Do we have time because we are moving?

[Dale]

Thank you, that's exactly what I wanted to ask. Not necessarily at a center but the difference between space and before/outside (completely hypothetical interest).

In modern cosmology there is no center of the universe, nor is there any "before" or "outside" the universe. It is not that it exists and has no gravity, it simply doesn't exist. We can discuss time dilation within the universe, but not before or outside it.

 

[Taragond]

Thanks Dale,

that's why it is hypothetical I guess? I know all that and I thought I mentioned it, too.
I just felt that if time runs slower the faster you move and we only know our own time personally, then time must run faster if you were slower.
For example:
our galaxy moves in a direction with some million km/h.
what If we could travel off with that speed relative to it in the opposite direction?

We would still have that speed relative to the milky way but relative to the surroundings we would get slower by that factor.
So...the galaxy's time is slowed down compared to it's surrounding space?
We are slowed down compared to the galaxy?
but shouldn't we get closer to the timeframe of the surrounding space by flying in the opposite direction?

Well I guess there just is no "surrounding space" as our galaxy's "speed" is just perceived as such by stretching space? Is that was I was missing? But in that case wouldn't it de facto have no speed?

 

[Dale]

that's why it is hypothetical I guess?

It isn't just hypothetical, it is contradictory to the theory. Since you are asking this question here I presume that you would like an answer which is consistent with the theory of relativity and modern cosmology. It simply is not possible to use a theory to answer a question which presupposes something contradictory to the theory.

I just felt that if time runs slower the faster you move and we only know our own time personally, then time must run faster if you were slower.
For example:
our galaxy moves in a direction with some million km/h.
what If we could travel off with that speed relative to it in the opposite direction?

This is a question which can be answered by the theory. Assuming that you are still close enough to our galaxy to ignore spacetime curvature, then in the galaxy's inertial frame you would be time dilated and in your inertial frame the galaxy would be time dilated.

We would still have that speed relative to the milky way but relative to the surroundings we would get slower by that factor.
So...the galaxy's time is slowed down compared to it's surrounding space?
We are slowed down compared to the galaxy?
but shouldn't we get closer to the timeframe of the surrounding space by flying in the opposite direction?

You would be at rest relative to a local "comoving"* observer. In the comoving observer's frame the galaxy would be time dilated and you would not. However, I don't think that anyone would call the comoving observer's reference frame "surrounding space" nor would anyone speak of speeds relative to "surrounding space".

*by comoving I mean an observer which is at rest relative to the FLRW coordinates, i.e. one that sees no dipole anisotropy in the CMB. See: http://en.wikipedia.org/wiki/Comoving_distance#Comoving_coordinates

 

[Taragond]

This is a question which can be answered by the theory. Assuming that you are still close enough to our galaxy to ignore spacetime curvature, then in the galaxy's inertial frame you would be time dilated and in your inertial frame the galaxy would be time dilated.

That sounds a little like from my perspective, galaxy-time would be slowed down, from the galaxy's perspective, mine would be slowed down.

 

[ghwellsjr]

That sounds a little like from my perspective, galaxy-time would be slowed down, from the galaxy's perspective, mine would be slowed down.

This is only true as long as you understand that "from my perspective" means "from the Inertial Reference Frame" in which I am at rest. It is not true "from the way things look to me". You can't perceive or see or observe Time Dilation either from your rest IRF or from the galaxy's rest IRF. Although things do look slowed down in a galaxy that is moving away from you, that's not just Time Dilation, that's Relativistic Doppler. Time Dilation is Relativistic Doppler with light transit time mathematically removed which requires you to make an assumption about how long it takes for the light to traverse from the Galaxy to you and that requires an assignment of an IRF.

 

[Taragond]

argh always that problem with perspective

proposal:
3 synchronized clocks, one stays at A while two are send with B at .6c away from A
after one year on B, C gets with one of those clocks on B in a lifeboat and heads at .9c away from B in direction of A.
Now the relative speeds are:
A:B => .6c
B:C => .9c
A:C => .3c
how much would the three clocks differentiate after 1, 2, 3 years?

 

[Dale]

That sounds a little like from my perspective, galaxy-time would be slowed down, from the galaxy's perspective, mine would be slowed down.

Yes. (and ghwellsjr is correct in describing what "from my perspective" means in relativity-speak)

 

[Dale]

how much would the three clocks differentiate after 1, 2, 3 years?

In which reference frame?

 

[ghwellsjr]

argh always that problem with perspective

proposal:
3 synchronized clocks, one stays at A while two are send with B at .6c away from A
after one year on B, C gets with one of those clocks on B in a lifeboat and heads at .9c away from B in direction of A.
Now the relative speeds are:
A:B => .6c
B:C => .9c
A:C => .3c
how much would the three clocks differentiate after 1, 2, 3 years?

There's no frame in which your scenario can be carried out. Speeds don't add algebraically. At least I cannot find a way to make your three relative speeds consistent.

I really think you need to focus on very simple scenarios until you get some basic understanding of how Time Dilation and Length Contraction work. Simple scenarios lead to simple explanations.

 

[Taragond]

maybe that's what I am trying to do...

If I had 2 synchronized clocks A&B and send B away with.9c for one year after which it decellerates and flies back. would the clocks still be synchronized? As I understand it, B would show less time has passed?

If that's correct, I would expect that in C time runs slower than in B, but from A's perspective in C time runs less slow than in B...so no...probably not...
But if it is a measurable effect with clocks there must be a solution to this?

I guess it would be too much to clarify here why speeds don't add algebraically? not sure, what to search for...

 

[ghwellsjr]

maybe that's what I am trying to do...

If I had 2 synchronized clocks A&B and send B away with.9c for one year after which it decellerates and flies back. would the clocks still be synchronized? As I understand it, B would show less time has passed?

If that's correct, I would expect that in C time runs slower than in B, but from A's perspective in C time runs less slow than in B...so no...probably not...
But if it is a measurable effect with clocks there must be a solution to this?

I guess it would be too much to clarify here why speeds don't add algebraically? not sure, what to search for...

Of course there's a solution for any problem that's consistently and completely described but you have combined parameters that are inconsistent and left out other important details so that there is not a single interpretation of your scenario.

You could have said:

3 synchronized clocks, one stays at A while two are sent with B at .6c away from A.
After one year according to B's time, C gets with one of those clocks on B in a lifeboat and heads at .9c away from B relative to B in direction of A.

Then you could ask your questions like how fast is C traveling in A's rest frame and what time is on each clock in the different rest frames.

 

[Taragond]

You could have said:

3 synchronized clocks, one stays at A while two are sent with B at .6c away from A.
After one year according to B's time, C gets with one of those clocks on B in a lifeboat and heads at .9c away from B relative to B in direction of A.

Then you could ask your questions like how fast is C traveling in A's rest frame and what time is on each clock in the different rest frames.

well...that one!

I am really sorry for my lack of precision in my questions. Maybe it's because english is not my native language, maybe lack of experience in discussions on these matters. Probably a bit of both...

But what does happen if you remove the difference in rest-frames by decellerating before comparing the clocks?

 

[ghwellsjr]

You could have said:

3 synchronized clocks, one stays at A while two are sent with B at .6c away from A.
After one year according to B's time, C gets with one of those clocks on B in a lifeboat and heads at .9c away from B relative to B in direction of A.

Then you could ask your questions like how fast is C traveling in A's rest frame and what time is on each clock in the different rest frames.

well...that one!

Ok, I'm going to do it from the Inertial Reference Frame (IRF) of the B clock after it leaves the A clock at 0.6c because then I can simply specify the speed of the C clock after 1 year to be -0.9c according to the same IRF. I have made a spacetime diagram showing all the timings you asked about. Note that the dots mark off one-month intervals of Proper Time along each of the three clocks' worldlines. I have marked in some of the Proper Time values to help you determine the intervening ones. I have also annotated significant events along the way. Start at the bottom of the diagram and work your way up:

In the above diagram, there are three significant speeds:

1) All three clocks start out at -0.6c with synchronized clocks up until time zero. The A clock (blue) continues on an inertial path.

2) The B clock (red) along with the C clock accelerates to 0.6c with respect to the A clock so it comes to rest in the IRF that I am calling B's IRF (even though it is only B's IRF after time zero).

3) At B's Proper Time (which is also the Coordinate Time) of twelve months, C accelerates away from B at -0.9c toward A.

I have also annotated the Proper Times along each of the worldlines corresponding to the 1-, 2- and 3-year intervals of Coordinante Time as you requested.

Does that make perfect sense to you?

Now you were also interested in the speed of C relative to A. You had said that it would be 0.3c since C originally departed away form A at 0.6c and then returned at 0.9c. But a simple algebraic solution doesn't work. Instead, we can see the answer by using the Lorentz Transformation on the Coordinates of all the events (dots) in the original diagram to create a new diagram moving at -0.6c with respect to the first one. This now becomes the IRF in which the A clock (blue) is at rest:

We can see that the speed of the C clock (black) as it is approaching the A clock (blue) is about -0.65c. Do you know how to determine this speed from the diagram? We can also determine this speed exactly using the relativistic velocity addition formula with v=0.6 and u=-0.9:

s = (v + u)/(1 + vu) = (0.6-0.9)/(1 + (0.6)(-0.9)) = -0.3/(1-0.54) = -0.3/0.46 = -0.652

I have also marked in the Proper Times along each of the clocks' worldlines corresponding to the 1-, 2-, and 3-year intervals of Coordinate Times as per your request. Note that these Proper Times are completely different than those that were determined according to the first IRF.

Even though the Proper Times at which the Coordinate Times are different in the two IRF's, the Proper Times at which identifiable events occur along the worldlines are the same in both IRF's. For example in both diagrams:

1) the last Proper Times at which all three clocks are together is 0.

2) the Proper Times on clocks B (red) and C (black) when they separate is 12 months.

3) the Proper Times on clocks A (blue) and C (black) when they pass each other is 28.8 months for clock A and 22.46 for clock C.

I am really sorry for my lack of precision in my questions. Maybe it's because english is not my native language, maybe lack of experience in discussions on these matters. Probably a bit of both...

But what does happen if you remove the difference in rest-frames by decellerating before comparing the clocks?

Here's another example of lack of precision: are you wanting the C clock to decelerate to the same speed as the A clock when they reunite? But then what about the B clock? It just keeps on going away from the A clock. When do you want it to decelerate and to what speed in which IRF do you want this to happen?