# A quick question

1. Dec 6, 2003

### Poy

I think this is the right place to put this question
Special reletivity says light always seems to move at c. But wouldn't it (relative to you) be moving faster than c if its moving away from you and you are moving away from it? I might misundestdn something here, but if someone could clearify for me, thanks.

2. Dec 6, 2003

### Staff: Mentor

No. Light moves at speed c in any reference frame. No matter how fast you move, you'll measure any light passing by to have speed c. It's a feature of the "spacetime" structure of the universe.

3. Dec 6, 2003

### Poy

I know how that ussually works, but I wasn't sure in this one condition... thanks.

4. Dec 7, 2003

### turin

You may want to reconsider your conceptualization. SR says that light literally moves at c for every observer, not just appearing to do so.

5. Dec 7, 2003

### Integral

Staff Emeritus
If you want to be nit picky it is Maxwell's Equations which state that the speed of light is constant to all observers. Einstein was able to state a constant c as a postulate, because it was derived as such by Maxwell in 1867. The Constancy of c was a major dilemma for Physicists in the last half of the 1800s.

6. Dec 7, 2003

### turin

I wasn't trying to be "nit picky." There is a lot of misconception about what SR says concerning the nature of space-time, i.e., "time dilation is a consequence of the delay of light due to its finite speed," being such a misconception. From the wording, I thought that Poy might have such a misconception. Sorry for being presumptuous.

I'll be nit picky only to point out that a statement by a set of equations does not exclude a statement by SR.

7. Dec 7, 2003

### jcsd

In special relativity velocities ($u$ and $v$)are summed:

$$w = \frac{u + v}{1 + \frac{uv}{c^2}}$$

take the case where $u = c$

$$w = \frac{c + v}{1 + \frac{v}{c}} = \frac{c(c + v)}{c + v} = c$$

So the sum of the two velocites when one is equal to $c$ is always equal to $c$.

8. Dec 7, 2003

### Integral

Staff Emeritus
There is a big difference between a derivation from basics and a postulate. Maxwell dervives, Einstein postulates. He was able to do that based on Maxwell's derivation.

Special Relativity does not contain a reason for the constancy of c, it is the starting point. From there SR explores the results of a constant c.

9. Dec 8, 2003

### turin

So what? I didn't claim a derivation of any sort. SR says that the speed of light is literally the same in every inertial frame of reference. Do you disagree with this?

I thought that Poy misunderstood this statement, based on his wording. I didn't want to make an issue of whether or not SR is true, and why.

Last edited: Dec 8, 2003