Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Bill S on 27/03/2018 13:31:32
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In his book “From Eternity to Here” Sean Carroll introduces a “gate into yesterday”. He describes it as an ordinary looking gate in a field, but if you walk through it (say) from right to left, you step into the previous day. He does concede that it could not be used to create paradoxes, but it would take you to the previous day, and back if you so chose. You could even meet your older/younger self.
Fellow posters will not be surprised that I have reservations about this. Of course, Sean has a PhD, whereas I, as far as physics is concerned, scarcely have ABC. However, I like to try to understand what the experts write about when they are addressing lay readers. I tried to engage Sean on his blog on this subject, but got no response.
Let’s see how it works. It’s 12.00 on 20th June. You have never been in that field before. You step through the gate, and you are at 12.00 on 19th June. The problem with this is that 12.00 on 19th June, in that field, is one of those immutable spacetime events. It cannot have happened without you, then with you; yet by stepping through the gate, you have “changed” that event.
Somehow, this does not seem to fit with Carroll’s assertions that: “Paradoxes do not happen.” or that “You can no more change events in your past in a spacetime with closed timelike curves than you can change events that have already happened in an ordinary, no-closed-timelike-curves spacetime.”
This is just the first of my concerns, and I would welcome comments.
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The Novikov self-consistency principle would assert either that (1) There is no record of you ever having been in that field in the past and as such any attempts to time travel will be thwarted in one way or another or (2) There is a record of you having been in that field in the past and your travel into the past is inevitable.
Of course, we don't know for sure that Novikov's self-consistency principle actually applies in reality.
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There is another way to avoid paradoxes using the multiverse theory: One who travels through the gate doesn't go to an earlier time in the same universe, but rather into another universe that is identical to the first in all ways except that it is 1 day delayed (or a year, or whatever).
Using these splitting timelines circumvents all time loop-related causality paradoxes. However, then there is the problem of how an event in one universe can cause one in the other, or is conservation of matter/energy within each universe violated when you disappear from one and appear in the other (what about information?).
Ultimately, time travel into the past remains pure hypothetical at this time (snort), and only through experiment will we be able to advance/cull/refine theories in any meaningful way.
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http://discovermagazine.com/2010/mar/02-the-real-rules-for-time-travelers
I just found this, which I hope to read sometime today - or yesterday :). It might add something.
I must also look at the Novikov self-consistency principle. Thanks Kryptid.
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There is another way to avoid paradoxes using the multiverse theory: One who travels through the gate doesn't go to an earlier time in the same universe,
I wondered how soon the multiverse would rear its head (heads?). Somewhere in the back of my mind there lurks a doubt that the multiverse really does resolve the problems. I'll have to come back to that.
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My (somewhat amateur) take on backwards time travel is that it is impossible unless there are some major errors in our current conceptions of time, thermodynamics, information theory, causality, chaos etc. (there probably are some errors, but there is also no guarantee that revised theories will allow for time travel either). It is therefore my inexpert opinion that any backward time travel will end up in "cuckoo land"
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It is therefore my inexpert opinion that any backward time travel will end up in "cuckoo land"
100% agreement there, Chiral. Trouble is, there are prominent scientists who think differently, and I tend to want to know why. A prime example must be J Richard Gott. Then there are all those people investigating retro-causality. Why?
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If it is possible to travel back in time then either a) no one will ever find out how to do it or b) it has been discovered somewhere else in the universe that is so remote from us that we will never know about it. This implies that no matter how hard we try we are never going to manage it.
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Jeffrey, that, too, makes sense to me, but it does introduce the often cited idea that if past directed time travel were achievable it would be possible to go back only to the point at which the first time machine operated. Does this imply that if there were a time machine anywhere in the Universe, that would influence the distance into the past that another being, in a remote part of the Universe, could travel into the past, if she/he/it built a time machine?
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Returning to the question of whether or not a multiverse scenario would really solve the problems of past directed TT; consider the following:
Sue, a young scientist, built a time machine which could take her to the past. She owned a gold sovereign and devised a plan to multiply it. She travelled back to the previous day, taking her sovereign with her. There she met her younger self and handed over the sovereign. The younger Sue then continued with her life and, the next day, travelled back, with her two sovereigns, which she gave to her younger self. I leave the rest to your imagination.
In a single universe it becomes difficult to explain where Sue’s additional sovereigns came from, but, at first glance, a multiverse scenario would seem to solve this. Each time Sue travels back in time, she moves to a different universe, thus, the total number of sovereigns in the multiverse remains the same, although the number in any one universe varies as Sue moves them about.
Closer inspection, though, reveals the “old enemy” of past-directed time travel: the immutable spacetime event.
Consider what happens. Sue, in universe A has one sovereign, and Sue in universe B has one sovereign. Sue, A travels back in time to meet a slightly younger Sue B, and gives her a sovereign. We now have a spacetime event in universe B at which Sue has, first one, then two sovereigns. Multiverse supporters find ways round this, by "rationalising" the hand-over time, but the fact remains that Sue B lives through the “period of travel” twice; once with one sovereign, then with two.
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Multiverse supporters find ways round this, by "rationalising" the hand-over time
Trying to put myself in the position of a multiverse supporter, I argue that if we designate the point to which Sue returns in universe B as point X, then at, for example, X minus one second Sue has only one sovereign, while at point X, plus one second, she has two sovereigns, and, in that universe, has always had two. However, for that to be a valid argument, not only would Sue A have to change universes in the act of time travelling, but Sue B would have to change universes when Sue A arrived at point X.
It becomes complicated, especially as claiming that Sue B had always had two sovereigns in that universe would suggest that she should have three when she receives one from Sue A. Of course, this makes no sense because, Sue B has two sovereigns only as a result of the point X hand-over.
I think I'm rambling :)
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The science-fiction author Isaac Asimov (also a biochemist) wrote a story about a molecule that could time-travel.
At the time, he was working on his Phd dissertation, and had his science-fiction stories published under a pen-name.
However, this story got published under his real name, and the final question from his Phd review panel was about thiotimoline.
See: https://en.wikipedia.org/wiki/Thiotimoline#Background
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... tables, and citations of fake articles from fake journals...
Reminds me of my A Level Sociology papers back in the 60s. Surprisingly, I got the best pass at the exam centre. I'm not sure what that says about sociologists. :)
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Sean’s gate allows a time traveller to move from today to yesterday by passing through the gate. He may then walk round the gate to the side of the field he just left, without activating the gate. It seems he can repeat this until passing through the gate takes him to the day before the gate was set up. This raises the questions:
If the gate is not there, how did the traveller exit it?
Can he get back to the day the gate was erected, or beyond?
If, because the gate was not there, he had been unable to exit it, what might have stopped him?
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Anyone can assemble words into a paradox. So far, nobody has assembled molecules into one. Something to do with entropy.
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The above post is very pertinent. Since, even though the time traveller would be travelling to the past, they are still in their own future pointing timeline. So entropy would have to reverse along their world line. A neat trick.
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So entropy would have to reverse along their world line.
Only when viewed from the RF of the time traveller.
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So entropy would have to reverse along their world line.
Only when viewed from the RF of the time traveller.
And that's a potential problem. From the perspective of the time traveller, the rest of the universe becomes more ordered. I don't know how much the entropy of the universe changes in a day, but I would wager it would take at least the power output of a small galaxy to reverse it (and possibly infinite energy, for an infinite universe).
Or maybe this is where thermodynamics breaks down, because we're just basing it on what has been observed so far, and actually the universe is even stranger than we thought possible.
I dunno, but reverse time travel violates almost every smell test I have for it. That it isn't expressly forbidden by QM or relativity doesn't mean it's possible. There are plenty of questions that are so poorly constructed that there is no way to give an answer other than, "bad question." Like, if I ask, "Is blue hunger taller than a hole?" It is grammatically correct, and all of the terms are well-defined on their own, but the implied relationship is just so unrelated to reality that no theoretical basis could even be formed to address the question. This may well be the case with questions of, "what if my time is going reverse to your time?"
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"Is blue hunger taller than a hole?"
Difficult to resist a challenge like that!
Hunger = effect of lack of food.
A blue moon is the second full moon in a calendar month; so blue hunger could be the second experience of hunger in a month.
Taller = higher above a specified level; most commonly the ground.
Hole: if this is taken to be a hole in the ground, it extends below ground.
Therefore, anything that is above ground must be taller than a hole.
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Anyone can assemble words into a paradox. So far, nobody has assembled molecules into one. Something to do with entropy.
Should one infer from this that you find no paradox in any of the double slit results?
I see no paradox, but I acknowledge that my conclusion might be based on very incomplete knowledge, or wrong thinking.
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And that's a potential problem. From the perspective of the time traveller, the rest of the universe becomes more ordered. I don't know how much the entropy of the universe changes in a day, but I would wager it would take at least the power output of a small galaxy to reverse it (and possibly infinite energy, for an infinite universe).
In the RF of the chrononaut, the Universe becomes more ordered; but in the RF of the Universe it does not. All that happens is that an insignificant object (somehow) transfers to a time when the Universe was more ordered. (?)
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The more ordered universe no longer exists. All the individual states of the constituents of the universe have changed. Time is a measuring device. It only relates to change. If everything were absolutely motionless, even the vacuum fluctuations, then time would have no meaning. Thus undoing these changes has nothing to do with time. It is undoing all the changes in state.
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The more ordered universe no longer exists.
I suspect that this is correct, but there are certainly those, with more knowledge than I have, who disagree. One “danger” here is that the “infinite cosmos” could creep in, and I’m keen to keep that out of this thread.
All the individual states of the constituents of the universe have changed. Time is a measuring device. It only relates to change
I see this as a good argument against past directed TT. Possibly the second best argument. :)
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All the individual states of the constituents of the universe have changed. Time is a measuring device. It only relates to change
Wouldn't this militate against the idea of CTCs?
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All the individual states of the constituents of the universe have changed. Time is a measuring device. It only relates to change
Wouldn't this militate against the idea of CTCs?
Yes.
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Jeffrey, I'd value your take on CTCs.
In fact anyone's views would be welcomed.
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You might find this article useful which discusses Stephen Hawking's paper on the subject.
https://en.m.wikipedia.org/wiki/Tipler_cylinder
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Thanks for the link, Jeffrey. I’d not seen that.
It still doesn’t give me your take on CTCs, though. I’m looking forward to that.
This is from my notes when I first met the idea of Tippler’s cylinder.
“There is, of course, the serious snag that Tippler’s cylinder has to be of infinite length, and even if an infinite length is theoretically achievable, it would be very difficult to attain if our Universe happens to be finite. In fact, it would be impossible to achieve, even in an infinite Universe, for reasons I considered earlier. This problem alone could be enough to take the rotating cylinder out of the equation, in the real world.”
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Can anyone explain in simple terms why a Tipler cylinder needs to be infinite in length in order to function?
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Can anyone explain in simple terms why a Tipler cylinder needs to be infinite in length in order to function?
No, but I think it has something to do with the maths being simpler if the cylinder were infinite. I'm hoping to find something about it when I follow Jeffrey's link.
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I particularly like this quote from http://www.andersoninstitute.com/tipler-cylinder.html
"There are problems, though. For the mathematics to work properly, Tipler’s cylinder has to be infinitely long. Also, odd things happen near the ends and you need to steer well clear of them in your timeship."
How do you find the end of an infinite cylinder? I guess this is right on topic in a thread involving cloud cuckoo land!
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One thing I wondered was if this could be linked to the rotating universe model. This includes vortices, and I wondered if these vortices could possibly take the place of rotating cylinders. Assuming that they could, and given that we could locate them, they would seem to present fewer problems than would trying to construct a cylinder, or anything else for that matter, of infinite length.
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The cylinders can be of a finite length if they are rotating fast enough. What you have to bear in mind is that the CTCs arise from exact solutions of the Einstein field equations. Just because a mathematical solution can be found does not mean it can exist in reality. There may be constraints that we have not found yet.
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I have found another page discussing the Gödel universe. I haven't read this through properly yet. It may be of interest to the curious.
http://iopscience.iop.org/article/10.1088/1367-2630/15/1/013063
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I've not had a chance to follow that link yet, Jeffrey, but, as I recall, Gödel proposed solutions to the equations of GR whereby time-like world lines could bend back on themselves, thus forming closed time-like loops.
How likely is it that Gödel’s universe is similar to Schrodinger’s cat?
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There is no issue with Schrodinger's cat. It was his way of showing how absurd he thought the uncertainty principle was. Uncertainty or indeterminacy happens whether Schrodinger liked it or not. In the case of the Godel universe we have no way of experimentally determining whether or not it is correct. It is well beyond our capabilities to engineer. To say the least.
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There is no issue with Schrodinger's cat. It was his way of showing how absurd he thought the uncertainty principle was.
Exactly. Could Gödel have been drawing attention to the problems with CTCs?
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Just an aside, but I like Terry Pratchett's observation about the possible states of Schrodinger’s cat.
“,,,,,three determinate states the cat could be in: these being Alive, Dead, and Bloody Furious.”
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There is no issue with Schrodinger's cat. It was his way of showing how absurd he thought the uncertainty principle was.
Exactly. Could Gödel have been drawing attention to the problems with CTCs?
He found a solution to the Einstein field equations. I have no idea what he thought about it.
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We're drifting away from Sean's gate. He says:
"You walk up to the gate, where you see an older version of yourself waiting for you. The two of you exchange pleasantries. Then you leave your other self behind as you walk through the gate into yesterday.”
Why would there be an older self there? Where would he have come from?
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He has lived a whole day more. Since he already went through the gate and came back to it to meet you.
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The more ordered universe no longer exists. All the individual states of the constituents of the universe have changed. Time is a measuring device. It only relates to change.
He has lived a whole day more. Since he already went through the gate and came back to it to meet you.
So, hasn’t he moved from a less ordered to a more ordered universe?
Does the more ordered universe still exist?
If not, how can "you" be in it?
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I'm just following the line of reasoning.
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Good luck with that, Jeffrey. I go round in circles with it, but I always seem to come back to the conclusion that past-directed time travel not on.
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He has lived a whole day more. Since he already went through the gate and came back to it to meet you.
So, hasn’t he moved from a less ordered to a more ordered universe?
Does the more ordered universe still exist?
If not, how can "you" be in it?
How's the line of reasoning going, Jeffrey?
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In Feynman diagrams anti particles are shown traveling backwards in time. Would the guy who walks through the gate be converted to anti matter and immediately annihilate with the matter around them? That would solve the grandfather paradox and fit with Feynman. ;)
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Good try Jeffrey. :)
The idea that Feynman said that an antiparticle is just a particle travelling backwards in time, is so often found that it is easy to see why some people think it’s true.
In fact, what he said was that the mathematics governing the evolution of a particle as time moves forwards is the same as the mathematics governing the evolution of an antiparticle as time moves backwards.
Would the guy who walks through the gate be converted to anti matter and immediately annihilate with the matter around them?
Would that happen only if he met a “matter” version of himself?
This, of course, would not happen because the version he met after passing through the gate, would also have passed through…..B*gg*r! this isn’t getting any better.
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The traveler would have to ensure that, one day later, every single atom in his body was in precisely the right place to join up smoothly with his past self.
Here, Sean seems to be saying, not only that a second version is somehow created, but that the two versions have to re-unite. This might present problems if, as he says, the traveller’s (travellers’?) memories are eraised.
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There is another way to avoid paradoxes using the multiverse theory
Marshall Barnes seems to agree with you.
http://www.science20.com/temporal_mechanic/blog/%E2%80%9Cnew_rules_time_travel_%E2%80%9D_and_sean_carroll%E2%80%99s_gate_2
“The solution is the many-worlds model. Logic dictates it. The math reveals it and the geometries describe it. It is impossible to construct, design, conceive or imagine a time travel scenario where parallel universes do not emerge as a solution.”
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One of the problems with having only a few minutes at a time to devote to on-line activity is that I often return to subjects that others may consider dead and buried. I apologise if that is the case here.
So, does everyone agree with Marshall Barnes, or is Sean's reasoning "pseudo-science"?
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One of the problems with having only a few minutes at a time to devote to on-line activity is that I often return to subjects that others may consider dead and buried. I apologise if that is the case here.
So, does everyone agree with Marshall Barnes, or is Sean's reasoning "pseudo-science"?
Better than starting a new thread each time you come back to :)
If one thinks that the many worlds interpretation is scientifically valid (I do not find it particularly compelling, and as far as I can tell it is not falsifiable by current and immediately foreseeable technology, which places it outside f the realm of science), then it offers a fairly obvious way around most of the paradoxes of backwards time travel. It's kind of a cop-out answer, if you ask me...