Talk:Twin paradox

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[edit] The symmetrical clock paradox

Some people get lost in the twin paradox presented here (with accelerations and all), so maybe a simpler, symmetrical paradox is in order. So, here is that "gedanken" experiment:

Let's have two identical rockets, carrying two identical high precision (in today's technology that would be atomic) clocks, with digital displays. The displays show the number of ticks (cycles) that each clock recorded (this could be for instance number of nanoseconds - a simple integer) since last reset. The rockets and clocks include a mechanism that fires the thrusters on detection of blue light and resets the clocks on detection of red light.

These two rockets are placed in space, away from any gravitational fields, so that they are pointed to one another, on a straight line. The rockets are far away from each other and we shall call the left one A and the right one B. Midway between rockets A and B we place a source of light we shall call S, capable of emitting blue or red light, each simultaneously to left and right, toward rockets A and B.

Rockets A and B, together with S form a single inertial frame of reference S for now (in other words, they are all stationary to one another).

Now we send blue light simultaneously from our source S toward rockets A and B. This starts the thrusters and the rockets burn their fuel, therefore being accelerated toward one another, on a collision course. The rockets burn all their fuel and each reaches constant velocity v in relation to our stationary frame of reference S, but in opposite direction. The speed at which the rockets are travelling is "normal" speed (i.e. not nearly close to the speed of light, but something a normal rocket could do).

After the burn is completed, we send red light simultaneously from our source S in the direction of rockets, therefore resetting the counters of our clocks. Now we have a system of three inertial reference frames, A - for rocket A, B - for rocket B and S - for our source of light S, in linear motion in relation to one another. And we have our clocks on rockets A and B synchronised, as observed from S.

Just before the collision occurs (i.e. when rockets A and B are in immediate vicinity of light source S) and if the trip of rockets is sufficiently long, we shall have the following situation in relation to clocks, as observed from different frames of reference, due to time dilation as per special theory of relativity:

  • clock A displays N as observed from reference frame A
  • clock A displays M (different from N) as observed from reference frame B
  • clock A displays L (different from N and M) as observed from reference frame S

And due to symmetry of motion and identical construction of clocks:

  • clock B displays N as observed from reference frame B
  • clock B displays M (different from N) as observed from reference frame A
  • clock B displays L (different from N and M) as observed from reference frame S

In comparison, such a system would show N on both clocks as observed from any frame of reference using only Newton's mechanics. --Jimbo2x (talk) 20:19, 3 January 2008 (UTC)

Since this is entirely original research we can't of course put it in an article, and it doesn't really belong on this talk page either. You might try Usenet. As a matter of fact it just so happens that someone (or you?) just asked exactly the same question on Usenet in sci.physics.relativity.
Anyway, I'll give you two hints here. (1) You write: "... in linear motion in relation to one another. And we have our clocks on rockets A and B synchronised." When they are in relative motion, they are not synchronized. They both see the other one's clock "run slower". (2) You created a symmetric situation, so you get symmetric results. Good luck on Usenet :-) - DVdm (talk) 21:44, 3 January 2008 (UTC)
Point taken about original research. I thought someone must have done this before (it seemed unlikely that this would be original). And it wasn't me on Usenet - so obviously not all that original ;-)
In terms of "synchronised" - they are synced in the sense that they are both running equally different as observed from S. About symmetric - yeah, that was my point. I think it would be useful to show that observers in different reference frames would read the same clock differently in immediate vicinity of each other according to SR.
--Jimbo2x (talk) 21:58, 3 January 2008 (UTC)
The idea of symmetry is of course not new, but I had never seen this kind of setup with the central light.
Little note on "... synchronised in the sense that...". Sure, but that's another kind of synchronization. From the three clocks in the setup, no pair of clocks is synchronized in any way. You could perhaps say that the clocks A and B are sort of equally desynchronized according to S, but as soon as you forget S, A and B are just two systems in relative motion - nothing special about them. In fact, one can construct a "central S" for every pair of systems in relative motion. That can't possibly make the pair physically special.
Do try the Usenet thread - it might become interesting :-)
Cheers, DVdm (talk) 22:25, 3 January 2008 (UTC)
There are only two clocks in the setup - A and B. If two equidistant stationary clocks can be synced by passing a beam of light simultaneously from the midway to them to "reset" them, so can the same two equidistant clocks in uniform opposing motion be synced as observed from S, unless the beam of light somehow travels to the left at different speed than to the right :-) Sure, from A, B is observed to be different (due to relative motion) and vice versa, but for an observer in S, they will be in sync (i.e. explicitly, both clocks display L when rockets meet, as observed from S). I never thought nor do I think that S would make A and B special. S is just another inertial frame of reference and an observer, just like you said.
--Jimbo2x (talk) 23:07, 3 January 2008 (UTC)
Well, you really do have a third clock in there. The minute you mention "reference frame S", you have it. No reference frame without a clock :-)
Do try Usenet for this - it's better suited for this kind of thing. This page is for discussions about the format and content of the article, not the subject. Cheers, DVdm (talk) 08:59, 4 January 2008 (UTC)
No worries. --Jimbo2x (talk) 09:41, 4 January 2008 (UTC)
Actually, all observers in the above example would see one and the same value on both clocks. That is to say, my example does not reach correct conclusions. --Jimbo2x (talk) 23:19, 14 February 2008 (UTC)

The problem is: whenever somebody tries to make a setup, which clearifies, what one is talking about, this "problem" is solved with the axe original research. The twin paradox has to be a mystery. There are so many ways to misunderstand what is argued. Simple example: what is a clock. It could be so simple: Two identical laboratories are connected by a loaded spring. This mechanism is released and according to conservation of momentum both labs gain the same speed, relative to each other. Now the have the following instruments: a source of light, that is a resonator, amplifying a certain spectral line of a certain gas of atoms and a prisma or better, grating. This light is directed to a semi reflecting mirror, twisted 45°, so 50% of the light is directed to the grating, 50% is send in direction of the second "twin" lab. So, the grating deflects two different beams: one from the local source and one from the distant source. It should be possible, by elementary calculation to determine the ratio of the detected deflection angles. And it should be possible to have a shutter which modulates the outgoing beam with this determined ratio on and of. And again it should be possible to compare the on/off ratio of the received and send signals. I have to point out: this is no OR, it is just a try to find a common language of what is a clock, how to measure a frequency..., how to measure distance and time in an environment, where nothing is absolute, except "c". ErNa (talk) 06:32, 9 May 2008 (UTC)

[edit] Accelerated rocket calculation

IMO the "Accelerated rocket calculation" doesn't add anything valuable to the article, and certainly doesn't add enough to justify its length.--76.93.42.50 (talk) 15:36, 17 March 2008 (UTC)

Seconded. Oddity- (talk) 09:14, 7 May 2008 (UTC)

[edit] Making the article easier to understand

The twin paradox is much easier to understand if one imagines a long line of stationary, evenly spaced, and synchronized clocks extending from the stationary twin to the point where the other twin turns around. Imagine that as these clocks pass the moving twins window a strobe flashes so he can read off the elapsed time. Even though the non-moving twins clock seems to the moving twin to be ticking at half the rate of his own, the elapsed time, as told by the clock immediately outside his window, is passing at twice the rate of his own. More importantly, just before he stops, in order to turn around, the line of clocks are, from his perspective, out of synch but the moment he stops the line of clocks will be perfectly synchronized again which means that the nonmoving twins clock now reads the same as the clock he is next to. That means that his calculation of what the nonmoving twins clock said jumps suddenly while he decelerates (which leads to general relativity). Of course, this doesnt add anything to the article that wasnt there before but it does make it easier to understand. Em3ryguy (talk) 08:17, 20 May 2008 (UTC)

There are language and reference [frame] problems in your essay. Also sounds like WP:OR because most of the time dilation issues in the article are discussion points on old/accepted papers and books on relativity, and you quote none. Jok2000 (talk) 02:55, 21 May 2008 (UTC)
First, its a suggestion not as essay. Second, it can be rewritten by anyone to any form they like. Third, since it doest change anything or add anything to the article that wasn't there before and moreover since every statement made is trivial I dont see why a citation would be necessary. I am simply suggesting it as a way of illustrating what is being talked about. Em3ryguy (talk) 03:57, 21 May 2008 (UTC)
Where do you draw the line between stating the obvious and publishing original research? If one can do nothing but repeat what has already been published and one cant even make a trivial observation then I have to question the usefulness of WP. People can read the published articles themselves. They look to wikipedia to illustrate and explain these complex ideas in simple terms. At least, I do. Em3ryguy (talk) 08:45, 24 May 2008 (UTC)