# What are your best arguments for time dilation, so duration is different ?

1. Feb 16, 2012

### digi99

I fully understand time dilation if I already could believe it, but I am not convinced yet.

What is the most convincing argument for you, that you are sure time dilation exist and so duration is different in locations (when duration is always the same, I believe in time dilation too, that's not the problem, its just a definition of Simultaneity)?

2. Feb 16, 2012

### Staff: Mentor

The experimental evidence for it is overwhelming: http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html

My favorite is the muon lifetime experiments:

"They stored muons in a storage ring and measured their lifetime. When combined with measurements of the muon lifetime at rest this becomes a highly relativistic twin scenario (v ~0.9994 c), for which the stored muons are the traveling twin and return to a given point in the lab every few microseconds. Muon lifetime at rest: Meyer et al., Physical Review 132, pg 2693; Balandin et al., JETP 40, pg 811 (1974); Bardin et al., Physics Letters 137B, pg 135 (1984). Also a test of the clock hypotheses (below)."

3. Feb 17, 2012

### Staff: Mentor

My favorite is GPS.

4. Feb 17, 2012

### bcrowell

Staff Emeritus
The Hafele-Keating experiment has the most charisma.

5. Feb 18, 2012

### bahamagreen

Are you looking for experimental evidence that confirms time dilation in the data, or are you looking for an explanation of how time dilation could be an existential reality?

The data received indicates dilation; but that data is always received locally, the interpretation of this data and extension of its implications to the existential state of the source of the data is a theoretical process.

I think most probably take the "philosophy" that the data represents the receiving observer's local existential reality, and use SR principles to infer the local existential reality of the data source in it's own reference frame... not much else to work with unless or until new experimental evidence could indicate otherwise... ?

6. Feb 19, 2012

### digi99

@bahamagreen

In fact I understand since yesterday what time dilation is, its not what I expected. Its just local time or remote time. The time you loose is the time from light being overbridged (or just time).

That C is always constant is something I already understand in fact (but now for sure).

So I was looking for data (and shall read the articles) but in fact I will not expect that C is not found.

The last problem is, time dilation is really to understand in the moving direction, but in the other directions is it still mysterious.

I would expect:

1) in other directions is no time dilation because there is no length contraction and so your ruler is not smaller
2) in other directions is length contraction as well, and so the same time dilation

7. Feb 19, 2012

### ghwellsjr

Time dilation has nothing to do with local time or remote time. I'm afraid you misunderstood my comment on another thread:
I was trying to get you to read Einstein's method of synchronizing the time on a remote clock to the time on a local clock as part of the definition of a Frame of Reference where all the clocks are synchronized to the same time and are the basis for Coordinate Time. Since all these clocks remain stationary with respect to one another and to the definition of the spatial coordinates in the Frame of Reference, none of them are time dilated.

If you go on to read the rest of his paper, he eventually gets to the dilation of time on a moving clock, that is, a clock moving with respect to the coordinate system, that is, a clock moving in the Frame of Reference. The time on the moving clock is called Proper Time. The ratio of the Proper Time on a clock moving with respect to the clocks displaying Coordinate Time is the reciprocal of gamma. It's as simple as that.

So please don't say the time dilation is just local time or remote time. You should say that time dilation is what happens to a moving clock (Proper Time) compared to a stationary clock (Coordinate Time).

8. Feb 20, 2012

### digi99

Yes you are right, sometimes I will mix the concept or definitions (not a professional in physics, but not on my website, I shall check it all again because I finish this subject and rewrite my blog and website), but I understand it very well. Besides I gave you an answer in the other thread (deleted and written again a time ago). So there you don't find this confusion anymore ..

Last edited: Feb 20, 2012
9. Feb 21, 2012

### harrylin

Time dilation is not at all a definition of simultaneity! See recurrent explanations in this recent thread:
https://www.physicsforums.com/showthread.php?t=575332&page=2 (you can start reading the comments by ghwellsjr, dalespam and myself from post #30 which describes time dilation without a definition of simultaneity).

The first positive detection of time dilation was the Ives-Stilwell experiment:
http://en.wikipedia.org/wiki/Ives–Stilwell_experiment

It makes use of the fact that "relativistic Doppler" is the combined effect of classical Doppler and time dilation.

Also interesting was the first indirect experiment as it was completely independent of relativity of simultaneity:
http://en.wikipedia.org/wiki/Kennedy-Thorndike_experiment

Harald

10. Feb 21, 2012

### digi99

Thanks Harald, I go to read it all but am short in time because of work, and this all takes a lot of time. So takes a while. Sometimes I think was I not started it for myself because it will be more and more complicated and so takes time (it is like a never ending story).

If I consider only my own example to make it simple (for my own) I just see with length contraction the car and ruller are smaller in frame B, but they both arrive at the same moment in as well frame B and A. But if there is communication between measuring persons between frame A and B, and B would give a light signal with his position to frame A and that signal arrives at the same moment the car arrived on time t in frame A, the car was on 1/γ . t in frame B. So I see it now, its just a measuring problem. And that time of the light signal, we call it time dilation.

You can say there is also time dilation when there is no length contraction, but you need a clock and ruler to prove that, and do you have in fact than length contraction too while measuring ?

Than you can say a moving clock runs slower, thats not only because it moves. Its just the moments you compare with a stationary clock. So in my example I could say send the time in the signal, it would be 1/γ. t from frame B when the light signal arrives on t in frame A. So the clock does not run slower, its just a measuring problem because you need it to be sure what the location is from the moving person (and vice versa).

You can make 1000 of errors in thinking with time dilation I guess .... who is right, I think the future and better experiments with expensive equipment impossible to have in your kitchen ...

11. Feb 22, 2012

### harrylin

No, not at all! Measuring problems and time of the light signal existed before relativity; time dilation is an effect in addition to that. If you rapidly (but without shaking it much) transport a clock away and bring it back, it will be found to lag behind on a "stationary" clock - this should not happen according to classical theory, or if it were just a measuring problem. And yes, such experiments with moving clocks have also been done (mentioned in post #4).

12. Feb 22, 2012

### digi99

Ok, could you than explain in my (classical) example what happend ? The car is in frame A on time t, after 1/γ . t it is in frame A when you measure with a ruler from frame B (1/γ shorter) or seen in frame B from the start point in frame A. If you could explain exactly what happend here in this very clear and short example, maybe I will understand more of it ? So what is the relation between t and 1/γ . t in the way one talks about time dilation ?

Maybe you have not seen that thread, so in frame A a car drives between start- and endpoint and takes t in time (measured in frame A), seen from frame B on a specified moment it is on 1/γ . t (because of length contraction), but on that time it is also on 1/γ . t in frame A.

Strange in all this theory is the fact how long it takes before it is clear, because everybody comes with the same examples, time after time and yet its not clear for me ... I am curious to your answer ...

Suppose the car is in frame A 1 meter long, its speed is 10m / s, so ten cars after each other in 1 second, in frame B ten cars of 1/γ m after each other since the start point, but remember the car was already shorter in frame A but measured with the unit 1 m, in frame B with the unit 1/γ m ...

Last edited: Feb 22, 2012
13. Feb 22, 2012

### harrylin

I will wait for your comments on the thread that I mentioned in post #9. For starters, do you understand the explanation by ghwellsjr in post #30 of that thread?
As it is in another thread, I will here just comment - as others likely did - that you have it completely wrong; and until you understand how it works, it's useless to continue this thread.

14. Feb 22, 2012

### JDoolin

I think that the best way to phrase this question is to voice what our intuitive expectations should be, regarding relative motion. Should we expect a Galilean Transformation to apply?

Then when you see the Galilean transformation, and compare it to the Lorentz Transformation, and compare the two to the rotation transformation.

Since the Lorentz Transformation looks just like rotation, but with hyperbolic sines and cosines instead of circular sines and cosines, there's a certain symmetry to it that just "seems" right. The most convincing argument, to me, is neither logical nor experimental; it's more aesthetic.

Last edited: Feb 22, 2012
15. Feb 22, 2012

### digi99

Thanks for the answer JDoolin. Also relating to the previous answer, this is a topic for many, as a usefull collection to convince somebody (not only for me).

I have a little question for you (if you want to answer), if a car is driving in frame A where somebody standing still (its rest frame A) is measuring its speed, than you have length contraction in the cars rest frame B. Is the car already smaller in frame A while measuring its speed because going smaller is something really physical or is this not real ?

16. Feb 22, 2012

### ghwellsjr

If a car is driving in frame A where somebody standing still (its rest frame A) is measuring its speed, then you don't have length contraction in the cars rest frame B, the car has length contraction in the rest frame A.

In the car's rest frame B, the person "standing still" is length contracted.

Pick one frame from which to define lengths. When you use a different frame, with its own definition of lengths, things have different lengths.

17. Feb 22, 2012

### Passionflower

To me time dilation is the situation where different observers measure a different duration between two events even after they discount the effect of light travel time.

18. Feb 22, 2012

### digi99

Thanks Ghwellsjr. A little bit confused about length contraction, maybe it is as I think.

So in frame A is the unit 1 meter and is the rest frame A for a person measuring the car when it was standing still. Suppose the car was 2 meter long.

Now the car has a speed in rest frame A, with speed v = x / t (x = distance in t seconds).

There is no time dilation for the car in frame A but a length contraction 1/γ.

In the rest frame B where the car is standing still (same car driving in the measuring persons rest frame A), there is time dilation 1/γ . t, suppose a ruler in the car, 1 meter is now 1/γ meter and the car is 2/γ meter long. Or is the car length contracted in A and has same new length in B ? What is the lenght from the car in frame B ?

What is the new unit in frame B for as well time as length expressed in units from frame A ?

Last edited: Feb 22, 2012
19. Feb 23, 2012

### ghwellsjr

I think a major part of your confusion is that you keep trying to deal with two frames at the same time. I keep telling you to pick one frame and describe everything in that one frame. I told you in post #7 that a clock moving in a frame will be time dilated. I told you in post #16 that a car moving in a frame will be length contracted.
When the car is at rest in Frame A, its length is 2 meters, that is, it is twice as long as the meter stick which is also at rest in Frame A. When the car accelerates to speed v in Frame A, both the meter stick and the car contract along the direction of motion and time dilates for them. Why do you say there is no time dilation for the car in Frame A when I just told you in post #7 that there was? Don't you read the posts that people respond to you?

OK, now you want to switch to Frame B:
In Frame B, you said the car is at rest, therefore, there is no time dilation and no length contraction for the car or the meter stick.
In Frame A, since the car is moving, it is length contracted and time dilated. In Frame B, since the car is not moving, it is not length contracted and not time dilated. Its length is 2 meters.
They are the same. A stationary meter stick in Frame A is the same length as a stationary meter stick in Frame B. A one-second tick in Frame A is the same duration as a one-second tick in Frame B.

20. Feb 23, 2012

### JDoolin

If you look at a coin from an angle, it looks oval. Is it really an oval, or does it just "appear" to be oval. Is the oval appearance of the coin really physical? Or is it not real?

If you are driving a car in the snow, the snowflakes look like they are coming toward you, rather than falling toward the ground. Is the appearance of motion toward you really physical? Or is it not real?

If you see a meter-stick going by at near the speed of light, viewed from the right angle, it will appear length contracted. Is its appearance of length contraction really physical, or is it not real?

What's missing here is another category. Yes it's really physical. Yes, it is real. But it is not real in the sense that it affects the snow, or affects the coin, or affects the meter-stick. However, it does really affect the observer's real physical measurements of the snow, the coin, the meter-stick. What you have here is observer dependent changes. That is, they do not model any real change in the properties (shape, size, direction of motion) of the snow, coin, or meter-stick, but instead, they model a real change in the properties of the observer (position, facing, direction of motion).

Last edited: Feb 23, 2012