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Time dilation. Um, what?

  1. Jan 6, 2015 #1
    I've been trying to get my head around time dilation in order to understand the relationship between space and time. I do not have a maths brain, I tend to understand things better as narrative concepts.

    I've read a lot of introductory material, but I'm having trouble making the link between the theory and reality.

    For example, my understanding of spacetime is that it is a mathematical model - grid-like in nature - that allows people to plot, measure, and predict space and time interacting. Time is shown as a dimension along with the three spatial dimensions and this makes it easier for people to... do various math things, that I accept are beyond my understanding at this point. So far, so fabulous.

    Then things move along to gravity, and I start to lose the plot. Because apparently gravity is caused by objects with mass warping spacetime. But an object - say a planet - is real, but spacetime is just a model that describes space and time in a way that makes it easier to understand. It's abstract. I can see the earth. I can feel gravity. Spacetime is an idea.

    So I go back and try to strengthen my understanding of the actual relationship between space and time. I read the light clock thought experiment. There's a lot of threads around this forum grappling with it and the less mathy ones helped me get my head round it.

    So the crux of my problem is this.

    I can understand that a light beam travelling between two mirrors could be seen to be traveling slower if the mirrors are moving, due to the extra distance travelled by the beam, and that it will depend on where you are in relation to the clock.

    But a clock - any clock at all - is just a device that measures time, not time itself. To me time is the way that humans experience events occurring in sequence.

    A person standing next to the clock on a horizontally moving spaceship and pouring a cup of tea should not take longer to fill the cup than a person making tea and viewing the moving clock from earth. The light beam may have to move longer between bounces but the tea should not take longer to hit china just because it's travelling horizontally, regardless of where you're seeing it from. A person returning to earth after a space journey should not be years older than their twin - the beam of light had longer to travel on the spaceship but how does horizontal movement affect how long it takes your cells to decay? Does the clock have magic powers?? What? What am I missing here???
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  3. Jan 6, 2015 #2


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    The point is, the person on the spaceship will, from your point of view, be pouring tea slower than usual. He will, in fact, age slower than usual. This is not due to "light bouncing", this is due to "time slowing". Literally, when you see something that is moving fast relative to you, you perceive its time to be slower. ALL process are slower, including nuclear decay, and cellular decay. The light clock is just one way to realize this, but it's not due to the light clock that time dilates.
  4. Jan 6, 2015 #3


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    Yeah, its not a simple concept to grasp, I understand. The problem is those thought experiments like the light clock are for people who were exposed to the fact that why we need time to slow down in SR in the first place.
    As you like narrative concepts, let me tell you a story! Imagine Edd and Liz are a happy couple who are both physicists. They decide to measure the speed of light and so they go to get funding for it. Also they want to do it very far from any planet or star, in free space. But when they are struggling to get funding, the hard condition affects their relationship and so they break up after a quarrel. Now things are different. Each wants to do the experiment sooner and better than the other. So they each go to get funding separately and they both succeed but with the difference that Liz gets a little less than Edd and so has to use a spaceship which is a bit slower than Edd's. Now that's annoying for Liz of course but she can do nothing about it. So they both go to deep space and start measuring the speed of light while Edd is moving with speed V and Liz with speed V-w.
    Now Edd measures the speed of light and gets c. Then Liz, uses some spying devices to get Edd's result and finds out that he measured c as the speed of light. She thinks to herself that if he measured c, so why should I bother?! I'm moving with speed V-w and so I should find c+a(the sign and magnitude of a depends on the orientation of Liz's speed w.r.t. the beam of light they're using to measure the speed of light.) So let's just accept c+a and get back soon. But then Liz realizes that this is not what a scientist do. She things she shouldn't let her personal affairs affect her scientific integrity. So she stays and measures the speed of light as being c. She's surprised by this result and also a bit sad because she thinks she's wrong somewhere. But again she says to herself, as a scientist, she should report what she found. So she gets back and reports her result. Now its really tough for her because Edd is famous because of his result and she's just someone who got the same thing later and also probably wrong. But Liz is not someone who just goes to a corner and starts crying. She starts to work on her data to see what was wrong with her experiment.
    But she finds out that every thing was right. So why she didn't get c+a, but instead c? How can that be? So she thinks, what if actually speed of light isn't the same as other speeds and doesn't actually depend on which frame of reference you're measuring it in? Let's see what happens if we assumed that! Also in her studies, she found realized that equations in Newtonian mechanics have the same form no matter in what frame of reference you are. But Maxwell's equations don't have this property. These two facts make Liz think that things are not as simple as they thought. So she says to herself, I demand that all physical laws have the same form in all frames of reference and speed of light is the same in all frames of reference and try to see what are the implications. After a week of hard work, she finds out that the Galilean transformations between frames don't satisfy these requirements and she finds out the right transformations. The transformations imply that time gets slower for someone who is moving w.r.t. you and also lengths are smaller and these effects are somehow that completely make up that extra a thing that she expected to get. So Liz was moving with speed w w.r.t. Edd and her c+a was converted to c because of the effects she discovered. She also finds out that if she was the one who measured it first and found out c and then Edd spied on her and though it should be c+b, Edd also would measure c and found what she found. So she finds out that its all symmetric between them. So she understands that because the speed of light should remain the same in all frames, it should be that time and space change accordingly to make up for the relative motion. So she publishes her results and gets even more famous than Edd.
    Now Liz thinks about that symmetry between herself and Edd and this makes her go back and think about their relationship. She remembers her mistakes and is now ready to accept if Edd comes by and asks to start their relationship anew. Also when Edd finds out how smart is Liz and how she kept her scientific integrity and this led her to success, and this makes Edd think more too. But now Edd thinks if he goes to Liz, she may think its because of her success. Now this is a tough condition because both want to return but Edd thinks this may be understood badly and Liz wants Edd to ask for starting the relationship again. This is now a friend of both who invites both of them to a party and arranges everything so that everything goes smoothly and now Edd and Liz are again a happy couple, this time also famous scientists.
    Did you enjoy the story?
  5. Jan 6, 2015 #4


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    For both people (let's call them Alice and Bob), time will proceed as normal from their own perspective. Neither will notice the tea taking longer to fill his or her own cup. But if Alice watches Bob pouring tea, she will say it takes longer (as measured by her) for Bob to fill his cup. Similarly, he will say the same thing about Alice because she is moving relative to him. From his perspective, she takes more time (as measured by him) to fill up her cup.

    Why does this happen? Because our universe works in such a way that the speed of light is the same for both Alice and Bob. Space and time adjust in just the right way so that this paradoxical fact holds true. In other words, Alice and Bob don't experience space and time in the same way.
  6. Jan 6, 2015 #5
    the fact is as you say the time a cup actually should take to fill anywhere shouldnt vary ... and actually it doesnt vary ... it depends on the frame of reference you are talking about ..
    suppose you are filling up the cup and another person is doing the same except you are travelling in a vehicle ...... then what you will see is that the other person is pouring tea slower than you are and the other person will observe just the opposite ....
    the fact that yes a clock is just a machine and it just does 1 tick per second as its made to do ... suppose you are in a space ship and your twin is on earth then if you synchronize those two clocks ( suppose you send electromagnetic pulse every second ) then the person standing on earth will count that your time is passing slower as the more distance it needs to travel .... and if u are travelling with v velocity the person on earth travels with -v velocity from your frame of reference .... so if she sends a pulse you will also get it a bit later than she wanted you to get ..... so its pretty funny because both of you will calculate wrong and think that the other person is younger than you ......
    now the fact that the earth has a gyroscopic motion unlike rockets and a rocket doesnt always move with a constant speed and u wont be able to keep contact with others for long in a spaceship ... say if u whirl around the galaxy for a year according to your calender and clock when you land on earth you will see a bit more time has passed than u counted ... the other people wont also feel the same that u have passed a bit more time than they have ......
    i've tried my best ...
  7. Jan 7, 2015 #6


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    I'd like to clarify that what you "see" and what you "measure" are not the same in these scenarios. What you "see" will also include Doppler effects due to the person moving towards or away from you (as well as abberation, etc.) and as such, a person moving towards you at high speed will actually "look" sped up instead of slowed down. But once you take this Doppler effect into account, you will find the usual time dilation.
  8. Jan 7, 2015 #7


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    No, the other person will observe the same that you are observing. If you see the other person pouring tea slower than you are then that other person will see you pouring tea slower than they are.

    Why do you say both of you will calculate wrong? It's true that according to each of your frames of reference, the other person is younger than each of you. There's nothing wrong with that.

    I'm not sure what you're saying here. I hope you are saying that you will see a bit (actually a lot) more time has passed on earth than for you and the other people will agree that you have passed a bit less time than they have.
  9. Jan 7, 2015 #8
    ...... yes i meant exactly what you meant ... both will count the other persons time slower .. i was trying to say that she ll say " u r slow " and the other person will say , "no , u are slow ."....... and yes of course both will calculate wrong in the sense that if the spaceship werent accelerating in any point (which is not possible) and if the other twin didnt know that she was on a space craft , then after her arrival they will look the same ( not that one will be an aged lady and the other will be pretty young ) .... and this doesnt mean that the whole calculation we do for time dilation is wrong .... time dilation is more real than anything ..... its just i meant it in the naked eye it seems wrong to a person without the knowledge of frame of reference
  10. Jan 7, 2015 #9


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    Physics doesn't make a distinction between "measured time" and "time itself", because it's only concerned with stuff that you can measure.

    Two ways to understand this:

    In general, If a light clock and some other (say biological) process (at relative rest to each other) would not be both affected in the same way, you would get actual paradoxes: What if the light clock releases food rations to the human? For observes at rest to the two, the human would be well fed. But for those who observe the light clock to slow down extremely, the human would starve. Assuming that all processes are slowed down in the same manner, is the simplest way out of this dilemma.

    In this specific case, you can also argue that the biological processes are based on chemical reactions and thus electromagnetic forces, which are affected in the same way electromagnetic waves (light) are.

  11. Jan 7, 2015 #10
    You already received some great answers, so here's a little addition:
    Yes indeed; however, it's a little more than an idea: "space" and "time" in a "spacetime plot" account for actual measurements with rulers and clocks. The "warping of spacetime" expresses that the presence of a massive object such as the Earth affects our measurements of height and time. According to general relativity, objects near a heavy mass are deformed (they are shrunken in the direction of the gravitational field) and natural processes are slowed down compared to objects and processes far away from heavy mass.
    Verifying the deformation is extremely difficult and I think that it has not yet been done, but verifying the slowdown of natural processes is done as a routine with atomic clocks.
    Last edited: Jan 7, 2015
  12. Jan 7, 2015 #11


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    I'm still not sure what you are saying. Are you saying that it is possible for one twin to accelerate away, take a long trip, turn around and come back to the other twin and they won't see an obvious difference in their ages? Are you denying that one will be an aged lady and the other will be pretty young? Are you saying that the naked eye is not enough to see a difference in their ages but rather they need to understand Special Relativity and have knowledge of frames of reference in order to establish Time Dilation?
  13. Jan 7, 2015 #12
    yes i am saying that there wont be any visible difference between them if she always travels with a constant velocity ...but if thats not possible then the space traveller might look younger .
    actually traveling in space with constant velocity is not possible in real life and a point on earth is nowhere near an ideal frame of reference for a rocket ( there are all sorts of motion going on ) ... if u observe very carefully , in real life every second the rocket might be accelerating with a variable acceleration if u take the point from which it left the earth as the frame of reference ...
  14. Jan 7, 2015 #13
    What matters is speed with respect to inertial reference systems, and not with respect to the Earth which indeed is not exactly an inertial reference system (I hope that that is what you meant!). In such examples the effects of the Earth's speed and gravitation are commonly neglected compared to the effect of a very high speed of the traveller's rocket.
  15. Jan 7, 2015 #14


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    ...and this clock is doing just that - measuring the passage of time. What's special about the light-clock is that you only need high school maths to show that it must be ticking at different rates according to different observers. Therefore time must be passing at different rates for different observers.
  16. Jan 7, 2015 #15


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    But I didn't ask you about the traveling twin going at a constant velocity, I asked if she accelerates away, that's not constant velocity. And then I said she turns around, that's not constant velocity. And please, let's not get stuck on the fact that we don't have the technology to actually do this experiment.

    Let me ask again, suppose both twins are 20 years old when one twin takes a trip away from earth at 99% of the speed of light where she spends 5 years according to her clock traveling away at 99%c and another 5 years getting back. She will be and look 30 years old when she gets back and her twin will be and look 91 years old, according to Special Relativity. Do you agree?
  17. Jan 7, 2015 #16
    general relativity says nothing about an accelerating frame of reference ... note that this post's about general relativity ... and yes they ll look same with acceleration being neglegible ...and u cant prove what u r saying
  18. Jan 7, 2015 #17


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    We're not talking about general relativity and we're not talking about an accelerating frame of reference. This thread is not just about general relativity. The OP asked about Time Dilation and "A person returning to earth after a space journey". That's what you discussed in your first post and what we have been discussing ever since.

    Now you are claiming that for the scenario I described, the twins will look the same. That's wrong.

    Maybe a spacetime diagram will help:


    Are you saying that my spacetime diagram is correct but that the twins will look the same age when they reunite? In other words, are you saying that the red twin's clock will register 30 years since she was born but she will look like she is 91 years old, just like the blue twin?
  19. Jan 7, 2015 #18


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    "Constant velocity" isn't a complete specification of her journey and hence of the amount she ages on that journey. To know how much she ages, you have to specify the point in spacetime where the journey starts, the point where it ends, and the path between those two points.

    Furthermore, to compare the age of the traveller with her stay-at-home twin both twins must be at the same place at the same time (although not necessarily at rest relative to one another, which makes it easier to construct acceleration-free thought experiments) at the beginning and end of the comparison period. This is why the problem is often posed using idealized twins - we can reasonably assume they were at the same place at the same time at the moment of birth.
  20. Jan 7, 2015 #19
    u r talking about minkowski diagram ? yes ... special relativity kinda tries to solve this... i agree with u .... but also raise further many more questions ...
    what if both of them are travelling at exactly opposite directions with high speed and meets on earth after some time what will we see ?
  21. Jan 7, 2015 #20


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    What do you mean, "kinda tries to solve this"? Special Relativity exactly solves this, no equivocation.

    Obviously, they will end up at the same age. What age do you think that will be if they both take the path of the red twin but in opposite directions?

    That was one question that was totally unrelated to anything discussed before. What are your many more questions?
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