Unifying Quantum and Relativity

[Makep]

While relativity deals with the macrocosm, the quantum mechanics deal with the microcosm. Is there any vague possibility that these two can unite to give us a single unified theory? If so, where do we start, and how?

 

[Vanadium 50]

There is such a thing - relativistic quantum mechanics (and its descendant, quantum field theory).

 

[Fredrik]

Just to clarify what Vanadium said... Quantum field theories are quantum theories of matter formulated in the framework of special relativity. So we're using special relativity to describe the properties of space and time, and quantum mechanics to describe the behavior of all physical systems (matter/energy) in that spacetime. We're also specifically considering those physical systems that can be described by quantum fields (elementary particles). The view of quantum field theory that's held by most theorists these days (see e.g. Weinberg's QFT book) is that consistency with special relativity will make any quantum theory of matter look like a quantum field theory when the energies are low.

Maybe there is an intermediate energy scale that's too high for quantum field theories to be useful and still too low for gravity to be important. I don't know. But I do know that when the energies get high enough, it's necessary to take gravity into account. (E.g. what if the energy density during a collision between particles is greater than the density that general relativity says is sufficient to create a black hole?).

So what physicists want to do is to unify quantum mechanics with general relativity. There are some partial successes, like quantum field theory in curved spacetime, and there are some serious attempts to really unify the two, like string theory and loop quantum gravity. None of those attempts have really succeded yet, but people are still working on them, so I guess we'll have to wait and see.

 

[DrGreg]

Just to clarify what Vanadium said... Quantum field theories are quantum theories of matter formulated in the framework of special relativity. So we're using special relativity to describe the properties of space and time, and quantum mechanics to describe the behavior of all physical systems (matter/energy) in that spacetime. We're also specifically considering those physical systems that can be described by quantum fields (elementary particles). The view of quantum field theory that's held by most theorists these days (see e.g. Weinberg's QFT book) is that consistency with special relativity will make any quantum theory of matter look like a quantum field theory when the energies are low.

Maybe there is an intermediate energy scale that's too high for quantum field theories to be useful and still too low for gravity to be important. I don't know. But I do know that when the energies get high enough, it's necessary to take gravity into account. (E.g. what if the energy density during a collision between particles is greater than the density that general relativity says is sufficient to create a black hole?).

So what physicists want to do is to unify quantum mechanics with general relativity. There are some partial successes, like quantum field theory in curved spacetime, and there are some serious attempts to really unify the two, like string theory and loop quantum gravity. None of those attempts have really succeded yet, but people are still working on them, so I guess we'll have to wait and see.

That is a good answer.

An interesting novel idea in this area is described by Jerzy Jurkiewicz, Renate Loll and Jan Ambjorn (2008), "Using Causality to Solve the Puzzle of Quantum Spacetime"[/color], Scientific American, June 2008. In essence they propose that spacetime is a quantum superposition of fractal metrics, which are self-similar with fractal dimension 2 on the quantum scale (but with a traditional dimension of 4 on the macroscopic scale).

But we'll have to wait to see if this approach is any more or less successful than other approaches.