[Lewis Thomson (OP)]

Ken would like help finding answers to this.

"What is the acceleration a light? I know the speed of light, but I never hear of light traveling at anything BUT the speed of light. When I turn on my lamp, I'm assuming the photons start at rest and reach the speed of light instantly. Is the acceleration infinite?"

What do you think? Leave your answers in the comments below...

[Halc]

What is the acceleration a light?

That doesn't parse. Not sure if the question was mistyped or copied incorrectly. Light doesn't accelerate, at least not relative to an inertial frame.

I know the speed of light, but I never hear of light traveling at anything BUT the speed of light.

Relative to any inertial frame, the speed of light in a vacuum is constant at c.  That means that there are two spaceships going the same way but one much faster than the other, they'll both still measure light as travelling at c.

All frames are inertial locally, so no local experiment will show a different speed, but non-local experiments will:
In alternate frames like the rotating frame of my house, or accelerating frame, speed of light is c only locally and varies with distance. So with the rotating frame, speed of eastbound light gets slower with altitude. So relative to say Singapore, eastward light speed drops to zero at an altitude that doesn't quite reach Neptune.
In such a frame, light can come in, slow to a complete stop, and then accelerate back up again, to indefinite high speed. That's acceleration of light, at least in a non-inertial frame.

In accelerating frames, and also its equivalent of varying gravitational potential, light moves faster in all directions at higher potentials, as measured by any one non-local observer. So if I have a ship accelerating at 1 g, the equivalent of us here on the surface of Earth, then light will move a tiny bit faster 10 meters closer to the front of the ship, or 10 meters higher in altitude. So some observer shines light at the moon that gets reflected back is measured to make the round trip in a little less time than it would if it moved at a constant c the whole way. This is because the gravitational potential is higher the entire way than it is on the surface of Earth where our observer is.
Meanwhile, it moves slower to the rear (or lower altitude) in an accelerating frame. In the example of our ship accelerating at a constant 1 g at the observer, light speed (relative to the ship's accelerating frame) falls to zero at about one light-year behind the ship, which means that light emitted further behind than that will never reach the ship, but light emitted closer will eventually reach the ship.

When I turn on my lamp, I'm assuming the photons start at rest and reach the speed of light instantly.

No, a photon cannot be at rest. It doesn't exist at all until emitted (a packet of energy) and it travels at c, or slightly under it since your lamp is probably not in a vacuum. So it doesn't accelerate at all in that sense.

Light can be bent around a gravity source via gravitational lensing. That's acceleration of sorts. It doesn't change the local speed, but it does change the direction the light is going (at least relative to coordinate space), so that's acceleration of a sort, just like the gravity of Earth continuously accelerates the ISS downward at nearly 1G, but its speed relative to Earth remains essentially unchanged at 7.66 km/sec.

[alancalverd]

Classic misunderstanding of photons as particles.

A photon isn't a particle or a wave, we just model electromagnetic radiation as though it were one or the other depending on the nature of the interaction we are discussing. EM radiation can only propagate at one speed, determined by Maxwell's equations,  in a vacuum.

 

[yor_on]

That one was funny, somewhat as if asking how many elephants are needed for earth to stand on. Light don't have a acceleration, it has a recoil which has been proven in experiments but it has no acceleration. It's always locally defined 'c'. It makes sense to assume it to have one though, being a 'particle' and all, but all of that builds on classical science and the way we see things macroscopically. We call it a propagation but it might be a field.
=

It's weirder than you might think, that recoil. If doesn't build on our classical definitions of 'action and reaction', it builds on symmetries and conservation laws. Whatever 'leaves/change' something else must 'produce' a recoil, and it becomes a sort of symmetry. Look up Emmy Noether for that one

https://www-users.cse.umn.edu/~olver/t_/noetherpub.pdf