is the speed of light always constant?

Answer 1

According to general relativity, in a vacuum "Yes." Light does slow down when it is traveling through materials.

Aloha

Answer 2

The speed of light in a vacuum is constant. Gravity does not alter the speed of light, it warps the space-time light travels through. Light travels more slowly through media other than vacuum, and has been slowed to a crawl in the laboratory.

There is some question as to whether the speed of light has remained constant for all time. A study published a couple of years ago claimed that it had increased very slightly over the last two billion years. However, other studies have contradicted this result and found no change over even longer periods of time.

Answer 3

Yes, but constant at different velocities depending on the stuff the light is traveling in. When saying that c = 186,000 mps, what is often overlooked is that c = 186,000 mps IN A VACUUM.

In something else, like water for example, c is a different, slower rate. This is why a pencil immersed in water appears to bend at the water's surface. Light passes slower through the water than it does in air (which is close to a vacuum speed by the way). So the bottom, immersed half of the pencil's image gets to your eyeball a little later than the top half, above water image does.

PS: One answerer invoked the so-called inflationary epoch of the big bang to point out faster than light speed. That is not a valid example because it was the universe, not the stuff (the mass and energy) in the universe, that expanded faster than light speed. And the speed of light constant applies only to the mass and energy within the universe.

Answer 4

NO it is definitely not. The speed of light is only constant in a vacuum. When light travels through an transparent substance such as water or glass it slows down. The amount it slows down is related to the refractive index of the medium. Don't worry about space-time if you are doing school physics.

Answer 5

Hi. There was a VERY brief time during the Big Bang (assuming that current theories are close to correct) when the universe grew at a rate faster than the speed of light. So "always constant?", maybe not. And Micky T may want to check out this link : http://hep.bu.edu/~superk/cherenkov.html

Answer 6

No. Light can be slowed by many objects. Something like a lens or prism slows down light. The ratio of the speed of light in a vacuum and the speed through an object is called the refractive index.

Mass can also slow down light as shown by gravitational lensing by large mass stars and other stellar objects.

Answer 7

No, no definitely no. Ignore any answer that says otherwise.

The speed of light *in a vacuum* is a universal constant.

The speed of light changes according to the refractive index of any medium (i.e. non-vacuum, e.g. glass, water) that it passes through. It will always be slower travelling through a medium than travelling through a vacuum.

Answer 8

Speed of a moving particle mass structure is dependent on its density. The denser the mass the faster it moves. So the densest mass of the Universe is what moves the fastest.
The formula to determine velocity in a structured medium is as follows;
V^2= Pressure of the structured medium divided by the densitiy to the moving mass.
V^2= P x volume per unit mass

Answer 9

speed of light in vaccum is always contant but if the medium is different the light's speed will change

Answer 10

yes but be careful if its through lets say a water rays will take longer to appear due to rays refracting in different directions so take longer to appear at the other side but technically through no refractions of the rays yes it is constant.