Galileo can rest easy as Chinese atom dropping experiment fails to disprove law of free-fall

Mainland Chinese scientists have taken Galileo’s gravity experiment on the leaning tower of Pisa to the next level by tracking the free fall of two atoms with unprecedented accuracy.

Lead scientist professor Wang Jin of the Chinese Academy of Sciences' Wuhan Institute of Physics and Mathematics in Hubei province said the experiment far outstripped previous studies.

“The accuracy of our result is ten times to the previous record, which has not been renewed for almost a decade,” he said.

“Galileo can smile in his grave. More than four centuries after, his theory still holds, at least for now.”

The work was detailed in a paper on the latest issue of Physical Review Letters, a journal run by the American Physical Society.

In 1589, Italian scientist Galileo Galilei was said to have climbed to the top of the famous tower in Pisa from which he dropped two iron balls, one large and one small.

To the surprise of spectators, the balls hit ground almost simultaneously, which was contradictory to the common sense belief at the time that heavier objects would fall faster.

Galileo’s law of free-fall was later adopted and expanded by Newton and Einstein, and is now known as the equivalence principle, a corner stone of modern physics.

The equivalence principle asserts that the force of gravity accelerates all objects equally, regardless of their mass, size or material. Einstein’s general theory of relativity was based largely on this assumption.

But Einstein's theory has long had a major problem, it conflicts with what we understand about quantum physics.

For decades physicists have tried to develop a grand theory to unify Einstein’s universe of relativity and the elusive world of quantum mechanics.

Some counted their hope on the collapse of the equivalence principle, which, in theory, could make a bridge to connect the two physical systems.

Scientists have tried to measure the possible change of gravitational pull on different objects, from celestial bodies such as stars to the very smallest particles such as atoms. They also tried to push the accuracy of the measurements to the extreme, with the hope of detecting any distortion of the equivalence principal by quantum mechanics.

The margin of error in the Chinese experiment was refined to hundred millionths, which meant if there was a free-fall time difference between the atoms down to a hundred millionth of a second, it could be detected.

“We did not,” said Wang, whose team had been repeating the experiment for the past six years.

Wang and his colleagues have built the world’s tallest atom free-fall tower. From the height of about a dozen meters, the atoms of two Rubidium isotopes with different masses are released simultaneously to the ground. The team then use very fine laser beams to measure the atoms' speed, similar to how speed guns work to track cars on a motorway.

“It is simple to say but difficult to do,” Wang said.

For instance, the atoms must be held at near-zero speed before release, while the electromagnetic and gravitational disturbance from the surrounding environment must be eliminated to ensure a straight fall of the atoms.

Wang said the Chinese team was in race with many laboratories around the world to push the accuracy to the next level, and their goal was to prove Galileo wrong.