What is causing time dilation: Speed or acceleration?

[teo del fuego]

I have the same question as the original poster and it's one none of you have seemed to grasped. I'll restate it: ignoring acceleration, deceleration, and massive objects, all of which I understand cause time dilation, would there be any time dilation due solely to constant velocity. Please don't trot out the trope about how would we know unless you turned around and returned to the stationary frame of reference. Hypothetical I've adjusted the space ship's clock so that as it cruises by Earth it is in synchs with Earth bound clock, but proceeds on its path at a constant non-accelerating velocity. Ignoring difficulties surrounding the ability to compare both clocks without decelerating and accelerating, is there any time dilation solely to a constant velocity?

 

[Ibix]

Ignoring difficulties surrounding the ability to compare both clocks without decelerating and accelerating, is there any time dilation solely to a constant velocity?

You can't ignore them. That's the point. If you ignore them you aren't talking about the real world and you can conclude whatever you like. If you don't ignore them then the answer depends on your choice of simultaneity convention.
[mentor's note: lightly edited to remove a sidebar conversation]

 

[teo del fuego]

You can't ignore them. That's the point. If you ignore them you aren't talking about the real world and you can conclude whatever you like. If you don't ignore them then the answer depends on your choice of simultaneity convention.

Okay, spent two hours last night and I believe that time dilation occurs with constant velocity and that acceleration /deceleration contributes to it, but I t occurs even without it. I humbly, respectfully, suggest, that you are on the wrong path by avoiding the question and saying it cannot be answered without a "Copenhagen-esque" approach that "we'll never know unless we can get the two clicks back together at the point of origin."

The answer to this question was contained in a lengthy article at Einstein Online. I haven't done research into the author's credentials, but everything he/she said I have read in physics books for years. Yet, he/she specifically created a thought experiment similar to the one I proposed where any effects of accel/deceleration, turns, etc. are factored out so that only constant velocity is considered. Yes, constant unchanging velocity does cause time dilation which can be measured without experiencing gravity or acceleration.

 

[Nugatory]

Hypothetical I've adjusted the space ship's clock so that as it cruises by Earth it is in synchs with Earth bound clock, but proceeds on its path at a constant non-accelerating velocity. Ignoring difficulties surrounding the ability to compare both clocks without decelerating and accelerating, is there any time dilation solely to a constant velocity?

We can't just "ignore" these because they're real difficulties; we have to do something about them, or at least construct a thought experiment in which they don't get in the way.

But with that said... Do measurements of the half-lives of particles moving at relativistic velocities not meet your requirement?

 

[Dale]

Okay, spent two hours last night and I believe that time dilation occurs with constant velocity and that acceleration /deceleration contributes to it, but I t occurs even without it. I humbly, respectfully, suggest, that you are on the wrong path by avoiding the question and saying it cannot be answered without a "Copenhagen-esque" approach that "we'll never know unless we can get the two clicks back together at the point of origin."

Ibix is correct, and this has nothing to do with QM, the uncertainty principle, or anything else even remotely related to Copenhagen.

If I were to give you two parameterized curves in a plane $f_1(s_1)=(x_1(s_1),y_1(s_1))$ and $f_2(s_2)=(x_2(s_2),y_2(s_2))$, and ask you which is longer, how would you determine it?

 

[PeterDonis]

The answer to this question was contained in a lengthy article at Einstein Online.

Please provide a link. We can't discuss references if we don't know what they are.

 

[secur]

Okay, spent two hours last night and I believe that time dilation occurs with constant velocity and that acceleration /deceleration contributes to it, but I t occurs even without it. I humbly, respectfully, suggest, that you are on the wrong path by avoiding the question and saying it cannot be answered without a "Copenhagen-esque" approach that "we'll never know unless we can get the two clicks back together at the point of origin."

Supposing a spaceship going by the Earth, you're figuring the spaceship has the velocity, not the Earth. Why not the other way around? In special relativity there's no way to say one is holding still, the other moving; or rather you can say either, depending on reference frame. So you would have to say they both have the same time dilation, so they're equal after all ...

That's why the detail of bringing them back together, so you can compare them, is vital. If the spaceship turns around and comes back, it will show dilated time. On the other hand if the Earth runs after the spaceship fast enough to catch it - it will show the reduced time instead! (By SR, ignoring GR). You can imagine the spaceship sends a rocket or a light signal with a time stamp, or that the Earth does so; or that there's an intermediate space station receiving, and comparing, the signals; and so forth. If you carefully take everything into account (calculating with actual speeds and distances in your thought experiments) you'll find SR always comes through with a sensible, consistent answer.

Given your doubts, this is a very worthwhile exercise.

Now the "Einstein online" reference you mention is, most likely, claiming we can know that the spaceship is the one which actually experiences time dilation. So it must assume the existence of an absolute reference frame, such as the microwave background radiation. And, that the Earth is almost at rest with respect to it, compared to the spaceship at very high velocity.

I don't say the article is wrong, especially since you haven't given a reference so I can look at it. But it's probably no good. If it presented a real proof, it would overturn 100 years of accepted physics, so we should have heard about it. Nevertheless post a ref and no doubt people will take a look.

 

[teo del fuego]

The addition of "-esque" to Copenhagen was a tip off that I knew the solution had nothing to do with QM. Rather, I was making an analogy. I'm not sure what the equations were about, except to demonstrate superior math skills. As I peck away on my iPhone keyboard creating affronts to the English language, I hope I'm not coming off as rude. I genuinely am fascinated by the topic and everyone's input

 

[teo del fuego]

Supposing a spaceship going by the Earth, you're figuring the spaceship has the velocity, not the Earth. Why not the other way around? In special relativity there's no way to say one is holding still, the other moving; or rather you can say either, depending on reference frame. So you would have to say they both have the same time dilation, so they're equal after all ...

That's why the detail of bringing them back together, so you can compare them, is vital. If the spaceship turns around and comes back, it will show dilated time. On the other hand if the Earth runs after the spaceship fast enough to catch it - it will show the reduced time instead! (By SR, ignoring GR). You can imagine the spaceship sends a rocket or a light signal with a time stamp, or that the Earth does so; or that there's an intermediate space station receiving, and comparing, the signals; and so forth. If you carefully take everything into account (calculating with actual speeds and distances in your thought experiments) you'll find SR always comes through with a sensible, consistent answer.

Given your doubts, this is a very worthwhile exercise.

Now the "Einstein online" reference you mention is, most likely, claiming we can know that the spaceship is the one which actually experiences time dilation. So it must assume the existence of an absolute reference frame, such as the microwave background radiation. And, that the Earth is almost at rest with respect to it, compared to the spaceship at very high velocity.

I don't say the article is wrong, especially since you haven't given a reference so I can look at it. But it's probably no good. If it presented a real proof, it would overturn 100 years of accepted physics, so we should have heard about it. Nevertheless post a ref and no doubt people will take a look.

This has been the most helpful of replies so far in that it makes the return trip integral to the frames of references and not just a practical dilemma of "how could we ever compare the clocks without doing so?"

The Einstein online article wa

Please provide a link. We can't discuss references if we don't know what they are.

Please provide a link. We can't discuss references if we don't know what they are.

I'll try, but I'm using a phone not a computer.

The Einstein Online article was published by the Max Planc Institute for Gravitational Physics so I am not quick to dismiss it. Until I can post a link, it was found by Googling "Time Dilation" and if one throws in "www.Einstein-Online," one will come close.

 

[teo del fuego]

Thanks to everyone for replying and bearing with me. I think I've synthesized the helpful replies and will try to answer the original question.

The reason I was keen on an answer was to understand the mechanical causal link that would actually retard the aging process of the space bound twin. I understand and easily accept that gravity would cause a slowing of all atomic, molecular, and chemical processes in the traveling twin as well as slow everything else in her spaceship. I understand the equivalence of acceleration to gravity. What I could not understand is how mere constant velocity could also cause a slowing of the age ming process of the space bound twin. So, here is my attempt at an answer. Correct me where I am wrong.

Constant, non-accelerating velocity causes what I will call a "reciprocal" time dilation such that when two astronauts approach each other they both validly observe the other person's clock to be running "slow."

The time dilation involved in the twin paradox is not symmetrical. Upon return the traveling twin is, in fact, demonstrably younger than the earthbound twin. This cannot occur without a return trip which, necessarily, involves accelerating away from Earth, decelerating, turning around, accelerating back to Earth and deceleration. So mere constant velocity does not give the type of time dilation at the heart of the twin paradox. That is caused by the gravity-like effects of acceleration and deceleration.

Am I close, or still off track? And please, no fancy equations. Einstein didn't use any math for those wonderful thought experiments that gave him his initial insights which he later flushed out and proved with equations. (Racing a beam of light, anyone?)

 

[Ibix]

You are off track. Start with triplets. One stays on a space station. The second sets off at 0.8c to a star 2 light years away, instantaneously turns round and comes back. The third sets off at 0.8c to a star 1 light year away, instantaneously turns round and comes back again and waits for the second twin to return.

When they all meet up at rest in the station they will all be different ages. But two of them underwent the same accelerations - 0 to 0.8c, 0.8c to -0.8c, -0.8c to 0. So it cannot be just the effects of acceleration.

The easiest way to understand it is that the time you personally experience is a measure of the length of the path you followed through spacetime. Your wristwatch measures this in much the same way as the odometer in your car measures the distance you have traveled through space. The reason the twins/triplets/clocks have different ages is that they have followed paths with different lengths. That's all.

You can interpret the difference in path lengths as being due to the effects of a gravity field that turned the traveling twin around if you wish, but it seems overcomplicated to me. The underlying reason is that the distance round two sides of a triangle is different from along the third.

The reason that there is ambiguity about time dilation without acceleration is that there is no universal way to define "now" for anywhere other than "here". Everyone agrees on what the traveling twin's watch will read when she arrives at the star (because they can determine the path she followed through spacetime and calculate its length). They just don't agree on what time it is for them when she gets there, and none of their answers is wrong. They're just based on different assumptions about what "now, over there" means.

One final observation. If you really want to understand this you will need to get to grips with the maths. Sure, Einstein came up with imagery. But he had no way to know that his imagery was even self consistent, let alone relevant to physics, until he worked out the maths.

 

[weirdoguy]

And please, no fancy equations. Einstein didn't use any math for those wonderful thought experiments that gave him his initial insights which he later flushed out and proved with equations.

Math is the language of physics so you can't do physics fully and properly without it. Einsteins thought experiments could have also been incorrect, and he wouldn't even know that without doing the math.

 

[Stephanus]

Perhaps I can help you?
I have the very same confusion as you do the first time I try to understand Twin Paradox from my point of view (High school graduate)
Twins Paradox happens because of these rules that you have to accept.
1. The speed of light is the same for all observers.
2. Because of no 1, so length and time as seen from other observers are differents.

Let's drop Twins Paradox first. But just this simple question.
Can you work it out.
B travels from A at 0.6 times the speed of light,
How slow is B's clock as seen by A ACTUALLY?

Consider this
If at first
A read his clock, it shows: 00:00
than A read B clock from a distance, it shows, say, 00:20 (00:20 is irrelevant here, it could be 00:50, 00:60, but the most important thing is its proper time)
Now if A clock shows 00:10 and A looks at B's clock
What would A see?
A: 00:30?
B: 00:25?
C: 00:28?
D: Other?
and why?
Just drop twins paradox first. Forget twins paradox.
Perhaps if you can answer this question, I can lead you to twins paradox.

 

[Stephanus]

@teo del fuego
For solving non accelerating object in SR. I only use simple algebra. I have difficulties, too for understanding accelerating in SR. But to understand Twins Paradox, you only need simple algebra, don't worry.

 

[Dale]

I'm not sure what the equations were about, except to demonstrate superior math skills. As I peck away on my iPhone keyboard creating affronts to the English language, I hope I'm not coming off as rude.

The equations were a challenge. You are claiming that we are avoiding the issue and that you (with two hours reading an unknown website) are able to grasp and resolve the issue that the rest of us were avoiding.

The math of finding the time on a clock is the same as the math of finding the length of a curve. If you can do one then you can do the other. If you can't do either then it is both rude and ignorant to assume that those who can are wrong.

 

[Dale]

The thread is closed. If teo de fuego wants to have a less confrontational conversation then he may start a new thread.