# Simple explanation for time dilation

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Time dilation is usually explained in a simple way using a "light clock" (1, 2).

Here I started a discussion about gravitational time dilation, offering a simple explanation I found on internet, but in that place is off-topic and, anyway, I want to extend this discussion, including time dilation in SR. This one was explained in a simple fashion (see here) and rewarded, although the explanation is not only naive but also incomplete.

So, I want to discuss this further. First, I want to "hear" how the light clock explanation for gravitational time dilation (see link above) works for up and down movement of the light. Second, I want to "hear" how the longer time needed for the light to complete the cycle, seen in light clock explanations, explains time dilation in general. In Ryan’s video (the one rewarded) there is a beginning of an explanation ... What do you think about it?

Please keep in mind, first, that I totally agree with relativity's results/predictions and second, that I don't expect from you the mathematical models and equations used to make the predictions, I want simple answers to the questions above, the physical explanation of time dilation.

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So, I want to discuss this further. First, I want to "hear" how the light clock explanation for gravitational time dilation (see link above) works for up and down movement of the light.

So his description is based on the idea that gravity curves spacetime and therefore the light has to take a longer path, and therefore each "tick" (reflection) takes longer. This is an interesting perspective although I don't know how realistic it is. However, why would it be any different for left-right versus up-down paths? They would be equally curved by gravity (ignoring the minute effect of the varying height).

Second, I want to "hear" how the longer time needed for the light to complete the cycle, seen in light clock explanations, explains time dilation in general.

The path of the light is longer, therefore the "ticks" are slower, therefore time is seen as running slower.

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Relativity predicts a relative frequency shift that depends on your position in a gravitational well.

For the light, the upper detector sees a redshift and the lower detector sees a blueshift. Or, viewed another way, the upper clock runs fast while the lower one runs slow, relative to the light.

For light, this is fairly easy to show, as it has to do with conservation of energy. If the light did not undergo a frequency shift, it would have the same energy at the top and bottom of the well. You could, in principle, take a photon, move it higher in the well, convert the energy to mass and have the mass drop down, which would gain mgh of energy per cycle. You then use the mass energy to create more photons and you would have an over-unity device. So the frequency shift has to happen.

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The true heart of the matter is that the space-time interval is invariant. Loosely, although observers may not agree on duration and length they will all agree on the difference which is the essentially the space-time interval. This explains why there is a difference in the clocks as measured by some 'stationary' observer and an observer that makes some round trip. Both the paths in space-time are different, but they must still agree on the space-time interval. Thus they will not agree in the duration of the trip.

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...why would it be any different for left-right versus up-down paths? They would be equally curved by gravity...

It's hard for me to see this, but probably you are right.

The path of the light is longer, therefore the "ticks" are slower, therefore time is seen as running slower.

This is right for the light clocks. Actually this is the light clock explanation ...

I asked how this explains time dilation in general, for any clock or thing. Did you watch Ryan’s video?

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This is right for the light clocks. Actually this is the light clock explanation ...

I asked how this explains time dilation in general, for any clock or thing. Did you watch Ryan’s video?

The light clock explanation shows that time runs slower for a moving object. Why do you need another explanation?

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... the upper detector sees a redshift and the lower detector sees a blueshift. Or, viewed another way, the upper clock runs fast while the lower one runs slow, relative to the light...

The light clock I wrote about (see the first post) has 2 mirrors/detectors, one up and one down. So, the up and down detectors form one clock, the light clock.

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I asked how this explains time dilation in general, for any clock or thing.

The key is to look at the space-time interval. Time dilation applies for any kind of clock, including your body clock.

It may be the case that this explanation is too mathematical for your taste, but in my opinion it gets to the heart of special relativity.

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The light clock I wrote about (see the first post) has 2 mirrors/detectors, one up and one down. So, the up and down detectors form one clock, the light clock.

You gave no examples of the light clock being used to explain gravitational time dilation. The observers at different potentials would disagree on the length of the travel of the light (and thus also for the time) but not for the same reason as a moving clock.

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The light clock explanation shows that time runs slower for a moving object. Why do you need another explanation?

Because we, and our real clocks, are made of atoms. Ryan begun to apply the light-clock explanation to all that's made of atoms, but he didn't elaborate. I did ... before him, but also too naive, so I wasn't able to publish in a journal.

Anyway, first I want to hear your opinion about his (rewarded) idea.

You gave no examples of the light clock being used to explain gravitational time dilation.

...a discussion about gravitational time dilation, offering a simple explanation I found on internet...

The observers at different potentials would disagree on the length of the travel of the light (and thus also for the time) but not for the same reason as a moving clock.

Not the reason of the "length of the travel of the light (and thus also for the time)" is important, but the effect on atoms, molecules, people, etc..

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Because we, and our real clocks, are made of atoms. Ryan begun to apply the light-clock explanation to all that's made of atoms, but he didn't elaborate. I did ... before him, but also too naive, so I wasn't able to publish in a journal.

If one is able to show that time is affected, not just the device for some mechanical reason (i.e. this not the action of a force), then everything is affected. The light clock is used because it has no mechanical parts that could be the cause. Once you have shown that, you shouldn't need an explanation with some other more complex device or system.

Anyway, first I want to hear your opinion about his (rewarded) idea.

For a high-schooler, it was a decent effort, but there are some obvious flaws. He mentions inertial frames, but all of his subsequent examples were of non-inertial frames — showed the earth orbiting, for example. The earth is inertial only to the extent you can ignore its rotation and revolution about the sun. Sometimes that's a reasonable approximation, but not always.

He mentions speeding up and slowing down as accelerations, but ignores rotation. Acceleration does not require a change in speed. That's a rookie mistake, but he's a rookie, so it's understandable.

I would have preferred he use an active example for the physics in two different frames, like tossing a ball in the air, or bouncing it on the floor. The passive example isn't bad, but I think an active one is more illuminating.

His example of light having to catch up to a moving target as a reason for time dilation doesn't work if you change direction, such that the target is moving toward the source. So that explanation fails under closer examination.

Not the reason of the "length of the travel of the light (and thus also for the time)" is important, but the effect on atoms, molecules, people, etc..

Time moves slower. Figuring out the ramifications is left an exercise.

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This is right for the light clocks. Actually this is the light clock explanation ...

I asked how this explains time dilation in general, for any clock or thing.

I think that is the problem with this description. It is trying to explain time dilation in terms of a mechanical process. Like all analogies, it fails when taken too literally.

Did you watch Ryan’s video?

No.

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...It is trying to explain time dilation in terms of a mechanical process. Like all analogies, it fails when taken too literally.

So why did they give 400,000 dollars for a failed explanation? You should watch the video

If one is able to show that time is affected, not just the device for some mechanical reason (i.e. this not the action of a force), then everything is affected. ...

Time moves slower. Figuring out the ramifications is left an exercise.

This is a good and needed exercise

You said "time is affected". Ok, and how atoms, molecules, etc. are "informed"/afected by it?

Don't you think that physics should explain things? Mathematically anything is possible. String teories use more than 10 dimensions and give good results. There is a theory with only 2 spatial dimensions (our 3D world beeing a holografic projection of the 2D) and it's mathematically correct. There is a working model with only 1 dimension, time. Also, there are scientists claiming that spacetime has no time dimension. Which model is better and why?

We, physicists, should investigate all the implications, as Ryan begun in his video ... ( I did, before seeing the video, and obtained amaizing things)

The key is to look at the space-time interval. Time dilation applies for any kind of clock, including your body clock.

It may be the case that this explanation is too mathematical for your taste, but in my opinion it gets to the heart of special relativity.

In my opinion, the explanation you mention is about how to understand Einstein's mathematical model and apply it in order to obtain the results, in this case, the "amount" of time dilation. This is not explaining what hapens in the atom if the path taken by the light between two objects (let's say the nucleus and one electron) increases, as both light-clock explanations suggested. We know that electromagnetic force is transmitted with the speed of light ...

See this:

and imagine one ship as the nucleus ad the other as the electron.

Anyway, if you love math, maybe you can answer this: http://www.scienceforums.net/topic/93377-more-tests-for-relativity/#entry904167

Edited by DanMP
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In my opinion, the explanation you mention is about how to understand Einstein's mathematical model and apply it in order to obtain the results...

That is what physics is all about, building models and seeing how they relate to nature. Any proper understanding of any physical phenomena will require some effort in understanding the mathematical structures of the theory used.

Anyway, as far as any atom itself is concerned nothing peculiar happens. You only see effects of time dilation when different observers compare things. Each observer sees no change with respect to himself.

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That is what physics is all about, building models and seeing how they relate to nature. Any proper understanding of any physical phenomena will require some effort in understanding the mathematical structures of the theory used.

Anyway, as far as any atom itself is concerned nothing peculiar happens. You only see effects of time dilation when different observers compare things. Each observer sees no change with respect to himself.

Mathematical models give results, not answers. Otherwise we couldn't have so many completely different working models (see above).

About atom, at speeds very close to c, electromagnetic interaction almost stop, because of the longer path. Maybe this is time dilation. Or you think that time is something out there, in the spacetime fabric, and the atoms somehow "take/read/use" it?

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Mathematical models give results, not answers.

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Was there a link to Ryans video? Could it be posted again please?

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Don't you think that physics should explain things?

Explain things? Sure. Explain all things? No.

Physics explains that time runs at different rates in two reference frames and/or gravitational potentials. It explains that simultaneity is relative. Beyond that, you need to look at the issue on a case-by-case basis, and such discussion is not necessarily part of physics.

In my opinion, the explanation you mention is about how to understand Einstein's mathematical model and apply it in order to obtain the results, in this case, the "amount" of time dilation. This is not explaining what hapens in the atom if the path taken by the light between two objects (let's say the nucleus and one electron) increases, as both light-clock explanations suggested. We know that electromagnetic force is transmitted with the speed of light ...

In the frame of the atom, nothing has changed for the atom — it's at rest. There are implications, though. e.g. any nucleus in the other frame is length-contracted, which has to be accounted for in heavy-ion accelerator experiments.

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Questions like: how the atom knows that it should run slower/faster, according to time dilation we calculate?

Was there a link to Ryans video? Could it be posted again please?

The video is here:

http://www.iflscience.com/physics/teenager-wins-400000-his-video-explaining-einsteins-theory-relativity

from 5:13 is the best part

Edited by DanMP
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Questions like: how the atom knows that it should run slower/faster, according to time dilation we calculate?

The video is here:

http://www.iflscience.com/physics/teenager-wins-400000-his-video-explaining-einsteins-theory-relativity

Nothing is dilated in its own frame. It is other frames in relative motion that are dilated or length contracted when compared to the local frame - the local frame is always normal to the observer within the local frame (ie the definition of local)

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Questions like: how the atom knows that it should run slower/faster, according to time dilation we calculate?

The atom doesn't know anything of the sort. You are currently moving at over 99% of the speed of light relative to something; is your watch running slow? No.

It is the effect of transforming coordinates from one frame of reference to another.

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The atom doesn't know anything of the sort. You are currently moving at over 99% of the speed of light relative to something; is your watch running slow?

GPS clocks on orbits are running faster than "my" clock. How they know to do it?

Did you watch the rewarded video after 5:13? Any comment?

Edited by DanMP
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GPS clocks on orbits are running faster than "my" clock. How they know to do it?

Did you watch the rewarded video after 5:13? Any comment?

i'll try again

Nothing is time dilated in its own frame. Things are only dilated in other frames. The clock is NOT running fast or slow in its own frame - it is only dilated when viewed from my frame which is at a different gravitational potential and in relative motion.

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They don't know.

We see many many video postings here. Have to start budgetting your time at some point.

Edited by Endy0816
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GPS clocks on orbits are running faster than "my" clock. How they know to do it?

It is because you are looking at them from a different frame of reference. If you were sat on the satellite it would appear to be running at normal speed.

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