# Can somebody explain time dilation to me please?

Discussion in 'Physics & Math' started by maxjojo, Mar 13, 2017.

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1. ### maxjojoRegistered Member

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You said ''Proper time is definitely physical. That is what clocks measure.''

A vacuum is empty of everything except ''space'' , what do you suggest is waiting to be measured? If there is nothing in the vacuum with physicality to measure but time definitely still exists, then surely this implies that Newton was part correct and absolute time existing in a vacuum?

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3. ### arfa branecall me arfValued Senior Member

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Time dilation pertains to the measurement of local times in local, but moving relatively, frames of reference. The difference, or dilation, in the rate of aging in one frame compared to the other, is something that can be compared in a third "stationary" frame. In fact this stationary frame can be chosen anywhere, what's relevant are the measurements and their comparison.

You assume that a metre stick, or unit of distance, will have the same length in any frame where the distance is a local measurement. From another frame moving relative to where that local measurement is made, that distance can appear to be less than a metre. The same thing happens with time, except time appears to "move farther" or speed up relative to the other frame.

In the twin paradox, the whole thought experiment depends on comparing times in the two twin's local frames. This means the travelling twin has to change course and return to where they started, and this involves them changing their coordinate system, the other twin is "stationary" relatively.

Moreover, accelerations aren't really relevant except at the start, where one twin accelerates to some fixed velocity, and when the course change is made. Since this thought experiment can be drawn as a spacetime diagram with mostly straight lines, the effect is because of velocities, not accelerations.

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5. ### maxjojoRegistered Member

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I have watched a video on youtube explaining the twin paradox. I can not understand how the travelling twin ages less but returns to the ground state twin in the ''now'' of that twin. If the ground state twin has a timeline of ten years gone by, the travelling twins clock runs at half the rate of the ground state twins clock, the travelling twin having only a five year time line, then the return to the ''now'', to the ground state twin, would surely be impossible?

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7. ### arfa branecall me arfValued Senior Member

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Not only possible, but actual. This "ground state" you refer to is actually two distinct nows, one for each twin. Bring them physically back together and they share the same now.

rpenner has told you that your daughter is in her own present, when she's going to the shop, than you are staying home (with your own coordinate time and proper time), these are different but not by very much, not measurable with ordinary stopwatches, for instance.

Last edited: Mar 15, 2017
8. ### maxjojoRegistered Member

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Did you even read what I wrote?

How can it be possible with two different length time lines in my scenario?

9. ### arfa branecall me arfValued Senior Member

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It can be because the lengths of the timelines are frame-dependent.

10. ### Confused2Registered Senior Member

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Imagine you have a transmitter that can bounce a signal off the Moon and beam it back to you. It takes nearly three seconds for radio waves (and light) to make the round trip. You start counting ... "One Mississippi... Two Mississippi ... Three Mississippi.." and as you get to Four Mississippi the "One Mississppi" you sent three seconds earlier comes through (unchanged) on your receiver. The idea that something sent earlier can arrive back later and be unchanged by time is a fairly simple concept. In the same way, if your daughter could travel to the Moon and return at the speed of light she would come back exactly as she left 3 seconds earlier, for her no time would have passed, she would still be on "One Mississippi" while for you three seconds (three Mississippis) would have passed.

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11. ### rpennerFully WiredRegistered Senior Member

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Incorrect. A "frame" is an imaginary inertial coordinate system. Saying that lengths of segments of world-lines are trajectory-dependent avoids introducing any coordinates into the discussion.

Another geometrical way to say this is: In Euclidean geometry the straight line is the shortest curve connecting any two points A and B. In the Minkowski geometry, the inertial trajectory is the time-like curve with longest measure connecting any to time-like associated events A and B. Since stay-at-home mom is moving with constant velocity = 0, that's the inertial trajectory connecting A=(x=home, t=25 minutes ago) and B=(x=home, t=now).

12. ### rpennerFully WiredRegistered Senior Member

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If space-time can be said to have a geometry, then that four-dimensional geometry is composed of points where and when events could potentially happen; thus we can speak of the invariant interval between these points even in the absence of actual events. Likewise, you can speak of a volume of vacuum inside a container, but all real examples of which, that container must be completely evacuated for a non-zero temporal duration. Space and time don't just vanish because nothing interesting fill them or nothing interesting happens.

That's not what Newton's concept of absolute time is.

13. ### The GodValued Senior Member

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Maxjojo knows more than a normal curious mother out to help her daughter!

Enjoy the ride guys.

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14. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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I agree with you assessment.

15. ### maxjojoRegistered Member

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I must of misunderstood the below paragraph.

''Unlike relative time, Newton believed absolute time was imperceptible and could only be understood mathematically. According to Newton, humans are only capable of perceiving relative time, which is a measurement of perceivable objects in motion (like the Moon or Sun).''

I assumed Newton meant space without motion or matter and time was so subtle to notice it passes by at an immediate rate?

Last edited: Mar 15, 2017
16. ### maxjojoRegistered Member

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I could imagine that, but I would not overlook the pre-existing space that was there before you sent the signal. The existing space being in the ''now' along with the Moon and the Earth.

17. ### RJBeeryNatural PhilosopherValued Senior Member

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Agreed. *unsubscribe*

18. ### maxjojoRegistered Member

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Like I said earlier, only a male would have such ego as to believe a women was incapable of learning a subject in relative short time. Do you fear adversaries in some way? Do my questions challenge your own ego?

I knew nothing about the subject until members here guided in the correct direction and understanding of the simplistic relativistic state of matter.

19. ### exchemistValued Senior Member

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No I don't imagine anybody's "ego" feels challenged by you, though you have to understand we do get quite a lot of trolls, masquerading under false pretences in order to wind up a scientist or two (people find amusement in such odd ways!), so we are a suspicious lot. Remember, we don't know who you are, and it takes time for you to build up a reputation for honest enquiry.

In fact, though, I'd have thought if you are stuck at home with a child, it might well be the time you get to thinking - and maybe reading a bit- about such things as this. But I'd suggest laying off the sex wars stuff. It is going to get in the way.

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20. ### sweetpeaRegistered Senior Member

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Yes, I think Ms Origin was taken aback with that outburst.

21. ### originIn a democracy you deserve the leaders you elect.Valued Senior Member

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LOL.

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22. ### maxjojoRegistered Member

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Being judgemental in only one thread I have engaged in, is too fast of an opinion about myself. I of course have been reading articles and watching documentaries so I could try to understand dilation. However my question is still unanswered.

I assumed Newton meant space without motion or matter and time was so subtle to notice it passes by at an immediate rate?

23. ### exchemistValued Senior Member

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Yes, it was too quick by some of my co-contributors, and I am not judging you yet. But as I'm a chemist who only understands Special Relativity on a good day, and General Relativity barely at all, only its effects, I'll leave others to field your questions.