Exploring the frontier of knowledge is what physicists do.
Now, there are many ways to do that, from smashing together very high energy beams of
particles to studying the cosmos.
But another way to do that is to make ultra-precise predictions and measurements. The more precise
the comparison, the more subtle the physics that’s being studied.
The most precise particle physics theory that we’ve ever devised is called quantum electrodynamics
or QED. I’ve made videos on both the calculation and experiment side of this theory.
At any particular moment in history, particle physics has a handful of predictions and measurements
that don’t quite agree with one another. The disagreement could be caused by an inaccurate
calculation or an imprecise measurement. A more exciting option is that the disagreement
could mean that the experiment is seeing something that wasn’t predicted by the theory. If
that’s true, we might be talking about a discovery.
One such discrepancy has arisen in studies of the magnetic moment of subatomic particles
called muons. Muons are kind of like heavy cousins of the electron and the magnetic moment
is basically how strong a magnet each individual muon is.
There are many things wrong with this mental image, but you can kind of think of the muon
as a tiny and spinning ball of electric charge. Take a charge and spin it and you’ve got
a magnet. It’s as simple as that.
There are many ways you can write down the magnetic moment of a particle, but those are
details that only experts care about. Perhaps the clearest way to do it is to define an
ideal and see the differences. For instance, the muon has the same electrical charge and
quantum mechanical spin as an electron and we can say that an ideal particle with those
properties has a magnetic moment of 1. Any deviation from 1 means that the particle you’re
studying isn’t ideal.
As it happens, when you measure the magnetic moment of the muon, you find that it isn’t
exactly 1. In fact, it’s about 1.001 or 0.1% too high. That turns out to not be a
big deal, as the muon isn’t an ideal particle. That small deviation is predicted by the theory
of QED. So no problem. The mystery arises when we measure the magnetic moment ultra-precisely.
During the late 1990s, an experiment at Brookhaven National Laboratory called g-2 studied the
magnetic moment of the muon and determined a value of 1.00116592091. And to give you
a basis of comparison, the prediction is 1.00116591803.
So we see that these two numbers agree pretty well, although not to as many digits as for
an electron. That’s not surprising- it’s harder to work with muons.
What’s more interesting is when you take into account the uncertainties of the prediction
and the measurement.
I’ve decided to show you what’s going on by using these curves. The blue one to
the left is the prediction and the orange one to the right is the measurement. The width
of each curve indicates the uncertainty of each. If the prediction and measurement agreed,
these two bell shape curves would lie on top of one another, like this. But they don’t.
They’re offset and the distance between them is bigger than seems possible given each
curve’s uncertainty.
What I just did was take away the experimental curve to help you understand what you are
seeing. This curve illustrates what the theory predicts. In high-end, fancy-schmancy theoretical
calculations, you don’t get a single number as your prediction. You get a most-likely
number and an uncertainty, which is the range of what is possible. Where the curve is highest,
this is where it is most likely that you’ll find the right predicted answer; where it
drops down a bit is less likely; and when it goes to zero, it’s not likely at all.
So now, I put the experimental curve back on and the same rules apply. The first thing
you notice is that they don’t overlap very much. That means that the theory and the experiment
don’t agree very well. And when prediction and measurements don’t agree, that means
that maybe you’ve discovered something.
So does that mean that these muon researchers have discovered something? Well, no… or,
more accurately, we can’t be sure. The reason is that if you look really carefully, you’ll
notice that there is a little spot where the two curves overlap just a little bit. So it’s
possible that they agree.
On the other hand, the overlap is really tiny. This could be a discovery. Or it could be
that either the existing calculation or measurement has an error in it.
So what do you do? Well, you make a better measurement.
And, of course, that’s what scientists are doing. They’ve borrowed the g-2 detector
from Brookhaven Laboratory. They put it on a barge, headed down the east coast of the
U.S., around Florida, and up the Mississippi river. The g-2 detector is a string of magnets
weighing 17 tons. It’s fifty feet across, and it couldn’t flex more than a tenth of
an inch during the journey. After a trek of 3,200 miles, the g-2 detector arrived at Fermilab.
Fermilab has more intense muon beams, so we can combine the existing precise detector
and the more intense beams to get a better measurement.
It was then installed in the experimental hall, and scientists are working to get the
equipment operational. That’s going to take a couple of years still, but they’re working
on it. I’m not personally involved, but the scientists involved are among the world’s
finest. This really is a hard measurement.
So what are they going to find? Well… and I don’t know how to say this… but that’s
a particularly dumb question. If they knew the answer, it wouldn’t be called research.
If the earlier measurement was accurate, the new measurement will be somewhere inside the
earlier measurement’s bell shaped curve. I don’t know where, but- but somewhere.
What is really important is that the new measurement should be four times more precise. We can
get a sense of the impact we can expect to see from the improved precision. If we assume
that the theoretical prediction doesn’t change and that the new measurement is the
same as the old measurement with smaller uncertainties, what you see is that the two curves now don’t
overlap at all.
As I said before, no overlap means a discovery. It means that the measurement is seeing something
not predicted by the Standard Model, which is currently our best model to describe the
universe. This g-2 experiment could well be the biggest scientific discovery of the decade.
Even more exciting, it could be the experiment that points the scientific community at the
next big breakthrough.