# The Naked Scientists Forum

### Author Topic: A question on time  (Read 2004 times)

#### acecharly

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##### A question on time
« on: 12/06/2013 12:46:30 »
When an object moves in relation to another and for example the stationary object is observed by the moving object to be one year older and vice versa does this show us that there is not one time but that time is almost in layers?

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Ace

#### yor_on

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##### Re: A question on time
« Reply #1 on: 13/06/2013 02:29:33 »
How do you measure a time? You use a clock, right? Something giving you even chunks of 'time' from where you define processes. You can prove time dilations, but to do it you need acceleration/decelerations to create that 'twin experiment'. Using solely uniform motion it becomes impossible, as far as I know, to prove a 'twin experiment', as you can't get back to earth to compare the result.

But you can measure time on Earth, in a uniform motion. And you can compare that clock you use to your metabolism (life span). Doing so you won't find your lifespan to gather more, or less, 'chunks of time', no matter what uniform motion you define yourself to have. It stays the same as always locally measured.

But your measurements of other (light) clocks will change with what uniform motion you define yourself to have, relative what you measure. The arrow of time is a local definition to me, and a 'constant' in all uniform motion. And those 'layers' you saw comparing, disappear as soon as you join whatever 'frame of reference' you compared that local clock to before.

So, the arrow will be locally the same for you, everywhere, as I think. And if it is, then it becomes hard discussing it as positional layers in space and time.

From comparing positions in SpaceTime it's observer dependent, so if you define some position to some specific 'layer of time', it will only be true relative you. The next 'observer' might define that same position differently, depending on its local mass and 'motion'.
« Last Edit: 13/06/2013 10:41:19 by yor_on »

#### yor_on

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##### Re: A question on time
« Reply #2 on: 13/06/2013 02:39:52 »
Comparing, a arrow becomes a relation, you comparing between frames of reference. But locally, imagining each 'frame of reference' as a defined position in SpaceTime, you can see the arrow as something 'ticking' at a same rhythm for you, wherever you go. You can in fact consider all dimensions as some sort of 'constants' locally, related to you, your ruler and a clock unchanging locally, defining that space and time you compare them against.

#### damocles

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##### Re: A question on time
« Reply #3 on: 13/06/2013 02:50:15 »
From Yor-On:
Quote
You can prove time dilations, but to do it you need acceleration/decelerations to create that 'twin experiment'. Using solely uniform motion it becomes impossible, as far as I know, to prove a 'twin experiment', as you can't 'get back' to earth to compare the result.

Actually you can perform the "twin experiment" (in principle) by the following ruse: A is a timekeeper in inertial motion. She co-ordinates setting her timepiece with B who is dashing past at 0.99c. B transfers the reading on her timepiece to C who is dashing past in the other direction at 0.995c. When C is passing A each transfers the reading on her timepiece to the other, and both can agree that C's timepiece has recorded a smaller interval of time than A's

#### yor_on

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##### Re: A question on time
« Reply #4 on: 13/06/2013 03:47:32 »
Haven't seen that one Damocles :) But how do they coordinate their clocks? In a co moving frame it's fairly easy to do so, you just bounce a light signal - A to B - (it bouncing back to A), B set his clock at null receiving it. The signal bounces back to A who just have to split it into two, to get the time it took to reach B, then setting his clock to that number. But here you have A inertial, B at 0.99, then meeting C at 0.95. then both (A and C) transferring their readings to each other, simultaneously?

It seems difficult to do that one? But it would be sweet if it worked.

#### yor_on

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##### Re: A question on time
« Reply #5 on: 13/06/2013 04:00:42 »
Apropos Clocks :)

This one EinsteinClocks. is rather nice. A little pearl I think, everything laid out logically, with the mathematics explained too.

#### damocles

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##### Re: A question on time
« Reply #6 on: 13/06/2013 06:35:46 »
From yor-on:
Quote
Haven't seen that one Damocles :) But how do they coordinate their clocks? In a co moving frame it's fairly easy to do so, you just bounce a light signal - A to B - (it bouncing back to A), B set his clock at null receiving it. The signal bounces back to A who just have to split it into two, to get the time it took to reach B, then setting his clock to that number. But here you have A inertial, B at 0.99, then meeting C at 0.95. then both (A and C) transferring their readings to each other, simultaneously?

It seems difficult to do that one? But it would be sweet if it worked.

yor-on that does not really arise, because the trick is in the specification "as they pass", and to avoid the need to allow for any doppler shift we could stipulate that the transfer of a time reading be digitally encoded, and that the length of voyage should be much longer than the time taken to transfer the digitally coded reading.

So, working through the detail, A and B can both set their clocks to null as they pass. That need not involve any error at all -- they can be within a light microsecond of one another if they want. Similarly B and C can pass arbitrarily close to one another and although there will be disagreement about the time, there will be no disagreement about the spatial separation.

The transfer of clock reading is effectively co-ordinating C-time with B-time. It can again be done without error: C might be a little astonished about the reading she has just received because it will not match her previous reckoning. But never mind that-- she will reset her clock. As C passes A, they can again transfer clock readings rapidly and without error, and both A and C will agree that the travellers (B>>C) have recorded a smaller time lapse than the stay-at home (A).

#### yor_on

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##### Re: A question on time
« Reply #7 on: 13/06/2013 10:38:45 »
Ahh, fair enough Damocles :)

#### acecharly

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##### Re: A question on time
« Reply #8 on: 13/06/2013 10:47:16 »
So is there is no absolute time then when we use relativity? just a measure of time? ie a second to me is a second to the man on the spaceship but we perceive each others as being different. If the man on the spaceship returned to earth to find himself one year younger than the man on the ground neither moving with relation to the other wouldn't thy be living in a different of time?

#### yor_on

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##### Re: A question on time
« Reply #9 on: 13/06/2013 14:54:43 »
time dilations exist, and they can be described from two points of view. Globally, as assuming a 'whole common universe' same for us all, the 'place' in where we live. From that point of view a 'time dilation' is observer dependent, related to mass, 'energy' and (relative) motion, as well as accelerations/decelerations. So they measurably 'exist', but related to you measuring. If all objects in a universe only had (relative) uniform motion accessible, they would still exist, and using Damocles definition, also being 'real', not a illusion. From that point of view the other definition, using 'locality' makes sense too. That as you from that point of view do have a unchanging arrow locally, related to your life span. Nothing you do will change that measurably and locally, as I think. But from that point of view 'frames of reference' becomes something 'real' in themselves, and the commonality of a 'same universe' questionable.

But it is still the simplest approach I've found. And it defines a arrow that points one way, not ticking backwards. You can't even find a arrow tick backwards astronomically, comparing your clock to a event horizon. The best you will find is a definition of some other frame 'stopping to tick' relative your own clock. Mathematically you labor with time reversibility, and treat a arrow by itself. Einsteins definition though, is one in where the arrow is one of four 'real' dimensions called SpaceTime, all dimensions 'fused/joined' together. You tweaking one parameter by mass or motion should then have a effect on the other, which it has, although when it comes to 'length, width and height' they all goes together. You can separate a 'space' theoretically into three dimensions, but you can't do it practically. Using 'degrees of freedom' instead of dimension you can define a two dimensional lattice, the atoms losing one (direction) degree of freedom, but in reality all such 'systems' still are embedded in a four dimensional reality called SpaceTime. Using such a definition ('fused together' dimensions) and also imagining a arrow to tick backwards, what result would that have on the other three dimensions?

Degrees of freedom are the better concept to me, though. Dimensions becoming weird in my thoughts.
=

It would be good if you got five, ten minutes (real local time:) to correct spelling etc, before getting your post 'fixed' by eternity :) I think TNS, or rather the developers, tested it locally, not really considering locations outside. and slow connections, because the time estimate I see after correcting is not the same as mine. A 'time dilation' possibly :)
« Last Edit: 13/06/2013 15:06:35 by yor_on »

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##### Re: A question on time
« Reply #9 on: 13/06/2013 14:54:43 »