# What is the physical cause behind time dilation?

• I

## Main Question or Discussion Point

Time dilation can be shown mathematically but what is the physical phenomenon due to which it occurs?

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phinds
Gold Member
2019 Award
Time dilation can be shown mathematically but what is the physical phenomenon due to which it occurs?
The constancy of the speed of light in all inertial frames. If you are thinking that things actually change size in their own frame of reference, then that is your problem, since that does not happen.

• DaveE and FactChecker
Grinkle
Gold Member
what is the physical phenomenon due to which it occurs?
If the speed of light is the same for all observers, then the passage of time (the measured duration between events) cannot be the same for all observers. Its impossible for both of those things to be the same - its an over-constrained situation. Satisfy yourself of that logic as a first step. Once you have comprehended the impossibility of two observers not at rest with respect to each other agreeing on both the speed of light and the passage of time, then study the experiments that show indeed all observers do agree on the speed of light.

There is no mechanistic explanation for why time moves at different rates for different observers. You might as well ask (and its a better / more fundamental question imo) why is the speed of light constant for all observers. Time dilation is just a consequence of that experimentally confirmed feature of the universe. Once a constant c is established, time dilation must exist. These are not at all independent phenomena, and the one we can most easily observe is / measure is c.

This forum is littered with in depth discussions of the workings of clocks. Examining time dilation from that perspective is really a big distraction, imo.

edit:

The constant speed of light is the physical phenomena due to which time dilation occurs.

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• DaveE, Deepblu and russ_watters
Ibix
Time dilation can be shown mathematically but what is the physical phenomenon due to which it occurs?
There isn't one. Time dilation is much the same effect as putting a ruler diagonally on a piece of graph paper and realising that the ruler markings don't match up with the grid. That's because the distance the ruler is measuring isn't in the same direction as the distance the grid is measuring.

That's as close to a "physical phenomenon" as you are going to get. Two frames in relative motion define "time" to be a slightly different direction in spacetime, like the ruler and the grid. If you analyse the function of a moving clock using relativity you'll find that it doesn't measure your time, but rather a mix of time and distance, just as the ruler measures a combination of x-distance and y-distance according to the graph grid.

As @phinds says, this can all be seen as a consequence of Einstein's two postulates - the principle of relativity and the constancy of the speed of light. Minkowski put that on a nice footing by observing that Einstein's maths following from that could be understood as implying the existence of 4d spacetime, and how you split that into space and time depends on your choice of "stationary" object and clock synchronisation method.

• Nugatory
Mentor
Time dilation can be shown mathematically but what is the physical phenomenon due to which it occurs?
Another way of understanding "Why time dilation?" is to start with the relativity of simultaneity (google for "Einstein train simultaneity"), which does have a clear physical explanation - it follows from the constant speed of light. Once you understand relativity of simultaneity, you have your explanation of time dilation:
- A and B are moving relative to one another, and both set their clocks to zero as they pass one another.
- If we consider A to be at rest, we find that at the same time that A's clock reads 10 seconds B's clock reads 5 seconds. Therefore, A considers B's time to be dilated by a factor of two.
- However, if we consider B to be at rest, the events "A's clock reads 10 seconds" and "B's clock reads 5 seconds" do not happen at the same time; at the same that B's clock reads 5 seconds A's clock reads 2.5 seconds so B considers A's time to be dilated by a factor of two.

Many non-serious treatments of special relativity focus too much on time dilation and length contraction and too little on relativity of simultaneity. This leads to endless confusion and misunderstanding, as all three are required to make sense of the subject.

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• FactChecker and PeterDonis
The constancy of the speed of light in all inertial frames. If you are thinking that things actually change size in their own frame of reference, then that is your problem, since that does not happen.
I know that things do not actually change size in their refernce frames but only is observed to be doing so. I am asking if time dilation occurs due to the time taken by the light to reach us or something of that sort.

A.T.
I am asking if time dilation occurs due to the time taken by the light to reach us or something of that sort.
No, time dilation is what remains after you accounted for signal delay.

• FactChecker
phinds
Gold Member
2019 Award
I know that things do not actually change size in their refernce frames but only is observed to be doing so. I am asking if time dilation occurs due to the time taken by the light to reach us or something of that sort.

PeroK
Homework Helper
Gold Member
I know that things do not actually change size in their refernce frames but only is observed to be doing so. I am asking if time dilation occurs due to the time taken by the light to reach us or something of that sort.
Some of the introductory material on SR may give the impression that the delay in light signals reaching an observer is of some physical significance, but it is not. Observations of the time of an event must take into account how long the signal takes to reach the observer, whether the information is sent by light, sound or carrier pigeon!

Thanks to everyone.Can you all suggest me some books for understanding special relativity?I have just started my undergraduate studies and cannot handle the rigorous maths required.

PeroK
Homework Helper
Gold Member
Thanks to everyone.Can you all suggest me some books for understanding special relativity?I have just started my undergraduate studies and cannot handle the rigorous maths required.
I like " Special Relativity" by Helliwell. The maths required for SR should be fairly modest, as is the case with this book.

If the speed of light is the same for all observers, then the passage of time (the measured duration between events) cannot be the same for all observers. Its impossible for both of those things to be the same - its an over-constrained situation. Satisfy yourself of that logic as a first step. Once you have comprehended the impossibility of two observers not at rest with respect to each other agreeing on both the speed of light and the passage of time, then study the experiments that show indeed all observers do agree on the speed of light.

There is no mechanistic explanation for why time moves at different rates for different observers. You might as well ask (and its a better / more fundamental question imo) why is the speed of light constant for all observers. Time dilation is just a consequence of that experimentally confirmed feature of the universe. Once a constant c is established, time dilation must exist. These are not at all independent phenomena, and the one we can most easily observe is / measure is c.

This forum is littered with in depth discussions of the workings of clocks. Examining time dilation from that perspective is really a big distraction, imo.

edit:

The constant speed of light is the physical phenomena due to which time dilation occurs.
This has always been the most obvious logical explanation for me. But the more I dive into special relativity, the more I'm beginning to see why all the physics instructors are trying to teach it from a Minkowski spacetime and spacetime diagram perspective. Still, I find my mind recoils at that way of looking at it even now.

Ibix
Thanks to everyone.Can you all suggest me some books for understanding special relativity?I have just started my undergraduate studies and cannot handle the rigorous maths required.
For SR you can get away with algebra, as long as you don't want to think about realistic acceleration, which needs you to be able to integrate and differentiate. I could handle the maths by the time I left school - I'd be surprised if you couldn't.

I rather like Taylor and Wheeler's Spacetime Physics - the first chapter is free online if you want to tryy before you buy.

• Klystron and Nugatory
For SR you can get away with algebra, as long as you don't want to think about realistic acceleration, which needs you to be able to integrate and differentiate. I could handle the maths by the time I left school - I'd be surprised if you couldn't.

I rather like Taylor and Wheeler's Spacetime Physics - the first chapter is free online if you want to tryy before you buy.
Right now I have a good grasp over calculus in single variables.Would that be enough to read introductory books on SR?

Ibix
Right now I have a good grasp over calculus in single variables.Would that be enough to read introductory books on SR?
Easily. General paths through 4d spacetime probably require some partial differentiation, but as long as you stick to motion in a line (as most introductory SR does) you're fine.

Easily. General paths through 4d spacetime probably require some partial differentiation, but as long as you stick to motion in a line (as most introductory SR does) you're fine.
Thank you so much.

Thank you so much.
Current SR learner myself, and I strongly suggest learning linear algebra if you want to go beyond the semester 1 physics version of SR. Here's a free online text on linear algebra:

http://linear.ups.edu/html/section-WILA.html

• Mentz114
…things do not actually change size in their reference frames but only is observed to be doing so.
Is there a difference between being observed to change size and actually changing size?

I'm beginning to see why all the physics instructors are trying to teach it from a Minkowski spacetime and spacetime diagram perspective. Still, I find my mind recoils at that way of looking at it even now.
Einstein also initially rebelled against Minkowski space. He called it unnecessary learnedness. He believed in comprehension before calculation.

• nitsuj
Is there a difference between being observed to change size and actually changing size?
Yes.

Einstein also initially rebelled against Minkowski space. He called it unnecessary learnedness. He believed in comprehension before calculation.
Einstein soon changed this view and immersed himself in 4D spacetime for 10 years before coming up with GTR.

• m4r35n357 and PeroK
...and the constant speed of light for all inertial frames is necessary for the laws of physics to be the same in all inertial frames.

Ibix
...and the constant speed of light for all inertial frames is necessary for the laws of physics to be the same in all inertial frames.
A little care is needed with this statement. The principle of relativity leads to two distinct self-consistent theories. Both have an invariant speed, one which is the same in all inertial frames of reference. In one case it is finite, and this is relativity, and in one case it is infinite, and this is Newtonian physics.

So the truth of your statement depends on what you mean by "the speed of light". If you mean the invariant speed (whatever it is) then its true. If you mean the speed at which light travels then that depends on a theory of light consistent with Newtonian physics, and we don't have one. Maxwell's equations were relativistic from the outset, even when we didn't know about relativity. That's how the whole business got started.

Not sure if this post will be helpful, so please take it with a grain of salt, from someone still very much learning the ropes...

I’ve often pictured the physics of time dilation this way (amalgamated from several sources): from the perspective of a ground based observer looking at a person in a spaceship flying by, the action potentials (for example) of the neurons of the person in the spaceship must be moving slowly. That’s because I take the motion of the action potentials as basically orthogonal to the motion of the spaceship, so that if they did not slow down, the sum of their motion plus the rocket ship moving at like .999c would be greater than c. So from the ground, one second for the rocket observer would be a long time. Of course the rocket observer would see the ground observer in motion, and so would say that it is the ground observer’s action potentials that move slowly...

So it is because of c. I’ve never been certain that the above logic completely squares with the usual pedagogy (and if it doesn’t, where the problem is). And I’m becoming more convinced these days that Lorentz invariance is the more important thing to try to understand...

Ibix
That’s because I take the motion of the action potentials as basically orthogonal to the motion of the spaceship,
If you are talking about three-velocities here, I don't think this is correct. If you are talking about four-velocities then it makes more sense. Note that they aren't orthogonal, but they are non-parallel.
And I’m becoming more convinced these days that Lorentz invariance is the more important thing to try to understand...
All directly measurable things are Lorentz invariant, so yes. I gather Einstein said that he'd have preferred the whole thing to be called the Theory of Invariants, in fact.

Time dilation can be shown mathematically but what is the physical phenomenon due to which it occurs?
Rotation. Anything moving with respect to you rotates out of a space dimension and into the time dimension. What it gains in time, it loses in length.

Grinkle
Gold Member
Rotation. Anything moving with respect to you rotates out of a space dimension and into the time dimension. What it gains in time, it loses in length.
It sounds like you are invoking a mechanical metaphor by using the word rotation, but I can't follow what you are saying (and I am not implying that is your fault). Can you clarify?