We’ve all seen the demonstrations. Even in high school science class, we learn about how gravity affects all objects equally. In 1589 the Italian scientist Galileo Galilei is said to have dropped two spheres of different masses from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass. Sure enough, both spheres hit the ground at the same time.
The concept is known as the universality of free fall, and while it’s been verified via a variety of systems and methods – including one famous demonstration by Apollo 15 astronaut Cmdr. David Scott, who dropped a feather and a hammer while standing on the surface of the moon. The feather and hammer, of course, struck the ground at the same time.
The world of quantum mechanics, however, is wild, counter-intuitive, and sometimes just plain unpredictable. Many physical effects way take for granted simply stop working at the sub-atomic level, or behave very differently. Our understanding of quantum physics implied that there should be some kind of interaction between quantum effects and gravitation.
What They Did
Last June, Zhong-Kun Hu, from Huazhong University of Science and Technology in China, and co-workers published their work testing the principle using rubidium atoms of opposite spin orientation. They found that spin orientation doesn’t matter—the atoms still fall at the same rate.
The two rubidium atoms were cooled to a few millionths of a degree above absolute zero to make them more stable (atoms move more at higher temperatures), and were placed in a vacuum tube.
Using laser beams, they propelled the atoms upwards and measured the rate at which they fell using a technique called “atom interferometry.”
The experiment showed that gravity paid no heed to the atoms’ opposite quantum spins and gave them the same treatment as all other objects: The equivalence principle still prevailed. The atoms fell at pretty much identical rates, meaning that even at quantum scales, the laws of classical physics for gravity still apply.
This is a problem in that there is still no way to connect the two aspects of physics to each other, making the attempt to arrive at a unified theory (seemingly) a vain one.
Hu and colleagues used atom interferometry to measure the free-fall acceleration of the rubidium-87 atoms. In this technique, atomic beams are split and then recombined to produce an interference signal from which the atoms’ gravitational acceleration can be deduced. The team showed that the free-fall acceleration of rubidium-87 atoms of opposite spin is the same to within 1 part in 10 million. The study comes on the heels of other atom interferometry experiments that verified universality of free fall for different atomic species and isotopes to a similar level of precision.
What This Means
Testing the universality of free fall isn’t just a fun exercise. Almost all quantum theories of gravity, which attempt to describe gravity using quantum mechanics, predict the violation of the principle, with free-fall acceleration depending on a quantum particle’s properties, such as its spin. Unfortunately, the experiment was a glorious failure. Since the two atoms fell at precisely the same rate, to within 1 part in 10 million, we now know that we’ve been going in the wrong direction.
Or we think we are. There could be quantum effects on gravitation that we simply lack the ability to measure, though that isn’t likely. The general consensus, though, is that theories involving gravity and quantum effects are probably all wrong – and that, in turn, throws a big wrench into the Grand Unified Theory of Everything.
This is what science is about, though. Keep trying things, write down the results, hypothesize about what ought to happen in related situations, and try it. See what works and what doesn’t. Science isn’t an oracle. It’s a journey of discovery, and there is apparently so much more to discover than we thought there was.
Original source: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.023001
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