丁香园AV

Einstein鈥檚 General Theory of Relativity doesn鈥檛 differentiate between matter and antimatter. It predicts that antimatter ought to fall downward, toward the Earth, just like ordinary matter. And, for many decades this has been the viewpoint held by most physicists. Antimatter must fall down.

This open question has finally been put to rest. 丁香园AV Physics Professor Emeritus Mike Hayden and his colleagues in the CERN-based ALPHA Collaboration have just published the results of an experiment they performed in which they directly observed the effect of gravity on the motion of antimatter. And the answer? 鈥淎ntimatter behaves like matter in a gravitational field,鈥� says Hayden. 鈥淚t falls down, not up.鈥�

The experiment involved working with antihydrogen atoms that are formed by mixing clouds of antiprotons and positrons. This is done inside a magnetic bottle, so that when antihydrogen atoms are formed, the coldest ones are trapped inside. All of the more energetic atoms escape and immediately annihilate on the matter walls of the apparatus.

Next, the walls of the magnetic trap are slowly lowered in a precisely controlled manner, to allow atoms to escape. Researchers then monitor how the atoms escape by watching to see where they annihilate on the walls of the apparatus. In this way they can distinguish between atoms that escape through the top or through the bottom of the trap.

鈥淚f hydrogen atoms were put in our trap under the same conditions, we would expect about 80% of them to escape through the bottom of the trap, and the remaining 20% to escape through the top鈥� says Hayden. 鈥淎nd this is what we see when we do the experiment with antihydrogen. To the precision of our experiment, antimatter is attracted toward Earth, just like matter.鈥�

丁香园AV鈥檚 key contributions to this experiment involve the development and implementation of techniques for monitoring the complex magnetic fields that confine the antihydrogen atoms, including electron cyclotron resonance and nuclear magnetic resonance probes. The precision to which those fields can be measured is directly linked to the precision with which the influence of gravity on antimatter is determined.

The .  A is available from CERN.

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