In a study published yesterday in Science, physicists describe their attempts to study the overlap between these two theories–by dropping really cold rubidium (only billionths of a degree warmer than absolute zero) from a great height (480 feet). The cold rubidium behaves as an observable, quantum mechanical system and since gravity is a main driver in general relativity, watching gravity’s pull on that system might give researchers glimpses into how to tie the two theories together.
“Both theories cannot be combined,” said researcher [and coauthor of the paper] Ernst Rasel of the University of Hannover in Germany. “In that sense we are looking for a new theory to bring both together.” [Live Science]
Here’s what they did:
Step 1 — Cool it
Physicists first made super-cold Bose-Einstein condensates of rubidium. Since heat is really the random jostling of molecules, to cool things down, experimenters had to make those molecules sit still. They used an elaborate system of lasers to hold the molecules steady.
When rubidium atoms get that cold, they exhibit quantum mechanical behaviors that researchers can observe, acting like one giant particle-wave.
The idea is to chill a cluster of atoms to a temperature that is within a fraction of absolute zero. At that extreme, the atoms all assume the same quantum-mechanical state and begin to behave collectively as a sort of super-atom, known as a Bose-Einstein condensate (BEC). [Nature News]
In this study, researchers contained that complicated system in a two-foot diameter and seven-foot tall cylinder.
Step 2 — Drop it
To test the effects of gravity on that cold glob of atoms, researchers wanted to watch them as they experienced free fall. That’s why they dropped the experiment in a tower at the Center of Applied Space Technology and Microgravity in Bremen, Germany.
The drop shaft, located at the Center of Applied Space Technology and Microgravity in Bremen, is pictured . . . in all its phallic glory. The sample area is magnetically shielded and can have the air evacuated. Samples dropped from the top will experience nearly five seconds at 10-6g before experiencing a cushy landing in an eight meter deep pool of loose polystyrene packing foam. [Ars Technica]
Because the fall time is fairly short, researchers repeated the drop 180 times. During the tests they systematically eliminated other effects on the cold atoms, like magnetic fields in the laboratory, to make sure the atoms only felt gravity’s sway.
The idea was to see whether quantum objects break the rule that says that gravity works on all objects in the same way:
It explains why a pebble and a piano fall at the same speed if dropped from the same roof, despite their different masses. It’s also a necessary first step toward describing the effects of gravity as curvature in spacetime. “It’s a very important cornerstone,” said physicist Ernst Rasel of the Leibniz University of Hannover in Germany. But, he added, the equivalence principle “is just a postulate — it’s not coming out of a law.” So of course, physicists have spent the past century trying to break it. [Wired]
Step 3 — Send it into Space?
The experiment didn’t find evidence that gravity acted differently on a quantum scale–but Rasel and his colleagues are justly proud of creating the experimental conditions that can test such a thing. Because this research created a robust little setup of these very special quantum mechanically behaving atoms, one possible next step would be to watch the atoms during an even longer amount of time in free fall, for example, in orbit around the Earth on the International Space Station.
Rasel is just happy that the experiment survived the first drop:
“I was very worried,” Rasel says of the moments before his team first dropped their experiment. “It was coming towards the end of a PhD thesis of a student,” he adds, explaining that it would have caused serious problems if anything went wrong. [Nature News]
Wired has a video of the experiment, here.
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Image: flickr / sludgegulper