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

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

## Answers and Replies

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

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

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

The galaxies are not moving near the speed of light!!!
quasars might be.

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?

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

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

ZapperZ
Staff Emeritus
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
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.

DaveC426913
Gold Member
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

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

The term "seeing" needs some qualification and also the direction of movement should be supplied to avoid ambiguity.
He said "moving along with us", did you miss that?

DaveC426913
Gold Member
He said "moving along with us", did you miss that?
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.

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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????????????????????

ZapperZ
Staff Emeritus
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????????????????????
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.

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HallsofIvy
Homework Helper
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????????????????????
"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.

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.

DaveC426913
Gold Member
First of all superstonerman, 600,000 meters per second is not double the speed of light.
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.

HallsofIvy
Homework Helper
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????????????????????
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.

DaveC426913
Gold Member
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.
So:
3 Galaxies coincidentally named 'Left', 'Middle' and 'Right' are in a line and moving relative to each other.

Galaxy M (in the middle) looks left and sees L receding at .5c its residents are appropriately aging slower, M looks right and sees R receding at .5c, also appropriately aging slower.

Galaxy L looks right and sees M receding at .5c and its residents appropriately aging slower. Beyond M, Galaxy L sees R receding at .8c, also appropriately aging even slower.

Galaxy R looks left and sees M receding at .5c and its residents appropriately aging slower. Beyond M, Galaxy R sees L receding at .8c, also appropriately aging even slower.

There is no universal viwepoint; it depends on where you are as an observer; it is relative.