GPLindsey

Question I've been pondering for anyone who knows the answer--

Light travels at 186,000 miles per second in a vacuum. However, when light first leaves a light source--such as a flashlight--is it instantaneously travelling at that speed, or does it accelerate to reach that speed? When we say a car is travelling at 60mph or a plane at 400mph, we know that they did not start out at that speed. An object at rest transitions through lower speeds before it achieves its maximum velocity. So if a photon of light is "at rest" before a flashlight is turned on, does it transition through lower speeds as it accelerates to 186,000 miles per second?

Coragyps

I'm sure that light starts out at full speed - after all, there's no mass to be accelerated. But then, it's been thirtysomething years since I took physics....

Undercurrent

Instead of thinking of light as a particle being "thrown" by a flashlight the way a pitcher throws a baseball, think of it as a being a wave emitted by the flashlight, the way a wave ripples away when you throw a rock into a pond.

The baseball accelerates, the wave emitted from the rock's impact site does not, and is instantly moving at the speed of the wave (although the individual molecules of water accelerate, the wave doesn't.)

Back before QM and reletivity, when people were arguing over the nature of light, this was one of the arguments for light being a wave, instead of a particle. The velocity of a wave is determined entirely by the medium it's in, so we'd expect a wave phenomenon to have a constant velocity in all milieux. A particle's velocity depends on the nature of its source, so if light was a particle, we (if we were late ninteenth century physicists,) would expect to see all different speeds of light.

Of course, the hunt for the medium that light propogates over (the equivalent of the water in the pond) and the attempt to determine earth's motion reletive to it led to the dicovery that light, and the universe, is more fucked up even than that.

So no, light doesn't accelerate.

m.

Kenny

Photons can only travel at c (the speed of light) – no more, no less. That’s because the relativistic momentum of an object is given by the equation p = mu/sqrt(1-(u/c)^2) where m is mass of the object, u is the velocity of the object in one’s chosen frame of reference, and c is the speed of light. As you can see, objects with zero mass have zero momentum (i.e. no mass plus no momentum = no measurable existence), with one and only one possible exception. If u =c and m = 0, then p = 0/0 which is mathematically undefined and potentially equal to any value. This little mathematical loophole is what allows particles like photons to have a zero mass and still maintain a nonzero momentum.

God Bless,
Kenny

[ June 28, 2002: Message edited by: Kenny ]</p>

tronvillain

So, would you say that when light travels through water photons are still travelling at c, even though the light appears to be travelling at less than c? It's possible are taking the long way around I suppose - that when travelling through a medium they do not take a direct path.

Utnapishtim

Well, light from the centre of the sun takes roughly a million years to exit, because it is bounced around inside the star. So the medium can affect the path light takes, but not the speed.

Shadowy Man

The speed of light propagation through different materials is different (and wavelength dependent too, hence dispersion), but always slower than the speed through a vacuum.

rebelnerd

Dumb question time, what about light from distant stars that is blue-shifted due to the star moving toward us? Would this count as acceleration or is the light-speed constant and merely compressed to the blue end of the spectrum?


Filo

Abacus

The speed is the same. The wavelength is different, however.

Leones Homus

Im pretty sire you are right there as light has no mass then the light will not accelerate.