# Is speed of light relative to eather flow?

1. Jun 24, 2012

### schiz0ai

Is speed of light relative to "eather" flow?

If 2 rockets fly from earth in opposite directions.
Both end up flying at 60% the speed of light.
Does that mean they flay faster then the speed of light, compared to eachother?
Or is it relative to spacetime or eather or whatever?

2. Jun 24, 2012

### Staff: Mentor

Re: Is speed of light relative to "eather" flow?

No. In relativity, velocities don't "add" the same way as in classical physics.

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html

The result is always less than the speed of light.

3. Jun 24, 2012

### schiz0ai

Re: Is speed of light relative to "eather" flow?

so would an observer standing on 1 rocket be able to see light coming from the other rocket?

4. Jun 24, 2012

### Staff: Mentor

Re: Is speed of light relative to "eather" flow?

Why not?

5. Jun 24, 2012

### schiz0ai

Re: Is speed of light relative to "eather" flow?

how fast would light travel from 1 rocket to the other? and compared to what? the earth? the rocket emitting the light? or the one recieving it?

6. Jun 24, 2012

### HallsofIvy

Re: Is speed of light relative to "eather" flow?

The basic concept of relativity is that the speed of light is the same in any coordinate system. So the answer to all of your questions is "c".

7. Jun 25, 2012

### schiz0ai

Re: Is speed of light relative to "eather" flow?

so does that mean that when we think we shot something at 90% speed of light.
That seen from the center of the universe, the earth was moving so fast to begin with, that the thing we shot went only little bit faster?

8. Jun 25, 2012

### Staff: Mentor

Re: Is speed of light relative to "eather" flow?

Something like that. Say we shoot a spaceship at 90% of the speed of light with respect to earth. Then those on that spaceship shoot a rocket in the forward direction at 90% of the speed of light with respect to the spaceship. According to us on earth, that rocket is moving at about 99% of the speed of light (not 180%).

9. Jun 25, 2012

### phinds

Re: Is speed of light relative to "eather" flow?

There is no "center of the universe".

10. Jun 25, 2012

### schiz0ai

Re: Is speed of light relative to "eather" flow?

i thought the radius of the universe was the speed of light times the time since the big bang, and asumed herefor it had a calculatable center, but something tells me i really need to do some serious years of school to understand that kinda stuff. And hopefully some day il get the time to dive into it.

11. Jun 25, 2012

### phinds

Re: Is speed of light relative to "eather" flow?

Try this for a start:

www.phinds.com/balloonanalogy

12. Jun 29, 2012

### tensor33

That is correct. However, it is true for any point in space. There is no special point that is the center of the universe, any point in space can be considered the center. This is called the cosmological principle.

13. Jun 29, 2012

### phinds

Re: Is speed of light relative to "eather" flow?

No, it is NOT true. You are using sloppy terminology. What you have described is NOT "the universe", which is what the statement was about, but the "OBSERVABLE universe". There is a big difference.

14. Jun 29, 2012

### tensor33

You're right. That was a sloppy choice of words. Let me rephrase that. Any point in space can be cosidered the center of the universe it can observe (The observable universe)

15. Jun 29, 2012

### phinds

Re: Is speed of light relative to "eather" flow?

Excellent recovery

16. Jun 29, 2012

### Austin0

Re: Is speed of light relative to "eather" flow?

I am not sure I understand the distinction you are making here.
It would seem that unless there is an assumption that our observable sector of a total universe is somehow privileged that it would follow that we (or any other point) are effectively at the center of the whole shebang whatever it's extent.
What am I missing?

17. Jun 29, 2012

### phinds

Re: Is speed of light relative to "eather" flow?

Yeah, I think I got carried away by the use of the terms "radius" and "universe" together and immediate said to my self, NO ... that's the OBSERVABLE universe (which HAS a radius), not "the universe" which does not have a radium, but you are of course correct. Any point anywhere is the center of its own observable universe

18. Jun 29, 2012

### Austin0

Re: Is speed of light relative to "eather" flow?

Just a point of clarification. There is an obvious distinction between observable universe and total universe. But if we assume global (total universe) isotropy and homogeneity it seems to me that tensor33's unqualified statement above, is not only valid as it stands but is a logical necessity. IMHO
I.e. Any point must be considered the center of the total universe.
What do you think?

19. Jun 29, 2012

### Dickfore

What's this?

20. Jun 29, 2012

### phinds

Re: Is speed of light relative to "eather" flow?

Yes, if the universe is infinite in extent. My own belief is that it likely is, but I figure my personal belief with no facts to back it up is worthless, and I don't have any. Since the topology of the universe is unknown, I'm hesitant to say that EVERYWHERE is the center although it may well be.

If the universe is finite but unbounded, I don't know enough about possible topologies to understand whether the statement would always hold or not.