Talk:Time travel
Talk about the problems of time travel 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.
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.
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.
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. :)
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
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.
_____________________________________
Who is John Titor?
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
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)
Screwball theories
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)
Wormhole Time Machine
In you article on time travel you cover the proposal the 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 is 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
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?
Recent edit
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)