# Speed of Light vs Light Years

1. Oct 25, 2007

### jacksnap

Hi,

This is probably a beginners question, but ill ask it anyway as it has me confused.

I have read quite a few posts from clever people here (so I assume its correct) that the closer to light speed you get, the slower time gets, and also distances get shorter.
Effectively meaning that, say for a photon, the universe is very small in its direction of travel and can be traversed in, practically, no time at all.

Then, why do we always measure distances in light years? we say the Andromeda Galaxy is roughly 2.5 million light years away, so travelling at the speed of light it would take us 2.5 million years.

Dont these 2 statements conflict, surely if we went at the speed of light (which I know is impossible) we would get there in a VERY short time.

Also if the first statement is true, then why does it take time for light from the sun to get to us etc.

I suspect its to do with the reference frames, and perhaps the zero time is what the photon 'feels', but from our reference frame it takes the longer time?
I personally would love to measure everything from the frame of the object moving, but I guess humans want to measure everything from their own reference frame and thats why we say its 2.5 million years.

Any extra info would be great.

Jason

Last edited: Oct 25, 2007
2. Oct 25, 2007

### Staff: Mentor

That's not quite right. A better way to say it might be: Moving clocks will be measured to run slowly, moving objects/distances will be measured to have contracted along the direction of motion.

So, if you are traveling very fast with respect to Earth, in a spaceship say, then to you time (and your clocks) run just like they always do. But if you were to make measurements (using your onboard clocks) of the Earth clocks, then you'd say that Earth clocks were running slow. And if you measured the distance between Earth and Mars, say, you'd say that the distance was shorter.

Let's leave the photon out of this, since it doesn't have a reference frame. But your point is correct: If your spaceship could reach a speed of 0.999c, then you could traverse ordinarily great distances in a very short amount of time as measured by your clocks.

According to Earth clocks, not according to your spaceship clocks. According to your clocks, it wouldn't take much time to get there.
To avoid insanity, we naturally measure things with respect to ourselves. (Just like folks on the spaceship would naturally measure time and distance using their moving spaceship clocks and metersticks.) Of course, we know that things will appear quite different if we were moving with respect to the Earth.