# Q & A: light acceleration

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Most recent answer: 03/07/2017
Q:
You anwsered a Question about the acceleration of light and said " Light travelles at a constant speed in a vacume Therefor no acceleration" But Light has to get to this speed so what is the inital acceleration of light
- Stuart
N.S.W.
A:
it's not actually true that "light has to get to this speed". You've assumed that light has to start at some other speed- presumably zero, i.e at rest. However, there really isn't any such state. Anything at rest with respect to one observer is not at rest with respect to another. Nature gives no sign of knowing which thing is "really" at rest.

So light need not start at one particular low speed and accelerate up. As it happens, any blip of light already starts at the speed of light as it is emitted by some object.

Mike W.

(published on 10/22/2007)

## Follow-Up #1: light bending

Q:
Hello there Well you sure did answer the question about light speeding up well. You're right - in terms of forward acceleration light needs none because its mass is 0 therefore its momentum is infinite therefore the "push" of time causes it to permeate eather with no work. But here's something else to wrap your mind around. When light rays approach a black hole, something funny begins to happen. At the EVENT HORIZON, light is orbiting a black hole so it cannot escape. This orbit is assumed to be very brief and unstable, but still an orbit nonetheless. Newtonian physics dictates that a change in vector is caused by acceleration forces. Tell me then, how light can "bend" with gravity and not accelerate. The only way I think this could happen is in one analogous to refraction.
- Joshua Kellerman (age 19)
Milwaukee, WI, USA
A:
The full answer is complicated, because it involves a spacetime with different properties than the flat one we imagine we live in. However, a partial answer is simple. For a path to bend, there must be acceleration at right angles to the motion. Acceleration in that direction neither speeds up nor slows down the motion. So even in nice classical circular orbits at low speeds, there's acceleration without a change in speed.

I'm not sure I understand any of your first part about infinite momentum and such.

Mike W.

(published on 05/15/2009)

## Follow-Up #2: light traveling through material

Q:
I understand that light is emitted already traveling at "c", so no acceleration. Does this mean that you can describe light traveling through dense objects as being constantly absorbed and re-emitted at "c"? Does this mean that light never actually travels at less than 299,792,458 m/s?
- Aaron
Missouri, USA
A:
Your description really does convey some of the feel for light traveling though material. Ordinary absorption/emission isn't quite the right phrasing, since that usually implies that the outgoing light has a random phase shift from the incoming light, and often a little frequency difference. It's more like a collection of particles systematically scattering the light. But you're right that to the extent that there are any parts of the material which don't have some charged-particle wave in them, the equation describing the light propagation has just the standard vacuum speed. So long as those regions are much smaller than a wavelength and arranged in a regular lattice, the net effect on the light propagation is still just a slowing down.

Mike W.

(published on 10/27/2010)

## Follow-Up #3: light speeding up

Q:
When bending light through a medium like glass or water, light can be slowed down, so when exiting the medium, it must have to return to the "speed of light". Acceleration takes time, thus the speed at which light would return to It's "constant" speed should be measurable... Yes?
- Aj Wills (age 24)
Windsor, Ontario, Canada
A:

You can think of it this way. The slowing of light comes from the plain light wave combining with waves in the material (especially the electrons) to make a net wave that travels more slowly. On the way out of the material, the electron wave functions gets weak over distance scales of about 1 angstrom (10-10 m). So the light wave speeds up as it travels that distance, getting free of the electrons. The time scale would be about that distance over c, or about 10-18 seconds.

Mike W.

(published on 03/07/2017)