For
those still holding out hope that antimatter levitates rather than falls in a
gravitational field, like normal matter, the results of a new experiment are a
dose of cold reality.
Physicists studying antihydrogen—an
anti-proton paired with an antielectron, or positron—have conclusively shown
that gravity pulls it downward and does not push it upward.
At least for antimatter, antigravity
doesn't exist.
The experimental results have been
reported in the Sept. 28 issue of the journal Nature by
a team representing the Antihydrogen Laser Physics Apparatus (ALPHA)
collaboration at the European Center for Nuclear Research (CERN) in Geneva,
Switzerland.
The gravitational acceleration of
antimatter that the team comes up with is close to that for normal matter on
Earth: 1 g, or 9.8 meters per second per second (32 feet per second per
second). More precisely, it was found to be within about 25% (one standard deviation) of normal gravity.
"It surely accelerates downwards,
and it's within about one standard deviation of accelerating at the normal
rate," said Joel Fajans, a UC Berkeley professor of physics who, with colleague
Jonathan Wurtele, a theoretician, first proposed the experiment more than a
decade ago. "The bottom line is that there's no free lunch, and we're not
going to be able to levitate using antimatter."
The result will not surprise most
physicists. Albert Einstein's theory of general relativity, though conceived before antimatter was discovered in
1932, treats all matter identically, implying that antimatter and matter
respond the same to gravitational forces. All normal matter, such as protons, neutrons and
electrons, have anti-particles that bear the opposite electrical charge and,
when they encounter their normal matter counterpart, annihilate completely.
An artist's conceptual rendering of antihydrogen
atoms falling out the bottom of the magnetic trap of the ALPHA-g apparatus. As
the antihydrogen atoms escape, they touch the chamber walls and annihilate.
Most of the annihilations occur beneath the chamber, showing that gravity is
pulling the antihydrogen down. The rotating magnetic field lines in the
animation represent the invisible influence of the magnetic field on the
antihydrogen. The magnetic field does not
rotate in the actual experiment. Credit: Keyi "Onyx" Li/U.S. National
Science Foundation
"The
opposite result would have had big implications; it would be inconsistent with
the weak equivalence principle of Einstein's general theory of
relativity," said Wurtele, UC Berkeley professor of physics. "This
experiment is the first time that a direct measurement of the force of gravity
on neutral antimatter has been made. It's another step in developing the field
of neutral antimatter science."
Fajans noted
that no physical theory actually predicts that gravity should be repulsive for
antimatter. Some physicists claim that, if it were, you could create a
perpetual motion machine, which is theoretically impossible.
Nevertheless, the idea that
antimatter and matter might be affected differently by gravity was enticing
because it could potentially explain some cosmic conundrums. For example, it
could have led to the spatial separation of matter and antimatter in the early
universe, explaining why we see only a small amount of antimatter in the
universe around us. Most theories predict that equal amounts of matter and
antimatter should have been produced during the Big Bang that birthed the
universe.
Gravity is incredibly weak
According to
Fajans, there have been many experiments, all indirect, that strongly suggest
that antimatter gravitates normally, but these experiments have been relatively
subtle.
"You
might ask, why not do the obvious experiment and drop a piece of antimatter, a
sort of leaning tower of Pisa experiment? You know, the experiment that Galileo
didn't actually do—it was apocryphal—where he supposedly dropped a lead ball
and a wooden ball from the top of the tower and proved that they both reached
the ground at the same time," he said.
"The real
problem is that the gravitational force is incredibly weak compared to
electrical forces," Fajans added. "So far, it has proved impossible
to directly measure gravity with a drop-style measurement with a charged
particle, like a bare positron, because any stray electric field will deflect
the particle much more than gravity will."
In fact, the
gravitational force is the weakest of the four known forces of nature. It
dominates the evolution of the universe because all matter—theoretically—is
affected by it over immense distances. But for a tiny piece of antimatter, the
effect is minuscule. A 1 volt/meter electrical field exerts a force on an
antiproton that is about 40 trillion times larger than the force of gravity
exerted on it by planet Earth.
The ALPHA
collaboration at CERN suggested to Wurtele a new approach. By 2010, the ALPHA
team was trapping significant quantities of antihydrogen atoms, and in 2011,
Wurtele insisted to Fajans that since antihydrogen is charge neutral, it would
not be affected by electric fields, and they should explore the possibility of
a gravity measurement.
Fajans
dismissed the idea for many months, but was eventually persuaded to take it
seriously enough to perform some simulations that suggested Wurtele's ideas had
merit. UC Berkeley lecturer Andrew Charman and postdoctoral fellow Andrey
Zhmoginov became involved and realized that a retrospective analysis of prior
data could provide very coarse limits on antimatter's gravitational
interactions with Earth.
With help from
their ALPHA colleagues, this led to a paper that concluded that antihydrogen
experiences no more than about 100 times the acceleration—in the up or down
direction—due to Earth's gravity, compared to regular matter.
That
underwhelming start nevertheless convinced the ALPHA team to build an
experiment to make a more precise measurement. In 2016 the collaboration began
to construct a new experiment, ALPHA-g, which conducted its first measurements
in the summer and fall of 2022.
The results published in Nature are based on simulations and a statistical analysis of what the team observed last year and puts the gravitational constant for antimatter at 0.75 ± 0.13 ± 0.16 g, or, if you combine the statistical and systematic errors, 0.75 ± 0.29 g, which is within error bars of 1 g. The team concluded that the chance of gravity being repulsive for antimatter is so small as to be meaningless.
UC
Berkeley postdoctoral fellow Danielle Hodgkinson, right, running the ALPHA-g
experiment from the control room at CERN in Switzerland. Credit: Joel Fajans,
UC Berkeley
At least a dozen UC Berkeley
undergraduate physics majors participated in the assembly and running of the
experiment, Fajans and Wurtele said, many of them from groups not well
represented in the field of physics.
"It's been a great opportunity
for many Berkeley undergraduates," Fajans said. "They're fun
experiments, and our students learn a lot."
A balance
The plan for ALPHA-g that Wurtele
and Fajans proposed was to confine about 100 antihydrogen atoms at a time in a
25-centimeter-long magnetic bottle. ALPHA can only confine antihydrogen atoms
that have a temperature less than half a degree above absolute zero, or 0.5
Kelvin.
Even at this extremely low
temperature, the antiatoms are moving at speeds averaging 100 meters per
second, bouncing hundreds of times per second off the strong magnetic fields at the ends of the bottle. (The magnetic dipole
moment of an antihydrogen atom is repelled by the pinched 10,000 Gauss magnetic
fields at each end of the bottle.)
If the bottle is oriented
vertically, the atoms moving downward will accelerate due to gravity, while
those moving upward will decelerate. When the magnetic fields at each end are
identical, that is, balanced, those atoms moving downward will have, on
average, more energy. Thus, they will be more likely to escape through the
magnetic mirror and hit the container, annihilating in a flash of light and
producing three to five pions. The pions are detected to determine whether the
antiatom escaped upward or downward.
The experiment is like a standard
balance used to compare very similar weights, Fajans said. The magnetic balance
makes the relatively tiny gravitational force visible in the presence of much
larger magnetic forces, much the same way that a normal balance makes visible
the difference between 1 kilogram and 1.001 kilograms.
The mirror magnetic fields are then
very slowly ramped down, so that all the atoms eventually escape. If antimatter behaves like normal matter, more antiatoms—about
80% of them—should escape out the bottom than the top.
"The balancing allows us to
ignore the fact that the antiatoms are all of different energies," Fajans
said. "The lowest energy ones escape last, but they're still subject to
the balance, and the effect of gravity is enhanced for all antiatoms."
The experimental setup also allows
ALPHA to make the bottom magnetic mirror stronger or weaker than the top
mirror, which gives each antiatom a boost in energy that can cancel or overcome
the effects of gravity, allowing equal or greater numbers of antiatoms to go
out the top than the bottom.
"This gives us a powerful
experimental knob that allows us, basically, to believe the experiment actually
worked because we can prove to ourselves that we can control the experiment in
a predictable manner," Fajans said.
The results had to be treated
statistically because of the many unknowns: The researchers couldn't be certain
how many antihydrogen atoms they'd trapped, they couldn't be sure they detected
every annihilation, they couldn't be sure there were not some unknown magnetic
fields that would have affected the antiatom trajectories, and they couldn't be
sure they'd measured the magnetic field in the bottle correctly.
"ALPHA's computer code
simulating the experiment could be subtly wrong because we don't know the
precise initial conditions of the antihydrogen atoms, it could be wrong because our magnetic fields aren't
correct, and it could be wrong for some unknown unknown," Wurtele said.
"Nonetheless, the control provided by adjusting the balance knob lets us
explore the extent of any discrepancies, giving us confidence that our result
is correct."
The UC Berkeley physicists are
hopeful that upcoming improvements to ALPHA-g and to the computer codes will
improve the instrument's sensitivity by a factor of 100.
"This result is a group
effort, although the genesis of this project was at Berkeley," Fajans
said, "ALPHA was designed for spectroscopy of antihydrogen, not
gravitational measurements of these antiatoms. Jonathan's and my proposal was
completely orthogonal to all the plans for ALPHA, and the research would likely
not have happened without our work and years of lonely development."
And while the null result could be
dismissed as unexciting, the experiment is also an important test of general
relativity, which to date has passed all other tests.
"If you walk down the halls of
this department and ask the physicists, they would all say that this result is
not the least bit surprising. That's the reality," Wurtele said. "But
most of them will also say that the experiment had to be done because you never
can be sure. Physics is an experimental science. You don't want to be the kind
of stupid that you don't do an experiment that explores possibly new physics
because you thought you knew the answer, and then it ends up being something
different."
Undergraduate students who participated
include Josh Clover, Haley Calderon, Mike Davis, Jason Dones, Huws Landsberger,
Nicolas Kalem James McGrievy, Dalila Robledo, Sara Saib, Shawn Shin, Ethan
Ward, Larry Zhao and Dana Zimmer.
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