# B Time dilation understanding

1. Nov 11, 2017

### Edem

Is time dilation defined by Special Relativity, General Relativity or both?

The topic of time dilation seems to come up often. So I thought I would pose a question, and give my summary and conclusions.
I've recently been struggling to understand time dilation. While researching this topic l became very confused and frustrated by the many different ways that this is explained.
The widely used twins paradox example and it's variations (as explained), are often overly complicated and sometimes contradictory.
It appears that even the experts are confused.
I've seen and read that to explain it; you only need SR, you only need GR, or that you need both.
I have made sense of it in the following way from what I've learned in hours of research and thought.
Please correct me if I'm wrong on any point.

To start with, I see time dilation as defined by SR as follows;

Two observers moving at any constant velocity (an inertial state, no acceleration/deceleration), will observe time dilation. It will be observed when moving at the same speed in different directions (any direction other than parallel), or different speeds in any direction. They will observe it in the same way (meaning the other has slowed or quickened). From their perspective both observers view the other as moving and their self as still.
This slowing/quickening will not be observationally agreed upon, which is a paradox.
Both frames of reference are equally valid under the principle of equivalence.
Both would see the others time as slowing if the distance between them is increasing, and see the others time as quickening if the distance is decreasing.
The amount is determined by their relative speed and direction of travel.
If they were parallel and moving in the same direction at the same speed, there would be no dilation. They would both appear to be still to each other.

My conclusion;
Time dilation in SR is an observational distortion and will always be reconciled when two observers meet (which could only happen in real life with a collision if no deceleration is used).

Now for time dilation in GR.
I hold the following to be true (again, inform me if I'm wrong);

For two observers moving in any reference frame, the one in the greater gravitational field will have their time slowed. This will be observationally agreed upon (barring any additional dilation due to SR). This is not paradoxical, and this difference will not be reconciled when they meet.
The degree of dilation is determined by the difference of gravitational force and duration of time.
Acceleration and deceleration create a gravitational force.
The force of acceleration/deceleration is equal to the force of gravity.

My conclusion;
That the slowing of time under GR is actual, it will not be reconciled when two observers meet.
In real life this slowing of time is felt as "g force" when accelerating or decelerating at more then one gravity (the earth's inertial state, which is what our biological bodies are evolved to live in). It's not very pleasant to have your time slowed by more than a little. And deadly to have it slowed quickly.

My final conclusion;
While both are observationally relevant, only SR is a paradox, and only GR relates to actual physical change (any age difference). Hence, all the confusion in the twins paradox.
In the real world there couldn't be any parting or any meeting (except for a collision) without velocity change (acceleration/deceleration).
GR is the only thing that explains an age difference in the twins paradox. But this would only be possible to a noticeable degree if the traveler were in a much stronger gravitational field for a very long period of time (which wouldn't be good).

To sum up;
Time dilation in SR is observational and not actual.
Time dilation in GR is observational and actual.
It seems that few explain this distinction between the two.
Einstein apparently did so by addressing them in separate theories.

Once again, please advise where I'm in error. I must be wrong somewhere! Because I could've put a question mark after half of the above statements.

2. Nov 11, 2017

### Edem

Okay, but gravitational time dilation isn't, if I understand correctly.

3. Nov 11, 2017

### jbriggs444

Upon re-uniting, the pocket watches of the two twins in the twin paradox will have different readings. Both twins will correctly predict the reading of the other twin's pocket watch at the time and place of the reunion. Yes this means that all disagreements during the journey will be reconciled when they rejoin at its end.

The twins could compare watches as they whiz past one another. No need to stop.

4. Nov 11, 2017

### vanhees71

I also don't know, why "experts" tend to teach introductory relativity as a collection of paradoxes, instead of introducing the adequate math right away. It seems as if they think it makes the subject more attractive to students, if they have to struggle with apparent (but completely absent!) paradoxes. Although amusing, they are not helping the beginner. What's unclear with my posting #8? That's all you need to understand the different aging of the twins in the socalled twin paradox. It's not a paradox at all; it's a well established empirical fact if you accept muons or other unstable particles as substitutes for twins. One has measured that muons or unstable nuclei running at high speeds in an accelerator are indeed living longer by a Lorentz factor $\gamma$ than when they are at rest.

https://th.physik.uni-frankfurt.de/~hees/pf-faq/srt.pdf

5. Nov 11, 2017

### vanhees71

Sure, everything related with gravity is described by GR. SR is GR in absence of gravitation!

6. Nov 11, 2017

### Edem

I'm not sure I understand. Are you agreeing with me?

7. Nov 11, 2017

### Edem

If this had read; Apparently in the quoted paragraph, Einstein is addressing the time dilation of GR in SR. This would be correct?

8. Nov 11, 2017

### Staff: Mentor

Probably not. You weren't very specific, but you seemed to imply that with SR alone, the twins' watches would show the same elapsed time upon their later meeting. This isn't correct: their watches show different elapsed times.

9. Nov 11, 2017

### Edem

Sorry, but I don't know the math and can't follow it. So, at this point I'm trying only to understand the concept, not the mathematical formulas used to derive precise predictable quantities.
The muons in above example are accelerating (exposed to g force). So their dilation is due to time dilation as described by GR.

10. Nov 11, 2017

### PeroK

Much of what you say is either wrong or misses the point.

My advice to you is to learn SR from a single, reputable source. The picking and choosing from all different sources has led to your confusion. The sources probably agree on more than you realise, but there are so many different ways to present a subject that you've ended up picking up on the differences, rather than on the core facts of SR (and GR).

11. Nov 11, 2017

### Edem

Can you explain simply how this can be without using gravity (acceleration/deceleration). If both are at a constant velocity (inertial state) than their watches should agree when they meet.

12. Nov 11, 2017

### Staff: Mentor

No, it can't be explained without acceleration/deceleration because they can't separate or meet again without acceleration/deceleration.

13. Nov 11, 2017

### Edem

The advice is appreciated, but the point to me is the actual physical change and not the illusionary observational distortion.

14. Nov 11, 2017

### PeroK

It's all physical. Time dilation has nothing to do with "illusionary observational distortion". It has to do with the nature of time itself.

15. Nov 11, 2017

### Edem

It's only observationally physical. You can't physically touch it.
Something you observe as shortened will not be shortened when it comes to rest.

16. Nov 11, 2017

### Staff: Mentor

That's meaningless.
That's false and/or irrelevant. What you suggest is like taking a different route home from work and expecting the odometer reading not to show a different distance traveled.

Do you really want to learn or do you already understand and just choose not to accept?

17. Nov 11, 2017

### Edem

The odometer analogy doesn't work for me, it introduces a measurement of distance not time.
I'm trying to understand. My conclusions are recent and not carved in stone.
I'm now trying to understand how constant speed (no gravity) can effect time without violating the principles of equivalency I've recently learned.

18. Nov 11, 2017

### PeroK

Ah, but time is distance. That's the fundamental point about spacetime, rather than space and time.

A clock, in fact, measures the distance it travels through spacetime. And that is at least one definition of time: "proper time".

Also, in GR, you can measure mass as a distance. The Sun's mass, for example, can be given as 1.5 km. See, for example:

https://en.wikipedia.org/wiki/Geometrized_unit_system

19. Nov 11, 2017

### Staff: Mentor

Length [contraction] *is* distance!
Carved in stone or not, you shouldn't enter a learning exercise already having conclusions.
What principle of equivalence? You mean symmetry? I'm afraid you can't reconcile this. You are demanding symmetry exist where it doesn't.

20. Nov 11, 2017

### SiennaTheGr8

@Edem

1) SR can handle accelerated motion just fine. You don't need GR for that.

2) Time dilation in SR happens regardless of whether the travelers whose times are mutually dilated ever meet up to compare wristwatch-readings (which would require one or both travelers to accelerate).

3) The "relativistic effects" of SR (time dilation, length contraction, the relativity of simultaneity) are every bit as real as the gravitational effects of GR (e.g., gravitational time dilation). They are not optical illusions.

In fact, length contraction is in some sense the very opposite of an optical illusion: If something were approaching you at 90% the speed of light and you took a photograph of it before it reached you, it would NOT appear length-contracted in the picture. To the contrary, its length in the picture would be GREATER than it would be if you took a picture of it at the same distance while it was at rest! Length contraction is measurable, but what you actually see (or photograph) depends on the behavior of light, and that's a different question altogether. See: https://en.wikipedia.org/wiki/Terrell_rotation