Talk:Time travel

Okay, I've just redesigned the entire talk page here Please try to keep the talk page readable in this (or a similar manner) (Way too many Horizontal Rules "----")

Faster than Light

and causality loops

This brings another objection. According to special relativity, traveling faster than light is equivalent to traveling backwards in time, according to some observers. In particular, if faster than light travel is possible without too many arbitrary restrictions, it is possible to have events in the future cause events in the past. This is called a causality loop.

In relativity there are two kinds of intervals, time-like and space-like, the former corresponding to sublight speeds and that latter to supralight speeds. The two are non-equivalent so faster than light objects won't have a rest frame. This is not the same thing as traveling backwards through time, but I'm not really sure how this paragraph should be edited, without trying to explain the whole theory here.

There are also light-like intervals, which correspond to (!) light speed.

For anything travelling faster than light, there exists a reference frame in which it goes back in time. This does not cause paradox unless it or a signal from it returns to the starting point before the start of travel, thereby creating a causality loop.

Difficulty of traveling FTL

Not sure about this.

First, exceeding the speed of light wouldn't result in a reversal of time.

Second, this isn't true if you have an object of a rest mass of zero.

This would lead one to posit that exceeding the speed of light would result in a reversal of time. While this is mathematically sound, as one approaches the speed of light, exponentially more force must be applied to accelerate, until finally an infinite amount of energy must be applied to reach the speed of light exactly. The only people who suggest that we must only develop technology to exceed the speed of light to develop time travel are people who do not understand the concept of mathematical limits.

But that's exactly what it is: a mathematical limit, in a mathematical model of reality. Mathematical models are constrained by what we know of the universe. Reality isn't. :)

Re Mathematical limit. It was shown (I believe proven mathematically) that we would never be able to exceed the speed of sound because of the pressure built up in front of the craft. As speed increases toward the speed of sound, this buildup increases. There was a large portion of the mathematical community convinced of this fact. We were able to overcome it and exceed that barrier (several times over now). While I admit that the concepts involving increasing math is substantially different, I do not underestimate the human mind. McKay 00:27, 27 Oct 2004 (UTC)

Objects of zero mass have to travel at the speed of light. There can be no exception.

Objects that go faster than the speed of light to travel backwards in time. And while the reasoning above is valid, it doesn't deal with the possibility of something that comes into existence travelling faster than the speed of light.

What is the difference between the Revelation of John and the Book of Revelation? - Zoe

Stuffed if I know, Zoe. I was just trying to remove the preaching implicit in the last version. -Robert Merkel 07:18 May 7, 2003 (UTC)

An important issue for time travel is the movement of information, and here it crosses over with prophecy. Harry Potter

Why Faster than light is Time Travel

Question

This is an interesting topic that I never really did understand much. The part that I am still a bit confused about is how it is believed that travelling faster than the speed of light will actually result in reversal of time. Is it because we mark time based on what we are able to see? I imagine not, since, if we were all blind, then sound would be our measure, and clearly going faster than the speed of sound doesn't cause time to reverse.

It seems clear that light cannot travel faster than the speed of light (although I've recently heard tell that the speed of light may not actually be as constant as once thought), and clearly there are other small particles that travel close to the speed of light that seem to slow down timewise. However, I can still see that it has clearly taken light a long time to reach from the outside of the galaxy back to earth, and we are seeing events that happened billions of years ago, but if something was able to go faster than the speed of light from the edge of the galaxy to reach here (say a billion times faster) it seems to me that it would still be years behind in reaching us, and not ahead of us.

Some of the arguments that I have heard (I apologize for not having any references here) seem to be similar to the arguments that we should not be able to move, since in order to reach a destination, we would have to get half way there first, and in order to get half way there, we would have to get half way to the half way point ad infinitum. We would have to traverse an infinite distance just to move. Clearly we are capable of moving over an infinite number of points in space, so could it be possible that if we found a way to travel faster than light, it would not necessarily cause a regression of time? I have other thoughts on this matter, but very little factual information to back them up, so I will quit while I'm behind.

(I'm still a bit new to wiki, so if this conversation seems very out of place, please let me know if there is a more appropriate place to move it to.)

Thank you --Chris

Answer

It all comes down to the speed fo light being a constant. Understanding why time dilation occurs because both moving and stationary obsesrvers both have to clock the speed of light at c is the first part understanding this. Second imagine a traveler exceeding the speed of light (from a rest frame). In order for both a rest frame observer and the traveling observer to observer the light traveling at the speed of light. The light would actually have to travel backwards to "catch up" to where it needs to be. McKay 06:24, 27 Oct 2004 (UTC)

As an added note, the relationship between time and the speed of light in Special Relativity is expressed as a formula which shows that the observed time of an object (t) slows down as it approaches the speed of light (c) - time dilation. When the velocity is equal to c, t equals zero. If you plug in a velocity faster than c, t goes into negative figures. -khaosworks 08:11, 27 Oct 2004 (UTC)

Khaos, What equation are you referring to? You aren't referring to the Lorentz equation are you? Can you provide the equation here? McKay 20:23, 29 Oct 2004 (UTC)

It's Special Relativity's equation for time dilation, <math>t' = {t\over1-\sqrt{(v^2/c^2)}}<math>, where <math>t'<math> is the fraction of <math>t<math> that is dilated, <math>v<math> is velocity and <math>c<math> is the speed of light.

So, if an object is at rest, <math>v = 0<math> and time is moving at a normal pace, <math>t = 1<math>, then time dilation is <math>0<math>. Keeping the flow of time constant at <math>1<math>, as <math>v<math> increases, <math>t'<math> decreases accordingly, until finally, when <math>v<math> is equal to <math>c<math>, then <math>t' = {1\over0} = 0<math>. So if <math>v<math> exceeds <math>c<math>, then <math>t'<math> becomes negative, indicating that the object is actually observed going backwards in time. -khaosworks 00:31, 30 Oct 2004 (UTC)

While I see what you are doing, I wonder where you got that equation. That looks similar to the Lorentz Factor, but is actually quite different. the lorents factor shows time dilation as <math>t' = {t\over\sqrt{1-(v^2/c^2)}}<math> Unless I'm mistaken. In this case, plugging numbers for <math>v<math> higher than c return an undefined solution (in real space). Am I missing something somewhere.McKay 05:34, 30 Oct 2004 (UTC)

Well <math>t' = {1\over0}<math> does not equal <math>0<math> anyway. That's undefined. PaulHammond 20:07, 6 Jan 2005 (UTC)

Hmm. A bit of research shows two versions of the time dilation equation. I'm not sure which one is correct now. -khaosworks 06:31, 30 Oct 2004 (UTC)

Well, I can prove mine mathematically, and wouldn't mind doing so, (want me to do it here?) so where did you find the other equation, somewhere in wikipedia? I'd like to follow their math.

I'm not a physicist, and to be honest my math is pretty basic - I'm just a relatively well-read amateur. You might want to Google for it and have a look yourself. -khaosworks 21:32, 30 Oct 2004 (UTC)

I decided to provide a proof at Lorentz factor feel free to take a look, I'd be happy to provide any questions if you want.McKay 07:35, 31 Oct 2004 (UTC)

Thanks. Looking at it, it seems to me that either way, <math>t'<math> appears to go into negative figures for velocities exceeding <math>c<math> -khaosworks 08:54, 31 Oct 2004 (UTC)

Not quite. In the formula that I provided, velocities cannot exceed <math>c<math>, because this provides a value less than zero under a radical with an even root, so the time dilation according to the Lorentz factor is undefined, not negative.McKay 18:58, 31 Oct 2004 (UTC)

Wormhole Time Machine

not just winding back clock?

In your article on time travel you cover the proposal to use of a wormhole for the creation of a time machine. The basic idea is to take one end of a wormhole through a relativistic journey - using either Special Relativity i.e. using pure speed to slow down time or General Relativity i.e. using a strong gravitation field to slow down time.

This article ( and other like it in popular scientific magazines ) generally dismiss this approach on purely practical grounds - such as the amount of energy required to create such a wormhole; but I would like to question the whole theoretical basis of the argument. To make things simpler I would like to start by considering a far more practical experiment as a first step.

Instead of one end of a wormhole I would like to use a camera with a built-in clock ( the sort of digital camera that can put a timestamp on the picture ). Take this camera on the sort of relativistic journey as proposed above. To make the discussion comprehensible let's assume on 1st January 2000 we set of on a ten year ( Earth time ) journey in such a way that the camera only experiences nine years elapsed time. When the camera arrives back on Earth all the local calendars will be showing 1st Jan 2010 but the internal clock for the camera will show 1st January 2009. Now the big question. If I take a photograph of the calendar on my wall do I get a picture of the 2009 calendar or the 2010 calendar? As I see it the picture must be of the 2010 calendar - even though the camera will give it a 2009 timestamp.

Now simply replace the camera with the end of the wormhole and attach a clock to the exit of the wormhole. When the end of the wormhole returns to Earth the very fabric of the space in the wormhole may have only aged by nine years and the clock will read 2009 but anyone looking out will see the 2010 calendar on the wall and if you step out of the hole you will be stepping into 2010 and not 2009. Time will then continue to progress for the exit of the wormhole and the outside world at the same rate so if you step into the entrance in 2020 you will see the wormhole exit clock reading 2019 but you will step out into 2020 - back when you started. So as a means of time travel it is no more effective that winding your wristwatch back.

RLS, 2004-08-04

Hah, I figured it out. This one had me stumped for a few weeks. I tried explaining it in my head, but couldn't quite get the words out right. As I was explaining a couple of TT concepts to a roomate, I figured out how to describe it.

Draw two paralell lines on a sheet of paper (or just visualize yourself doing it). Place a straightedge perpendicular to both parallel lines. This straightedge represents the wormhole. Currently, the two different lines represent the time lines for the two locations in space where the ends of the wormhole are (seperated by however much 3-D distance you wish). Under normal circumstances, the ruler will move forward, torque free, with a vector of motion paralell to the two lines. As one end of the wormhole is accelerated to a fraction of the speed of light and experiences time dilation, the other end of the wormhole does not. The "stationary" end of the wormhole continues to move forward along its timeline at the same rate as before, while the "moving" end of the wormhole doesn't move forward as fast as it used to. If under the same speed and time conditions you presented above, imagine you are the one moving the end of the wormhole in your spaceship. Leaving in 2000, making what appears to you to be a 9 year journey, the rest of the world experienced ten years. Your calendar will read 2009, and the worlds will read 2010. Being someone who doesn't live in the past, you move your calendar forward a year to match theirs, but you are now biologically one year younger than your twin brother. But where is the other end of the wormhole? If you were still moving it forward at the same rate, and you quickly enter the end of the wormhole that was accelerated to a fraction of the speed of light, the other end is now in 2011.111111... (just after noon on Feb 9 if I did my math right), so your calendar is now 1.111 years behind normal earth time, and your brother is 2.11111 years older than you are.

Does this all make sense? Should this be included in the main article to alleviate questions like yours (and mine ;) )?

Thankyou for your response. The part that troubles me is the claim that the entrance end ( the one that remained on Earth all the time ) will be in 2011. The journey for the exit end ( the one that did the travelling ) was specified as lasting ten years as measured by Earth time. As the entrance end remains stationary relative to the Earth it will have aged ten years and the entrance clock will read 2010 and any camera attached to the entrance will photograph the 2010 calendar and give it a 2010 timestamp, essentially the entrance will be in 2010. When the exit end returns to Earth in 2010 ( Earth time ) the wormhole exit camera will be able to photograph both the laboratory calendar and the wormhole entrance clock and both will show 2010 but be given a timestamp of 2009. Let us take the case of a subject who travelled with the moving end. He will have aged only nine years and his own watch will read 2009 but he will be back on earth in 2010. If he immediately goes through the wormhole in a reverse direction ( i.e. enters the exit end and emerges from the entrance end ) he will come out in 2010. The fact that his watch still read 2009 does not mean he travelled back a year, simply that his watch was wrong as a result of his original relativistic journey. To make this clearer assume that when he first arrived back on Earth, after his trip with the wormhole exit, he meets his brother and takes him on the reverse wormhole trip. The brother would have been on Earth for the ten year duration of the original trip; his watch would say 2010; he would see the calendar and wormhole entrance clocks both saying 2010; he would see the wormhole exit clock saying 2009. After his reverse trip through the wormhole his watch would still say 2010 and he would still see the laboratory calendar saying 2010.

RLS, 2004-11-20

I think you're getting a little confused ebcause you are switching reference frames when you shouldn't. I'm not sure though. i'm going to explain what each of the perspectives, Wormhole, and stationary and moving twin. I will do all of this relative to the earth (stationary) time.

Moving Twin. He's the guy who gets accelerated to the fraction of the speed of light, with the exit of the wormhole. For simplicity, assume the acceleration is very fast, and the time spent doing such is negligible. On January 1st 2000, he begins his trip beginning with an acceleration to the fraction of the speed of light (0.19<math>c<math> I believe), and remaining at that speed for some time. While on the ship, he practices StarCraft for the matchup with his brother when he returns. When his calendar says (is about to say) January 1st 2009, he begins the rapid deceleration and lands just about where he left. His clock says Jan 1st 2009, but the world clock says January 1st 2010.
Stationary Twin. He's the guy who just sits around playing StarCraft the entire time. He's watching the clock when his twin leaves at 1 January 2000, and watches him come back on 1 January 1 2010. He then proceeds to kick his brother's trash because he's got an extra year of practice on him.
Wormhole. Now I think its important to realize that a wormhole isn't really space, nor does it "age" per se. The wormhole exists out of space-time. For our purposes, it was created|discovered some time ago, and was prepared for this jorney. Initially, it maps between two points at the same time. Uhh, one end at Blizzard's headquarters (very close to the twin's house), and the other has been placed in a special device that allows it to move when the spacecraft moves, but has access ports that allow anyone to go through it still. This is where the straightedge example comes in. before 2000, both ends of the wormhole were moving through the timelines of each of their respective associated points in space at the same rate. When one end is acclerated, that end experiences time slower. In this example, after 1 Jan 2000, and before 1 Jan 2010 (remember we're using Earth Time, because the wormhole itself doesn't have frame of reference within a time dimension) 9/10 as fast. The one end of the wormhole that was accelerated to the fraction of the speed of light, is now 10 years in the future compared to when it began it's journey. The other end, must be 10/9 times as far into the future, because it was stationary, so that would be 100/9 years, or 11.1111111... years. The wormhole doesn't have an age. Had an observer (a long lost triplet perhaps) entered the stationary end of the wormhole on 1 Jan 2010, he would be put on the ship, (9/10 * 10 years) 9 earth years into the journey, which would put him in the spaceships time of (9/10 * 9 years), 8.1 years past the departure, or 2008 (feb ish)

One minor point. I do not agree that the wormhole exist outside spacetime - it is part of spacetime. In general I still thing you are confusing local aging with passage through time as measured by a reference frame. To try to make my ideas clearer I have created a web page with a detailed presentation of my argument. See :- [my webpage (http://homepage.ntlworld.com/rlsinclair/physics/wormtime.htm)] I hope this will either convince you I am right or allow you to pinpoint exactly where I went wrong.

RLS, 2004-11-27

I read your paper, and I think I understand what you are saying now, but I still am not sure I entirely agree. Lemme make a couple of assumptions, before I make my statement, you let me know where I go wrong.

Wormholes are essentially a mapping between two points in normal space-time (actually it's an einstein-rosen bridge, but it wouldn't be hard to mathematically state the 4 dimensional coordinates of both locations.
Wormholes can exist that travel through time. (Whether such procedure stated by Kip could do such a thing, is yet to be determined)
Moving ends of a wormhole is concievable
Moving ends of a wormhole is difficult, because this is not just the motion of matter, but the alteration of space/time.

Having said all of that, my conclusion is that while devices that move wormholes are very difficult, and might exhibit the exact behaviour that you describe, it could also be concievable that the device moves the wormhole through time as well as through space. I don't think that Kip's method would prevent it, as such a device might bind the exit to matter, which could be modified in such a way.

Minor comments on your paper:

- typos all over the place.
- you mention only moving one end of the wormhole, but between figures 1 and 2, you show both ends moving.

McKay 04:30, 28 Nov 2004 (UTC)

I am not sure I agree with the first assumption. Normal space is continuous through the wormhole ( I could conceive of a situation where the universe looks like an erythrocyte with the two faces touching, but this is not how the Thorne wormhole is configured ). Personally I doubt that wormholes can travel through time but I am sure that my argument shows that the Thorne method won’t produce them. Sorry about all the typos, I have removed a lot of them but every time I look more appear. As for the figure; only one end is shown, the other is at some indeterminate point of the diagram - I have changed the caption to make this ( slightly ) less confusing. Early next week I intend to float the idea on the sci.physics.relativity newsgroup; so you can have the fun of seeing me being savaged by the mob.

RLS, 2004-12-04

Vehicles and positioning

In science fiction and elsewhere, I've encountered 3 different types of time machine. Please improve the suggested names of each of these types.

Vehicular type This time machine travels with the traveller(s) to the destination time and place (e.g. the TARDIS of Doctor Who).
External type This time machine does not travel with the traveller(s). The travellers arrive at the destination without the time machine (e.g the Time Tunnel or the time machine of Quantum Jump).
History Type This time machine does not travel in time with the traveller(s), but the traveller(s) always emerge from the time machine at some time in its history (e.g. wormhole time machine). The travellers can not go further back in time than the earliest time in the time machine's history or go anywhere where the time machine was not located.

li An interesting case is the H. G. Wells Time Machine. It looks like a History type machine, because it does not move, during the time travel. However, it is however moved in the far future and upon return to 1900, it arrives outside the house, rather than inside the house where it was in 1900. This makes it a vehicular type.

Someonewhat unrelated, there is an Isaac Asimov story (I think) about a murder committed with a type of antigravity machine that decouples the movement of an object from all the forces acting on it - the assumption in the story is that a billiard ball on the table (for that is what the story revolves around) will slowly lift off the table as forces around it stop acting on it, but, of course, what happens is that it shoots off at high speed and kills someone. It is stationary relative the the movement of the sun, earth etc.

--- Karl Palmen

Taking in account Earth, Sun, and Milky Way's movement don't make much sense for me. In physics, position in space (and time) it's only a relative measure. Taking something off the "coordinate system" makes as sense as taking it off the Universe. The "Earth movement" problem is the same as teleporting: the travelling device needs physical means to transmit matter, energy and information (first of all, information) in this universe (as a Clifford D. Simak book does with teleportation, with a web of machines through the galaxy which send information about the traveller's particles, so they can be destroyed at departure and reconstructed at arrival, making a bit of creepy kind of voyage). If you give means to do it (as did Wells, who nailed it), it necessary will provide a clear arriving point in space and time. There is nothing as dissapearing from here and pop there, whithout nothing underlying doing the work to link one event to another. If it's something magic, unespecified, or through another universe, it could be anything. That would be more of fantasy than science (or science-fiction).

One way to have a "paradoxical" universe is to have two time dimensions. For example, model the entire present universe as the internal state of a computer. In that case, when someone goes into the "past", that just tells the computer to rewind and alter a backup copy of the universe's state. In this case, there's the time of the computer and the time inside of the computer. And of course, this is exactly how Star Trek and other such shows model time travel.

Screwball theories

TMaxine?

I moved the following from the article as it did not fit into the article where it was posted. I could not find any google hits for it either, so either it is nonsense or it needs rewording (context? Maybe a SF story I don't know) to be understandable by a normal reader.

The TMaxine Project is designing a console and Multidimensional Training in order to develop experience in the practicalities.

andy

_____________________________________

Who is John Titor?

The purveyor of the Tmxxine stuff, deleted last month, has reposted it; I've reverted that edit. This month's newcomer from the realm of ideas that, to put it charitably, don't have general acceptance, is the "Chronovisor" stuff. My impression is that this one actually has a few adherents, with a book published about it, so I'm reluctantly leaving it in, the way we leave in other pseudoscientific drivel. The Chronovisor article to which it links does say "still unconfirmed," "alleged existence," etc. JamesMLane 08:52, 2 Jul 2004 (UTC)

Recent edits

Religious traditions

From opening paragraph of article:

"...the religious traditions of Jesus Christ, descended from the Kingdom of God, which exists out-of-time-and-space..."

Can anyone give some more specific reference for this? I am not aware of any traditions of time travel episodes involving Jesus, certainly not comparable to the explicit Mohammad story. There are some (not particularly systematic) modern metaphysical constructions about god's kingdom being outside time, but I don't think an off-hand reference like this is justified. As far as Jesus is concerned the nearest would probably be mystical statements in John's Gospel such as "before Abraham was born, 'I am'" (Jn. 8.58) but that doesn't seem to amount to the same thing. FrankP 10:21, 17 Nov 2004 (UTC)

I think that they really meant to link to Kingdom of Heaven, which doesn't explain it either. I'm not a believer in this particular trait of heaven's "location", but some believe that heaven exists outside of time and space, I think the necian creed, or the trenton creed or something may mention something like this. I don't belive it, so I wouldn't know where to look. I don't like the implication, so I'm not going to go to all the trouble to find the correct info and fix it either, if someone does, fine by me though. You are correct that nothing states this directly in the scriptures AFAIK. McKay 13:34, 17 Nov 2004 (UTC)

Let's leave it out then, unless somebody else can justify it. I don't believe religious stories either (I'm an atheist with an unhealthy tendency towards Biblical scholarship), but it seems to me there's enough fiction in the Bible already without inventing more that isn't there. There's some airborne transport (Elijah, Ezekiel) and some flying by Jesus (Acts 1:9), also predicted for his followers on his return (1 Thess 4:17), but no time travel. I'd have to check the wording for the Nicene and Tridentine creeds, but from memory there is no very specific church teaching on spatial or temporal whereabouts of heaven. FrankP 15:25, 17 Nov 2004 (UTC)

Anon Technobabble

An anon recently added this paragraph:

Consider this,We all know the fundamental cells are made of atoms.SO inorder to travel time 
too we need time molecule.But time molecule is not actually a molecule,but an assumption of 
a great energy.Consider Einstein's theory of relativity M/[t*(1-(v^2/c^2))^1/2],where 
M=mass,v=velocity,c=velocity of light,t=time. here if the body travels equal to the speed of 
light then its mass will be at infinity.But if it travels faster than speed of light then 
its  mass decreases.Therefore an utmost potential form of energy is created in itself.It 
also  posses enormous kinetic energy.This energy if harnessed could provide either necessary 
time or  dimension travel. NOTE:-This is just one of the visions for time travel.There may be
a  slight truth in this theory. So if this is true when you travel through time you travel 
through each and every second of a day.So if you travel through a YEAR PAST or FUTURE you are
crossing each and every day at unimaginable(may be reality in future)speeds.SO time travel is
not safe even if you think of wormholes.

Obviously, it has to be cleaned up, but I have no idea if the information is correct or not. Can someone who understands this decide whether we should renovate it/keep it/etc.? --pie4all88

It sounds like psuedo-scientific technobabble to me. What the passage is describing is the tachyon, but the inference that the tachyon possesses enormous kinetic energy is patent nonsense. -khaosworks 05:48, 3 Oct 2004 (UTC)

Ok, I think I'll remove the paragraph. If someone wants to put it back in, they can just go to the article's history and clean it all up. Paragraphs with this many errors reflect badly on Wikipedia anyways. --pie4all88 18:42, 3 Oct 2004 (UTC)

Going faster than light does not take you backward in time. Plug in such a velocity in the time dilation expression and see that it results in an imaginary temporal displacement. Going backward in time requires a timelike catastrophe into the nonprincipal conjugate, at any velocity.lysdexia 02:57, 7 Oct 2004 (UTC)

Cernobov

It appears there was a revert war about this Cernobov paragraph. The paragraph sounds like crackpotism to me and I've deleted it, not doing a simple revert because some normal changes were made in the interim. JamesMLane 20:04, 4 Apr 2004 (UTC)

Help on Edits

Possibility of paradoxes

The article states under "The possibility of paradoxes"

If his results can be generalized they would suggest, curiously, that none of the supposed
paradoxes formulated in time travel stories can actually be formulated at a precise physical
level: that is, that any situation you can set up in a time travel story turns out to
permit of many consistent solutions.

The last part of the sentence, "turns out to permit of many consistent solutions" has me baffled--the "of" shouldn't appear here. I don't know, however, if it's supposed to permit one of many or just many. Can someone who knows help me out here?

Examples of Time Travel Vehicles

When I rewrote the part involving the name of those two popular time travel vehicles, I wrote it in such a way that didn't require usage of the name of the time travel device. That's because in Wells's novel, the ship isn't given a name, so saying "Wells's novel of the same name" doesn't work.McKay 06:24, 27 Oct 2004 (UTC)

Heinlein/Paradox reference isn't

"Probably the most elaborate demonstrations of supposed time travel paradoxes are in Robert A. Heinlein's All You Zombies... and By His Bootstraps"

Actually, these are examples of a lack of paradox, as is the movie Twelve Monkeys.

In 'All You Zombies' Heinlein describes a character who is a natural and functional hermaphrodite, raised as an orphan, impregnated by a mmysterious man, who disappears. The female reproductive organs are destroyed in the one birth. Later the child is stolen from the new mother-father. The person grows older and meets up with an older man who introduces them to the ability to travel in time, whereupon they travel back to confront the mysterious man and, instead, are the man, then travel forward again to retrieve the child and drop it off back in time as an orphan, then travel forward in time to instroduce their younger self to time travel.

Likewise, 'Byt his Bootstraps' describes a similar closed loop. Mind boggling as the story may be, NO paradox is encountered.

Compare this to the movie 'Twelve Monkeys' wherein Bruce Willis's character is sent back time to prevent a plague that wipes out most of humanity and not only does he completely fail to stop it, he inadvertently causes it (indirectly) AND sees his younger self from the opposite end of a suppressed memory in his own childhood of seeing a man who would later be him.

These three stories are not demonstrations of paradox but, quite the contrary, demonstrations of The Novikov self-consistency principle.

See predestination paradox. -khaosworks 20:47, 5 Dec 2004 (UTC)

Parallel universes need clarification

If we make an assumption that each of these parallel universes is a dimension

Universe is not a dimension, at least not in the physical sense (only in the sense of "another dimension"). Thus I don't think that this paragraph actually makes sense even gramatically, much less physically. Please, someone more familiar with the history of this article, clear this up. Paranoid 22:51, 18 Dec 2004 (UTC)

Deleted reference to Superman

Superman (1978),

Because I don't think it's a "Key example" of a film involving time travel concepts PaulHammond 19:57, 6 Jan 2005 (UTC)

In reference to the film, it is mentioned that film implied that Supermen prevented bombs from exploding, which is incorrect - bomb exploded in space, and that was important part of the plot for Superman II, IIRC. saigon_from_europe

The bomb that exploded in space from Superman II and released the Phantom Zone criminals was in the elevator of the Eiffel Tower. Different nuclear device entirely. --khaosworks 21:51, 14 Mar 2005 (UTC)

Religious edit

"Prophecy as decision

The most simple explanation is that prophecy is merely the announcement of future events that will happen because god chooses to make them happen and the fulfillment is merely the event taking place, which was announced earlier. This is the most likely explanation and meets Einstein's demand that explanations should be as simple as possible, but not simpler."

Attributed to 213.23.248.77 on April 8, 2005.

Comments:

The first sentence this seems to be a re-statement of the material right after it.

The confused reference to Occam's Razor - which was not in any way "Einstein's demand" - does not make sense. Applying Occam's Razor would in my opinion result in the explanation that observers choose to remember prophecies that appear to have been fulfilled over prophecies that were not fulfilled. This sentence also fails to be neutral POV, especially with the blanket statement: "This is the most likely explanation".

WCFrancis 20:02, 8 Apr 2005 (UTC)

(Reverted while I was writing the above. Not the first time this has happened and I doubt that it will be the last.)

User:213.23.248.77 is at it again. This nonregistered user at same address as above changed "It is interesting to note that any religion which postulates the existence of fulfilled prophecy requires, at the very least, an agent which can move information from the future into the past." by changing to "It is not interesting...., because that is a logical fallacy which ignores that god may just be deciding on a specific event and announce it ahead of time, which leads to the more interesting question if he should be doing this at all and what the ethical foundation for this is."

Already reverted. This begins to appear as an agenda well beyond any reasonable neutral point of view.

Please note: it is my understanding (limited though it may be due to my "newby" status) that an article is a totally inappropriate place for philosophical discussion and/or argument. That's what Talk is for.

WCFrancis 03:09, 9 Apr 2005 (UTC)

Actually, it does raise an interesting point; if a sufficiently powerful entity issues the statement "In X number of years event Y will happen," and then X years later it makes event Y happen, or if a sufficiently clever entity figures out that event Y is almost certain to happen in X years and predicts it, that would have the appearance of prophecy without requiring any time travel to happen. We therefore can't say that prophecy requires time travel. The anon is wording it poorly, however. Bryan 18:13, 9 Apr 2005 (UTC)

Can this be expressed w/o treating time as medium?

Intro now says

It has been confirmed that travelling at speeds approaching the speed of light can cause time dilation, the effects of which cause the individual travelling to pass through time more slowly. From the perspective of the traveller, external time would be going much faster, causing the traveller, upon stopping, to arrive at a place farther in the future than their subjective elapsed time would indicate.

pass through time more slowly suggests a preferred frame of reference
instead of external time & subjective time why not just talk about frames of reference?
subjective time suggests it is just a matter of personal perception rather than a measurement that even tools would make

Can this idea be expressed w/o talking about time as though it were a medium (like water or the aether)? --JimWae 20:27, 2005 Jun 11 (UTC)

Can length (space) be measured apart from the length of something? Can time be measured apart from measuring the time for an event(s)? Can either space or time be measured, or are they not conceptual constructs (perhaps unavoidable, perhaps because of how our brains work) with which we measure events & objects?--JimWae 20:27, 2005 Jun 11 (UTC)

The difficulty Jim sees seems to me to be the result of not paying attention to the maths. The contrast between length and time is misguided - it should be duration vs. length; time vs distance. Creeping idealism, perhaps - is Jim having trouble understanding time travel from an idealist viewpoint? Banno 21:35, Jun 11, 2005 (UTC)

You persist in making this personal, don't you Banno? You want to solve the problem by Banno's fiat of what "it should be"? Perhaps instead you could explain on what basis you claim it is improper to treat length & time as 2 of 4 dimensions. Your dodge does not affect the relevance of my point - my point remains releavent whether you talk about measuring length or measuring distance. We do not measure distance itself either, we measure the distance between 2 "points". If you think YOU do understand time travel, Banno, I suggest you are fooling yourself. --JimWae 21:56, 2005 Jun 11 (UTC)

The wording of the time dilation remarks in the introductory paragraph was indeed unclear (though not as bad as the rest of the paragraph used to be). I've reworded it into something more appropriate. My main concern is with going into too much detail in the introduction (that's what the "physics" section of the page is for). --Christopher Thomas 22:53, 11 Jun 2005 (UTC)

Info Box

As an anonymous user said; the info box at the top of the article makes it look like a magazine. The user removed it and it was almost immediately replaced by someone else. Well IT DOES make this article look like a magazine article! It's completely out of place in an encyclopaedia. It looks stupid; it's banal. I vote it be removed. Arcturus 10:58, 12 Jun 2005 (UTC)

As I said when I reverted the anon's edit, I think it is fine. It is no completely out of place - other articles have similar boxes; I don't see why why "looking like a magazine" is a bad thing; It is eye-catching. So I say keep it. But this is a trivial issue. Banno 21:34, Jun 12, 2005 (UTC)

Yes, I didn't realise it, but this template is used in numerous articles concerning unsolved problems in physics. However, I still maintain that its use trivialises these important topics. It might be eye catching, but it serves no useful purpose here. I hope other people contribute to this discussion. Arcturus 21:45, 12 Jun 2005 (UTC)

I think that, far from trivializing it, it directs the lay reader's mind as to why the subject is worthy of contemplation. It would be a boring article indeed if it were only a dry, academic treatise — Wikipedia should be useful, and articles should inform not just the interesed, but also interest the random reader. As Plutarch said, the mind is not a vessel to be filled, but a fire to be lighted. --khaosworks 21:53, Jun 12, 2005 (UTC)

Place and time

if time travel was possible, would you end up in the same place you started. say you were in london england in 2005, and wanted to time travel to the year 3000 a.d., if you started your journey in england would u end up in england? or could you end up in another country? (--Nicksinif, 21 June 2005)

The short answer is that you could end up just about anywhere you wanted to. What constitutes "the same place" depends on your reference frame, and picking that is pretty arbitrary. A reasonable constraint to keep movement in space and time of the same order would be to say you couldn't teleport more than 10 light-years if you were travelling forward or backwards 10 years in time (as the two represent comparable distances in spacetime from the point of view of someone stationary in your original reference frame), but that's an arbitrary restriction. Another way of looking at it is to realize that by performing time travel, you're already moving in space from the point of view of someone in a different reference frame (this is covered in more detail at faster than light and special relativity; you actually look like you're doing a FTL teleport from some reference frames).--Christopher Thomas 07:42, 21 Jun 2005 (UTC)

in back to the future 3, marty was living in a 1985 la suburb, but somehow ended up 6 hrs north of la in jamestown ca when he went back to 1885. is that possible? he starts out in la and ends up in jamestown ca? (--Nicksinif, 21 June 2005)

Helpful note - you can sign your name (and a datestamp) by putting four ~ signs at the end of your comment. This makes it less confusing for others to read talk threads. --Christopher Thomas 07:45, 21 Jun 2005 (UTC)

The dates changed, so maybe the Earth orbits the sun in a very slightly different plane than space-time does? ;)Still the sun is not where it was 100 years ago either. Or maybe it's a work of fiction ;) --JimWae 07:46, 2005 Jun 21 (UTC)
- Spacetime doesn't actually orbit anything; it's what makes things orbit :). More seriously, part of the principles behind relativity are that spacetime doesn't impose a preferred reference frame, so whether you tag along with the earth or follow a different path is mostly arbitrary (it just takes work to change which inertial frame you're in). A time machine is also very handy as a distance-teleporting machine. --Christopher Thomas 07:51, 21 Jun 2005 (UTC)

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