The concept of spacetime

  • Thread starter Gammaboy
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Hello, I'm new to the forums here but I just wanted to ask a couple of questions.

Is the theory of Relativity unique in its proposal that space and time are interwoven? Basically, was it the first theory to make such a claim that there exists not only space and not only time but spacetime?

Also, is this spacetime believed to be present in all levels, even in the subatomic level?

Does Quantum Mechanics mention anything about space and time or is spacetime something unique to Relativity?

Sorry about the long series of questions and sorry if they seem a little trivial. Thanks.
 

mathman

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The theory of relativity applies to everything. In particular the complete quantum theory required the inclusion of special relativity (Dirac - 1927).
 
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Relativity states that between two events (two handclappings, two blinks of a light or whatever you choose for two events) there is a observer-independent invariant interval, the so- called spacetime interval. With observer we mean inertial (non-accelerating to each other) observers. Relativity was novel in proclaiming this kind of interwoven concept of space and time.
Relativity applies for the entire universe at all levels, even though it can be neglected in calculations where only small velocities/ energies are involved (or little mass, in the case of general relativity). In these cases non-relativistic quantum mechanics is being used.
But to have a complete description of nature, relativity definitely has to be accounted for in quantum mechanics. Then you have quantum field theory.
 
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Ratzinger:
So this "quantum field theory" only includes Special Relativity?
 

pervect

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Gammaboy said:
Ratzinger:
So this "quantum field theory" only includes Special Relativity?
You can do "quantum field theory" in curved space-time. This sort of theory is where Hawking radiation comes from. This is short of a full marriage of GR and quantum mechanics, though, because gravity is not quantized in this theory, it's still treated classically.

Special relativity introduces the concept of space-time as unified idea because space and time get "mixed together" by the Lorentz transform, much as North and East get "mixed together" by rotational transforms.

GR introduces the idea that space-time must be "curved" to explain gravity.
 
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As pervect said - the unification of space and time into spacetime is consequent to the transforms - although Minkowski is frequently credited with being the first to state the principle per se - his contribution was simply a convenient way to describe things graphically.
 
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So this "quantum field theory" only includes Special Relativity?

Yes. It's the marriage of Quantum mechanics and special relativity.
Combining quantum mechanics and general relativty has so far not worked out. Leading canditate for solving that problem is for many string theory.
 

pervect

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Gravity cannot currently be unified with the other forces. The best treatment to date models gravity as curved space-time, and other forces with a fully quantum mechanical model.

This approach combines general relativity and QFT, but because it does not quantize gravity, it is not a "theory of everything" such as M-theory is supposed to be.

The approach does involve the use of GR, because it requires considering space-time as curved, which is a hallmark of GR and not included in SR.

This approach (QFT in curved space-time) is what predicts the Hawking radiation at the event horizon of a black hole.
 

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