How can it be that light moves at the same speed to all observers if the observers are moving?

Question

Please explain thoroughly.

It just does, c'mon man :P

Answer 1

Welcome to the wonderful world of relativity.  It just does.

Answer 2

The simplest way to explain it, is that the speed of light is constant, and the speed of time isn't.

Let's say that there are two spaceships traveling along parallel paths. One is traveling at the rather slow speed of 1/10,000 the speed of light. The other spaceship is traveling at very near the speed of light. As they pass a given point in space they each switch on their headlights. From within each spaceship they each see the light traveling away from them at the speed of light. To us, an outside observer, it appears as though the light is doing just what it is supposed to do. It is traveling at the speed of light. We see it very rapidly outdistancing the slow moving spaceship, and very very slowly pulling away from the fast moving spaceship.  The two are moving at almost the same speed.

The question is, how can we each see the light as moving at the exact same speed. For the people in the fast moving spaceship, shouldn't they be able to see that the light is only slowly moving away from them. If they're going fast enough, they should almost be able to keep up with it. How is it that they see the light pulling away from them at 186,000 miles per second, yet to us they're almost keeping up with it.

The reason that the people in the fast moving spaceship still see the light as moving away from them at the speed of light, while to us, the observer, it is just barely outpacing them, is because time is moving differently for each of us. For the people in the fast moving spaceship time has slowed way down. Their time is barely moving at all compared to ours. To us it may take hours or days for the light to get 186,000 miles ahead of the fast moving spaceship, but to them, it will take one second.

The faster that we move through space, the slower that we move through time. So light is always moving away from us at 186,000 miles per second.

If we get in our spaceship and go flying around, it doesn't matter how fast we go, or how slow we go, light will always be moving 186,000 miles per second faster than we are. The speed of light remains constant, but the passage of time changes to compensate for our changes in speed.

That's basically how light can always be seen to move at the same speed for all observers. The speed of light is the one constant in our universe. The speed of time isn't constant, in fact space itself isn't constant. It contracts and expands with speed, just like time does. But that's a whole other story.

Answer 3

We do not know why light should travel through a vacuum at a constant speed and do so for every observer regardless of his or her own motion. I suspect, for no reason other than why not, that it has something to do with the fact that light has no rest mass but I am only wildly guessing.

Actually "it just does" is not that bad an answer. It is straight to the point and clearly states the current lack of understanding of this basic fact.

Answer 4

because time and distance are not constant, only the speed of light is. see wikipedia "special theory of relativity"

Answer 5

There are folks out there responsible for getting shuttles up and down and landing equipment on other planets who do not get relativity.

Trying to get it boiled down to a Yahoo! Answer! is a lost cause.

Best I can offer you is that mass, movement and times flow are not constants. They are the three human viewable aspects of reality that plays weird games with mass, speed and time. The only constant that is measurable from this three-way nightmare of maths gone mad is that light always moves at the same speed regardless of the observers status.

Answer 6

The speed of light is designated as the only constant in the universe, it makes no difference if an observer is moving away from, or toward, a source of light, its speed will always be measured at 186000 miles per second. No difference can be detected because time is a factor in the measurement, and time dilates at light speed.

Answer 7

Light moves so fast (at about 300,000 km. per second) that it really does not matter whether you are moving or at rest. the difference totally negligible.