I How time dilation is a permanent change and length contraction not?

[AlephClo]

Comparing high precision atomic clocks, one flying in a airplane and one on the ground had demonstrated permanent time dilation presumably due to special relativity (speed) and/or general relativity (gravity) .

While similar experiment with a ruler would demonstrate length contraction only when flying due to be measurement and simultaneity but not permanently changed when compared with the ground ruler?

Should I understand that SR does not permanently cause time dilation nor length contraction, while gravity does change length and causes time dilation permanently as per general relativity does? Consequently, gravitation has a real effect on length contraction locally.

Thank you, AlephClo

 

[PeterDonis]

Comparing high precision atomic clocks, one flying in a airplane and one on the ground had demonstrated permanent time dilation

No, it demonstrated differential aging, which is indeed "permanent" in the sense of being invariant--all observers agree on which clock has shown more elapsed time when two clocks separate and then come back together again after traveling on different trajectories. "Time dilation" is not the correct term to describe that; "time dilation" refers to a frame-dependent concept which is, indeed, the analogue of frame-dependent "length contraction".

There is no "permanent" analogue to differential aging for lengths.

Should I understand that SR does not permanently cause time dilation nor length contraction, while gravity does change length and causes time dilation permanently as per general relativity does?

No. See above.

 

[Ibix]

Neither time dilation nor length contraction is permanent. Two clocks or rulers, at rest with respect to each other, will always measure the same (edit: will always tick at the same rate, or their 1cm marks will be the same spacing) whatever their history.

However, if the two clocks move apart and return together they may well show different elapsed times for the time apart. This is usually called "differential aging" rather than time dilation, although the effects are related.

The closest everyday analogy turns out to be odometers on cars. If we put our cars next to each other and roll along our odometers will accumulate distance identically. If you stay put for a bit while I drive around the country for a while, when I return my odometer reading will be different from yours, but side-by-side we still accumulate distance at the same rate.

 

[PeterDonis]

If you stay put for a bit while I drive around the country for a while, when I return my odometer reading will be different from yours

Or, since it's not possible to "stay put" in spacetime, a better analogy would be that if you and I start out with the same odometer readings and drive from point A to point B by different routes, our odometer readings when we meet up again at point B will not be the same.

 

[AlephClo]

Neither time dilation nor length contraction is permanent. Two clocks or rulers, at rest with respect to each other, will always measure the same whatever their history.

However, if the two clocks move apart and return together they may well show different elapsed times for the time apart. This is usually called "differential aging" rather than time dilation, although the effects are related.

I am confused.
1) I assume with are discussing SR effect only here.
2) In the first paragraph you say will always measure the same whatever their history. While in the second paragraph, if one clock move apart (I add: as part of its history) they may well show different elapsed times.
3) What is the difference between differential aging and time dilation, how are they related? Any reading references?

Thank you.

 

[PeroK]

Comparing high precision atomic clocks, one flying in a airplane and one on the ground had demonstrated permanent time dilation presumably due to special relativity (speed) and/or general relativity (gravity) .

While similar experiment with a ruler would demonstrate length contraction only when flying due to be measurement and simultaneity but not permanently changed when compared with the ground ruler?

Should I understand that SR does not permanently cause time dilation nor length contraction, while gravity does change length and causes time dilation permanently as per general relativity does? Consequently, gravitation has a real effect on length contraction locally.

Thank you, AlephClo

Note that time dilation and length contraction, although related, are different in that time dilation is a difference in a rate of change that accumulates.

If you take the example of a clock that accelerates, travels at relativistic speed relative to your clock for a while, then decelerates back to rest then:

The rate at which the moving clock runs in your reference frame will be different only while it is moving. The rate goes back to normal once it has come to rest.

Imagine a clock simply recording the rate at which time is passing: just flashing every second, say. Like the ruler, once it comes to rest it will be back to "normal".

But, the clock itself measured not just the rate of time passing but the total time elapsed. This is actually a measure of the spacetime distance travelled. When it comes to rest it shows a record of its journey by its elapsed time, which is permanent.

A ruler, on the other hand, does not keep a permanent record of anything.

 

[phinds]

I am confused.
1) I assume with are discussing SR effect only here.
2) In the first paragraph you say will always measure the same whatever their history. While in the second paragraph, if one clock move apart (I add: as part of its history) they may well show different elapsed times.
3) What is the difference between differential aging and time dilation, how are they related? Any reading references?

Thank you.

He was implying that they will measure the same RATE, not have the same value

 

[PeroK]

3) What is the difference between differential aging and time dilation, how are they related? Any reading references?

Thank you.

Try:

https://en.wikipedia.org/wiki/Twin_paradox#Role_of_acceleration
As time dilation is symmetrical, it's really differential ageing in the twin paradox.

 

[AlephClo]

I now have a better understanding. Thank you all

 

[Ibix]

In the first paragraph you say will always measure the same whatever their history.

I've added a minor edit to post #3 to clarify. In any case, @phinds was correct in his interpretation of what I wrote. I think all your other points have been answered by others.

 

[Dale]

The closest everyday analogy turns out to be odometers on cars.

This is my take on it also. Clocks are analogous to odometers and rulers are analogous to metronomes.

 

[PAllen]

Another odometer example showing relativistic differential distance:

Consider that Alice travels from Paris to Berlin on geodesic (given such a road has been built), at 20 meters per second. Bob travels the same route at 10000 meters per second. Then, using ideal identically constructed odometers, Bob will measure less distance traveled from Paris to Berlin than Alice.

 

[Dale]

Another odometer example showing relativistic differential distance:

Consider that Alice travels from Paris to Berlin on geodesic (given such a road has been built), at 20 meters per second. Bob travels the same route at 10000 meters per second. Then, using ideal identically constructed odometers, Bob will measure less distance traveled from Paris to Berlin than Alice.

And the difference will be permanent.

 

[Ibix]

And the difference will be permanent.

And the odometers will function the same if tested next to each other in Berlin (assuming it survived traveling at 36,000kph).