Most physicists take it as given that spacetime must be quantized to fit neatly into the framework of quantum field theory. Jonathan Oppenheim isn’t so sure.
Jonathan Oppenheim has been described as a “heretic” in theoretical physics for his unconventional theory of spacetime. But his brand of heresy seems to be gaining ground.
“Initially, 99 percent of my colleagues thought I was a crackpot,” jokes Oppenheim, a professor at University College London. “Now, 70 percent of them think so.”
Oppenheim’s theory remains controversial for good reason: it suggests that the central problem in theoretical physics — a unification between the fundamental theories of gravity and quantum mechanics — might be the wrong target.
The big question: Is spacetime quantum?
Whereas prevailing wisdom suggests that achieving this unified theory requires spacetime to be described through a quantum framework, Oppenheim questions whether spacetime needs to be quantized at all.
What if, he asks, gravity is unique from all the other forces in that it simply defies quantization?
“General relativity is completely different from the forces of nature that we’ve been able to quantize,” he explains on a video call from Vienna, where he’s working with experimental colleagues to test his theories.
“General relativity is not a gauge theory in the usual sense. Gravity is not even a force in the usual sense — it’s spacetime geometry.”
This distinction is important because it opens the door to alternative theories of gravity — theories that preserve spacetime’s classical essence while still addressing the tension between general relativity and quantum mechanics.
The experimental squeeze
Oppenheim and his colleagues have developed an experimental framework to detect (or rule out) quantum properties of spacetime. Central to this work are stochastic fluctuations — random disturbances in spacetime which he and his students have proven must exist in any theory in which spacetime remains classical.
“Precision measurements establish an upper bound on noise, while interference experiments set a lower bound,” he says. “Together, we call this the ‘squeeze.’”
One side of this “squeeze” comes from experiments like the LISA Pathfinder, which measures minuscule accelerations between two test objects in space. “These experiments may see noise due to the fluctuations in spacetime that the theory predicts, or they may have other causes,” says Oppenheim. “Either way, these set an upper bound on stochastic fluctuations in spacetime.”
On the other side are interference experiments with dense particles, which establish a lower bound for the coupling constant governing these fluctuations. By narrowing the window of possibilities from both directions — squeezing upper and lower bounds closer together — Oppenheim hopes to determine whether spacetime truly displays quantum properties or something else.
Heretical ideas and friendly wagers
Oppenheim is well aware that his ideas often put him at odds with the mainstream physics community – but he’s OK with those odds.
In fact, he accepted a friendly wager with physicists Carlo Rovelli and Geoff Penington, who bet him (with 5,000:1 odds) that spacetime is better explained through a quantum framework than Oppenheim’s proposed non-quantum formulation.
The wording of their wager reads:
“Whereas Carlo Rovelli and Geoff Penington firmly believe that gravity is better described by a quantum theory. And whereas Jonathan Oppenheim is more sympathetic to other possibilities (i.e., doesn’t have a clue), Oppenheim and Rovelli/Penington accept a wager at 1:5,000 odds that spacetime is described by quantum theory.”
The prize to the winning side of the wager has yet to be finalized, though the parties agreed in principle to rewards along the lines of “some crisps, a bazinga ball, a small amount of olive oil, balsamic vinegar, or wine.”
While the wager adds a bit of levity to the pursuit, Oppenheim is serious about his research agenda and believes it will continue to gain traction, especially if it continues to be consistent with experimental observation.
While Oppenheim has been referred to as a heretic, he insists his ideas are conservative. “We now have a theory of gravity which is mathematically consistent with quantum theory, and which thus far appears to be consistent with observation, so I feel compelled to take it seriously,” he says.
“What’s more, if spacetime is classical, several long-standing puzzles in physics, such as the measurement problem and the black hole information paradox, lose much of their enigmatic nature. And the stochastic fluctuations predicated by the theory may even be an alternative explanation for dark matter.”
In pursuit of truth
For Oppenheim, the pursuit of these questions is as much about personal fulfillment as scientific discovery. He has always been a problem solver and unorthodox thinker; as a student at the University of British Columbia, for example, he founded the Edible Ballot Society, which satirically endorsed eating ballots to protest problems in electoral politics.
Accustomed to sitting outside the mainstream, Oppenheim believes in the value of challenging conventional wisdom.
“I’m drawn to this work because it feels very clear to me that these questions are worth asking, even if some researchers seem certain they know the answers,” he says. “Physics requires a diversity of approaches. We should remain humble about how strongly we hold certain beliefs – nature has a way of exposing our prejudices as foolish.”
As Oppenheim continues to refine his experiments, he remains optimistic about the possibility of a breakthrough that could push physics in a radical new direction.
“I don’t know how far we are from the truth,” he admits. “But what's exciting is that we can now turn to experiments to guide us. So, we'll find out the truth of it soon enough.”
