Understanding the Paradox of Backward Time Travel: Why We Can't Go Back

[chosenone]

Tachyons are the particle i think your talking about.If tachyons traveled backwards in time then anywhere in the time line moving forward tachyons would tavel back to the beginning of the universe,so all tachyons from all time periods would stop moving backwards when they reach there,but that would also mean that at the big bang all tachyons in the entire existence of the universe would be there before the universe ever go here,thats kind of hard to believe!

 

[chosenone]

Since you like trying to debate time travel,and if is possible,try this one on for size.if you killed someone accidentally by something you did,what you could do is send a telepathic signal back in time telling your self what happen by giving your past self a preminition,of the up coming event that will cause the person death.why there seems to be no paradox in this one is because changing time has the problem about when it first happens then you go back to change it,what happens if it was suppose to be like that in the time line,what happened when you come up to the event the first time before it happened,when you changed it.this one has the ability to get around that,when you come up the the event the first time you had a precognitive experience where you saw your self accidentally killing someone,and you would just think its a warning to stop it before it happened and would believe that was meant to be not changing time,so when you send the signal back to do it,in the time line how would you know what happened first,if the first time you had a preminition,and that's that!

 

[Royce]

In the other thread I said that our language hense our logic is not built to handle time travel in either direction. We can go on forever trying to decide if we came back in time then our coming back in time is already in our past, present and future thus proving a deterministic universe without freewill. Going ahead in time implys the same thing for when the future does arrive we would already be there having in our past traveled to our future thus the future determins the past and the past determins our future. There is also the possibility that there is only one time, NOW and that the sequentail flow of time is an illusion caused by the limitation of our minds.
John Gribbins said that and electron traveling back in time was indistingquishable from a positron traveling forward in time, again Feynman diagrams. He also mentioned tachyons as going backwards in time and that time in Einstein's equations showed yup with a negative sign. Does that mean that we are actually traveling in negative time?
I was jokinly referring to those concepts. I just can't take any discussion of time travel seriously but it makes great SF which I've loved most of my life.

 

[Royce]

No, they travel backwards in time on a Feynman Diagram according to John Gribbin. A positron is indistinguishable from an electron moving backward in time. That is vertually a quote from the last book of his that I read on QM. In the same book, the name of which I can't remember right now, he mentioned the possibility of tachyons and said that they would have to move backward it time since any object approaching the speed of light has its time slowed down and a photon moving at the speed of light has 0 time or is outside of time anything such as a tachyon traveling faster that C must move backward in time or have negative time.

As you may have noticed I am brand new to this forum having just found it yesterday and I am enjoying it thoroughly. Thank you all.

 

[Zefram]

People have pointed out the idea that there are anti-particles that move backward in time. I don't think these people realize the consequences of such travel - viewing it as just like traveling backward through a spatial dimension. This is not so because traveling backwards through time = arriving before leaving.

Precisely. Just like the retarded light/advanced light deal.

IOW, I would have to get to point B (my supposed destination), without ever having left point A (my supposed starting point) - without, in fact, every having been on point A (my supposed starting point). This makes no logical sense - unless someone would care to prove otherwise - and thus the idea of particles that travel backwards in time cannot be true.

Which part are you having trouble with? A to B forward in time is equivalent to B to A backward in time (at least on a very simple quantum level; I've never considered a macroscopic scenario). Neither scenario involves never having been at point A. They simply swap departure point for destination and flip the time direction around.

 

[chosenone]

well tachyons are hypothesised to be hitting the planets surface as cosmic radiation,there traveling faster than light because,light is being slowed down as it is passing though the thick atmosphere,and the tachyons are'nt.but if they are traveling backwards intime,they would be hitting the surfaces before they entered the atmosphere.

 

[Zefram]

From a few relativistic arguments I've heard, tachyons could not be charged. So antimatter (if it was traveling back in time) would not be tachyonic (assuming that's a word).

 

[DrChinese]

Originally posted by Mentat
1. Perhaps you could give the reason why you think this is possible?

2. Also, if Feynman was such a strong supporter of Relativity, why did he suggest something that contradicted one of GR's principles?

1. Feynman Diagrams work in either time direction. There is nothing - at the quantum level - which inhibits photons from going from the future to the past. Indeed, I believe this is a part of the calculations for Feynman's path integrals - i.e. considering the effects of virtual anti-photons. Anyone?

2. Relativistic QM respects Special Relativity, not General Relativity.

As to GR itself: Probably everyone has speculated as to whether gravity flows from GR, or whether there is a quantum description which has GR as its approximation. And if that were the case, all kinds of strange things might be the case. So I'm not sure what you are getting at about Feynman.

 

[quantumcarl]

Originally posted by Mentat
was there a genuine point that you were trying to make and that I failed horribly at noticing?

Yes.

If you don't understand what I said... you may want to go back and re-read all those books in the elementary school library you've bragged about reading in grade 4.

My point is simple and well known throughout the western world.

Let me repeat. Just for you.

Today is tommorrow's yesterday and yesterday's tommorrow.

In this axiom we can see that the past, present and the future exist now.

Therefore we are in a unique, quantum position to travel through, experience and change all three conditions, simultaniously... past, present and future, now.

 

[ElectrikRipple]

so u think that all of the universes in which are created from decisions made are still traveling on the same time line and tehrefore there can only be one time line or whatever and that the same amount of time passes, no matter what universe/dimension/whatever u are in? this makes since if that's what ur saying if not then let me know before i reply to the theory itself
P.J. <<<<tired [zz)]

 

[ElectrikRipple]

Originally posted by Alias
Is there evidence that the past continues to exist somewhere, such that we might go there? I don't think so.

i like this idea
P.J. <<<<tired [zz)]

 

[ElectrikRipple]

Originally posted by quantumcarl
Yes.

If you don't understand what I said... you may want to go back and re-read all those books in the elementary school library you've bragged about reading in grade 4.

My point is simple and well known throughout the western world.

Let me repeat. Just for you.

Today is tommorrow's yesterday and yesterday's tommorrow.

In this axiom we can see that the past, present and the future exist now.

Therefore we are in a unique, quantum position to travel through, experience and change all three conditions, simultaniously... past, present and future, now.

carl...i don't care if u take offense to this bro but dude i don't know what's wrong with u and ur thinking process or u know what wow...

P.J. <<<<tired [zz)]

 

[kristofer]

Hi, guys.

I'm Kristofer's brother. He has an interest in learning about physics and science. When he told me this, I suggested this forum as a place to begin. He's lots of ideas about solving world problems, particularly in the area of alternative fuels and the like but he's only eleven and needs to learn the rudiments. Please be patient with him, as you have time. Many of you can answer his questions much better than I can.

I've encouraged Kris to post here. He is only just learning but he'd like to learn a lot, and this is about the best place for learning what he really wants.

Thanks, everyone. Have a great Sunday!

 

[mouseman]

Ar matey, welcome aboard! We hopes ye'll find our humble ship to yer likin's. Batten down, bucco's! We sail fer the Sea of Knowledge!

Man, I'm such a dufus.

 

[ElectrikRipple]

mouseman lol wut was that?
P.J.

 

[Royce]

Originally posted by Mentat
Hold on a second, Royce. You are saying that anti-matter paticles travel faster than the speed of light through space?

Mentat,
I posted a reply earlier then thought later about it. After reading other posts I guess that moving backward in time does mean moving faster that the speed of light also. I hadn't realized this implication before.
Then I read another post about tachyons moving backward and coming together at the Big Bang. Sure that would be a logical conclusion but where did they come from, The Big Crunch or Rip? Had to be, logically.
Then I had another thought. If space is curved and a closed curve at that wouldn't that mean also that time too is curved and a closed curve also implying that the Big Band is the same as the Big Crunch?
The Beginning is the same event as the End in an endless loop of time.
Of course everything would be wiped clean at the Crunch/Bang, no information could be passed on to the next cycle so it wouldn't really be a loop, would it?
I then remembered that one of the problems with the BIG BANG is that anti-matter should have been created at the same time and rated as "normal" matter. They would collide and annihilate each other nearly as fast as they were created but obviously this didn't happen because we are here in a "normal" matter universe. Could it be that matter travels slower than light in one direction in time and that anti-matter travels faster than light in the opposite direction in time thus they never meet except in the end/beginning. Are they separated by the light barrier and time direction? This would also imply the sum total of enery/matter = 0. It could be that all of this is nothing but a logal fluctuation of the vacuum.

What do you think? Is it possible or am I really as crazy as I and others think?

 

[Mentat]

Everyone, this is a very important point that nearly everyone has missed: For something to get from point A to point B (or point B to point A, it doesn't matter) implies eventuality, eventuality, in turn, necessitates that a certain amount of time expire. For a certain amount of time to expire (or pass) you have to get from the present to the future (or at least what used to be considered the future), not from the present to the past..

I think that this is the big hurdle that people just aren't getting past. People speak of anti-particles that travel backward in time. This is utter foolishness, because to travel from point A to point B implies eventuality. When I said that you cannot arrive before leaving, this (the statement in red) is what I was trying to express.

 

[Mentat]

The problems with "pretzel" time.

I will now show why I don't think that the "Pretzel time" works:

1) One dimension
Time is a dimension. It is not a set of dimensions. Because of this, it cannot curl backward. To curl necesitates an extra dimenion (a curled one).

2) Not the past, but the future:
Even if time were pretzeled, one would not be going to the past, but the future. You see, the point in time which corresponds to 5:30 pm, July 4th 1776 (for example) does not exist anymore (as Alias has been stressing) - it didn't exist at 5:31 of that date, and will in fact never exist again. So, when I travel "back" to this point, I am actually traveling to a new point - one in which I was there - and since it is new, it must have come into existence after the time I started traveling; and if it came into existence after I started traveling, then I am in the future, not the past.

 

[Zefram]

For a certain amount of time to expire (or pass) you have to get from the present to the future (or at least what used to be considered the future), not from the present to the past..

This is the point being challenged (at least by myself); the idea I'm arguing is that time has no intristic "flow" and that the distinction between "past" and "future" is a matter of perspective as in spatial coordinates (and that there need not be a preferred direction along the time axis that everything must move in--things can travel along the time axis in either direction). If you happen to be traveling in the direction that human perception seems to prefer and see an electron traveling the other way along the time axis, it will appear to you to be a positron.

If, continuing on with that example, you move between two spatially and temporally (is that the word I want?) separated points, A and B, you must, of course, be at one before (from your point of view) the other. So starting from A and moving to B (along what we'll call the + time axis (in addition, obviously, to the spatial axis or two or three you'll need to traverse)), you move from present to future (your words). An electron starting from B and traveling to A along the - time axis would appear to you as a positron traveling along the + time axis from A to B; in other words, it would appear (to you) to start and end at the same points that you do and also to be taking the same route as you along the time axis between the two points (yet some of its familiar properties would be reversed).

This backwards traveling electron (positron), would (pretending it sees and interprets time like human beings like you and I do) say that it was at B before it reached A. Since you and the positron are traveling in opposite directions (along this time axis), you might both record the time the other is taking as negative--from your own point of view then the electron took a negative time to travel from B to A (however, since that sounds funny, you'd just say that it traveled from A to B as you did in a positive time).

You can see (if you followed that) that I'm treating this time axis very similarly to a spatial axis; looking at a simple spacetime diagram with a worldline on it you get the impression that time moves like the needle of a seismograph (or rather the paper sliding by underneath that). I don't see it like that but rather as a two-way street with some quirk of the human mind providing either the "preferred" direction of travel or the illusion of some kind of movement (which one, I don't know). That does imply some kind of determinism and I haven't thought long enough on it to see the other deep consequences of it, but that view serves me well when thinking about time travel.

Do you see now where I'm coming from?

 

[Alias]

Originally posted by Zefram
This is the point being challenged (at least by myself); the idea I'm arguing is that time has no intristic "flow" and that the distinction between "past" and "future" is a matter of perspective...

STOP! You are exactly correct up to that point. This idea that we all seem to have that says that 'time flows' in a particular direction, is an artifact of our perception. I believe that time does not flow, but that the universe renews. The universe does not 'endure' in time. Time is what you get with the repitition of renewing universes. Again the analogy is the frames of a movie film. You get no movie unless you look at the frames consecutively. No motion is apparent, no action occurs, unless you view the film dynamically. The same may be true with our universe. This hypothesis, if true, would also indicate quantized space and thus, non-continuous motion. These 'would be facts' conveniently explain the limiting speed of light and the fact that no mass can travel faster than this. I think that from this point of view, the Lorentz transformations can tell us much about the qualities of space.

 

[ElectrikRipple]

Originally posted by Mentat
I will now show why I don't think that the "Pretzel time" works:

1) One dimension
Time is a dimension. It is not a set of dimensions. Because of this, it cannot curl backward. To curl necesitates an extra dimenion (a curled one).

There's your problem. Why do u have to have two dimensions before u time can be "pretzeled"? A slinky isn't made up of multiple strips. Am i missing something?
P.J.

 

[ElectrikRipple]

Originally posted by Zefram
This is the point being challenged (at least by myself); the idea I'm arguing is that time has no intristic "flow"

People often view time as a river flowing towards the future. Which i guess make since. Time travels into the future. The present becomes future and the future becomes present. But can u not swim against the current of the river of time?
P.J.

 

[Royce]

Originally posted by Zefram
This is the point being challenged (at least by myself); the idea I'm arguing is that time has no intristic "flow" and that the distinction between "past" and "future" is a matter of perspective as in spatial coordinates (and that there need not be a preferred direction along the time axis that everything must move in--things can travel along the time axis in either direction).

Zefram,
I agree. Everything that I have read about relativity and QM including Hawking and Feynman says that time is just another dimension with no preferred direction. From what I ave read all of Newton's, Lorentz's, Maxwell's and Einstein's formula work equally well no matter whether time is + or -. In fact Einstein's formulas ended up with time as - which I mentioned before. Everyone seems to ignore that fact as insignificant, but is it. Nor is time constant nor linear and may well be quantumized (is that a word?) as well as space. Some seem to think that space may be at the Plank level as someone else mentioned in this forum. To me, in my humble opinion EVERTHING IS RELATIVE and QUANTUM.
Consider the photon. It moves only at the speed of light. It cannot stop or slow down or speed up. If it does then it is no longer a photon. It is it's own anti-particle. And at the speed of light it is outside of time. In its perspective it travels instantaneously from one point to another. Which means that it is everywhere along its path at the same time even though that path may be thousands of light years long.

 

[Mentat]

Originally posted by Zefram
This is the point being challenged (at least by myself); the idea I'm arguing is that time has no intristic "flow" and that the distinction between "past" and "future" is a matter of perspective as in spatial coordinates (and that there need not be a preferred direction along the time axis that everything must move in--things can travel along the time axis in either direction). If you happen to be traveling in the direction that human perception seems to prefer and see an electron traveling the other way along the time axis, it will appear to you to be a positron.

If, continuing on with that example, you move between two spatially and temporally (is that the word I want?) separated points, A and B, you must, of course, be at one before (from your point of view) the other. So starting from A and moving to B (along what we'll call the + time axis (in addition, obviously, to the spatial axis or two or three you'll need to traverse)), you move from present to future (your words). An electron starting from B and traveling to A along the - time axis would appear to you as a positron traveling along the + time axis from A to B; in other words, it would appear (to you) to start and end at the same points that you do and also to be taking the same route as you along the time axis between the two points (yet some of its familiar properties would be reversed).

This backwards traveling electron (positron), would (pretending it sees and interprets time like human beings like you and I do) say that it was at B before it reached A. Since you and the positron are traveling in opposite directions (along this time axis), you might both record the time the other is taking as negative--from your own point of view then the electron took a negative time to travel from B to A (however, since that sounds funny, you'd just say that it traveled from A to B as you did in a positive time).

You can see (if you followed that) that I'm treating this time axis very similarly to a spatial axis; looking at a simple spacetime diagram with a worldline on it you get the impression that time moves like the needle of a seismograph (or rather the paper sliding by underneath that). I don't see it like that but rather as a two-way street with some quirk of the human mind providing either the "preferred" direction of travel or the illusion of some kind of movement (which one, I don't know). That does imply some kind of determinism and I haven't thought long enough on it to see the other deep consequences of it, but that view serves me well when thinking about time travel.

Do you see now where I'm coming from?

Your idea'd be fine, if it weren't for General Relativity. Relativity states that we are always moving at the speed of light. However, our motion is apportioned between spatial and temporal motion. Thus, if you accelerate to the speed of light in space, you stop moving in time. However, to move "backward" in time, you have to go faster than c, and that is impossible.

Also, if something were traveling in the other [temporal] direction, I would never see it (as you say I would), because it would appear for an instant, and thus instantly be gone (well, I guess that argument will get philosophical, if I pursue it, so I'll just leave that alone).

 

[Mentat]

Originally posted by ElectrikRipple
There's your problem. Why do u have to have two dimensions before u time can be "pretzeled"? A slinky isn't made up of multiple strips. Am i missing something?
P.J.

You ask why you need to dimensions, in order to pretzel time? Let's think spacially, for the moment. If all of space was one-dimensional (as straight line, with no depth or width), then it would not be able to curve back on itself, would it? If you say it would, I'd like you to consider the fact that curling requires movement on the y axis. I posit that it should be the same with time (as time is also a dimension).

 

[Hurkyl]

However, our motion is apportioned between spatial and temporal motion. Thus, if you accelerate to the speed of light in space, you stop moving in time. However, to move "backward" in time, you have to go faster than c, and that is impossible.

The metric is symmetric with respect to reflections in all axes; traveling forward and backward in time yields the same speeds, just like traveling east vs west.

The problem is that to smoothly go from forward in time to backward in time that your trajectory has to go from timelike to spacelike (spacelike = FTL in the classical sense)... but non-smooth motion can transition from one to the other, and the same is true if you've always been going backwards in time and never forwards.


The same, incidentally, is true of energy. Antiparticles were theoretically discovered because there was nothing in the metric to force the energy of a particle to be positive. To have negative energy, p = m v forces either mass to be negative or the particle to travel backwards in time (which simply means you parametrize its trajectory in the direction opposite coordinate time flow).

Let's think spacially

You're presupposing a space in which to think spatially. Euclidean thinking doesn't always help understand non-Euclidean geometries.


Hurkyl

 

[ElectrikRipple]

wish i knew what that stuff meant
P.J.

 

[Raavin]

something silly.

I have a fantastic (as in fantasy) theory that matter is actually a wave phenomenon and that space is an illusion created by the interaction of the 'waves'. There is more detail but I won't go into it here. Suffice it to say that the idea implies that past events are retained in the present as residual vibrations. That's how we can remember things as there is still a residual of the original wave. Time in this model is a change in the waves and only moves in a direction that we see as being from present to future. It doesn't matter how the waves move, change is change and can only be perceived as moving in this direction. Just like the collapsing universe idea that time would seem to be moving in the same direction. The residual wave bit implies though that there is, in the present, a 'memory' of the past that resides in the present. The retarded and advance waves idea of Feynman in regards to explaining the phenomenon of radiation resistance (I wish I understood it fully) implies that particles can travel backward in time but, it would seem that this is restricted to very small spaces and timespans (am I wrong). Perhaps there is a 'Plank timespan' and matter bounces back and forth between the two extremes or perhaps time, as I said before, is just change.

Not very helpful to the discussion I know but...

Raavin

 

[quantumcarl]

Originally posted by ElectrikRipple
carl...i don't care if u take offense to this bro but dude i don't know what's wrong with u and ur thinking process or u know what wow...

Look who's talking.

 

[Loren Booda]

If time progresses according to the outward expansion of our universe, then regression in time corresponds to inward observations of compactified dimensions within the Planck length, the stasis of spacetime.

 

[Alias]

Originally posted by Loren Booda
If time progresses according to the outward expansion of our universe, then regression in time corresponds to inward observations of compactified dimensions within the Planck length, the stasis of spacetime.

Could you elaborate a bit more Loren? I lost you half way through.

 

[Mentat]

Originally posted by Hurkyl
The metric is symmetric with respect to reflections in all axes; traveling forward and backward in time yields the same speeds, just like traveling east vs west.

The problem is that to smoothly go from forward in time to backward in time that your trajectory has to go from timelike to spacelike (spacelike = FTL in the classical sense)... but non-smooth motion can transition from one to the other, and the same is true if you've always been going backwards in time and never forwards.


The same, incidentally, is true of energy. Antiparticles were theoretically discovered because there was nothing in the metric to force the energy of a particle to be positive. To have negative energy, p = m v forces either mass to be negative or the particle to travel backwards in time (which simply means you parametrize its trajectory in the direction opposite coordinate time flow).




You're presupposing a space in which to think spatially. Euclidean thinking doesn't always help understand non-Euclidean geometries.


Hurkyl

Now this is a point I hadn't considered. I don't know why, but I haven't heard it put quite like this before. You make a very good point, Hurkyl. There is one thing that bugs me - and maybe you could clear it up for me -, and that is when people talk about the particle and the anti-particle "meeting" at a certain point in time. It seems like this would have to, literally, be instantaneous.

 

[Mentat]

Originally posted by Loren Booda
If time progresses according to the outward expansion of our universe, then regression in time corresponds to inward observations of compactified dimensions within the Planck length, the stasis of spacetime.

Please expound on this, Loren Booda, I'm afraid I didn't quite understand what you were trying to say.

 

[ElectrikRipple]

Originally posted by quantumcarl
Look who's talking.

?

 

[Zefram]

Now this is a point I hadn't considered. I don't know why, but I haven't heard it put quite like this before.

I've been trying to get that point across for over a week.

Thank you, Hurkyl.