# B Uniform motion and time dilation - real or perspective based

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1. Apr 16, 2017

### Rob-NYC

Does time dilation in relative UNIFORM motion REALLY happen or is it merely a perspective from one reference frame relative to another as in the simple case where objects appear smaller the further away they are from the perspective of an observer in a particular reference frame?

Putting it another way, how can both clocks really be ticking slower than the other in two relative frames of uniform motion? Since both reference frames are part of one reality, how can they contradict each other by such an illogical assertion? Shouldn't all perspectives reveal a consistency in the same Universe instead of a stark contradiction as if the observers were in separate Universes? As for evidence, how can the clocks on an orbiting GPS satellite be affected by time dilation due to the component of its UNIFORM motion (the other component being acceleration, where it is constantly falling towards Earth), since such motion is relative and, so, Earth's clocks can be viewed as moving instead of the satellite's and therefore be affected by time dilation (leaving only time dilation due to the difference in the gravitational fields explaining the time difference when the two clocks are compared)?

An explanation invoking relativity of simultaneity only explains starting time differences of the clocks (affecting their synchronicity) but not the portion of the overlapping period when each clock is supposedly ticking slower than the other.

Mathematical explanations don't necessarily prove what is actually occurring. The math merely provides relationships between different frames of reference.

Only a precise, common sense, logical explanation will suffice for me otherwise I will maintain the possibility of a fundamental flaw in Relativity Theory.

Thanks

Last edited: Apr 17, 2017
2. Apr 17, 2017

### Staff: Mentor

There is no such thing as absolute proof in physics or any other branch of science. Absolute proofs only exist in mathematics. We use relativity because it provides a way to make accurate predictions and solve problems better than anything else.

We can give you a precise and logical explanation, but it will not be "common sense" in any way. Common sense doesn't even exist in the first place, or, at minimum, common sense varies so widely between people that it is pointless to try to use in almost anything related to science.

There's nothing illogical about it once you delve into the math and look at the available evidence supporting it. In fact, Special Relativity solved a number of problems in physics known about at the beginning of the 20th century. I suggest looking into the history of SR here: https://en.wikipedia.org/wiki/History_of_special_relativity

Here's a link to a number of tests of SR: https://en.wikipedia.org/wiki/Tests_of_special_relativity

3. Apr 17, 2017

### Orodruin

Staff Emeritus
Already your assertion of "really happen" shows that you somehow want to keep an absolute time. Absolute time does not exist in relativity and what is simultaneous will vary between inertial frames. This is a crucial part of time dilation and you cannot understand time dilation without it.

I suggest reading my PF insight on time dilation (see link in my signature).

4. Apr 17, 2017

### Ibix

Two cars are moving at 30mph on straight roads that cross at an angle $\theta$. One driver looks out of his side window at the other car and notes that it is falling behind because its speed in his direction is only $30\cos\theta$. The other driver looks out of her side window at the first car and notes that it is falling behind because its speed in her direction is only $30\cos\theta$. Is that paradoxical? No. The two drivers just have slightly different notions of which direction is "behind".

This is the basis of time dilation. Two reference frames define "time" as different directions in spacetime. The ticks of a clock at rest in either frame are equally spaced, but they lie on non-parallel lines. So the projection of the spacing of one line on to the other is different, but you get the same difference whichever clock you start with. The only complication is that you need to use hyperbolic functions when working with the time direction instead of trigonometric ones. That's why time and space are different.

One note: have you ever thought about what common sense actually is? It's your own personal set of rules about how you think the world works. The only difference between "common sense" and science is that science is very formal about checking for consistency between the rules and making explicit what the limits of the rules are. When we can't make the rules make sense we keep poking at them until we understand why not, then update the rules. Demanding a common sense explanation is either tautological (science is common sense with equations) or doomed to end in disappointment (the world is more complex than you think - and you regard that as the world's problem).

5. Apr 17, 2017

### PeroK

And, yet, your GPS works based on time-dilation calculations!

6. Apr 17, 2017

### Staff: Mentor

What does it mean for something to REALLY happen? How is it different from something that just happens, or something that only really happens instead of REALLY happening. Relativity can describe the outcome of measurements, so does the distinction between "happening", "really happening", and "REALLY happening" lead to any measurable differences?

Last edited: Apr 17, 2017
7. Apr 17, 2017

### vanhees71

It's "really happening" in the sense that you really observe, e.g., as many muons from the atmosphere as predicted by relativistic time dilation rather than what you expect from Newtonian abolute time. The point is that these muons travel with quite a large velocity and thus the $\gamma$ factor, $\gamma=1/\sqrt{1-v^2/c^2}$, is pretty large, and their lifetime is longer by this factor according to relativity theory than when they are at rest (and in the restframe it's where the lifetime of a particle is defined), and indeed, what's observed is the prediction from time dilation.

8. Apr 17, 2017

### Rob-NYC

Vanhees71, nope. If X is smaller than Y, then to say that Y is smaller than X is a CONTRADICTION! The only way we can make sense of this is if the former relationship happens in one Universe and the latter in another one. Now think about this; if an observer in this Universe could measure the elapsed time on both clocks (only due to uniform motion and NOT acceleration/gravitational field), it would be nonsensical to suppose that each of the two clocks would show, upon examination by such observer, a shorter elapsed time than the other.

9. Apr 17, 2017

### weirdoguy

No it is not, because "being smaller" is relative. And that is a FACT not CONTRADICTION.

10. Apr 17, 2017

### Rob-NYC

I meant this reply for Orodruin.

11. Apr 17, 2017

### Rob-NYC

So is uniform motion relative.

12. Apr 17, 2017

### Staff: Mentor

Agreed. If you look into the math then you find that relativity never says that.

What you do find is $t_{AB}<t_{BB}$ and $t_{BA}<t_{AA}$, which is not a contradiction, let alone a CONTRADICTION!

13. Apr 17, 2017

### Rob-NYC

But these muons you speak of are subject to Earth's gravitational field. I'm not questioning time dilation due to non-uniform motion. I'm only questioning time dilation due to relative UNIFORM motion.
Mathematics is a tool with which we test hypotheticals. Change the hypothesis and you change the math.

14. Apr 17, 2017

### Ibix

Yes. No one is claiming otherwise. The claim is that X>Y and Y'>X'. Which is not contradictory since X, X', Y and Y' are different things.
It's easy enough to read two clocks. All you need is eyes.

There's no contradiction in time dilation. The two clocks are measuring different things and using different definitions of "now" for at least one measurement (the relativity of simultaneity). That means that the time period between the start and end of the experiment for one clock is different according to that clock and the other one.

What you are trying to do, whether you realise it or not, is force a Newtonian notion of time into relativity. That will definitely yield contradictions, but that's not a problem with relativity. That's you trying to force together contradictory assumptions.

15. Apr 17, 2017

### Ibix

Gravitational time dilation is completely negligible in this experiment. It makes a difference on the 11th decimal place, or something like that. The muons live three or four times their proper lifetime.

Yes. But what you were linked to was a collection of experimental results in agreement with the predictions of special relativity. As you say, if you change the hypothesis you change the maths and make different predictions. But what we predict is what we see. What does that say about our hypothesis?

Edit: see also the sticky thread at the top of the relativity forum for more experimental evidence.

16. Apr 17, 2017

### Staff: Mentor

Um, no... Experiment is how you test hypotheses.

I think you may have meant something like "math is a tool for proving theorems based on axioms, change the axioms and you change the proof"

In either case, relativity is on solid footing. It has been validated experimentally to high precision, and the relevant mathematical theorems follow from well established axioms.

You didn't respond to the question about what experiment could be done to identify if something REALLY happens.

17. Apr 17, 2017

### Rob-NYC

You say "easy to read the two clocks." You're missing a crucial consideration. The clocks would have to be slowed down to examine, introducing time dilation due to non-uniform motion which I'm not questioning.

18. Apr 17, 2017

### Staff: Mentor

Why? I can read the license plate of a car as it goes by without slowing it down. We can do lots of experiments without slowing the clock. The muon one is a good example.

19. Apr 17, 2017

### Rob-NYC

Your assessment of what I actually meant regarding mathematics as a tool is perfect. But, if we want to compare the elapsed time on the two clocks to prove each had undergone time dilation SOLELY due to their relative UNIFORM motion, such would not be possible because one or both of them would have to be slowed down and subjected to dilation due to non-uniform motion, thereby masking the evidence of we are seeking.

20. Apr 17, 2017