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June 27, 2017

Wrinkles in Spacetime
Searching for defects in the fabric of the cosmos could help physicists home in on the correct theory of quantum gravity.
by Colin Stuart
FQXi Awardees: Sabine Hossenfelder
March 31, 2016
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Sabine Hossenfelder
Frankfurt Institute for Advanced Studies
Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies, in Germany, is widely known within the physics community for speaking her mind. A prolific blogger, she writes under the pseudonym "bee." As she has recounted, it was a nickname given to her by her mother because the last two syllables of Sabine sound like the German word for the insect, and she admits that a creature which stings is an apt moniker for her words.

Her characteristic sharpness and no-nonsense attitude may well give Hossenfelder just the right qualities for discerning which research avenue is best for physicists to follow, when faced with many competing options. In particular, Hossenfelder is searching for a good theory of quantum gravity—a framework that would bring together Einstein’s theory of gravity, general relativity, which describes how cosmic bodies move, and quantum theory, which governs the behaviour of particles on the smallest scales. "Some people work on problems that I don’t think are problems at all," she says. "But the question of how to find a consistent theory that combines gravity with quantum field theory is one that everyone agrees is a problem—and one that has to have a solution."

A working theory of quantum gravity is needed to explain events in the very early universe and within the core of black holes, which are both instances where strong gravity is confined in a small region. So far, describing what happens in these regimes has defied physicists. "We know the theories we have right now are inconsistent—when you combine them the answer is nonsense," she says. "It is clearly not how nature works, there has to be a better answer."

Shooting Down Theories

The trouble is that there are many contenders for that better answer. Hossenfelder—who has worked on black holes, particle physics beyond the standard model, cosmology, quantum foundations and, most recently, condensed matter physics—is hoping she can shoot some of them down, leaving a smaller range of possibilities to pursue further. The key to unlocking these mysteries of quantum gravity could involve investigating whether spacetime is continuous or discrete, when you zoom in to look at it closely. This is a question that she will spend the next two years investigating thanks to an FQXi grant of $126,000.

People like to talk
about ’atoms of
- Sabine Hossenfelder
Spacetime is the four dimensional fabric conceived of by Albert Einstein in his theories of relativity. Before relativity, the standard picture of gravity said that, for example, the planets in our solar system orbit the Sun because the latter pulls on them with a gravitational force. But Einstein argued that the Sun’s presence warps spacetime around it and the Earth is simply following the curvature of the fabric. Various attempts at a theory of quantum gravity treat this spacetime differently, with around half a dozen suggesting that space is discrete, just as ordinary matter is made up of individual building blocks. "People like to talk about ’atoms of spacetime,’" says Hossenfelder.

Hossenfelder is working on identifying experiments that may be able to probe whether spacetime is indeed discrete, allowing physicists to rule out rival theories that have it pinned as continuous. She’s particularly focussing in on potential imperfections in the discreteness. "If spacetime is not fundamentally continuous, then the smoothness we use in general relativity must have defects in it due to quantum effects in its discrete structure," she says. It is a bit like the defects you find within the lattice of a crystal such as a diamond—the underlying structure does not repeat perfectly. And if a particle travelling through spacetime encounters one of these defects then its energy and momentum will be altered. It is these changes that Hossenfelder is hoping to find experimental evidence for. (See also, "Journeying Through the Quantum Froth.")

Defect Detectors
Particles traveling from distant quasars could reveal whether spacetime is
continuous or discrete.

Credit: ESO/M. Kornmesser
So far she has been working on models involving flat spacetime because it is easier to do the math (arXiv:1401.0276v1). Now she is using the FQXi grant to extend her research into looking at defects in spacetime that is curved. "We don’t live in flat spacetime—we know that the universe expands and that spacetime is curved in the solar system. So if you really want to compare the model to data then you need to get the curved case right," says Hossenfelder. Once the model is in place, experimental physicists could look for particles that have travelled a very long way across the universe—perhaps from distant quasars or the cosmic microwave background radiation, the afterglow of the big bang—to see if their behaviour matches what you’d expect if they’d encountered spacetime defects along the way.

According to Frans Klinkhamer, a theoretical physicist from the Karlsruhe Institute of Technology in Germany, Hossenfelder is taking a "healthy approach" to the problem. He also adds a word of caution, however. "How much progress can be made remains to be seen," he says. But if successful, the pay off would be huge. Measurements that reveal the size and spacing of the defects could tell us much about the underlying theory of quantum gravity, says Klinkhamer. "It would show how classical spacetime emerges from quantum theory."

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