Your guess
Since when do we know from measurement that antimatter falls down?
Why it needed testing
According to Einstein's general relativity, all masses fall the same way, whatever they are made of. But in 1915 Einstein didn't know about antimatter. Most theorists expected it to be attracted too. Some, however, worked out what it would mean for the cosmos if matter repelled it.
An atom made of antiparticles
Gravity is by far the weakest of the four fundamental forces. That makes antihydrogen ideal for the test: electrically neutral and stable, a positron orbiting an antiproton. A magnetic trap holds the anti-atoms so they don't touch matter and annihilate.
Opening the trap
In the vertical ALPHA-g apparatus, the team trapped about 100 anti-atoms at a time and released them slowly over 20 seconds. For ordinary matter, simulations predicted that about 20% would escape upwards and 80% downwards. Antihydrogen did the same.
Exactly the same? Still open
The measured value was 0.75 g – g being ordinary gravitational acceleration – with an uncertainty of about 0.2 g. That fits normal gravity and rules out repulsive "antigravity". Whether both fall in exactly the same way is for a more precise measurement with laser-cooled anti-atoms to show.
