Talk:Twin paradox/Archive 3

message: :Some might object that, while the travelling twin experiences acceleration, so does the stay-at-home twin, both by ordinary motion and by being in a gravity field. Why one has the "better" acceleration which controls the direction of time dilataion is not apparent. Also, the journey can be arranged so that the stay-at-home twin experiences exactly the same accelerations as the traveller (by moving around on a stationary gravitating planet to exactly match the accelerations of the traveller whose rocket ship always accelerates at one planet gravity).
On the one hand, this seems to me to be a reasonable objection coming from someone trying to understand special relativity. On the other hand, it is all on its lonesome, not properly integrated with the rest of the article, and it disrupts the textual flow. I will therefore remove it, but am including it here, in case someone wants to integrate it properly later on.--Susurrus 01:47, 16 May 2006 (UTC)
thread_title:  A Stray Objection 

message: I don't understand why the lines in the Minkowski diagram are coloured the way that they are. The terrestrial observer is supposed to be moving upwards along the y axis, yet the lines are blue on the bottom and red on the top. When the lines are on the bottom, this is when the stellar observer is moving away from the terrestrial observer. Shouldn't this mean that the lines would be red-shifted, hence red, not blue? Conversely, when the lines are on the top, this is when the stellar observer is moving towards the terrestrial observer, and the lines should therefore be blue-shifted, hence blue, not red. After all, this is the way that the lines are coloured in the two diagrams below it. --Susurrus 03:21, 16 May 2006 (UTC)
replies: text: I don't think the colors were meant to have meaning, just to be distinguishable. —wwoods 06:43, 16 May 2006 (UTC)
thread_title:  Twins paradox Minkowski diagram 

message: Consider an alternate example: A space ship plans to go to the nearest star system a distance d = 4.45 light years away, at speed v = 0.866 c (i.e. 86.6% of the speed of light). Assume that this star system is at rest relative to earth. This space ship first travels in the opposite direction from this star system, then turns around and passes earth at speed v = 0.866 c and both twins record the time of this pass. A space station has already been set up at this destination that will record the time of arrival of the space ship, and relay a message from the travelling twin to earth containing all of his measurements. There will be no acceleration of the space ship between earth and the nearest star system. The one way trip will take t = d/v = 5.14 years in Earth time. Those on earth expect the travelling twin to be 0.5×5.14 = 2.57 years older when he reaches the space station than he was when he passed earth.
The ship's crew calculate how long the trip will take them. They realize that the distant star system and the earth are moving relative to the ship at speed v during the trip. Therefore, the distance between the earth and the star system will be shortened (by the length contraction) to  \epsilon d = 0.5d = 2.23 light years. The journey takes   2.23/v = 2.57 years. The twin on the space ship aged 2.57 years, just as those on Earth expected. This information, along with the time of arrival as measured by the space station is relayed back to earth by the space station. Of course there will be a delay in this transmission of 4.45 years. This will not matter as long as both twins do their calculations correctly. 
Notice that this example is almost identical to the first example given above. The main difference is that neither twin accelerates or changes reference frames between the time that the spaceship passes earth and the time it passes the space station. Yet the effect of time dilation is exactly the same. And there is no reason for the twins to reunite, since all of the measurements taken on the space ship is relayed to earth in a message. Obviously, in this example the difference in the amount of time that passed for each twin cannot be due to acceleration or either twin changing reference frames, yet this example is still not symmetrical. 
Notice that both twins use earth and the space station as reference point for the distance travelled, and that this space station is in the same inertial frame as the earth. The twin on earth is using reference points in his own reference frame to measure the distance traveled and mark the start and stop times. The twin on the space ship is also using the same reference points, but they are in relative motion to him. The fact that these reference points (earth and the space station) are at rest relative to one twin and in motion relative to the other is the reason for the asymmetry. The twin on the space ship will measure the distance travelled to be shorter, and the time it took to be shorter than that measured by the twin on earth. 
Also notice there is no reason to use General Relativity, since Special Relativity can account for the asymmetry of the twin paradox, while ignoring any acceleration or change of reference frames, and show that there is no paradox at all.
*Alan, the problem here is that we cannot ignore acceleration.  SR only works insofar as the matter under consideration has uniform motion (i.e. constant zero or non-zero speed, without any changing direction).  SR is called 'special' for a reason: it is a restrictive case of physical law and when unraveling the physical nature of the universe, we can only apply special restrictions when certain conditions apply.  Since the twin in the spaceship must change his/her speed to move away from the twin on Earth, and then slow down, and then turn around, and then change his/her speed to move back toward the twin on Earth, the spaceship twin is accelerating multiple times (change of speed and change in direction).  SR therefore does not, and can not, solely apply to this thought experiment.  This article accurately reflects this fact therefore.  I know this may not be a satisfying answer, but when we apply the laws of physics we must do so correctly.  If you would like to discuss these ideas further, send me an email at shale_d2010 @yahoo .com.  Let's reserve this talk page for article improvements. Cheers, Astrobayes 17:09, 21 August 2006 (UTC)
thread_title:  Alan's ''Alternate Resolution with non-accelerating twins'' 

message: if a force is applied to a rod 10 light yrs. long, it would require 10 yrs @ c to 
propagate to the other end before the whole thing could be in motion. You have to add
40 yrs for two accelerations and two decelerations. Phyti 05:09, 9 June 2006 (UTC)
replies: text: In fact we should do little more than represent what the literature provides. From which publication was that example taken? Harald88 21:57, 9 June 2006 (UTC)
thread_title: my mistake but

message: from Alan's previous example, but I don't think it really matters because there is so
much confusion about this subject from the reader's point of view.Phyti 00:12, 10 June 2006 (UTC)
replies: text: Please express yourself more clearly. Harald88 07:17, 10 June 2006 (UTC)
thread_title: the rod=

message: From a previous post by 'Alan Hill may 22 2006' who proposed a rod 10 light yrs long attached to his space ship. He did not consider the physics of the trip. The acceleration forces are not instantaneous, but would require 10 years to transmit to the other end of the rod (inertia). With two accelerations and two decelerations, he has to add 40 yrs to the trip(earth time). The second  point from (Alan's Alternate Resolution with non-accelerating twins) a quote:
"The ship's crew calculate how long the trip will take them. They realize that the distant star system and the earth are moving relative to the ship at speed v during the trip. Therefore, the distance between the earth and the star system will be shortened (by the length contraction) to εd = 0.5d = 2.23 light years, for both the outward and return journeys. Each half of the journey takes 2.23 / v = 2.57 years, and the round trip takes 2×2.57 = 5.14 years. The crew arrives home having aged 5.14 years, just as those on Earth expected." 
The amount of energy used to accelerate the ship if applied to the earth would obviously not produce an equivalent result in the opposite direction even by classical standards of momentum conservation, therefore the ship's speed has changed and not the speed of the earth. We can always have the ship make an orbit to start at the earth with a uniform speed and stay within SR. One thing left out of all the discussions is, the time dilation is not the result of the relative speed of the ship to the earth, it's the relative speed of the ship to light. The ship contracts according to the Lorentz theory, which the earth twin observes. The twin on the ship is subject to time dilation because of his speed, but he is not aware of this because his brain and all the elements of his body and every  particle on the ship that depends on internal light signals is effected the same way. It's the same state as the contracted length of the ship, his ruler contracts also therefore he can't detect any change. Obviously his motion is not going to change the size of the universe, but the effect of the time dilation makes his unit of time longer than on the earth. He arrives at locations with a known distance earlier than expected (by his clock). His conclusion, things are shrinking in his direction of motion. This is the difference between actual and perceived contraction. The reference "by length contraction" in the above quote usually will refer to the Lorentz factor but as shown is time dilation. There are two different principles at work.
Hopefully this is clear enough. Phyti 01:41, 11 June 2006 (UTC)
thread_title:  for clarity 

message: , aka the Jazztel triple play services anon from Spain, made some changes which made a rather bad article worse:
#"(In GR the word gravitational is used frecuently to speak about fictitious forces)" This is very misleading.
#"With this effect in mind we can explain the point of view of the travelling twing. When he starts accelerating towards his brother he will see a fictious field, and the whole universe will be free-falling in this field, including the brother at home." This is nonsense.
---CH 23:32, 13 June 2006 (UTC)
replies: text: I find that sentence rather obscure. Perhaps the author tried to say that in GRT the laws of nature are valid for inertial frames and accelerating frames alike? Harald88 18:18, 6 July 2006 (UTC)
thread_title: Edits by 87.217.89.114

message: I have to agree with the article when it states that "this occurs because of faulty assumptions". Obviously there has to be a faulty assumption somewhere.
Unfortunately the faulty assumption has not been correctly identified. It is in fact 
: 2AB/(t'A-tA) = c,
significant because in relativity, light travels from A to A in time t'A-tA, which is of course a faulty assumption. That it is indeed an assumption is stated in the English translation of Einstein's paper, "Zur Elektrodynamik bewegter Körper", where the translation reads : 
"In agreement with experience we further assume the quantity 2AB/(t'A-tA) = c,
to be a universal constant--the velocity of light in empty space.
Certainly no mathematician would identify himself with a reversal of direction and call that "velocity", not even a schoolboy. 
(First posted to 
http://groups.google.co.uk/group/sci.physics.relativity/msg/4674b29449f08fb0?hl=en
Wed, Mar 17 1999 9:00 am when Schaefer, a major wikipedia editor and proponent of Einstein's relativity, was a USEnet correspondent and should have been aware of it.)
Der alte Hexenmeister 21:39, 11 July 2006 (UTC)
replies: text: I'm afraid that your own WP:OR isn't a valid source for information in Wikipedia. Harald88 22:11, 11 July 2006 (UTC)

text: Velocity is a vector expressing the speed of a mass, as well as its direction.  To reverse a direction is an acceleration because the velocity is changing, and this is a simple physics 101 statement.  The pejorative "schoolboy" comment above is unfortunate because any "schoolboy" (or girl) of physics learns within the first week or two that a change in direction is an acceleration, so your comment is out of place. This entire concept of acceleration coming from a speed or direction change is what allows us to exploit angular momentum to stabilize satellites.  I would be interested to see any physics textbook you can cite that does not call a change of direction of a moving object an acceleration.  You can bet such a text isn't used in any university physics course. Astrobayes 17:43, 21 August 2006 (UTC)
thread_title:  Good faith agreement 

message: From the article:
#The twin paradox, sometimes called the "clock paradox", stems from Paul Langevin's 1911 thought experiment.
#In 1911, Langevin discussed the evolution of the concepts of space and time in physics and presented the principal aspects of special relativity.
#The usual resolution of the paradox as presented in physics text books ignores its origin (it only surfaced with general relativity, see above) 
:  General relativity was not written in 1911, how could Paul Langevin's paradox have surfaced with general relativity?
Der alte Hexenmeister 22:04, 11 July 2006 (UTC)
replies: text: As explained in the article, it wasn't regarded paradoxical in 1911 when Langevin discussed the example of differently aging people. And as the article also points out, neither was it considered paradoxical when Einstein presented the basic calculation with clocks in 1905. Perhaps another clarifying sentence is needed in the article to spell it out? Harald88 22:17, 11 July 2006 (UTC)

text: General relativity was not written in 1911, how could Paul Langevin's paradox have surfaced with general relativity? 
Der alte Hexenmeister 08:14, 12 July 2006 (UTC)
replies: text:  I can only advice you to read the article. Harald88 22:16, 12 July 2006 (UTC)
thread_title: Conflicting and/or ambiguous information