Talk:Time travel

traveling faster than light to travel back in time

This is complete rubbish. anything travelling FASTER than light by definition is still moving forwards in time. the fact that some may see things as if they were moving backwards in time does not mean they are actually moving backwards in time. This is only a consequence of the fact that the data stream can only arrive at light speed no matter how fast the object is moving... the frames of data will be arriving in reverse order but never before that data was actually produced. you will never see a signal prior to its actual transmission... the actual moment of transmission can still be calculated via Lorentz transformations and adjusted for all frames of reference. 2ndly the notion of faster than light leading to reversed time is primarily based on a mis-reading of the very same Lorentz transformation. It appears to have a negative time factor after light speed, but this is not quite the case. what it actually has is a squareroot of a negative number which is not the same as a negative number. its called an imaginary number and in electronics represents a phase shift. in hyper dimensional space it would represent a negative number which is 90 degrees out of phase with the 3 d space, or the 4th spacial dimension, hyperspace. that means that any object moving faster than light would be in hyperspace behind the expanding 3 d space and would move backwards away from the 3 d spacial bubble... not backwards in time, but backwards down a wormhole that still exists in current time... that wormhole does not link to any existant past, because the past does not exist, its only memories of where the current arrangement of things came from... the worm hole could link to other hyperspacial objects or if large enough to some other part of the current 3 d space. —Preceding unsigned comment added by Jiohdi (talk • contribs) 14:17, 22 December 2009 (UTC)

If you can find a good, and verifiable, reference to all of that above then it would make for a good section in the article. HumphreyW (talk) 14:27, 22 December 2009 (UTC)

Gott, J. Richard (2002). Time Travel in Einstein's Universe. pp82-83 is sited in the Faster-than-light article as to why FTL results in backwards time travel. Also one of the ideas about tachyons is that their FTL nature means they either have imaginary mass or travel backwards in time. So the idea that FTL and backwards time travel is not rubbish as shown by cited sources in the relevant articles.--BruceGrubb (talk) 13:43, 23 December 2009 (UTC)

Is this an appropriate source?

I've found a talk (http://www.youtube.com/watch?v=vR1vyqM4KkY ) given at a respectable, but not scientific conference (http://www.tedxbrussels.eu/beta/html/2009.html). Can it be used as a source?--Seador (talk) 16:22, 3 February 2010 (UTC)

Serguei Krasnikov is a respectable physicist himself so probably anything he says is OK to include. What did you want to use the video as a source for, though? Might be able to find the same claim/argument in one of Krasnikov's papers... Hypnosifl (talk) 23:14, 4 March 2010 (UTC)

The talk seems much easier to understand :) --Seador (talk) 20:49, 5 March 2010 (UTC)

sleeping is *not* time travel

The examples of Rip Van Winkle and Honi HaM'agel as time travelers should be removed. 99.35.33.233 (talk) 09:52, 3 March 2010 (UTC)

It is identical to a one-way trip to the future. I think it's awfully close. --RLent (talk) 18:11, 29 March 2010 (UTC)

Since we are all currently making a one-way trip to the future, then we might as well list ourselves as examples. Seriously, if that's your best argument, we should delete it. It makes a mockery of the concept. 71.57.108.198 (talk) 20:24, 6 April 2010 (UTC)

With that logic you can consider existing a form of time travel. 95.109.102.252 (talk) 22:03, 5 April 2010 (UTC)

Hi. I wandered here after deleting the "time travel story" tag that somebody put on _Marooned in Real Time_. One way travel into the future is NOT time travel as most people understand it. If you want to discuss it, make a new entry under "stasis" or something. Really. DavidHobby (talk) 03:56, 17 April 2010 (UTC)

Different authors define "time travel" differently, certainly plenty would say a story of travel into the future via time dilation is "time travel". See for example here, here, here, here, here, and here. Hypnosifl (talk) 23:35, 18 April 2010 (UTC)

Incidentally, for those who argue that one-way travel into the future can't be considered time travel since by that logic we're all traveling into the future, the third link addresses this: In one sense we are all travelling in time since our experience is of a sequence of moments stretching from the distant past to the far future. But what we usually mean by time travel is a difference in the amount of time we experience compared to others. In this respect high-speed motion gives a sure-fire way of travelling into the future. Recall the rocket twin in the example above. He has experienced ten years while his brother (and everyone else on Earth) has lived through twenty years. The rocket twin has travelled ten years into the Earth's future.

Of course, even if you are willing to consider time dilation a form of time travel (and there is no 'right answer' here, it just depends on how you define things, the point is just that wikipedia should respect widespread usage and plenty of people do define things this way) it's still debatable whether something like Rip Van Winkle can be classed as time travel, since his body has not physically elapsed less time than anyone else even if he was unconscious for a long time and didn't subjectively experience all those years. But then the section is titled "origins of the concept", it isn't necessarily saying that all the stories there could actually be classed as time travel stories, just that can be considered somehow relevant to the development of the idea. Hypnosifl (talk) 00:01, 19 April 2010 (UTC)

Hypnosifl-- I still think it's not time travel, and that most people agree with me. But O.K., I'll leave the tag on _Marooned in Real Time_. For consistency, why don't you tag Niven's _A World out of time_, as well? (http://en.wikipedia.org/wiki/A_World_Out_of_Time) DavidHobby (talk) 16:46, 19 April 2010 (UTC)

Time Travel changed to Time Control\Manipulation?

In the article it mentions slowing down time and other forms of time manipulation so why is it only time travel and not time manipulation? Also, I personally think their should be a section for stopping time which, under the current article name, would be irrelevant. I know it is a strange thing to ask but I just wanted to gather some other opinions on this. Benevolinsolence (talk) 03:47, 17 March 2010 (UTC)

If the Theory of Relativity is correct wouldn't that make (ghosts) humans from the past and (aliens) humans from the future, but both still beings who have attempted to traveled through time. Neither can exsist in our world long enough to be study (unless Roswell is real) and time travel is cannot be sustained, the injustices of the past will not be forgotten in 2012. —Preceding unsigned comment added by 172.130.32.125 (talk) 21:02, 25 May 2010 (UTC)

Proposal for an illustration

Here is a animated gif I made using synfig :

I think it would be nice to have such animated gifs to illustrate all different kinds of time travels. I also put the original synfig's source code on the corresponding page. --Grondilu (talk) 18:42, 10 April 2010 (UTC)

Concerning this animation :

I think it would be nice to have the traveller changing history in the other universe. For instance, he might be hurt on the head by a falling stone, and decide to go back in the past to tell his alter ego to step aside. I've started to create this but I'm getting lazy, so if someone wants to do it... --Grondilu (talk) 20:38, 12 April 2010 (UTC)

Here it is. I'm not very happy with the result, so I'm not sure I will put it on the article.

--Grondilu (talk) 09:56, 13 April 2010 (UTC)

neat idea, but the animation is a bit slow-moving...also, doesn't your old parallel universe animation already show "changing history", since in the second universe the traveler is prevented from stepping into the time machine? Hypnosifl (talk) 15:06, 13 April 2010 (UTC)

Yeah indeed, this animation has bad timing. The file is also too big to get animated on thumbnail. It's true also that both animations show history change. My mistake. I meant that in the second animation, the traveler has a real purpose in going back in time. He wants to do something precise for a reason. He wants to change history in some kind of a subjective meaning. It's more what we expect from time traveling : there is something we don't like in the past and we want to change it.--Grondilu (talk) 13:09, 14 April 2010 (UTC)

I also consider making a simpler version, where the traveler would just push his alter ego in order to prevent him from getting hurt by the falling stone. This would allow to avoid the need of a speech balloon.--Grondilu (talk) 13:22, 14 April 2010 (UTC)

Here it is. I'm not sure it is much better. I let other people to decide.

--Grondilu (talk) 20:43, 16 April 2010 (UTC)

These are hilarious, in a good way! :) Keep working on them. Kingturtle (talk) 05:08, 1 August 2010 (UTC)

Some maths for faster-than-light travel ?

I think some mathematical justification should be given for faster-than-light travel, and the reasons why this implies backward time travel.

I suggest considering two space ships, A et B. A travels at infralight speed $(1-\alpha )c$, B travels to supralight speed $(1+\beta )c$.

Special relativity allows only to express coordinates in A spacetime reference. For this, the relativistic coefficient $\gamma$ is :

$\gamma ={1 \over {\sqrt {1-({\frac {v_{A}}{c}})^{2}}}}={1 \over {\sqrt {1-(1-\alpha )^{2}}}}\approx {\frac {1}{\sqrt {2\alpha }}}$

We change from the earth coordinates $(x,t)$ to the coordinates $(x',t')$ in the A spaceship refererence, with these equations :

$x'=\gamma (x-vt)\approx {\frac {1}{\sqrt {2\alpha }}}(x-(1-\alpha )ct)$

$t'=\gamma (t-{\frac {v_{A}}{c^{2}}}x)\approx {\frac {1}{\sqrt {2\alpha }}}(t-(1-\alpha ){\frac {x}{c}})$

The coordinates of the B spaceship in Earth reference are $x=v_{B}t=(1+\beta )ct$.

So, in the A spaceship reference, those coordinates are :

$x'\approx {\frac {1}{\sqrt {2\alpha }}}((1+\beta )ct-(1-\alpha )ct)\approx {\frac {1}{\sqrt {2\alpha }}}(\beta +\alpha )ct$

$t'\approx {\frac {1}{\sqrt {2\alpha }}}(t-(1-\alpha ){\frac {(1+\beta )ct}{c}})\approx {\frac {1}{\sqrt {2\alpha }}}(\alpha -\beta )t$

Therefore when we choose $\alpha <\beta$, then $t'$ decreases when $t$ increases, in other words the spaceship B is going forwards in time in the Earth reference, but backwards in the spaceship A reference.

--Grondilu (talk) 14:57, 12 April 2010 (UTC)

I don't think it's good form to use approximations in a proof like this. For an exact proof, suppose in the $(x,t)$ frame you have some signal moving at speed $V=c^{2}/v'$ which is faster than light ($v'$ represents some slower-than-light speed...the usefulness of writing it this way will become apparent in a moment). Suppose the signal is emitted at the origin $x=0,t=0$ of the $(x,t)$ frame, so the signal has position as a function of time given by $x(t)=Vt=tc^{2}/v'$, and the receiver detects the signal at some later time $t=t_{0}$, so the event of the signal being received must have coordinates $x=t_{0}c^{2}/v',t=t_{0}$. Then if we transform to a frame with coordinates $(x',t')$ moving at speed $v$ relative to the first frame, then according to the Lorentz transformation the signal was still sent at the origin of this frame, i.e. $x'=0,t'=0$, but the coordinates of the signal being received would be:

$x'=\gamma ((t_{0}c^{2}/v')-vt_{0})$

$t'=\gamma (t_{0}-(vt_{0}/v'))$

It's easy enough to see from the second equation that if $v>v'$ then the time coordinate $t'$ of the signal being received will be negative, meaning that the signal was received at an earlier time coordinate in this frame then the time coordinate $t'=0$ when it was sent, so that the signal moved backwards in time in this frame (and if $v=v'$ then the signal will have been received at the same time $t'=0$ that it was sent, so in this case the signal moved with infinite speed).

Not sure if the existing references already give the math for why tachyons imply causality violations, but if they don't it might be easier to just find another reference that does rather than put a bunch of equations in the article. Hypnosifl (talk) 16:54, 12 April 2010 (UTC)

Hawking says forward only -- 2010-MAY

http://news.nationalpost.com/2010/05/03/time-travel-possible-stephen-hawking-says/

If a reckless explorer wanted to travel into the distant future and meet Morlocks or intelligent apes or something, it just might be possible, says physicist Stephen Hawking. The scientist, one of the leading voices in theoretical astrophysics, said that in the future humanity could develop spaceships capable of travelling near the speed of light. But at these speeds, time would slow down for those on board, hurtling them into the future.

Hawking burst another science-fiction bubble and said that travelling backwards through time would be impossible because it could create situations where the fundamental rule of cause and effect got broken.

Hawking also weighed in last week on the possibility of alien life and warned us that we probably shouldn’t try to contact them. Canadian alien enthusiast and former defence minister Paul Hellyer defended aliens and called Hawking’s views irresponsible. He also suggested that aliens have been visiting Earth for decades and contributed to our technology.

--JimWae (talk) 18:49, 3 May 2010 (UTC)

Do "laws of physics" allow things to happen?

The lede currently says:

it is currently unknown whether the laws of physics would allow backwards time travel.

Do the laws of physics "allow" things to happen, or do we decide what the laws of physics "are" by interpreting what happens? I think the answer is definitely the latter. I think that part of the lede should say something like:

Time travel at different rates into the future is predicted by the theory of relativity and has been experimentally verified for small amounts of time. Travelling forward for larger amounts of time is, so far, physically difficult, requiring large amounts of energy. Travelling backwards in time has never been verified, and presents many theoretic problems.

--JimWae (talk) 19:09, 3 May 2010 (UTC)

I think it is correct to say that we don't really know if laws of physics allow backwards time travel. According to me, part of this lack of knowledge is related to the fact that it depends on what we mean by "time travel".

I'm pretty sure that time travel does exist in every day physics, since according to Feynman's interpretation of antimatter, positrons are backward time travelling electrons. Moreover, as far as I know, all laws of physics are symetric as far as time is concerned, and the reasons why there is a time arrow, and a second principle of thermodynamics, are still open subjects.--Grondilu (talk) 15:02, 4 May 2010 (UTC)

Curious Text

This article states: "I will find your family and take them. The bike riding gorillas will take care of them." While this appears to be in keeping with the general quality of information in Wiki articles, perhaps the author(s) should up their medication levels before continuing. Jmdeur (talk) 13:03, 4 May 2010 (UTC)

It was only there for a few minutes. I already deleted it about 15 minutes before you posted your comment. And never mind the vandals, they will always be here. No need to waste time trying to psychoanalyse them. HumphreyW (talk) 13:26, 4 May 2010 (UTC)

ON RECENT REVERTED ON Time Travel

(Copied by Ckatz’s request from his talk page)

Hi Ckatz, you've just reverted my smal addition to Time Travel on "notability". I am truly confused why is it done, it has reliable references to scientific papers, which are also based on previous many years research. Can you kindly explain what is wrong? --AarCart (talk) 21:07, 12 May 2010 (UTC)

Hello... you should post your thoughts on the article talk page to see if other editors feel it is suitable for inclusion. After reviewing the text, and the related article, there does not appear to be wide acceptance for the theory. Do you have information that suggests otherwise? --Ckatz^chat_spy 08:06, 13 May 2010 (UTC)

I did some research in my spare time on the issue of wiki notability. I am not a veteran wiki editor by any means, but I did some digging and as I understand it, notability is a guideline that exists to improve the quality of available articles. It is not a wiki policy, and it does not imply that ideas have to be widely accepted to appear in an article. In fact, this idea is not new, but go back to at least the year 1990. The idea is very well sourced, and is entirely verifiable with references.

Besides, I found that notability as a guideline is extremely controversial as evidenced by the following Slate.com (http://www.slate.com/id/2160222) and Washington post (http://www.washingtonpost.com/wp-dyn/content/article/2007/02/23/AR2007022301738.html) articles. The idea is, as I said, not new, and is verifiable. As such, this idea is a service to the readers of wikipedia because everyone has a right to know as much as possible about the world they live in; it is a basic freedom.

Actually, it’s a pretty simple idea that is mostly self-evident. Regardless, it is still verifiable and is interesting enough to be expressed on wikipedia.

This is one of the sections that was deleted:

“Valentin Koulikov explains the absense of time travelers by the same reason as the lack of evidence on extraterrestrial civilizations (Fermi Paradox): advanced civilizations tend to use light speeds in their common life to avoid time relativity differences with their many space and time travelers . This causes huge time flow tempo difference with our own civilization, effectively prohibiting any kind of communication and making their visits extremely rare on our time scale.”

As the above is well-sourced in academic journals, I invite any and all editors who read this to please weigh in with their opinions. --AarCart (talk) 14:25, 16 May 2010 (UTC)

What academic journals are you referring to? philica.com is not a real academic journal, just a self-published internet site, and it's full of crackpot articles like this one (see wikipedia's guidelines on fringe theories and reliable sources, and feel free to start a thread on Wikipedia:Reliable_sources/Noticeboard if you disagree that philica.com cannot be treated as a reliable source for mainstream scientific information) Hypnosifl (talk) 17:28, 20 May 2010 (UTC)

Time Travel Solution?

I believe a good way to think about time is to think of a "moment in time" as a certain configuration of the universe. This configuration would include things like the locations of the universe's objects and also things like the momentums of the objects and in which direction the objects are "currently" moving. In my view, time is a much more encompassing dimension (piece of information) than width, length and height. If you know what time it is, you'll know the current configuration of the universe and therefore, the width, length and height of all the objects.

If a moment in time is a configuration of the universe, then time travel is possible. Clearly, the configuration of the universe is always changing. Going "back" to a certain moment in time, according to my definition of time, would mean changing the configuration of the universe to it's configuration in the given moment in time. Of couse, it will be quite difficult to travel "back" in time if the configuration of the universe you wish to achieve is quite different from the current one, as almost any two universe configurations are likely to be.

But, if you're content with changing your "local time" - the configuration of the universe in a local area - well, at least getting it to match to a high enough degree, then it appears that you can travel to (change to) a time (configuration) that is sufficient for your purposes. For example, if you wanted to bring someone back to life, you'd have to reconstruct their body with their DNA sequences, etc. You see, it gets difficult! But, you can construct a clock that at least appears to cycle through the same local times. Using System Restore on a Windows machine is another example - the settings get changed back. It appears that you've gone back in time.

Do these definitions of time and time travel make sense enough to include a new section on them in the article? What do you think? Synesthetic (talk) 06:15, 8 July 2010 (UTC)

This trick is indeed not exactly a time travel, but does something similar. It might be classified as "artificial/synthetic universe" solution. It relies on the ability for the traveler to forge the universe, or at least a local area of it, to whatever he needs, and in particular to an exact replica of what it was some time ago. It is quite a huge hypothesis though, since such a person would have full power over matter and space, pretty much as a god. But why not. However, such a definition migh be included only if relevant references can be given, whether it is in science or fiction. It is pretty much similar to the cyclical universe section I added, after the broadcast of a Futurama episode dealing with this subject.--Grondilu (talk) 21:11, 2 August 2010 (UTC)

By the way, I think this kind of time travel is more or less what Superman does in the first movie, when spinning Earth backwards. Even if such a method would not reverse events on the surface, it's what the scenario suggested. I don't know if adding a section for this would be relevant.--Grondilu (talk) 21:22, 2 August 2010 (UTC)

Time Travel In Literature

I don´t want to change the main article myself. Imo any kind of prophecy is time travel. The tales of Oedipus (the prophecy failed to prevent that future by travelling back in time). Revelation of John (the prophecy prevented that future by travelling back in time). The turning point in time travel literature is 'Chronic Argonauts'. Because man builds and uses the time travel device, instead of some supernatural entities. —Preceding unsigned comment added by 78.35.55.66 (talk) 07:46, 6 August 2010 (UTC)

Indeed relating time travel to prophecy/divination has some interest. One of the reasons is that both share some paradox problem, and are confronted to causality violations. But mentionning this on this article would be tricky, since prophecy is not time travel. IMO we can mention it if and only if we can find some famous observer/analyst writing about this comparaison.-- Grondilu (talk) 14:00, 10 August 2010 (UTC)

Parallel universe and free will

Someone has modified my caption on the parallel universe hypothesis time travel illustration, and have it say that this version has the potential of preserving free will.

I disagree. IMO, this version of time travel does preserve free will.

I guess it might look different since in this version, a person can prevent himself from doing something, which might be interpreted (wrongly imo) as an obstacle to free will.

According to me, when the traveller goes back and prevents his double from doing something, he's acting not so much differently from anyone who would act this way. The possibility for someone to be prevented from acting by someone else is just usual every day life phenomenum. And from the point of view of the traveler wannabe, his double coming from another universe's future is another person as any other else.

IMO Free will conflicts occur when someone is confronted to the result of his own action in the future. By being aware of what he is supposed to be doing, he suddenly becomes unable to do anything else. That's an other way of expressing the predestination paradox, I guess.

Anyway, I suggest reverting this modification and put back the idea that the paral. univ. hypoth. version of time travel does preserve free will.

--Grondilu (talk) 06:23, 4 September 2010 (UTC)

I changed it to "has the potential" because many people don't believe in "free will" regardless of their views on time travel (at least not the non-compatibilist version), I think this would be particularly true of scientists who tend to believe human behavior is purely a product of the physical brain (plus the input from its environment) which is assumed to follow the same fundamental laws of physics as all other physical systems (and while quantum physics opens up the possibility of randomness, 'free will' is generally conceived as something more than simple randomness, there's supposed to be an element of choice involved). So certainly you are free to believe in the parallel-universe version of time travel but still deny the existence of free will for scientific or philosophical reasons, thus saying that this type of time travel "preserves free will" seems too strong since it makes it sound like there is a necessary connection between the two. Hypnosifl (talk) 18:02, 4 September 2010 (UTC)

Also, what would you think of changing it to this? "For those who believe in a non-compatibilist version of free will, this version of time travel can preserve free will." Hypnosifl (talk) 19:57, 4 September 2010 (UTC)

Indeed this would be more appropriate, although such a sentence is quite inelegant (imo). So I suggest : this version of time travel preserves the concept of free will. Mentioning free will as a concept allows us not to assume its scientific or philosophical validity.--Grondilu (talk) 01:03, 5 September 2010 (UTC)

It is not clear for me wether it's the compatibilist or non-compatibilist version that makes Parall. univ. time travel compatible with free will. I have to think about it. Anyway, mentionning compatibilism here is a good idea. So the caption might be : this version of time travel preserves free will, or at least its (non-?)compatibilist version.--Grondilu (talk) 01:40, 5 September 2010 (UTC)

How about something like this: "For those who believe in a non-compatibilist idea of free will, this version of time travel avoids the problems of the fixed timeline version"? Or "Unlike the fixed timeline idea, this version of time travel presents no conflict with the non-compatibilist notion of free will"? Something along those lines, comparing with the fixed timeline theory? Hypnosifl (talk) 03:07, 5 September 2010 (UTC)

Something in this idea indeed, but the sentences you're suggesting are a bit long, imo. AFAIK I'd go for a simple : "This version of time travel preserves a non-compatibilist notion of free-will". Comparaison with the previous time travel version (fixed timeline), doen't have to be explicit.--Grondilu (talk) 07:41, 5 September 2010 (UTC)

(un-indenting) The problem with that is it kind of reads like this version of time travel doesn't preserve a compatibilist notion of free will, and it also makes it sound a bit like this form of time travel is intended by to "preserve a non-compatibilist notion of free will". So, I'd prefer a sentence that specifies it's talking about how this form of time travel might be viewed by "advocates of non-compatibilist notions of free will" or something along those lines, rather than looking like its talking about the relation between parallel universes and free will in general (another option would be to say something along the lines of 'while all version of time travel are consistent with compatibilist notions of free will, this version is also consistent with non-compatibilist free will'). I don't see how the nuances can really be conveyed in a single short sentence, but maybe break up the comment into two shorter sentences? Hypnosifl (talk) 18:17, 5 September 2010 (UTC)

Again, I confess I don't undertand very well how compatibilism can intervene here (even if I understand it does intervene), so I'm not in good position to chose the appropriate sentence. I let you to decide. However, I'd like to say that as far as I'm concerned I've never seen much incompatibility between determinism and free will, until I considered those two time travels versions. Whatever my conception of free will is, to me it is obvious that the "single timeline" version is incompatible with it, while the "parallel universe" is compatible. And yet, both time travel versions are fully deterministics (apart from the break of symetry, which can't occur in a deterministic way, but that's an other matter).--Grondilu (talk) 21:53, 5 September 2010 (UTC)

Hang on, I think I have an idea about how to make things clearer. Imagine we add a self-destruction button on the time machine. In the single timeline version, during the time when both traveler are presents, it's absolutely impossible for any of them to press this button. If one of them does, then the time machine can't be used to do what it is supposed to be doing, and we have an free lunch paradox (a doubling of one person for no reason). In the parallel universe version however, it is possible to press this button at any time, and in any universe. That's the difference, imo. The fact that the impossibility of pressing a button exists and is manifest, is no different than if there was a sentence such as Do not press this button written other it. It is this instruction, implicit but very real (since it is implied by logic), that acts as some kind of a supernatural coercitive force that is contradictory with the free will concept. IMO.--Grondilu (talk) 22:13, 5 September 2010 (UTC) I'm considering creating a modified version of the fixed timeline illustration with such a button. I might make it blink during the time when it is forbidden to press it.--Grondilu (talk) 22:46, 5 September 2010 (UTC)

Well, I don't see why a compatibilist view of free will would be ruled out by the self-consistent single timeline version of time travel. Sure, you can't press the button, but it would also be true in a deterministic universe that if the initial conditions of the universe are such that you won't press the button, then you can't press the button in that case either (though i suppose it depends on the definition of "can't", see below), the difference is just that in the time travel scenario you know you won't press the button. Doing some searching on the internet, I can find examples of compatibilists who don't see a conflict with single-timeline time travel (see the discussion of David Lewis' position on time travel here and here for example, along with his actual paper on the subject here, and see here and here for discussion of Lewis' arguments in favor of compatibilism), but none who do see a conflict. The closest might be something like this paper (which the author also discusses in this blog post) which argues that time travel has some "strange" consequences in terms of what we can and cannot do--see the discussion on p. 3-4 of what compatibilists and incompatibilists mean when they say we "can" do something or other, and then the argument on p. 13 about how a time traveler "cannot" kill their own baby self even in a compatibilist sense, despite the fact that they "can" do other things in a compatibilist sense, like pinching their own baby self. But it doesn't seem like she's actually arguing that this would rule out the compatibilist notion of free will, just that she's disagreeing with other philosophers like Lewis that a time traveler still "can" do things like killing their younger self in the compatibilist sense of the word. So unless there are some examples of philosophers who feel that fixed-timeline time travel would actually rule out the compatibilist notion of free will, I think the article should be written as though there is no conflict between them, otherwise we'd be doing original research. Hypnosifl (talk) 00:36, 6 September 2010 (UTC)

Errors in Time Travel page

The following text (second paragraph under the Theory heading) is inaccurate and misleading:

Relativity states that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future" (according to relativity there is no single objective answer to how much time has 'really' passed between the departure and the return, but there is an objective answer to how much proper time has been experienced by both the Earth and the traveler, i.e. how much each has aged). On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that problems of causality be resolved. The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? But some scientists believe that paradoxes can be avoided, either by appealing to the Novikov self-consistency principle or to the notion of branching parallel universes (see the 'Paradoxes' section below).

I edited it this morning, as suggested in your page about dealing with factual errors ('It's a wiki - you can change it yourself'):

Relativity states that if one were to move away from the Earth at relativistic velocities and return, it would appear to an observer on Earth that more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future". However, since the Special Theory of Relativiy allows no preference for any frame of reference over any other, from the viewpoint of the traveller the reverse would seem to happen, with time processes being slowed on the Earth. Although according to relativity there is no single objective answer to how much time has 'really' passed between the departure and the return, it seems probable that the apparent paradoxical contradictions introduced would be cancelled out on return, through the processes of acceleration and deceleration. Many in the scientific community believe that backwards time travel is highly unlikely. Any theory which would allow time travel would require that problems of causality be resolved. The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? But some scientists believe that paradoxes can be avoided, either by appealing to the Novikov self-consistency principle or to the notion of branching parallel universes (see the 'Paradoxes' section below).

- but returning to the page this afternoon, I find that the original has been restored. What's point of allowing people to make corrections, when the original incorrect version will just be restored and the changes lost?

Ed Addis (talk) 15:42, 13 October 2010 (UTC)Ed Addis

Thanks for opening a discussion. I wasn't the person who reverted your edit, but I can tell you that your edit was incorrect. The situation for the two observers is not symmetrical, because one of them has to undergo an enormous acceleration when changing directions. One observer is in a single inertial reference frame, the other in two very different intertial frames. This was all thoroughly worked out decades ago. Looie496 (talk) 17:27, 13 October 2010 (UTC)

I was the editor who changed it back (with a slight modification to make it more clear). You might want to read about the twin paradox--if you have two clocks set to the same time when they are next to each other, then they move apart and later reunite with one clock moving inertially between the two meetings while the other clock accelerates to turn around after they've been moving apart for a bit, then the clock that moved non-inertially will show less elapsed time when they are compared at the same location than the clock that moved inertially. It is true that if two clocks just move apart inertially, then in each clock's inertial rest frame the other clock must be running slower, but the rule that moving clocks run slower only applies in inertial frames, in order for the two clocks to compare readings at a single location one needs to accelerate to turn around and this breaks the symmetry. Hypnosifl (talk) 12:10, 14 October 2010 (UTC)

Response to Looie496

I'm sorry but you're simply incorrect about this. The point is that while the two observers are separating at constant velocity (over a hypothetical long journey through space, the situation is entirely symmetrical, and each will observer the other's time processes to be slowed. The evident paradox here is corrected because the accelerations necessary to reunite the two observers cancel out the paradoxical differences.

Response to Hypnosifl

As explained above the apparent differences will be cancelled out if the observers are reunited. The reason that the symmetry is not broken is that when acceleration takes place, each observer will appear to the other to be accelerating. The notion that the acceleration itself acts directly on the time processes of the accelerating observer is incorrect, and represents a misunderstanding of the basics of relativity. None of these 'relativistic' effects are 'real' to the observer, only to the third party observer, relative to whom the high velocities or accelerations are taking place. If you believe that reunited observers in this situation would retain any of the differential time effects, please cite evidence for this. I'm not at all happy about your removal of my edit - can you please explain how I take this further, in order to have my edit re-instated? —Preceding unsigned comment added by Ed Addis (talk • contribs) 18:26, 27 October 2010 (UTC)

Ed Addis (talk) 18:28, 27 October 2010 (UTC)Ed

Ed, it's you who simply misunderstands basic relativity, and I already cited a reputable web page (on the site of a theoretical physicist, John C. Baez) explaining the so-called "twin paradox" above--did you look at it? It clearly states in the "Introduction" section that "Not to keep anyone in suspense, Special Relativity (SR for short) plumps unequivocally for the first answer: Stella ages less than Terence between the departure and the reunion." On your talk page I have also added some links to discussions of the twin paradox in published relativity textbooks, with the relevant sections being viewable on google books, those links again are this book and this one. One can also find a discussion of a similar scenario at the end of section 4 of Einstein's original 1905 paper, where he writes:

If we assume that the result proved for a polygonal line is also valid for a continuously curved line, we arrive at this result: If one of two synchronous clocks at A is moved in a closed curve with constant velocity until it returns to A, the journey lasting t seconds, then by the clock which has remained at rest the travelled clock on its arrival at A will be ${\frac {1}{2}}tv^{2}/c^{2}$ second slow.

(actually Einstein is using a Taylor series approximation here, the clock that moved away from A and back to A at speed v would be behind the inertial clock at rest at A by $t(1-{\sqrt {1-v^{2}/c^{2}}})$, see his comment slightly earlier about "neglecting magnitudes of fourth and higher order"...but the basic point is that the clocks will no longer be synchronized when they reunite, with the non-inertial one moving on a 'closed curve' having elapsed less time than the inertial one that remained at position A)

As to your final question "I'm not at all happy about your removal of my edit - can you please explain how I take this further, in order to have my edit re-instated?", I hope a careful examination of my citations above will convince you that your ideas conflict with the standard understanding of special relativity by physicists, but if they don't and you don't think further discussion here will help, you could try looking into Wikipedia:Dispute_resolution. Hypnosifl (talk) 07:11, 29 October 2010 (UTC)

I don't have the luxury of being able to assert conclusively that either my or your view on this is more widely held. I guess that as an editor, yours will prevail, and I certainly have no wish to promote my ideas in contradiction of the 'standard view'. You will no doubt sense a 'however' on its way. The view you, and your references, espouse does not accord with the basic tenets of Special Relativity. When two observers are separating at near light speed (as measured by each other), each will see the others' clocks running slower. Surely you don't deny this? Why would you suppose that some 'magical' process occurs, as a result of the accelerations necessary to reunite them, having the effect of making one observer's clocks change from appearing to be behind the other's to being ahead of them? Think about the process - when do you think this change takes place? Your only argument to explain this is to invoke broken symmetry in the situation, but this is just a fantasy. Just because one observer is sitting in a rocket that uses motors to accelerate it, doesn't make for an asymmetric situation - to the traveller, the supposedly 'stationary' planet he's come from is accelerating away from him by exactly the same amount. The only reason that the planet-bound observer doesn't feel this is that the effect is diluted because of the mass of the planet, but it is no less real, and the planet-bound observer is no more inertial during the process than is the 'traveller'. However you argue it, you end up having to justify treating the planetary frame of reference as being more 'special' than the traveller's, which is in total conflict with basic relativity. The only sustainable solution to the paradox is that the differences are wiped out through the process of re-uniting the two observers. If you don't believe that's what happens, please cite evidence (not theory) to prove your case.

82.33.52.106 (talk) 22:19, 29 October 2010 (UTC)Ed Addis

"When two observers are separating at near light speed (as measured by each other), each will see the others' clocks running slower. Surely you don't deny this?"

Only if by "measured" you mean assigning coordinates to events on the other's worldline using the coordinates of their own inertial rest frame. It's not like time dilation represents what either observer actually sees with their eyes, the notion of "measuring" time dilation always presupposes some spacetime coordinate system, and the usual equation for time dilation only works in an inertial coordinate system.

"Why would you suppose that some 'magical' process occurs, as a result of the accelerations necessary to reunite them, having the effect of making one observer's clocks change from appearing to be behind the other's to being ahead of them?"

If by "appearing" you mean what the observer sees visually, then no, they don't see the clock suddenly jump forward, but each observer will see the other's clock running faster than their own when they see the other moving towards them, due to the Doppler effect (this section of the page on the twin paradox page I linked to earlier discusses these visual appearances). Again, the time dilation equation is not supposed to represent what either sees, but rather the rate a clock is ticking relative to coordinate time in some inertial frame.

As for why it makes sense to invoke broken symmetry due to acceleration, consider an analogy with 2D Euclidean geometry. Suppose we pick two points A and B on a plane, and draw two paths between them, one of which is a single straight segment from A to B, while the other looks like a "V" shape consisting of two straight segments at different angles joined by a bend in the middle--we can label the point of the bend as C. Now suppose we have two little cars driving along each path with their odometer running, measuring the total length of each path from A to B. Do you agree that since a straight line is always the shortest distance between two points, the car that drives on the straight segment from A to B will have a shorter accumulated odometer distance than the car that drives on the V-shaped paths consisting of two straight segments AC and CB? Here, the amount a car's odometer increases along a given path through 2D space is analogous to the amount a clock ticks along a given path through spacetime.

But now suppose we draw a pair of cartesian coordinate axes on this same plane, and want to know the rate the car's odometer increases relative to the increase in y-coordinate of the car (analogous to wanting to know the rate a clock's reading increases relative to the t-coordinate in some inertial frame). In that case, the answer will depend on how we orient the y-axis. if we use a coordinate system where the y-axis is parallel to the segment AB, then for a given increase in y-coordinate dy, the odometer of the car moving along AB only increases by dy since it's going parallel to the y-axis, while the odometer of the car moving along AC increases by a greater amount ${\sqrt {dy^{2}+dx^{2}}}$ since AC is not parallel to the y-axis and so the car moving along it also has a changing position on the x-axis. On the other hand, if we instead orient the y-axis parallel to the segment AC, then for a given increase in y-coordinate dy, the odometer of the car moving along AC only increases by dy while the odometer of the car moving along AB increases by ${\sqrt {dy^{2}+dx^{2}}}$. Obviously this is analogous to the fact that when the two observers are moving apart inertially, then you can use either observer's inertial rest frame, and the two different frames will disagree about which observer's clock increases by more for a given increase in the t-coordinate of that frame (i.e. the two frames disagree which clock is ticking slower)

But despite the symmetry in the 2D example when we are just considering two straight segments AB and AC, when we consider the entire path the symmetry is broken--regardless of how we orient our Cartesian coordinate axes, we'll always find that the bent path consisting of AC and CB has a greater total length than the straight path AB. Specifically, if point A is assigned coordinates $x_{A}$ and $y_{A}$, point B is assigned coordinates $x_{B}$ and $y_{B}$, and point C is assigned coordinates $x_{C}$ and $y_{C}$, then by the pythagorean theorem we know the total length of the straight path is ${\sqrt {(y_{B}-y_{A})^{2}+(x_{B}-x_{A})^{2}}}$ while the total length of the bent path is ${\sqrt {(y_{C}-y_{A})^{2}+(x_{C}-x_{A})^{2}}}+{\sqrt {(y_{B}-y_{C})^{2}+(x_{B}-x_{C})^{2}}}$, and all Cartesian coordinate systems will agree on the value of these two lengths (and agree that the straight one is shorter), even though they disagree on the specific coordinates assigned to A, B, and C. And it's exactly the same in SR, although different frames can disagree which of two clocks is ticking slower when they are moving at constant speeds, if two clocks move apart and later reunite they'll always agree on the total time elapsed on each clock, and they'll always agree that the one that moved at constant velocity has more total elapsed time than the one that changed velocity to turn around, even though they disagree on the specific values of the velocities.

"Just because one observer is sitting in a rocket that uses motors to accelerate it, doesn't make for an asymmetric situation - to the traveller, the supposedly 'stationary' planet he's come from is accelerating away from him by exactly the same amount."

No, the observer who accelerates knows objectively that he didn't continue moving inertially, because he feels G-forces that could be measured by an accelerometer, and in generally he will see that equations of motion for objects in his lab aren't the same when he's accelerating as they are when he's moving inertially (for example, if a ball is floating in front of him in zero G while he's moving inertially, then if the lab accelerates one of the walls will suddenly start getting closer to the ball until it hits it).

"The only reason that the planet-bound observer doesn't feel this is that the effect is diluted because of the mass of the planet, but it is no less real, and the planet-bound observer is no more inertial during the process than is the 'traveller'."

When considering the twin paradox one typically makes the simplification of ignoring gravity which depends on curved spacetime in general relativity (in curved spacetime the notion of an 'inertial' observer is more subtle, an inertial frame can only be defined in a small local region where the effects of curvature are negligible, but you can still analyze the motion of two clocks which move apart and later reunite using the full equations of general relativity, and you typically find that one elapses more time than the other--see the Hafele–Keating experiment for example). Feel free to imagine that we're dealing with two spaceships in deep space, and one has its thrusters off so it moves inertially between the two meetings, while the other moves away, then turns its thrusters on (and feels G-forces) to turn around, then returns to meet with the first ship.

"The only sustainable solution to the paradox is that the differences are wiped out through the process of re-uniting the two observers."

Be realistic for a moment, do you really believe it's likely that you have discovered a basic flaw in relativity which has somehow escaped the attention of thousands and thousands of smart professional physicists (including Einstein) for over a century? Or are you willing to consider the possibility that you, as an amateur with no technical training in relativity, might be making some sort of conceptual error? Beware the Dunning–Kruger effect!

"If you don't believe that's what happens, please cite evidence (not theory) to prove your case."

The aforementioned Hafele–Keating experiment is one example of evidence for differential aging (though it's based on a combination of velocity-based time dilation and gravitational time dilation unlike the purely SR-based thought experiment above). Some others are discussed here. Hypnosifl (talk) 06:50, 30 October 2010 (UTC)

"If by "appearing" you mean what the observer sees visually, then no, they don't see the clock suddenly jump forward, but each observer will see the other's clock running faster than their own when they see the other moving towards them, due to the Doppler effect"

A schoolboy error. When two observers have a relative velocity of near light speed, each will observe (clearly in his own inertial frame - how else would he carry out measurements?) the time processes of the other to be slowed. It doesn't make any difference whether they are approaching or separating - the relevant term in the relativistic time dilation formula depends on the square of the velocity.

"No, the observer who accelerates knows objectively that he didn't continue moving inertially, because he feels G-forces that could be measured by an accelerometer, and in generally he will see that equations of motion for objects in his lab aren't the same when he's accelerating as they are when he's moving inertially (for example, if a ball is floating in front of him in zero G while he's moving inertially, then if the lab accelerates one of the walls will suddenly start getting closer to the ball until it hits it).

The planet-bound observer has exactly the same experience as the traveller - he is experiencing the same effects as the traveller anyway through the gravitational field he's sitting in. The effect of the traveller's acceleration (both at turn-around and at re-uniting) is to change fractionally the gravitational effects experienced by the other. In his deep gravitational well, this is sufficient to cancel out the time dilation effects in his clocks as measured by the traveller. There isn't a huge amount of point in using the planet-bound example since the effects of special relativity are invalidated by the presence of the strong gravitational field. When two observers each with his own spaceship, and in the absence of any nearby gravitational fields, are considered, the symmetry of the situation becomes self-evident.

Thanks for the reference to the Dunning–Kruger effect. It's good to know that one is discussing issues with people who have respect for one's abilities! Of course I'm an amateur, but at least I have an Oxford degree in Physics - albeit from some time back.

As to the Hafele–Keating experiment, see my remarks above on strongly non-inertial situations. I do realise that it's going to be difficult to generate an example in a 'laboratory' where special relativity can be expected to hold rigorously, to confirm or otherwise the view you've expressed that there is a residual time discrepancy between the two observers' clocks, but without the proof, that view is still in the realms of speculation, as for any hypothesis with no direct experimental confirmation. For myself, I have never understood why the physics establishment continues to promulgate this prediction, in the face of the arguments to the contrary. Of course much of the heat generated by this topic arises from loosely phrased assertions in the media by people with no understanding of the physics - eg 'you can't go faster than the speed of light', 'a person travelling at near the speed of light ages more slowly' - bald statements that press the right buttons for sensational reporting and sci-fi narrative, but omit all the important qualifying clauses to make sense of them.

195.28.224.59 (talk) 16:06, 2 November 2010 (UTC)Ed Addis

"A schoolboy error. When two observers have a relative velocity of near light speed, each will observe (clearly in his own inertial frame - how else would he carry out measurements?) the time processes of the other to be slowed. It doesn't make any difference whether they are approaching or separating - the relevant term in the relativistic time dilation formula depends on the square of the velocity."

No Ed, it's you who's making a basic error here--I clearly stated that I was talking about what's seen visually, and you don't seem to understand that when physicists talk about what is "observed" in a given frame (or just what occurs in that frame), they are not talking about visual appearances but rather what coordinates are assigned to events in that frame, with the coordinates being assigned in a way that factors out delays in seeing events due to the finite speed of light. The usual way of doing this, as discussed in Einstein's original 1905 paper which I linked to above and in virtually any physics textbook (also see the discussion on this page) is to imagine a grid of inertial rulers and clocks at rest relative to myself, with the clocks synchronized according to the Einstein synchronization convention, and then to imagine I assign coordinates to events in a local manner by looking at the reading on the clock that was right next to the event as it happened. Another option would be to just subtract the distance/c from the time I saw the event, for example if an event happens 10 light years away and I see the light from that event in 2010, I can say that I "observed" the event to occur in 2000 in my rest frame.

So, let's consider the Doppler effect. Suppose a clock is traveling towards me at 0.6c in my frame, and each time the clock ticks forward by 20 seconds it sends a pulse towards me. Because of time dilation, it is slowed down by a factor of ${\sqrt {1-0.6^{2}}}=0.8$ in my frame, so it takes 20/0.8 = 25 seconds between sending successive pulses in my frame (so here you can see I am assuming that the clock is 'really' running slow in my frame). During these 25 seconds, since it's moving at 0.6c it will have moved 25*0.6c = 15 light-seconds closer to me. So suppose at t=0 seconds in my frame it is at a position of x=100 light-seconds, at which point it sends a pulse, and then 25 seconds later at t=25 seconds in my frame it's at a position of x=85 light-seconds, 15 light-seconds closer to the origin where I am sitting, at which point it sends a second pulse towards me. Since the first pulse was emitted at a distance of 100 light-seconds from me, I don't actually see the pulse until 100 seconds later when the light has actually crossed that distance to reach me, so I see the first pulse at t=0 + 100 = 100 seconds. The second pulse was emitted at a closer distance so it only takes 85 seconds to reach me, so since it was emitted at t=25 seconds in my frame I will see it at t=25 + 85 = 110 seconds. So I see the first pulse at t=100 seconds and the second pulse at t=110 seconds, meaning I see the clock sending a pulse at 10-second intervals, in spite of the fact that the clock is sending pulses every time it goes forward by 20 seconds, and it "really" takes 25 seconds between sending the pulses in my frame due to time dilation. Thus I am visually seeing the clock running fast as it moves towards me, in spite of the fact that it's "really" running slow in my rest frame. And you can see that this calculation agrees with the relativistic Doppler shift equation, $f_{observed}=f_{source}{\sqrt {\frac {1+v/c}{1-v/c}}}$, since in this cace v/c=0.6 and $f_{source}$ = 1 pulse/20 seconds, so we have $f_{observed}=1/20s{\sqrt {\frac {1.6}{0.4}}}=1/20s{\sqrt {4}}=1/10s$, i.e. the equation predicts I should see the pulses arrive with a frequence of 1 pulse every 10 seconds just as derived above.

If you disagree with any of the above I hope you will show where you think I have made an error in my calculations, and will give your own corrected calculations showing both the position and time coordinates where the clock sent successive pulses and the time coordinates that the light from these pulses reaches me. If you just doubt what I'm saying is correct but don't have the technical understanding to do your own calculations, you can verify that other authorities agree that a clock moving towards an inertial observer will be visually sped up even though it's "really" running slow in the observer's frame, for example see The Doppler Shift Analysis from the Twin paradox page I linked to earlier (which is hosted on the site of a theoretical physicist, and is part of an FAQ that a number of physicists worked on). And for a published reference, you might look at this book (viewable on google books) which says on pages 73-74: If the clock is moving away from you, the light from each successive flash has further to go before it reaches you, so you see the clock running more slowly than it is actually running in your frame. On the other hand, if the clock is moving toward you, the light from each successive flash has less distance to cover, so you see the clock running faster than it actually is running in your frame. It turns out that when the clock moves toward you, this effect is more important than the fact that the clock is running slowly, so you see it running fast.

"The planet-bound observer has exactly the same experience as the traveller - he is experiencing the same effects as the traveller anyway through the gravitational field he's sitting in."

As I said in my previous comment, I'm just talking about the analysis of this situation in the context of special relativity where there is no gravity, if you want to take gravity into account you have to deal with curved spacetime and general relativity, and no coordinate system covering a large region of curved spacetime can be considered "inertial" so the analysis is more complicated (though in curved spacetime there is the concept of a 'locally inertial' frame in a very small patch of spacetime, thanks to the equivalence principle). Did you read the part where I said "Feel free to imagine that we're dealing with two spaceships in deep space, and one has its thrusters off so it moves inertially between the two meetings, while the other moves away, then turns its thrusters on (and feels G-forces) to turn around, then returns to meet with the first ship"? Do you agree that this is the type of situation that can be analyzed theoretically using special relativity, and that SR predicts that only the observer who accelerates will feel G-forces, and that this observer will be younger than his inertial twin when they reunite? In no way does this prediction contradict any of the basic assumptions of special relativity as you seemed to suggest earlier, instead it follows logically from them.

"Thanks for the reference to the Dunning–Kruger effect. It's good to know that one is discussing issues with people who have respect for one's abilities! Of course I'm an amateur, but at least I have an Oxford degree in Physics - albeit from some time back."

Well, your understanding of special relativity is obviously lacking if you think anything in the theory is inconsistent with the theoretical conclusion that in the twin paradox scenario, the twin that accelerates will be younger when he reunites with the inertial twin. You also misunderstand other basic aspects of the theory, like the above-mentioned difference between visual appearances (which includes the Doppler effect) and coordinates assigned to events in a given frame.

"As to the Hafele–Keating experiment, see my remarks above on strongly non-inertial situations. I do realise that it's going to be difficult to generate an example in a 'laboratory' where special relativity can be expected to hold rigorously, to confirm or otherwise the view you've expressed that there is a residual time discrepancy between the two observers' clocks, but without the proof, that view is still in the realms of speculation, as for any hypothesis with no direct experimental confirmation."

You are free to argue that the theory of special relativity might not be experimentally correct even in the absence of gravity (although since general relativity reduces to special relativity locally, I doubt you'd be able to actually construct a theory that agrees with GR's predictions about experiments like the Hafele-Keating experiment yet disagrees with special relativity in predictions about experiments far from any source of gravity). But your argument seemed to go further, and say that on a purely theoretical level there is some conflict between the basic assumptions of special relativity and the prediction that the non-inertial twin will be younger when the two twins reunite in the twin paradox. This is a completely untenable position, as special relativity is mathematically well-defined and self-consistent, and I explained to you how the twin paradox prediction is consistent with the symmetry between inertial frames by the analogy with Euclidean geometry and the symmetry between different Cartesian coordinate systems, which you didn't respond to at all. If you continue to defend the idea that you have found a purely theoretical flaw, you are also supposing that you as an amateur have discovered some basic theoretical contradiction in SR that has somehow escaped the attention of all the thousands of smart physicists who have studied the theory. To suppose that this possibility is more likely than the possibility that you have just misunderstood some aspect of the theory is exactly the sort of over-inflated confidence in one's own ability to understand a subject one hasn't studied in any depth that led me to mention the danger of falling prey to the Dunning-Kruger effect.

In any case, since we now seem to be mainly discussing relativity itself rather than improvements to the article (and you seem to acknowledge that the sentence that started this discussion does accurately represent the view of mainstream physicists about what relativity would predict, even if you think the prediction is wrong), if you want to continue the discussion we should probably choose some other forum to do so. I often post in the relativity forum at physicsforums.com, there are a lot of posters with expertise in the subject there. Hypnosifl (talk) 18:11, 2 November 2010 (UTC)

OK - I have little more to say on this anyway. Final effort. Your hypothesis rests on the supposition that the acceleration 'felt' by the traveller at turn-around in some way 'operates' on the time processes in his frame. This view is still rooted in the notion that there is a 'real' slowing effect at work in the physical environment of the traveller, relative to the planet-bound frame, whereas in fact its only reality is as observed from the latter frame. This must be obvious from the fact that, whilst the traveller may be travelling at near light-speed relative to the planet-bound observer, he has different relative velocities with respect to other objects in the universe. Without such a 'real' effect locally, there is nothing for the acceleration to 'work on'. Consequently, any argument that the acceleration is only being experienced by the traveller, and not by the planet-bound observer is irrelevant. What's important about the acceleration in this context is only the change of relative velocity observed by the two observers, which of course is identical viewed from either standpoint. So to summarise, it's not the asymmetrical relation between the locally experienced gravity or acceleration that affects the outcome, but the symmetrical remotely experienced changes in relative velocities. I haven't checked through your maths, and I have no doubt at all that it's accurate, given your assumptions about the physics, but I don't agree with those assumptions. I don't pretend to have found a flaw in special relativity, as you suggest. I just don't think that you are applying it correctly. However, as your mind is clearly made up on the subject, and as I have no expectation of persuading you to see the problem in a different light, I will rest my case here, and wait for valid experimental data to prove or disprove my case.

195.28.224.59 (talk) 13:29, 4 November 2010 (UTC)Ed Addis

"Final effort. Your hypothesis rests on the supposition that the acceleration 'felt' by the traveller at turn-around in some way 'operates' on the time processes in his frame. This view is still rooted in the notion that there is a 'real' slowing effect at work in the physical environment of the traveller, relative to the planet-bound frame, whereas in fact its only reality is as observed from the latter frame. This must be obvious from the fact that, whilst the traveller may be travelling at near light-speed relative to the planet-bound observer, he has different relative velocities with respect to other objects in the universe. Without such a 'real' effect locally, there is nothing for the acceleration to 'work on'. "

Ed, it isn't my hypothesis, it's just a very basic standard conclusion in special relativity which would be taught to anyone in an introductory class on the subject. I gave you several links to textbooks and websites discussing the standard understanding, including a link to Einstein's original 1905 paper where he discussed how the conclusion of differential aging follows from the basic premises of SR. If you can't be bothered to do basic research like clicking on links given to you (or doing some searching on your own--just type "twin paradox" or "clock paradox" into google books and you will find plenty of material), but just want to spout off uninformed opinions about things you obviously have no technical training in but are nevertheless totally confident about (Dunning-Kruger effect again), then you will never learn anything.

Anyway, it's hard to make sense of your argument since it relies on odd verbal formulations rather than anything mathematical--what does "operates on the time processes in his frame" even mean? Of course I agree that you can analyze the situation from the perspective of any inertial frame you like, there's no need to use the Earth's frame, but the point is that all frames agree on the amount each twin ages over the course of their entire journey, and all agree that the one who accelerates ages less than the inertial twin. If you like I could give you the math analyzing the same trip from the perspective of two different inertial frames, one of which defines the inertial twin to be at rest the whole time but with the other defining the inertial twin to be moving at constant velocity while the non-inertial twin is at rest during one of the legs of his trip (either the outbound leg before the turnaround or the inbound leg after the turnaround), to show that despite the fact that the different frames disagree about which twin was aging slower during a given leg of the trip (and also can disagree on whether the non-inertial twin's clock starts running slower after the acceleration or whether it was running slower before the acceleration than after), they still both calculate the same thing for total amount each twin has aged at the time they reunite. But I suspect you would just ignore it like you ignore everything else I tell you.

"I don't pretend to have found a flaw in special relativity, as you suggest. I just don't think that you are applying it correctly."

I am applying it the same way every physicist applies it, as you would know if you bothered to do some basic research. There isn't really any alternate way to "apply it" that would be consistent with the basic postulates of SR--for example, everything I am telling you can be derived from the fact that during a time-interval of $\Delta t$ in any inertial frame, a clock moving at constant velocity v in that frame will tick forward by a proper time of $\Delta \tau =\Delta t{\sqrt {1-v^{2}/c^{2}}}$, so if a given clock moves on a path made up of two inertial segments lasting for time-intervals of $t_{1}$ and $t_{2}$, with velocities of $v_{1}$ and $v_{2}$ (and with the change in velocity at the moment of acceleration being treated as instantaneous), the total elapsed time on that path must be $\Delta t_{1}{\sqrt {1-v_{1}^{2}/c^{2}}}+\Delta t_{2}{\sqrt {1-v_{2}^{2}/c^{2}}}$. More generally, a path with continuously-varying velocity v(t) can be treated as a sum of infinitesimally brief inertial segments each lasting dt, so if you're familiar with the notion of an integral you should be able to see that the generalization of the above equation is that if the clock has a known velocity v(t), the time elapsed on the clock between two times $t_{1}$ and $t_{2}$ will be given by $\int _{t_{1}}^{t_{2}}{\sqrt {1-v(t)^{2}/c^{2}}}\,dt$ (you can see this equation given in this section of the twin paradox page I linked to earlier, and also in textbooks like equation 4.8 in on p. 103 of this book or equation 8.101 on p. 242 of this one). It's just a mathematical matter to show that if you $t_{1}$ represents the time the two twins depart from one another and $t_{2}$ represents the time that they reunite and compare ages, then if one twin has a constant v(t) while the other has a varying v(t), the integral will always give a smaller elapsed aging for the one with varying v(t).

Something similar is true in 2D Euclidean geometry, where a straight line between two points (one with constant slope in any Cartesian coordinate system, analogous to constant velocity in an inertial spacetime coordinate system) always has a shorter distance than a path between the same points with varying slope--that was the point of my geometric analogy earlier which you totally ignored. In fact, if you know the instantaneous slope as a function of x-coordinate S(x) for a given path, then the total length of the path between two points with x-coordinates $x_{1}$ and $x_{2}$ can be calculated with the integral $\int _{x_{1}}^{x_{2}}{\sqrt {1+S(x)^{2}}}\,dx$, which you can see is very similar to the previous integral except for the plus sign where the other had a minus sign, and since we know a straight line is the shortest distance between two points, we know this integral must always give a shorter total length for a path with constant S(x) than for a path with varying S(x) between the same pair of endpoints at $x_{1}$ and $x_{2}$. See this post of mine on physicsforums for more mathematical details on the geometric analogy. Hypnosifl (talk) 09:06, 7 November 2010 (UTC)

Parallel universe time travel and break of symmetry

I wrote that the parallel universe version necessarly breaks symmetry between universes. To me it was kind of obvious when I wrote it.

But now that I'm thinking about it, I'm not so sure. Does it make any sense to consider an operation that would switch between two alternate universes ?? I doubt it would, unless you consider some kind of a meta-universe that includes all alternate universes. Which is kind of weird.

Therefore, if someone removes all remarks concerning the breaking of symetry in the article, I would understand.

--Grondilu (talk) 16:37, 27 October 2010 (UTC)

No Causality Problem

If time travel is possible, and you do travel to the past, then you already have been in the past and it's a part of the present already. For whatever reason, if you travel to the past to change something that contributed to the present as you know it, you can't, because it didn't get changed. From the perspective of the present, events in the past are immutable, and the most a time traveler could accomplish is to create the future that he knows already exists. Nothing else can happen because it didn't happen. No paradox, no causality issues.

Personally, I think time travel is impossible due to the conservation of mass/energy. Something sent into the past would look like destruction of mass/energy in the present and creation of mass/energy in the past.Jlodman (talk) 05:29, 29 October 2010 (UTC)

Well, your first paragraph suggests you believe only in the self-consistent time travel version (see article). Conservation of mass/energy is not an issue in the continuous time travel version. Anyway, let me remind you that this place is not a forum about the article's subject. Do you have anything to say concerning the article itself, and not just about time travel ? --Grondilu (talk) 00:50, 2 November 2010 (UTC)

Time travel into the future

From the lead section:

"...one-way travel into the future is arguably possible..."

Why arguably? I thought this effect had been experimentally confirmed (albeit, given current technology, by only minute amounts). 86.179.118.226 (talk) 21:58, 13 February 2011 (UTC)

Please show your sources for the experiments done and results thereof. HumphreyW (talk) 04:47, 14 February 2011 (UTC)

I thought this was common knowledge. See e.g. Time_dilation#Experimental_confirmation. I think I also remember reading that the GPS system has to be adjusted to compensate for the different speed that the clocks run at while whizzing round Earth. Are we talking about the same thing here? 86.160.222.121 (talk) 12:33, 14 February 2011 (UTC) Edit: Sorry, the GPS thing is mentioned at the link given...

I suppose it's "arguably" because there are some people who don't consider differential aging via relativistic time dilation to qualify as "travel into the future"--perhaps they think that travel into the future would have to match the science fiction convention of just disappearing from the present and reappearing in the future, having skipped over the intervening years completely. And there are also others who don't think one-way travel into the future can ever be called "time travel", see Talk:Time_travel#sleeping_is_.2Anot.2A_time_travel. Still, this isn't the point of view of any authority I know of (a physicist, a science fiction critic, etc.) so I wouldn't object to removing the "arguably". Hypnosifl (talk) 17:07, 15 February 2011 (UTC)

I think you may be right about why "arguably" was used. However, I read it as implying that the existence of relativistic time dilation effects is debated (rather than, as I understand it, being an established scientific fact). How about we change "arguably" to "theoretically"? If I could go arbitrarily fast, then 100 world-years could be swallowed up in a couple of seconds of my time -- is that right? If so, then I think most people would accept that as being "one way travel into the future". 86.181.174.241 (talk) 18:37, 15 February 2011 (UTC)

Only problem with "theoretically" is that it might suggest it's purely theoretical, when in fact it's been verified with atomic clocks that space shuttles and the like do age slightly less than clocks on the ground. Maybe something like "and a form of one-way time travel into the future is possible given..." ('a form' would suggest that other forms which appear in sci-fi, like jumping forward in time and skipping the intervening years, may not be possible). Hypnosifl (talk) 00:11, 19 February 2011 (UTC)

Yes, I see what you mean about "theoretically". However, regarding your wording, I'm not sure exactly what other "forms" you are thinking of that are not possible. Why is "jumping forward in time and skipping the intervening years" not possible (in principle)? In principle, can I not, for example, condense 500 world-years into what seems to me like 1/10 second? That sure seems like skipping the intervening years to me. I thought the only thing I couldn't do was condense those years into exactly zero seconds. However, that seems an unusual and severe requirement. I think most putative time-travellers would be pretty happy with a "journey time" of a second or a few seconds. How about "and one-way time travel into the future is in principle possible given..."? 86.181.174.240 (talk) 18:46, 23 February 2011 (UTC)

I meant "skipping" as in actually not being physically present in the intervening years, like in "Back to the Future" where the DeLorean just vanishes from one time and place and appears in another, a pretty common convention in sci-fi time travel. In relativity even if you traveled 500 years into the future in Earth's frame in only 1/10 of a second of your own time, in the Earth's frame your ship would still be present at every moment in those 500 years, it's just that all processes aboard your ship would be running incredibly slow in that frame. Anyway, "in principle" would be OK with me, though I'd suggest "possible in principle" rather than "in principle possible", I think it's the more common phrase (and the two p's in a row sounds a little awkward). Hypnosifl (talk) 00:01, 24 February 2011 (UTC)

Very true. I know this place is not a forum about time travel, but I can't help noticing that, since the ship's internal clock is running extremely slow from Earth's perspective, it can not move at any substantial speed, because such a speed would be ridiculously high in the ship's reference. Thus, the ship has to resist any external force applied to it, which shows how mass and time dilatation are closely related. --Grondilu (talk) 18:19, 24 February 2011 (UTC)

Actually, despite the time dilation, speed is completely symmetric--if the ship measures the Earth to be moving away at 0.6c (60% the speed of light) relative to itself, then the Earth also measures the ship to be moving at 0.6c relative to the Earth. And as long as both are moving inertially (not accelerating), time dilation itself is symmetric, in this example the ship's clock will be running slow by a factor of 0.8 in the Earth's frame, but the Earth's frame will be running slow by a factor of 0.8 in the ship's frame. It all has to do with the fact that each one is using their own set of rulers and clocks to measure the other one, and each one thinks the other guy's rulers are shrunk due to length contraction and the other guy's clocks are slowed-down due to time dilation and out-of-sync due to the relativity of simultaneity. Here is a thread on physicsforums.com where I illustrated how it would look in both frames if you had two ruler/clock systems moving at relativistic speed next to each other, you might find that helpful in understanding how it can all work out. Hypnosifl (talk) 18:34, 24 February 2011 (UTC)

reducing the size of the article

The article is currently tagged as too long, and indeed I think it is.

In order to shorten the article, I think we shoud discuss which parts should stay in it, and which part should be put in a separate article.

Time travel is still a very imaginary experiment. Actually science fiction is by far the most common place for time travel to occur. Therefore, I think the ideas from fiction should be the most part of the article, and it should be one of the first sections. Rigourus science hypothesis should be regarded as digressions, and should be put at the end of the article, or in a separate article.

--Grondilu (talk) 16:47, 28 February 2011 (UTC)

I don't think it's that bad, currently the readable text (as opposed to references and the like) is about 73 KB, and Wikipedia:Article_size only offers loose guidelines...judging from that I would say pages only slightly larger than 50 KB could go either way, they only say an article "almost certainly should be divided" if it exceeds 100 KB, and elsewhere they say "Articles of about 200 KB (~30 pages) are not uncommon given that many topics require depth and detail, but it's typical that articles of such size get split into two or more sub-articles." There may be certain sections that could be merged into other articles, but completely splitting apart the physics content and the fiction content would seem a little excessive to me. Hypnosifl (talk) 04:08, 8 March 2011 (UTC)

reducing article size - move fiction stuff to merge with "Time Travel in Fiction"?

To reduce the length of this article, perhaps the section of ideas from fiction should be merged into the related article Time travel in fiction. That article has taken a science fiction history approach to the subject, which perhaps could be nicely complemented by the approach taken in THIS article. ChrisBaker (talk) 17:31, 6 March 2011 (UTC)

The problem is that most of the "ideas from fiction" is about the theory of how time travel would work, just using fiction to illustrate these ideas, and so it fits in naturally with the earlier stuff concerning ideas about how to resolve paradoxes from physics (Novikov self-consistency principle, parallel universes, etc.) and from philosophy. So I think that stuff fits better in this article than in "Time travel in fiction". The section on "Origins of the concept" might make more sense in that article, though. Hypnosifl (talk) 03:58, 8 March 2011 (UTC)

time traveling train?

i have heard somewhere that if a train travels at the speed of 7 rounds of earth in a sec it would go 100 years forward in just 1 week.is it possible in future?can a train can travel at that much speed on earthronitd 09:28, 25 March 2011 (UTC) — Preceding unsigned comment added by Ronitd (talk • contribs)

I highly doubt a train can sustain such a speed without overheating. Even if a train could be that fast it would not go forward in time. A train ride with that speed would be like arriving at your destination the time you went aboard the train. . . . I guess? Matthew Goldsmith 22:11, 27 March 2011 (UTC) — Preceding unsigned comment added by Lightylight (talk • contribs)

Fatalism And The Grandfather Paradox

According the the grandfather paradox if I were to travel to the past to kill my grandfather and succeed I would have never existed in the first place. Killing my grandfather would result in my parents never being born and without any parents to give birth to me I wouldn't exist. According to fatalism what has already happened cannot be changed because it was fated to happen. Since my grandfather had married, raised my parent, and my parent raised me, I cannot kill my grandfather. He was fated to live so he will live regardless of any murder attempt. This is all hypothetical because I cannot travel back in time and I have no motive to kill anybody. Matthew Goldsmith 22:33, 27 March 2011 (UTC)