# Maybe im onto something or i dont have a clue what im on ablout

1. Feb 13, 2007

### superstonerman

Iv been reading up about einsteins relativity business. I currentley understand that the faster an object moves the slower time will pass for that object relative to a staionairy observor. I understand the twin parodox where if one object is traveling closer to the speed of light it will age slower than an object that is staionairy relative to earth. If this is the case then the following has me a bit mistifed, appreciate someone clearing this up:

The sun moves at 600,000 metres per second relative to the milky way this is double the speed of light (i thought matter could not travel faster than the speed of light??). This must mean that earth is in a time dilation field relative to the rest of the universe or at least other galaxies. Also when we use telescopes to look into the sky and observe distant stars and galaxies they too are also moving near the speed of light relative to us. So might we be looking at these stars in the past relative to our present?????

Maybe im talking jibburish can soemone set me straight. Thanks for your help

2. Feb 13, 2007

### imaplanck

Light speed is a smidgen less than 3x10^8, not 3x10^5.

3. Feb 13, 2007

### imaplanck

If I recall, the gamma of a velocity 300,000m/s is negligible to say the least.

4. Feb 13, 2007

### Magister

The galaxies are not moving near the speed of light!!!

5. Feb 13, 2007

### rbj

quasars might be.

6. Feb 13, 2007

### superstonerman

the galaxies arnt moving at the speed of light?

are you sure the galaxies arnt moving at the speed of light even relevent to us?

7. Feb 13, 2007

### ZapperZ

Staff Emeritus
1. Please check your numbers. Your original post is now moot due to the error in comparision that you made.

2. Rather than arguing back and forth about "galaxies moving at the speed of light", you should find a proper source first to back this claim.

3. You migth want to brush up on the postulates of Special Relativity first before coming up with this. Many people on here who tried to violate this physics didn't make it out alive.

Zz.

8. Feb 13, 2007

### superstonerman

yes i relise now that i was wrong about my speed of light when using the suns intertial frame but please verify this statement for me:

Other galaxies experience time diliation due to their speed of movment relative to our galaxy

9. Feb 13, 2007

### ZapperZ

Staff Emeritus
But this is irrelevant to the rest of what you are proposing. First of all, we really don't care about the "time dilation" going on in other system. It has no bearing on our observations. The light coming from those source does not change in speed just because that galaxy happens to be "time dilated". Secondly, WE are not in a "time dilation field", since our proper time doesn't charge.

Zz.

10. Feb 13, 2007

### DaveC426913

We have a super-powerful telescope and we point it at a few galaxies.

- a galaxy that's moving along with us through space at a similar speed: no see no time dilation on their planets
- a galaxy that's moving fast relative to us: we see them slow down significantly
- we fly to that 2nd galaxy (the one moving fast relative to us) and we point our scope back at the Milky Way: we see the Milky Way slowed down relative to our new location

There is no preferred viewpooint in the universe, it's all based on the frame of reference we are in, and what we are looking at

11. Feb 13, 2007

### MeJennifer

The term "seeing" needs some qualification and also the direction of movement should be supplied to avoid ambiguity.

For instance the validity of the statement:

"a galaxy that's moving fast toward us relative to us: we see them slow down significantly"

would completely depend on what you mean by "see".

12. Feb 13, 2007

### nakurusil

He said "moving along with us", did you miss that?

13. Feb 13, 2007

### anantchowdhary

14. Feb 13, 2007

### DaveC426913

If it's moving along with us, then it's not moving at all (since the Milky Way is also not moving), thus it experiences no relativistic effects.

But MeJennifer was referring to my examples where I talk about galaxies that ARE moving relative to us. I deliberately left out the direction so as not to complicate the issue for the OP.

Last edited: Feb 13, 2007
15. Feb 14, 2007

### superstonerman

Ok say you had two galaxies moving away from each other at exactely half the speed of light. If each galaxie had a clock assigned to it after 1 year would both the clocks stay synched? As both the galaxies are moving at the same speed but away from each other. But im confused because if this is the case how do you know one galaxy isnt moving at the speed of light and the other galaxie is staionairy in this case time would come to a stop for the moving galaxy. Soemone help me out hre cuz im confused????????????????????

16. Feb 14, 2007

### ZapperZ

Staff Emeritus
Look up http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html" [Broken] in special relativity. If all SR does is adds velocity the SAME way, it wouldn't have been THAT revolutionary.

Zz.

Last edited by a moderator: May 2, 2017
17. Feb 14, 2007

### HallsofIvy

Staff Emeritus
"how do you know one galaxy isnt moving at the speed of light and the other galaxie is staionairy " Relative to what? The whole point is that velocity is always relative to something.

You first postulated "two galaxies moving away from each other at exactely half the speed of light." I had assumed you meant the usual convention "half the speed of light relative to each other. That would mean that galaxy A sees galaxy B moving away from it at c/2, galaxy B sees galaxy A moving away from it at c/2. It would make no sense to add their speeds.

If, instead, you mean "two galaxies are moving, in exactly opposite directions, each at half the speed of light relative to some third frame of reference, kit would make sense to add their speeds but not in the way you think. If two objects are moving directly away from one another with speeds relative to some third frame of reverence v1 and v2 then the speed of each one relative to the other is given by
$$\frac{v_1+ v_2}{1+ \frac{v_1v_2}{c^2}}$$

In particular, if two galaxies are moving directly away from each other at, relative to some third frame of reference, c/2, then the speed of each, relative to the othe is
$$\frac{c/2+ c/2}{1+ \frac{c^2}{4c^2}}= \frac{c}{5/4}= \frac{4}{5}c$$
still less than the speed of light.

18. Feb 18, 2007

### Quantumgravity

First of all superstonerman, 600,000 meters per second is not double the speed of light. The speed of light is about 186,000 MILES per second or 299, 792,458 meters per second. So the sun is not going anywhere near the speed of light relative to the Milky Way.
But I commend you for thinking about the time dilation for galaxies speeding away from each other. I've never thought about that. It does seem that they should be experiencing different rates of time. That's a really cool thought. Thanks.

19. Feb 19, 2007

### DaveC426913

That should have been 600,000 KILOmeters per second.

BTW, in a thread elsewhere I'd posted that our speed around the Milky Way according to Wiki is about 200km/s, so that's ~1/3000th of c.

20. Feb 19, 2007

### HallsofIvy

Staff Emeritus
The crucial point to make is that motion is relative! It makes no sense to say "how do you know one galaxy isnt moving at the speed of light and the other galaxie is staionairy" . All motion must be measured relative to some frame of reference. I assume that when you say "two galaxies moving away from each other at exactely half the speed of light", that is relative to some third frame of reference. Of course, in a frame of reference attached to either of the two galaxies, that galaxy is stationary. The other galaxy would not be moving at the speed of light in that galaxy's frame of reference but at
$$\frac{\frac{1}{2}c+ \frac{1}{2}c}{1+ \frac{\left(\frac{1}{2}c\right)\left(\frac{1}{2}c\right)}{c^2}}= \frac{c}{1+ \frac{1}{4}}= \frac{4}{5}c$$
still well under the speed of light.