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My mom is a, now-retired, high school teacher. As teenager I thought this was a great job and wanted to become a teacher myself. To practice, I made money giving homework help but discovered quickly I hated it for a simple reason: I don’t like to repeat myself. I really don’t like to repeat myself.But if I thought spending two years repeating how to take square roots - to the same boy - was getting me as close to spontaneous brain implosion I ever wanted to get, it still didn’t quite prepare me for the joys of parenthood. Only the twins would introduce me to the pleasure of hearing Jingle Bells for 5 hours in a row, and re-reading the story about Clara and her Binky until the book mysteriously vanished and will not be seen again unless somebody bothers to clean behind the shoe rack. “I told you twice not to wash the hair dryer,” clearly wasn’t my most didactic moment. But my daughter just laughed when the fuse blew and the lights went off. Thanks for asking, we got a new dryer.
And so I often feel like I write this blog as an exercise in patience. Nobody of course bothers to search the blog archives where I have explained everything. Sometimes twice! But today I will try to be inspired by Ethan who seems to have the patience of an angel, if a blue one, and basically answers the same questions all over and over and over again. So here are answers to the questions I get most often. Once and forever I hope...
- Is string theory testable?
The all-time favorite. Yes, it is. There is really no doubt about it. The problem is that it is testable in principle, but at least so far nobody knows how to test it in practice. The energy (densities) necessary for this are just too high. Some models that are inspired by string theory, notably string cosmology, are testable with existing experiments. That it is testable in principle is a very important point because some variants of the multiverse aren’t even testable in principle and then it is indeed highly questionable whether it is still science. Not so though for string theory. And let me be clear that I mean here string theory as the candidate theory of everything including gravity. Testing string theory as means to explain certain strongly coupled condensed matter systems is an entirely different thing.
- Do black holes exist?
Yes. We have ample evidence that supermassive black holes exist in the centers of many galaxies and that solar-sized black holes are found throughout galaxies. The existence of black holes is today generally accepted fact in the physics community. That black holes exist means concretely that we have observational evidence for objects dense enough to be a black hole and that do not have a hard surface, so they cannot be a very dim stars. One can exclude this possibility because matter hitting the surface of a star would emit radiation, whereas the same would not happen when the matter falls through the black hole horizon. This horizon does not have to be an eternal horizon. It is consistent with observation, and indeed generally believed, that the black hole horizon can eventually vanish, though this will not happen until hundreds of billions of years into the future. The defining property of the black hole is the horizon, not the singularity at its center, which is generally believed to not exist but for which we have no evidence one way or the other.
- Why quantize gravity?
There is no known way to consistently couple the non-quantized theory of general relativity to the quantum field theories of the standard model. This only works in limiting cases. The most plausible way to resolve this tension is to quantize gravity too. It is in principle also possible that instead there is a way to couple quantum and classical theories that has so far been missed, or that the underlying theory is in some sense neither classical nor quantum, but this option is not favored by most researchers in the field today. Either way, the inconsistency in our existing theories is a very strong indication that the theories we have are incomplete. Research in quantum gravity basically searches for the completion of the existing theories. In the end this might or might not imply actually quantizing gravity, but Nature somehow knows how to combine general relativity with quantum field theory, and we don’t.
- Why is it so hard to quantize gravity?
It isn’t. Gravity can be quantized pretty much the same way as the other interactions. It’s just that the theory one arrives at this way cannot be a fundamental theory because it breaks down at high energies. It is thus not the theory that we are looking for. Roughly speaking the reason this happens is that the gravitational equivalent of a particle’s charge is the particle’s energy. For the other known interactions the charge and the energy are distinct things. Not so for gravity.
- Is quantum gravity testable?
Again, yes it is definitely testable in principle, it’s just that the energy density necessary for strong quantum gravitational effects is too high for us to produce. Personally I am convinced that quantum gravity is also testable in practice, because indirect evidence can prevail at much lower energy densities, but so far we do not have experimental evidence. There is a very active research area called quantum gravity phenomenology dedicated to finding the missing experimental evidence. You can check these two review papers to get an impression of what we are presently looking for.