Can General Relativity and Quantum Mechanics be two separate things?

[Genecks]

I'm reviewing the Theory of Everything wikipedia page, and I have this question: Can GR and QM be two separate things?

 

The argument is that there may be something that meshes them together. However, can they be two separate things? And if so, wouldn't there be some underlying thing that enables them to be two seperate things (that itself could be argued to be a ToE, I think).

[swansont]

I'm reviewing the Theory of Everything wikipedia page, and I have this question: Can GR and QM be two separate things?

 

The argument is that there may be something that meshes them together. However, can they be two separate things? And if so, wouldn't there be some underlying thing that enables them to be two seperate things (that itself could be argued to be a ToE, I think).

 

GR fails to explain gravity and related phenomena at quantum scales. You need something new to do that, and since that's the scale where quantum effects are important, the answer is likely something that involves QM.

 

I don't know enough about the details to how long the odds are that a modification to GR would work that didn't involve QM, other than it would probably be a massive longshot.

[StringJunky]

 

GR fails to explain gravity and related phenomena at quantum scales. You need something new to do that, and since that's the scale where quantum effects are important, the answer is likely something that involves QM.

 

I don't know enough about the details to how long the odds are that a modification to GR would work that didn't involve QM, other than it would probably be a massive longshot.

Are gravitational effects significant within quantum domains i.e atoms in the presence of the other three forces?

Edited July 7, 2014 by StringJunky

[mathematic]

Are gravitational effects significant within quantum domains i.e atoms in the presence of the other three forces?

Inside an atom gravitational effect is not important. However when trying to understand what is going on inside a black hole, the two theories, as they stand, give contradictory results.

[imatfaal]

The gravitational interaction of two protons a/o electrons is in the order of 10^37 (I think) weaker than the electromagnetic

[latex]|F_{grav}|= G \frac{m_1m_2}{r^2}[/latex]

 

[latex]|F_{em}|=\frac{1}{4 \pi \epsilon_0} \frac{q_1 q_2}{r^2}[/latex]

 

using protons and orders only

 

[latex]|F_{grav}|= 10^{-11} \frac{10^{-27} 10^{-27}}{r^2}[/latex]

 

[latex]|F_{em}|=10{^{10}} \frac{10^{-19}10^{-19}}{r^2}[/latex]

 

Which is 37 orders of magnitude difference. Electrons would be even more difference

[StringJunky]

Should it follow that gravity has to be quantised because the other three are ...may scientists be trying to group a vegetable with fruit?

[imatfaal]

Should it follow that gravity has to be quantised because the other three are ...may scientists be trying to group a vegetable with fruit?

 

Yes quite possibly - but science is based on intuitive thinking backed up by rigorous experimentation and formalised in maths. This part of physics is based on the leap of imagination that all four forces are actually manifestations (an avatar in the old sense - only kidding) of a single force (would only be single at very very high energies). We have managed to model the other three forces together through the use of quantised fields, gauge bosons, etc. - so why not gravity. Even if we are wrong we will still learn. And the beauty is that this all ties in with cosmology as the only time we can think of that this might be really relevant is soon after the big bang - from the smallest to the largest field of study.

[StringJunky]

 

Yes quite possibly - but science is based on intuitive thinking backed up by rigorous experimentation and formalised in maths. This part of physics is based on the leap of imagination that all four forces are actually manifestations (an avatar in the old sense - only kidding) of a single force (would only be single at very very high energies). We have managed to model the other three forces together through the use of quantised fields, gauge bosons, etc. - so why not gravity. Even if we are wrong we will still learn. And the beauty is that this all ties in with cosmology as the only time we can think of that this might be really relevant is soon after the big bang - from the smallest to the largest field of study.

Yes. With my little knowledge I'm also inclined to think it has a property in common with the other three and they mutually emerged via some phenomenological path from a more unified source. Still, as you say, the cognitive leap is still large and gravity is smooth whereas the other three are not ...a very big bridge to reconcile I would imagine for those that are trying to. Is this the primary problem, that it won't chop into pieces?

[imatfaal]

Yes. With my little knowledge I'm also inclined to think it has a property in common with the other three and they mutually emerged via some phenomenological path from a more unified source. Still, as you say, the cognitive leap is still large and gravity is smooth whereas the other three are not ...a very big bridge to reconcile I would imagine for those that are trying to. Is this the primary problem, that it won't chop into pieces?

 

Well it is 37 orders of magnitude smaller than the EM - and we have only just (within 100 years) realised that the photon is a quantum messenger particle of the EM force. We need to be looking at the gravitational interaction of individual particles (like we can with the EM) before we will be able to recognise the implicit quantum nature of gravity; if it exists that is. My history is not good - but I think the lumpiness of the photon was discovered whilst the other two forces were found to be (or to be excellently modelled as) lumpy through the development of the respective quantum field theories rather than observationally.

[Sensei]

My history is not good - but I think the lumpiness of the photon was discovered whilst the other two forces were found to be (or to be excellently modelled as) lumpy through the development of the respective quantum field theories rather than observationally.

Cloud chamber was showed to public in 1911.

Wilson saw the first time traces leaved by cosmic rays in 1894.

 

Oil drop experiment, to measure quantization of charged particles, was done in 1909. Published in 1913.

Edited July 9, 2014 by Sensei

[mathematic]

What do you mean by "lumpiness" of photons?

[imatfaal]

What do you mean by "lumpiness" of photons?

 

Light and all EM-Radiation is not continuous - it comes in little packets, quanta, discrete bits that we call the photon. Classical theories of light would allow a smooth scale of absorption and emission of light - after Einstein (and Planck) it was shown that to fit experimental evidence we should consider light as made up of lots of discrete quanta.

 

http://en.wikipedia.org/wiki/Photon

The joining of this model of the photon as a discrete object and Maxwell's unbelievebly accurate and neat wave theory of electromagnetic radiation is called quantum electro dynamics - it is one of the quantum field theories I referred to above. QED, along with QCD (quantum chromo dynamics), allows us to describe the Strong, the Weak, and EM forces. We are searching for the Quantum Theory of Gravity. Once we have a quantum theory of gravity we hope to be able to show that it can be seen as a manifestation of a unified force (like the other three).

[swansont]

The quantization represented by the photon was proposed well before QED was developed. That was in 1905, with the photoelectric effect paper that won Einstein the Nobel.

[mathematic]

 

Light and all EM-Radiation is not continuous - it comes in little packets, quanta, discrete bits that we call the photon. Classical theories of light would allow a smooth scale of absorption and emission of light - after Einstein (and Planck) it was shown that to fit experimental evidence we should consider light as made up of lots of discrete quanta.

 

http://en.wikipedia.org/wiki/Photon

The joining of this model of the photon as a discrete object and Maxwell's unbelievebly accurate and neat wave theory of electromagnetic radiation is called quantum electro dynamics - it is one of the quantum field theories I referred to above. QED, along with QCD (quantum chromo dynamics), allows us to describe the Strong, the Weak, and EM forces. We are searching for the Quantum Theory of Gravity. Once we have a quantum theory of gravity we hope to be able to show that it can be seen as a manifestation of a unified force (like the other three).

My confusion was not because of the discrete nature of photons, but simply the use of the word "lumpy" to describe it.

 

 

 

 

 

 

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to describe it.

[StringJunky]

My confusion was not because of the discrete nature of photons, but simply the use of the word "lumpy" to describe it.

He used that word because in my post I described gravity as 'smooth' and he understood what I meant. replying in the same analogous way.

[MigL]

At Planck scales/energies all four forces are expected to be equivalent in strength, ie unified. At this scale even gravity, or more appropriately, space-time, is thought to be 'discontinuous or 'granular'.