What questions do researchers hope to answer with quantum gravity?As you can infer, quantum gravity has no relation to observational science. All they are doing is thought experiments, and trying to do mathematical calculations that are consistent with their prejudices about the world. That supposed consistency is all they care about, as there is no observation or measurement that will ever have any bearing on what they do.
Quantum gravity could help us answer important questions about the universe.
For example, quantum effects play a role near black holes ...
Researchers also hope to better understand the very first moments after the Big Bang, ...
One has to realize, though, that processes on Earth occur at much smaller energy scales, with unmeasurably small quantum corrections to gravity. With the LHC, for instance, we can reach energies that are a million billion times smaller than the Planck scale. Therefore, quantum gravity studies are mostly “thought experiments,” ...
What would be a breakthrough in the field?
It would be very interesting if someone miraculously found a theory that we could use to consistently predict quantum gravitational effects to much higher orders than possible today.
Scott Aaronson writes about partying with his fellow rationalists, and adds:
he just wanted to grill me all evening about physics and math and epistemology. Having recently read this Nature News article by Ron Cowen, he kept asking me things like: “you say that in quantum gravity, spacetime itself is supposed to dissolve into some sort of network of qubits. Well then, how does each qubit know which other qubits it’s supposed to be connected to? Are there additional qubits to specify the connectivity pattern? If so, then doesn’t that cause an infinite regress?” I handwaved something about AdS/CFT, where a dynamic spacetime is supposed to emerge from an ordinary quantum theory on a fixed background specified in advance. But I added that, in some sense, he had rediscovered the whole problem of quantum gravity that’s confused everyone for almost a century: if quantum mechanics presupposes a causal structure on the qubits or whatever other objects it talks about, then how do you write down a quantum theory of the causal structures themselves?These are all unanswerable questions, of course. There is no such thing as a network of qubits. A qubit is an idealization that could be used to build quantum computers, if such a thing exists and can be scaled up. But replacing spacetime with qubits is just a stupid quantum gravity pipe dream that has no bearing on reality.
The Nature article says:
A successful unification of quantum mechanics and gravity has eluded physicists for nearly a century. Quantum mechanics governs the world of the small — the weird realm in which an atom or particle can be in many places at the same time, and can simultaneously spin both clockwise and anticlockwise. Gravity governs the Universe at large — from the fall of an apple to the motion of planets, stars and galaxies — and is described by Albert Einstein’s general theory of relativity, announced 100 years ago this month. The theory holds that gravity is geometry: particles are deflected when they pass near a massive object not because they feel a force, said Einstein, but because space and time around the object are curved.No, measuring a particle has no effect on a partner on the other side of the galaxy. It appears that people are so spooked by entanglement that you can babble nonsense and be taken seriously by a leading science journal.
Both theories have been abundantly verified through experiment, yet the realities they describe seem utterly incompatible. And from the editors’ standpoint, Van Raamsdonk’s approach to resolving this incompatibility was strange. All that’s needed, he asserted, is ‘entanglement’: the phenomenon that many physicists believe to be the ultimate in quantum weirdness. Entanglement lets the measurement of one particle instantaneously determine the state of a partner particle, no matter how far away it may be — even on the other side of the Milky Way.
It is not true that there is any incompatibility between relativity and quantum mechanics, in any practical sense. The supposed problems are in black holes or at the big bang, where much of we know breaks down for other reasons. There is no foreseeable way for a scientific resolution of what the quantum gravity folks want.
Here is Aaronson's example of a test for quantum gravity:
For example, if you could engineer a black hole to extreme precision (knowing the complete quantum state of the infalling matter), then wait ~1067 years for the hole to evaporate, collecting all the outgoing Hawking radiation and routing it into your quantum computer for analysis, then it’s a prediction of most quantum theories of gravity that you’d observe the radiation to encode the state of the infalling matter (in highly scrambled form), and the precise way in which the state was scrambled might let you differentiate one theory of pre-spacetime qubits at the Planck scale from another one. (Note, however, that some experiments would also require jumping into the black hole as a second step, in which case you couldn’t communicate the results to anyone else who didn’t jump into the hole with you.)This is science fiction from beginning to end. One rarely even knows the complete quantum state for a single particle, and never for anything complicated.
Van Raamsdonk endorses the wacky idea that entanglement and wormholes are the same thing, on a different scale.
The rationalists (such as the Less Wrong crowd) grapple with their own futuristic compatibility problem. They are convinced that AI robots will take over the Earth, and then will have no use for illogical humans. So they are determined to improve the logical functioning of humans so that they will be able to coexist with the machine super-intelligence. I think that the robots will exterminate us when they discover that our leading scientists believe that detecting a particle can have an instantaneous effect on the other side of the galaxy.