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

• I
• AlephClo
In summary: So, if you compare the clock that was flying to one on the ground, the flying clock will show a time dilation (permanent difference) as it was moving.Differential aging is a difference in rate of change that accumulates over time.
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

Umang Soni
AlephClo said:
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.

AlephClo said:
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.

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.

Last edited:
Klystron and russ_watters
Ibix said:
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.

cianfa72 and Ibix
Ibix said:
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.

AlephClo said:
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.

AlephClo
AlephClo said:
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

Ibix
I now have a better understanding. Thank you all

AlephClo said:
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.

Ibix said:
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.

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
PAllen said:
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.

Dale said:
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).

Dale

## 1. How does time dilation cause a permanent change?

Time dilation is a phenomenon in which time appears to pass slower for an object in motion than for an object at rest. This is due to the fact that as an object moves faster, its relative velocity to the observer increases, causing time to appear to pass slower for the moving object. This change in the perception of time is permanent because it is based on the fundamental principles of relativity and cannot be reversed.

## 2. Why is length contraction not a permanent change?

Length contraction is a phenomenon in which the length of an object appears to decrease in the direction of its motion. This is due to the fact that as an object moves faster, its relative velocity to the observer increases, causing the object to appear shorter in the direction of its motion. However, this change in length is not permanent because it is based on the relative motion between the object and the observer. If the object were to stop moving, its length would return to its original value.

## 3. How is time dilation related to the speed of light?

Time dilation is directly related to the speed of light. According to Einstein's theory of relativity, the speed of light is constant and is the same for all observers regardless of their relative motion. This means that as an object approaches the speed of light, its relative velocity to the observer increases, causing time to appear to pass slower for the moving object. This is the basis for time dilation and the reason why it is a permanent change.

## 4. Can time dilation and length contraction be observed in everyday life?

Yes, time dilation and length contraction can be observed in everyday life, although the effects are very small at everyday speeds. GPS satellites, for example, experience time dilation due to their high speeds and the fact that they are further away from the Earth's gravitational pull. This effect must be taken into account in order for the GPS system to function accurately. However, in everyday situations, the effects of time dilation and length contraction are too small to be noticeable.

## 5. How do time dilation and length contraction impact our understanding of time and space?

Time dilation and length contraction have greatly impacted our understanding of time and space. These phenomena are fundamental principles of relativity and have been confirmed by numerous experiments. They have challenged our traditional understanding of time and space as absolute and have shown that they are relative concepts that are dependent on the observer's frame of reference. Time dilation and length contraction have also played a crucial role in the development of technologies such as GPS and particle accelerators.

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