Struggles With The Continuum - Part 2 - Comments

[john baez]

The LIGO experiment is very important, but it says nothing at all about quantum gravity, gravitons or string theory. I explained why in the discussion thread here:

• John Baez, Gravitational waves, 11 February 2016.

If you go there you'll see a long conversation with at least 113 comments. I must have answered at least 40 interesting questions about LIGO and gravitational waves. It was quite fun!

 

[atyy]

The LIGO experiment is very important, but it says nothing at all about quantum gravity, gravitons or string theory. I explained why in the discussion thread here:

• John Baez, Gravitational waves, 11 February 2016.

If you go there you'll see a long conversation with at least 113 comments. I must have answered at least 40 interesting questions about LIGO and gravitational waves. It was quite fun!

But if we take GR to be a low energy effective quantum field theory of a spin 2 particle, then in that sense, wouldn't LIGO say something about gravitons in the same way it says something about GR?

 

[vanhees71]

At least it gives an upper limit of the graviton mass.

 

[john baez]

But if we take GR to be a low energy effective quantum field theory of a spin 2 particle, then in that sense, wouldn't LIGO say something about gravitons in the same way it says something about GR?

Okay, if you insist. What I meant is that we don't know anything more about quantum gravity than we did before LIGO discovered gravitational waves. This was a classical experiment, not a quantum one.

At least it gives an upper limit of the graviton mass.

Okay. If someone thought the graviton had a nonzero mass they might be less convinced of that now. Of course we already knew that either the graviton mass is zero or general relativity is wrong.

I would prefer to say LIGO's first result can help us test a prediction of purely classical general relativity: namely, that gravitational waves don't disperse, at least if they're not too strong and their wavelengths are much shorter than the curvature length scale of the spacetime they're in. You can interpret this in terms of gravitons if you like. But we're no more (or less) sure that gravitons exist now than we were a few weeks ago.

 

[vanhees71]

Sure, perhaps one should say that it gives a limit on a mass of the gravitational field. It's analogous to the measurement of a "photon mass" in the context of electromagnetics. You can test this also by, e.g., high-accuracy measurements of Coulomb's law, i.e., with classical em. field situations.