Light speed

1. May 20, 2006

Strategiest

If we have two airships going at speed 0.6c in the opposite directions. then they will be seperated at a speed higher than light.

let's name the airships , A and B
My question is , Let's say we have the two airships pull an elastic material.
take a point of the material on Airship A name it a , and a point on airship B. name it b. if they go at a speed close to light , then point a and b will be going at a speed faster than light ?

If so. then shouldn't all the points on the elastic material be going at a speed faster than light ?

Last edited: May 20, 2006
2. May 20, 2006

AlphaNumeric

You don't add relativistic speeds in the same way you do with everyday speeds.

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

Let v = 0.6c and u = 0.6c, that gives V = 0.88c, so they aren't moving apart at a speed greater than light, but less than light.

3. May 20, 2006

Staff: Mentor

Besides...
If the spacecraft are going .6C, then quite obviously, the fastest any part of the material will be going is .6C - right where it is connected to a spacecraft.

And....
Also quite obviously, a point at the center of an elastic band is stationary wrt where the stretching started. Try it with a broken rubber band - put a mark at the center and pull the ends apart. The mark doesn't ever move.

4. May 20, 2006

Thank you.