# Speed of light

1. Sep 6, 2007

### Swatje

Hey lads,

This might be a stupid question, but I've been thinking about it.

Let's say, we make a visit to CERN particle accelerator... They accelerate a particle (or a football) to about 298 thousand km/s. (c = 300 thousand rounded). And then I accelerate a particle, or something else, to about 3000 km/s. And at the exact same moment, they move in opposite direction, so their relative speed is faster than 300 thousand km/s.

Is this possible?

Sorry if it's a stupid question...

Francis.

2. Sep 6, 2007

### neutrino

If you add them using the rules of Galilean relativity, then you will get answer that suggests the particle should be travelling at a speed greater than c. But it is observed that the rules of Galilean relativity does not work at speeds close to c.*

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html#c1

*Actually, the Einstein velocity-composition rule gives the 'right' answer in all cases, but at low speeds, the differences between Galilean and Einsteinian are negligible.

3. Sep 6, 2007

### Swatje

Wow thanks alot for that website!

*i'm gonna read* :D

4. Sep 6, 2007

### cristo

Staff Emeritus
Yes, it's possible, but no, the relative velocity is not greater than the speed of light. You need to invoke special relativity, and cannot simply add the velocities like you would in everyday life. Take a look at this webpage on Addition of velocities in Special Relativity

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