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hey, since you might be familiar with this: so quantum field theory is actually a departure from QM instead of its continuity ? I'm confused, i thought QM was t

by bsaul 5y ago

hey, since you might be familiar with this: so quantum field theory is actually a departure from QM instead of its continuity ? I'm confused, i thought QM was the best thing we had regarding quantum behaviors... Also, i sometimes saw theoretical physicists talk about "quantum gravity" as if it was a solid theory (i tried to look at some presentation on the topic, but it was way beyond my level)... That made me even more confused, because i was under the impression that chord theory was the only thing we had to unify quantum mechanic and general relativity, and that it was far from accomplished... Could you (or anyone reading this) help me out of my confusion ?

Koshkin 5y ago
QM alone fails to account for certain facts. QFT does this by using somewhat “weird” methods in order to bring Special Relativity and QM together. It has been working pretty well so far, and it is indeed the best we have. String Theory, on the other hand, while being a consistent theory of quantum gravity, uses a model (or models) that we may never be able to verify experimentally.
jfengel 5y ago
Quantum mechanics is the low-energy case of quantum field theory. They have the same kind of relationship as Newtonian mechanics have to general relativity: a very, very different basis that just happens to have a convenient low-energy formulation, where the differences between truth and experiment are too small to be observed. Quantum mechanics can't even begin to talk about quarks. The weak and strong forces just don't exist. And they're very, very different kinds of forces from gravity and electromagnetism. It requires a completely different kind of basis, one based on fields rather than particles. As for "quantum gravity", there's a lot to unpack. There are very sound and useful theories of Quantum Field Theory in curved spacetime, such as semiclassical gravity. That's what Stephen Hawking was working on when he produced his black hole results. But those theories have limitations and aren't a full integration of quantum mechanics with gravity. Unfortunately, that seems to require yet another complete rewrite of the fundamental basis of things, such as strings or loop quantum gravity. And more unfortunately, the differences only occur at stupid high energy levels, so it's practically impossible for us to do experiments to help figure out which one is right.

by bsaul 5y ago

Koshkin 5y ago

QM alone fails to account for certain facts. QFT does this by using somewhat “weird” methods in order to bring Special Relativity and QM together. It has been working pretty well so far, and it is indeed the best we have. String Theory, on the other hand, while being a consistent theory of quantum gravity, uses a model (or models) that we may never be able to verify experimentally.

jfengel 5y ago

Quantum mechanics is the low-energy case of quantum field theory. They have the same kind of relationship as Newtonian mechanics have to general relativity: a very, very different basis that just happens to have a convenient low-energy formulation, where the differences between truth and experiment are too small to be observed. Quantum mechanics can't even begin to talk about quarks. The weak and strong forces just don't exist. And they're very, very different kinds of forces from gravity and electromagnetism. It requires a completely different kind of basis, one based on fields rather than particles. As for "quantum gravity", there's a lot to unpack. There are very sound and useful theories of Quantum Field Theory in curved spacetime, such as semiclassical gravity. That's what Stephen Hawking was working on when he produced his black hole results. But those theories have limitations and aren't a full integration of quantum mechanics with gravity. Unfortunately, that seems to require yet another complete rewrite of the fundamental basis of things, such as strings or loop quantum gravity. And more unfortunately, the differences only occur at stupid high energy levels, so it's practically impossible for us to do experiments to help figure out which one is right.