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.
