Freethought & Rationalism Archive

Jesse · 07-12-2003, 02:31 PM

Quote:

Originally posted by sophie
tronVillain says time travel is not "illogical at its core." It may or may not be impossible, but it is not inherently illogical. Anyone claiming that it is is simply experiencing a failure of imagination.

ME : My imagination fails me when the science is unfounded. It seems tronvillain places more value on his imagination rather than hard facts, science facts or natural facts of life.

But as I've already pointed out a few times on this thread, it is a "science fact" that the laws of general relativity permit time travel into the past in certain circumstances. This prediction has not been empirically verified, so time travel should have about the same status that black holes did when they were just a prediction of general relativity that hadn't been directly observed (and we still haven't empirically verified a number of predictions about black holes, like that they have 'event horizons' beyond which nothing can escape).

By the way, here's a cool recent "Wired" article on a few of the different types of time machines permitted by general relativity (written by Michio Kaku, author of Hyperspace):

http://www.wired.com/wired/archive/1...imetravel.html

tronvillain · 07-12-2003, 04:13 PM

sophie:

Quote:

My imagination fails me when the science is unfounded. It seems tronvillain places more value on his imagination rather than hard facts, science facts or natural facts of life.

Ah, so you are one of those people who confuses physical impossibility with logical impossibility. Pathetic.

tronvillain · 07-12-2003, 04:28 PM

Jesse:

Quote:

Even this is too weak, I think, because it still allows for the possibility of "changing" the past, which would suggest some sort of physically implausible "meta-time" and also allow for small paradoxes. The resolution of paradoxes favored by most physicists who speculate about time travel in the context of general relativity is the fixed block time idea in which the whole of spacetime is a fixed four-dimensional entity, with the constraint that history must be self-consistent (see the Novikov self-consistency principle), so any interaction a time traveler has with the past was already part of this self-consistent history all along. This does open up the possibility of weird causal loops, like a time traveler who grew up fascinated by the mysterious disappearance of Amelia Earhart, until finally he goes back to investigate and the shock of the appearance of a time machine causes Amelia Earhart to crash the plane. Still, there is nothing inconsistent about such loops.

Oh, I am aware that such a solution still allows for "changing" the past and so requires the existence of a "meta-time", but I am afraid I do not see the problem "small paradoxes" would present. Such a universe would "eventually" (in meta-temporal terms) settle down into a self-consisent fixed four-dimensional entity anyway.

Quote:

The self-consistency principle also suggests that any attempt to change the past is bound to fail--if you try to assassinate Hitler in 1935, the gun will misfire, or you'll kill the wrong guy, or your time machine will blow up in transit, or you'll have second thoughts before going through with it. People sometimes object to this on the grounds that it would seem to require some sort of intelligence or strategy on the part of the universe, but perhaps it's better to say that the laws of physics demand that among all logically possible sets of events, only the self-consistent ones can become a reality.

Ah, but the beauty of the solution I described is that it explains the self-consistency principle. Say, you go back in time and assassinate Hitler, but this is an unstable change since it results in you never going back in time to assassinate Hitler, and the universe snaps back to roughly the way it was "before"... repeat until you get a stable change to the past (firmly attach the future to the past so that it cannot snap back).

Jesse · 07-12-2003, 05:41 PM

tronvillain:
Oh, I am aware that such a solution still allows for "changing" the past and so requires the existence of a "meta-time", but I am afraid I do not see the problem "small paradoxes" would present. Such a universe would "eventually" (in meta-temporal terms) settle down into a self-consisent fixed four-dimensional entity anyway.

A paradox is a logical impossibility--it is logically impossible that at any point in meta-time a self-contradictory state of affairs could exist.

tronvillain:
Ah, but the beauty of the solution I described is that it explains the self-consistency principle. Say, you go back in time and assassinate Hitler, but this is an unstable change since it results in you never going back in time to assassinate Hitler, and the universe snaps back to roughly the way it was "before"... repeat until you get a stable change to the past (firmly attach the future to the past so that it cannot snap back).

First of all, it only "explains" self-consistency by positing something even weirder, a kind of "self-repairing" effect on the part of the timeline, which unlike a simple global self-consistency constraint would appear to require some active intelligence or teleology to make it work (not to mention that the global self-consistency idea actually has some basis in known laws of physics, while meta-time and self-repair are complete fantasies). But I don't think the "self-repairing" idea works anyway, since as I said it seems that at some intermediate point in meta-time you'd have a logically contradictory state of affairs. Perhaps it'd help if you fleshed out your idea with a more detailed example...what it sounds to me like you're saying is this:

Timeline 1: (earliest point in meta-time)

1925: Hitler does not meet any time travellers, is not assassinated.

2025: I read about Hitler and decide to stop him, step into my time machine and vanish.

Timeline 2: (later in meta-time)

1925: Hitler both is assassinated by me, since I came from the future to do so after reading about him in the history books, and simultaneously isn't assassinated by me because I didn't read about him in the history books and therefore didn't travel back in time. (logical contradiction)

2025: I simultaneously do read about Hitler in the history books and go back in time to assassinate him, and don't read about him in the history books because he was assassinated in 1925 before coming to power, so I don't go back in time. (logical contradiction)

Timeline 3: (final point in meta-time)

1925: Hitler is not assassinated by me, because even though I come back in time to stop him after reading about him in the history books, my gun misfires when I try to kill him. (timeline has self-repaired and found a consistent solution)

2025: I read about Hitler in the history books (missing the footnote where they mention an unsuccessful assassination attempt in 1925) and decide to go back to 1925 to stop him. (timeline has self-repaired and found a self-consistent solution)

If this is not what you actually meant, can you provide an example of a meta-time sequence like this one to show what you did mean?

B. H. Manners · 07-12-2003, 08:24 PM

To a certain extent, isn't all time present time?

sophie · 07-13-2003, 10:02 AM

Mr. Manners, I must agree with you. If my consciousness moved back into the past relative to the extrinsic world at this moment, my consciousness would indeed have a present in your past. The problem would be that would be my future._My future lies in the past which is an all encompassing present. WHEW. That would really be a crunch.

Jesse, the entrophic conditions for time travel seems illogical.

TronVillain, before logical plausability can be achieved the premise should be valid. To back your case you should use a BIG IF.

Jesse · 07-13-2003, 11:01 AM

Quote:

Originally posted by sophie
Jesse, the entrophic conditions for time travel seems illogical.

Do you mean "entropic"? The physicists who talk about the possibility of time travel don't seem to think it contradicts the laws of thermodynamics, and they're the ones who would be expected to have the most informed opinion on this issue.

This article talks a bit about negative energy (required to hold wormholes open) and entropy--see the section on "Cosmic Flashing and Quantum Interest":

http://www.physics.hku.hk/~tboyce/sf.../wormhole.html

sophie · 07-13-2003, 11:35 AM

The things one can do with math. It seems plausible then to wrap oneself up in a math equation and post the entry behind the negative sun. Of course it will take a lot of energy, it may take even more energy to split the positive energy from the negative energy, iff the shutter could flow faster than the speed of light. Yep, then we could get there before it happened and control the results accordingly...

Yep, I'm sure Mother Nature has more surprises in store for us in the form of humans.

Jesse · 07-13-2003, 12:16 PM

Quote:

Originally posted by sophie
The things one can do with math. It seems plausible then to wrap oneself up in a math equation and post the entry behind the negative sun.

There is already an experimentally-confirmed method of creating a region of space with a negative energy density--the Casimir effect.

tronvillain · 07-13-2003, 03:42 PM

Jesse:

Quote:

A paradox is a logical impossibility--it is logically impossible that at any point in meta-time a self-contradictory state of affairs could exist.

Oh, I know what a paradox is, I just fail to see how they would present a problem for such a solution.

Quote:

First of all, it only "explains" self-consistency by positing something even weirder, a kind of "self-repairing" effect on the part of the timeline, which unlike a simple global self-consistency constraint would appear to require some active intelligence or teleology to make it work (not to mention that the global self-consistency idea actually has some basis in known laws of physics, while meta-time and self-repair are complete fantasies). But I don't think the "self-repairing" idea works anyway, since as I said it seems that at some intermediate point in meta-time you'd have a logically contradictory state of affairs.

All that such "self-repair" would require would be a certain view of causality: a causal chain beginning in 2025 stretches back to 1925 and alters events in 1925, which severs the causal chain at 2025, which restores events in 1925 to their previous state. If there is no variation in space-time then the loop repeats indefinitely and if there is variation in space-time then the loop repeats until a self-consistent timeline exists (the time machine explodes, the change to the past eventually causes itself to be made, and so on). Both are a form of "self-repair" and they do not seem to require anything unusual other than a "meta-time."

This is the version of the detailed example I would give:

Timeline 1: (earliest point in meta-time)

1925: Hitler does not meet any time travellers, is not assassinated.

2025: I read about Hitler and decide to stop him, step into my time machine and vanish.

Timeline 2: (later in meta-time)

1925: Hitler is assassinated by me, since I came from the future to do so after reading about him in the history books.

2025: I do not read about him in the history books because he was assassinated in 1925 before coming to power, so I don't go back in time.

Timeline 3: (later in meta-time)

1925: Hitler does not meet any time travellers, is not assassinated.

2025: I read about Hitler and decide to stop him, step into my time machine and vanish.

Timeline 4: (later in meta-time)

1925: Hitler is assassinated by me, since I came from the future to do so after reading about him in the history books.

2025: I do not read about him in the history books because he was assassinated in 1925 before coming to power, so I don't go back in time.

Repeat with minor variations:

Timeline n: (final point in meta-time)

1925: Hitler is not assassinated by me, because even though I come back in time to stop him after reading about him in the history books, my gun misfires when I try to kill him. (timeline has self-repaired and found a consistent solution)

2025: I read about Hitler in the history books (missing the footnote where they mention an unsuccessful assassination attempt in 1925) and decide to go back to 1925 to stop him. (timeline has self-repaired and found a self-consistent solution)

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07-12-2003, 05:41 PM	#34
Jesse Moderator - Science Discussions Join Date: Feb 2001 Location: Providence, RI, USA Posts: 9,908	tronvillain: Oh, I am aware that such a solution still allows for "changing" the past and so requires the existence of a "meta-time", but I am afraid I do not see the problem "small paradoxes" would present. Such a universe would "eventually" (in meta-temporal terms) settle down into a self-consisent fixed four-dimensional entity anyway. A paradox is a logical impossibility--it is logically impossible that at any point in meta-time a self-contradictory state of affairs could exist. tronvillain: Ah, but the beauty of the solution I described is that it explains the self-consistency principle. Say, you go back in time and assassinate Hitler, but this is an unstable change since it results in you never going back in time to assassinate Hitler, and the universe snaps back to roughly the way it was "before"... repeat until you get a stable change to the past (firmly attach the future to the past so that it cannot snap back). First of all, it only "explains" self-consistency by positing something even weirder, a kind of "self-repairing" effect on the part of the timeline, which unlike a simple global self-consistency constraint would appear to require some active intelligence or teleology to make it work (not to mention that the global self-consistency idea actually has some basis in known laws of physics, while meta-time and self-repair are complete fantasies). But I don't think the "self-repairing" idea works anyway, since as I said it seems that at some intermediate point in meta-time you'd have a logically contradictory state of affairs. Perhaps it'd help if you fleshed out your idea with a more detailed example...what it sounds to me like you're saying is this: Timeline 1: (earliest point in meta-time) 1925: Hitler does not meet any time travellers, is not assassinated. 2025: I read about Hitler and decide to stop him, step into my time machine and vanish. Timeline 2: (later in meta-time) 1925: Hitler both is assassinated by me, since I came from the future to do so after reading about him in the history books, and simultaneously isn't assassinated by me because I didn't read about him in the history books and therefore didn't travel back in time. (logical contradiction) 2025: I simultaneously do read about Hitler in the history books and go back in time to assassinate him, and don't read about him in the history books because he was assassinated in 1925 before coming to power, so I don't go back in time. (logical contradiction) Timeline 3: (final point in meta-time) 1925: Hitler is not assassinated by me, because even though I come back in time to stop him after reading about him in the history books, my gun misfires when I try to kill him. (timeline has self-repaired and found a consistent solution) 2025: I read about Hitler in the history books (missing the footnote where they mention an unsuccessful assassination attempt in 1925) and decide to go back to 1925 to stop him. (timeline has self-repaired and found a self-consistent solution) If this is not what you actually meant, can you provide an example of a meta-time sequence like this one to show what you did mean?

07-12-2003, 08:24 PM	#35
B. H. Manners Veteran Member Join Date: Jan 2002 Location: Texas Posts: 1,213	To a certain extent, isn't all time present time?

07-13-2003, 10:02 AM	#36
sophie Veteran Member Join Date: May 2003 Location: On the road to extinction. . . Posts: 1,485	Mr. Manners, I must agree with you. If my consciousness moved back into the past relative to the extrinsic world at this moment, my consciousness would indeed have a present in your past. The problem would be that would be my future._My future lies in the past which is an all encompassing present. WHEW. That would really be a crunch. Jesse, the entrophic conditions for time travel seems illogical. TronVillain, before logical plausability can be achieved the premise should be valid. To back your case you should use a BIG IF.

07-13-2003, 11:35 AM	#38
sophie Veteran Member Join Date: May 2003 Location: On the road to extinction. . . Posts: 1,485	The things one can do with math. It seems plausible then to wrap oneself up in a math equation and post the entry behind the negative sun. Of course it will take a lot of energy, it may take even more energy to split the positive energy from the negative energy, iff the shutter could flow faster than the speed of light. Yep, then we could get there before it happened and control the results accordingly... Yep, I'm sure Mother Nature has more surprises in store for us in the form of humans.

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