# Is it ture that max speed

1. Aug 11, 2004

### expscv

i was curious ,

is it ture that nothing can exceeds the speed of light, such all object (except photon) max speed is "speed of light" ?

2. if both object travel close to light of speed, but in oppsite direction

so realvtive to object 1, is object 2 travel at double of "speed of light"??

2. Aug 11, 2004

### HallsofIvy

Staff Emeritus
Yes, it is true that no material object (with non-zero rest mass) can travel at (or faster than) the speed of light.

If two objects are moving with, say, 0.99c (99% of the speed of light) relative to some third object (every speed has to be relative to something) going in opposite directions then the speed of each one, relative to the other, is (0.99c+ 0.99c)/(1+ (.99c)(.99c)/c2)= 1.98c/(1.9801)= 0.9999c.

3. Aug 11, 2004

### Gonzolo

The formula HallsofIvy used is from Einstein's Special Relativity, which is the theory we must use to deal with such speeds.

4. Aug 11, 2004

Here is the full formula in case you wanted to know:

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

5. Aug 11, 2004

### whydoyouwanttoknow

All particles with a rest mass greater than 0 travel at below the speed of light.
Particles with 0 rest mass only travel at light speed.
Particles with less than 0 rest mass should, if they exist, travel at only greater than light speed.

6. Aug 12, 2004

### expscv

oh? so photon is equal to 0 rest mass?

becasue it can travel exactly at speed of light?

7. Aug 12, 2004

### whydoyouwanttoknow

Yeah, photons have no mass.

8. Aug 17, 2004

### Mk

not exactly, *ahem*

A photon can travel at the speed of light because it has zero rest mass

9. Aug 17, 2004

### Gonzolo

Are you sure there is "causality" here? c speed and 0 mass are pretty much hand in hand IMO. I would say either way to say it is good.