Einstein was right: Antimatter falls under gravity
Experiments at CERN have now demonstrated that antimatter falls, validating yet another aspect of the general theory.
A long-standing dispute in physics has just been settled: how would antimatter, the counterpart of matter, behave under the influence of gravity? Einstein’s general theory of relativity implied that antimatter would fall just like matter does, but some physicists have long been suggesting that it should rise upward. Experiments at CERN have now demonstrated that antimatter falls, validating yet another aspect of the general theory.
The results have been published in Nature.
To be sure, antimatter was not known about when Einstein published his general theory in 1915. It was discovered only in 1932. But the general theory treats all matter as identical, which would imply that antimatter and matter should respond to gravitational forces in the same way.
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To define it simply, antimatter is the opposite of matter, or matter with some of its properties reversed. In matter, all elements have negatively charged particles called electrons orbiting their nuclei; in antimatter, the counterpart of the electron is the positron. For protons inside atomic nuclei, the antimatter version is the antiproton.
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When the universe was created as a result of the Big Bang, it resulted in equal amounts of matter and antimatter, or so the theory goes. Antimatter is like matter in most respects, but when the two come into contact, they annihilate each other and their combined mass is converted to energy.
Small amounts of matter are present all over the universe, but their quantities are very small compared to “regular” matter. That is why it has been difficult to test how gravity affects it. At CERN, a collaboration called ALPHA studies antimatter by creating it. This is where the gravity test was conducted.
The ALPHA collaboration has an Antimatter Factory, where special machines create antihydrogen atoms. A hydrogen atom consists of a single electron orbiting a nucleus that contains a single proton; an antihydrogen atom, therefore, has a single positron in orbit around a nucleus that has a single antiproton. After binding antiprotons and positrons together, the Antimatter Factory confines these antihydrogen atoms in a trap that prevents them from coming into contact with regular matter. If they did, they would annihilate each other.
For the test, the magnetic field was switched off, so that the antimatter would indeed get annihilated. An apparatus called ALPHA-g, commissioned in 2021, measured the vertical positions at which the annihilation happened. Researchers trapped groups of about 100 antihydrogen atoms, and then slowly released the atoms over a period of 20 seconds. Then they compared the results with what would have happened if the same operation was carried out with “regular” matter.
Computer simulations predicted that 20% of matter atoms would exit through the top of the trap and the remaining 80% would exit through the bottom. The difference would be the result of gravity, with the bulk of the matter atoms falling downward.
In the experiment, the researchers released groups of 100 antihydrogen atoms several times. They then averaged the results of seven such releases. The results showed that the antimatter atoms exiting through the top and bottom were in the same proportion as predicted by simulations for matter atoms. In other words, the effect of gravity was the same for both: antimatter falls just like matter does.
“It has taken us 30 years to learn how to make this anti-atom, to hold on to it, and to control it well enough that we could actually drop it in a way that it would be sensitive to the force of gravity,” ALPHA spokesperson Jeffrey Hangst said in a statement.