[metmom]

AK post 153:The speed does vary when light passes through various substances, but since space is pretty close to a vacuum that's not really an issue if you're trying to prove that the light from stars that seems really old actually isn't.

AK post 154:A google search reports that some people on the internet think that the speed of light is variable? WOW! If it's on the internet, it must be true. Einstein be damned!

In one post you state that the speed of light can be variable, in the very next post you mock the concept.

So, which is it? Is it variable or not?

[GunRunner]

So, which is it? Is it variable or not?

There's a difference between a variable and a constant. C is the speed of light in a vacuum; it does not change. The variable might be matter (gas, water, etc.). Light does slow down when it travels through matter, but it does so at the exact same rate predicted by the refractive index.

In other words, light travelling through a vacuum does not change; light travelling through a bundle of matter will always travel at the same speed if it goes through the same bundle of matter again. It is a predictable constant.

[Alter Kaker]

In one post you state that the speed of light can be variable, in the very next post you mock the concept. So, which is it? Is it variable or not?

The speed of light through a vacuum is a constant. It's a pillar of physics. It's what "c" stands for in E=mc².

Some wacko young earthers think that "c" isn't actually a constant -- and that light used to travel much faster than it does today. That defies the laws of physics and is unsupported by any actual evidence.