want it. Yeah, it takes a it's a lot
of work to divert from a path in the real
world back into academia. It's much harder than just like
going from undergrad to grad school. So I really respect that.
You know, you gotta really want it cool. So if
there are any there, any listeners out there who want
to change careers and get a PhD, they should contact you,
right and you have a spot for them. I'm not
(10:51):
sure I just offered anybody funding, but I would encourage you.
If you have a deep passion for physics and you're
finding yourself in a dead end job and you wish
you had gone to physics grad school to unraveled mr
of the universe, I encourage you, sir and ma'am, if
you'd like to be in another dead end of of
awesome knowledge, then be a cartoonist. Is that what you're
(11:12):
gonna say? Yes? Yes, alright, So Daniel, let's break it
down for people. What is quantum spin? We don't know?
Done podcast over, why we had to diversely so much?
Just do fill fill in the air time? Um, So
(11:34):
people don't know because I don't know what quantum spin is.
We don't really know what it is. Now we know
it's a thing, right, So there's this thing, this property
of particles. We don't really understand what it is, what
it's doing. But there's this thing we've observed and we
call it quantum spin. So let's do that. Let's talk
about what this thing is that we can observe, and
then we'll talk about why we call it quantum spin.
(11:54):
And you know whether it's actually spinning and stuff. Right, Well,
what's the origin of this? How did this come to
be a thing? And it came to be a thing
basically because of magnets, right, and you remember that charged particles.
So a little particle with the charge on it, like
an electron, if you move it in a circle, right,
like through a wire loop or something, then it makes
a magnetic field. Right, you can turn it on and
(12:16):
off like electro magnets. This is how they work, right,
like motors everyday electric motors, motors in your car and
in your electric car, and you're um even your phone, right,
there's probably a little motor of solenoid doing the vibrations
when you put it on vibrate mode. That that's the principle.
Like they just pump electrons through a coil a little loop,
(12:36):
and then that creates a magnetic magnetic field which moves something.
That's right. And the cute cool thing there is you
have a magnet you can control electronically or digitally. So
that's pretty awesome. But the important concept there is that
things moving in a circle. Charges moving in a circle
give you a magnetic field. Right, So that's something we
know about, right, something we understand. And so then people
were asking the question, um, well, do electrons themselves like
(13:01):
individual particles? Do they have little magnets on them, like
not just moving in a circle, but is there a
magnetic field just due to the particle? And this is
the kind of thing physicists do. They're like, well, we
don't expect this to be this way, but let's just check, right,
let's see if this happens. Because back then maybe they
thought particles were like little little balls, right maybe yeah,
well they didn't know. Right, this is in the nineteen twenties,
(13:23):
this is almost a hundred years ago. The whole idea
of a particle was still pretty new. People discovered electrons
and neutrons and you know, it was a crazy era
of discovery. They were maybe asking, like, is one electron
maybe like a magnet itself? Yes, exactly right, And that's
what they were wondering about. And so if it's a magnet,
it would have a direction, right, like a field exactly
(13:43):
what it have its own little magnetic field. So that
was the question they were trying to answer, like does
an electron or a silver atom or whatever, a little
particle have its own magnetic field. So what they did
is they built this device that would um that. Basically,
they put particles in this device and the device has
a magnet nick field on it, and they move the
particles through the device, not in a circle, just in
(14:04):
a straight line. And the idea is that the device
has magnets in it. So if the particles have their
own magnetic field, then they'll get pushed to one side
because the magnets on the particle would interact with the
magnets um from this device and would push the particles
to one side or the other, kind of like like
if you not through an electron, but if you through
like an actual magnet, like a fridge magnet, if you
(14:25):
threw it at some other magnets, it would get deflected, right, yeah, exactly,
it would get deflected. Now, most magnets are balanced, right,
Most magnets have a north and a south. So say
you set up like a really strong pair of magnets
and then you threw a fridge magnet through it, probably
wouldn't get deflected because it has a north and a south,
and so the polls would get balanced. So what they
did was they set up one really big magnet and
(14:48):
then a weaker magnet on the other side, so there's
like an uneven magnetic field through it, so that if
your fridge magnet goes through it, depending on the angle
of the magnet, depending on the angle of the north
and the south, they'll get pulled the one direction or
the other. So they build the particle version of this, Right,
They an uneven magnetic field, and they shot some particles
through it, and what they found was really surprising, Right,
(15:12):
it was really shocking what they discovered. They found that
particles do have magnetic fields. Yeah. Well, first of all,
they found two things that were surprising. One, particles do
have their own little magnetic fields. I mean, first they
did it with silver atoms, which is sort of the
easiest thing they could do to to make a beam.
Then this is a long time ago before you could
easily make like a beam of particles. They just put
a bunch of silver in an oven and like some
(15:33):
of it boiled off and they collimated it and got
a beam of silver atoms. And then later they did
the same thing with electrons, and they found that these
things have their own little magnets. Like an electron is
a magnet, which is kind of perplexing, like what does
that mean? Like where does this magnet come from? Right?
But what does it mean that it's a magnet? Like
if I just look at an electron, it has like
a north and south poll to it. Yes, exactly. Electrons
(15:56):
have their own little magnetic fields, even when they're not
moving in a circle, even if they're not moving at all,
Like if you suspended a like current, can you can
you do it? Yeah? Yeah, exactly. If you have an
electron just floating motionless in mid air, it will have
its own little magnet. And that's really weird to us now, right,
because we know that part of electrons and particles, they're
(16:17):
not like little balls or just points. Yes, exactly, and
so the question of where does that magnet come from?
That's where this whole idea of spin came from. But
the other really weird thing they found was that the
magnets didn't point in every direction, Like if you just
throw a bunch of magnets through this device, if they're
all pointed in different directions. Right is just randomly oriented,
(16:39):
you'd expect them to get deflected in random directions. This
one goes left, this one goes right, this one goes
a little bit up, this one goes a little bit down.
But you know, if they're randomly organized, they should go
in all sorts of directions, right, But what they found
was that they either went left or they went right.
There was not nothing in between, Like it's either left
or right and nothing else, like, those are your two options, really,
(17:01):
and no variations and how much right or how much left? Yeah,
they all went exactly the same amount left or this
exactly the amount right? What, Yeah, exactly. And that's why
we call it quantum. Right, they have this little quantum
magnetic field. The amazing thing is that, say, then you
rotate the device, you're like, okay, rotated ninety degrees. Maybe
you were measuring it on the X axis. Now you're
(17:22):
measuring on the y axis, right, so you're imagining, okay, well,
maybe these particles just you know, we're somehow weirdly oriented
along the x axis, so they either go left or right.
So then rotate your device ninety degrees. Then all the
particles either move up or down right. No matter how
you orient the device, the particles either go along the
(17:42):
magnet or against it. Right. So it's not like a
it's not like a real magnetic field, right, it's something weirder.
It's not like a real magnetic field. Exactly like a
magnetic it's like a magnetic field. And that's that's exactly
where we what they struggled with. They're like, Okay, this
has some property. It's something is generating this magnet, and
(18:04):
it's definitely quantum mechanical in some weird way because we
have all these weird properties. Like another weird thing about
this magnetic devices. So you send a bunch of electrons
through and you split them um left and right. Okay,
then you take the left beam, only the left beam,
and you split them through a device that's rotated ninety
degrees that goes up and down. Then they split up
(18:24):
and down right, even though beforehand they would only split
left and right. Wait, what if you take the left beam,
the one that the ones that went left? What if
you try to split them up again? Horizontally. Do they
all go left again, Yes, they do. But if you
split them left right, and you take the left beam,
then you split them up down, you take the upbeam,
(18:45):
and then you try to split it left and right again,
then they mix they go both left and right. And
that's why it's quantum mechanical, yeah, because you can't you
can't measure this weird little quantum magnet that the electron has.
You can't measure it both in X and in y
at the same time. It's the old Heisenberg uncertainty principle.
You can't know too much information about the universe. So
(19:07):
when you measure it in up, down, it mixes it
up again and left right. Whoa. It's like, oh, I see,
you can't measure the up and down and the left
and right at the same time. It's like them that
you can't know a particle where it is and where
it's going at the same time. The same thing applies
to the magnet exactly. And that's how we knew it
was a quantum mechanical property. Right, So we discovered this
(19:28):
weird thing about particles that they have their own little magnets,
and somehow this magnet is quantum mechanical, and so they
were like, what could this be? All right, let's get
into what it could be and some of the weird
things about that. But first let's take a quick break, okay, Daniel.
(19:55):
So people were measuring electrons and they found that they
have sort of like an inherent and inherent magnet inside
of electrons, and that it's quantum mechanical, meaning that it's
like really weird and you can't measure up and down
and left and right. That's right um, and so people
that's why people decide to call quantum spin. Yeah, even
though it's not really spinning, they just went for that name.
(20:17):
That's right. They call it quantum spin. And so let's
try to figure out what do they mean by quantum spin?
What why did they call it quantum spin? Alright, Well,
quantum is pretty straightforward. It's definitely it's a property of particles,
and it behaves like other quantum things that you can't
know too much about it at the same time or
you can't measure the x and the y um direction
of at the same time. And also it's either up
(20:38):
or down, right, there's no in between, So it's quantized.
So the quantum stuff all right, I buy it. Right,
you should definitely call this quantum something. If you're gonna
call it quantum spin or quantum banana, you know, that's
another question. But it should definitely have the quantum label. Okay, Well,
I feel like maybe they should have called it quantum
pole or something, you know, like something that references that,
(20:58):
because really you're trying to you're trying to capture this
kind of the magnetic the direction of the magnetic field
of the electron, right, which is kind of like the pole. Yeah, well,
what they what they did is they thought about what
generates magnetic polls, right, what generates magnetic fields. And we
know that a particle moving in a circle, right, like
orbiting for example, will generate a magnetic field. So then
(21:19):
they thought, maybe the particle is spinning. Maybe it's like
physically spinning. And if you imagine electron like it has
charge on it, think about the surface of the electron
has like little bits of its charge distributed across the surface.
This is the image they had back then, not the
image that we use now. If that was spinning, then
you can imagine that the spinning surface of the electron
(21:42):
could be basically particles moving in a circle and now
would generate a magnetic field. So they were like, ah,
maybe we've discovered that electrons can spin, right, but why
didn't they just call it like quantum poles and that?
Because because you were born a hundred years too late. Man,
I know, that's what I'm saying. Totally. I'm officially nominating
(22:02):
you to be on the Secret Physics Naming Committee. I
think you're you're great at this. I mean in the
sense of people out there who might be trying to understand,
is you know, I mean they're enough is they don't
really um care what they call it a perthennial thing.
But if if they had called the quantum the quantum
magnetic pole or something like that, would that still be
accurate and also maybe be easier to understand. I don't
(22:23):
think it's accurate because it's describing the effect and not
the cause. Right. We think something is happening which generates
the magnetic field and these other properties. But what we'd
like to do is figure out what the cause of
it is, Like, what is the particle doing that generates
the magnetic field? Right? What is the source of it?
And does that give us insight into other stuff? And um,
(22:44):
it turns out it does. Right, If we like to
think about the particles having this thing we call spin,
and we call it spin because it generates the magnetic field,
but also because the way the mathematics of the spin
is really similar to the mathematics of electrons in orbit.
M What do you mean it's similar? Well, the like
the mathematical language we use to like to describe it
(23:05):
is very very similar, right, And when you see it's
like some phenomenon A and you describe it mathematically, and
you see phenomenon being you describe mathematically and you notice
how look they're described by the same math, then you
have to wonder are they two sides of the same
coin or are they really the same thing? And so
people's people thought, oh, look at all this release relationships
(23:26):
between quantum between this thing we call quantum spin and
orbital angular momentum, you know, the angular momentum of something
moving around in a circle. And it was a lot
of connections there, but they turned out to be wrong, right,
Like I know, I know we were trying to do,
but you sort of missed kind of well, yes and no, right,
that's the first way you think about quantum mechanics like
(23:46):
theoretically it does work, Physically it doesn't. So like theoretically
it does, it works in this beautiful, really deep way
because um, what we know, for example, is that angular
momentum is conserved. Right, Like momentum is this property of
particles to keep going in the direction they're going. Angular
momentum is property particles to keep spinning the way they
(24:08):
were spinning, right, And we know that angler momentum is conserved.
If you start something spinning, it's going to keep spinning
until you stop it. Now, what's what's conserved is total
angle momentum, not the anglementum of like one thing. So
you spin a top right, and you can stop it
by touching it against another top, which then takes its
spin right. So the total spin of those two tops
(24:29):
is conserved. Well, the fascinating thing is that while we
don't know what spin is, we know that orribital, angular
momentum and spin are conserved together, like the some of
them is conserved, so you can change the spin of
some particle and and it will influence its angular momentum. Right,
(24:49):
what do you need to conserve is the some of
those two things, which tells you that spin really is
like a kind of angular momentum. Fundamentally, theoretically, these really
are related thing, Like a particle has that kind of
quite angular momentum, and so it's approble to call it spin. Yes,
it's some kind of intrinsic angular momentum. Like you can
(25:11):
transfer angular momentum from orbital from the orbital kind to
the spin kind and back right, which tells you that
they really are two kinds of the same thing, that
in some way the division between them is just in
our minds. It's just mathematical. Oh, I see, So you're
saying it's some kind of spin, it's some kind of
a rotation. Yes, exactly. So it's like spin. It's like
(25:33):
spin exactly. And so it's really I got you, I
got you. Yeah, you admit it on air that horror
Chand was right. They should be called like spin. It
should be called like spin. I completely in totally agree
with you. There you go. But I guess that the
question is, like, so they called they decided to call
(25:53):
it spin because it's sort of like it, like real spin.
But I guess the question is are these particles actually spinning? Right?
And that's the fascinating part is that they can't I
mean particles are points, right, they have no volume. We
were talking earlier about the idea of a particle with
a surface and maybe bits of it on the surface
we're spinning like the surface was rotating and the bits
(26:15):
of charge removing in a circle to generate a magnetic field.
That's hogwash, Like that can't happen because particles don't have
a size. Right, Electronics, as far as we know, has
zero size. Like the one side of it is exactly
the same place as the other side of it, so
there's nothing to spin. You can't turn around a point
and has no direction. It's like a vector of zero length,
(26:37):
Like there's no the distance between one end of the
particle and the other end of the particle for them
to be sort of moving at different in different directions. Right,
exactly because it's a point particle. Yeah, you spin the
particle and it's exactly the same as it was before. Right,
there's no direction to it. Right, But what isn't it
kind of maybe a philosophical question, like maybe a particle
(26:58):
a point can't spin? Who's is a point can't spin?
You just can't see it. I just said it. That's
not enough for you. I'm only a like physicist. You're liked.
You're well liked, Daniel. Um. No, imagine a imagintive vector right,
imagintive vector right, which is a length and a direction. Okay, Um,
that can spin right, you can turn it ninety degrees
(27:20):
or degrees or whatever like an arrow. Like an arrow exactly.
But if an arrow has zero length, what direction is
it pointing in the one that I told, the one
that it decides to have. I don't know this quantum stuff. No,
it has no directive, has no length. It can't have
a direction, right, because the direction would imply a length. Right,
But you're sort of telling me that it does kind
(27:41):
of have a direction, right, It kind of has like spin, Yeah,
it has some property. It can't spin physically, like, it
can't spin in the way that we would spin a
tennis ball. Right. It definitely cannot do that. Also, these
other problems, like if you imagine an electron, if you say, well,
we don't really know. Maybe an electron does have a size, right,
you haven't seen it. It maybe does. Well, you know,
(28:01):
if you say, we know the size of electron has
to be less than like ten to the minus twenty meters,
because that's the most best resolution we have we have
on our biggest particle accelerators, and then you calculate, like, well,
how fast would the service the electron be spinning? Will
it be spinning faster than the speed of light? So
it's definitely not happening. And these are not tiny, little
(28:21):
actually spinning balls, right. But this always happens when we
try to describe something quantum mechanical in terms of something
that's not right. You have the tennis ball or the
baseball spinning in your head and you're trying to use
that as a model, and it works for a while
until it doesn't. And it doesn't work because this thing
is not a tiny ball, right, It's some weird thing
and it has some weird property. The amazing thing is
(28:43):
this spin property is really similar to this other thing
we do understand, right. I think maybe the problem is
that you're saying that um like precision wise, like where
it's constituent matter is can't rotate in space, right, But
you're saying that it has other properties other than position
(29:05):
of its constituent matter that do sort of have a
preferred direction. Yes, exactly, it has. That's why we call
it intrinsic spin because it has some property which is
very similar mathematically to spin, but we know it's not
actually spinning. So like intrinsic is like is the physics
version of like right, because well, it's intrinsic spin. It's
(29:25):
like some kind of spin. Right, So you're saying that
that it's a point, but it is kind of spinning.
I'm saying it's a point and it has some weird
property which is related to physical spin, but it's not.
But mathematically it's kind of equivalent. Before we keep going,
let's take a short break. It's really quite fascinating, and
(29:56):
you know it's amazing too. It's to look at the
history because they did these experiments in twenties and the
twenties was also when they were figuring out quantum mechanics,
like they're like, how does this thing work? And they're
still trying to put the math together. And when they
were trying to put together the math for relativistic quantum mechanics,
I was like the quantum mechanics of tiny particles moving
really fast, they discovered it didn't work. It only worked
(30:18):
if you added some new hidden variable to the electrons,
like there had to be two kinds of them, not
just anti particles and particles, but every electron also had
to have this other weird hidden property and then they
called it spin, and it's the same. It turned out
to be the same spin that the other physics we're
looking at, Yes, exactly. It all converged beautifully, and they
(30:39):
were like to ching, look at this, Oh my god,
it all makes sense now we understand those experiments, right,
and so you need spin, like quantum mechanics doesn't work
without spin, Like the Lorenz group and quantum field theory,
all that has has been deeply built inside of it,
so we know it's a thing and theoretically it makes sense.
(31:00):
Do you mean it needed like a hidden variable? What
does that mean? Like it needed needing it like an
extra space, like an extra variable attached to it for
the math to like. Yeah, think about an electron is
having labels, right, like it has certain mass, that has
a certain position, has certain direction whatever. Every electron also
has to have this other label. You know, it's either
(31:22):
up or down. Right, Remember this is spin, so it's
not like you're spinning at some random speed. It's quantum spin.
So there's only two options up or down. And so
every electron has to have this weird label you're spinning
up or you're spinning down, and that makes a difference,
like when you fill out atomic orbitals. Right, No, two
electrons can be in the same orbital because their fermions
(31:43):
they don't like to share. But an electron that spin
up is different from an electron that spin down. So
you can have two electrons in the ground state because
they have this weird sort of hidden thing that's different
about them, like black on the red or it's like
the exception to the probably exclusion, right, it's like it
lets you go around it, right. It's why you can
have two atoms in the ground state where otherwise the
(32:07):
poll exclusion principle, well tell you can't. It's because, well,
they're not really in the ground state. There's two ground states.
You can have a ground state for up and a
ground state for down. Right, But it's not really up
and down, right, it's only up and down if you
measure it up and down. It's up and down along
whatever direction you measure. So if you measure it in X,
every electron will say I'm up or I'm down. If
(32:28):
then you measure it in why, every electron will say, oh,
I'm up or I'm down, but they get mixed up. Right,
So there are the quantum mechanically confusing because they're either
up or down in X and then later your upper
down and why which makes misses up your upper downness
and X. It's very complicated. So you're saying that electrons
can like talk essentially it's a way to communicate. Yeah, um.
(32:52):
And and this is why um. This comes up all
the time in like quantum computing, because you can use
electrons as sort of a cube bit. Right is it
spin up or is it spin down? Electron can be
in two states and it's like a nice map from
a classical bit which is zero or one. So these
quantum mechanical properties are nice because they have two states. Right,
(33:13):
So electrons are spin up or spin down. Um. So
that's why it comes up all the time. And also
in quantum entanglement, like you you have some particle, create
two electrons, well to conserve anglo momentum, one has to
be spin up and one has to be spinned down.
Oh really Yeah, when you when you create them out
of nothing or out of something else, they can they
can't both come out the same spin. Yeah. Well, for example,
(33:34):
z bosons can have spin zero. WHOA, what does that
even mean, Daniel? It has no things, which is not
really like spin. It's actually it's actually even more complicated.
Z Bosons have a total spin of one. That's like
the length of their spin. But this is a vector,
so it can point in different directions, which means they
have three waves to spin. So particles like electrons are
(33:57):
called spin one half particles. They have one half of
a unit to spin, which means they can be spin
up one half or down one half. Z Bosons have
spin one, right, so they can be spinned plus one
or minus one or zero. So z bosons have three
different ways to spin, whereas electrons have two ways to spin.
It's pretty weird it considered like spin one way or
(34:18):
the other way or not at all. Yes, exactly. But
if you have a z boson with spin zero and
it decays into an electron and a positron, then one
of them has to be spin up and the other
one has to be spinned down so that they add
up to the original angular momentum of the z boson,
which was zero. What if a plus one then it
(34:38):
turns into an electron and a positron which are spinning
in the same direction. So they add those two one
halves add up to one. I'm gonna pretend I understood that. Well,
that's the cool thing about it is that the math
of this is really similar. You can use all the
math you developed for like angler momentum and understanding spin
orbitals and stuff like that, you can use that same
(34:59):
math to understands been which is really compelling to me.
Tells me that theoretically we're dealing with a very similar topic, right,
or maybe the math we had to understand angular momentum
matches the physics of quantum particles, right, yes, exactly when
the Mathew describing matches the physics, then that success, right,
that says, oh, look I've described it. I've gotten some inside.
(35:20):
I mean, that's all we can ever do writ is
hope that the math describes the physics, right, You know
what I mean is like um, maybe maybe if you
hadn't called it spin, you called it quantum blukity book, right,
and then reconsidering nominating you for that committee after that?
What do you have against the blukaty books? I don't
even know how to spell it anyway. All right, So
(35:42):
let's say we had quantum blukaty blok. Yeah, and then
later you find out that angular momentum behaves like quantum
blukity bluk, then that would be which one would be
more correct? Right? We don't work correct? Well, if you're
using the same math for both of them, then you're done.
And there it's really just a question of how you
name it. Right. In the end, it's the math, right,
(36:03):
Then the physics is really about the math and not
the names. In the end, it's about the equations on
the paper and and and they're the structures and however
about it. So really you could have called it anything,
but you picked spin because it's sort of related to
something that we people had knowledge about, or people have
kind of intuitive understanding about, that's right, And because we
think it really is a kind of angular momentum. What
(36:26):
kind is it? And are these things really spinning? And
why do electrons have intrinsic angle momentum? That we have
no idea, but it seems to be necessary to make
quantum field theory work. It seems to be a kind
of angular momentum. It's definitely a real thing, but it's
kind of a mystery, and it's something I like to
think about like, why are you doing the electron, why
are you spinning this way? What? What is making you
(36:48):
generate that magnetic field? Yeah, exactly exactly, Well I guess so.
And then the answer is what it's quantum? Say, it's um.
It's some property of electrons um that sort of behaves
similar to rotation. But we don't really know what it is,
but it's there, it's real, and it's definitely not actually spinning.
And it's not just electrons, right, All particles have some
(37:11):
kind of spin. There's particles with half integer spin we
call them fermions, and all the matter particles and like that,
electrons and quarks, and then there's particles with integer spin,
like bosons, like photons and ws and zs and gluons
and those kind of particles. And that's actually the way
we distinguish them. Right, Fermions have half endeger spin and
bosons have integer spin. So it's a it's a it's
(37:31):
an important deal. Like in the particle world, it's a
big deal. Right, And you meet a new particle, you
want to know what is its spin. The Higgs, for example,
is spin zero. It's the only particle we know that
has no spin at all and never can spin. It's
the only particle we've ever found that can never spin. Wow,
now you're just messing with basic arithmetic. Man. You're like,
(37:52):
if it's zero, can only be zero. If it's one,
it can be zero when mine is you know what
I mean, Like you're using the same words to describe
things that means anyways, Okay, I should be more careful
when I say what it means for a particle have
a certain amount of spin. When we say a particle
spin one, what we mean is that the length of
its spin vector is one, and that vector can point
in different directions, and the individual particle can have spin
(38:14):
plus one, zero or minus one if it's a spin
one particle. If a particle spin one half, that's like
the length of its spin vector, then it's spin vector
con point either plus one half or minus one half.
Those guys can't be zero. It's like, it's like arithmetic.
It's not really Yeah, that's what I mean. It would
just be so much easier to understand, you guys if
(38:34):
you just said, instead of saying it's quantum spin, it's
it's like spin. All right, I'm gonna say it's like
spin from now on, and we'll see how many weird
eyebrow races I get in my physics conversations. Yeah, totally.
Well you'll probably get weird eyebrows in your physics department.
But I'm saying, if you're talking to people out there
in the street, are you suggesting there's like more unibrows
in the physics department than in your average street more unibrows,
(38:58):
one weird eyebrows man. All right, well, that's what quantum
spin is. I know it's confusing and it's complicated, but
we hope we at least brought you up to speed
to where the physics community is. And remember, even physicists,
we don't really know what quantum spin is. And all
(39:20):
those grad students I asked, what, how would you explain
quantum spin to a random person on the street, They
got themselves tangled up and by their tongues as well.
So it's a confusing topic. But if you still have
questions about quantum spin, send us an email to feedback
at Daniel and Jorne dot com. That's right. You can
even send us like emails or like questions or emails
(39:40):
that about how much you like us. See you next time.
If 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 at Facebook, Twitter,
(40:01):
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 Explain
the Universe is a production of I heart Radio. For
more podcast from my heart Radio, visit the i heart
Radio app, Apple Podcasts, or wherever you listen to your
(40:22):
favorite shows.
Ythaniel. Do you ever get caught using a word that
you don't really understand? Do you actually always know what
you're talking about? I don't think I'm willing to devote
that here on the podcast, and maybe we can have
that conversation over a beer sometimes. But the thing that
(00:22):
I love is that there are some topics in physics,
some idea, some words that I'm pretty sure nobody actually
really understands. I feel like this is taking us to
another episode about quantum physics that's right and wrong at
the same time. No, I mean, yes, exactly what at
(00:47):
the same time. Hi'm Jorge. I'm a cartoonist and the
creator of PhD Comics. Hi I'm Daniel, I'm a particle physicist,
(01:11):
and Welcome to our podcast, Daniel and Jorge Explain the Universe,
a production of I Heart Radio in which we take
crazy and fascinating and amazing and hot and wet and
nasty things about the universe and try to explain them
to you. Today's episode is dedicated to a Lana, whose
boyfriend Nick is a local fan of the podcast here
in Irvine. Happy Birthday, Alana. Today's topic is a really
(01:38):
fun one, and in preparation for it. Not only did
I go and do the normal street interviews I usually
do with random people, but I actually went around the
halls of my physics department and I asked a bunch
of grad students if they could explain today's topic for me,
and they found it pretty tough. Wow, that's a pretty
interesting spin on the topic in our process. That's right,
(02:00):
that's right to be on the podcast. We're going to
be talking about quantum spin exactly. Quantum spin? What is it?
What does it mean? Are things actually spinning? Why is
it quantum um? We've got a lot of people sending
in requests to talk about this topic. And I think
when people are interested in physics and digging around in
(02:23):
videos on the internet and listening to podcast you hear
this come up a lot. It comes up in quantum entanglement,
It's in science fiction everywhere, and of course people want
to know what does it mean, what's going on? Why
is it so important? That's right? What is spin? How
fast is it spinning? What's the spin on it? Exactly?
Is it's spinning out of control? That's right? Is it
(02:43):
different from political spin? Um? It's definitely a thing, right,
It's something fascinating and more importantly, if you're an expert
in this field, does that make you a spin doctor
doctor of spin? Yes, exactly. You know, we have sort
of a bad track record in physics of naming things
um naming things using familiar words, like we give corks flavors, right,
(03:06):
the up cork and the charm cork and the top
cork a different flavors of corks, colors and colors, right
when they don't really have color, they don't really have color,
and they don't really taste different. I mean, I don't know,
I've never actually tasted a top cork, So there you go.
I'm speaking on something I don't really understand, but we
don't mean it in that way. So sometimes we're like
adopting existing words and just using them totally inappropriately. Other
(03:32):
times we're trying to use words that are similar to
some other concepts because specifically because we want to call
up structures and ideas from those concepts. Like color is
actually a pretty cool one because while the particles are
not colored, there's something about quantum color which is similar
to color, and so we want to express that. You're
(03:52):
sort of trying to grab onto an intuitive idea, even
if you're trying to like a reference, you're trying to
reference an intuitive idea in our everyday experience and and
kind of latch that onto a physics concept exactly. And
we do that all the time, right, That's basically what
physics is is try to explain the unknown in terms
of the known, right, Like when we try to talk
(04:14):
about particles, we're doing that all the time. You know,
we're saying, oh, it's a particle, No, it's a wave. Right,
It's really neither, and it's both and it's something else
more complicated, And we're just trying to patch together a
description of using ideas in your head, right, we weave
these together into some sort of understanding. Here's such a
suggestion for you guys. Maybe you should just instead of
calling being bold and calling it like colors or flavors,
(04:37):
just at the word, like at the beginning in your
official definition. So the announcement should be like, like we're
calling this color, I mean, like let's call flavor. No,
I just call it like a quantum light color. It's
light color. It has a color like property or a
spin like property, or you know, I mean, if you
(04:58):
want to get mathematically, just with a little Tilda in
the front right, and then nobody would be confused. Yeah,
I like that idea a lot good. You like like it? Yeah,
I totally like it, and I think, um, I hope
on Twitter it gets a lot of likes. There you go.
I think I want to something. I think I think
you should be nominated onto the secret International Physics Naming Committee.
(05:23):
I don't know where it meets or when it meets,
or who's in charge of it, but they have been
making some dubious decisions recently and they need some fresh blood. Well,
I think if I was in charge, I would just
put the tilt in front of physics itself. You know,
what are you study till the physics? Physics? I'm studying
something like physics, but it's not actually, I mean, isn't
(05:44):
everything like physics? Pretty next thing, you're gonna be calling
me like a physicist, not actually a physicist, but something
like approximating a physicist. You know, you're gonna spread this
Tilda Daniels sort of like a Daniel into it. Lee,
You're Daniel. You know I'm actually a Daniel. I am
the definition of me, right, You're Daniel sub zero exactly.
(06:09):
I'm not like me I am me, right, you are,
I think therefore I am Daniel. Well, anyways, before we
spin out of control here on this um side conversation, um,
we're talking about quantum spin, and so it's it. It
plays a big deal in quantum physics and quantum entanglement, right,
and atomic orbitals. I mean it's it's sort of it's
important in everybody's atoms, which um which are a kind
(06:32):
of important to people, that's right. I like my atoms,
you know, they're not like atoms. I just like them.
But exactly, spin is everywhere, and you hear come up,
and especially when you're hearing explanations of quantum entanglement or
quantum computing or or orbitals and this kind of stuff,
you hear about it. And when I was a student
and I was learning about quantum spin, I was like, okay,
(06:54):
but what is it like? Are these things actually spinning?
Why do we call it spin? Right? How could a
particle spin? And so let's dig into all that today. Well,
as usual, Daniel went out there and asked people in
the street if they knew what quantum spin was. And
you actually have kind of an interesting spin twid this time, right, Yeah,
I went out and I asked people on the U
C Irvine campus and then a few other folks at
(07:16):
the at an Irvine mall Um what they thought quantum
spin was, if they could describe it, and if they
did understand quantum spin. I also asked them, are these
particles actually spinning or is that just like spin? Is
that just like a word we use. Well, here's what
people had to say, um, so energy, I just think
of energy. Oh yes, what's the quantum spin? Uh? Like
(07:39):
quantum spin as an like spin up, spin down with
within molecular orbitals and electrons um. So are those particles
like actually spinning gesa Uh No, I haven't quantum spin? No?
(08:00):
Uh yeah, it's electron spin um that it can be
positive or negative um. And then how's it different from
normal spin? Like a tennis ball can spin? Is electron
spain the same thing? Are they physically spinning? Uh? I'm
not sure. I know some stuff about Polly's principle that
(08:20):
the spin should be in a specific way, and they're prepared,
so I know some way in distunt stuff, but I'm
not sure about the people. Right. Yeah, isn't because um
every electron has a different type of spin. It's like
either a plus one half a minus one half. I
forgot what it would Yeah, and are they like actually spinning.
Is it like the same thing as a tennis ball spinning?
(08:41):
Or is it's um? No, because it's not like because
you can't treat it like it's a particle, right, because
it's um like an electron is a mix of a
particle and a way. I've heard of it, but I'm
not familiar with the meaning. Okay, No, I don't know
what that is, all right. Yes, there's up and down spin, right,
and it's in me. Uh well, I don't know much
(09:03):
more than that, Okay, all right. It seemed very binary.
Some people had had no idea, never even heard of it,
and some people had a lot to say about this topic. Yeah,
and some people knew some stuff sort of in the
vicinity of the topic, but not actually like reluing to
the question the vicinity. Yeah, yeah, exactly. I like when
when I asked somebody question and I can tell they
(09:23):
haven't thought about this in a long time, and they
have sort of like a free association going off in
their mind that wait, this is connected to that idea,
to that idea of that idea, and then they go no, Actually,
I don't know. I don't know the answer. That's really fun,
but you can see their process. Yeah, because a lot
of people to be heard about it or read about
it a long time ago, maybe in high school physics
or something, and so you sort used to feel like
(09:45):
it's in there in your brain. You just need some
time to you know, boot up the hard drives. Yeah, exactly.
But I think my favorite part of this experience asking
people these questions was that one gentleman after I asked him, um,
if you knew about quantum spin, he then asked me,
he said, is this some thing you need for your dissertation?
Said yes, yes, sir, Yes, sir, I am a twenty
(10:08):
five year old graduate student. Absolutely, are you saying that
you can be a grad student at forty something? There
are fewer of them. Yes, there are a few or
forties something year old grad students. But I have to
say the ones that are forty years old, like folks
that went out into the real world and worked in
real estate or law or something and then came back
to do physics grad school, they're really good students. They
(10:29):
really want it. Yeah, it takes a it's a lot
of work to divert from a path in the real
world back into academia. It's much harder than just like
going from undergrad to grad school. So I really respect that.
You know, you gotta really want it cool. So if
there are any there, any listeners out there who want
to change careers and get a PhD, they should contact you,
right and you have a spot for them. I'm not
(10:51):
sure I just offered anybody funding, but I would encourage you.
If you have a deep passion for physics and you're
finding yourself in a dead end job and you wish
you had gone to physics grad school to unraveled mr
of the universe, I encourage you, sir and ma'am, if
you'd like to be in another dead end of of
awesome knowledge, then be a cartoonist. Is that what you're
(11:12):
gonna say? Yes? Yes, alright, So Daniel, let's break it
down for people. What is quantum spin? We don't know?
Done podcast over, why we had to diversely so much?
Just do fill fill in the air time? Um, So
(11:34):
people don't know because I don't know what quantum spin is.
We don't really know what it is. Now we know
it's a thing, right, So there's this thing, this property
of particles. We don't really understand what it is, what
it's doing. But there's this thing we've observed and we
call it quantum spin. So let's do that. Let's talk
about what this thing is that we can observe, and
then we'll talk about why we call it quantum spin.
(11:54):
And you know whether it's actually spinning and stuff. Right, Well,
what's the origin of this? How did this come to
be a thing? And it came to be a thing
basically because of magnets, right, and you remember that charged particles.
So a little particle with the charge on it, like
an electron, if you move it in a circle, right,
like through a wire loop or something, then it makes
a magnetic field. Right, you can turn it on and
(12:16):
off like electro magnets. This is how they work, right,
like motors everyday electric motors, motors in your car and
in your electric car, and you're um even your phone, right,
there's probably a little motor of solenoid doing the vibrations
when you put it on vibrate mode. That that's the principle.
Like they just pump electrons through a coil a little loop,
(12:36):
and then that creates a magnetic magnetic field which moves something.
That's right. And the cute cool thing there is you
have a magnet you can control electronically or digitally. So
that's pretty awesome. But the important concept there is that
things moving in a circle. Charges moving in a circle
give you a magnetic field. Right, So that's something we
know about, right, something we understand. And so then people
were asking the question, um, well, do electrons themselves like
(13:01):
individual particles? Do they have little magnets on them, like
not just moving in a circle, but is there a
magnetic field just due to the particle? And this is
the kind of thing physicists do. They're like, well, we
don't expect this to be this way, but let's just check, right,
let's see if this happens. Because back then maybe they
thought particles were like little little balls, right maybe yeah,
well they didn't know. Right, this is in the nineteen twenties,
(13:23):
this is almost a hundred years ago. The whole idea
of a particle was still pretty new. People discovered electrons
and neutrons and you know, it was a crazy era
of discovery. They were maybe asking, like, is one electron
maybe like a magnet itself? Yes, exactly right, And that's
what they were wondering about. And so if it's a magnet,
it would have a direction, right, like a field exactly
(13:43):
what it have its own little magnetic field. So that
was the question they were trying to answer, like does
an electron or a silver atom or whatever, a little
particle have its own magnetic field. So what they did
is they built this device that would um that. Basically,
they put particles in this device and the device has
a magnet nick field on it, and they move the
particles through the device, not in a circle, just in
(14:04):
a straight line. And the idea is that the device
has magnets in it. So if the particles have their
own magnetic field, then they'll get pushed to one side
because the magnets on the particle would interact with the
magnets um from this device and would push the particles
to one side or the other, kind of like like
if you not through an electron, but if you through
like an actual magnet, like a fridge magnet, if you
(14:25):
threw it at some other magnets, it would get deflected, right, yeah, exactly,
it would get deflected. Now, most magnets are balanced, right,
Most magnets have a north and a south. So say
you set up like a really strong pair of magnets
and then you threw a fridge magnet through it, probably
wouldn't get deflected because it has a north and a south,
and so the polls would get balanced. So what they
did was they set up one really big magnet and
(14:48):
then a weaker magnet on the other side, so there's
like an uneven magnetic field through it, so that if
your fridge magnet goes through it, depending on the angle
of the magnet, depending on the angle of the north
and the south, they'll get pulled the one direction or
the other. So they build the particle version of this, Right,
They an uneven magnetic field, and they shot some particles
through it, and what they found was really surprising, Right,
(15:12):
it was really shocking what they discovered. They found that
particles do have magnetic fields. Yeah. Well, first of all,
they found two things that were surprising. One, particles do
have their own little magnetic fields. I mean, first they
did it with silver atoms, which is sort of the
easiest thing they could do to to make a beam.
Then this is a long time ago before you could
easily make like a beam of particles. They just put
a bunch of silver in an oven and like some
(15:33):
of it boiled off and they collimated it and got
a beam of silver atoms. And then later they did
the same thing with electrons, and they found that these
things have their own little magnets. Like an electron is
a magnet, which is kind of perplexing, like what does
that mean? Like where does this magnet come from? Right?
But what does it mean that it's a magnet? Like
if I just look at an electron, it has like
a north and south poll to it. Yes, exactly. Electrons
(15:56):
have their own little magnetic fields, even when they're not
moving in a circle, even if they're not moving at all,
Like if you suspended a like current, can you can
you do it? Yeah? Yeah, exactly. If you have an
electron just floating motionless in mid air, it will have
its own little magnet. And that's really weird to us now, right,
because we know that part of electrons and particles, they're
(16:17):
not like little balls or just points. Yes, exactly, and
so the question of where does that magnet come from?
That's where this whole idea of spin came from. But
the other really weird thing they found was that the
magnets didn't point in every direction, Like if you just
throw a bunch of magnets through this device, if they're
all pointed in different directions. Right is just randomly oriented,
(16:39):
you'd expect them to get deflected in random directions. This
one goes left, this one goes right, this one goes
a little bit up, this one goes a little bit down.
But you know, if they're randomly organized, they should go
in all sorts of directions, right, But what they found
was that they either went left or they went right.
There was not nothing in between, Like it's either left
or right and nothing else, like, those are your two options, really,
(17:01):
and no variations and how much right or how much left? Yeah,
they all went exactly the same amount left or this
exactly the amount right? What, Yeah, exactly. And that's why
we call it quantum. Right, they have this little quantum
magnetic field. The amazing thing is that, say, then you
rotate the device, you're like, okay, rotated ninety degrees. Maybe
you were measuring it on the X axis. Now you're
(17:22):
measuring on the y axis, right, so you're imagining, okay, well,
maybe these particles just you know, we're somehow weirdly oriented
along the x axis, so they either go left or right.
So then rotate your device ninety degrees. Then all the
particles either move up or down right. No matter how
you orient the device, the particles either go along the
(17:42):
magnet or against it. Right. So it's not like a
it's not like a real magnetic field, right, it's something weirder.
It's not like a real magnetic field. Exactly like a
magnetic it's like a magnetic field. And that's that's exactly
where we what they struggled with. They're like, Okay, this
has some property. It's something is generating this magnet, and
(18:04):
it's definitely quantum mechanical in some weird way because we
have all these weird properties. Like another weird thing about
this magnetic devices. So you send a bunch of electrons
through and you split them um left and right. Okay,
then you take the left beam, only the left beam,
and you split them through a device that's rotated ninety
degrees that goes up and down. Then they split up
(18:24):
and down right, even though beforehand they would only split
left and right. Wait, what if you take the left beam,
the one that the ones that went left? What if
you try to split them up again? Horizontally. Do they
all go left again, Yes, they do. But if you
split them left right, and you take the left beam,
then you split them up down, you take the upbeam,
(18:45):
and then you try to split it left and right again,
then they mix they go both left and right. And
that's why it's quantum mechanical, yeah, because you can't you
can't measure this weird little quantum magnet that the electron has.
You can't measure it both in X and in y
at the same time. It's the old Heisenberg uncertainty principle.
You can't know too much information about the universe. So
(19:07):
when you measure it in up, down, it mixes it
up again and left right. Whoa. It's like, oh, I see,
you can't measure the up and down and the left
and right at the same time. It's like them that
you can't know a particle where it is and where
it's going at the same time. The same thing applies
to the magnet exactly. And that's how we knew it
was a quantum mechanical property. Right, So we discovered this
(19:28):
weird thing about particles that they have their own little magnets,
and somehow this magnet is quantum mechanical, and so they
were like, what could this be? All right, let's get
into what it could be and some of the weird
things about that. But first let's take a quick break, okay, Daniel.
(19:55):
So people were measuring electrons and they found that they
have sort of like an inherent and inherent magnet inside
of electrons, and that it's quantum mechanical, meaning that it's
like really weird and you can't measure up and down
and left and right. That's right um, and so people
that's why people decide to call quantum spin. Yeah, even
though it's not really spinning, they just went for that name.
(20:17):
That's right. They call it quantum spin. And so let's
try to figure out what do they mean by quantum spin?
What why did they call it quantum spin? Alright, Well,
quantum is pretty straightforward. It's definitely it's a property of particles,
and it behaves like other quantum things that you can't
know too much about it at the same time or
you can't measure the x and the y um direction
of at the same time. And also it's either up
(20:38):
or down, right, there's no in between, So it's quantized.
So the quantum stuff all right, I buy it. Right,
you should definitely call this quantum something. If you're gonna
call it quantum spin or quantum banana, you know, that's
another question. But it should definitely have the quantum label. Okay, Well,
I feel like maybe they should have called it quantum
pole or something, you know, like something that references that,
(20:58):
because really you're trying to you're trying to capture this
kind of the magnetic the direction of the magnetic field
of the electron, right, which is kind of like the pole. Yeah, well,
what they what they did is they thought about what
generates magnetic polls, right, what generates magnetic fields. And we
know that a particle moving in a circle, right, like
orbiting for example, will generate a magnetic field. So then
(21:19):
they thought, maybe the particle is spinning. Maybe it's like
physically spinning. And if you imagine electron like it has
charge on it, think about the surface of the electron
has like little bits of its charge distributed across the surface.
This is the image they had back then, not the
image that we use now. If that was spinning, then
you can imagine that the spinning surface of the electron
(21:42):
could be basically particles moving in a circle and now
would generate a magnetic field. So they were like, ah,
maybe we've discovered that electrons can spin, right, but why
didn't they just call it like quantum poles and that?
Because because you were born a hundred years too late. Man,
I know, that's what I'm saying. Totally. I'm officially nominating
(22:02):
you to be on the Secret Physics Naming Committee. I
think you're you're great at this. I mean in the
sense of people out there who might be trying to understand,
is you know, I mean they're enough is they don't
really um care what they call it a perthennial thing.
But if if they had called the quantum the quantum
magnetic pole or something like that, would that still be
accurate and also maybe be easier to understand. I don't
(22:23):
think it's accurate because it's describing the effect and not
the cause. Right. We think something is happening which generates
the magnetic field and these other properties. But what we'd
like to do is figure out what the cause of
it is, Like, what is the particle doing that generates
the magnetic field? Right? What is the source of it?
And does that give us insight into other stuff? And um,
(22:44):
it turns out it does. Right, If we like to
think about the particles having this thing we call spin,
and we call it spin because it generates the magnetic field,
but also because the way the mathematics of the spin
is really similar to the mathematics of electrons in orbit.
M What do you mean it's similar? Well, the like
the mathematical language we use to like to describe it
(23:05):
is very very similar, right, And when you see it's
like some phenomenon A and you describe it mathematically, and
you see phenomenon being you describe mathematically and you notice
how look they're described by the same math, then you
have to wonder are they two sides of the same
coin or are they really the same thing? And so
people's people thought, oh, look at all this release relationships
(23:26):
between quantum between this thing we call quantum spin and
orbital angular momentum, you know, the angular momentum of something
moving around in a circle. And it was a lot
of connections there, but they turned out to be wrong, right,
Like I know, I know we were trying to do,
but you sort of missed kind of well, yes and no, right,
that's the first way you think about quantum mechanics like
(23:46):
theoretically it does work, Physically it doesn't. So like theoretically
it does, it works in this beautiful, really deep way
because um, what we know, for example, is that angular
momentum is conserved. Right, Like momentum is this property of
particles to keep going in the direction they're going. Angular
momentum is property particles to keep spinning the way they
(24:08):
were spinning, right, And we know that angler momentum is conserved.
If you start something spinning, it's going to keep spinning
until you stop it. Now, what's what's conserved is total
angle momentum, not the anglementum of like one thing. So
you spin a top right, and you can stop it
by touching it against another top, which then takes its
spin right. So the total spin of those two tops
(24:29):
is conserved. Well, the fascinating thing is that while we
don't know what spin is, we know that orribital, angular
momentum and spin are conserved together, like the some of
them is conserved, so you can change the spin of
some particle and and it will influence its angular momentum. Right,
(24:49):
what do you need to conserve is the some of
those two things, which tells you that spin really is
like a kind of angular momentum. Fundamentally, theoretically, these really
are related thing, Like a particle has that kind of
quite angular momentum, and so it's approble to call it spin. Yes,
it's some kind of intrinsic angular momentum. Like you can
(25:11):
transfer angular momentum from orbital from the orbital kind to
the spin kind and back right, which tells you that
they really are two kinds of the same thing, that
in some way the division between them is just in
our minds. It's just mathematical. Oh, I see, So you're
saying it's some kind of spin, it's some kind of
a rotation. Yes, exactly. So it's like spin. It's like
(25:33):
spin exactly. And so it's really I got you, I
got you. Yeah, you admit it on air that horror
Chand was right. They should be called like spin. It
should be called like spin. I completely in totally agree
with you. There you go. But I guess that the
question is, like, so they called they decided to call
(25:53):
it spin because it's sort of like it, like real spin.
But I guess the question is are these particles actually spinning? Right?
And that's the fascinating part is that they can't I
mean particles are points, right, they have no volume. We
were talking earlier about the idea of a particle with
a surface and maybe bits of it on the surface
we're spinning like the surface was rotating and the bits
(26:15):
of charge removing in a circle to generate a magnetic field.
That's hogwash, Like that can't happen because particles don't have
a size. Right, Electronics, as far as we know, has
zero size. Like the one side of it is exactly
the same place as the other side of it, so
there's nothing to spin. You can't turn around a point
and has no direction. It's like a vector of zero length,
(26:37):
Like there's no the distance between one end of the
particle and the other end of the particle for them
to be sort of moving at different in different directions. Right,
exactly because it's a point particle. Yeah, you spin the
particle and it's exactly the same as it was before. Right,
there's no direction to it. Right, But what isn't it
kind of maybe a philosophical question, like maybe a particle
(26:58):
a point can't spin? Who's is a point can't spin?
You just can't see it. I just said it. That's
not enough for you. I'm only a like physicist. You're liked.
You're well liked, Daniel. Um. No, imagine a imagintive vector right,
imagintive vector right, which is a length and a direction. Okay, Um,
that can spin right, you can turn it ninety degrees
(27:20):
or degrees or whatever like an arrow. Like an arrow exactly.
But if an arrow has zero length, what direction is
it pointing in the one that I told, the one
that it decides to have. I don't know this quantum stuff. No,
it has no directive, has no length. It can't have
a direction, right, because the direction would imply a length. Right,
But you're sort of telling me that it does kind
(27:41):
of have a direction, right, It kind of has like spin, Yeah,
it has some property. It can't spin physically, like, it
can't spin in the way that we would spin a
tennis ball. Right. It definitely cannot do that. Also, these
other problems, like if you imagine an electron, if you say, well,
we don't really know. Maybe an electron does have a size, right,
you haven't seen it. It maybe does. Well, you know,
(28:01):
if you say, we know the size of electron has
to be less than like ten to the minus twenty meters,
because that's the most best resolution we have we have
on our biggest particle accelerators, and then you calculate, like, well,
how fast would the service the electron be spinning? Will
it be spinning faster than the speed of light? So
it's definitely not happening. And these are not tiny, little
(28:21):
actually spinning balls, right. But this always happens when we
try to describe something quantum mechanical in terms of something
that's not right. You have the tennis ball or the
baseball spinning in your head and you're trying to use
that as a model, and it works for a while
until it doesn't. And it doesn't work because this thing
is not a tiny ball, right, It's some weird thing
and it has some weird property. The amazing thing is
(28:43):
this spin property is really similar to this other thing
we do understand, right. I think maybe the problem is
that you're saying that um like precision wise, like where
it's constituent matter is can't rotate in space, right, But
you're saying that it has other properties other than position
(29:05):
of its constituent matter that do sort of have a
preferred direction. Yes, exactly, it has. That's why we call
it intrinsic spin because it has some property which is
very similar mathematically to spin, but we know it's not
actually spinning. So like intrinsic is like is the physics
version of like right, because well, it's intrinsic spin. It's
(29:25):
like some kind of spin. Right, So you're saying that
that it's a point, but it is kind of spinning.
I'm saying it's a point and it has some weird
property which is related to physical spin, but it's not.
But mathematically it's kind of equivalent. Before we keep going,
let's take a short break. It's really quite fascinating, and
(29:56):
you know it's amazing too. It's to look at the
history because they did these experiments in twenties and the
twenties was also when they were figuring out quantum mechanics,
like they're like, how does this thing work? And they're
still trying to put the math together. And when they
were trying to put together the math for relativistic quantum mechanics,
I was like the quantum mechanics of tiny particles moving
really fast, they discovered it didn't work. It only worked
(30:18):
if you added some new hidden variable to the electrons,
like there had to be two kinds of them, not
just anti particles and particles, but every electron also had
to have this other weird hidden property and then they
called it spin, and it's the same. It turned out
to be the same spin that the other physics we're
looking at, Yes, exactly. It all converged beautifully, and they
(30:39):
were like to ching, look at this, Oh my god,
it all makes sense now we understand those experiments, right,
and so you need spin, like quantum mechanics doesn't work
without spin, Like the Lorenz group and quantum field theory,
all that has has been deeply built inside of it,
so we know it's a thing and theoretically it makes sense.
(31:00):
Do you mean it needed like a hidden variable? What
does that mean? Like it needed needing it like an
extra space, like an extra variable attached to it for
the math to like. Yeah, think about an electron is
having labels, right, like it has certain mass, that has
a certain position, has certain direction whatever. Every electron also
has to have this other label. You know, it's either
(31:22):
up or down. Right, Remember this is spin, so it's
not like you're spinning at some random speed. It's quantum spin.
So there's only two options up or down. And so
every electron has to have this weird label you're spinning
up or you're spinning down, and that makes a difference,
like when you fill out atomic orbitals. Right, No, two
electrons can be in the same orbital because their fermions
(31:43):
they don't like to share. But an electron that spin
up is different from an electron that spin down. So
you can have two electrons in the ground state because
they have this weird sort of hidden thing that's different
about them, like black on the red or it's like
the exception to the probably exclusion, right, it's like it
lets you go around it, right. It's why you can
have two atoms in the ground state where otherwise the
(32:07):
poll exclusion principle, well tell you can't. It's because, well,
they're not really in the ground state. There's two ground states.
You can have a ground state for up and a
ground state for down. Right, But it's not really up
and down, right, it's only up and down if you
measure it up and down. It's up and down along
whatever direction you measure. So if you measure it in X,
every electron will say I'm up or I'm down. If
(32:28):
then you measure it in why, every electron will say, oh,
I'm up or I'm down, but they get mixed up. Right,
So there are the quantum mechanically confusing because they're either
up or down in X and then later your upper
down and why which makes misses up your upper downness
and X. It's very complicated. So you're saying that electrons
can like talk essentially it's a way to communicate. Yeah, um.
(32:52):
And and this is why um. This comes up all
the time in like quantum computing, because you can use
electrons as sort of a cube bit. Right is it
spin up or is it spin down? Electron can be
in two states and it's like a nice map from
a classical bit which is zero or one. So these
quantum mechanical properties are nice because they have two states. Right,
(33:13):
So electrons are spin up or spin down. Um. So
that's why it comes up all the time. And also
in quantum entanglement, like you you have some particle, create
two electrons, well to conserve anglo momentum, one has to
be spin up and one has to be spinned down.
Oh really Yeah, when you when you create them out
of nothing or out of something else, they can they
can't both come out the same spin. Yeah. Well, for example,
(33:34):
z bosons can have spin zero. WHOA, what does that
even mean, Daniel? It has no things, which is not
really like spin. It's actually it's actually even more complicated.
Z Bosons have a total spin of one. That's like
the length of their spin. But this is a vector,
so it can point in different directions, which means they
have three waves to spin. So particles like electrons are
(33:57):
called spin one half particles. They have one half of
a unit to spin, which means they can be spin
up one half or down one half. Z Bosons have
spin one, right, so they can be spinned plus one
or minus one or zero. So z bosons have three
different ways to spin, whereas electrons have two ways to spin.
It's pretty weird it considered like spin one way or
(34:18):
the other way or not at all. Yes, exactly. But
if you have a z boson with spin zero and
it decays into an electron and a positron, then one
of them has to be spin up and the other
one has to be spinned down so that they add
up to the original angular momentum of the z boson,
which was zero. What if a plus one then it
(34:38):
turns into an electron and a positron which are spinning
in the same direction. So they add those two one
halves add up to one. I'm gonna pretend I understood that. Well,
that's the cool thing about it is that the math
of this is really similar. You can use all the
math you developed for like angler momentum and understanding spin
orbitals and stuff like that, you can use that same
(34:59):
math to understands been which is really compelling to me.
Tells me that theoretically we're dealing with a very similar topic, right,
or maybe the math we had to understand angular momentum
matches the physics of quantum particles, right, yes, exactly when
the Mathew describing matches the physics, then that success, right,
that says, oh, look I've described it. I've gotten some inside.
(35:20):
I mean, that's all we can ever do writ is
hope that the math describes the physics, right, You know
what I mean is like um, maybe maybe if you
hadn't called it spin, you called it quantum blukity book, right,
and then reconsidering nominating you for that committee after that?
What do you have against the blukaty books? I don't
even know how to spell it anyway. All right, So
(35:42):
let's say we had quantum blukaty blok. Yeah, and then
later you find out that angular momentum behaves like quantum
blukity bluk, then that would be which one would be
more correct? Right? We don't work correct? Well, if you're
using the same math for both of them, then you're done.
And there it's really just a question of how you
name it. Right. In the end, it's the math, right,
(36:03):
Then the physics is really about the math and not
the names. In the end, it's about the equations on
the paper and and and they're the structures and however
about it. So really you could have called it anything,
but you picked spin because it's sort of related to
something that we people had knowledge about, or people have
kind of intuitive understanding about, that's right, And because we
think it really is a kind of angular momentum. What
(36:26):
kind is it? And are these things really spinning? And
why do electrons have intrinsic angle momentum? That we have
no idea, but it seems to be necessary to make
quantum field theory work. It seems to be a kind
of angular momentum. It's definitely a real thing, but it's
kind of a mystery, and it's something I like to
think about like, why are you doing the electron, why
are you spinning this way? What? What is making you
(36:48):
generate that magnetic field? Yeah, exactly exactly, Well I guess so.
And then the answer is what it's quantum? Say, it's um.
It's some property of electrons um that sort of behaves
similar to rotation. But we don't really know what it is,
but it's there, it's real, and it's definitely not actually spinning.
And it's not just electrons, right, All particles have some
(37:11):
kind of spin. There's particles with half integer spin we
call them fermions, and all the matter particles and like that,
electrons and quarks, and then there's particles with integer spin,
like bosons, like photons and ws and zs and gluons
and those kind of particles. And that's actually the way
we distinguish them. Right, Fermions have half endeger spin and
bosons have integer spin. So it's a it's a it's
(37:31):
an important deal. Like in the particle world, it's a
big deal. Right, And you meet a new particle, you
want to know what is its spin. The Higgs, for example,
is spin zero. It's the only particle we know that
has no spin at all and never can spin. It's
the only particle we've ever found that can never spin. Wow,
now you're just messing with basic arithmetic. Man. You're like,
(37:52):
if it's zero, can only be zero. If it's one,
it can be zero when mine is you know what
I mean, Like you're using the same words to describe
things that means anyways, Okay, I should be more careful
when I say what it means for a particle have
a certain amount of spin. When we say a particle
spin one, what we mean is that the length of
its spin vector is one, and that vector can point
in different directions, and the individual particle can have spin
(38:14):
plus one, zero or minus one if it's a spin
one particle. If a particle spin one half, that's like
the length of its spin vector, then it's spin vector
con point either plus one half or minus one half.
Those guys can't be zero. It's like, it's like arithmetic.
It's not really Yeah, that's what I mean. It would
just be so much easier to understand, you guys if
(38:34):
you just said, instead of saying it's quantum spin, it's
it's like spin. All right, I'm gonna say it's like
spin from now on, and we'll see how many weird
eyebrow races I get in my physics conversations. Yeah, totally.
Well you'll probably get weird eyebrows in your physics department.
But I'm saying, if you're talking to people out there
in the street, are you suggesting there's like more unibrows
in the physics department than in your average street more unibrows,
(38:58):
one weird eyebrows man. All right, well, that's what quantum
spin is. I know it's confusing and it's complicated, but
we hope we at least brought you up to speed
to where the physics community is. And remember, even physicists,
we don't really know what quantum spin is. And all
(39:20):
those grad students I asked, what, how would you explain
quantum spin to a random person on the street, They
got themselves tangled up and by their tongues as well.
So it's a confusing topic. But if you still have
questions about quantum spin, send us an email to feedback
at Daniel and Jorne dot com. That's right. You can
even send us like emails or like questions or emails
(39:40):
that about how much you like us. See you next time.
If 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 at Facebook, Twitter,
(40:01):
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 Explain
the Universe is a production of I heart Radio. For
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(40:22):
favorite shows.
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