As part of an experiment to measure—to an extremely precise degree—the
charge-to-mass ratios of protons and antiprotons, the RIKEN-led BASE
collaboration at CERN, Geneva, Switzerland, has found that, within the
uncertainty of the experiment, matter and antimatter respond to gravity in
the same way.
Matter and antimatter create some of the most interesting problems in
physics today. They are essentially equivalent, except that where a particle
has a positive charge its antiparticle has a negative one. In other respects
they seem equivalent. However, one of the great mysteries of physics today,
known as "baryon asymmetry," is that, despite the fact that they seem
equivalent, the universe seems made up entirely of matter, with very little
antimatter. Naturally, scientists around the world are trying hard to find
something different between the two, which could explain why we exist.
As part of this quest, scientists have explored whether matter and
antimatter interact similarly with gravity, or whether antimatter would
experience gravity in a different way than matter, which would violate
Einstein's weak equivalence principle. Now, the BASE collaboration has
shown, within strict boundaries, that antimatter does in fact respond to
gravity in the same way as matter.
The finding, published in Nature, actually came from a different experiment,
which was examining the charge-to-mass ratios of protons and antiprotons,
one of the other important measurements that could determine the key
difference between the two.
This work involved 18 months of work at CERN's antimatter factory. To make
the measurements, the team confined antiprotons and negatively charged
hydrogen ions, which they used as a proxy for protons, in a Penning trap. In
this device, a particle follows a cyclical trajectory with a frequency,
close to the cyclotron frequency, that scales with the trap's magnetic-field
strength and the particle's charge-to-mass ratio. By feeding antiprotons and
negatively charged hydrogen ions into the trap, one at a time, they were
able to measure, under identical conditions, the cyclotron frequencies of
the two particle types, comparing their charge-to-mass ratios. According to
Stefan Ulmer, the leader of the project, "By doing this, we were able to
obtain a result that they are essentially equivalent, to a degree four times
more precise than previous measures. To this level of CPT invariance,
causality and locality hold in the relativistic quantum field theories of
the Standard Model."
Interestingly, the group used the measurements to test a fundamental physics
law known as the weak equivalence principle. According to this principle,
different bodies in the same gravitational field should undergo the same
acceleration in the absence of frictional forces. Because the BASE
experiment was placed on the surface of the Earth, the proton and antiproton
cyclotron-frequency measurements were made in the gravitational field on the
Earth's surface, and any difference between the gravitational interaction of
protons and antiprotons would result in a difference between the cyclotron
frequencies.
By sampling the gravitational field of the Earth as the planet orbited the
Sun, the scientists found that matter and antimatter responded to gravity in
the same way up to a degree of three parts in 100, which means that the
gravitational acceleration of matter and antimatter are identical within 97%
of the experienced acceleration.
Ulmer adds that these measurements could lead to new physics. He says, "The
3% accuracy of the gravitational interaction obtained in this study is
comparable to the accuracy goal of the gravitational interaction between
antimatter and matter that other research groups plan to measure using
free-falling anti-hydrogen atoms. If the results of our study differ from
those of the other groups, it could lead to the dawn of a completely new
physics."
Reference:
Stefan Ulmer, A 16-parts-per-trillion measurement of the
antiproton-to-proton charge–mass ratio, Nature (2022).
DOI: 10.1038/s41586-021-04203-w
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