# Light when going as fast as the speed of light

1. May 19, 2012

### d3aj1986

Let's ignore the impossibility of it and assume you are in a spaceship going at the speed of light. If you shine a light from the spaceship how would an observer see it? Just light? What if the observer is shinning light parallel to the spaceship.. How would you see it?

2. May 19, 2012

### D H

Staff Emeritus
"Let's ignore the impossibility of it ..." In other words, "what do the laws of physics say would happen if one were to violate the laws of physics". You cannot ignore the impossibility of it. A spaceship cannot move at the speed of light.

3. May 20, 2012

### Filip Larsen

You may instead want to ask what an observer at rest in front or behind a spaceship sees if the spaceship shines a light out the front or back window as the speed of the spaceship relative to the observer approaches the speed of light.

The short answer to that question is that the observer will measure the speed of the light from the flashlight as moving with the speed of light(1), but the frequency will be increased (blue-shiftet) when the light is approaching the observer and decreased (red-shiftet) when the light is moving away from the observer. In the limit where the spaceship and flashlight travels infinitely close to light speed the frequency of the received light will go towards infinity (when approaching) or zero (when moving away).

(1) This follows directly as one of the axioms of the special theory of relativity: light in vacuum will always be measured to travel with the same constant speed, namely the speed of light (in vacuum).

Last edited: May 20, 2012