July 30, 2020 • 42 mins
Can physicists discover new particles by watching how muons wiggle in magnetic fields?
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Speaker 1 (00:08):
Hey, or hey, I have a really practical question for you. Well,
you know I'm an engineer, so practical is my middle name.
All right, Well, here's the question. Who would you trust
more to build a spare room off of your house?
A physicist, board engineer. I think you know the answer
to that, Daniel, Not to physicists. All right, but tell
me why. You know. Physicists are awesome, but I wouldn't
(00:31):
say they're very precise. You know, they approximate everything. You know,
everything's like plus or minus a galactic idea. So you
want to know how big this bedroom is going to
be in advance, for example. I just don't want like
a spherical room or like a quantum on certain room.
You know, I want to know where I'm sluting. Well,
you might appreciate a surprise, but I think that's fair,
that's reasonable. But you know that sometimes physicists can actually
(00:53):
get really hardcore about the detail. You're still not building
my spare room. I am more hammy cartoonists and the
(01:15):
creator of PhD comics. Hi, I'm Daniel. I'm a particle
of physicist and you definitely don't want me building your
spare room. Is that because of you or because you're
a physicist, or is it all packaged together. You know,
there's a whole spectrum of physicists. There's the kind that
really likes to build stuff, crawl around on the detector
with a hammer and a wrench and and get dirty.
And then there's the kind of like to sit in
(01:35):
front of a laptop and analyze data and think about statistics.
And I'm definitely more of that second kind. Well, you
can check out my house in your laptop and think
about it for a long time. I'll build you a
virtual spare room. There you go. I'll program another spare
room in the simulation of your universe. It sometimes exists
in this universe and sometimes it doesn't. It's theoretical. That's right.
(01:58):
Just step through this black hole into your new spare
rooms and cozy but welcome to our podcast Daniel and
Jorge Explain the Universe, a production of our Heart Radio
in which we talk about all the amazing things that
are happening in this universe, all the things we'd like
to understand from the very very large, very very dense,
all the way down to the very very small and
the very very weird. Yeah, because the universe has a
(02:19):
lot of amazing things and all kinds of skills, you know,
galactic cosmological skills, But there are also amazing things happening
at the smallest scales of reality and nature. That's right,
and these really tiny things they give us an amazing opportunity.
They let us test our understanding and not just do
we mostly get things right, They let us really push
(02:41):
our limit to understand exactly how these things are working.
Do our models predict what's happening or is there something
a little bit wrong? And that kind of raises a
question of how well do we know what's happening at
these really tiny scales, Like we can measure distances from
here to the moon, for example, or here to maybe
the next star, But how do you measure things that
(03:01):
are that small? And how do you know if you're right?
It definitely takes a certain skill. You have to come
up experimentally with really clever devices, things that isolate individual particles,
or get a bunch of particles and get them all
aligned and then separate them from any other effect. It's
a really particular skill in science to devise an experiment
that forces nature to reveal something for you that pushes
(03:24):
everything else away. Like we learned about the Lego experiment
that measures gravitational waves. There's a huge amount of cleverness
involved in isolating those things so you can see tiny
little wiggles in space. Well, this is sort of experimental cleverness,
and when you deal with particles, you need this sort
of the same kind of skill. You need to set
the universe up in a way that it has to
reveal to you very precisely the answer to your question. Right,
(03:47):
But sometimes a problem, right, Daniel, is that you measure
something and you don't get what you expect. You measure something,
you think it's going to be this big or this
long or this heavy, and then when you measure in reality,
it's different. Yeah, well that's not a problem. That's fantastic,
that's exciting. That's an opportunity, you know, Like, yeah, because
(04:07):
we have two branches of our work. We have the
experimental side that's going out and doing stuff and measuring
things and answering questions, you know, sort of asking the universe,
what do we conclude from that? How do we interpret
that and build a model of the universe in our heads,
and then turn around and predict future measurements and when
those predictions disagree with the things we observe. That gives
(04:28):
us an opportunity to update that model, to say, oh,
something was wrong. There's a new particle, or this particle
works differently, or black holes are actually bigger than we thought.
Those are the moments when we learn. So when theory
and experiment disagree, I smell opportunity. But how do you
know who's right. You never do, And usually they're both wrong,
and they're both wrong in different ways because there are
(04:50):
very very different challenges, you know, split the difference. Calculating
something theoretically has challenges of computing time and getting minus
signs right and sort of organized in your mind, and
and getting answers experimentally has all sorts of different challenges,
making things clean, making them distinct, getting a big sample
of something, getting enough material, you know, we're sometimes just
(05:11):
getting enough money to build the device that you need.
That's the hard part. But there is one of these
big mysteries in nature that it has to do with
a weird kind of discrepancy between what the theory predicts
and what we actually measure. So today on the podcast,
we'll be talking about mystery of the muance magnetic moment.
(05:37):
That sounds marvelous and magnificent. It is one of the
most amazing and marvelous moments in magnetic field history. You know,
it's an opportunity for physics to learn something because it's
something that we know how to calculate very very very precisely.
You know, if you want to find out what's wrong
with your theory, you need to find something that you
can predict very accurately and then measure you're very very accurately,
(06:01):
so you can compare the two and that tells you
if your theory is right or and or if you're
measuring device is working right and not just giving you
weird things. Yeah, and if you hope that your experiment
is correct, then you know, if you see a discrepancy,
tells you that your theory is wrong. And sometimes we
do this as a way to detect the presence of
new particles or you know, just to see if anything
(06:23):
is right, because some of these calculations are very very sensitive,
so it's a very good way to tell whether there's
anything missing in your ideas. You know, it's sort of
like if you walk around your house and you could
take a really precise measurement, you could see where everything was,
and you compared that to the drawings you have of
your house. Right, that would tell you, like, you know,
whether your house is well described by your idea of it,
(06:45):
or whether it was a mistake to hire you to
clearly actually probably not. How are you to measure it
also build it? I would say, Wow, this is perfect work.
You should pay your contract or double. I guess that's
a big question. And the question is how good or
physics isn't measuring things? And so we were wondering, we
were curious about how many people out there. So if
I've thought about how good our measurements of the universe are,
(07:08):
and in particular, what's the most precisely measured quantity in physics? So,
as usual Daniel went out there into the wilds of
the Internet to ask people what's the most precisely measured
quantity in physics? That's right, And if you're interested in
answering random internet questions without any preparation, please right to
me at feedback at Daniel and Jorge dot com and
(07:30):
I'll send you some questions to answer. So think about
it for a second. If you were asked, what's the
most precisely measured thing in physics. What would you answer?
Here's what people had to say. My best guess is
increments of time, accusing an atomic clock. I don't know.
I don't know, Sorry that I don't know. Is this
something to do with the plank Clint? Maybe aliens? I'm
(07:53):
not sure what you mean about quantity, whether that be
amount of things, how many bananas it takes to create
a black hole. I'm going to guess it's mass. Maybe
I like this question. Well, I think first we have
to define what do we mean by precisely? Maybe temperature
(08:13):
mass would be the most precisely measured quantity in physics
because it holds a tangible value. I reckon they can
measure pretty small, like maybe an adam. All ride a
broad range of answers, from aliens to bananas to the
plank scale. I feel like our audience is very much
in tune with what we cover here in the podcast. Yeah,
these are great answers, and I have to confess I
(08:35):
think that some of these answers make me rethink how
I should have asked this question, because I asked, what's
the most precisely measured quantity in physics? Like you go out,
you do an experiment, you measure something. But I think
really the question we should have asked is what's the
most precisely calculated quantity in physics? And there's a difference.
For example, the atomic clock answer is a really good one.
(08:58):
You know, atomic clocks are precise us too, like one
part and tend to the sixteen. You know, it takes
like ten to the sixteen seconds before they're off by
one second. So you would agree with a lot of
these alien, Yeah, some of these are really very accurate.
And for example, Lego, like we mentioned before, gravitational waves.
To detect gravitational waves, they have to measure, you know,
(09:19):
the change in length of something by one part in
tend to the twenty or tend to the twenty one,
which is really incredibly precise experimentally. But what I was
going for was a question of, like, what's the most
precise test we have of our theories, which requires not
just a really precise experiment, but also a really precise prediction. Oh,
I see, like what's the most precise that physicists have
(09:42):
been right about stuff? Is that kind of what you mean? Yeah,
because for gravitational waves, for example, that's a very precise
experimental measurement but we didn't know in advance how big
it would be, and we don't know necessarily how big
those should be. It depends on the size of the
black holes that are the same with the tomic clocks.
We can't calculate the those things as well as we
can measure them. In order to get some insight into
(10:04):
the universe, you need something where you can calculate it
really well and you can predict it really precisely. Interesting,
so you're kind of talking about like, according to the
laws of physics, we think that dis quantity should be this,
and then how well does it match with what we
actually measure of it exactly? Because it's those discrepancies we
need to learn something. It doesn't matter if you measure
(10:24):
the length of your house to one picometer, because we
don't know how big your house should be and doesn't
really tell us anything about the universe. But if you
measure something really precisely that we can also predict that
we can calculate that has to be a certain value
because of our understanding of physics. Then measuring it and
finding out it's something else gives you a clue that
something is wrong about our model. Right, Well, it seems
(10:47):
like there's one such thing that we're trying to predict
and measure at the same time, and that there's a
big mystery about why those two things don't match. And
that's the magnetic moment of muance, which is a great
alliteration there. I'm so pleased to have some positive feedback
for a name in particle physics from you. That's a high,
(11:09):
high standard for your poetic writing. Here, the mystery of
the muon magnetic moment. Yes, it's really marvelous, all right.
So this is a quantity that we have predicted using
theory and that we've measured using big machines. But those
two things don't match. That's right, all right, So let's
get into it, Daniel. Let's start with the first m
What is the magnetic moment. So when you think about particles,
(11:30):
remember we like to think of them as little dots
in space that have labels, and those labels can be
like what's the spin or what's the charge or how
much mass do they have. We don't think of particles
is like little physical balls that actually do these things.
They're weird quantum objects and they have these labels. And
so this is one of the labels of a particle.
But it's a little weird because it's not like a
(11:52):
direct label. It's not like something you can put right
on the particle. Because particles they don't have a magnetic charge.
They have electric charge. That's how they feel electric fields.
But as we talked about on the podcast before, there
are no particles that just have like a north or
a south magnetic charge on their own. See, they have
an electric charge, but they don't have like a pole
(12:13):
like you say, like a magnet, like a north and south.
That's right, they don't have just a north and just
a south. What they have is this weird magnetic field.
It's a dipole to have a north and a south,
just like every magnet we've ever discovered has a north
and a south. And that comes from the combination of
having charge and having spin because charge and spin together
gives you some sort of magnetic interaction. Okay, so particles
(12:36):
have spin and charge and together they have a like
a pole, like a magnet, little magnet inside of it. Yeah,
and that's what we call the magnetic moment. It's the
part of the muan that is affected by a magnetic field.
And you know, fundamentally it comes from having charge and
from spinning. And that's because it has a magnetic moment.
It doesn't have a magnetic charge. It's not a north
or a south, but it is affected by the magnetic field.
(12:59):
And that's what we mean when we say the magnetic
moment of the muan. How a muan is affected by
a magnet. It's not the moment for like an electron
looks at a positron and they feel that attraction towards. No,
it's not. It's not a dramatic moment. It's not something
exists in like theory of screenplays or anything like that,
unless you're writing a movie about particles, in which case
there probably is an electrifying moment for the muan. Wow,
(13:22):
you would totally watch that movie. Um, I totally have
that movie script already in a drawer in my house.
It's been sent to several Hollywood agents, but nobody seems
to be writing. Consider this podcast my pitch for this project.
I would definitely watch that movie, but I have not
yet written the script anyway. So we're interested in you know,
(13:43):
what happens when you put a magnetic field on a muan.
And this is something we can measure because we can
do that experiment, and it's also something we can calculate,
and it turns out to be really sensitive to exactly
what's happening into some other big questions about how particles
work well, And maybe let's go back a step and
cover the other m which is the muon. So muan
(14:04):
is like an electron, Is it like a quirk? Yeah,
So we are made out of quirks and electrons. Right,
We have quirks that make up the protons and neutrons
inside our atom, and then we have electrons whizzing around them.
But each of those particles have other copies. There are
other kinds of quirks, and there are also other kinds
of electrons. So there's a heavier version of the electron.
We call that a muan, exactly the same as the electron,
(14:27):
except has a lot more mass. And there's another one
even called the towel. So the electron has these two cousins,
the muon and the towel that have all the same
interactions and all the same properties, like the same charge,
the same spin, but just heavier match, just heavier mass. Yeah,
and it's weird. We don't know why they exist, Like
why do we have the muan. Why do we have
(14:48):
the tow Why does the electron have two cousins and
not nine cousins or seventeen cousins or any cousins like
my cousin? Are they good for anything? I'm not going
to get in the middle of that family dispute. We
have cousins, so we want specified which one I'm talking about.
But what I guess what I mean is like, is
it good for anything? Like does it form part of
(15:09):
you know, can you make an atom out of them?
Or do we just know them kind of theoretically, or
we know that they formed, but then they disappear quickly.
That's right. They're not stable, so you can form atoms
out of them. You can take a proton and put
a mu on around it and form a bound state.
But the muan lasts for you know, a few microseconds.
Remember that heavy particles don't survive very long in the universe. Actually,
(15:31):
in its reference frame, if you were riding on the
back of a muan, you'd see that it lasts a
few microseconds. But because they move so fast, their clocks
are slowed down. So as we watch the muan, we
see them live their three microsecond lifetime. Over a longer
period because of time dilation, so they don't last terribly long.
It's still you know, seconds or minutes. But muans don't
(15:51):
last in our universe because they're heavy. They effectively turn
into electrons. All right, So there's a big mystery regarding
the magnetic moment of the eu on. So let's get
into the theory and the experiment and talk about what
it means. But first let's take a quick break a right, Daniel,
(16:19):
we're talking about the magnificent mystery of the marvelous muon
magnetic moment. Momentarily, hi rolls in your mouth. It's delicious. Um,
So yeah, so tell me about this mystery. So we
know about the muan and you're saying that we can
the theory predicts its magnetic moment. How can the theory
(16:40):
predict something like that, Well, we think about it in
terms of particles, right, We're talking about how the muon
is affected by a magnetic field. But a magnetic field
we know is really carried by photons. Like when things
interact electromagnetically, we can imagine that as being done by
photons moving through space carrying information. Remember, every force that
we think about electromagnetism, the strong force the weak force
(17:03):
has these particles to sort of do its job, and
in the case of the electromagnetic interaction, it's the photon.
So when you think about how a muon is affected
by a magnetic field, really on the sort of particle level,
what you're thinking about is what happens when a photon
hits a muon, or how does a photon interact with
a muon. That's sort of like the basic tinker toy
(17:25):
element of particle physics that lets muans be affected by
magnetic fields, right, because magnetic fields are transmitted by photons. Yeah,
magnetic fields are basically photons. We can think about like
our fields particles or particles fields, but they're very tightly connected.
So like if I throw a muon at a bunch
of magnets and it curves one way, it's not because
(17:47):
it's something in it. It's because it's like hitting and
interacting with photons. Yeah, exactly, it's getting bent by the
magnetic field. And very natural way to think about that
is in terms of photons being generated by you know
whatever where the source of your magnetic field is and
pushing the muan. All right, So then we think of
its interactions is hitting photons, and so how does that
(18:09):
help us predict its magnetic moment. Well, it's fascinating because
there's a whole bunch of different ways that a photon
can hit that mu on. Like the simplest thing is
photon hits the muon and bounces off, right, So you
have photon muan interaction very simple, Like in your mind,
you have a couple just little lines of particles that
intersect and then they go their separate ways. That's the
(18:30):
simplest thing, and you can use that to calculate, all right,
what's the strength of the magnetic moment of the muan?
And if you did that calculation, you get a pretty
simple answer. This was done first by a guy named
Julian Schwinger, and he was so proud of this calculation
that he actually had this number. It's alpha and the
fine structure constant over to pie. He put this number
(18:50):
on his tombstone. He's like, don't forget I came up
with this. Seriously, it's like it's a beautiful calculation. He
was so proud. This guy did a huge amount of
physics in his lifetime. He's basically the person who proved
that Fynman's theory of quantum electro dynamics actually work. Finding
like sketched a bunch of doodles and had a few ideas,
but never like actually made it work. And Julian Schwinger
(19:12):
was like, all right, let's do all the calculations and
see if this is right. But that benefit in his
tool stone, I guess this is a really succinct way
to just sort of like sum up the guy's life. Anyway,
the point is that there are other things the photon
can do also. It doesn't just have to bounce off
the muan on its way there. It could like split
into an electron and positron and then convert back into
(19:34):
a photon and then go off. Or it can emit
a particle and then reabsorb that first particle. So if
you'd like drawing these Fineman diagrams these ideas for how
this happens, all you have to do is add a
couple more lines, and all these things describe totally valid
things the photon could do as it interacts with the muon,
and those change effectively the muan's magnetic moment. See, so
(20:00):
it's kind of like the muon doesn't really have a
magnetic moment. How does it interact with a photon? Its
interaction with the photon is essentially what determines how it
reacts to magnetic fields, which is its magnetic moment. And
photons are crazy, they're like always turning into other stuff
and spewing off particles and reabsorbing them. And the real
actual thing that happens between a mean and a photon
(20:22):
is some some of all those things, all those things
mixed together, which you can and that you can predict
with the theory. Like your theory, you can like write
this down in a piece of paper, like what happens
if a photon hits the muan, and you can in
a piece of paper you can work out how that
muan should bend its path or how we should get deflected.
(20:42):
And then you can say, well, what if it was
a little bit more complicated, what if it also emitted
another particle at the same time, Then it would change
your calculation. And you know, as you make these things
more complex, there are more and more possibilities, so it
becomes very challenging theoretically to account for all the different things.
But that's also gives you an opportunity because if there
(21:04):
are crazy particles out there that you would never considered,
then the photon could be turning into them, could be
like interacting with them, could be like popping into existence
some weird new particle you never imagined. And that would
change how it interacts with the muan because it would
lose some energy. It would just it would change its angle,
it would change its direction, it would change the probability
(21:27):
of this thing happening at all. And so in this way,
the photon interacting with the muan is sort of like
a probe of the whole universe because along the way,
the photon can do all sorts of crazy stuff. You
can do anything that quantum mechanics lets it do, and
what happened affects how it interacts with the muon. And
so by calculating this quantity and then measuring it, you
(21:47):
can ask, like, is there anything else that the photon
is doing along the way that's changing how it interacts
with the mu want to see, like how good are
we predicting what photons actually do. Yeah, it's like you
said to photons, hey, go crazy, do anything you want
to do, and then we're going to try to calculate
all the things we think you can do, and then
let's compare. And you know, if it turns out you're
(22:08):
dancing with a new kind of particle we've never heard
about before, we're gonna know you're like stalker fans. Yeah,
and you know, people like me, I like to discover
new particles by sort of making them concretely, like pouring
enough energy into a collider so that we have enough
energy to make this new particle. And see it's sort
of directly, but this is another way to do it,
is to like look for these particles just sort of
(22:30):
like briefly popping into existence as photons do their crazy
dance with muans. And I guess my question is why
the mean like couldn't I mean, all these questions and
all these magnetic moment ideas we should work for any
other particle, right, So why are we focusing on the
muon specifically? And you can do these calculations also for
the electron and also for the town, right, but the
(22:52):
muon is sort of in a sweet spot because it's
a little bit heavier, it's sort of easier to handle.
The new physics should happen to all of these particles. Right,
but it has essentially a proportionately larger effect on the
muan because it has a larger mass. I see, So
lemon is like the guinea pig. Yeah, the muan is
like the best place to get the universe to reveal
all these little details, all right, And so you can
(23:15):
run the math and it should tell you how the
muon should bend in a magnetic field. And you can
also measure how, like you can throw me on at
a magnetic field and see how it bends. That's the
experimental side. Yeah, But before we move on to the
experimental side, I gotta sort of shout out to the
theory here, because this is what I meant earlier about
being really precise. On the theoretical side, this quantity, the
(23:36):
magnetic moment of the muan, is the number that theorists
know best. It's the most precisely calculated quantity basically in
the universe as far as we know, unless there are
alien physicists doing it out there. What how can something
theoretical be precise? Doesn't precision mean like how right you are.
It doesn't mean how right you are. And when we
(23:57):
do these calculations, we start with the simplest idea as
we say, well, what's the simplest thing of photon can do?
And that gets you mostly right, and they think, well,
what if it does one weird thing along the way,
And there's like nineteen ways for that to happen, So
you add nineteen calculations. Well what if it did two
weird things along the way. Okay, now there's nineteen squared
ways to do that, and each of these gives us
smaller and smaller effect. And so as you add up
(24:20):
more and more of these ideas you're considering, you get
closer to the true answer, but also becomes harder. And
so now they're at the point where they're calculating like
millions and millions of possibilities. Maybe first to turn to
ano electron, and that electron did some weird thing, which
turn to into a photon, which then did some weird thing.
And so they've estimated sort of theoretically how precise this is,
(24:40):
Like it's impossible to get it exactly right because you
need to do an infinite number of calculations, so they
can estimate how close they get based on how much
is the answer changing as they add more ideas, So
they're asthm topically approaching the deep truth. I guess there's
a you know, an engineering, there's always this issue about
the different between accuracy and precision, like accuracies how right
(25:04):
you are, and precision is like how sure you are?
So is the thing that's happening here? Is it that
theories are pretty sure they know what the moment of
the muan is, like they think they've covered all the angles,
so they're pretty sure, but maybe they don't know if
it's the actual value. Yeah, you know, I have equivalent
with theoretical physics here because experimentalists trying to be really
(25:25):
formal about the statistical statements we make. If we say, okay,
there's an uncertainty here, that means something very specific. Statistically,
it means if you did the same experiment a hundred times,
you would get the answer within your uncertainty bounds sixty
percent of the time or something like that, or a
different answer if your Bayesian. That's precision. There is are
a lot more hand wavy, you know, they're like, well,
(25:47):
we tweaked a couple knobs and got different answers, and
so you know that's the uncertainty. We multiplied some things
by two just to see how things would change. So
that's what we're calling the uncertainty. And you know, it's harder,
it's different. They're not measuring things about the universe. They're
just trying to like guess how closely they are the
right answers. So I guess maybe the title should really
(26:07):
be the most precisely guessed that theory quantity ever, you
know what I mean. Like they put a lot of
attention into the They've covered every angle on so they're
they're pretty sure that this is what the plan is. Yeah,
I suppose so, although you know, there have been moments
in this history and this is a decades long project
to make the theory more precise and make the experiment
(26:28):
more precise. It's a bit of an arms race to
see like who's getting more and more precise. There was
a moment in the nineties when the theorists discovered that
they had gotten the sign wrong, like that a minus
sign where there should be a positive sign, and it
changed the answer kind of a lot. So they're they're
definitely mistakes in there. Oh my gosh, who made the mistake?
Are they going to put that in their tombstone as well?
(26:49):
One more minus sign? Know, there are different groups, and
they're cross checking each other, and so you know, that's
another way they try to estimate how correct these things.
All right, well, let's get in to now the experiment
part of it and how well these two things match up.
Who's more precise or less accurate or more marvelous. But
first let's take a quick break. All right, we're talking
(27:23):
about the magnetic moment of the muan as the most
precisely guest at quantity ever, and now we're going to
measure it with an experiment, and that just involves throwing
a muan at a magnetic field and seeing where it
goes or is there is there something special going on?
You know, that would work, But what you want is
a really precise measurement. You want a measurement which is
(27:44):
accurate to like one part in ten to the twelve
or ten to the thirteen, and so to do that
you need a really clean setup. And so what you
described would work, but it's sort of hard to measure
it's a single particle. And so what you want is
a lot of muans. You want them all basically doing
the same thing. So you can get a bunch of
measurements and divide by a big number and it sort
(28:05):
of averages out some of the mistakes. And so what
they do is they get a huge pile of muans,
a big blob of muans, and they point the spin
of the muan, which is the thing that determines again
where this magnetic field is going, and they get them
to spin in the direction they're moving, and they move
them in a circle. So they have this ring in
Chicago where they have a bunch of muans and they
(28:25):
move them in a circle. And when muans move around
in a circle in a magnetic field, their spin will precess,
it will rotate around the access of motion. Because that's
how the physics work out. Like if you try to
ban the muon, it will also sort of change in
other ways. Yeah, Like one thing that happens to a
particle and you put it through a magnetic field is
that it bends. But a particles moving in a circle
(28:46):
through a magnetic field will process, It will will change
the direction in which they're pointing. So that's what they
can do, is they can measure the difference between the
direction of the magnetic field that they're putting on these
particles and the direction of the spin of the ones
which affects their magnetic moment, And so they have come
up with really clever ways to measure these things and
to reduce all sorts of uncertainties. And you know, if
(29:08):
you're a visual person, it's really very similar in spirit
to the experiment that looks for gravitational waves. What you're
trying to do is isolate this experiment from any other effect.
You know, like, is that the microwave oven in the
break room that's changing the answer? You do we understand
all the electromagnetic fields nearby. Is the radiation from the
ground affecting our result? It's this kind of experiment you're
(29:31):
like really isolating any source of noise or uncertainty. All right,
So they're spinning these muans in a circle in Chicago,
and again not in Minnesota or Milwaukee or Montana. No,
it's being done at Fermulab, the accelerator complex just outside
Chicago between Batavia and Naperville, where I did my pH
d thesis hometown plug. All right. So they're spinning these
(29:54):
in a in a circle, and they're measuring how they're
processing or changing in the direction of their moment, and
that tells you the magnetic moment of the muan experimentally,
and now the problem is how well does it match
with what the theorists. Yeah, that's right, that's the questions.
That we have the number from the theory and the
number from the experiment, and if you write these two
numbers down on a piece of paper, they agree to
(30:15):
the first what is it, like eight or nine digits
before they disagree. So it's like it's really a testament
to an incredible amount of work. I mean, you call
it guessing, but like these theorists have done a huge
amount of work to really nail this down, and the
experimentalists have done a different, difficult pile of work, and
now they have these two numbers. It's incredible to me
that they agree this closely at all. All right, so
(30:38):
let's maybe sign out the number for the audience here.
So the experimentalists say that the magnetic moment of the
muan is two point zero zero two three three one
eight four one eight close our mind is some small quantity,
and what are the units of these These are dimensionless units. Yeah,
so okay, that was from the experimentalist. Theorists say it
(31:00):
should be two point zero zero two three, three, one
eight three six to not fo, that's right. So they
agree on you know, after the decimal place, they agree
to seven digits, and then they disagree. One of them
says four one, eight and the other one says three
six two, which is not a huge difference. It's like
(31:21):
twelve zeros and then like, yeah, it's a bunch of
zeros and fifty six. But the fascinating thing is that
both of them are pretty confident in their results. So
there's a gap between them, very tiny gap between them,
but the uncertainty is smaller than the gap. Right, the
difference between them is fifty six, and the uncertainty is
like fifteen, So the difference is like three or three
(31:43):
and a half times the uncertainty. It seems really it's
so weird to me that they're so confident, you know
about these numbers, Like you know, I've done experiments, and
you know, to get that kind of position is really hard,
Like if they ran this experiment next year and the
year after that, but they still get the same exact. Yeah.
These uncertainties reflect statistical limitations, so like you haven't runned
(32:04):
for an infinitely long time, and also systematic uncertainties like
things you think will contribute to mismeasurement or or bias
on your result, and you know these are estimates. It
could be that they're wrong. It could be just a
basic mistake somewhere. But this is what we're trying to learn.
Like we're trying to learn, like do we understand how
to do these precision measurements or do we understand how
to do these calculations, or is there a new particle
(32:27):
out there that we're not factoring into our calculations that's
playing with a magnetic moment of the muant finding a
little bit? Is this the hint of the discovery of
some new particles, some new supersymmetric particle which is too
heavy to make a particle colliders and only appears very
briefly and gives these little hints to the mule, like
is there something hiding in that zero point zero zero
(32:52):
five six difference between the experiment and the theory, or
because they're both pretty sure of their numbers, there's now
like they're both pretty sure of their numbers. Yeah, it
couldn't be like a wire missing here or a plus
sign missing over there. They certainly could be and their
independent checks, their independent experiments, and we'll talk about that
in a moment, but they're both pretty confident. And I
(33:14):
remember learning about this in college and I was still
learning about quantum mechanics and how it all worked. And
at the time, I thought of physics as sort of
like a description of what we see about the universe,
just like sort of a human internal to our minds
approximation of what's happening in the universe. And I read
about this calculation, like, wow, it agrees to you know,
nine or ten decimal places. That's amazing. And I at
(33:37):
this moment where I thought, wait a second, maybe physics
isn't just describing approximately what's happening. Maybe we've discovered like
the source code, Like maybe this is what the universe
itself is doing. Because to get that accurate, to get
that precise, it's sort of shocking, you know, to imagine
there could be another theory that could also be that precise.
(33:57):
So I see, it's like, what if we actually uncover
at the code of the simulation of the universe, because
it's so we're so right, and we're so right. Yeah,
maybe the universe does run on a computer using these equations?
Is that kind of what you mean? Sort of you know,
but in a more universal way, like maybe the universe
does follow laws and it does calculations, and it follows
(34:17):
these rules when it does those calculations, you don't have
to be embedded in some meta universe and simulated on
a computer. Maybe the universe is doing calculations though. Anyway,
it's an incredible testament in my mind to the work
involved here, and it's amazing that it works at all.
I agree, right, But there is sort of an interesting mystery,
And I guess the weird thing is that you're telling
me that for the electron there's no difference between the
(34:41):
experiment and the theory. That's right, Like this difference only
shows up in the melee. We can do the same
measurement for the electron. We actually a similar number, but
there's no discrepancy. Like the electron. When they do the
theoretical calculation and they do the experimental measurement, they get
those two things to agree to within uncertainty. Now we
expect that new is it, new particles whatever, would have
(35:01):
a bigger effect on the muan, so it's not a
surprise that it doesn't appear there for the electrons. And
that's quite fascinating. So maybe there's something going on with
the muan that you wouldn't see in the electron. So
the electron you check that box or like the theory,
and both groups have gone at the electron with the
same kind of intensity and precision, and you can do
(35:23):
all the same kinds of theoretical calculations for the electron
and get a really precise number. And then you can
go measure the magnetic moment of the electron, because electrons
also bend in magnetic fields, and you can make that
measurement really, really precise, and those two numbers agree. Electrons,
we understand them like. There are no mysteries hiding under
the rug for the magnetic moment of the electron, but
(35:44):
for the muan, which is exactly where we would expect
to see something weird. First we start to see something weird,
all right, But it's different for the muan, which means
that it might be hiding a secret. So what does
that mean, Daniel, What could be hiding underneath the marvelousness
of the muant? Well, you know, we suspect that there
are other particles out there that we have not yet discovered.
(36:05):
We found six particles that are corks, six particles that
are leftons, and then a few of the particles that
mediate the interactions between them, and so we have this
pile of particles, but we don't know if those the
only particles out there. And actually it would make a
lot more sense if there were more particles, because there
are these weird patterns we found that are unexplained, and
(36:25):
some of them would click together really nicely if there
were new particles. Like some of the particles we've seen
are called fermions. They have spin one half, and the
other ones are called bosons because they have spin one.
There's one idea that maybe every fermion has a boson version,
like the muan has another version of it called the
smeu on, and the photon is another version of it
(36:47):
called the photino. And these are like just one idea
of how there could be new particles out there that
sort of solve deep problems in theoretical physics, but we
haven't seen them yet, so they could just be too big,
too heavy for us to discover them. In particle colliders,
remember to see something in the collider, you have to
put in enough energy, which means you have to make
(37:08):
the collider big enough, which means you have to get
enough money from the government to build a really big tunnel,
So there's a limitation there. This might be another way
to like sneak around that limitation and see these new
particles for the first time, at least hint that they're there.
And if you do the calculations and what do you
expect to see if there are these new particles, this
(37:29):
is kind of exactly what you expect to see. So
is the idea then that maybe there's a new part
that we don't know about that the photon is turning
into or like transforming into before it interacts with me.
Because the photon can interact with anything that has electric charge.
And so if there's some new heavy particle out there
that does have electric charge but it's never really exists
(37:51):
in the universe because it's too massive, well, occasionally the
photon can turn into it or pairs of it, like
it's particle and it's antiparticle, and that would change how
it interacts with the muan because you have to include
it in all of these calculations. Like maybe it admits
this new heavy particle and then it interacts with the
muan and then it reabsorbs that particle, and that would
(38:11):
change the way it interacts with the muan. And so
the presence of weird heavy particles changes the basic interaction
between two very simple particles, which I think is fascinating.
It's like a it's a clever way to leverage you
know something about the universe, to force the universe to
tell you about what's going on, even if you don't
have the energy to build that com eider. I think Danny,
what you're saying is that the experimentalist are right and
(38:33):
the theories are wrong. Well, you know, the experimentalists are
probably wrong in different ways from the theorists. Experimentalists definitely
make mistakes. It's really hard to do these things and
to get them right and to remove all sources of air.
And that's why it's fascinating as a cross check, because
if they're wrong, they're probably wrong in different directions or
different amounts. And so it's a great way to cross check,
(38:55):
and you know, to improve experimental physics and to improve
our theoretical understanding of the universe and maybe find new
particles in between. And yeah, and maybe make us stop
off in Stockholm to collect your Noobile prize. And that's
why you know this isn't over. It's not just like, oh, hey,
we saw this discripancy were done, because it's kind of indirect, right,
This is not like we make these particles, we see them,
(39:17):
we understand them. It's just sort of like a clue
that the particles are there. And so what they want
to do is make these things more precise. They want
to get better experimental measurements. They want to push the
theoretical measurements to see are these things wrong? Do these
stack up? Can we improve this uncertainty? Can we make
these things ten times as precise? And does it stick
around or disappear? All? Right? Well, I think it all
(39:38):
speaks to just again this idea that there may still
be amazing things hiding even in tiny little gaps of
point zero zero zero zero zero five six. That's right,
some of the biggest clues in the universe turn out
to be on the smallest numbers. And there is news
to come because there's an experiment happening right now again
in Chicago that's going to give a measurement of the
(39:59):
muan magnetic moment that's going to be four times as
precise as the one that we have now. They took
this big magnet and they shipped it from Long Island
where they did the experiment first, and moved into Chicago
and that a cleaner beam with more muans and they're
running those results right now. And in eighteen they said
that they would have results quote sometime in t and
(40:19):
so here we are in no results yet, but we
expect any day, any day. These things are hard. We
expect any day they'll come out with the new measurement.
And the whole physics community is waiting, like, what's going
to happen. How's the number going to change? So it's
a big deal. They're like, hey, we said we'd be
accurate about the muan, not about when we would tell
you about the plus or minus five years, plus the
(40:44):
minus fifty six years. Oh man, I'm sure that there
are some graduate students out there on this experiment it's
called muan G minus two, that they are sweating and
working hard to get this number out. Well, hopefully we
added a little bit more pressure because now I'm curious
about what's gonna happen. We're all curious because this is
how we learned about the universe. We corner it and
force it to tell us what is the answer to
(41:05):
this number? We think we know what it should be.
Tell us what the real truth is, tells the universe,
don't keep it to yourself. Experimental physics is basically a
modern day oracle, right, We actually do get to ask questions,
will be horrible, and it gives us answer and then
it chops off your head or something something Greek and
classical like that. It kills your mom. Probably all right, Well,
(41:27):
we hope you enjoyed that, and think about all the
amazing secrets that could be hiding in the smallest of quantities.
That's right. In one of these days, one of these
secrets will reveal something deep and true about the universe.
See you next time. Thanks for listening, and remember that
(41:48):
Daniel and Jorge Explain the Universe is a production of
I Heart Radio. Or more podcast from my Heart Radio
visit the i heart Radio app, Apple Podcasts, or wherever
you listen to your favorite
Hey, or hey, I have a really practical question for you. Well,
you know I'm an engineer, so practical is my middle name.
All right, Well, here's the question. Who would you trust
more to build a spare room off of your house?
A physicist, board engineer. I think you know the answer
to that, Daniel, Not to physicists. All right, but tell
me why. You know. Physicists are awesome, but I wouldn't
(00:31):
say they're very precise. You know, they approximate everything. You know,
everything's like plus or minus a galactic idea. So you
want to know how big this bedroom is going to
be in advance, for example. I just don't want like
a spherical room or like a quantum on certain room.
You know, I want to know where I'm sluting. Well,
you might appreciate a surprise, but I think that's fair,
that's reasonable. But you know that sometimes physicists can actually
(00:53):
get really hardcore about the detail. You're still not building
my spare room. I am more hammy cartoonists and the
(01:15):
creator of PhD comics. Hi, I'm Daniel. I'm a particle
of physicist and you definitely don't want me building your
spare room. Is that because of you or because you're
a physicist, or is it all packaged together. You know,
there's a whole spectrum of physicists. There's the kind that
really likes to build stuff, crawl around on the detector
with a hammer and a wrench and and get dirty.
And then there's the kind of like to sit in
(01:35):
front of a laptop and analyze data and think about statistics.
And I'm definitely more of that second kind. Well, you
can check out my house in your laptop and think
about it for a long time. I'll build you a
virtual spare room. There you go. I'll program another spare
room in the simulation of your universe. It sometimes exists
in this universe and sometimes it doesn't. It's theoretical. That's right.
(01:58):
Just step through this black hole into your new spare
rooms and cozy but welcome to our podcast Daniel and
Jorge Explain the Universe, a production of our Heart Radio
in which we talk about all the amazing things that
are happening in this universe, all the things we'd like
to understand from the very very large, very very dense,
all the way down to the very very small and
the very very weird. Yeah, because the universe has a
(02:19):
lot of amazing things and all kinds of skills, you know,
galactic cosmological skills, But there are also amazing things happening
at the smallest scales of reality and nature. That's right,
and these really tiny things they give us an amazing opportunity.
They let us test our understanding and not just do
we mostly get things right, They let us really push
(02:41):
our limit to understand exactly how these things are working.
Do our models predict what's happening or is there something
a little bit wrong? And that kind of raises a
question of how well do we know what's happening at
these really tiny scales, Like we can measure distances from
here to the moon, for example, or here to maybe
the next star, But how do you measure things that
(03:01):
are that small? And how do you know if you're right?
It definitely takes a certain skill. You have to come
up experimentally with really clever devices, things that isolate individual particles,
or get a bunch of particles and get them all
aligned and then separate them from any other effect. It's
a really particular skill in science to devise an experiment
that forces nature to reveal something for you that pushes
(03:24):
everything else away. Like we learned about the Lego experiment
that measures gravitational waves. There's a huge amount of cleverness
involved in isolating those things so you can see tiny
little wiggles in space. Well, this is sort of experimental cleverness,
and when you deal with particles, you need this sort
of the same kind of skill. You need to set
the universe up in a way that it has to
reveal to you very precisely the answer to your question. Right,
(03:47):
But sometimes a problem, right, Daniel, is that you measure
something and you don't get what you expect. You measure something,
you think it's going to be this big or this
long or this heavy, and then when you measure in reality,
it's different. Yeah, well that's not a problem. That's fantastic,
that's exciting. That's an opportunity, you know, Like, yeah, because
(04:07):
we have two branches of our work. We have the
experimental side that's going out and doing stuff and measuring
things and answering questions, you know, sort of asking the universe,
what do we conclude from that? How do we interpret
that and build a model of the universe in our heads,
and then turn around and predict future measurements and when
those predictions disagree with the things we observe. That gives
(04:28):
us an opportunity to update that model, to say, oh,
something was wrong. There's a new particle, or this particle
works differently, or black holes are actually bigger than we thought.
Those are the moments when we learn. So when theory
and experiment disagree, I smell opportunity. But how do you
know who's right. You never do, And usually they're both wrong,
and they're both wrong in different ways because there are
(04:50):
very very different challenges, you know, split the difference. Calculating
something theoretically has challenges of computing time and getting minus
signs right and sort of organized in your mind, and
and getting answers experimentally has all sorts of different challenges,
making things clean, making them distinct, getting a big sample
of something, getting enough material, you know, we're sometimes just
(05:11):
getting enough money to build the device that you need.
That's the hard part. But there is one of these
big mysteries in nature that it has to do with
a weird kind of discrepancy between what the theory predicts
and what we actually measure. So today on the podcast,
we'll be talking about mystery of the muance magnetic moment.
(05:37):
That sounds marvelous and magnificent. It is one of the
most amazing and marvelous moments in magnetic field history. You know,
it's an opportunity for physics to learn something because it's
something that we know how to calculate very very very precisely.
You know, if you want to find out what's wrong
with your theory, you need to find something that you
can predict very accurately and then measure you're very very accurately,
(06:01):
so you can compare the two and that tells you
if your theory is right or and or if you're
measuring device is working right and not just giving you
weird things. Yeah, and if you hope that your experiment
is correct, then you know, if you see a discrepancy,
tells you that your theory is wrong. And sometimes we
do this as a way to detect the presence of
new particles or you know, just to see if anything
(06:23):
is right, because some of these calculations are very very sensitive,
so it's a very good way to tell whether there's
anything missing in your ideas. You know, it's sort of
like if you walk around your house and you could
take a really precise measurement, you could see where everything was,
and you compared that to the drawings you have of
your house. Right, that would tell you, like, you know,
whether your house is well described by your idea of it,
(06:45):
or whether it was a mistake to hire you to
clearly actually probably not. How are you to measure it
also build it? I would say, Wow, this is perfect work.
You should pay your contract or double. I guess that's
a big question. And the question is how good or
physics isn't measuring things? And so we were wondering, we
were curious about how many people out there. So if
I've thought about how good our measurements of the universe are,
(07:08):
and in particular, what's the most precisely measured quantity in physics? So,
as usual Daniel went out there into the wilds of
the Internet to ask people what's the most precisely measured
quantity in physics? That's right, And if you're interested in
answering random internet questions without any preparation, please right to
me at feedback at Daniel and Jorge dot com and
(07:30):
I'll send you some questions to answer. So think about
it for a second. If you were asked, what's the
most precisely measured thing in physics. What would you answer?
Here's what people had to say. My best guess is
increments of time, accusing an atomic clock. I don't know.
I don't know, Sorry that I don't know. Is this
something to do with the plank Clint? Maybe aliens? I'm
(07:53):
not sure what you mean about quantity, whether that be
amount of things, how many bananas it takes to create
a black hole. I'm going to guess it's mass. Maybe
I like this question. Well, I think first we have
to define what do we mean by precisely? Maybe temperature
(08:13):
mass would be the most precisely measured quantity in physics
because it holds a tangible value. I reckon they can
measure pretty small, like maybe an adam. All ride a
broad range of answers, from aliens to bananas to the
plank scale. I feel like our audience is very much
in tune with what we cover here in the podcast. Yeah,
these are great answers, and I have to confess I
(08:35):
think that some of these answers make me rethink how
I should have asked this question, because I asked, what's
the most precisely measured quantity in physics? Like you go out,
you do an experiment, you measure something. But I think
really the question we should have asked is what's the
most precisely calculated quantity in physics? And there's a difference.
For example, the atomic clock answer is a really good one.
(08:58):
You know, atomic clocks are precise us too, like one
part and tend to the sixteen. You know, it takes
like ten to the sixteen seconds before they're off by
one second. So you would agree with a lot of
these alien, Yeah, some of these are really very accurate.
And for example, Lego, like we mentioned before, gravitational waves.
To detect gravitational waves, they have to measure, you know,
(09:19):
the change in length of something by one part in
tend to the twenty or tend to the twenty one,
which is really incredibly precise experimentally. But what I was
going for was a question of, like, what's the most
precise test we have of our theories, which requires not
just a really precise experiment, but also a really precise prediction. Oh,
I see, like what's the most precise that physicists have
(09:42):
been right about stuff? Is that kind of what you mean? Yeah,
because for gravitational waves, for example, that's a very precise
experimental measurement but we didn't know in advance how big
it would be, and we don't know necessarily how big
those should be. It depends on the size of the
black holes that are the same with the tomic clocks.
We can't calculate the those things as well as we
can measure them. In order to get some insight into
(10:04):
the universe, you need something where you can calculate it
really well and you can predict it really precisely. Interesting,
so you're kind of talking about like, according to the
laws of physics, we think that dis quantity should be this,
and then how well does it match with what we
actually measure of it exactly? Because it's those discrepancies we
need to learn something. It doesn't matter if you measure
(10:24):
the length of your house to one picometer, because we
don't know how big your house should be and doesn't
really tell us anything about the universe. But if you
measure something really precisely that we can also predict that
we can calculate that has to be a certain value
because of our understanding of physics. Then measuring it and
finding out it's something else gives you a clue that
something is wrong about our model. Right, Well, it seems
(10:47):
like there's one such thing that we're trying to predict
and measure at the same time, and that there's a
big mystery about why those two things don't match. And
that's the magnetic moment of muance, which is a great
alliteration there. I'm so pleased to have some positive feedback
for a name in particle physics from you. That's a high,
(11:09):
high standard for your poetic writing. Here, the mystery of
the muon magnetic moment. Yes, it's really marvelous, all right.
So this is a quantity that we have predicted using
theory and that we've measured using big machines. But those
two things don't match. That's right, all right, So let's
get into it, Daniel. Let's start with the first m
What is the magnetic moment. So when you think about particles,
(11:30):
remember we like to think of them as little dots
in space that have labels, and those labels can be
like what's the spin or what's the charge or how
much mass do they have. We don't think of particles
is like little physical balls that actually do these things.
They're weird quantum objects and they have these labels. And
so this is one of the labels of a particle.
But it's a little weird because it's not like a
(11:52):
direct label. It's not like something you can put right
on the particle. Because particles they don't have a magnetic charge.
They have electric charge. That's how they feel electric fields.
But as we talked about on the podcast before, there
are no particles that just have like a north or
a south magnetic charge on their own. See, they have
an electric charge, but they don't have like a pole
(12:13):
like you say, like a magnet, like a north and south.
That's right, they don't have just a north and just
a south. What they have is this weird magnetic field.
It's a dipole to have a north and a south,
just like every magnet we've ever discovered has a north
and a south. And that comes from the combination of
having charge and having spin because charge and spin together
gives you some sort of magnetic interaction. Okay, so particles
(12:36):
have spin and charge and together they have a like
a pole, like a magnet, little magnet inside of it. Yeah,
and that's what we call the magnetic moment. It's the
part of the muan that is affected by a magnetic field.
And you know, fundamentally it comes from having charge and
from spinning. And that's because it has a magnetic moment.
It doesn't have a magnetic charge. It's not a north
or a south, but it is affected by the magnetic field.
(12:59):
And that's what we mean when we say the magnetic
moment of the muan. How a muan is affected by
a magnet. It's not the moment for like an electron
looks at a positron and they feel that attraction towards. No,
it's not. It's not a dramatic moment. It's not something
exists in like theory of screenplays or anything like that,
unless you're writing a movie about particles, in which case
there probably is an electrifying moment for the muan. Wow,
(13:22):
you would totally watch that movie. Um, I totally have
that movie script already in a drawer in my house.
It's been sent to several Hollywood agents, but nobody seems
to be writing. Consider this podcast my pitch for this project.
I would definitely watch that movie, but I have not
yet written the script anyway. So we're interested in you know,
(13:43):
what happens when you put a magnetic field on a muan.
And this is something we can measure because we can
do that experiment, and it's also something we can calculate,
and it turns out to be really sensitive to exactly
what's happening into some other big questions about how particles
work well, And maybe let's go back a step and
cover the other m which is the muon. So muan
(14:04):
is like an electron, Is it like a quirk? Yeah,
So we are made out of quirks and electrons. Right,
We have quirks that make up the protons and neutrons
inside our atom, and then we have electrons whizzing around them.
But each of those particles have other copies. There are
other kinds of quirks, and there are also other kinds
of electrons. So there's a heavier version of the electron.
We call that a muan, exactly the same as the electron,
(14:27):
except has a lot more mass. And there's another one
even called the towel. So the electron has these two cousins,
the muon and the towel that have all the same
interactions and all the same properties, like the same charge,
the same spin, but just heavier match, just heavier mass. Yeah,
and it's weird. We don't know why they exist, Like
why do we have the muan. Why do we have
(14:48):
the tow Why does the electron have two cousins and
not nine cousins or seventeen cousins or any cousins like
my cousin? Are they good for anything? I'm not going
to get in the middle of that family dispute. We
have cousins, so we want specified which one I'm talking about.
But what I guess what I mean is like, is
it good for anything? Like does it form part of
(15:09):
you know, can you make an atom out of them?
Or do we just know them kind of theoretically, or
we know that they formed, but then they disappear quickly.
That's right. They're not stable, so you can form atoms
out of them. You can take a proton and put
a mu on around it and form a bound state.
But the muan lasts for you know, a few microseconds.
Remember that heavy particles don't survive very long in the universe. Actually,
(15:31):
in its reference frame, if you were riding on the
back of a muan, you'd see that it lasts a
few microseconds. But because they move so fast, their clocks
are slowed down. So as we watch the muan, we
see them live their three microsecond lifetime. Over a longer
period because of time dilation, so they don't last terribly long.
It's still you know, seconds or minutes. But muans don't
(15:51):
last in our universe because they're heavy. They effectively turn
into electrons. All right, So there's a big mystery regarding
the magnetic moment of the eu on. So let's get
into the theory and the experiment and talk about what
it means. But first let's take a quick break a right, Daniel,
(16:19):
we're talking about the magnificent mystery of the marvelous muon
magnetic moment. Momentarily, hi rolls in your mouth. It's delicious. Um,
So yeah, so tell me about this mystery. So we
know about the muan and you're saying that we can
the theory predicts its magnetic moment. How can the theory
(16:40):
predict something like that, Well, we think about it in
terms of particles, right, We're talking about how the muon
is affected by a magnetic field. But a magnetic field
we know is really carried by photons. Like when things
interact electromagnetically, we can imagine that as being done by
photons moving through space carrying information. Remember, every force that
we think about electromagnetism, the strong force the weak force
(17:03):
has these particles to sort of do its job, and
in the case of the electromagnetic interaction, it's the photon.
So when you think about how a muon is affected
by a magnetic field, really on the sort of particle level,
what you're thinking about is what happens when a photon
hits a muon, or how does a photon interact with
a muon. That's sort of like the basic tinker toy
(17:25):
element of particle physics that lets muans be affected by
magnetic fields, right, because magnetic fields are transmitted by photons. Yeah,
magnetic fields are basically photons. We can think about like
our fields particles or particles fields, but they're very tightly connected.
So like if I throw a muon at a bunch
of magnets and it curves one way, it's not because
(17:47):
it's something in it. It's because it's like hitting and
interacting with photons. Yeah, exactly, it's getting bent by the
magnetic field. And very natural way to think about that
is in terms of photons being generated by you know
whatever where the source of your magnetic field is and
pushing the muan. All right, So then we think of
its interactions is hitting photons, and so how does that
(18:09):
help us predict its magnetic moment. Well, it's fascinating because
there's a whole bunch of different ways that a photon
can hit that mu on. Like the simplest thing is
photon hits the muon and bounces off, right, So you
have photon muan interaction very simple, Like in your mind,
you have a couple just little lines of particles that
intersect and then they go their separate ways. That's the
(18:30):
simplest thing, and you can use that to calculate, all right,
what's the strength of the magnetic moment of the muan?
And if you did that calculation, you get a pretty
simple answer. This was done first by a guy named
Julian Schwinger, and he was so proud of this calculation
that he actually had this number. It's alpha and the
fine structure constant over to pie. He put this number
(18:50):
on his tombstone. He's like, don't forget I came up
with this. Seriously, it's like it's a beautiful calculation. He
was so proud. This guy did a huge amount of
physics in his lifetime. He's basically the person who proved
that Fynman's theory of quantum electro dynamics actually work. Finding
like sketched a bunch of doodles and had a few ideas,
but never like actually made it work. And Julian Schwinger
(19:12):
was like, all right, let's do all the calculations and
see if this is right. But that benefit in his
tool stone, I guess this is a really succinct way
to just sort of like sum up the guy's life. Anyway,
the point is that there are other things the photon
can do also. It doesn't just have to bounce off
the muan on its way there. It could like split
into an electron and positron and then convert back into
(19:34):
a photon and then go off. Or it can emit
a particle and then reabsorb that first particle. So if
you'd like drawing these Fineman diagrams these ideas for how
this happens, all you have to do is add a
couple more lines, and all these things describe totally valid
things the photon could do as it interacts with the muon,
and those change effectively the muan's magnetic moment. See, so
(20:00):
it's kind of like the muon doesn't really have a
magnetic moment. How does it interact with a photon? Its
interaction with the photon is essentially what determines how it
reacts to magnetic fields, which is its magnetic moment. And
photons are crazy, they're like always turning into other stuff
and spewing off particles and reabsorbing them. And the real
actual thing that happens between a mean and a photon
(20:22):
is some some of all those things, all those things
mixed together, which you can and that you can predict
with the theory. Like your theory, you can like write
this down in a piece of paper, like what happens
if a photon hits the muan, and you can in
a piece of paper you can work out how that
muan should bend its path or how we should get deflected.
(20:42):
And then you can say, well, what if it was
a little bit more complicated, what if it also emitted
another particle at the same time, Then it would change
your calculation. And you know, as you make these things
more complex, there are more and more possibilities, so it
becomes very challenging theoretically to account for all the different things.
But that's also gives you an opportunity because if there
(21:04):
are crazy particles out there that you would never considered,
then the photon could be turning into them, could be
like interacting with them, could be like popping into existence
some weird new particle you never imagined. And that would
change how it interacts with the muan because it would
lose some energy. It would just it would change its angle,
it would change its direction, it would change the probability
(21:27):
of this thing happening at all. And so in this way,
the photon interacting with the muan is sort of like
a probe of the whole universe because along the way,
the photon can do all sorts of crazy stuff. You
can do anything that quantum mechanics lets it do, and
what happened affects how it interacts with the muon. And
so by calculating this quantity and then measuring it, you
(21:47):
can ask, like, is there anything else that the photon
is doing along the way that's changing how it interacts
with the mu want to see, like how good are
we predicting what photons actually do. Yeah, it's like you
said to photons, hey, go crazy, do anything you want
to do, and then we're going to try to calculate
all the things we think you can do, and then
let's compare. And you know, if it turns out you're
(22:08):
dancing with a new kind of particle we've never heard
about before, we're gonna know you're like stalker fans. Yeah,
and you know, people like me, I like to discover
new particles by sort of making them concretely, like pouring
enough energy into a collider so that we have enough
energy to make this new particle. And see it's sort
of directly, but this is another way to do it,
is to like look for these particles just sort of
(22:30):
like briefly popping into existence as photons do their crazy
dance with muans. And I guess my question is why
the mean like couldn't I mean, all these questions and
all these magnetic moment ideas we should work for any
other particle, right, So why are we focusing on the
muon specifically? And you can do these calculations also for
the electron and also for the town, right, but the
(22:52):
muon is sort of in a sweet spot because it's
a little bit heavier, it's sort of easier to handle.
The new physics should happen to all of these particles. Right,
but it has essentially a proportionately larger effect on the
muan because it has a larger mass. I see, So
lemon is like the guinea pig. Yeah, the muan is
like the best place to get the universe to reveal
all these little details, all right, And so you can
(23:15):
run the math and it should tell you how the
muon should bend in a magnetic field. And you can
also measure how, like you can throw me on at
a magnetic field and see how it bends. That's the
experimental side. Yeah, But before we move on to the
experimental side, I gotta sort of shout out to the
theory here, because this is what I meant earlier about
being really precise. On the theoretical side, this quantity, the
(23:36):
magnetic moment of the muan, is the number that theorists
know best. It's the most precisely calculated quantity basically in
the universe as far as we know, unless there are
alien physicists doing it out there. What how can something
theoretical be precise? Doesn't precision mean like how right you are.
It doesn't mean how right you are. And when we
(23:57):
do these calculations, we start with the simplest idea as
we say, well, what's the simplest thing of photon can do?
And that gets you mostly right, and they think, well,
what if it does one weird thing along the way,
And there's like nineteen ways for that to happen, So
you add nineteen calculations. Well what if it did two
weird things along the way. Okay, now there's nineteen squared
ways to do that, and each of these gives us
smaller and smaller effect. And so as you add up
(24:20):
more and more of these ideas you're considering, you get
closer to the true answer, but also becomes harder. And
so now they're at the point where they're calculating like
millions and millions of possibilities. Maybe first to turn to
ano electron, and that electron did some weird thing, which
turn to into a photon, which then did some weird thing.
And so they've estimated sort of theoretically how precise this is,
(24:40):
Like it's impossible to get it exactly right because you
need to do an infinite number of calculations, so they
can estimate how close they get based on how much
is the answer changing as they add more ideas, So
they're asthm topically approaching the deep truth. I guess there's
a you know, an engineering, there's always this issue about
the different between accuracy and precision, like accuracies how right
(25:04):
you are, and precision is like how sure you are?
So is the thing that's happening here? Is it that
theories are pretty sure they know what the moment of
the muan is, like they think they've covered all the angles,
so they're pretty sure, but maybe they don't know if
it's the actual value. Yeah, you know, I have equivalent
with theoretical physics here because experimentalists trying to be really
(25:25):
formal about the statistical statements we make. If we say, okay,
there's an uncertainty here, that means something very specific. Statistically,
it means if you did the same experiment a hundred times,
you would get the answer within your uncertainty bounds sixty
percent of the time or something like that, or a
different answer if your Bayesian. That's precision. There is are
a lot more hand wavy, you know, they're like, well,
(25:47):
we tweaked a couple knobs and got different answers, and
so you know that's the uncertainty. We multiplied some things
by two just to see how things would change. So
that's what we're calling the uncertainty. And you know, it's harder,
it's different. They're not measuring things about the universe. They're
just trying to like guess how closely they are the
right answers. So I guess maybe the title should really
(26:07):
be the most precisely guessed that theory quantity ever, you
know what I mean. Like they put a lot of
attention into the They've covered every angle on so they're
they're pretty sure that this is what the plan is. Yeah,
I suppose so, although you know, there have been moments
in this history and this is a decades long project
to make the theory more precise and make the experiment
(26:28):
more precise. It's a bit of an arms race to
see like who's getting more and more precise. There was
a moment in the nineties when the theorists discovered that
they had gotten the sign wrong, like that a minus
sign where there should be a positive sign, and it
changed the answer kind of a lot. So they're they're
definitely mistakes in there. Oh my gosh, who made the mistake?
Are they going to put that in their tombstone as well?
(26:49):
One more minus sign? Know, there are different groups, and
they're cross checking each other, and so you know, that's
another way they try to estimate how correct these things.
All right, well, let's get in to now the experiment
part of it and how well these two things match up.
Who's more precise or less accurate or more marvelous. But
first let's take a quick break. All right, we're talking
(27:23):
about the magnetic moment of the muan as the most
precisely guest at quantity ever, and now we're going to
measure it with an experiment, and that just involves throwing
a muan at a magnetic field and seeing where it
goes or is there is there something special going on?
You know, that would work, But what you want is
a really precise measurement. You want a measurement which is
(27:44):
accurate to like one part in ten to the twelve
or ten to the thirteen, and so to do that
you need a really clean setup. And so what you
described would work, but it's sort of hard to measure
it's a single particle. And so what you want is
a lot of muans. You want them all basically doing
the same thing. So you can get a bunch of
measurements and divide by a big number and it sort
(28:05):
of averages out some of the mistakes. And so what
they do is they get a huge pile of muans,
a big blob of muans, and they point the spin
of the muan, which is the thing that determines again
where this magnetic field is going, and they get them
to spin in the direction they're moving, and they move
them in a circle. So they have this ring in
Chicago where they have a bunch of muans and they
(28:25):
move them in a circle. And when muans move around
in a circle in a magnetic field, their spin will precess,
it will rotate around the access of motion. Because that's
how the physics work out. Like if you try to
ban the muon, it will also sort of change in
other ways. Yeah, Like one thing that happens to a
particle and you put it through a magnetic field is
that it bends. But a particles moving in a circle
(28:46):
through a magnetic field will process, It will will change
the direction in which they're pointing. So that's what they
can do, is they can measure the difference between the
direction of the magnetic field that they're putting on these
particles and the direction of the spin of the ones
which affects their magnetic moment, And so they have come
up with really clever ways to measure these things and
to reduce all sorts of uncertainties. And you know, if
(29:08):
you're a visual person, it's really very similar in spirit
to the experiment that looks for gravitational waves. What you're
trying to do is isolate this experiment from any other effect.
You know, like, is that the microwave oven in the
break room that's changing the answer? You do we understand
all the electromagnetic fields nearby. Is the radiation from the
ground affecting our result? It's this kind of experiment you're
(29:31):
like really isolating any source of noise or uncertainty. All right,
So they're spinning these muans in a circle in Chicago,
and again not in Minnesota or Milwaukee or Montana. No,
it's being done at Fermulab, the accelerator complex just outside
Chicago between Batavia and Naperville, where I did my pH
d thesis hometown plug. All right. So they're spinning these
(29:54):
in a in a circle, and they're measuring how they're
processing or changing in the direction of their moment, and
that tells you the magnetic moment of the muan experimentally,
and now the problem is how well does it match
with what the theorists. Yeah, that's right, that's the questions.
That we have the number from the theory and the
number from the experiment, and if you write these two
numbers down on a piece of paper, they agree to
(30:15):
the first what is it, like eight or nine digits
before they disagree. So it's like it's really a testament
to an incredible amount of work. I mean, you call
it guessing, but like these theorists have done a huge
amount of work to really nail this down, and the
experimentalists have done a different, difficult pile of work, and
now they have these two numbers. It's incredible to me
that they agree this closely at all. All right, so
(30:38):
let's maybe sign out the number for the audience here.
So the experimentalists say that the magnetic moment of the
muan is two point zero zero two three three one
eight four one eight close our mind is some small quantity,
and what are the units of these These are dimensionless units. Yeah,
so okay, that was from the experimentalist. Theorists say it
(31:00):
should be two point zero zero two three, three, one
eight three six to not fo, that's right. So they
agree on you know, after the decimal place, they agree
to seven digits, and then they disagree. One of them
says four one, eight and the other one says three
six two, which is not a huge difference. It's like
(31:21):
twelve zeros and then like, yeah, it's a bunch of
zeros and fifty six. But the fascinating thing is that
both of them are pretty confident in their results. So
there's a gap between them, very tiny gap between them,
but the uncertainty is smaller than the gap. Right, the
difference between them is fifty six, and the uncertainty is
like fifteen, So the difference is like three or three
(31:43):
and a half times the uncertainty. It seems really it's
so weird to me that they're so confident, you know
about these numbers, Like you know, I've done experiments, and
you know, to get that kind of position is really hard,
Like if they ran this experiment next year and the
year after that, but they still get the same exact. Yeah.
These uncertainties reflect statistical limitations, so like you haven't runned
(32:04):
for an infinitely long time, and also systematic uncertainties like
things you think will contribute to mismeasurement or or bias
on your result, and you know these are estimates. It
could be that they're wrong. It could be just a
basic mistake somewhere. But this is what we're trying to learn.
Like we're trying to learn, like do we understand how
to do these precision measurements or do we understand how
to do these calculations, or is there a new particle
(32:27):
out there that we're not factoring into our calculations that's
playing with a magnetic moment of the muant finding a
little bit? Is this the hint of the discovery of
some new particles, some new supersymmetric particle which is too
heavy to make a particle colliders and only appears very
briefly and gives these little hints to the mule, like
is there something hiding in that zero point zero zero
(32:52):
five six difference between the experiment and the theory, or
because they're both pretty sure of their numbers, there's now
like they're both pretty sure of their numbers. Yeah, it
couldn't be like a wire missing here or a plus
sign missing over there. They certainly could be and their
independent checks, their independent experiments, and we'll talk about that
in a moment, but they're both pretty confident. And I
(33:14):
remember learning about this in college and I was still
learning about quantum mechanics and how it all worked. And
at the time, I thought of physics as sort of
like a description of what we see about the universe,
just like sort of a human internal to our minds
approximation of what's happening in the universe. And I read
about this calculation, like, wow, it agrees to you know,
nine or ten decimal places. That's amazing. And I at
(33:37):
this moment where I thought, wait a second, maybe physics
isn't just describing approximately what's happening. Maybe we've discovered like
the source code, Like maybe this is what the universe
itself is doing. Because to get that accurate, to get
that precise, it's sort of shocking, you know, to imagine
there could be another theory that could also be that precise.
(33:57):
So I see, it's like, what if we actually uncover
at the code of the simulation of the universe, because
it's so we're so right, and we're so right. Yeah,
maybe the universe does run on a computer using these equations?
Is that kind of what you mean? Sort of you know,
but in a more universal way, like maybe the universe
does follow laws and it does calculations, and it follows
(34:17):
these rules when it does those calculations, you don't have
to be embedded in some meta universe and simulated on
a computer. Maybe the universe is doing calculations though. Anyway,
it's an incredible testament in my mind to the work
involved here, and it's amazing that it works at all.
I agree, right, But there is sort of an interesting mystery,
And I guess the weird thing is that you're telling
me that for the electron there's no difference between the
(34:41):
experiment and the theory. That's right, Like this difference only
shows up in the melee. We can do the same
measurement for the electron. We actually a similar number, but
there's no discrepancy. Like the electron. When they do the
theoretical calculation and they do the experimental measurement, they get
those two things to agree to within uncertainty. Now we
expect that new is it, new particles whatever, would have
(35:01):
a bigger effect on the muan, so it's not a
surprise that it doesn't appear there for the electrons. And
that's quite fascinating. So maybe there's something going on with
the muan that you wouldn't see in the electron. So
the electron you check that box or like the theory,
and both groups have gone at the electron with the
same kind of intensity and precision, and you can do
(35:23):
all the same kinds of theoretical calculations for the electron
and get a really precise number. And then you can
go measure the magnetic moment of the electron, because electrons
also bend in magnetic fields, and you can make that
measurement really, really precise, and those two numbers agree. Electrons,
we understand them like. There are no mysteries hiding under
the rug for the magnetic moment of the electron, but
(35:44):
for the muan, which is exactly where we would expect
to see something weird. First we start to see something weird,
all right, But it's different for the muan, which means
that it might be hiding a secret. So what does
that mean, Daniel, What could be hiding underneath the marvelousness
of the muant? Well, you know, we suspect that there
are other particles out there that we have not yet discovered.
(36:05):
We found six particles that are corks, six particles that
are leftons, and then a few of the particles that
mediate the interactions between them, and so we have this
pile of particles, but we don't know if those the
only particles out there. And actually it would make a
lot more sense if there were more particles, because there
are these weird patterns we found that are unexplained, and
(36:25):
some of them would click together really nicely if there
were new particles. Like some of the particles we've seen
are called fermions. They have spin one half, and the
other ones are called bosons because they have spin one.
There's one idea that maybe every fermion has a boson version,
like the muan has another version of it called the
smeu on, and the photon is another version of it
(36:47):
called the photino. And these are like just one idea
of how there could be new particles out there that
sort of solve deep problems in theoretical physics, but we
haven't seen them yet, so they could just be too big,
too heavy for us to discover them. In particle colliders,
remember to see something in the collider, you have to
put in enough energy, which means you have to make
(37:08):
the collider big enough, which means you have to get
enough money from the government to build a really big tunnel,
So there's a limitation there. This might be another way
to like sneak around that limitation and see these new
particles for the first time, at least hint that they're there.
And if you do the calculations and what do you
expect to see if there are these new particles, this
(37:29):
is kind of exactly what you expect to see. So
is the idea then that maybe there's a new part
that we don't know about that the photon is turning
into or like transforming into before it interacts with me.
Because the photon can interact with anything that has electric charge.
And so if there's some new heavy particle out there
that does have electric charge but it's never really exists
(37:51):
in the universe because it's too massive, well, occasionally the
photon can turn into it or pairs of it, like
it's particle and it's antiparticle, and that would change how
it interacts with the muan because you have to include
it in all of these calculations. Like maybe it admits
this new heavy particle and then it interacts with the
muan and then it reabsorbs that particle, and that would
(38:11):
change the way it interacts with the muan. And so
the presence of weird heavy particles changes the basic interaction
between two very simple particles, which I think is fascinating.
It's like a it's a clever way to leverage you
know something about the universe, to force the universe to
tell you about what's going on, even if you don't
have the energy to build that com eider. I think Danny,
what you're saying is that the experimentalist are right and
(38:33):
the theories are wrong. Well, you know, the experimentalists are
probably wrong in different ways from the theorists. Experimentalists definitely
make mistakes. It's really hard to do these things and
to get them right and to remove all sources of air.
And that's why it's fascinating as a cross check, because
if they're wrong, they're probably wrong in different directions or
different amounts. And so it's a great way to cross check,
(38:55):
and you know, to improve experimental physics and to improve
our theoretical understanding of the universe and maybe find new
particles in between. And yeah, and maybe make us stop
off in Stockholm to collect your Noobile prize. And that's
why you know this isn't over. It's not just like, oh, hey,
we saw this discripancy were done, because it's kind of indirect, right,
This is not like we make these particles, we see them,
(39:17):
we understand them. It's just sort of like a clue
that the particles are there. And so what they want
to do is make these things more precise. They want
to get better experimental measurements. They want to push the
theoretical measurements to see are these things wrong? Do these
stack up? Can we improve this uncertainty? Can we make
these things ten times as precise? And does it stick
around or disappear? All? Right? Well, I think it all
(39:38):
speaks to just again this idea that there may still
be amazing things hiding even in tiny little gaps of
point zero zero zero zero zero five six. That's right,
some of the biggest clues in the universe turn out
to be on the smallest numbers. And there is news
to come because there's an experiment happening right now again
in Chicago that's going to give a measurement of the
(39:59):
muan magnetic moment that's going to be four times as
precise as the one that we have now. They took
this big magnet and they shipped it from Long Island
where they did the experiment first, and moved into Chicago
and that a cleaner beam with more muans and they're
running those results right now. And in eighteen they said
that they would have results quote sometime in t and
(40:19):
so here we are in no results yet, but we
expect any day, any day. These things are hard. We
expect any day they'll come out with the new measurement.
And the whole physics community is waiting, like, what's going
to happen. How's the number going to change? So it's
a big deal. They're like, hey, we said we'd be
accurate about the muan, not about when we would tell
you about the plus or minus five years, plus the
(40:44):
minus fifty six years. Oh man, I'm sure that there
are some graduate students out there on this experiment it's
called muan G minus two, that they are sweating and
working hard to get this number out. Well, hopefully we
added a little bit more pressure because now I'm curious
about what's gonna happen. We're all curious because this is
how we learned about the universe. We corner it and
force it to tell us what is the answer to
(41:05):
this number? We think we know what it should be.
Tell us what the real truth is, tells the universe,
don't keep it to yourself. Experimental physics is basically a
modern day oracle, right, We actually do get to ask questions,
will be horrible, and it gives us answer and then
it chops off your head or something something Greek and
classical like that. It kills your mom. Probably all right, Well,
(41:27):
we hope you enjoyed that, and think about all the
amazing secrets that could be hiding in the smallest of quantities.
That's right. In one of these days, one of these
secrets will reveal something deep and true about the universe.
See you next time. Thanks for listening, and remember that
(41:48):
Daniel and Jorge Explain the Universe is a production of
I Heart Radio. Or more podcast from my Heart Radio
visit the i heart Radio app, Apple Podcasts, or wherever
you listen to your favorite
Hosts And Creators
Daniel Whiteson
Jorge Cham
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