Quantum Physics: A Physicist Makes the Case for a Classical Approach to Gravity
Deep disagreements remain at the heart of physics. General relativity, which describes gravity, clashes with quantum physics. In an attempt to bridge this divide in physics, countless physicists have spent their lives working to build a theory of quantum gravity.
But one physicist is backing a completely different approach. Jonathan Oppenheim believes that gravity may be fundamentally classical, meaning it is not quantum at all. It’s an unconventional idea, to put it mildly.
“When we started, probably 99 percent of our colleagues thought we were crazy, and now that’s down to 70 percent,” says Oppenheim from University College London.
All known forces except gravity are formulated in quantum physics. The prevailing opinion is that gravity will need to be integrated with its quantum peers. But gravity is different, Oppenheim says. While other forces evolve within the fabric of spacetime, gravity is the curvature of spacetime itself. Thus, Oppenheim asserts, “it’s pretty clear that it has to have a quantum nature, in my opinion.”
Physicists have proposed several “no-go theorems” that seemingly prohibit a classical theory of gravity. These theorems highlight contradictions that arise when classical gravity is applied to quantum particles. But these no-go theorems can be circumvented by adding some randomness to how spacetime bends in response to quantum particles, Oppenheim says in his report on December 4 in Physical Review X.
Consider the famous quantum physics experiment, the double-slit experiment (SN: 5/3/19). Particles are sent towards a detector, separated by a barrier with two slits in it. When those particles reach the detector, they create a patterned result known as an interference pattern. This pattern arises because a particle in quantum physics is not restricted to passing through one slit or the other. Instead, it can exist in a superposition state, taking a quantum combination of both possible paths. If the scientist makes a measurement to determine which slit the particle passed through, this pattern vanishes. When particles, in this case light particles called photons, are sent toward a barrier with two slits in it, the particles produce an interference pattern (bands) due to quantum effects. Dorling Kindersley / Getty Images
If the standard classical picture of gravity is correct, it would be possible to measure the gravitational field of those particles precisely enough to determine which slit the particle passed through. This possibility would destroy the interference pattern, even without actually making the measurement. Because scientists observe interference patterns in the lab, this is a major blow to the standard classical theory of gravity.
But the randomness built into Oppenheim’s theory means that the particle has no defined gravitational field, but rather the field fluctuates. This means, contrary to the standard classical version of gravity, that it is not possible to determine which slit the particle passed through by accurately measuring its gravitational field. Particles can pass through the slits in a superposition state, restoring the interference pattern, thereby reinstating the possibility that gravity may be classical.
Experiments can test this theory by searching for evidence of those random fluctuations in gravity, Oppenheim and his colleagues say in their report on December 4 in Nature Communications. “In general, you would measure the mass’s response to a gravitational field precisely,” says Zach Wheeler-Davis, a co-author of the study who completed the work at the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
This is not the first time scientists have proposed a way to reconcile classical gravity with quantum physics. But Oppenheim has been “leading a renaissance,” according to physicist Sudhir R. V. S. V. from the Massachusetts Institute of Technology. Sudhir hopes to test the theory using another type of experiment, which involves measuring the correlations between the motions of two masses interacting gravitationally, he and his colleague said in their report on September 16 on arXiv.org.
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The theory has features that some physicists may find unsatisfactory. For example, the inherent randomness means that the theory is not reversible: unlike other theories, there is no way to start from the end of an interaction and trace its steps backward.
However, even believers in quantum gravity believe that the work has value. “The reason this work is interesting to me is not really because I think gravity is classical,” says Flaminia Giacomini from the Zurich Institute of Technology. The result, she says, is interesting regardless of whether gravity is considered classical or quantum. This is because in order for the experiment to confidently declare that gravity is quantum, scientists need to understand the possibilities of classical gravity. “Only in this way will we be able to strongly prove that gravity is not compatible with a classical description.”
Source: https://www.sciencenews.org/article/gravity-quantum-mechanics-physics-theory
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