# Speed of light is constant?

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1. Apr 24, 2015

### Swetha.M.L

Is light's speed constant? The universe is expanding, i think the distant b/w the planets&stars are increasing is that true? is it true then is the time taken to reach sun light to earth now from past change? if the time is constant then how the speed of light is constant?

2. Apr 24, 2015

### phinds

In a vacuum, yes.

yes
EDIT: I was a bit fast on that one. No, the things in the solar system are not moving apart. I was thinking you were talking about galaxies which ARE moving apart (except in clusters). I suggest you read the link in my signature.

yes
EDIT: again, I was thinking you were asking about really distant objects such as other galaxies. Since the distance from Earth to Sun is not changing, neither is the light travel time.

First off, all speed is relative to something so you need to be careful how you think about things in relative motion.

Suppose you have a set of three cars. the first car is moving South at 10 mph and the second is moving North at 10mph. Now the third car starts moving North at 20 mph and just keeps going at 20mph. The first and second cars start speeding up and slowing down. Why would you think that has any effect on the speed of the 3rd car? [all those speeds, by the way, are implied as being relative to the ground, which is taken as the frame of reference]

3. Apr 25, 2015

### Swetha.M.L

i am very sorry for disturbing you with my stupid question. what do you mean by the expansion of universe?

4. Apr 25, 2015

### phinds

You would really be better off reading some basic cosmology instead of asking random questions on an internet forum. The expansion of the universe is one of the first things you will learn about.

5. Apr 26, 2015

### mrspeedybob

Here might be a good place to start.

Leonard Susskind is well respected physicist. You can consider him credible. Also, the way he presents cosmology here, the math is kept as simple as is possible considering the subject.