How can we test quantum gravity?

If you have good eyes, the smallest objects you can make out are about a tenth of a millimeter, roughly the width of a human hair. Add technology, and the smallest structures we have measured so far are approximately 10^-19m, that’s the wavelength of the protons collided at the LHC. It has taken us about 400 years from the invention of the microscope to the construction of the LHC – 400 years to cross 15 orders of magnitude.

Quantum effects of gravity are estimated to become relevant on distance scales of approximately 10^-35m, known as the Planck length. That’s another 16 orders of magnitude to go. It makes you wonder whether it’s possible at all, or whether all the effort to find a quantum theory of gravity is just idle speculation.

I am optimistic. The history of science is full with people who thought things to be impossible that have meanwhile been done: measuring the light deflection on the sun, heavier-than-air flying machines, detecting gravitational waves. Hence, I don’t think it’s impossible to experimentally test quantum gravity. Maybe it will take some decades, or maybe it will take some centuries – but if only we keep pushing, one day we will measure quantum gravitational effects. Not by directly crossing these 15 orders of magnitude, I believe, but instead by indirect detections at lower energies.

From nothing comes nothing though. If we don’t think about how quantum gravitational effects can look like and where they might show up, we’ll certainly never find them. But fueling my optimism is the steadily increasing interest in the phenomenology of quantum gravity, the research area dedicated to studying how to best find evidence for quantum gravitational effects.

Since there isn’t any one agreed-upon theory for quantum gravity, existing efforts to find observable phenomena focus on finding ways to test general features of the theory, properties that have been found in several different approaches to quantum gravity. Quantum fluctuations of space-time, for example, or the presence of a “minimal length” that would impose a fundamental resolution limit. Such effects can be quantified in mathematical models, which can then be used to estimate the strength of the effects and thus to find out which experiments are most promising.

Testing quantum gravity has long thought to be out of reach of experiments, based on estimates that show it would take a collider the size of the Milky Way to accelerate protons enough to produce a measureable amount of gravitons (the quanta of the gravitational field), or that we would need a detector the size of planet Jupiter to measure a graviton produced elsewhere. Not impossible, but clearly not something that will happen in my lifetime.

One testable consequence of quantum gravity might be, for example, the violation of the symmetry of special and general relativity, known as Lorentz-invariance. Interestingly it turns out that violations of Lorentz-invariance are not necessarily small even if they are created at distances too short to be measurable. Instead, these symmetry violations seep into many particle reactions at accessible energies, and these have been tested to extremely high accuracy. No evidence for violations of Lorentz-invariance have been found. This might sound like not much, but knowing that this symmetry has to be respected by quantum gravity is an extremely useful guide in the development of the theory.

Other testable consequences might be in the weak-field limit of quantum gravity. In the early universe, quantum fluctuations of space-time would have led to temperature fluctuation of matter. And these temperature fluctuations are still observable today in the Cosmic Microwave Background (CMB). The imprint of such “primordial gravitational waves” on the CMB has not yet been measured (LIGO is not sensitive to them), but they are not so far off measurement precision.

A lot of experiments are currently searching for this signal, including BICEP and Planck. This raises the question whether it is possible to infer from the primordial gravitational waves that gravity must have been quantized in the early universe. Answering this question is one of the presently most active areas in quantum gravity phenomenology.

Also testing the weak-field limit of quantum gravity are attempts to bring objects into quantum superpositions that are much heavier than elementary particles. This makes the gravitational field stronger and potentially offers the chance to probe its quantum behavior. The heaviest objects that have so far been brought into superpositions weigh about a nano-gram, which is still several orders of magnitude too small to measure the gravitational field. But a group in Vienna recently proposed an experimental scheme that would allow to measure the gravitational field more precisely than ever before. We are slowly closing in on the quantum gravitational range.

Such arguments however merely concern the direct detection of gravitons, and that isn’t the only manifestation of quantum gravitational effects. There are various other observable consequences that quantum gravity could give rise to, some of which have already been looked for, and others that we plan to look for. So far, we have only negative results. But even negative results are valuable because they tell us what properties the sought-for theory cannot have.

[From arXiv:1602.07539, for details, see here]

The weak field limit would prove that gravity really is quantized and finally deliver the much-needed experimental evidence, confirming that we’re not just doing philosophy. However, for most of us in the field the strong gravity limit is more interesting. With strong gravity limit I mean Planckian curvature, which (not counting those galaxy-sized colliders) can only be found close by the center of black holes and towards the big bang.

(Note that in astrophysics, “strong gravity” is sometimes used to mean something different, referring to large deviations from Newtonian gravity which can be found, eg, around the horizon of black holes. In comparison to the Planckian curvature required for strong quantum gravitational effects, this is still exceedingly weak.)

Strong quantum gravitational effects could also have left an imprint in the cosmic microwave background, notably in the type of correlations that can be found in the fluctuations. There are various models of string cosmology and loop quantum cosmology that have explored the observational consequences, and proposed experiments like EUCLID and PRISM might find first hints. Also the upcoming experiments to test the 21-cm hydrogen absorption could harbor information about quantum gravity.

A somewhat more speculative idea is based on a recent finding according to which the gravitational collapse of matter might not always form a black hole, but could escape the formation of a horizon. If that is so, then the remaining object would give us open view on a region with quantum gravitational effects. It isn’t yet clear exactly what signals we would have to look for to find such an object, but this is promising research direction because it could give us direct access to strong space-time curvature.

There are many other ideas out there. A large class of models for example deals with the possibility that quantum gravitational effects endow space-time with the properties of a medium. This can lead to the dispersion of light (colors running apart), birefringence (polarizations running apart), decoherence (preventing interference), or an opacity of otherwise empty space. More speculative ideas include Craig Hogan’s quest for holographic noise, Bekenstein’s table-top experiment that searches for Planck-length discreteness, or searches for evidence of a minimal length in tritium decay. Some general properties that have recently been found and that we yet have to find good experimental tests for are geometric phase transitions in the early universe, or dimensional reduction.

Without doubt, there is much that remains to be done. But we’re on the way.

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[This post previously appeared on Starts With A Bang.]