Why is quantum spacetime necessarily random?

[flufffrost]

I was just wondering. I have been looking all over for an answer to this question but I can't seem to find any. I read a biography about Einstein recently and it said that when quantum mechanics first came about he said (something like): "I will not deny its usefulness, only the conclusions people draw from it. God does not play dice with the world." Another time he said something like, "to a man that does not understand lightning, determining just where it will strike is impossible, he would only be able to observe and estimate the probability of it striking one place or another." Can't this apply to quantum mechanics? Or is there some way to prove that what happens at the quantum level is completely random? Couldn't we just be observing the effects of some force we do not know about and cannot describe?

 

[Fredrik]

It's possible, but there are theorems restricting the form of the underlying theory. Its variables would have to be non-local, unobservable, or have some other undesirable property. There's also no experimental evidence that suggests that we might need such a theory.

The thread title asked about spacetime, but your post is about something else entirely. Spacetime must have quantum properties because we know that matter changes the properties of spacetime, and that matter isn't classical.

 

[torquil]

Spacetime must have quantum properties because we know that matter changes the properties of spacetime, and that matter isn't classical.

I disagree. It is not known that spacetime can exist in a superposition of different states just because this is true for matter. That would be a consequence of a theory of quantum gravity, and therefore speculation at the moment.

Nothing is known about how the quantum properties of matter affect gravity, since it is experimentally impossible to check it at this moment. So I think it is a completely open question.

Torquil

 

[Fredrik]

It is not known that spacetime can exist in a superposition of different states just because this is true for matter.

I didn't say anything that specific, but you're right. The correct conclusion isn't that we need a quantum description of space and time, it's that GR can't be right. I do however think that this is an exellent reason to think that spacetime isn't classical.

 

[Frame Dragger]

This is similar to a different thread about 'entangling' spacetime. The best answer is that no current definition of spacetime is one which is useful in answering this question. Frederik and torquil are making this point as well, by way of the nature of their interaction on this point.

I would add: Frederik: Spacetime may be neither Classical NOR Quantum. There's no guarantee that quantum-gravity will do more than open new questions on THAT particular topic. I respect the predictions made, but they are pure guesses.