# Light speed

1. Aug 31, 2004

### no idea

Can someone explain how it is possible that the speed of light is constantly 670 million mph irrelevent of relative speed? how is it that you can be going 669 million mph yet light still accelerates toward you at the same rate??

2. Aug 31, 2004

### HallsofIvy

Staff Emeritus
Yes, by explaining relativity! But not on this forum- it would take much too long. Any library ought to have a good, fairly elementary, introduction to the theory of relativity. If you are interested, it is certainly worth the time to read such a book.

3. Aug 31, 2004

### Staff: Mentor

The gist of it is that time and space vary in such a way as to make the speed of light always constant.

4. Aug 31, 2004

### LURCH

Here's a place to start;

Speed is the amount of time it takes to go a certain distance. Light's speed is always measured to be the same in every experiment, no matter which way the the observer is travelling, or at what speed. So it was concluded that, in order for light's speed to remain 670 Million mph (relative to the observer) even when the observer's speed changes (relative to whatever reference frame the observer is using), either the miles or the hours must change. So, at 669 million mph, you would still measure the speed of light to be 670 million mph, because your miles are shorter (which is called "length contraction), or your hours are longer (which is called "time dilation").

That's where all the wierdness of Relativity comes from.

5. Aug 31, 2004

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