# Going the speed of light, possibly.

1. Sep 11, 2013

### DrWafflez Jr.

I have a question, my bestfriends dad said that if we were driving at 70 mph and then honked the horn, the sound would go 70 mph + the speed of sound. So the obvious question, if that is true can we go the speed of light? But sound is the vibrations of atoms? Does that mean the atoms would have to bump into each other at the speed of light? Last but not least can this be accomplished with a gun being shot or something of the sort? Thanks in advance

2. Sep 11, 2013

### DennisN

No. At relativistic speeds, you can not add velocities like you usually do, because of special relativity. See Velocity-addition formula (Wiki) and Einstein Velocity Addition (HyperPhysics).

Last edited: Sep 11, 2013
3. Sep 11, 2013

### Staff: Mentor

Your friend is incorrect, the speed of sound through a medium is not affected by the speed of the emitter.

4. Sep 11, 2013

### Staff: Mentor

Honk the horn and the sound will travel at the speed of sound no matter the speed of the source - sound waves have a particular speed in the air and that's the speed the move at.

Fire a bullet forward from a moving car and the speed of the bullet relative to the ground will be given by the velocity addition formula DennisN mentions ($\frac{u+v}{1+uv/c^2}$) where $u$ is the speed of the car relative to the ground and $v$ is the muzzle velocity of the bullet. For cars and bullets and other slow-moving objects (here "slow-moving" means not tens of thousands of miles a second) this formula reduces to the $u+v$ that common sense tells us to expect.