View Full Version : quantum gravity

apolloman

2010-Mar-22, 01:56 PM

Hello,

I understand Einstein said that gravity is the warping of the space-time fabric due to the presence of mass (rather simplistic maybe but I believe a correct statement).

If so, would the laws of quantum gravity be somehow related to what the fabric looks like at the quantum level ? Do we know what the fabric is made of ?

Third question, would somebody be kind enough to give me a mathematical example of how GR and QM break down when merged ?

Thank you for any knowledge you feel like imparting.

ShinAce

2010-Mar-22, 02:33 PM

Hello,

I understand Einstein said that gravity is the warping of the space-time fabric due to the presence of mass (rather simplistic maybe but I believe a correct statement).

If so, would the laws of quantum gravity be somehow related to what the fabric looks like at the quantum level ? Do we know what the fabric is made of ?

Third question, would somebody be kind enough to give me a mathematical example of how GR and QM break down when merged ?

Thank you for any knowledge you feel like imparting.

In GR, you can look at smaller and smaller distances and the equations are the same.

In QM, everything happens in steps.

I look at it like a staircase. GR is like an accessibility ramp, smooth but a burden to use if your legs are good.

QM is like a staircase, stepped but inaccessiblt to everyone.

Argos

2010-Mar-22, 04:20 PM

Third question, would somebody be kind enough to give me a mathematical example of how GR and QM break down when merged ?

Quantum mechanics has a statistical nature. Thatīs a fundamental distinction, as cited above.

Green44

2010-Mar-22, 05:59 PM

Hello,

I understand Einstein said that gravity is the warping of the space-time fabric due to the presence of mass (rather simplistic maybe but I believe a correct statement).

If so, would the laws of quantum gravity be somehow related to what the fabric looks like at the quantum level ? Do we know what the fabric is made of ?

Third question, would somebody be kind enough to give me a mathematical example of how GR and QM break down when merged ?

Thank you for any knowledge you feel like imparting.

The fabric in a sense is virtual , it only has a presence if mass is present. the only real proof of it is when light with no mass bends around mass,. we can witness this, but it has no real definitive proof, its only a understanding of a theory. Is there Still a fabric if there's no mass? now thats the million dollar question.

GR is analog , QM is digital. at some point when we observe smaller an smaller things the analog GR will switch to digital, and that is when it breaks down,. Im sure you know that analog is consistent and could be found at any location using GR, but QM is digital and it can only be found at specific values/locations, This killed Einstein and he died in his bed trying to find the missing equation that would link the two.

trinitree88

2010-Mar-22, 06:10 PM

Apolloman Consider the free fall of a coconut from your overhead coconut palm while you sleep on a blanket below on a sandy beach. Classically in field theory,it picks up speed continuously as it falls, while in a quantum theory only incrementally allowed speeds would occur, similar to the energy level diagram in the hydrogen atom.The issue is nobody has a measure of the increment....yet.

Green44

2010-Mar-22, 06:18 PM

http://quantummechanics.ucsd.edu/ph130a/130_notes/img1944.png

http://farm4.static.flickr.com/3331/3278124162_b79298b2ea.jpg

hope the links work, but this should shed a bit of light on how QM is digital. Keep in mind that the second link is a 3d object sliced in half,. 2d they would say.

ShinAce

2010-Mar-22, 08:47 PM

To add to the thread.

Quantum mechanics deals with particles. All forces have a corresponding particle that carries the force. We have not found a particle for gravity. It's expected, but currently untested.

If we can't prove a particle for gravity, a quantum theory of gravity stays as a theory.

I think Einstein spent something like the last 30 years of his life finding a field theory for particles and forces. A smooth, continuous theory. This has not been found. Rather, it has been abandoned. I think....

loglo

2010-Mar-22, 08:53 PM

One reason for the failure of Quantum Gravity has to do with the predicted properties of the gauge boson which would carry the gravitational force. A graviton is predicted to have spin +2, but quantum theory does not have a method in which the equation of state of a spin +2 particle can be "renormalised", which means the probabilities can't be arranged to exclude the infinities that arise in most QM calculations.

DrRocket

2010-Mar-24, 05:09 AM

Hello,

I understand Einstein said that gravity is the warping of the space-time fabric due to the presence of mass (rather simplistic maybe but I believe a correct statement).

If so, would the laws of quantum gravity be somehow related to what the fabric looks like at the quantum level ? Do we know what the fabric is made of ?

Third question, would somebody be kind enough to give me a mathematical example of how GR and QM break down when merged ?

Thank you for any knowledge you feel like imparting.

No one knows what a successful theory of quantum gravity will be. There is a lot of ongoing research and not a lot of tangible progress.

The "fabric" of spacetime is an analogy. There is no actual fabric and it is not made of anything so far as is known. But take a look at the notion of the vacuum in quantum field theories. (Google "vacuum state" or something like that).

The most notable breakdown between QM and GR is that QM is a stochastic (probabilistic theory) that predicts only the probabilities of potential outcomes and GR is very much a deterministic theory that makes precise predictions of what must happen, not what might happen.

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