Re: Crap physics

Hal Finney (
Thu, 4 Jun 1998 18:43:36 -0700

Michael Lorrey, <>, writes:
> Easy. Since a solid substance has its atoms tightly packed, the gravity
> field exerted by each indvidual atom is very high at a very short distance
> from the nucleus. As gravity field density will change c relative to an
> observer outside that local field density, and the gravity field density
> inside matter is higher than that outside matter, photons traveling
> through solid, or liquid matter, will be observed to slow down while
> traveling inside the matter, then resuming the normal c when outside
> that matter, while if you were the photon, you would notice no difference
> between the two....

I don't believe this is a correct explanation for why photons travel
more slowly through matter than through empty space. It has nothing to
do with gravity, which would be inconceivably small at this scale.

Rather, the photons interact with the electrons around the atom.
In effect, the photons are constantly being absorbed and almost instantly
re-emitted. Phase and other state information is preserved, but the
photons end up being slowed down. Once the photon re-enters vacuum it
is able to proceed unimpeded at c.

It is possible to travel faster than photons within matter. If a charged
particle does so will it will be slowed down by the electrons and will
emit Cherenkov radiation, something like a "sonic boom" but of light.