Millenium falcon firing a laser

1. Sep 26, 2008

nick227

you are on the millenium falcon, going at the speed of light, and you fire a laser. does the laser go anywhere?

if you were on a truck moving at 50m/s and threw a ball forward at 20m/s, then the ball would go at 70m/s. not the same when dealing with c, right?

so then how can the laser go anywhere?

2. Sep 27, 2008

Staff: Mentor

Special Relativity has two postulates:

1. The laws of physics are the same in all inertial frames of reference.
2. The speed of light is constant in all inertial frames of reference.

That means, if you are in a truck moving at 50m/s and throw a ball forward at 20m/s, the resulting speed of the ball is not 70 m/s and when a spacecraft fires a laser, it travels away from the spacecraft at the speed of light, regardless of the speed of the spacecraft. If you are moving much slower than the speed of light, these effects are not noticeable, but they are still there.

3. Sep 27, 2008

Fredrik

Staff Emeritus
u=50, v=20. The ball's velocity relative to the ground: u+v=70.

One thing that special relativity tells us is that this way of adding velocities is only approximately correct. This is the right way to do it, according to SR:

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

When the velocities are small compared to the speed of light, the naive way to add velocities works extremely well, as you can verify yourself.

Millennium Falcon: u=0.99c (relative to "planet X")
Laser: v=c (relative to the MF)

The laser beam's speed relative to planet X:

$$\frac{u+v}{1+uv/c^2}=\frac{0.99c+1c}{1+0.99\cdot 1}=c$$

4. Sep 27, 2008

Staff: Mentor

Of course it does! Otherwise the reviews of the movie would be terrible. And besides, who would pay George Lucas to make special effects that can't be seen.