How quantum mechanics fits with Einstein’s theory of general relativity remains one of the biggest unanswered questions in physics today.
While the former explains the behavior of quarks, photons, and other quantum particles, the latter describes gravity as the warping of space-time. Both theories are widely accepted by the scientific community. However, they appear incompatible.
But now it appears at least one part of Einstein’s theory holds up in the quantum world: the equivalence principle.
The researchers, writing in Science Advances, do not claim to have united the two theories or to have shown gravity is quantum, but they have demonstrated that an effect predicted by Einstein’s equivalence principle does occur on a quantum level — at least, in certain instances.
“This is a unique paper, in the sense that it combines a hard experiment with a far-reaching theoretical interpretation, about one of the most fundamental questions in physics: How can gravity (described by Einstein’s theory of relativity) and quantum theory be unified into one understanding of the universe?” lead author Ron Folman of Ben-Gurion University of the Negev said in a statement.
“These two pillars of modern physics have so far eluded all attempts at a unified theoretical framework, but this complex experiment gives more hints as to how such a unification may be achieved,” Folman added.
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The Equivalence Principle And Einstein’s Elevator
The atom chip used in the experiment was fabricated at Ben-Gurion University of the Negev. In the experiment, the chip was upside down, and the atoms were manipulated just beneath it.
(Image Credit: Ben-Gurion University of the Negev)
Einstein’s elevator is a thought experiment explaining the equivalence principle, according to an arXiv study. It’s based on the idea that gravity locally disappears for an observer who is in free fall.
Essentially, a person in a windowless elevator would be unable to tell if their feet are on the floor because they are on Earth and are experiencing gravity or if they are in space and weightless but are feeling the accelerating elevator’s floor pushing against them.
This theory holds when tested with ordinary objects but, until now, has not been observed in quantum objects.
Testing The Equivalence Principle On Quantum Objects
For this experiment, Folman and colleagues used clouds of ultracold rubidium atoms. These were placed near the surface of an atom chip — a device used to control and manipulate atoms with tiny electronic, optical, or, in this case, magnetic fields. Doing so enabled the team to put the atoms into a quantum superposition, meaning a single particle can exist in multiple forms or locations at once.
Under this scenario, one was placed in a stationary position — a state that was achieved by producing an upward force that canceled out the effect of gravity. Meanwhile, the second was flung upward with a magnetic flick before being allowed to fall under gravity.
Once the fall was complete, a magnetic pulse was used to unite the two parts, creating an interference that the team could use to measure small variations in quantum phase that had built up during earlier parts of the experiment. The measurements matched predictions based on Einstein’s equivalence.
“We have no consistent theory telling us why quantum physics should fail,” co-author Vlatko Vedral, a professor in the department of physics at the University of Oxford, U.K., said in a statement. “This experiment pushes quantum mechanics into one of its most intriguing frontiers, gravity, and shows that, once again, its predictions hold.”
The researchers are cautious in their claims, stressing that the result shows the equivalence principle holds in the precise scenario tested. It might provide clues to how quantum mechanics and Einstein’s theory of gravity connect, but it doesn’t solve the decades-old dilemma.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in Science Advances: Observation of the quantum phase of free fall and the consistency with the equivalence principle
- This article references information from a study published in arXiv : Einstein's Happiest Moment: The Equivalence Principle
