Can Time Travel into the Past Ever Be Possible?

[lvlastermind]

I'm interested in knowing anything about time travel into the past. If anyone has any insight about this it would be helpful.

 

[oldunion]

I'm interested in knowing anything about time travel into the past. If anyone has any insight about this it would be helpful.

I did some research on it a while back because i got interested in John Titor's rhetoric. From what my memory has now, you would have 2 Kerr black holes which would interact with each other in such a way as to form a singularity.

Although, they are doing some research in Rhode island and at cern with super miniature black holes created from smashing gold together in particle accelerators. they said the resulting plasma displayed black hole type properties consuming more energy than it released or something to that effect.

But i don't see how a black hole could get you to another time, i do not see a relationship.

 

[PeteSF]

Kip Thorne's book Black Holes and Time Warps is an excellent layman's resource on this topic.

 

[JesseM]

I did some research on it a while back because i got interested in John Titor's rhetoric. From what my memory has now, you would have 2 Kerr black holes which would interact with each other in such a way as to form a singularity.

Kerr black holes already are singularities. Anyway, Titor's explanation was basically just technobabble that doesn't make any sense in terms of physics, despite the inclusion of a few buzzwords like "Kerr black hole" that actually are used in physics.

 

[DrChinese]

One of the most practical issues with respect to "sci-fi" time travel...

Where would you travel to?

Not a simple question at all. Earth rotates, as does the solar system and the Milky Way. So we do not move even close to a straight line. Since we have no idea how fast we are moving through space, even in a relative fashion, it is impossible to estimate where we where an hour ago. So if you went back in time and expected to be on Earth, you better make provision to instead end up in outer space somewhere.

 

[PeteSF]

Since we have no idea how fast we are moving through space, even in a relative fashion

Do you understand what "relative" means?

But I agree that this is an oft-unaddressed problem with time travel to arbitrary times. It could be argued that the time machine follows an inertial path through space-time, but I think that this would mean that Earth-based time machines would end up in the centre of the planet!

I think it is more practical (it's funny to talk about practicality and time travel in the same sentence!) to imagine a time machine which requires a device at both ends of the journey. However, this would disallow travel to times before the device was built, which would upset the plot devices of a few SF authors.

Wormhole time travel as described in theory by Kip Thorne involves traveling between two ends of a spacetime "wormhole". It doesn't allow travel to an arbitrary time or place - the other end of the wormhole must be moved to the location you wish to go, and the time difference is fixed unless altered by time dilation.

 

[JesseM]

One of the most practical issues with respect to "sci-fi" time travel...

Where would you travel to?

Not a simple question at all. Earth rotates, as does the solar system and the Milky Way. So we do not move even close to a straight line. Since we have no idea how fast we are moving through space, even in a relative fashion, it is impossible to estimate where we where an hour ago. So if you went back in time and expected to be on Earth, you better make provision to instead end up in outer space somewhere.

To even say that there is a point in space now that uniquely coincides with "the position of the Earth an hour ago", regardless of whether we can know where that point is or not, is to assume the existence of absolute space, in contradiction with relativity. And as PeteSF says, you seem to be misunderstanding what "a relative fashion" would mean, it is certainly possible to say how far the Earth has moved relative to some inertial observer.

 

[lvlastermind]

Does anyone know of any websites with information on the possibilities of time travel into the past?

 

[JesseM]

Does anyone know of any websites with information on the possibilities of time travel into the past?

Here's an article from Scientific American by physicist Paul Davies:

How to Build a Time Machine

Some others:

http://science.howstuffworks.com/time-travel4.htm

http://www.lifesci.sussex.ac.uk/home/John_Gribbin/timetrav.htm

http://www.npl.washington.edu/AV/av_index_sub.html#6

http://www.aleph.se/Trans/Tech/Space-Time/#Time

 

[DrChinese]

To even say that there is a point in space now that uniquely coincides with "the position of the Earth an hour ago", regardless of whether we can know where that point is or not, is to assume the existence of absolute space, in contradiction with relativity. And as PeteSF says, you seem to be misunderstanding what "a relative fashion" would mean, it is certainly possible to say how far the Earth has moved relative to some inertial observer.

Just the opposite, I don't mean to imply there is or is not an absolute reference frame. What I said (or meant) was there is no relative position for the Earth to use as an anchor. Presumably, the place I would want to go back to - were I going back in time - would be the "same" place I am now. I certainly dispute the idea there are observers out there who could assist in this process as they will be just as confused. None of them would be sitting in a preferred position to assist us. Because of the various orbits we are a part of, I doubt we could ever obtain enough information to determine our effective velocity. It is very dynamic.

 

[JesseM]

Just the opposite, I don't mean to imply there is or is not an absolute reference frame. What I said (or meant) was there is no relative position for the Earth to use as an anchor. Presumably, the place I would want to go back to - were I going back in time - would be the "same" place I am now.

The "same" place relative to some physical landmark like the earth, or the "same" place in some absolute sense?

I certainly dispute the idea there are observers out there who could assist in this process as they will be just as confused.

Assist in what process? Other observers couldn't find a point that was objectively the same place in a frame-invariant way, but they could certainly find the point that was the same place relative to their own frame--normally when people talk about measuring positions or speeds "in a relative fashion", they mean measuring them relative to a particular observer's rest frame. For example, if you are driving at 50 mph down the road in my rest frame, then in my frame the point you were an hour ago is 50 miles behind your current position, while in your frame the point you were an hour ago is exactly the same as your current position.

None of them would be sitting in a preferred position to assist us. Because of the various orbits we are a part of, I doubt we could ever obtain enough information to determine our effective velocity. It is very dynamic.

What does "effective velocity" mean? If you think there is some absolute, frame-invariant concept of our "effective velocity", then you are saying the theory of relativity is wrong. It's not just a concept of us not being able to obtain enough information to determine it, it's that relativity says that in principle their can be no notion of a velocity that does not depend on an arbitrary choice of reference frame (except for the velocity of light, which is the same in all reference frames).

 

[oldunion]

Kerr black holes already are singularities. Anyway, Titor's explanation was basically just technobabble that doesn't make any sense in terms of physics, despite the inclusion of a few buzzwords like "Kerr black hole" that actually are used in physics.

Forgive the lacadasical usage of singularity, but it was a "relationship" of some sort between two kerr black holes. I found the paper i wrote about it last semester.

Also looking into CERN and the institute at rhode island (forget the name) would yield some interesting studies that may take to your liking.

 

[JesseM]

Forgive the lacadasical usage of singularity, but it was a "relationship" of some sort between two kerr black holes. I found the paper i wrote about it last semester.

In your paper you write:

With this laments understanding of physics, let us now observe the possibility that two black holes could theoretically collide. From research being done at the University of British Columbia by Luis Lehner, a representation of what this instance would "look" like has been calculated by a computer. The two black holes would merge into one, and among other events, radiate gravity waves from the area [2]. It is believed by some that this phenomenon is the basis for time travel and that it is possible for it to naturally happen.

Who are these "some" who believe that the merging of black holes would be the basis for time travel? I have never heard any physicists suggesting such a thing, only John Titor's technobabble.

Also looking into CERN and the institute at rhode island (forget the name) would yield some interesting studies that may take to your liking.

What studies? Is the institute in Rhode Island Brown University (where I went to college, coincidentally)? Note that physicists had suggested the possibility of micro-black holes being created in supercollider collisions well before John Titor posted his story, he was no doubt cribbing the idea from something he had read.

 

[DrChinese]

The "same" place relative to some physical landmark like the earth, or the "same" place in some absolute sense? Assist in what process? Other observers couldn't find a point that was objectively the same place in a frame-invariant way, but they could certainly find the point that was the same place relative to their own frame--normally when people talk about measuring positions or speeds "in a relative fashion", they mean measuring them relative to a particular observer's rest frame. For example, if you are driving at 50 mph down the road in my rest frame, then in my frame the point you were an hour ago is 50 miles behind your current position, while in your frame the point you were an hour ago is exactly the same as your current position. What does "effective velocity" mean? If you think there is some absolute, frame-invariant concept of our "effective velocity", then you are saying the theory of relativity is wrong. It's not just a concept of us not being able to obtain enough information to determine it, it's that relativity says that in principle their can be no notion of a velocity that does not depend on an arbitrary choice of reference frame (except for the velocity of light, which is the same in all reference frames).

I am not arguing the validity of relativity, and I think you are reversing my meaning. If someone went back in time 10 years, without traversing space, they would end up far away from Earth. If they went back 1000 years, they would be even farther away.

If you COULD measure the begin position and the end position of Earth, you MIGHT think you had determined the Earth's "effective" velocity - in other words the direction and rate the Earth is moving. You might use that to extrapolate to points even further in the past. But that would be false, because that value will change with each begin point and end point. And no observer can tell you any information that will change this. Because there is no absolute reference frame, there is no way to determine anything other than movement relative to some observer who is of course also moving. (And no one is moving in anything close to a "straight" line anyway, due to various orbits.)

So as I said, if you traveled to the past, where would you go back to? Empty space probably. And even if you did pop out near Earth, what direction would you be heading in? And how fast? If you were to go back in time, and didn't match velocity with the Earth, it might be a bit of a problem for you.

So I am not saying there is an absolute space coordinate system. But presumably practical time travel would need to adjust for a few extra critical factors that I don't believe you could ever learn.

 

[JesseM]

I am not arguing the validity of relativity, and I think you are reversing my meaning. If someone went back in time 10 years, without traversing space, they would end up far away from Earth. If they went back 1000 years, they would be even farther away.

Your phrasing is still implicitly assuming absolute space--what does "without traversing space" mean? Do you agree that in the example where you are driving down the road at 50 mph in my frame, then in my frame the "same point in space" that you are now is 50 miles ahead of the position your car was an hour ago? Do you agree that in your own rest frame, the "same point in space" that you are now was still inside the car an hour ago? So if you're in the car and you send your body back in time an hour "without traversing space", will you end up still inside the car, or will you end up 50 miles in front of the car? If you think there is a single true answer to the question of where you would end up if you traveled in time "without traversing space", then you are assuming there is a preferred reference frame. After all, in some inertial frame, the position coordinates of the Earth 10 years ago are exactly the same as the position coordinates of the Earth today--why is that frame any less valid than any other?

So as I said, if you traveled to the past, where would you go back to? Empty space probably.

Why "probably"? The question is just undefined, since there is no "natural" place that the laws of physics say you should expect to end up. The whole idea of instantaneously disappearing from one point in spacetime and reappearing in another has no basis in any known theory of physics--instead, physicists who speculate about the real possibility of time travel are thinking of something more like a wormhole, where you travel continuously through a weirdly-curved region of spacetime and end up inside your past light cone.

 

[oldunion]

In your paper you write: Who are these "some" who believe that the merging of black holes would be the basis for time travel? I have never heard any physicists suggesting such a thing, only John Titor's technobabble. What studies? Is the institute in Rhode Island Brown University (where I went to college, coincidentally)? Note that physicists had suggested the possibility of micro-black holes being created in supercollider collisions well before John Titor posted his story, he was no doubt cribbing the idea from something he had read.

If it wasnt a referenced link, i don't remember where it came from. I mentioned a computer graphic representation of the merging of two, it was these people who speculated on the possibility.

 

[loz]

regarding time travel into the past

Just read your post asking about time travel into the past so I thought I'd give you some info you may find useful. Time travel itself is relatively easy with the use of a tardis but since these are in short supply there is a more difficult way to accomplish backward travel in time.
Imagine the universe exploding and spiralling about itself, each particle or conglomeration of matter traveling at slightly different velocities. After countless eons this difference means that certain areas are much "older" than others. In effect these "older" particles exist in a universe which was created relatively longer ago than the "younger" particles.
Now instead of particles let's say stars and planets .Imagine a star system with a considerably greater velocity than our own comes around in a massive orbit allowing you to hitch a ride as it passes if you zoom into space in a straight line directly upwards. As you are caught in its gravitational field you manage to land on a planet orbiting one of its outermost stars. You would then be in a much younger universe (relatively speaking) than the one you had just left due to the time dilation effect of the velocity. Hitch a ride on a fast enough particle and you theoretically go to a very young universe(relatively). Of course the converse is true and you could hitch a ride on a slower planet and end up in a relatively older universe.
I think maybe you want travel in spacetime but since the timewars between the daleks and the timelords that technology seems to have been lost forever. Since I don't know your level of expertise or age i will give a very easy way to work out time dilation which really emphasises it's simplicity.
Take 2 pieces of square card and for the purpose of this example mark one with a 5X5 grid. If you were to travel at 60% the speed of light that would be 3/5 of the cards length (3/5 = 60%/100%) so cut out a square 3X3 from the cards corner. Now cut up the remaining 16 squares (5X5 - 3X3) and form a new square with them. The resulting square would be 4X4 so at 60% the speed of light one second to you would actually last 5/4 (oldside/newside) seconds or 1.25 seconds to the control(given that the observer is traveling at negligible speed). Multiple cards and velocities can be used and the sides measured against each other for relative time. The actual equation is as follows

(squareroot(Csquared - Vsquared))/C

hope you find this of help and if so drop me next weeks winning lottery numbers.

 

[chroot]

loz,

You have absolutely no idea what you're talking about. Let me welcome you by pointing out that personal theories are not welcome here. Virtually everything you've said is either gibberish, or simply wrong. Warning issued.

- Warren

 

[oldunion]

loz,

You have absolutely no idea what you're talking about. Let me welcome you by pointing out that personal theories are not welcome here. Virtually everything you've said is either gibberish, or simply wrong. Warning issued.

- Warren

Not to put what loz said on par with Einstein's work, or to say i agree with it , but much of physics was once someone's personal theory.

"curved space...such nonsense...how could space possibly be curved...the man belongs in a nuthouse."

Quote of a naysayer from Einstein's Dream

 

[loz]

hello this is not a personal theory it is derived from michelson morley or einsteins time dilation equation


the explanation as to relative age is infact true as countless experiments show so that chronological age can differ from actual age . I did use artistic license with regard to the explanation and used the star system as an example to demonstrate the idea but overall the argument is sound. A photon emitted in the bigbang would if it continued till now would not have aged and so could be said to be zero hours from creation. Or did I explain it in such a way as to make you believe that i was suggesting you could actually travel back in time rather than visiting an object with a different chronological perspective?

 

[arvin_amiel]

this thing really caught me up.. well for now we will think that time travel just can't happen. well what kind of logic can lead us to this?? maybe it is still not time for us to discover what really is behind time travel.. i think it interesting too.. but for now i don't really think this possible.. but who knows maybe tomorrow someone might discover how..:) arvin_amiel@yahoo.com just mail me

 

[loz]

Quote:

I do not take offense with the theory of relativity; I take offense with your attempted explanation of the theory, which was completely full of misconceptions.

- Warren

Above is the PM I received in respect to my second message regarding time travel. I made no attempt to explain the theory of relativity nor did I make such a claim. My message was intended for the original poster of the thread who asked for any insight into the subject. It is for this reason that I posted my original message, not to postulate any theory or attempt to explain relativity(as such an explanation would be very long indeed for a forum). While my explanation may have not been to your liking it was employed as a teaching tool and was necessarily simplistic. Regarding any misconceptions I may have please educate me since you seem to have the definitive understanding of the subject. However i disagree entirely that the misconception is mine as everything in the original post is backed by empirical data from numerous experiments from many independent parties. As an administrator I was a little surprised at your tone and lack of insight and suggest that, rather than insulting people who attempt to help posters, you spend your time attempting to understand the equations not just knowing them.

 

[Starship]

Relativity does not allow time travel. In fact, relativity does not allow motion in spacetime.

 

[selfAdjoint]

Relativity does not allow time travel. In fact, relativity does not allow motion in spacetime.

Welll then call it a closed timelike curve worldline.

 

[Starship]

Welll then call it a closed timelike curve worldline.

Once we postulate the physical existence of a time axis, motion immediately becomes an impossibility.

http://arxiv.org/PS_cache/gr-qc/pdf/0503/0503097.pdf

 

[robphy]

I haven't read this myself... but here might be a starting point
http://plato.stanford.edu/entries/time-travel-phys/

 

[selfAdjoint]

Once we postulate the physical existence of a time axis, motion immediately becomes an impossibility.

http://arxiv.org/PS_cache/gr-qc/pdf/0503/0503097.pdf

That paper has been rebutted by a famous physicist who knows its author well: http://www.arxiv.org/abs/gr-qc/0504039

 

[Starship]

That paper has been rebutted by a famous physicist who knows its author well: http://www.arxiv.org/abs/gr-qc/0504039

By far the majority of physicists consider time travel as pseudoscience. There just isn't any experimental evidence that it happens.

Relativity does not allow motion is space-time. It allows motion in space. Time is just the abstract inverse of change. Change (or motion) is primary, time is something we deduce from it.

 

[JesseM]

By far the majority of physicists consider time travel as pseudoscience. There just isn't any experimental evidence that it happens.

False. Pretty much all physicists acknowledge that general relativity in its present form allows for time travel, although most think this possibility will be eliminated by a theory of quantum gravity.

Relativity does not allow motion is space-time. It allows motion in space. Time is just the abstract inverse of change. Change (or motion) is primary, time is something we deduce from it.

False again. In relativity it's best not to think about "motion" at all, but just to think of "worldlines" frozen in 4D spacetime. And general relativity does allow worldlines which curve back and end up in the past light cone of some previous point on the worldine, or even worldlines which look like loops, known as "closed timelike curves". You might check out the book Black Holes and Time Warps by Kip Thorne (a well-known name in the field of general relativity, who often makes friendly bets with Stephen Hawking about various unresolved questions) for more details on this.

 

[Starship]

False. Pretty much all physicists acknowledge that general relativity in its present form allows for time travel, although most think this possibility will be eliminated by a theory of quantum gravity.

Few physicists know relativity and AFAIK relativity does not allow for time travel. Motion in space-time is itself impossible (by definition). If you are serious about this, then i really doubt the correctness of the theory.

 

[JesseM]

Few physicists know relativity and AFAIK relativity does not allow for time travel.

"Few physicists know relativity"? Along with quantum physics, it's one of the cornerstones of 20th-century physics! General relativity is the framework used to understand the big bang and black holes and gravitational lensing, for example. And yes, it does allow for time travel, that's exactly what I've been saying. Like I said, you can read details in Kip Thorne's Black Holes and Time Warps, or on some of the webpages I posted earlier in this thread (post #9).

Motion in space-time is itself impossible (by definition).

I didn't say anything about motion in spacetime. Here's an analogy I used on this thread:

Think of a block of solid ice with various 1-dimensional strings embedded in it--if you cross-section this block, you will see a collection of 0-dimensional points (the strings in cross-section) arranged in various positions on a 2-dimensional surface, and if you take pictures of successive cross-sections and arrange them into a movie, you will see the points moving around continuously relative to one another (in terms of this metaphor, the idea that there is no single universal present means you have a choice of what angle to slice the ice when you make your series of cross-sections). You shouldn't think of time travel as the points returning to precisely the same configuration they had been in at an earlier frame of the movie; instead, you should just imagine one of the strings curving around into a loop within the 3-dimensional block, what in general relativity is known as a "closed timelike curve".

 

[El Hombre Invisible]

Einstein supposed, did he not, that four dimensions of space and time are laid out like a map. You don't travel through time... something has those four co-ordinates or they do not. On the other hand, I just got off a mind-numbing topic about particles and antiparticles, the notion that antiparticles are particles traveling in the opposite direction in time. Then there's the positive/negative energy, space-curvature, twin universe thingy which I know little about but I think supposed that space contraction (such as gravity) was space trying to get back to the big bang and space expansion (the universe as a whole) was space trying to get away from it. Lots of high concept possibilities. I assume you will never get an answer to this question as if you did then why didn't you tell yourself huh? Maybe it's too dangerous. You might travel continuously through a weirdly-curved region of spacetime and end up inside your a- ha ha...

 

[Starship]

Few physicists know relativity"? Along with quantum physics, it's one of the cornerstones of 20th-century physics! General relativity is the framework used to understand the big bang and black holes and gravitational lensing, for example. And yes, it does allow for time travel, that's exactly what I've been saying. Like I said, you can read details in Kip Thorne's Black Holes and Time Warps, or on some of the webpages I posted earlier in this thread (post #9).

Few physicists know the complex math of general relativity. What is meant with time-travel? Does this mean that we can run the movie backwards? Can we go back to the stone age?

Anyway, here is a very good argument against time travel.

http://mb-soft.com/public/time.html

Time travel probably violates causality and energy conservation:

http://math.ucr.edu/home/baez/physics/Relativity/GR/time_travel.html

 

[quantumdude]

Few physicists know the complex math of general relativity.

Where do you get these ideas? Anyway, you are going to have to start backing up your assertions better if you want them to be tolerated here.

Anyway, here is a very good argument against time travel.

http://mb-soft.com/public/time.html

What's so good about it? It's a purely verbal argument. It doesn't even touch "the complex math of general relativity".

 

[Starship]

Where do you get these ideas? Anyway, you are going to have to start backing up your assertions better if you want them to be tolerated here.

No you have to back up your ideas if you think time travel is really a possibility.

 

[JesseM]

Few physicists know the complex math of general relativity.

I doubt it's true that "few" physicists know it--all cosmologists would have to, as should any physicist studying theories of quantum gravity. It may be that most physicists are required to study the basics as part of a general physics education in graduate school, I'm not really sure.

I'm not sure whether it's required in What is meant with time-travel? Does this mean that we can run the movie backwards? Can we go back to the stone age?

What does "run the movie backward" mean? The idea of slicing the block of ice into cross-sections and using them as frames in a movie was just a sort of schematic way of thinking about the difference between our ordinary notion of moving time and spacetime in relativity, the block of ice with the strings embedded in it is really the fundamental thing, just as spacetime as a whole with various worldlines embedded in it is really the fundamental thing in general relativity. You can sometimes do a "foliation" on spacetime which means cross-sectioning 4D spacetime into a series of 3D "spacelike hypersurfaces", but this isn't possible for all possible spacetimes, and in spacetimes where you can there's no unique way to do it (so two events which are part of the same hypersurface in one foliation would be part of different hypersurfaces in another).

But for any worldline, you can at least talk about the "proper time" experienced along that worldline, ie the time that would be read on a clock that followed that worldline. So backwards time travel in this context would mean that the clock's worline loops around in such a way that when it reads 4:00, it passes next to the point on the worldline where it read 3:00.

Anyway, here is a very good argument against time travel.

http://mb-soft.com/public/time.html

That's a pretty ill-informed argument actually. For example, they say:

Speculate now, that time travel into the past was possible, and that people a thousand years in our future learned how to do it and control it. For the moment, let's take a big example, that they decided to transport a medium sized hill from their time to ours, a thousand years earlier.

No matter WHAT process might be involved, it is certainly clearly necessary to know exactly where each atom was at some specific instant in time. This would be an astoundingly difficult accomplishment, but we will assume that those people a thousand years in our future would have figured out some way to do it.

Since we would need to somehow re-construct the structure of the hill, we would certainly need to know the position of each atom REALLY accurately. If two atoms were supposed to be right next to each other (as in a compound, like hydrogen and oxygen forming water molecules), The location of the atoms would obviously have to be known to about 10-12 cm (or one one-trillionth of a centimeter). This figure comes from nuclear physics, where this is a common distance of interaction between individual atoms.

But time travel as envisioned by physicists does not involve an object disappearing from one time and being reconstructed in another, like the transporter in star trek--instead, it involves a physical object taking a continuous trip through a twisty region of spacetime which results in it ending up in the past, there are no discontinuities where it disappears or reappears. Since the entire rest of the section of that paper is based on this disappear/reappear premise, it doesn't make sense. The paper also assumes that history could be "changed" by the presence of a time traveler, but in the "static spacetime" view, whatever the time traveler does in the past should have been part of history all along, so it wouldn't have changed anything. See this thread for a discussion of how paradoxes can be avoided by postulating that only self-consistent histories would be allowed by the laws of physics.

 

[quantumdude]

No you have to back up your ideas if you think time travel is really a possibility.

I didn't say anything about it one way or the other. The statement I quoted from you is that few physicists understand GR. I know that you are wrong about that.

 

[Myriad209]

By far the majority of physicists consider time travel as pseudoscience. There just isn't any experimental evidence that it happens.

That isn't true, many concepts in physics work forward and backward in time and it DOES happen in a few cases.

 

[JesseM]

That isn't true, many concepts in physics work forward and backward in time and it DOES happen in a few cases.

To say that an antiproton is mathematically equivalent to a proton moving backwards in time is not really evidence of time travel. True time travel is only possible in general relativity, and there's no evidence that this actually happens, or that this will still be possible once a theory of quantum gravity is found.

 

[El Hombre Invisible]

To say that an antiproton is mathematically equivalent to a proton moving backwards in time is not really evidence of time travel. True time travel is only possible in general relativity, and there's no evidence that this actually happens, or that this will still be possible once a theory of quantum gravity is found.

Why do you assume that the mathematical equivilence of an antiparticle to a particle traveling backwards in time (or vice versa) is not a consequence of its physical truth? If an electron exists in a certain point in space in the future and another in the past, why would it be any less true to say that it is a positron traveling from the future (further away from the big bang) to the past (nearer to it) than to say that it is an electron traveling from the past to the future? It is in the future and in the past and at every point in time in between. That's its trajectory.

 

[loz]

Time travel probably violates causality and energy conservation:

[/url]

While I cannot cannot argue with your statement about time travel violating causality I disagree that it violates energy conservation. Since space (here, there) and time (past, present and future ) could be regarded as within the same system, then energy conservation is not an issue. There are numerous experiments showing energy borrowing although the greater the energy borrowed, it seems, the quicker it must be repayed.

 

[JesseM]

Why do you assume that the mathematical equivilence of an antiparticle to a particle traveling backwards in time (or vice versa) is not a consequence of its physical truth?

I don't assume it's not, I'd just say it's not necessary to view it that way, thus this cannot be taken as "evidence" of time travel. After all, in Newtonian physics a ball falling down in a vacuum is mathematically equivalent to a time-reversed version of the same ball moving upwards in a vacuum after being thrown, but I don't think many people would say that this proves Newtonian physics allows backwards time travel.

 

[PeteSF]

Time travel probably violates causality and energy conservation

What is causality, exactly, and has anyone proved it can't be violated?

 

[Starship]

What is causality, exactly, and has anyone proved it can't be violated?

Causality says that every effect must have a cause (e.g, energy is needed to do work). What we have (and what we are of course) is a proof that causality is never violated.

 

[JesseM]

Causality says that every effect must have a cause (e.g, energy is needed to do work).

"Energy is needed to do work" would not really be an example of what physicists mean by causality--causality as they define it deals not with abstract concepts but with specific events, causality basically means that an event can only affect other events that are in its future light cone. Also, I don't think causality would demand that every event be caused by other events--I'm pretty sure quantum randomness is not considered to violate causality, for example.

What we have (and what we are of course) is a proof that causality is never violated.

There is no proof that causality is never violated. A universe that allowed closed timelike curves could still be perfectly lawlike since it would still be constrained by the laws of general relativity, it's just that events could be affected by both events in their past and events in their future. Of course there is also no evidence that causality is violated, and most physicists would probably guess that the most basic laws of nature (quantum gravity) will end up preserving causality.