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Old Oct 1st 2013, 01:31 PM   #1
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Quantum Mechanics and General Relativity

As someone who is in high school and hence not entirely educated in the methods of doing theoretical physics (but highly interested)... what parts specifically of quantum mechanics and general relativity need to be combined? Is it just a matter of incorporating probability into relativity (and, of course, adding in the other three forces)? Or is there other stuff that needs to happen?
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Old Oct 2nd 2013, 01:42 AM   #2
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Originally Posted by RelativityIsWrong View Post
As someone who is in high school and hence not entirely educated in the methods of doing theoretical physics (but highly interested)... what parts specifically of quantum mechanics and general relativity need to be combined? Is it just a matter of incorporating probability into relativity (and, of course, adding in the other three forces)? Or is there other stuff that needs to happen?
Probably the biggest problem with the Classical theory of gravity (GR) is that there are singularities...the black hole is the best known such. The gravitational force on an object near a black hole approaches the infinite as you get really really close to the singularity inside the event horizon.

Infinities are bad in Physics.

But there is an out to this: If we can come up with a Quantum theory of gravity (which has to be created in any event) we might be able to get rid of the infinities. So far I know of only two theories which will get rid of the infinities are N = 8 supergravity and string theory...in N = 8 the infinities cancel; in string theory they never appear in the first place as far as I know.

-Dan
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Old Oct 2nd 2013, 08:39 AM   #3
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Right, string theory causes it to start growing again below the Planck length (or from the perspective of something inside, it would start growing).

But my question is, what needs to be combined in a quantum theory of gravity, assuming you knew how to cancel things like gravitons interacting with each other?
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Old Oct 2nd 2013, 09:03 AM   #4
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Originally Posted by RelativityIsWrong View Post
Right, string theory causes it to start growing again below the Planck length (or from the perspective of something inside, it would start growing).

But my question is, what needs to be combined in a quantum theory of gravity, assuming you knew how to cancel things like gravitons interacting with each other?
For starters we need to consider the force of gravity between all the particles in a given system. Then we have to worry about field strengths around so-called "point particles." (Every elementary particle is defined as a point particle.) Those are the two biggies. I suppose you could count the properties of space-time itself which isn't really a clear concept in QFT: In QFT space-time points are integrated out so the result of a calculation doesn't depend on space-time. I have heard no really good ideas on that point. But some exist I'm sure.

I see no reason that gravitons have to be canceled out. In fact graviton emission is the same thing as gravity waves and I don't think anyone has any problems with being able to detect that...it's just that the waves have such low energy that we don't have sensitive enough equipment to measure it. We do have indirect proof: neutron stars orbiting each other are predicted to generate large amounts of gravitational energy. Such systems slow the orbital speeds of both neutron stars. Over time we should be able to detect this.

-Dan
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Old Oct 2nd 2013, 11:28 AM   #5
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Older (but not necessarily wiser)

My physics is similarly High School based + a general interest over many (many) years.
I am thus posting without the distraction of any real knowledge...
However, my impression of relativity and quantum theories is that they have a fundamentally incompatible view of reality.
Relativity is based on a Geometrical description.
Quantum Theory is based on Particle Interactions.
Both models are sensible (and extremely accurate) mathematical descriptions of the features they refer to.
However it is probable that BOTH models will have to be discarded in order to find a new model that covers both Quantum and Relativistic worlds.
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Old Oct 2nd 2013, 06:30 PM   #6
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Gravitons would, having energy, interact with each other, creating more gravitons, which interact with each other... etc. Imagine photons having an electromagnetic charge.
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Old Oct 2nd 2013, 06:31 PM   #7
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Oh, another question. What's the equation for time dilation in the presence of energy? Couldn't find it on Google.
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Old Oct 2nd 2013, 10:09 PM   #8
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Originally Posted by MBW View Post
My physics is similarly High School based + a general interest over many (many) years.
I am thus posting without the distraction of any real knowledge...
However, my impression of relativity and quantum theories is that they have a fundamentally incompatible view of reality.
Relativity is based on a Geometrical description.
Quantum Theory is based on Particle Interactions.
Both models are sensible (and extremely accurate) mathematical descriptions of the features they refer to.
However it is probable that BOTH models will have to be discarded in order to find a new model that covers both Quantum and Relativistic worlds.
Actually both are described by geometry. It's just that the Quantum geometries are described in a more complicated way (in general.) This is done for ease of defining and looking for symmetries. But yes, in certain cases there is a definite distinction between the realities they describe. The Quantum reality is the more basic...but to be correct, when we take on a large scale, the theory must predict what the Classical theory defines. (Proving this for Quantum theories is a really nasty task.)

-Dan
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Old Oct 2nd 2013, 10:16 PM   #9
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Originally Posted by RelativityIsWrong View Post
Gravitons would, having energy, interact with each other, creating more gravitons, which interact with each other... etc. Imagine photons having an electromagnetic charge.
Basically correct. Gravitons carry "gravitational charge," that is to say they carry energy. So they interact with themselves. But other theories that have that attribute, that strong and weak nuclear forces, can be handled using renormalization techniques. We have not been able to do this with gravity. The carriers of the electromagnetic force are photons and they do not carry an EM charge. Thus it is a lot easier to handle.

Originally Posted by RelativityIsWrong View Post
Oh, another question. What's the equation for time dilation in the presence of energy? Couldn't find it on Google.
What you are asking for is highly non-trivial. However you should be able to do a search for the "Schwarzschild" solution, the black hole solution to the Einstein equations. But I have to warn you...this is not easy Mathematics.

-Dan
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