Special Relativity: Light's Relative Speed Explained

[Poy]

I think this is the right place to put this question
Special relativity 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.

 

[Doc Al]

Originally posted by Poy
Special relativity 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?

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.

 

[Poy]

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

 

[turin]

Originally posted by Poy
Special relativity says light always seems to move at c.

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

 

[Integral]

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.

 

[turin]

Originally posted by Integral
If you want to be nit picky ...

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.

Originally posted by Integral
... it is Maxwell's Equations ...

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

 

[jcsd]

There really should be a sticky thread about this as variations of this one seems to be the most oft-asked question:

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 $c$.

 

[Integral]

Originally posted by turin

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

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.

 

[turin]

Originally posted by Integral
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.

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.