We have every reason to believe that gravity is inherently a quantum theory. But how can we prove this once and for all? Dr. Sabina Nossenfelder, a theoretical physicist, an expert in quantum gravity and high energy physics, talks about this. Further from the first person.
If you have good eyesight, the smallest objects you can see will be about one tenth of a millimeter: about the width of a human hair. Add technology, and the smallest structure that we have been able to measure so far was about 10-19 meters, which is the wavelength of protons colliding at the LHC. It took us 400 years to go from the most primitive microscope to the construction of the LHC - an improvement of 15 orders of magnitude over four centuries.
The quantum effects of gravity are estimated to become relevant at distance scales of about 10-35 meters, known as the Planck length. This is another 16 orders of magnitude path or another factor of 1016 in terms of collision energy. This makes you wonder if this is possible at all, or if all efforts in trying to find a quantum theory of gravity will forever remain idle fiction.
I am an optimist. The history of science is full of people who thought that much was impossible, but in fact it turned out to be the other way around: measuring the deflection of light in the gravitational field of the Sun, machines heavier than air, detecting gravitational waves. Therefore, I do not consider it impossible to experimentally test quantum gravity. It may take tens or hundreds of years - but if we keep moving, we might one day be able to measure the effects of quantum gravity. Not necessarily by directly reaching the next 16 orders of magnitude, but rather by indirect detection at lower energies.
But out of nothing, nothing is born. If we don't think about how the effects of quantum gravity might manifest and where they might appear, we will definitely never find them. My optimism is fueled by a growing interest in the phenomenology of quantum gravity, a research area devoted to the study of how best to look for manifestations of quantum gravity effects.
Since no one consistent theory has been invented for quantum gravity, current efforts to find observable phenomena are focused on finding ways to test the general features of the theory, by looking for properties that have been found in some different approaches to quantum gravity. For example, quantum fluctuations in spacetime, or the presence of a "minimum length" that will mark the fundamental limit of resolution. Such effects could be determined using mathematical models, and then the strength of these possible effects could be estimated and to understand which experiments could give the best results.
Testing quantum gravity has long been considered out of reach of experiments, judging by estimates, we need a collider the size of the Milky Way to accelerate protons enough to produce a measurable amount of gravitons (quanta of the gravitational field), or we need a detector the size of Jupiter to measure gravitons that are born everywhere. Not impossible, but certainly not something that should be expected in the near future.
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Such arguments, however, concern only the direct detection of gravitons, and this is not the only manifestation of the effects of quantum gravity. There are many other observable consequences that quantum gravity can raise, some of which we have already looked for and some of which we plan to look for. So far, our results are purely negative. But even negative ones are valuable, because they tell us what properties the theory we need may not have.
One testable consequence of quantum gravity, for example, may be symmetry breaking, fundamental to special and general relativity, known as Lorentz invariance. Interestingly, the violations of Lorentz invariance are not necessarily small, even if created at distances that are too small to be observed. Symmetry breaking, on the other hand, will seep through the reactions of many particles at available energies with incredible precision. No evidence of Lorentz invariance violations has yet been found. It may seem sparse, but knowing that this symmetry must be observed with the highest degree of accuracy in quantum gravity, you can use this in developing a theory.
Other testable consequences could be within the weak field of quantum gravity. In the early Universe, quantum fluctuations in space-time should have led to temperature fluctuations arising in matter. These temperature fluctuations are observed today, being imprinted in the background radiation (CMB). The imprint of "primary gravitational waves" on the cosmic microwave background has not yet been measured (LIGO is not sensitive enough for it), but it is expected to be within one to two orders of magnitude of the current measurement accuracy. Many experimental collaborations are working in search of this signal, including BICEP, POLARBEAR and the Planck Observatory.
Another way to test the weak field limit of quantum gravity is to try to introduce large objects into a quantum superposition: objects that are much heavier than elementary particles. This will make the gravitational field stronger and potentially test its quantum behavior. The heaviest objects that we have so far managed to tie into a superposition weigh about a nanogram, which is several orders of magnitude less than it takes to measure the gravitational field. But recently, a group of scientists in Vienna proposed an experimental design that would allow us to measure the gravitational field much more accurately than before. We are slowly approaching the quantum gravity range.
(Keep in mind that this term differs in astrophysics, where "strong gravity" is sometimes used to refer to something else, such as large deviations from Newtonian gravity that can be found near black hole event horizons.)
The strong effects of quantum gravity could also leave an imprint (other than weak field effects) in CMB (relict radiation), in particular in the type of correlations that can be found between fluctuations. There are various models of string cosmology and quantum loop cosmology that study observable consequences, and proposed experiments like EUCLID, PRISM, and then WFIRST may find early indications.
There is another interesting idea, based on a recent theoretical finding, according to which the gravitational collapse of matter may not always form a black hole - the entire system will avoid the formation of the horizon. If so, the remaining object will give us a view of the region with quantum gravitational effects. It is not clear, however, what signals we should look for to find such an object, but this is a promising direction of search.
There are a lot of ideas. A large class of models deal with the possibility that quantum gravitational effects endow spacetime with the properties of a medium. This can lead to light dispersion, birefringence, decoherence, or void space opacity. You cannot tell about everything at once. But, without a doubt, there is still much to be done. The search for evidence that gravity is indeed a quantum force has already begun.
ILYA KHEL