Is the speed of light constant?

Yes. Is there more to this question?

Why wouldn’t it be?

Yes. Not only is it constant, it is the constant. Time can change, slow down speed up, mass can alter but the speed of light does not.

@Lightlyseared I think you redeemed this question, cheers man!

As far as I know, speed of light in the vacuum (denoted by “c”) is constant.
In passing through transparent materials, the observed speed of light differs from c and is frequency-dependent; it is slower than c.
The speed of light in air is only slightly less than c. Denser media, such as water and glass, can slow light much more.

prasad’s got it… just wanted to add that apparently light can also be stopped and restarted…

Provided the light is not interfered with or passes through another medium, the speed of light remains constant and can never exceed its maximum speed of 299,792,458 meters per second.

Beam me up Scotty!

Does the medium through which it moves affect speed? I can’t imagine light moves through water at the same speed it does through air. Or in the airless vacuum of space?

@Foolaholic, yes it does. This is why it gets darker as you go deeper into the ocean – the light is slowed down by each bit of water it goes through until there is simply no energy left. The speed I listed above is the speed in the airless vacuum of space, thus logically its maximum speed.

a few Creationists have said that the speed of light has slowed down in order to justify their beliefs on cosmology. I don’t believe in Creationism, but at the same time, as a non-physicist, I can’t think of any reason why “constants” such as gravity or speed of light should not change. again, as a non-physicist.

Dynamicduo,prasad and funkdaddy gave great answers and i also want to add that gravitation slows light too.

@Christian95; gravitation does not slow light down, but it does distort spacetime, thereby ‘bending’ light. In effect, it changes the direction but not the speed of motion. A sufficiently dense object can distort spacetime to the point that all possible movement through it leads in towards the object. So, from the outside, light is in effect trapped.

@dynamicduo; that is not why the bottom of the ocean is dark. A decrease in the energy of a photon does not correspond to brightness, but rather a shift in color; brightness is the result of photon concentrations. Energy is quantized, or divided into indivisible bits, which mean that a photon cannot just lose a bit of its energy with each interaction. Rather, when a proton interacts with a particle, it has a certain chance of being absorbed as heat or re-emitted. Thus, as light passes through water, some percentage of its constituent photons are absorbed per unit time, ultimately leaving a very sparse concentration- darkness. There is no slowing of the light, although there does appear to be- rather, the re-emission of photons is not necessarily directly along their path before they were absorbed, and so they take an indirect route which, of course, takes longer even though they move at the same velocity. (That last bit after the hyphen may not be an truly accurate description of the cause of the phenomenon- that is what I learned in Sophomore physics, but a glance at Wikipedia seems to indicate that it is somewhat more complicated).

Yes. I believe the basis of the darkness of water as it deepens is purely a case of energy conversion. Conservation of energy states that energy cannot be destroyed, only changed to different kinds of energy. As light strikes water, the energy begins to be absorbed, and most (if not all) becomes heat. The less light is reflected, the more it is absorbed. For example, dark clothing and asphalt heating up more on a sunny day, while lightly colored clothing and surfaces reflect more light, keeping cooler. As for gravity effecting light, it is, as Christian95 stated, more of a side effect of space-time being, in effect, altered/warped/bent as mass increases. This is why light can’t escape a black hole, as it really has nowhere to go. The one caveat there is Kruskal–Szekeres theories involving white holes. Where black holes can be entered but not exited, white holes cannot be entered, only exited. This, however, is theoretical model that is not applicable to black holes caused by gravitational collapse, but rather within theoretical models of eternal black holes (in Kruskal–Szekeres coordinate systems. It gets even more confusing and in depth as the man Hawking suggests that a black hole and a white hole are actually the same thing, where (and forgive me if this is the wrong interpretation) a white hole expels matter in the past, and a black hole pulls it in in the future. I think there may be one or two decent documentaries on this subject if you look through some of PBS.org’s streaming video selections. Hawking discusses black holes a bit in one of his “Into the Universe” documentaries, namely the one on time (as one of maybe two certain ways to “time-travel” to the future), where a capable vessel could orbit a black hole, in effect slowing it’s own time, so when it left orbit, the outside universe will have aged much faster.