# Does time dilation cause the speed of light to be invariant?

• I
You're almost there. You just have to accept the fact that there is no special frame of reference in which to judge whether other clocks run slow or are out of sync, or that meter sticks are shortened. These things are true in all reference frames, so that means there is no basis upon which to claim that light really travels at different speeds in different frames. All you have are meter sticks and clocks, and when you use them to measure speeds you find that there is a maximum possible speed. It follows then, that such a speed must be the same in all reference frames.
I think this is an underrated post. "The Principle of Relativity."

I understand that C is constant, and using the Lorentz formulas you can work it out. What I have trouble grasping, and a few others on here also, is that when we travel towards a light source at 0.25C, we would expect to measure the light at 1.25C. When we ask why not we get told, "ah, it's because your clock is running slow". OK, I can get that, but when we move away from a light source at 0.25C and expect to measure the light at .75C, how can it be because of the same reason "your clock is running slow"?

People have have been talking about "different reference frames", but in this example I am in the same reference frame measuring a light source in front of me and one behind me.
I would say look at Mister T's post. But as a fun diversion:

When moving toward it you SEE the sun's clocking moving FAST, not slow. This is tied to the Doppler effect. You can see this if you examine what the twins paradox actually see rather than what some ideal clocks posted at places in space would read if someone was next to them at each event. As the twin leaves, both see the other as having slow clocks, and as he returns, both see the other as having fast clocks, but due to length contraction and relativity is simultaneity, "when" each twin sees the other have a fast or slow clock varies, and in doing the calculations in that thread it was found that this is actually why the "moving" twin ages less. This isn't difficult to work out. Just choose your relative speed, calculate your length contraction that earth sees and compare measurements they will make. A good example showing this is found here:

http://www.scientificamerican.com/article/how-does-relativity-theor/

Sorry to resurrect my own thread, but I've been thinking about this issue some more. The great paradox in this, it seems to me, is that the key to understanding the invariance of the speed of light is that you have to understand the nature, not of light, but of space and time. It is the flexibility of space and time (distance and time) that causes the speed (a ratio of distance over time) of light to be the same for all observers. This leads me to the following thought experiment, which I can't understand correctly:

Suppose I'm traveling in a spaceship at 95% c. I have a meter stick and a clock, and I decide to measure the speed of light in the ship's lab. Travelling at near c, the clock slows down (time dilation), and the meter stick contracts (length contraction). However, in order to get the same ratio (300,000), if the ruler contracts, shouldn't the clock speed up?
I would point you to Mister T's post (quoted above). All inertial frames are equally valid, which means for all intents and purposes you are AT REST, your meter stick is NOT contracted, there ISN'T any time dilation, and you measure c for the speed of light.