# Speed of light

1. Jan 17, 2009

### JKaufinger

I know there is an answer to this somewhere, so please forgive me for not being able to see it :)

When people say that light will always be measured at a constant speed, no matter where you are, what do they mean?

Say we are in a spaceship which is standing still. Inside the spaceship, we throw a ball at 5 m/s. Now the spaceship goes to a fast speed. We throw the ball, and inside the ship, we still see it as 5 m/s.

So are they saying that if you are in the fast-moving spaceship, you will measure light the same as in the standing still space ship, just as you would the ball? That doesn't make any sense, because that would imply that light, like the ball, is affected by the inertia of the ship. It also implies that if the ship is moving at 100 m/s and the ball is 5 m/s from inside the ship, then to the outside, the ball is 105 m/s. Thus, if the light was also affected, then that means that the light would be going at c+5 m/s to the outside observer.

You see where I have this confusion?

Thanks.

2. Jan 17, 2009

### mgb_phys

You can never know what speed you are moving at if you can't see some background object to measure against. So by this principle you can't use a measurement of the speed of light to tell your speed - whatever speed you are doing you will measure the same speed for light

No it doesn't - speeds don't actually add like that. At low speeds they almost do so the speed would be something like 104.99999999 m/s. As things get closer to the speed of light this effect is bigger.
At the speed of light the speed is the same measured by any observer.

3. Jan 17, 2009

### Staff: Mentor

4. Jan 18, 2009

### JinChang

In layman's terms, you can't apply conventional physics (Newtonian) to speed of light or anything that approaches such high speed. Einstein showed that speed of light (c) is constant and as you approach this limit, time-dilation and length-contraction get introduced. If an object were to reach the speed of light, which is impossible by the way (anything with mass can't), then in theory, it would have zero-length in the direction it's heading, gain infinite amount of mass, and literally be turned into pure infinite energy.

We are so used to relatively slow speeds in our everyday-experiences that we tend to automatically think in terms of additive velocities. What happens instead in high-speed situations is that "Doppler-Effect" shifts the light spectrum to lower frequency when the object is moving away from the observer (and vice-versa) while the speed of light remains constant. That is as simple as I can explain it.