# Are time dilation experiments conclusive?

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1. Aug 9, 2015

### Boro Petrovic

Before posting reply, please think about time as a concept that is used to describe a change relative to some other change. For example, a traveler will arrive on a destination after earth completes x rotations. At the beginning, people used earth rotation as a reference change and called it a "day". Today we use change processes on a mechanical clock or on atomic clock as a reference change . The main point here is that it is all about change.

Theory of relativity postulates that internal changes in an object are slowed down when that object moves very fast. For example, If one twin leaves the earth, travels very high speed then returns to the earth and finds that his twin aged 50 years more than he did then that means that all changes in his body (cells life cycles, electrons travelling through neurons, brain synapses building, etc) were slower compared to the changes in his twins body. One of the experiments that was used to prove it was comparison of changes on an atomic clock that orbits the earth with same kind of changes on an atomic clock that is on the earth.

Since our ability to observe changes on the subatomic level is very limited, what are the chances that some other factors caused the difference in between observed changes on the atomic clock on the earth compared to observed changes on the atomic clock that was orbiting earth?
Is this experiment sufficient to conclude that all subatomic changes , including the atom spin and subatomic particles movements , are slowed down compared to the changes in the atom that moves with slower speed.

Are time dilation experiments so far sufficient to allow general application on a complex system such as a living organism?

Last edited by a moderator: Aug 9, 2015
2. Aug 9, 2015

### phinds

It absolutely does NOT say that. It says that if an object is moving relative to you, then it APPEARS to you that its time is slowed down, and to it as if your time is slowed down. Both of your clocks, however, continue to tick away at one second per second (and yes, they are the SAME amount of "second").

No, that just proves that the AMOUNT of time is different for different paths through space-time, not that time passes at different rates for different objects.

Complexity has nothing to do with it, only relative motion and our measurement ability are relevant, and no, we do not have sufficiently good measurement techniques to measure that small an amount of time dilation.

EDIT: and by the way, this is a common misconception because popular science presentations do a rather poor job of explaining time dilation (and often just flat get it wrong and say what you said).

3. Aug 9, 2015

### Boro Petrovic

It seems to me that you are not thinking about time as a concept that describes change. For example, aging of our bodies is a cumulative result of extremely large series of change events in our bodies. If you can imagine two twins aged differently then that MUST mean that changes in one twins body were slower COMPARED to changes in other twins body. The twin who aged less obviously did not EXPERIENCE any slow down because all changes in his body , including his brain synapses connections that happen as result of his thinking, were slower compared to changes in his twins body.

4. Aug 9, 2015

### phinds

No, YOU are not understanding how the universe works. It is very un-intuitive, but it is the way it is. Clocks tick at the identical one second per second and biological processes run at the identical rates, regardless of your motion relative to someone else.

Just think about this. You, right now as you read this, are moving at .99999999c relative to a particle at CERN and according to that particle you are MASSIVELY time dilated. Do you feel slowed down? You are ALSO moving at ..5c relative to another frame of reference, so in THAT frame of reference you are only slightly time dilated. Can you be both at the same time?

As I told you in my original answer, the AMOUNT of time is different for different paths through space time.

5. Aug 9, 2015

### Staff: Mentor

We have clearly not observed time dilation in all known physical processes. But look at the issue from the opposite direction: of the several processes that have shown time dilation, none of them have any known mechanism by which speed could affect the rate of the processes (indeed, the ancient principle of relativity forbids it). So there is nothing that would imply anything but the amount of time passed was different for the traveling twin. So yes, the known evidence is quite sufficient.

6. Aug 9, 2015

### pervect

Staff Emeritus
The FAQ gives a number of papers on the topic, one of which, being a review paper, would be an ideal place to start looking at the literature. http://www.worldscientific.com/doi/abs/10.1142/S0217732305017202, Gwinner, "EXPERIMENTAL TESTS OF TIME DILATION IN SPECIAL RELATIVITY". Unfortunately it's paywalled, so if you want the full details rather than an off-the-cuff opinion you'll have to arrange access through your local library.

I did find results of another paper by the same author via google, searching for "heavy ion storage ring tests of time dilation", which was mentioned as the most precise test to date. The successful google hit had the full text of the article, and was at iopscience. How much longer it'll be up I can't say. Reading the full text, I can basically say that the modern experiment was a more accurate update of the Ives-Stillwell experiment (which you'll find numerous references to on wiki and in the litertaure), and that the conclusion was that the experimental results matched the predictions of SR at a level of accuracy considerably better than 1 part per million, when compared to the class of non-relativistic "test theories" that were considered.

So the very short answer is yes, it has been tested.

7. Aug 9, 2015

### Staff: Mentor

Phinds, the OP appears to understand how the theory works (what the theory says), but is asking about how we know the evidence couldn't support another conclusion. Just saying 'that's the way it is, accept it' does not answer the question.

8. Aug 9, 2015

### Boro Petrovic

Thank you for your reply. I assume that by "Amount of time passed was different" you meant that "amount of events that happened was different". That implies that rate of the processes were different. Right?

9. Aug 9, 2015

### Bandersnatch

I think I'd disagree with phinds' assertion in post #2 and #4 on the basis that it might be confusing time dilation (which is a symmetrical, observer-dependent effect) with differential ageing (which is what the OP is talking about).

Relativity predicts that observers in motion w/r to each other will see each other's clocks to move slower. A clock is understood as the rate of change of some physical process (relativity predicts all processes to be affected).
This observation is symmetrical. Each observer sees their own clocks to run just fine.

However, if one of the travellers gets back and meets the other one, the path through the space-time they will have travelled will be different, and one will be older than the other (accrued more time passage).

Similarly, an observer deeper in a gravity well will see clocks of an observer sent higher up to run at a different rate, and should they ever meet again their age would differ.

Last edited by a moderator: Aug 9, 2015
10. Aug 9, 2015

### Staff: Mentor

Closed pending cleanup.

EDIT: reopened, and merged with the previous thread. Some posts have been modified or deleted to make it consistent.

Last edited: Aug 9, 2015
11. Aug 9, 2015

### Staff: Mentor

That is not a postulate of relativity. It is a derived conclusion.

Such as what "other factor"? How do you propose that such other factors should conspire to produce exactly the time dilation predicted by relativity and not some other number. It is not enough for the other factors to affect the clock, but they must do so in exactly the correct way as to produce a "false positive".

No single experiment, in isolation, is sufficient to make much of a strong conclusion about anything. Reproducibility under different conditions is critical. The one you mention is called the Hafele-Keating experiment. By itself, no, it is not sufficient, but in conjunction with all of the other tests the preponderance of evidence is overwhelming.

Here is a review with a good list of experiments regarding SR:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

Yes. Living organisms are based on the EM force and the strong and weak nuclear forces. Time dilation has been proven for all three.

12. Aug 9, 2015

### Staff: Mentor

Yes.
When compared across frames (counting the events in one frame against the elapsed time in another frame), yes.

13. Aug 9, 2015

### phinds

Hm ... I didn't think my replies read that way, but if they did then I agree it was poorly stated. I was attempting to show him that time dilation is totally frame dependent and that all clocks tick at the same rate locally.

14. Aug 9, 2015

### phinds

Certainly no arguement there but I think we are promoting confusion in the OP by not fully emphasizing that processes do not run slow locally even while they appear different from different frames.

15. Aug 9, 2015

### Staff: Mentor

It may take longer to get there by following the OP down the path he wants to go (we assume), but in my experience it tends to be more convincing if we follow the line of logic until it fails than to just take a short-cut to the end.
Yes, I think the OP is already aware of that.

16. Aug 9, 2015

### phinds

Fair enough.

17. Aug 10, 2015

### Markus Hanke

It is worthwhile pointing out that in the context of physics, time is defined to be what clocks measure. Thinking of it in terms of "change" is misleading ( at best ) - you could have a system that is completely static and exhibits no change at all ( e.g. the rest frame of a free elementary particle in an otherwise completely empty region ), yet if you placed a clock into that frame it would still give you a non-zero reading.

18. Aug 10, 2015

### A.T.

No matter how complex you make your "clock", if it stays synchronized with a "simple" local clock in their common inertial rest frame, then they must have the same synchronization in any other frame, where they both move at the same speed. Otherwise you get paradoxes. So all "clocks" or processes must be affected by time dilation from movement in the same way.

19. Aug 10, 2015

### Markus Hanke

Indeed. Coming back to the original point, time dilation is something that is observed as a relationship between the readings on a clock; that can be a mechanical clock, but it could also be digital, or atomic, or a decaying particle. The point is that the dilation factor is the same in all cases, meaning time dilation has nothing to do with the mechanism of the clocks themselves, but everything with their relationship in space-time.

20. Aug 10, 2015

### phinds

I agree w/ the second part of this but don't understand what you mean by the first part "dilation factor is the same in all cases". Actually the dilation factor is DIFFERENT from the point of view of each object that is moving at different speeds than (i.e. relative to) other objects.

21. Aug 10, 2015

### Markus Hanke

Apologies for the confusion ! What I meant to say is that time dilation is the same no matter what type of clock is used - two mechanical clocks in separated frames will be dilated by the same factor as ( e.g. ) two atomic clocks in the same two separated frames. In other words, time dilation is independent from the mechanism of the clocks themselves.

Yes, I do understand that gravitational time dilation is of course not symmetric, unlike kinematic time dilation between inertial frames in flat space-time.

22. Aug 10, 2015

### phinds

OK, I agree w/ all that. Thanks.

23. Aug 10, 2015

### Markus Hanke

No, I should thank you, and all the rest of the regular contributors here - this forum is invaluable for an interested amateur like myself, who is wanting to test his existing understanding on matters of physics, and hopefully further it as I go along

24. Aug 10, 2015

### phinds

I'm sure I'm just as much an amateur as you are, I just happen to have found the site a few years earlier and can now parrot some of the stuff stated by the folks here who actually DO know what they are talking about

25. Aug 11, 2015