Talk:Time dilation/Archive 2011

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light clock example

I have seen some confusion resulting from the light clock section of the article on physics forums, although mostly from those that don't understand time dilation in the first place, but then, it is the purpose of the article to demonstrate such things, so I suggest that this particular section be edited somewhat to clarify, especially since it is the heart of the article. I have tried editting it myself, the first time being cut because it didn't agree with the diagrams and the second I cut myself because it introduced new variables and the resulting equations didn't agree without further modification and explanation, while the point is to keep things simple. Anyway, I think adding these next couple of paragraphs to the end should help with any concerns about that section. The first paragraph is as concise as it can be, I believe, while still agreeing fully with what has already been presented. The second paragraph may require some rewording, but it may important to include that as well so that readers know that the same time dilation can be found from the point of view of either of the frames.

"This is the time dilation found only by the frame of reference that is considered stationary as it observes the light pulse which is bouncing back and forth in the moving frame. To clarify this further, let us now place a mechanical clock at mirror A in the moving frame. All frames must agree upon the readings of the mechanical clock as the light pulse departs mirror A in the moving frame and then arrives back at mirror A again, since those two readings of the mechanical clock directly coincide in the same place as the two events of the light pulse departing and arriving, therefore all frames observe the difference between these two readings in the moving frame to be Δt as that frame measures itself. According to our frame of reference, however, the same light pulse travels a longer path in the time Δt', whereby the time dilation that the stationary frame of reference observes of the moving frame's clock, as found earlier, is the ratio of the time that the stationary frame observes passing upon the moving frame's clock as compared to the time that passes upon the stationary frame's own clock.

If we were to now reverse the roles of the observing and moving frames, making the original frame of the light clock the observing frame while considering the other frame to be moving, while also placing another light clock in the other frame with a second light pulse bouncing back and forth between two mirrors in the same way as in the original frame, then the first frame will measure the same time dilation of the second frame's clock as well, found in the same way as before from the other frame, demonstrating that each frame measures the same time dilation of the other." (Grav-universe (talk) 18:33, 4 January 2011 (UTC))— Preceding unsigned comment added by Grav-universe (talk • contribs) 18:31, 4 January 2011 (UTC)

Hi Grav, I don't think that it is the purpose of the article to demonstrate things to confused people on physics forums. When we add content to the article, the first thing we must do, is to have a reliable source (or sources) for the edit, and make sure that we do not add our own original research. As it currently stands, the section is backed by four sources. You can consider adding a new section to the article with some other point of view and perhaps with different graphics and/or equations, but that will need proper, and, very likely, reliable sourcing and a consensus on the talk page that it is indeed needed. That's how Wikipedia works - this is definitely not a physics forum. Cheers - DVdm (talk) 18:49, 4 January 2011 (UTC)

Thanks, DVdm. Here is a good reference about adding a mechanical clock to the light clock that was supplied to me. http://books.google.com/books?id=jYHtp6kx8qgC&lpg=PR1&pg=PA40#v=onepage&q&f=false Brian Greene's The Elegant Universe, pg 40

Here is a diagram from Wikimedia that can be added as well - http://commons.wikimedia.org/wiki/File:Light-clock.png.

It was also suggested by several knowledgable people that I avoid indicating which frame is moving and which is stationary, as either can be considered as such, edit the first sentence about which frame the light pulse is bouncing in, since the same pulse is also bouncing according to any inertial frame, and that the last paragraph is generally mentioned in the rest of the article, so I made it just a quick mention. Here is the result. What do you think? Will it work with the reference given?

"This is the time dilation found only from the frame of the observer who sees the clock in motion. To clarify further, let us now place a mechanical clock at mirror A. All frames must agree upon the readings of the mechanical clock as the light pulse departs mirror A and then arrives back at mirror A again, since those two readings of the mechanical clock directly coincide in the same place as the two events of the light pulse departing and arriving, therefore all frames observe the difference between these two readings of the mechanical clock to be Δt as that frame measures itself. According to the observing frame, however, the same light pulse travels a longer path in the time Δt', whereby the time dilation that the observing frame measures of the other frame's clock, as found earlier, is the ratio of the time that the observing frame views passing upon the other frame's clock to that of the time that passes upon the observing frame's own clock. In addition, if we were to now place another light clock in the second frame while reversing the roles of the frames, then the first frame will also measure the same time dilation of the second frame's clock, demonstrating that each frame measures the same time dilation of the other."

(Grav-universe (talk) 20:34, 4 January 2011 (UTC))

I have a copy of Greene's book here and have checked pages 37 thru 41 (section The effect on Time: Part II). I see (some) support for the above text up to about half way (see the Rolex-example in the book). However, starting at the second halve ("According to the observing frame...") the above is text is first inspired by the earlier part of the book that explains things exacly like it already is explained in the current article section (the thing with the longer path). Furthermore, subsequently Greene does not mention anything about a "ratio of the time that the observing frame views passing upon the other frame's clock to that of the time that passes upon the observing frame's own clock", nor about what you say in the last sentence "In addition, if we were to...". So I think it is clear that your proposed text is not supported by this source and that, since you use a result form higher up in the current article, your text is largely wp:SYNTHESIS and thus wp:OR. I'm even tempted to say that it looks like we could use the source rather to support the current section, but of course we won't add it to the list since the article section contains equations whereas the source obviously does not. DVdm (talk) 22:30, 4 January 2011 (UTC)

Too technical?

This article has been tagged as too technical (not by me). While I find most of the article pretty well written, someone who isn't familiar with special or general relativity might still wish to learn what is meant by time dilation in a general sense. There are a few different ways this article could be altered to promote this:

Perhaps a section at the beginning could be inserted to give a description which uses less technical language to give a sense of what time dilation is. This section might not have to capture all the details, or even that there are two different phenomena in which it arises.
A simple thought-experiment description, from an observer's perspective
A section which briefly defines the important terms: observer, reference frame, clock.

Deciwill (talk) 22:39, 3 February 2011 (UTC)

Comment from another person; I agree with the need for a thought experiment. Might I suggest two twins (A&B) in deep space with no acceleration, each with a clock. Relative to A's perspective, B travels a distance h with acceleration a, coasts a distance d, de-accelerates with acceleration -a until at rest with respect to A, and then performs a symmetric trip back to return to A. The difference in clock times will have an analytical solution that can be given and be graphed as a function of a*h for fixed d, and if there is any d dependence (?) vice versa, ie, as a function of d for fixed ah. There is way too much reliance on explaining how puzzling this sort of question is rather than just giving the correct answer. The twin paradox is too puzzling because it focuses only on the coasting part of the trip and the twin's telepathic knowledge of each other's clock speed. By making the two clocks undergo acceleration (as the twins ultimately do) you put them back in the same reference frame where the interpretation of accumulated clock differences is not so terribly difficult. Then you can graph clock A-clock B time (y-axis) relative to reference B time (x-axis), and then graph clock A - clock B time relative to reference A for the entire trip, and finally be reassured that the two graphs do come up with the same cumulative descrepancy in the end.

Since some people want a big reduction in technical level, they should get an early reference to a related Wikipedia article on the twin "paradox", which is probably where there interest lies. I think it is justified from the title "time dilation" that this article should get more technical after an mellow introduction, but a less technical for at least a few paragraphs. —Preceding unsigned comment added by 128.244.9.9 (talk) 21:10, 11 February 2011 (UTC)

173.59.245.69 (talk) 13:41, 10 February 2011 (UTC)

Equation 3 error RESOLVED ON 03/22/2011

On 03/22/2011 I personally added a minus sign in front of the last term of Eqn (3). I wrote the comments below prior to making this change. So as of now Eqn (3) is corrected, and the material below only makes sense if you imagine no minus sign in front of the last term, which was the case prior to my change on 03/22/2011. —Preceding unsigned comment added by 128.244.9.9 (talk) 13:58, 22 March 2011 (UTC)

The last radial velocity correction term in Eqn (3) seems to have the wrong sign. Clicking just above Eqn (3) on the Wikipedia Schwarzschild metric equation reference the radial term is;

-(dr^2)*(1-Rs/R)^(-1)

(1-Rs/R)^(-1) = 1+[-1+(1-Rs/R)^(-1)] = 1 + [ (-1+Rs/R)/(1-Rs/R) + 1/(1-Rs/R) ] = 1 + (Rs/R)/(1-Rs/R) = 1 + 1/(R/Rs-1) = 1 + (R/Rs-1)^(-1)

Rs/R=2GM/(Rc^2)=2U/c^2

-(dr^2)*(1-Rs/R)^(-1) = -(dr^2)*[1 + (c^2/(2U)-1)^(-1) ]

Divide by dt^2 to get radial velocity squared. The leading minus sign indicates the 1st radial velocity term above correctly contributes to Eqn (3)'s total velocity term, -(v/c)^2. The leading minus sign also hits the 2nd term but this minus sign mysteriously vanishes in Eqn (3). So either Eqn (3) is wrong or the Wikipedia Schwarzchild reference is wrong.

There are two references to Eqn (3). The TD Moyer reference neglects this higher order radial velocity term altogether. The Matolcsi reference sign error was acknowledged by Matolcsi himself, who emailed me at 7:00 AM (my time) 03/22/2011 to acknowledge that the sign in his Eqn (14) was in error. In this paper Eqn (14) serves as an intermediate step to Eqn (15), which removes this higher order term and is correct to the order of approximation stated.

At this point I will try to make the last term in Wikipedia "Time Dilation" Eqn (3) have a minus sign in front. If I succeed, this could make the prior material a bit confusing. —Preceding unsigned comment added by 128.244.9.9 (talk) 13:52, 22 March 2011 (UTC)

128.244.9.8 (talk) 15:10, 10 February 2011 (UTC)

Possible errors in article

I have used Wikipedia for years and have found it to be a very valuable source of information. Recently, however, I had reason to read the article entitled "Time Dilation". After carefully review I have come to the conclusion that this article is fatally flawed and should be withdrawn and replaced.

I should point out that I have a doctorate in mechanical engineering with no formal training in relativity theory. I do know, however, that the concept of time dilation is perhaps the most significant principle associated with the Special Theory of Relativity. Much of the content of the Wikipedia article contradicts basic details concerning time dilation and at least two equations are simply wrong. I do not feel qualified to rewrite the article but I would be glad to discuss my concerns with a qualified professional. This subject is very important to science and any relevant article should be prepared by a qualified professional. Unfortunately, the original author of the article, in my opinion, appears to be somewhat confused and unqualified.

68.62.216.239 (talk) 22:25, 14 February 2011 (UTC)2/14/11 Frank B. Tatom (email removed)

I've taken a look at the article, and the content seems to be correct at first glance. If you feel there are equations that are not correct, please state which ones they are and what you think the errors in them are. You will get a better response if you raise specific concerns about content, rather than using an all-caps thread title and saying that the last several editors to work on the article were un-qualified. --Christopher Thomas (talk) 00:39, 15 February 2011 (UTC)

Also please not only specify "what you think the errors in them are", but also on which verifiable reliable sources your opinion is based. DVdm (talk) 08:31, 15 February 2011 (UTC)

Response of Tatom

I will try to briefly explain my criticism of this article.

In Section 2.0, entitled “Simple Inference of Time Dilation Due to Relative Velocity”, a proof is offered which leads to the last equation for Δt' in that Section. This equation is fundamentally incorrect as can be determined from any standard physics text, such as shown by

1) Eq. 3, p. 11, Principles of Modern Physics, Robert Leighton, 1959, McGraw-Hill Book Co. or

2) Eq. 1.7.3, p. 94, Methods of Theoretical Physics, Morse and Feshbach, 1953, Part 1, McGraw-Hill Book Co.

Because the erroneous equation is derived from the “proof” preceding it, clearly the author did not understand what the basic point of Time Dilation represents. This error is fundamental and persists throughout the article. For example, in Subsection 4.1, entitled “Time Dilation Due To Relative Velocity”, the same erroneous equation is repeated with further confusing discussion. As another example in Subsection 6.1, entitled “Time Dilation At Constant Acceleration”, the fourth equation for proper time is also incorrect, consistent with the previous observation.

The errors previously noted are fundamental to the whole concept of Time Dilation and clearly demonstrate that the whole article is fatally flawed. I feel certain there are numerous other discrepancies in the article, but it is difficult to justify spending the time sorting out the logic or lack thereof.

I notice that the twenty-four preceding comments in the Technical Discussion appear several times greater in length than the original article itself. A sizeable portion of these comments reflect some sort of confusion and/or misunderstanding of the whole concept. Such confusion/misunderstanding is symptomatic of the basically flawed nature of the article.

The concept of Time Dilation is perhaps the most important basic principle associated with the Special Theory of Relativity. In my opinion this article is simply unacceptable for explaining such an important concept and should be withdrawn.

68.62.216.239 (talk) 21:01, 17 February 2011 (UTC)Frank B. Tatom

The equation and its derivation in section 2 are correct and backed by four sources in the text. You can verify all of them on-line for yourself by following the links.

I don't have the Leighton source here, but I am looking at the equations 1.7.3. of my Morse and Feshbach. This is the general Lorentz transformation in three spatial dimensions. In order for the source notation to match the notation and the thought-experimental setup we have here, you must take the 4th equation for t and t', and:

switch to delta notation (t -> Δt, t' -> Δt', x -> Δx, x' -> Δx'),
interchange the primed and the unprimed variables (t <--> t' and x <--> x', and, of course, u <--> v),

or, if you prefer, use the inverse tranformation, where v is replaced with -v, which doesn't matter since this value is part of a coefficient that will be set to zero—see next...

put Δx=0, since in the frame of the clock, the bounces of the light pulse have the same x-coordinate.

I'm sure that a similar remark holds for the Leighton source.

You will find some more insight in this in Consequences of special relativity#Time dilation and length contraction, where the equation for time dilation is directly derived from the Lorentz transformation. Hope this helps. DVdm (talk) 21:48, 17 February 2011 (UTC)

TATOM RESPONSE

I am puzzled by your response. I don’t understand your exchanging the primed and unprimed time terms. Maybe by walking through the first equation and comparing it term by term to Eq. (1.7.3) from Morse and Feshbach we can resolve the matter.

Please examine the left-hand term, Δt’, in the last equation in Section 2.0. Do you agree that Δt’ represents time as measured by the moving clock, and that Δt on the right hand side represents time as measured by the stationary clock? Do you further agree that the Δt term is multiplied by the Lorentz factor? If you do not agree with any point noted please explain your disagreement.

Now, please observe the term, t, on the left had side of Eq. (1.7.3). Do you agree that this term represents time as measured by Observer A, who is stationary, and that the term, t’, on the right hand side represents time as measured by Observer B, who is moving at a velocity , u, with respect to Observer A? Do you agree that the t’ term is multiplied by the Lorentz factor? If you do not agree with any point noted please, as before, explain your disagreement.

Now, do you agree that the first equation contradicts the second equation? If you do agree, then I rest my case. If you do not agree then I appear to be wasting my time, and I would suggest that you check with your physics professor.

68.62.216.239 (talk) 16:20, 21 February 2011 (UTC)Frank Tatom

In the last equation of the section Δt’ does not represent "time as measured by the moving clock", and Δt on the right hand side does not represent "time as measured by the stationary clock". It is vital to note and specify that the delta's represent the "time between two well-defined events as measured by a clock". In this case the events are two bounces of some light pulse on a mirror-clock, and

Δt on the right hand represents the "time between the bounces of the mirror-clock light signal as measured by the mirror-clock", whereas
Δt' on the left hand represents the "time between the same bounces of the same mirror-clock light signal as measured by another clock for which the mirror clock appears to be moving".

Likewise, the term, t, on the left hand side of Eq. (1.7.3), does not represent "time as measured by Observer A" and t’, on the right hand side, does not represent "time as measured by Observer B". It is vital to note that these values represent the "times of some particular well-defined event as measured by different observers". You must specify that

t on the left hand side represents "the time of some well defined event as measured by Observer A", whereas
t' on the right hand side represents "the time of the same well defined event as measured by Observer B".

You must decide in which frame the clock is at rest, i.o.w. according to which observer the bounces have the same x-coordinate. In this case the top figure of the section tells you that we have chosen the unprimed frame for that, which means that we must use Δx=0. Therefore, we cannot just use the 4rd equation of Eq. (1.7.3) for that, since it has a x'#0 (or a Δx'#0) and we must have x=0 (or Δx=0). If you want to use that 4th equation, you must also use the first equation and explicitly put x=0 (or Δx=0) in there. Then you have a system of two equations in Δx' and Δt', from which you can eliminate the non-zero Δx' to obtain the correct relation between Δt' and Δt, namely the one that is valid in the case at hand where it is demanded that Δx=0. That equation is Δt' = γ Δt.

Spelled out in detail:

The transformation equations of Eq. (1.7.3) in Morse and Feshbach are, in delta-notation: { Δx=γ (Δx'+u Δt') , Δy=Δy', Δz=Δz', Δt=γ (u/c² Δx' + Δt') } with γ=1/√(1-u²/c²).
With the condition Δx=0, the first transformation equation produces 0=γ (Δx'+u Δt'), and thus (since γ#0) we have Δx'=-u Δt'.
Inserting this value for Δx' into the fourth transformation equation, we get Δt = γ (-u²/c² Δt' + Δt') = γ (1-u²/c²) Δt' = γ 1/γ² Δt' = 1/γ Δt'
Grabbing the left and right hands of the previous identities, we get Δt = 1/γ Δt', or equivalently, Δt'=γ Δt — QED

Alternatively, you can first invert the system of equations to Δt' = γ (-u/c² Δx + Δt) and directly put Δx=0 in there, in which case you do not need the inverse counterpart of the first equation, namely Δx' = γ (Δx - u Δt). This is neatly explicitised in Consequences of special relativity#Time dilation and length contraction. Please have a careful look at that.

I hope this helps. If it does not help, then I'm afraid that we cannot just continue here, since the equations of section 2 are solidly backed by four sources, and the reader who has a problem with it, is supposed to accept the sources—combined with the stability of the article's content—at face value. On this article talk page we are not allowed to discuss the subject, or help a reader understand the basics of the subject, so in fact we are already going against policy here. This was already explained on your user talk page. DVdm (talk) 19:14, 21 February 2011 (UTC)

Space travel time paradox

Please address the clock paradox that occurs when our long gone space travelers return to earth and find that the earth's clocks were also dilated in their view due to their high speed travel relative to earth and so that they find the earth's clocks were also dilated (in their view) so that they are still in sync with earth time. —Preceding unsigned comment added by 71.59.129.181 (talk) 17:57, 23 February 2011 (UTC)

Please sign your talk page messages with four tildes (~~~~)? Thanks.

The article has two links to our article Twin paradox where this is handled. DVdm (talk) 18:28, 23 February 2011 (UTC)

On the parallel mirror derivation of the Lorentz factor, please point out that L can go to zero

A very fine article. However... The parallel mirror derivation of the Lorentz factor left me totally confused when it was first shown when I was a freshman physics student. It seemed that the light was traveling partially transversely to the direction of even the MMOVING observer and therefore there might be a non-unity factor for light propagating transversely to the motion. If you could please point out that the Lorentz result is the SAME even when the ratio 2L/(vdt') goes to zero (i.e. the angle between the moving frame light beam direction and the x axis is zero), then it would be clear that the direction of the light in the moving frame may be parallel, in the limit, to the direction of travel of the moving observer. In fact, I believe it MUST be parallel for the resulting Lorentz factor to be correct. 69.238.19.118 (talk) 20:30, 11 March 2011 (UTC)

Relativity postulates/ Frame of references/ Escape velocity

As we all know that the laws of physics are the same in all inertial frames of reference but since speed of light (pulse) is way greater than Escape velocity of any inertial frame of reference except black hole therefore shouldn't a pulse of light clock be moving freely or independent of frame of reference of spaceship or any other arrangement carrying the light clock mechanism74.198.150.224 (talk) 05:29, 13 March 2011 (UTC) Khattak#1-420.

Paradox of time spent in light speed travel ie "1 year at the speed of light"

When you travel at the speed of light, you are in actual fact teleporting in the sense that teleportation is understood in unscientific terms. From the traveller's point of view, it is impossible and paradoxical to travel for any amount of time at the speed of light, as time does not pass. If you traveled at the speed of light for billions of years earth-time, from your point of view, you will have merely teleported incredibly far. It totally breaks my brain how almost everyone goes about with the nonsensical analogy of traveling for 1 year at the speed of light and how many years would pass on earth. The paradox being that the 1 year of perceived time passage is intended for the observer - not the traveler. This faulty "explanation" is why most people find it extra hard to wrap their heads around this - because it is double confusing. What they should say is either: If you travel for one year at the speed of light, it will feel like no time has passed - you will simply be at your destination instantly - the year only passes for someone looking at you from where you left. Article updated accordingly. 196.215.118.171 (talk) 23:24, 23 November 2010 (UTC)

This has got to be wrong, I'm sure of it. Can anybody confirm/elaborate?

Light does not reach locations instantly; it has a finite and constant speed. Teleportation would require an object to greatly excceed the speed of light and leap instantaneously from one point in space to another. As an aside, objects with a nonzero rest mass cannot attain a velocity equal to c, so most realistic time dilation scenarios use speeds arbitrarily close to c. Time dilation, along other related effects, is very real and does not qualify as an analogy, nor is it truly a paradox. Time dilation and special relativity as a whole has has been rigorously and experimentally confirmed. —Preceding unsigned comment added by 75.82.216.196 (talk) 02:10, 12 April 2011 (UTC)

Time dilation is not about "time" It's just matter moving slowly because the medium

The understanding about time dilation it's a miss conception, physics say that when an object accelerate near C it's own time run slowly, but this is no a time issue, it's a matter "running" slowly in the universe, imagine that the matter need to "move" through the space in the universe, fast and more fast approaching to C, each particle need more "time" to reach they own tiny microspace, like each electron rotating the nuclei, the electron need to move no just in the macro space, but in it's own microspace. it need to accomplish more work and that is why need more energy

In our size-world, a human need to move above the surface of earth to reach on point. In the subatomic space where the electron rotates (and other sub atomic and sub sub atomic particles) it need to move arround the nucleus, is a very tiny space but it's space that each particle need to travel

So, if you start to moving faster, the medium (in this case the universe), need to be "traveled" and like in our human size world if and airplane move too fast the medium (the air) stoping it and need more energy to get faster, even so, that can cause the airplane to explode because the acceleration it's too fast for the medium, and in the space/universe an object approaching to C probably explodes, annihilating it self, stop it's own sub atomic particles motion (probaly permanently), become the moving negative, or cause some bizarre behavior, but not time stoping not even time travel

It's just matter moving slowly because the medium

sorry if I can't publish my mail, I'm new editing wikipedia...

--190.136.160.19 (talk) 12:08, 8 June 2011 (UTC)ultracrw@gmail.com

--190.136.160.19 (talk) 11:59, 8 June 2011 (UTC)[ultracrw]

[ultracrw] — Preceding unsigned comment added by 190.136.160.19 (talk) 11:53, 8 June 2011 (UTC)

I would suggest reading WP:RS to start with. Anything that gets typed into Wikipedia needs a reliable reference. So what you and I think matters not, it is what others wrote and published per WP:RS. History2007 (talk) 13:00, 8 June 2011 (UTC)

Energy needs of constant acceleration will not rise as speed increases

The energy required to constantly accelerate will stay a constant ignoring external resistance, e.g. gravitational fields. Speed is only relative in relation to other bodies. i.e. when you accelerate, you are not really moving at all as the universe has no frame of reference. So where it says with current propulsion it's impossible because as speed rises, energy needs rises... this needs to be changed as is inaccurate. Yes, accelerating constantly to faster then light (in relation to earth) is probably impossible at present, but it does not mean it wont be impossible if we design more efficient propellents. I've been over this debate so many times with people, and they really need to grasp the concept that speed is 'relative' i.e.is essentially irrelevant in the context of the entire universe. Accelerating at 1g, would require the same energy each time you pushed the accelerator, unless you suggest the universe 'knows' where you are flying away from and imposes a speed limit? I'm aware of time dilation, length contraction so please dont respond. Not to be rude, but the amount of times I've seen this statement made. Einstein when he said you need infinite energy to pass light, meant it as a hypothetical. You cannot technically pass the speed of light, but you can go a light year in less then a year. Of course, when you travel a light year for you maybe a day passed, but for the point you are aiming for (and any external observers) at least a year has passed.

You are using a reference frame that travels with the craft, which is not an inertial frame. Using an inertial (free-falling) frame, the energy needed does indeed increase. I agree that the example as-given is vague (in fact, I'm not sure it serves a useful purpose at all), but it's referring to the case where you have an arbitrarily large magnetic cannon or what-have-you, and the "1 g" acceleration is measured with respect to that device's reference frame. If you instead have a rocket that magically produces its own acceleration (relative to observers within it), an outside observer sees an acceleration of less than 1 g due to time dilation. --Christopher Thomas (talk) 00:22, 27 May 2011 (UTC)

I need to take issue with some of this. "Speed is only relative in relation to other bodies. i.e. when you accelerate, you are not really moving at all as the universe has no frame of reference." To a degree, this is technically correct, but if the concept of movement is to travel from A to B the speed at which the observer is moving between them has (at least) two frames of reference to use to determine speed. The above statement is also, and to a much greater degree, technically incorrect as it is implicit within "not really moving at all" that there is a condition of absolute rest with the same difficulties of how to reference that absolute rest.

It would be possible to remain stationary, relative to the Sun, to travel from the Earth to the Moon, or more correctly to wait until the moon has travelled to your location. But to achieve this one must accelerate away from the earth to achieve a state of rest, i.e. to not be moving relative to the Sun, until one is hit by the moon hurtling through space relative to the Sun. To achieve this you must accelerate to be standing still! By any consideration, if a body is accelerated, or decelerated, it must have changed the speed at which it is moving, relative to the speed it was moving before it was accelerated or decelerated.

"Accelerating at 1g, would require the same energy each time you pushed the accelerator, ... ?" At the speeds we have experienced this is certainly correct, but as speed increases, for the sake of accuracy lets use a common reference to determine speed and measure our speed against, C, anyway, as the speed of a body increases so does the mass of that body. That is the basic concept of mass - energy equivalence. IF we could accelerate to C our mass would have become infinite and the energy required to accelerate to C would, by necessity, also be infinite. Thankfully, this mass increase is realised gradually. At around 0.5 C mass increases by around 10% and at around 90% C mass has increased by around 50%. This follows approximately the same effect of special relativistic time dilation. When we develop a propulsion system that can accelerate something bigger than an atom to 50% C, that is ~334,800,000 mph we may start to gain some “real time” benefits from special relativistic time dilation, but it’s not until around 90% C, or ~602,640,000 mph, that the effects will become very significant. But, if we can achieve travel at that high a speed, we will be able to travel to the nearest star and return in less than ten earth years anyway (of course that ignores the rather extended periods of acceleration and deceleration that would be required to achieve a speed of over half a trillion miles per hour!)

195.89.38.156 (talk) 15:38, 11 October 2011 (UTC) Ken Dickson

simple diagram

Consider adding a diagram like this one: http://www.anselm.edu/homepage/dbanach/st9.jpg no fancy colors or animation needed, this is simple and very clear — Preceding unsigned comment added by 88.241.140.254 (talk) 17:39, 15 September 2011 (UTC)

Proposal to move sentence out of bullet

Original Content Copy: Time Dilation > Overview > Time dilation: special vs. general theories of relativity:

In Albert Einstein's theories of relativity, time dilation in these two circumstances can be summarized:

In special relativity (or, hypothetically far from all gravitational mass), clocks that are moving with respect to an inertial system of observation are measured to be running slower. This effect is described precisely by the Lorentz transformation.
In general relativity, clocks at lower potentials in a gravitational field—such as in closer proximity to a planet—are found to be running slower. The articles on gravitational time dilation and gravitational red shift give a more detailed discussion. Special and general relativistic effects can combine, for example in some time-scale applications mentioned below.

Thus, in special relativity, the time dilation effect is reciprocal:...

Suggested Content Copy Change: In Albert Einstein's theories of relativity, time dilation in these two circumstances can be summarized:

In special relativity (or, hypothetically far from all gravitational mass), clocks that are moving with respect to an inertial system of observation are measured to be running slower. This effect is described precisely by the Lorentz transformation.
In general relativity, clocks at lower potentials in a gravitational field—such as in closer proximity to a planet—are found to be running slower. The articles on gravitational time dilation and gravitational red shift give a more detailed discussion.

(Special and general relativistic effects can combine, for example in some time-scale applications mentioned below.)

Thus, in special relativity, the time dilation effect is reciprocal:... 76.186.76.58 (talk) 20:45, 4 October 2011 (UTC)John Christian Edwards, author/editor/publisher

Done. Good find. Cheers - DVdm (talk) 20:53, 4 October 2011 (UTC)