Talk:Time travel

WikiProject Time assessment rating comment

Needs a lot of cleanup, but basically a B.

Can someone Incorporate the LHC at Cern

Cern is the largest Physics project in history and employs the guys who have Ph.D's from MIT and the like. Like 2000 of the smartest guys in the world are employed over there and work full time using the fastest computers, strongest magnets, and brightest intellectual capital in our galaxy. http://en.wikipedia.org/wiki/Large_Hadron_Collider This is going to be finished within 6 months and has spent billions of dollars and over 13 years of construction. They plan to create new novelties, including technology that is related to time travel, such as manageable black holes. I do not know how to bring this section into the article, as this topic might be adminned by people who aren't famaliar with high level physics and think its vandalism, but if someone can begin to incorporate a paragraph about the LHC, I will smoothe it out and make the links work. Sentriclecub (talk) 08:21, 8 March 2008 (UTC) Can someone who is in charge of this page or moderates it, please reply below.

Begin here, thanks!

Want to help write or improve articles about Time? Join WikiProject Time or visit the Time Portal for a list of articles that need improving. -- Yamara 15:56, 10 January 2008 (UTC)

if time travel were true i would have met myself by now —Preceding unsigned comment added by 172.163.75.184 (talk) 03:34, 25 February 2008 (UTC)

Time Machine

I personally believe that any form of Time Machine cannot currently exist due to the lack of visitors from the future or past. I believe that if a time machine ever did exist it's location would not be on this Earth but in outer space, because all theories and principles of the Speed-of-Light are developed from the properties of light in a vacuum. I do however believe that it is possible for a time machine to be built on earth and implemented on earth using a Casimir Vacuum and a man made Black Hole formed in a Gigantic Particle accelerator(Gigantic so that a self sustaining Black Hole could be formed, or at least one that would not collapse in less than one second). I also believe that time travel is possible through the use of a space craft and an Alcubierre Warp Drive(although there is no method for creating one, Yet!)--Mjeden2006 20:07, 9 January 2007 (UTC)

• there is work currently being done useing an aray of lasers to accelerate a particle and send it back in time... John Doe or Jane Doe 14:06, 8 March 2007 (UTC)

Unfortunately, if it's ever built, it will only be able to send information back to the point in time this device is built, not before... —Preceding unsign ed comment added by 72.67.35.112 (talk) 05:10, 29 January 2008 (UTC)

Unless of course we are wrong about those laws of physics. 86.151.228.86 (talk) 22:15, 4 March 2008 (UTC)

---

vinamra mattoo--

well I think that there is an another method of making a time machine . we know that the speed of electrons is much higher than light .if we increase the kinetic energy of the atom to such an extend that its electronic shell becomes large then there can be an opening of a portal .the more the kinetic energy .more back we go.and if we change the charge of the electron {by converting it into positron }then we can go to the future

Guys, I'm sorry to enter this discussion with nothing to add, but I just have to put my two cents in on the above statement.

"we know that the speed of electrons is much higher than light"

It is impossible for electrons to travel faster than light.

"if we increase the kinetic energy of the atom to such an extend that its electronic shell becomes large then there can be an opening of a portal"

1. The kinetic energy of an atom has nothing to do with electron energy. Kinetic energy is simply a measurement of how fast an atom is moving. You would measure this with a thermometer ; ).
2. Increasing the energy of electrons would not increase the size of the "shell". It would simply move it into a higher quantum energy state.

Also, I don't think it's possible to convert an electron into a positron. However, antimatter is not my area of (relative) expertise.

Good thinking, but sadly time travel seems to be only a thing of science fiction. Perhaps you should look at some of the articles on positrons or quantum energy states for more information. Nice theory though! -- X Wild Irishman x (talk) 02:49, 14 May 2008 (UTC)

Safety

Are there any theories on the safety of time travel? --NEMT (talk) 20:30, 20 November 2007 (UTC)

Yes, its well known that when time travelling your safety is not guaranteed, especially when you push it to the limit. —Preceding unsigned comment added by 88.106.16.89 (talk) 01:21, 27 February 2008 (UTC)

Common wisdom suggests that one ought to bring one's own weapons when time traveling, especially if one has only time traveled once before. 76.192.168.161 (talk) 15:47, 16 August 2008 (UTC)

You could get lost in a completely different parrallel universe and never be able to return to your own. —Preceding unsigned comment added by 222.153.21.181 (talk) 10:34, 3 July 2008 (UTC)

make money on time travel

immagine this, you find a wormhole connecting a point in the past with the future. near the worm hole is a dollar. you pick up the dollar and throw it into the future end. now when you find the worm hole it has two dollars in front of it. you pick both up and repeat the process.soon you'll be very rich.what concerns me is that this can be done with anything. by this means you can in a sense create mass or produce exponentional amounts of energy. if this is done with energy would there eventually be a really big explosion from the infinite loop? —Preceding unsigned comment added by 70.111.221.127 (talk) 03:03, 14 February 2008 (UTC)

You would still only have one dollar. --RLent (talk) 20:24, 17 April 2008 (UTC)

That is probably impossible, given that wormholes are designed to travel space-time, not copy items. —Preceding unsigned comment added by 68.48.94.53 (talk) 11:45, 23 May 2008 (UTC)

Designed? BreathingMeat (talk) 01:07, 27 May 2008 (UTC)

Anything that can be used for time travel should also be usable for having at least the appearance of multiple "copies" of the same item--think of how if you went back in time and met with your younger self, it would appear as if there were two "copies" of you in the same place. But if something like the Novikov self-consistency principle is true, you could only actually spend the dollar that had been through the time machine the most times and was thus the "oldest"--if you tried to spend any of the "younger" versions of the dollar, you wouldn't be able to send it through the time machine again and this would be inconsistent with the fact that you can see "older" versions lying around. Hypnosifl (talk) 19:39, 27 May 2008 (UTC)

You will have zero dollar because the dollar that you just threw into the wormhole would appear in the past but would end up going into a completely different parallel universe. The guy who threw it into the wormhole is not the same guy receiving the dollar. —Preceding unsigned comment added by 222.153.11.81 (talk) 06:14, 4 July 2008 (UTC)

Time travel is not only possible; it is actionable.

Time travel will certainly become a reality; all things the human mind can comprehend can be accomplished. The claim that only photons can travel at the speed of light will be ridiculous to a first year student in the year 3008. Ask almost anyone on the planet 108 years about travelling from New York to San Francisco in 8 hours and they would have said its impossible. Ask someone 508 years ago about travelling 70 miles in one hour and they would have said thats impossible. Any problem is solvable given enough time and resources. The atom did not even exist in the year 1744, at least not to scientists of the day, yet in 1945 we smashed a few together over Japan with spectacular results. One new discovery leads to discovering something else which will lead to travel faster than the speed of light. If the human brain can think it, then it can be done. —Preceding unsigned comment added by 131.44.1.69 (talk) 11:03, 27 February 2008 (UTC)

You might want to consider the difference between science and wishful thinking

I can think of jumping to the moon, but I can't do it. There is a difference between that which is difficult and that which is impossible. Just because difficult things have been done does not mean that there are no impossible tasks.--RLent (talk) 20:27, 17 April 2008 (UTC)

I think that jumping to the moon is much more likely then travelling in time, but i believe both scenarios are accomplishable. - Thymo —Preceding unsigned comment added by 62.63.25.18 (talk) 12:11, 19 May 2008 (UTC)

We could evolve into creatures that can convert mass into energy somehow and jump to moon. —Preceding unsigned comment added by 222.153.21.181 (talk) 10:38, 3 July 2008 (UTC)

Time travel

We can theorize only about time travel in the past, because travelling in the future cannot be possible due to the future timeplace (spacetime) still not exists, so there isn't no place where to travel (Presentism). If we assume that future timeplace exists, then we also must assume that we are living in the past, what would sound incredible.
But speaking about travelling in the past, the problem is where the past spacetime information is stored. Because to see something in the past, there must exist adequate timeplace. But it's completely unimaginable how and where the all past states of all events and all materia can be stored, inclusive those not only on the Earth, but also in the Space.
We also must not mix up travelling in time and travelling through Wormholes (one over Bermuda Triangle), what isn't travelling in time, but travelling between different Universes. Roberts7 13:28, 29 February 2008 (UTC)

You might want to look into the relativity of simultaneity, which makes a pretty good argument against presentism. And note that the alternative to presentism, eternalism, does not say we are "living in the past", it says there is no absolute truth about whether a given time is "past" or "present" or "future", these terms are understood as purely relative to the speaker like "here" and "there" or "left" and "right". See also A-series and B-series. Hypnosifl (talk) 17:57, 8 March 2008 (UTC)

In a very real sense, we are living in someone's past, and living in someone's future. When I was a kid, the year 2000 was "the future", now it's the past. It used to be the present.--RLent (talk) 20:34, 17 April 2008 (UTC)

High gravity as a substitute for high speed relatistic time dilation effect

From the article:

Using time dilation under the Theory of General Relativity, for instance: Residing inside of a hollow, high-mass object;

Are you sure this is accurate? I was under the impression that according to the shell theorem, the net gravity inside a hollow, symmetrical sphere was zero. If the hollow object isn't symmetrical, there could be some gravity inside, but in that case it would be no different from (and probably less efficient than) standing on the outside of a high-mass object. —Preceding unsigned comment added by 67.117.209.190 (talk) 09:16, 10 March 2008 (UTC)

Apparently the observer inside would not feel any gravitational force (the spacetime inside the shell would be flat), but the time dilation (measured relative to an observer far away from the shell) would be the same on the inside as it is right on the outside of the spherical shell--see the post by pervect at the end of this thread, or this paper on arxiv.org. Hypnosifl (talk) 18:13, 11 March 2008 (UTC)

My Thoughts

It would be so cool if time travel was possible because if you embarassed yourself you could go back and change it.

so cool —Preceding unsigned comment added by 91.105.158.7 (talk) 14:08, 26 March 2008 (UTC)

• A little reminder of Talk page guidelines: The purpose of a Wikipedia talk page is to provide space for editors to discuss changes to its associated article or project page. Article talk pages should not be used by editors as platforms for their personal views. Thanks for reading. :) --Bisqwit (talk) 15:04, 27 March 2008 (UTC)

Not Possible

There are some physicists that think (sorry, no citations) time travel is not possible because time has no dimension. Since there is no there to go back to, or to go forward to, any talk of time travel is silly.

To be complete, this article should add a contravening section with this point of view. Shawncorey (talk) 00:17, 31 March 2008 (UTC)

You really need citations for a claim like this; it sounds more like the philosophical notion of "presentism" discussed in the article, I've never seen any physicist use this argument, and it doesn't sound like a genuinely physical argument either, so I would suspect you're just misremembering. Hypnosifl (talk) 03:41, 1 April 2008 (UTC)

what you ARE is a collection of energies that is no longer where you WERE, but has migrated to a new location, there is no travelling back to where they WERE and expecting to find them there. For some reason this absurdly simple thought has escaped some modern science due to a mis-reading of a space-time charting of changes as a map of some real places that still exist. The past is chartable but not there any more, and the future only exists as thoughts in the imagination of men, it has no REAL location and it should be absurdly obvious why no one has yet travelled back from there, because there is no one there yet.

Consider, if all moments in existance are said to be like a strip of 4-D film, then pray tell me who made the film strip and WHEN? secondly tell me how any of those moments can be considered by anyone as THE present moment? thirdly tell me what is moving? all those moments would appear static to a godlike being, yet we are fully aware of motion and that would not happen unless we were somehow more than 4 dimensional and moving through all these moments... and if that were so, it would mean that we were not human, but some sort of omni-dimensional soul moving through all those souless bodies statically awaiting our presence. And while that may be possible, it does not seem very likely!Jiohdi (talk) 21:49, 8 April 2008 (UTC)

Time travel is possible. And I have already proved myself right because we are travelling forward through time right now. If we can go forward, why not backwards? We just need to find out how to do it first. —Preceding unsigned comment added by 222.153.21.181 (talk) 10:49, 3 July 2008 (UTC)

Time travel, or space-time travel?

The first paragraph of this section is wrong. It is based on the assumption that the Earth does not undergo any acceleration. Its argument of relative velocity is overshadowed by the accelerations. The first acceleration is angular acceleration; the Earth is spinning. If I were to go back 12 hours in time (and ignore the other accelerations), I would end up on the opposite side; same latitude but 180 degrees difference in longitude. Not only that, the relative velocity of the place where I "started" would be 180 degrees from when I started. If I were to go back 6 months, the Earth would be on the opposite of the Sun and its velocity would be 180 degrees different. To come up with a successful argument, you would have to use general relativity, not special relativity.--Shawncorey (talk) 17:32, 16 May 2008 (UTC)

It's not correct that you need general relativity to deal with acceleration--special relativity can certainly deal with acceleration in flat spacetime where gravity is negligible, see the way SR deals with the twin paradox on this page for example. If you were standing on a spinning sphere of negligible mass, then if you take your position on the sphere when your clock reads 3:00 and your position on the sphere when your clock reads 4:00, you can certainly find an inertial reference frame in SR where both of these are the same point in space--just imagine a projectile fired from your position at 3:00 which moves at constant velocity and has just the right speed and direction so that it arrives at your position at 4:00. In the rest frame of this projectile, this means you have returned to the same position at 4:00 that you occupied at 3:00.

Of course, in the case of the Earth we do need general relativity, not because of accelerations but because of the fact that the Earth's gravity curves spacetime. However, in general relativity you are free to use arbitrary smooth coordinate systems, and the laws of physics obey the same tensor equations in all the coordinate systems, thanks to what's known as "diffeomorphism invariance" (also known as general covariance)--see this page for a discussion. So again, it would certainly be possible to find a coordinate system where your position at 4:00 is the same as your position at 3:00, even if you are standing on a spinning orbiting planet, and this coordinate system is physically no better or worse than any other in GR. Hypnosifl (talk) 22:44, 18 May 2008 (UTC)

OK, gone--Mr. Shawn H. Corey (talk) 00:54, 17 May 2008 (UTC)

Or, you could rewrite the section so that it makes better sense... instead of blanking a paragraph about a frequently-discussed issue in time travel SF, while leaving the paragraph full of cruft intact, baffling any new readers. --Yamara ✉ 02:25, 17 May 2008 (UTC)

No, I cannot.

Nor, you little worm, can you blame me for the lack of physics that agree with your imagination.

The paragraph is wrong...and it is gone.--Mr. Shawn H. Corey (talk) 04:51, 17 May 2008 (UTC)

The section is not wrong. There is no such thing as absolute space in relativity, and hence no absolute notion of which point in space at a later time is at the "same position" as a point in space at an earlier time. One can choose different coordinate systems which give different answers as to whether a given object is at rest or not (spatial coordinate not changing with time), and none is more "true" than any other in relativity. The paragraph already contains a reference for this in the form of a book by physicist Robert Geroch, if you doubt what I'm saying I suggest you read it. Hypnosifl (talk) 22:59, 17 May 2008 (UTC)

That doesn't matter. Even without an absolute reference, one might still materialize out in space, due to the fact that the earth is orbiting in an elipse, not moving in a straight line. Before you time travel, you are traveling with the earth. During time travel, will you still be influenced by the earth's gravity? Maybe/Maybe not. Who knows. It's possible that the earth will continue to be influenced by the sun and move on its orbital path, while you--the time traveler--would continue on your last known momentum, which would be a straight line. When you come out of time-travel, you will have diverged paths from the earth. This is different, than staying in an "absolute" (fixed) location during time travel, and letting the earth continue. 129.139.1.68 (talk) —Preceding undated comment was added at 19:53, 21 July 2008 (UTC)

after seeing mention of this on Wikipedia:Editor_assistance/Requests i decided to take a look, while im not an expert on relativity As the inclusion of something is based in the requirements of Verifiability and Notability, Relativity is defiantly notable and therefore comes down to is it verifiable. At first look the source appears to be reliable, but as I don`t have access to it I cant really tell, but can accept on good faith

I can't see any specific reason for its removal unless can be demonstrated that the cited source is not reliable or does not say what is stated in the article.

also the argument itself is not just dependent on relativity (general or special) but also the "method of time travel" involved and how that effects the start and end points, as ending up elsewhere makes the assumption that the time travel process does not take any such effects into account.

You might want to ask at WikiProject Relativity for someone more knowledgeable.

but as a general point stated in Verifiability wikipedia relies on is verifiability, not truth.

The threshold for inclusion in Wikipedia is verifiability, not truth—meaning, in this context, whether readers are able to check that material added to Wikipedia has already been published by a reliable source, not whether we think it is true

--Firebladed (talk) 13:51, 19 May 2008 (UTC)

Vote on paragraph

An editor has been removing the following cited paragraph, or parts thereof, over the past couple days:

Time travel, or space-time travel?

An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date. However, according to the theory of special relativity, this argument is based on a false premise. Special relativity rejects the idea of absolute time and space; there can be no universal truth about the spatial distance between events which occurred at different times (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time, because the distance depends on the observer's frame of reference.

[ref]Geroch, Robert (1978). General Relativity From A to B. The University of Chicago Press. p. 124.

I see no reason for it to be summarily removed, especially the cited section. Some arguments for or against are cited above, which may be helpful in making a consensus on whether it should remain. -Yamara ✉ 00:50, 19 May 2008 (UTC)

• Strong Keep - It's a common criticism of time travel in fiction, beginning with H.G. Wells' The Time Machine, and a discussion is clearly merited in the article. Special relativity's challenge is plainly part of this criticism. -Yamara ✉ 00:50, 19 May 2008 (UTC)

Again, you don't read what I have written.

I said the first acceleration is angular acceleration.

Until you can read, nothing else is important. —Preceding unsigned comment added by Shawncorey (talk • contribs) 23:53, 27 May 2008 (UTC)

As pointed out here, special relativity can handle acceleration just fine as long as gravity isn't involved. And in any case, the current version of the text covers both special and general relativity. Do you disagree that both theories reject the notion of absolute space, and hence they reject the notion that there is any unique definition of the "same place" at different times? Hypnosifl (talk) 01:42, 28 May 2008 (UTC)

OK, you are Absolutely Right.

Nothing I can say will change your minds.  I was foolish to think so.  —Preceding unsigned comment added by Shawncorey (talk • contribs) 01:54, 28 May 2008 (UTC) 

"The only sense in which special relativity is an approximation when there are accelerating bodies is that gravitational effects such as generation of gravitational waves are being ignored. But of course there are larger gravitational effects being neglected even when massive bodies are not accelerating and they are small for many applications so this is not strictly relevant. Special relativity gives a completely self-consistent description of the mechanics of accelerating bodies neglecting gravitation, just as Newtonian mechanics did."

Now why do I object to that because it involves gravity, and not angular acceleration?

That's because I'm not stupid!

I'm tried of blaming me for your lack of imagination.

Grow up!

Mr. Shawn H. Corey (talk) 02:58, 28 May 2008 (UTC)

I'm not sure I get your meaning here--when you say "because it involves gravity, and not angular acceleration", what is the "it" that you're referring to ? Do you mean the paragraph is talking about gravity? Yes, the paragraph is saying that you only need to use general relativity if gravity is involved, and that as long as you ignore the gravitational waves produced by acceleration, which in most ordinary circumstances would be extremely tiny, then special relativity is perfectly capable of dealing with acceleration. Just look at the opening paragraph of that same page:

"It is a common misconception that Special Relativity cannot handle accelerating objects or accelerating reference frames. It is claimed that general relativity is required because special relativity only applies to inertial frames. This is not true. Special relativity treats accelerating frames differently from inertial frames but can still deal with them. Accelerating objects can be dealt with without even calling upon accelerating frames."

So clearly they are saying that special relativity can handle accelerations, including angular acceleration--do you disagree that this is what they're saying? Hypnosifl (talk) 03:14, 28 May 2008 (UTC)

Accelerating objects can be dealt with by special relativity but only if the frame of reference is NOT tied to them. The frame of reference MUST be linear. Otherwise there are pseudo-forces to reckon with.

But, if you care to read the original page, the frame of reference is tied to the surface of the earth. Therefore it an accelerated frame. Therefore specially relativity does not apply. EVER.

Gravity is not "insignificant". If you "time travel" back for 6 months, the earth is on the other side of the sun; over 200 million kilometres away. This is the last time I expect to read that a displacement of over 16 light-minutes is insignificant!

--Mr. Shawn H. Corey (talk) 02:27, 5 June 2008 (UTC)

You're still missing the point. There was never any assumption in that paragraph of the wikipedia article that we were using the Earth's rest frame (I should know, because I wrote the section on relativity); the point is just that if you take two events which happen on the Earth at different moments, it is possible to find a coordinate system where they both happen at the same position, and another coordinate system where they happen at different positions. This is true regardless of whether we're using inertial frames in a flat SR spacetime or arbitrary coordinate systems in a curved GR spacetime.

"You're still missing the point. There was never any assumption in that paragraph of the wikipedia article that we were using the Earth's rest frame" Yes, there was. Forgive me for calling you a liar when you are. --Mr. Shawn H. Corey (talk) 22:03, 10 June 2008 (UTC)

No, there wasn't. The paragraph only says that you can find a frame where a pair of events at different times on Earth (such as an event now and an event in 1865) happened at the same position in that frame; there was nothing about this being the rest frame of the Earth, it could certainly be the rest frame of an inertial projectile that crosses the orbit of the Earth at those two times, as I discussed in my last comment. Read it again, and tell me where you think it says anything about the Earth's rest frame:

However, according to the theory of relativity, this argument is based on a false premise. Relativity rejects the idea of absolute time and space; there can be no universal truth about the spatial distance between events which occurred at different times[46] (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the "same position" that the Earth was at another time. Hypnosifl (talk) 23:14, 10 June 2008 (UTC)

When I talked about treating gravity as "insignificant", I was only talking about treating the curvature of spacetime caused by mass as insignificant (as you may know, GR allows spacetime to be curved by mass and energy, while SR assumes a 'flat' spacetime with Minkowski geometry). If we replace the Earth with a hollow sphere of negligible mass which is still rotating about its axis and still moving in an elliptical orbit around some central point (perhaps because we're swinging it around this point on a tether rather than because of the spacetime curvature created by the Sun), then we have a situation where the movement of the sphere is similar to the movement of the Earth but where the spacetime curvature caused by gravity is negligible so spacetime is close to "flat" and we can treat the situation using SR alone. In this case it may be true that if you take an event A on the sphere 6 months ago and an event B on the sphere today, events A and B would be 200 million kilometers apart in the inertial rest frame of the central point the sphere is orbiting around (the position of the 'Sun'). The point is, you can easily find a different inertial frame where events A and B happen at the same spatial coordinates! This new inertial frame would not be the rest frame of the sphere, since the sphere is not moving inertially. Rather, it would be the rest frame of an inertial projectile which was fired at the position and time of event A, and which moved in a straight line at just the right speed and direction to intersect the sphere's orbit again at exactly the position and time of event B.

Do you disagree that if we have an object moving on a non-inertial path in flat spacetime, and we have two events A and B on the object, it will always be possible to fire an inertial projectile at the right speed and direction so that it is right next to both A and B as they happen? Do you disagree that this means that in the inertial rest frame of the projectile, A and B both had the same space-coordinates? Do you disagree that in SR all inertial frames are equally valid, so if different inertial coordinate systems disagree about whether A and B happened at the "same position" or "different positions" (for instance, perhaps in one inertial frame the two events happened 0 km apart while in another inertial frame they happened 200 million km apart), there is no basis for saying one frame's answer has more physical validity than the other's? Hypnosifl (talk) 16:39, 5 June 2008 (UTC)

"GR allows spacetime to be curved by mass and energy" You are wrong. GR is not about mass and gravity; it's about acceleration. It's about the acceleration of gravity is equal to the acceleration of moment. It states you CANNOT DISTINGUISH THE TWO! "Spacetime" is curved by mass, energy, and acceleration.

Gravity cannot be locally distinguished from acceleration, according to the equivalence principle--but the equivalence principle only applies to very small "local" regions (technically, a 'local' region must actually be infinitesimally small), on larger scales there is certainly an objective difference between curved spacetime and accelerations in flat (minkowski) spacetime. In curved spacetime, if you look at any non-infinitesimal region of that spacetime, you will see tidal forces which are not seen by an accelerating observer in flat spacetime, for example. See the last section of this article, where tidal forces (and the need to restrict the equivalence principle to infinitesimally small regions) are discussed.

Even in flat spacetime, you can treat the pseudo-forces induced by acceleration as being a sort of "gravitational" force in an accelerating reference frame; whether or not this requires you to use "GR" or is still part of "SR" is a matter of definition. See this page on the 'GR explanation' for the twin paradox, which notes that modern physicists usually define the difference between GR and SR in terms of curved vs. flat spacetime:

This feature looms so large in the final formulation of GR, that most physicists reserve the term "gravitational field" for the fields produced by matter. The phrases "flat portion of spacetime", and "spacetime without gravitational fields" are synonymous in modern parlance. "SR" and "flat spacetime" are also synonymous, or nearly so; one can quibble over whether flat spacetime with a non-trivial topology (for example, cylindrical spacetime) counts as "SR". Incidentally, the "modern" usage appeared quite early. Eddington's book The Mathematical Theory of Relativity (1922) defines Special Relativity as the theory of flat spacetime.

So modern usage demotes the uniform "gravitational" field back to its old status as a pseudo-field, with all the pejorative connotations of the prefix "pseudo". And the hallmark of a truly GR problem (i.e., not SR) is that spacetime is not flat. By contrast, the free choice of charts --- the modern form of the General Principle of Relativity --- doesn't pack much of a punch. You can use curvilinear coordinates in flat spacetime. (If you use polar coordinates in plane geometry, have you suddenly departed the kingdom of Euclid?)

In any case, even if we do take into account the fact that spacetime is curved by the mass of the Sun and the Earth, and that therefore we must use GR to address the question of whether the Earth is in the "same position" now that it was in 1865, the answer is the same: it depends which of various equally valid coordinate systems you use. In GR all coordinate systems are on equal footing thanks to the principle of diffeomorphism invariance, not just inertial coordinates, so in GR we can say that a coordinate system in which the Earth stays at rest the whole time is no different than any other coordinate. If you haven't heard of diffeomorphism invariance, please read this article which I linked to in the updated version of that paragraph in the time travel article, and which includes the following:

Closely related to background independence is another basic ingredient of general relativity, known by the imposing name diffeomorphism invariance. It concerns the coordinates physicists use to describe space and time. The principle of diffeomorphism invariance implies that, unlike in theories prior to general relativity, there are no additional structures in physics that allow us to distinguish preferred coordinate systems. As far as the laws of physics are concerned, no coordinate system is better than another, and one is free to choose.

So, do you deny that diffeomorphism invariance in GR means that a coordinate system where the Earth remains at rest throughout its orbit is physically no better or worse than any other coordinate system? Hypnosifl (talk) 23:14, 10 June 2008 (UTC)

BTW, I'm saying there is no initial rest frame; all frames undergo acceleration; and therefore, have pseudo-forces that must be accounted for.

--Mr. Shawn H. Corey (talk) 22:03, 10 June 2008 (UTC)

Did you mean to say "inertial" rather than "initial"? In curved spacetime it is true that you can't have inertial frames, but in flat spacetime you can, this is the foundation of special relativity. In any case, as I said above, in GR all coordinate systems are on equal footing because of diffeomorphism invariance. Hypnosifl (talk) 23:14, 10 June 2008 (UTC)

"An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into the exact same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that "realistically" the time machine should actually reappear in space far away from the Earth's position at that date."

Wow, totally dis me! I said from the start it's not about displacement, it's about acceleration! I am getting really tired of the little worms who dis me. --Mr. Shawn H. Corey (talk) 22:22, 10 June 2008 (UTC)

Do you always take intellectual disagreements so personally? I'm not trying to "dis" you, I'm just saying I don't think your argument makes sense. The point is that if someone claims the time traveler "should" reappear at a certain location far from the Earth, that would only make sense if you think there's some absolute truth about which location today is at the "same position" as the Earth was in 1865, but both special relativity and general relativity say you can choose between different coordinate systems which are all on equal footing (inertial frames in flat SR spacetime, or arbitrary coordinate systems in curved spacetime) and which disagree about whether the Earth today is at the same coordinate position that it was in 1865.

Do you agree that in both SR and GR, there is no absolute truth about whether the Earth today is at the "same position" it was in 1865, and that there won't be an absolute truth about this regardless of whether the Earth is accelerating or not? Please answer this question yes or no...if you do agree with me on this point, then I don't see why you think "it's not about displacement, it's about acceleration" (and if you disagree with me on this point, you're just wrong). Hypnosifl (talk) 23:14, 10 June 2008 (UTC)

No, the earth is NOT in the same position as it was in 1865.

Again, you haven't read what I wrote. I'm saying that you cannot ignore acceleration; whether it's because of gravity or rotation. To answer you, NO, you are absolutely wrong; you cannot ignore acceleration. The earth is not is the "same position" because it has undergone accelerations between then and now.

The earth is accelerating! And everything on its surface is accelerating! Get used to it. This is a fact. Now explain yourself using general relativity.

--Mr. Shawn H. Corey (talk) 00:00, 11 June 2008 (UTC)

You don't seem to understand that since we are free to use any coordinate system in GR--not just inertial ones as are normally used in SR--then it is perfectly possible to find a coordinate system in which any given object is at rest in those coordinates (i.e. its coordinate position does not change with coordinate time), even if the object is accelerating in other coordinate systems, or even if it is accelerating in the physical sense of experiencing G-forces (in the coordinate system where it's at rest, the G-forces would be explained in terms of a pseudo-gravitational field acting on the object--see The Twin Paradox: The "General Relativity" Explanation). You can certainly choose a coordinate system in which the Earth is at rest at all times, and according to the principle of diffeomorphism invariance, this coordinate system is no better or worse than any other in GR.

If we ignore curved spacetime and just imagine the Earth as a massless object accelerating in flat spacetime, then we don't need to choose a coordinate system where the Earth is at rest at all times; we just need a coordinate system where its position today is the same as its position in 1865, even if it moved to other positions in between. It would be easy to find an inertial coordinate system where this is true--again, just imagine an inertial projectile fired from the position of the Earth in 1865, with just the right direction and speed so that it crosses the Earth's path again in 2008. In the inertial rest frame of this projectile, the projectile's position doesn't change over time, agreed? So if the projectile's position is the same as the Earth's in 1865, and the projectile's position is again the same as the Earth's in 2008, that means in the projectile's inertial rest frame, the Earth's position in 1865 is the same as its position in 2008, even though it moved to other positions in between.

Either way, the end conclusion is the same: there can be no basis in either GR or SR for saying that the Earth's position today is objectively different from its position in 1865, since you can find a coordinate system in which the positions are the same, and the coordinate system is just as valid as any other according to relativity (whether it's an inertial coordinate system in SR or an arbitrary coordinate system in GR). Hypnosifl (talk) 00:42, 11 June 2008 (UTC)

I wonder how much of this borders on original research. I thought Yamara was asking whether we should dismiss the paragraph or keep it. It seems that it is a well-written paragraph backed with a source- a book written by a professor at the Dept. of Physics at the University of Chicago. Seems like a decently reputable source, enough at least to provide citation for this article. Doesn't matter if we agree or disagree with Professor Geroch. Thus my vote is...

• Strong Keep unless I missed something that would invalidate our use of this cited academic perspective. WDavis1911 (talk) 03:14, 11 June 2008 (UTC)

Is threre a "Reader may require background on X subject to understand" template somewhere?

I've read a large amount of scientific articles including this one, that deal heavily with many concepts such as relativity and physics. In many places the article goes to and fro these subjects which can make it a bit confusing for a reader who does not have much of an understanding of the subjects (Such as myself). I know some basics about relativity, but as I do not have a college education on the subject can only guess what certain sentences mean.

I think I speak for me, and many others, when I ask if there's a template or tag we can add at the beginning of the article saying something like "Sections of this article may not be clear for readers without background knowledge of general relativity. Wikipedia has an introduction article to the subject here" just to let possible readers know that if they don't understand much of it, they need to read up more on that underlying subject. There seems to be a template for everything else from not enough sources, to wrong tone for the article. Surely it would be helpful to create one to let readers know what background information they need to understand?

And don't tell me that only people who already knew that background information would read the article, come on, the specifics of time travel can be interesting to a variety of people whether they have a degree in physics or not.

Thanks, Cody-7 (talk) 19:44, 18 July 2008 (UTC)

Bot report : Found duplicate references !

In the last revision I edited, I found duplicate named references, i.e. references sharing the same name, but not having the same content. Please check them, as I am not able to fix them automatically :)

• "Gott" :
  • {{cite journal | first = J. Richard | last = Gott | title = Time Travel in Einstein's Universe | year = 2002}} p.33-130
  • {{cite journal | first = J. Richard | last = Gott | title = Time Travel in Einstein's Universe | year = 2002}} p.76-140
• "Thorne1" :
  • {{cite book | last = Thorne | first = Kip S. | authorlink = Kip Thorne | title = [[Black Holes and Time Warps]] | publisher = W. W. Norton | date= 1994 | pages = p. 499 | id = ISBN 0-393-31276-3}}
  • {{cite journal | first = Thorne| last = Kip S. | title = [[Black Holes and Time Warps]] | year = }} p. 499
  • {{cite journal | first = Thorne| last = Kip S. | title = [[Black Holes and Time Warps]] | year = }} p. 499

DumZiBoT (talk) 06:20, 8 August 2008 (UTC)

There is no such thing

I don't care what you here, I think there is no such time as "time traveling." It is just a figment of people's imaganation. First of all, if you understand how time works, something happens that CAN NOT BE RE DONE. Period. It is permanant. That being said, the future is being made the minute you are reading this. I don't think the future is going on RIGHT NOW. YOU are CREATING the future. —Preceding unsigned comment added by 76.120.217.110 (talk) 18:50, 19 August 2008 (UTC)

Whether it's a scientific possibility or not does not change the fact that the concept exists in people's imagination, and is widespread. Therefore the article is justified. --Bisqwit (talk) 04:30, 21 August 2008 (UTC)

Time travel - allah, tibes and phophecies

I have only sckimmed through all the current threads but it has occured to me that there is little (or perhaps even no) mention of time travel in relation to religous beliefs, and in particular to God. Since fictional and philosophical accounts of (potential) time travel are at least partly accounted for it would only seem fair to at least mention scriptural accounts of time, also a greater balance would be allowed for being that religous answers to questions of time can and indeed sometimes do differ from scientific responses.

Monothestic religions, and perhaps too others, beilieve that God is the creator of both time and Space, of the laws too of both time and Space, and consequently also the creator of time-space, even in its modern conception. For Muslims time travel is already possible, though not necessarily for humans - it is explicitly mentioned in the Holy Horan that since Alla is omnipresent and is able to be in all things in all times he is able to move faster than the speed of light, indeed faster than anything that it is possible for the human imagination to realise - it also talks in deapth in the Koran about the relativity of time as we percieve it, that what seems like a thousand years to us is only a day or less to Alla.

The greeks themselves had many gods (although given Zeus, their god of gods this is not tottaly true) and indeed they themselves had a god specifically for time itself, this god was able to appear both from the future and within it. There are, obviously, many other religous and spiritual interpretations of time travel and I would encourage anyone that feels able to summarise a general sketch of these belief systems to do so, if only for the purposes of balance.

There is a further point, though not too related, that I wished to raise in relation to the fiction literature that the article mentions may refer to earliest accounts of time travel - I would suggest, not wishing here to patronise the author, that the articles mentioned are typically white-western and show no real cultural variation, it would be laughable to think that a few European authors arround the 17th Century somehow stumbled upon such concepts as Time machines, the grandfather-paradox and other time-travel related notions - the merest glance through any well-stocked library would easily reveal discussions of time travels and references to them littered through the history books, from the Greeks to the Persians, from Prussia to the Ottoman and Turkish Empires, right through to the European and American writers. Perhaps, even arguably, time travel was known of, though is a less technically abstract way to many of the world's great ancient tribes - some aboriginal tribes, for example, smoked or consumed certain intoxicating herbs while melodically chanting, at least some of these tribes did believe that there visions were actually their spirits travelling outside of their body, so too outside of their time and space, where it has even been claimed that they were able to communicate with alien species that are yet to have evolved or that have already become exstinct. I have not here suggested that it is the case that these tibal people really time-travelled, though for all I know they may have, but it is claimed from various anthropoligists that they did believe in time-travel.

Because tribal societies did not rely on written language as we do now, or at least it would seem that some of us do, evidence of time-travel belief from pre-organised religous societies can be difficult to come across, nevertheless it should still be at least reffered to. For one last example of a tribal belief in time-travel (note, for the spirit rather than for the body) I will tell you about the Hopi tribal phrophecy, the Hopi being one of the few remaining Native Indian tribes lefty alive in the Americas: Just in the last few years it was discovered by some scientific explorers that within certain Nevada desert rocks had been hidden the tribal drawings of the Hopi tribes, pictures of white men arriving in what looked like ships, carrying crosses, pictures of cars and squared houses and even planes too, this would not have been surprising except that acheologists dated the pictures to be pre-colonial, they even dated some of the images to be older than what they reffered to. This might be seen as evidence that the Hopi Spirits really could travel forwards to view a time of the future that was to come, but a little like Scruge in Dicken's a Christmas Carol they were not physically able to interact with the future but merely to spiritually observe. I am intesrested to ask others weather they think Phrophecies and visions of these kinds should be mentioned in the article.

Sorry my comments are a little long and wordy, i am fairly new to this site!