Acceleration of the universe

[Tony5856]

If Einsteins special relativity states that the speed of light is the same for everyone if you are traveling at a constant velocity, then this tells me that the universe, or all of the galaxies can't be accelerating away from each other, because that would mean that we all are accelerating. It's my understanding that special relativity has been tested and proved many times. So why is it still believed that the universe or the galaxies are accelerating? Any thoughts?

[swansont]

The expansion of space falls outside of special relativity. Space isn't a "thing" that SR restricts; you can add (or stretch) space between objects to that the appear to be moving apart, but the local effect is negligible. So distant objects can be receding faster than c, because the rate of expansion varies with the distance, without violating relativity. Locally, the motion is not exceeding c. Whether that added space is happening at an increasing rate, i.e. acceleration, is similarly not restricted by SR.

[Tony5856]

Thanks for your reply, I see now, but maybe you can help me with another question?

In the spaceship example here:

http://m.dummies.com/how-to/content/einsteins-special-relativity.html

If I am in the ship traveling very fast, relative to me, the ship standing still appears to be moving and the laser would travel in the diaginal direction. Meaning relative to me the time for the person in the ship standing still is slower than my time. But relative to the person standing still, my time is slower, and the time dialations would cancel each other. Please help me with this one

[swansont]

You each see time dilated in the other frame of reference, because the situation is symmetrical and neither of you has an absolute claim to being at rest. That sounds paradoxical, but the only way to compare clocks is to be in the same reference frame, and to do that one of you has to undergo an acceleration. This breaks the symmetry of the situation, and that person is the one whose time was affected. Their clock will be slow relative to the unaccelerated observer.

[Tony5856]

Yes, but both frames of reference can not be different in order to reconcile what has happened

I should have said, both frames of reference can not "conflict"

[Tony5856]

After further study I have concluded that in the above linked example and in the "twin paradox" others have added a third "inertial frame of reference." This still does not explain the symmetry and therefore the paradox between the two original frames of reference.

What am I missing?

[swansont]

No, there are only two frames, one for each of the two observers. What do you think the third one is?

[Strange]

Yes, but both frames of reference can not be different in order to reconcile what has happened

I should have said, both frames of reference can not "conflict"

 

They don't conflict (whatever that means).

[Tony5856]

"That sounds paradoxical, but the only way to compare clocks is to be in the same reference frame, and to do that one of you has to undergo an acceleration. This breaks the symmetry of the situation, and that person is the one whose time was affected. Their clock will be slow relative to the unaccelerated observer"

 

 

Ok, both people can not be in the same reference frame, they each have thier own, so if both people look at person A's reference frame, person B's clock is slower. And if both people look at person B's reference frame, person A's clock is slower. There is a paradox, and I'm not seeing what breaks the symmetry.

I am sorry, there are only two reference frames, I'm still trying to learn this stuff. It sounds to me like you are saying that we should ignore one of the reference frames, ignoring one frame gives one result, and ignoring the other gives a different result. That's what I meant by conflict, that was not the correct word to use. I guess I don't see how one reference frame can be ignored.

[swansont]

A sees B's clock running slow. B sees A's clock running slow by an identical amount. Each of them can claim they are at rest and the other is moving at speed v, because motion is relative. That's the symmetry — "you are moving at v and your clock is running slow by x" is a statement that can be made by either observer, since both can lay claim to the laws of physics working properly (one of the postulates of SR).

 

The only way to properly compare the clocks is to do it in the same frame. One of them must accelerate to do this, and acceleration is not relative: you can tell if you're accelerating (Newton's laws don't work properly, for example). The accelerating clock (let's say it was B's frame) will end up being the one that ran slow, because A is now the only one who has remained in a frame where all of the laws of physics still work properly — A can claim s/he is right.

[Tony5856]

You keep saying "accelerating" Are you saying that the acceleration of person B is causing the time dialation, not the constant linear motion??

The example I was referring to was supposed to be in empty space with no other forces applied. The twin paradox has two points of acceleration and two points of deceleration.

I am not denying that time dialation occurs, just that SR does not seem to explain it correctly.

Saying that in order to compare the clocks you must be In the same frame of reference, says to me that you are ignoring the other frame of reference and its data

And by the way, thank you for trying to help me see this

Earlier i was referring to a third inertial frame reference, I read that here

http://www.theoryofeverything.co.uk/time_dilation_clock_twin_paradox_relativity_theory/

 

But this only complicates matters because we are talking about two frames of reference

 

"A clear question arises. When considering only relative motion effects, where could one locate a standard atomic clock so that we may be certain that it will run at the fastest rate compared with all other identical clocks? Based on our present knowledge, this location will be in the inertial frame that is stationary against the overall background given by the most distant observable matter in the universe. The slowing of all moving clock readings must be referred to this standard preferred inertial frame. Hence, there is a unique preferred inertial frame of reference in our universe that establishes an absolute time standard."

Edited March 21, 2014 by Tony5856

[Strange]

Earlier i was referring to a third inertial frame reference, I read that here

http://www.theoryofeverything.co.uk/time_dilation_clock_twin_paradox_relativity_theory/

 

That is not a reliable source. (Which might account for some of your confusion.)

[swansont]

You keep saying "accelerating" Are you saying that the acceleration of person B is causing the time dialation, not the constant linear motion??

No. Time dilation is caused by relative motion. But acceleration allows you to "assign ownership" of it, as it were.

 

 

The example I was referring to was supposed to be in empty space with no other forces applied. The twin paradox has two points of acceleration and two points of deceleration.

(Deceleration is a subset of acceleration.)

 

The twin paradox really only has one acceleration that matters, that of the turnaround.

 

 

I am not denying that time dialation occurs, just that SR does not seem to explain it correctly.

Saying that in order to compare the clocks you must be In the same frame of reference, says to me that you are ignoring the other frame of reference and its data

No. You are always comparing the clocks, so you are using data from both frames.

 

And by the way, thank you for trying to help me see this

Earlier i was referring to a third inertial frame reference, I read that here

http://www.theoryofeverything.co.uk/time_dilation_clock_twin_paradox_relativity_theory/

 

But this only complicates matters because we are talking about two frames of reference

 

"A clear question arises. When considering only relative motion effects, where could one locate a standard atomic clock so that we may be certain that it will run at the fastest rate compared with all other identical clocks? Based on our present knowledge, this location will be in the inertial frame that is stationary against the overall background given by the most distant observable matter in the universe. The slowing of all moving clock readings must be referred to this standard preferred inertial frame. Hence, there is a unique preferred inertial frame of reference in our universe that establishes an absolute time standard."

Strange is correct: that is not a reliable site. There is no preferred frame of reference.