So exactly what is quantum gravity? Well, if you are a graduate student in physics or someone who has been studying modern physics and keeping up with all the latest development in cosmology, you might know what it is about. It sounded esoteric and as a matter of fact, it is sort of esoteric. Einstein formulated the general relativity in 1915 which revolutionized the world of science and it is still the corner stone in today cosmology. Quantum mechanics, on the other hand, has been developed by many other well known physicists ( Niers Bohr, Max Planck, Heisenberg, Schrodinger, etc) starting in the early 1900 and it is a very well established theory and surpassed every test there is in demonstrating the accuracy and precision of the theory even til this day. Both general relativity and quantum mechanics have served beautifully within their own regimes but when physicists tried to unify them together into a single theory, all hell break loose. ( sounded like our political parties or the different religions there are in this world), but I promise not to talk about those here in the blog, but focusing on the issues between quantum mechanics and general relativity and try to understand what might be the major roadblock between the two branches of physics. Do we really need to unify the two or can we just let them serve its own regime and leave it at that?
Physicists like to unify different theories into a single theory to explain our current state of the universe and we have been trying to do that for a long time but unsuccessful. The two branches simply refuse to cooperate and this has been the biggest problem we are facing. Most physicists believe there is a unified theory of everything but due to the broken symmetry at the very beginning of the universe, it has resulted into many different forms but we have been successful into unifying some of it such as the electroweak force, energy mass, electromagnetic field, but we are having trouble unifying the strong force with the electroweak, and when we use equations in quantum mechanics to solve problems in general relativity, we came up with answers that don’t make sense. ( infinity everywhere). As physicists, we tend to brainstorm and try to come up with some reasonable hypotheses and then put them to some tests and see if it is worthwhile to pursue with it and if that’s the case, we have to design experiments in order to verify our theories into scientific facts.
As of today, there are 3 potential candidates ( hypotheses) for unifying quantum mechanics and general relativity, so called theory of Quantum Gravity as general relativity is a law of gravitation using the idea behind curvature of spacetime rather than as an attractive force according to Newton.
The most popular one is string theory. It is based on quantum mechanics suggesting that spacetime is not fundamental but emerged from something that is even more fundamental. According to quantum mechanics, space isn’t empty but filled with virtual particles popping in and out of existence continuously. There is also a very strange phenomenon in quantum mechanics and that is entanglement where every particles regardless of the distances between them, have a mysterious correlation so called quantum entanglement which Einstein called it ” spooky action at a distance”. This entanglement associates with an entropy and string theory suggested that spacetime emerged from this quantum entanglement. In order for the mathematical model established for string theory to work, several ideas have been implemented. 1) There is a need for extra dimensions ( 11 to be precise).2) The existence of super symmetry meaning super particles must be observed. Unfortunately, all the experiments at the LHC have not observed extra dimensions or any of the super particles, but that doesn’t mean they don’t exist. It’s just that as long as those findings are not observed, the mathematical framework for string theory simply will evaporate away. As of now, string theory isn’t testable regardless of the elegant mathematical model and far from being a real science. However, many physicists are still pursuing with the string theory and continue to look for the super particles and finding the missing extra dimensions, but until then, string theory is still being put on hold. Another idea in the string theory is that spacetime emerges from fundamental reality called Ads ( anti- De Sitter space)/ CFT ( conformal field theory correspondence. We can think of CFT as a two dimensional surface of a three dimensional sphere and Ads as its interior and all the connections of the particles within the sphere are through a quantum process called entanglement on the surface. The stronger the entanglement, the closer the space regions are within the sphere. A big challenge called upon this idea of Ads/CFT is that we don’t live in an anti De Sitter space but rather we live in something much closer to de Sitter space where it describes an accelerated and expanding universe. Figuring out how to set up a correspondence for the kind of space that resembles the one that we live in is the most problematic for string theorists.
Another popular candidate which also suggests that spacetime is emergent and tends to unify relativity and quantum mechanics, known as loop quantum gravity. Unlike string theory, loop quantum gravity postulates that spacetime isn’t smooth and continuous the way it is in general relativity, instead it is made of discrete components, chunks or atoms of spacetime. These atoms of spacetime are connected in a network with one, and two dimensional surfaces to form spin foam. These spin foams are limited to two dimensions but gives rise to our four dimensional world, three of space and one of time. While string theory suggests that spacetime emerges from the behavior of seemingly unrelated systems in the form of entanglement, loop quantum gravity talks about the emergence of spacetime very much like sloping dune emerges from the collective motion of sand grains in wind. Both string theory and loop quantum gravity suggest that spacetime emerges from some underlying reality and like string theory which has failed to live up to its promise in unifying gravity and quantum mechanics, loop quantum gravity isn’t testable although it has recently gained lots of recognition. In order to accept these ideas as science, they must be put to tests but we already have seen the various obstacles that they are facing and this is precisely what we do as scientists to keep searching for the answers because those ideas about string theory and loop quantum gravity are actually very good and they do have the potentials in unifying general relativity and quantum mechanics into a theory of everything, but what we are lacking is solid evidence. Most physicists are not ready to give up on these ideas and continually looking for data or some other possible candidates which can do the same thing.
Causal set theory, another contender of quantum gravity, suggests that space and time are made of more fundamental components Physicists get really excited when different theories agree on at least something together. Modern physics is a victim of its own success. General relativity and quantum mechanics serve so beautifully well within their own territory but we need one theory by combining these two best theories together and we see lots of problems. Quantum gravity is needed to describe extreme conditions such as the center of a black hole or near the big bang but it would take a particle accelerator about the size of a typical spiral galaxy in order to generate the kind of power and energy to get to it and it is simply not possible. The lack of such experimental data is exactly what is taking so long for physicists to work on quantum gravity More experiments on entanglement are needed to study the Ads/ CFT correspondence for the string theory and the need to find those missing dimensions as well as the super particles in order for the super symmetry to come through.
Will we ever know the real nature of space and time? The fact that the ancient Greeks asked questions like ” what is space? ‘ and ” what is time?”. Despite the fact that we have come a long way in developing modern science to this 21st century, we are still asking these questions and we don’t really seem to have any definite answers, only finding it more difficult and complex to deal with these days!!!!