There is no such thing in relativity. In general relativity there is density of massenergy that leads to curvature of spacetime. (Which in turn tends to lead to increased density of massenergy. Hence black hole production.)

@ tamurphy:

“Electromagnetic energy has no upper limit for temperature and rate of propagation.”

- In all modern theories of EM it has the latter. I assume you are confusing this with the existence of virtual particles, but they are not real particles with real energy.

As for temperature it is a property of thermodynamics, and doesn’t affect speed of propagation.

- There are conjectures that says there is a maximum temperature, but it is an open question. It is all speculative physics.

FWIW, Smolin seems to be considered a joke by most other theorists. (Spurious comments from working theorists on blogs, so anecdotal impression only.) Maybe the book should have been titled “The Trouble With Smolin”.

@ vaivo:

“Since photons carry energy and momentum, how can they be considered massless?”

They are without invariant mass in relativity. That is allowed, and is the natural state for particles in that theory.

Particle invariant masses, which are also allowed, are thought to be created by the Higgs mechanism. See the recent ballyhoo over LHC and Tevatron results.

“C increases with density of space”

No change of density of space, see above. Massenergy curves space, and particles tries to follow that curvature. If they are invariant massless like the photon they succeed fully.

Curved trajectories, hence lensing possible. The result is indeed analogous to lensing with refractive lenses that slows light as it passes through them. But the physics is not the same. Wikipedia has a real nice article on gravitational lensing with movies and all.

(Can’t wait for the mass artifact to go to a museum!)

@ Jeff:

in very select cases this small deviation from the old value could have massive consequences on complex computer simulations dealing with chaos.

Naively I can’t see how that works. The very idea of chaos is that you can’t repeat all numerical experiments!

@ Koshka:

let’s hope it will help climate science geniuses narrow down the climate sensitivity uncertainty.

Why would they have to do that, the certainty is already high enough to convince of AGW replacing the old Greenhouse Theory? (In the same way GR replaced newtonian gravity; a slight improvement, much better understanding.)

As it happens the exponential rise of the signal over the noise will soon overtake ROI on improving theory and experiment. They can attribute global (and in some cases regional already) observations with 2 sigma certainty as of last year. There is ~ 50 % chance that it will be 3 sigma predictivity at the time of the next IPCC report 2014.

Then the theory will not only be validated beyond reasonable doubt as climate science, but as applied physics.

People, the tone was tongue in cheek. The “speed of light” hasn’t change. The radius of a photon hasn’t changed. None of these constants have fundamentally changed. We just got better measurements of them by using new and/or refined techniques. On any real level, this has absolutely zero impact on anything other than the uncertainty of measurements (that margin has gone down because we narrowed the uncertainty of the constants we use to make measurements) and the numbers posted in the covers of text books if they even go to that many decimal spots.

DON’T WORRY! You won’t outrun your headlights any time soon.