# Is time dilation relative?

1. Jul 19, 2014

### objecta99

If velocity is relative and if we cannot say which is moving away from what *objectivley, how do we say that time dilation is relative as well if we can tell who experienced the time dilation, as special relativity shows - and other experiments (muon concentrations etc). For example the jets clock is proven to run slower (and not the clock on the platform, in relation to the jet), gps atomic clocks are corrected for SR effects etc---these seem to me to argue that we do know which objects are undergoing time dilation not in a relative sense. Do experiments like the jet clocks or gps statellite clocks or muon concentration experiments get around the frame change symmetry breaking explanation any better? Does the frame change symmetry break per the twin experiment account for the reason why time dilation is relative and not objective and does this "frame change reason" account for why time dilation is relative for all other experiments (jet clocks, gps, muons)? I've heard some claim that time dilation is not relative bc we Know who will age less, (even if its after the fact and initially experienced as relative) and some say the twin paradox 'frame change' is not pertinent to how we understand the jet clock time dilation, gps satellite time dilation and high muon concentrations. thoughts?

Last edited: Jul 19, 2014
2. Jul 19, 2014

### Staff: Mentor

The term "time dilation" is ambiguous. Sometimes it refers to a situation where there is no invariant way of telling who is "moving" and who is not--for example, if you and I are in two separate spaceships moving relative to each other, and there is no other reference object nearby. But sometimes it refers to a situation where there *is* an invariant way of telling who is "moving": for example, if the motion is periodic. And sometimes it refers to a situation where all the clocks in question are co-located at the start and end of the experiment, so their readings can be compared directly.

GPS satellites are moving in periodic orbits around the Earth, so there is an obvious way to compare their elapsed times: just compare the time it takes for the satellite to complete one orbit, according to the satellite clock and according to the Earth clock. The satellite clock shows more elapsed time than the Earth clock, and this is an invariant fact; the Earth clock and the satellite clock both agree on it.

Similarly, in the Hafele-Keating experiments, atomic clocks were flown around the world and then returned to their starting point to be compared with clocks that stayed there. So the clocks were together at both the start and end of the experiment, which again gives an obvious way to compare them. The different elapsed times of the clocks are again invariant facts in this situation; all of the clocks agree on how much time elapsed on each one between the two events at which they were together.

3. Jul 19, 2014

### objecta99

That helps a lot regarding how we assess the clocks in orbit. I have a friend that has been trying to convince me that time dilation can be shown to be non-relative and doesnt need a "turn around" twin paradox type scenerio either, but is also not an orbital situation. He sets up a scenerio like the following to make his point:

We can show that time dilation is non-relative by syncing three clocks. Clocks at point A and B (planets or big rocks in space) and the clock in the "ship" clock C. If the clock in the ship is slower than the clock at point B, then it is also slower than the clock at point A (which can be verified as still synced with B). This is all without "turn-around" in the twin paradox. If both body A, body B , and spaceship C have synced clocks then when the spaceship reached body B it will have an off-clock than what A and B have - because C had a higher velocity of the 3 bodies traveling from one to the other. The spaceship that had a reference point going from body A to body B but lost all reference to those bodies "in between" still has velocity (and will still reach body B if lined up). It doesn't lose velocity once it loses the relationship (e.g. if body A gets antimatter wiped). hence we can say that the time dilation based on C's higher velocity is non-relative.

This may not be enough to go on, but this is pretty much the summary he gives me. I would never endorse such a scenerio but Is this correct or have u ever heard of a comparable scenereo to this that would make sense, i mean whats the glaring problem here? To me there could be many but I'm just such a physics novice that I'm to scared to say whats at work here with 3 clocks without an orbit either or twin paradox issue. im willing to delete this thread too if its not up to par. I don't mean to ask what might seem absurd questions to experts. thanx:-)

4. Jul 19, 2014

### ghwellsjr

Two rocks against one ship so the rocks win, is that how it works? Well let's just even up the score and make the scenario symmetrical, not by changing anything but simply by adding a second ship behind the first one the same distance apart in their mutual rest frame that the rocks are apart in their mutual rest frame. The clocks on the ship have been synchronized. Now whatever you want to say about the two rocks, you can also say about the two ships, correct?

5. Jul 19, 2014

### objecta99

ghwellsjr that sounds right to me as far as I can tell. I would say more but I better not bc I'll just end up probably saying something wrong or unimportant--I barely have the hang of twin paradox time dilation symmetry let alone 3 clocks. Yeah just to reiterate the scenario. ship C travels from rock A to B (regardless if A gets destroyed at some point during the trip), all were originally synced, once C reaches B and is shown to have a slow clock relative to B then its also slow relative to A since A and B are synced. Walla: C's time dilation is shown to be not relative. that's the 'gist'. sounds pretty bad, im just trying to really understand exactly why its so bad--there seems to be a lot of fast and loose playing with how the reference frames are being treated. thanx

6. Jul 19, 2014

### Staff: Mentor

I'm not entirely sure I understand the scenario, but I think the intent is that clocks A and B are at rest relative to each other, and clock C is moving relative to A and B, correct? If that's the case, this scenario doesn't show that time dilation is non-relative; it just shows that two clocks at rest relative to each other stay in sync, relative to each other. Relative to clocks A and B, clock C is running slower, yes; but relative to clock C, clocks A and B are running slower.

Yes, if clock C travels from clock A to clock B, it will have less elapsed time than the difference of clock A's reading when C passes A, and clock B's reading when C passes B. But since it only passes each clock once, there's no invariant way to say which clock(s) are "really" running slow. For that, C would have to meet up with the *same* clock (A or B) twice, or C's motion would have to be periodic so that there would be a common reference from which to compare.

Your friend might object that, since clocks A and B are in sync, reading one of them is equivalent to reading the other. But that's only true for an observer that is at rest relative to both clocks; there's nothing that requires clock C, which is moving relative to A and B, to consider their readings as equivalent. For example, say that, when C passes clock A, A's reading is exactly 12 noon, and when C passes clock B, B's reading is exactly 6 pm. B will say that A was also reading 6 pm at the same instant that C passed B; but C will not. C will say, using his own convention for clock synchronization, that clock A read something *earlier* than 6 pm at the same instant that C passed B.

In other words, from clock C's point of view, clocks A and B are *not* in sync with each other. This is an illustration of relativity of simultaneity: the definition of what events happen "at the same time", which is required for clock synchronization, is frame-dependent. C's definition of what events happen "at the same time" is different from A's and B's. And since there is no common reference from which to compare them, there is no way to say that either definition of simultaneity is the "right" one. That's how A and B can say that C is running slow, while C says that A and B are running slow.

7. Jul 19, 2014

### objecta99

that was awesome, you read the scenario correct. C travels from rock A to rock B and all three are "originally" synced, when space ship C reaches rock B and C is slow compared to B then its slow compared to A bc A and B are synced or at rest together. You guys summed this up well. I wont try to add anything I just wanted to at least convey the scenario best I could and let those who really know what they are talking about help clarify the scenario. thank you very much

8. Jul 20, 2014

### ghwellsjr

Time for some more spacetime diagrams to show you that Time Dilation is relative just like Velocity is relative to an Inertial Reference Frame (IRF). In this spacetime diagram, rock A is shown in blue, rock B is shown in black and ship C is shown in red traveling at 0.6c from blue A to black B. Since in this frame, C is traveling at 0.6c, its clock is Time Dilated by the factor 1.25 which means the dots marking off one-year increments of time are spaced 1.25 years of Coordinate Time:

As you can see, during the time that red ship C is traveling from blue rock A to black rock B, five years has transpired for A and B but only four years has transpired for ship C. But that's only true in the mutual rest frame of A and B. In other frames, the speeds of all objects can change and with them the Time Dilation factors.

To see this, we transform to the rest frame of the red ship C:

As you can see, the red ship C is not Time Dilated but the two rocks are Time Dilated.

As I suggested in post #4, you can make the scenario symmetrical by adding another ship D shown in green behind ship C and spaced the same distance apart in their mutual rest frame as rocks A and B are separated in their mutual rest frame:

If we ignore rock B, this looks just like a mirror image of the first IRF. We see blue rock A traveling from stationary red ship C to stationary green ship D in four years whereas both ships ticked off five years.

Can you see that Time Dilation is relative to your chosen IRF just like velocity is?

For completeness sake, we transform this frame back to the original IRF:

You commented that ship C had a higher velocity of the three bodies and I have already shown you that this is not true in the last two diagrams but now I want to show you that we can pick a frame in which all the bodies are traveling at the same speed (0.333c) and all of them are subject to the same Time Dilation:

For both red ship B and black rock A, it takes them each four years to reach their respective destinations, a completely symmetrical scenario.

Any more challenges for the notion that Time Dilation is not relative?

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9. Jul 20, 2014

### objecta99

ok heres some more challenge.

If we have a train car with a photon going up and down between two mirrors at the speed of light - the faster that train car is moving, the farther these photons are traveling to get from mirror to mirror. And since they cannot travel faster than the speed of light, time slows down in compensation.

On the platform, however, the distance is not elongated. It only appears elongated from inside of the train car. So from the perspective of each time is slowing down, but in reality, the clock in the train will have less ticks once the train stops and the count is reflected. If the platform was moving at the same speed (away from the train), the ticks would show as the same (as time would equally slow down).

How come I cannot say that as a consequence of relativity that the sun is rotating around the Earth just as much as the Earth is rotating around the sun, even though we know that the Earths orbit is due to the gravitation of the Sun and the direction and speed in which the Earth is traveling that compensates for the gravity so it does not just drop into it. From our perspective the Sun is rotating around the Earth, and from the Sun's perspective the Earth is rotating around the sun.

Last edited: Jul 20, 2014
10. Jul 20, 2014

### Staff: Mentor

Certainly you can do this. You can use any coordinate system you like, but of course the math will be simpler in a well chosen coordinate system.

11. Jul 20, 2014

### objecta99

thanx Dale. Yeah im having a real time debate back and forth with someone who wants to argue with me that time dilation and velocity are non-relative. so I am outsourcing for the heck of it to get any help I can. This guy (friend whos becoming more my enemy after every passing hour lol) takes the view that given the train platform hypothetical that there really is a time dilation on the trains clock but not for the platform clock though we see both as time dilated. I know its the old basic debates lol. My last comment was a little hubristic which was intentional lol. I don't hold the positions that im playing devils advocate for and I have also learned a lot from the help and diagrams I have been given on here just in the last 24 hours so I don't want to sound like an entitled naïve twit. so far I feel solid about how to symmetrically account for time dilation in a frame dependent manner but the train platform scenario has tripped me up somewhat, I wonder if its a consequence of the two never actually meeting but only theoretically meeting or what?? appreciate all the help iv'e been given.

12. Jul 20, 2014

### ghwellsjr

When you make statements like this, you should make it clear which frame they apply to. It is obvious that it's the frame of the railroad tracks for which your scenario is being described.

What distance are you talking about, the distance between the mirrors or the distance the train travels between successive reflections of the photon off of one mirror?

Less ticks than what? You have to be very clear when you are setting up a scenario.

The same as what? Are you forgetting what I have already told you? Pick an Inertial Reference Frame, any thing that is moving according to that IRF is Time Dilated compared to the Coordinate Time of that frame.

The Sun's rest frame is inertial, the earth's is not. It's far easier to describe and analyze everything from the Sun's IRF. That would show us that the moving clocks on the earth run slower than the Coordinate Time of the Sun's IRF and stationary clocks on the Sun run at the same rate as the Coordinate Time (neglecting any gravity influences). You cannot say the same thing about the earth's frame because it is not inertial.

13. Jul 20, 2014

### acegikmoqsuwy

$\displaystyle \Delta t = \frac {\Delta t_0} {\sqrt{1-\left(\frac {v} {c}\right)^2}}$

Why does it say $\displaystyle t' = \gamma \left(t-\frac{vx} {c^2}\right)$? Clearly $t' = \gamma t$

Unless of course, $v = 0$ or $x = 0$

Even though the distance of the platform doesn't change, the length of the train car does to keep the speed of light constant, and time dilation, in effect, backs this up.

You can say this, and in a certain perspective, the geocentric model was correct since in our perspective that is how the universe is. The reason why the other model is what we use, however, is because it simplifies everything down.

It's all just perspective. Anything can seem like anything else. The reason why certain things are accepted is because they work in all the perspectives. One exception, however, is the statement that we are the center of the universe. This appears to be true no matter where you stand in the universe, but that's because everything is moving away from everything else.

Last edited: Jul 20, 2014
14. Jul 20, 2014

### objecta99

these are a rephrasing of the general arguments posed to me by said friend that I have had the hardest time contending with. I hope it doesn't matter too much bc regardless if I supported the *correct interpretation of relativity or not, on my view what counts is that such support or contention involves qualitative understanding (that's probably something more appreciated in philosophy than science meh?) even if a qualitative understanding entailed a certain view rather than some other view on the matter.

Yeah I set that train platform scenario up poorly. Its just the usual question as to whether if we could compare the two clocks, the train clock would be slower when held side by side to the latter (or however that's achieved hypothetically via speed of light video transmission) and from this we could say that the time dilation was non-symmetrical (it only appears symmetrical) and non-relative. I am not endorsing this view but that is the argument made by some (maybe not here). I understand to the level I do, that an IRF must be chosen in order to assess relative time dilation, I have no disagreement there. Some ppl still want to poo-poo that and say that there is only one frame or clock where this is an "actual" time dilation taking place. I'm sure you are much more aware of the nuances in this debate than I am and have encountered such a challenge, but there it is. sorry, my response was hubristic in tone and I was going to respond with " I pity the fool that claims time dilation is non-relative" lol?

15. Jul 20, 2014

### Staff: Mentor

objecta99, it would be helpful if you would stop talking so much about your friend. Since we are not privy to your conversations with him/her we cannot realistically be expected to contribute to the conversation. Simply ask the questions that *you* have as directly as possible.

One thing that may help is to understand that in the theory of relativity not everything is relative. Even pre-relativity there was a long list of things which were relative (velocity, kinetic energy, momentum, etc.) and a list of things which were not relative (duration, length, mass, etc.), so the fact that some things are relative and others are not is nothing new.

All relativity did was expand the "relative list" to include duration, length, and simultaneity and also expand the "non-relative list" to include new concepts which were not discovered before (spacetime interval, proper acceleration, etc.).

To simply use the name of the theory to broadly claim that "everything is relative" is pure ignorance. Certain specific quantities (length) are relative, other specific quantities (invariant mass) are invariant, and the theory is completely silent on other concepts (moral/ethical values).

16. Jul 20, 2014

### objecta99

Pls no one get mad at me, i just want to rephrase what seems to be the biggest sticking point for "me". I am not disagreeing with any points raised by any of you folks i just am trying to rephrase the issue to make sure i am not missing anything. Pls do not think i am not getting what is being told me, if anyone wants to respond with a simple "i already explained this" or "go back to my former post" i will gladly and graciously accept such without qualms. Heres my line of questioning:

Just bc we need a relative frame to compare velocity does this mean that if one object has velocity then the other object relative to it has the same velocity and mirrors its velocity per relative velocity. can we have two objects with different velocities, and yet they are still relative to each other in that we cannot compare without the frames of reference.

consider the simplisitic hypothetical stated as such, i realize that it does potentially beg the usual "we need a IRF" :
"a clock aboard the plane moving eastward, in the direction of the Earth's rotation, had a greater velocity (resulting in a relative time loss) than one that remained on the ground, while a clock aboard the plane moving westward, against the Earth's rotation, had a lower velocity than one on the ground."

There are two frames of reference. The clock on the jet, and the clock on the ground.

1) Does the clock in one have greater velocity as experimentally shown?
2) Does the clock in one slow more than the other as experimentally shown?

basically what im wondering is can two velocities relative to each other have different velocities under SR and relative to eachother?
Back to the 'train platform' scenerio, do the implications of SR entail that the platform would need to carry the same velocity as the train just bc velocity is relative. If so does this mean that nothing could have more velocity than anything else, and all objects would be traveling the same velocity as the object they are referenced with relatively? Time slows the faster the velocity for the object traveling such (which as shown is not necessarily both objects - if it was then a clock on a plane does not have greater or lesser velocity than a clock on the ground). IOW how do I understand that there can be two different velocities (even when they are relative to each other) if relative velocity means that time dilation and velocity are symmetrical in intertial frames--does this symmetry mean that they are equal in velocity? or just share the same factor for Lorentz transforms--and what is the difference. Pls guys I know I am testing some patience perhaps its not my intent, im just really trying to make sure I fully understand and in order to do this I have to repeat a bit. thank you

Last edited: Jul 20, 2014
17. Jul 20, 2014

### Staff: Mentor

No, there is also a third one: a frame that is *not* rotating with the Earth. This is the frame with respect to which the velocity is given in what you quoted. The plane moving westward has a lower velocity relative to this frame because it is moving in the opposite direction to the Earth's rotation, so with respect to a non-rotating frame it is moving slower.

No. You just need to correctly identify the frame to which the velocities are referred.

18. Jul 21, 2014

### ghwellsjr

If two objects are traveling along the same line at any arbitrary speeds and they are inertial, then they can make some measurements to determine the velocity of the other object relative to itself without regard to any reference frame. One such method uses Doppler shifts, measuring the rate of the other object's clock compared to its own. Then it is easy to calculate the relative velocity. Both objects will determine that the other one is moving away or toward at the same speed (in opposite directions).

However, if you establish a reference frame, you can transform the worldlines of the objects to other frames that will increase the speed of the objects to much higher degrees, all the way to just under the speed of light.

Is that what you're wondering about?

19. Jul 21, 2014

### objecta99

yes and I am both glad you posted this and not glad for other reasons due to what I thought I understood. DOH! I have to ask for my own sanity and understanding that if that's the case then in that instance how come I can say that between two objects "traveling along the same line at any arbitrary speeds and they are interial" that they can make some agreed upon calculations about who's speed has greater magnitude without regard to any reference frame, and still claim that velocity is always relative. It seem that the relative velocity is addressing the problem of giving an absolute position and velocity etc but that doesn't mean that we cant in some cases tell and agree who is going faster than who. am I right here? is this a speed vs velocity thing or am I not understanding that relative velocity and in some cases velocity can be separated meaningfully.

can the arbitrary speeds not be the same, can they be assumed to be at least possibly different in these comparative and agreed upon cases of whos traveling faster.??

I hope I have missed your point bc if the preceding is more or less right then I got checkmated potentially.

Last edited: Jul 21, 2014
20. Jul 21, 2014

### objecta99

This is what I am trying to understand:
Is the following true or false: It is not the case that in all cases where two objects are moving at different proper times, in both reference frames there will not be an agreement about who is traveling faster then the other.