In a 25-foot pit in the basement of Varian Physics Building, Stanford researchers are running an experiment that could unravel Einstein’s most famous theory.

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Courtesy of Jason Hogan

Stanford physicist Mark Kasevich is heading an attempt to verify Einstein’s equivalence principle by dropping atoms down this chute in the Varian Physics Building.

Physics Prof. Mark Kasevich and his lab are testing the equivalence principle, a central tenet of general relativity.

“The equivalence principle says that if I take two objects of different mass and I drop them, they will accelerate at the same rate, assuming you get rid of air resistance and messy forces like that,” explained Jason Hogan, a physics doctoral student in the Kasevich lab.

The Apollo astronauts demonstrated the principle in dramatic fashion by dropping a hammer and a feather on the moon. With no air to interfere with their falls, the two objects hit the lunar dust at the same time.

The equivalence principle is an established part of physics.

“It has been verified to the 13th decimal place,” said Hogan. “Our goal is to push the limits to 15 decimals.”

If the equivalence principle breaks down at that level of precision, there may be serious consequences for the accepted laws of physics.

“The whole theory of general relativity depends on the equivalence principle being exactly true, not mostly true,” Hogan explained. “If the equivalence principle is broken, it means that Einstein’s theory would have to be replaced by something else.”

Einstein’s theory of general relativity explains gravity by saying that matter causes space to curve. Physics textbooks often give the analogy of a bowling ball on a trampoline. The heavy bowling ball makes the trampoline sag. If someone rolls an orange towards the bowling ball, the orange will fall down into the depression created by the heavier bowling ball.

Hogan explained why the equivalence principle is related to general relativity. General relativity states that objects move in paths determined by the curvature of the space around them regardless of their mass.

To test the equivalence principle at the finest level, Hogan and fellow grad student David Johnson will measure the acceleration of falling rubidium atoms. The deep pit in Varian was deemed a perfect place to set up the experiment.

“The longer you watch something fall, the better you can measure its acceleration,” Hogan explained.

The Kasevich team started modifying the pit for their experiment four and a half years ago. But the pit has been a part of Varian for longer than that, and is the subject of much physics department lore.

“I’ve heard rumors from people around campus,” Hogan said, explaining that the first use of the pit was magnetically shielding human subjects while sensitive magnetometers measured their blood flow.

The pit was also used by researchers working on Gravity Probe B, a satellite-based mission designed to measure the space-time curvature near Earth.

When the Kasevich lab moved into the room, Hogan said the pit was covered up with a false floor.

“We saw the blueprints and could see the pit was there,” he said. “We said, ‘That’s awesome, we have to use it.’”