Quantum Mech incompatible with Relativity

[Pjpic]

Is there a consise, yet accessible, description of how quantum mechanics and relativity are incompatible?

 

[f95toli]

Yes, standard quantum mechanics does not include a description of gravity, which is why it is not "compatible" with general relativity (which is basically the theory OF gravitaty).

Note that special relativity was integrated in QM a long time ago, the only "problem" with relativistic QM is that it is mathematically quite complicated which is why it is not covered in undergraduate courses etc.

 

[Pjpic]

Yes, standard quantum mechanics does not include a description of gravity, which is why it is not "compatible" with general relativity (which is basically the theory OF gravitaty).

Note that special relativity was integrated in QM a long time ago, the only "problem" with relativistic QM is that it is mathematically quite complicated which is why it is not covered in undergraduate courses etc.

Thanks for the answer. Is there a (understandable) reason QM does not include a description of gravity?

 

[w3390]

Every tuesday on the history channel a show called The Universe comes on and details cosmic quandaries in great detail. One episode talked about the incompatibility of general relativity and quantum mechanics. The episodes can be purchased on itunes as well.

 

[f95toli]

Thanks for the answer. Is there a (understandable) reason QM does not include a description of gravity?

From a mathematical point of view the problem is that you can't quantize gravity in the same way that you can quantize e.g electromagnetic radiation (which is mathematically actually quite easy to do).
Now, the reason we can quantize electromagnetic interaction is because it is mediated by particles (i.e. photons) and the same is true for all other interactions as well, except for gravity -which as you know- according to GR is just curved spacetime: i.e. in "classical" GR there is no mediating particle (and introducing the graviton doesn't really solve the problem either).

 

[jtbell]

But classical E&M also doesn't have particles (photons). The photons come from the quantization process.

 

[f95toli]

But classical E&M also doesn't have particles (photons). The photons come from the quantization process.

Yes, but the quantization procedure is quite straightforward and can be done by starting from classical EM and introducing a few concepts that are at least "reasonable".
Also, if is usually possible (albeit complicated)to use quantized EM (QED) for ordinary EM problems as well, there is no contradiction between QED and classical EM in limits where they both are useful. But we do not have a quantized theory of gravity that agrees with the predictions of GR.

Moreover, there is plenty of experimental evidence showing that EM really IS quantized, i.e. the photon is "real"; at least to the extent that a single photon detector will register discrete events (not to mention HB&T experiments etc).
But we do not yet -as far as I know- have any experimental evidence for quantized gravity.