John Page

I'd be very interrsted to hear whether other phenomena seem to acquire velocity instantaneously. Gravity for example - I've read a couple of articles claiming that the effect of gravitation is instantaneous over any distance with one stating that this is necessary to for the coherenece of newtonian physics to explain how centrifugal (or is it centripetal) force is balanced by gravity. On the other hand, another article seemed to say that measurements on GPS satellites confirmed general relativity results that the time of any effect at a distance would need to take into account the different inertial frames of reference. I'm curious about magnetism also.

Cheers, John

BTW the Cereneko Effect link was fascinating and pretty much made sense - if we don't have a means of detecting anything exceeding the speed of light then of course it would appear that c is an absolute limit. I've been trying to imagine if we had no knowledge of light whether physics would have concluded that the speed of sound could not be exceeded (absent clues from the sonic boom of a meteorite).

Cheers, John

Boro Nut

Wow!! And there's silly old me thinking it was only 1-barg less. Ony in America! Presumably.

Boro Nut

Answerer

Well, this is not quite true as photon do have momentum or particle's behaviour and even Einstein claimed that light have an effective mass(whatever that means).

Kenny

Yes, it is true that light travels slower in a medium such as water or air, but this has nothing to do with photons traveling faster than the speed of light. When light travels slower through a medium, it is due to the fact that the photons which make up the light are constantly being absorbed and reemitted by the atoms in the medium. It’s like having a car that instantly acquires 60 miles an hour when you hit the gas and instantly goes back to zero when you hit the breaks – you won’t waste anytime speeding up or slowing down, but you still might have to stop at traffic lights.

God Bless,
Kenny

Kenny

It is true that photons have momentum, but this does not mean they have mass. See my first post here to see how that it possible. It is sometimes said that photons have no “rest mass,” but do have “effective mass” as a result of their motion, but many physicists do not like the concept of rest mass preferring to view rest mass as simply mass and what you have called “effective mass” as merely a consequence of an object’s relativistic momentum. You can associate a sort of mass with photons. The energy of a photon is given by E = hf (where h is Plank’s constant and f is the frequency of the photon). You can also say since E = mc^2, mc^2 = hf, m = hf/c^2. But, this is already fudging things a little (the m in E = mc^2 properly refers to “rest mass” or just plain mass depending on how you want to look at things, not “effective mass” associated with motion) and this mass is a pseudo-mass at best.

God Bless,
Kenny

[ July 02, 2002: Message edited by: Kenny ]</p>

joshack

One utterly useless implication of light's instantaneous acceleration is that the light you see from distant stars is liable to be the exact same "age" as it was when it was created. That's brand spankin' new light!

Kachana

Kenny, I'm a little confused. You said:

And then gave the analogy:

Given that light can only travel at c, would it be more like a car that goes 60mph when you hit the gas, and then turns into something else when you hit the brakes? Or do photons stop for a bit when they are absorbed?

Kenny

Well, to make the analogy complete, I should have said the car ceases to exist and at every red light and then reappears when the light turns green. Photons cease to exist as photons when they are absorbed and become part of the energy of the system that absorbed them. That system can then reemit that energy as photons once again. The actual photons themselves however, never exist at rest or any other velocity besides c.

God Bless,
Kenny

Shadowy Man

Kenny:

Is this really the best way to describe it? If you really do have absorption, i.e. destruction of the photon, and re-emission, i.e. creation of a new one, how do you maintain the direction of
the photon, and more importantly, its phase?

beausoleil

If I may...

<a href="http://www.wikipedia.com/wiki/refractive+index" target="_blank">http://www.wikipedia.com/wiki/refractive+index</a>

Alternatively...

The quantum description is given by quantum field theory of solids: The
approximate Hamiltonian of the coupled system is in terms of the
creation and annihilation operators of light and electron modes with
interaction terms describing absorption of a photon and creation of an
electron mode and vice versa. This Hamiltonian may be diagonalized by a
Bogoliubov transformation replacing photon and electron operators by a
linear combination of them, which may be interpreted as quasi particle
operators of "polaritons", the quanta of the coupled oscillation. The
dispersion relation of the polaritons is a mixture of the dispersion
relations of the light and the polarization wave and is approximately
linear for small wavenumbers / large wavelengths with



\omega \approx ck/n



while the dispersion relation for photons and electrons remains unchanged.

Therefore I'd say one never measures a photon with \omega = ck/n for n
\neq 1 but one could measure polaritons with this relation.

(from <a href="http://www.lns.cornell.edu/spr/2001-04/msg0032182.html" target="_blank">http://www.lns.cornell.edu/spr/2001-04/msg0032182.html</a> )

I couldn't have put it better myself