Over the last three years, researchers at Fermilab gathered around
a 50-foot particle accelerator controlled by superconducting magnets.
They shot high-energy particles through the racetrack, looking to confirm a tiny discrepancy
between theory and experiment.
The object of interest was the muon.
This year, Fermilab published the results of the Muon-g-2 experiment–and the findings
have profound implications for new physics.
This is the Standard Model of particle physics.
For the last 50 years, it’s been our best description of the 17 known fundamental particles
and their interactions.
Muons are much like electrons, with the same electrical charge and spin properties.
But they’re different in one important way: they’re 200 times heavier.
Because of this, muons have more energy to briefly emit and reabsorb hidden particles.
They give us a special window into the subatomic universe, where unknown particles could be
lurking.
Each muon, because of its spin properties, is generating its own little internal magnetic
field.
And so when you take that muon and you put it in an external magnetic field, like one
you've generated in the laboratory, what that does is it changes the frequency
of their wobble ever so slightly.
This is where we talk about a quantity called the g-factor of the muon.
The g-factor essentially tells you the strength of that internal magnetic moment that the
muon has.
So by determining very precisely how that muon's internal magnetic moment is interacting
with the external magnetic field, what we're getting a glimpse into is this quantum mechanical
world where all the other particles are suddenly popping out for a moment in time.
The Standard Model predicts the muon’s g-factor.
But when scientists measured it 20 years ago at the Brookhaven National Laboratory, they
found a significant statistical deviation, which they call 3.7-sigma.
If you imagine flipping a coin a hundred times, that Brookhaven result would be about like
finding 67 heads or 68 heads, something like that.
And you think, well, that's kind of weird.
I wonder if there's something wrong with the coin.
The goal was to follow up on this Brookhaven experiment and do the experiment again at
a laboratory capable of producing 20 times the number of muons.
The results show an even more significant deviation from the Standard Model prediction–very
close to the 5-sigma difference needed to claim a scientific discovery.
You would have to do the two experiments 40,000 times over before you would expect to get
a result as anomalous as the one we see from the two experiments at this point.
And so the reason that that is so exciting is because it basically comes down to two
things: either there's an 18th particle that we haven't yet discovered directly, or there's
something we don't understand about the existing 17 particles and the forces that we know about
that are interacting with them.
Particles beyond the Standard Model could help explain some of the most enduring mysteries
in physics, such as dark matter–a substance we know exists but haven’t yet detected.
But shortly after Fermilab published its results, a group called BMW published state-of-the-art
supercomputer calculations that target the dominant source of error in
the theory.
The results bring the model's predictions in line with the g-2 measurements.
And that has other implications for other calculations we do in the Standard Model,
which makes that an interesting discovery in and of itself.
Whether the muon g-2 experiment points to an 18th particle or something we don’t understand
about the existing particles, the results are not the last word in particle physics.
This is a perpetual motion machine.
Once set into motion, this theoretical system works infinitely, requiring no additional
energy.
The quest to design these machines has captured the human imagination since the Middle Ages.
To build a perpetual motion machine, you’d have to evade the second law of thermodynamics,
which says that in a closed system, entropy, or disorder, always increases.
This makes perpetual motion machines impossible.
But this year, researchers at Stanford University and Google made a profound discovery.
They built a kind of perpetual motion machine inside a quantum computer.
This is a time crystal.
It’s a quantum object that forever cycles between different states without consuming
energy.
Time crystals are an entirely new phase of matter–the first to break a fundamental
law of symmetry in physics.
In physics, we tend to organize our understanding around symmetries and their breaking.
And the most fundamental of these symmetries is translation symmetry in space and translation
symmetry in time.
So translation symmetry in space means that the laws of physics look the same, whether
you do an experiment here or, you know, in, in a different country or just a meter from
here, and translation symmetry in time means that the laws of physics look the same whether
you do your experiment today or tomorrow or yesterday.
But nevertheless, there is this big difference between space and time, because while we can
freely move back and forth and space, we can't freely move back and forth in time.
Consider an ice crystal.
Its millions of atoms are arranged periodically across space.
This system is in equilibrium, so its properties won’t change with time.
Time crystals, on the other hand, are arranged periodically across space and time.
Because they are both stable and ever-changing, they don’t respect time translation symmetry.
For a decade, physicists have tried to reconcile this with the laws of thermodynamics.
But it turns out they were approaching it from the wrong angle.
Our entire understanding of phases of matter of many-body physics is based on the laws
of equilibrium–thermodynamics.
Everything we know about quantum matter right now is based on these fundamental laws.
My collaborators and I were working in this very different corner of physics.
We were thinking about phases of quantum matter out of equilibrium.
This is a class of systems that always remain stuck in the state in which it started.
And we were asking the question, you know, what are the new kinds of possibilities?
What are the new types of phases of matter that are allowed in an out-of-equilibrium
setting?
We outlined the properties of one of the phases that we found, which we call the pi spin glass.
It has a very unique kind of spatial temporal order.
And we had outlined those details in our paper, but hadn't made the connection with time crystals.
But then
in the course of our paper getting reviewed, one of the referees was like, “Hey, is this
phase of time crystal?”
We were like, “Yeah, you’re right!”
I think if we had set out to find the time crystal, we would have run into a lot of the
same objections that various people who were trying to create these time crystals ran into.
Like in a lot of physics, you know, a lot of things come serendipitously.
This year, Khemani collaborated with Google to create her time crystal in one of the company’s
quantum computers.
What's happening now is that we've gone from asking what exists in nature to what quantum
mechanics allows.
These new devices and new quantum computers are allowing us to prove the full scope of
what's allowed by quantum mechanics.
I think time crystals are just one very dramatic example of how systems out of equilibrium
can display fundamentally new types of physics as compared to systems in equilibrium.
There's so much rich physics to be discovered here.
And we've done so little.
In the 1960s, astronomers spotted a mysterious arc towering above the Milky Way.
They called it the North Polar Spur and assumed that it was stellar debris in our local galactic
neighborhood.
But a Japanese astronomer had a different theory.
Yoshiaki Sofue thought the arc was just a small part of a massive hidden structure:
a pair of bubbles at the galaxy's heart.
For decades, this was dismissed as a fringe idea.
There were a lot of papers explaining why there is no Southern counterpart–like
ever in science, people trying to explain what does not exist.
Peter Predehl spent 25 years developing the orbiting telescope e-ROSITA, which launched
in 2019.
Last year, e-ROSITA conducted the first large-scale
survey of the sky in X-ray–and saw something
astounding.
There is a Southern counterpart of what was known before as the Northern bubble.
And we saw it with e-ROSITA.
The Japanese people–they were the first in thinking about how this is probably something
which is connected to the galactic center.
The Milky Way is like a pancake and we are sitting in the pancake.
There are two bubbles touching each other at the center of this pancake, and they are
filled with hot gas.
These bubbles have a diameter of 50,000 light years.
And this is expanding, these bubbles.
And with the expansion, one can determine that the origin was about 15 or 20 million
years ago.
The culprit behind this ancient galactic calamity may have been Sagittarius A-star, the supermassive
black hole at the center of our galaxy.
Today, it’s mostly quiet.
But long ago, it could have consumed a cloud of hot gas, spewing energy in both directions
with the force of 100,000 supernovas.
With e-ROSITA and other telescopes, astronomers will continue to uncover the enduring mysteries
at the heart of the universe–and at the heart of our galaxy.