Time travel paradox resolved...I think

[kourama]

I apologize if this has come up before.

I had an insight once many years ago about time travel and forgot about it until it came up in conversation the other day.

I submit my idea for review:

The well-known time travel paradox works as follows:

An engineer (let's face it, scientists never actually BUILD anything) designs and builds a device that allows her to travel backwards into time. After completing the device she decides to save herself all the work of design and goes back in time to give herself the plans to build the device. She goes back in time and gives the plans to her former self.

If her former self has the plans given to her, then who actually designed the device?

My resolution for this problem works like this: if we look at the space-time continuum so that causality always works in a positive direction in time, then the initial event would be the engineer arriving in what is perceived to be her past. Once she is there, then her perception of a personal past is an illusion. She has a set of memories, but since what she perceives as a memory has not occurred, she is not remembering a past.

From that point forward, causality works as normal, and she operates as any entity in the universe. WHen she interacts with what she perceives to be her former self, she is, in fact, interacting with a completely seperate person, on whose future she does not depend.

So, the paradox is gone, but there is a new problem: if causality works only in a forward direction in time, then what causes the initial event, which is the arrival of the 'future' engineer in the universe?

Well, if I could answer that, I'd have made a killing in the stock market by now. ;)

Originally posted by kourama
Once she is there, then her perception of a personal past is an illusion. She has a set of memories, but since what she perceives as a memory has not occurred, she is not remembering a past.

From that point forward, causality works as normal, and she operates as any entity in the universe. WHen she interacts with what she perceives to be her former self, she is, in fact, interacting with a completely seperate person, on whose future she does not depend.

So, the paradox is gone, but there is a new problem: if causality works only in a forward direction in time, then what causes the initial event, which is the arrival of the 'future' engineer in the universe?

Well, if I could answer that, I'd have made a killing in the stock market by now. ;)

This doesn't seem to me to resolve anything. The freshly-existing engineer arrives with a full set of plans in her "non-memory" and a plan to communicate it to her earlier self. This violates causality any way you look at it, especially if we view causality as always happening in the forward direction. The fact that the device doesn't yet exist is irrelevant; just the existence of a plan in the engineer's head is enough to violate causality, it seems to me.

[mindless]

Even if time travel is possible the consequences far outweigh any advantages we could receive, and the potential for total time ripping chaos is high.

Time travel is a Pandora’s box that shouldn't ever be opened.

[kourama]

Originally posted by sundog


This doesn't seem to me to resolve anything. The freshly-existing engineer arrives with a full set of plans in her "non-memory" and a plan to communicate it to her earlier self. This violates causality any way you look at it, especially if we view causality as always happening in the forward direction. The fact that the device doesn't yet exist is irrelevant; just the existence of a plan in the engineer's head is enough to violate causality, it seems to me.

Yep, which means, as far as I can tell, that the only way to travel in time is to be lucky enough to simply appear in the universe, fully formed with all the illusion of memory in tact, a la Bertrand Russel (that was his idea, right?)

[Oso]

If time travel is possible, or will ever be possible, then it's already happening, and any consequences of such have already happened. However I'm not aware of any evidence that someone from the future is paying us visits.

Oh, of course, we're the the ones traveling on the front edge, so eventually we might be able to go into the past.

Hogwash.

[DrChinese]

Time travel is a normal part of the quantum world. In the quantum world, particles can travel forward or backward in time.

Accordingly, it would be difficult to categorize this as a paradox. It is real.

A more accurate question is: does the future influence the past as much as the past influences the future? As best as we can tell, there appears to be an asymmetry in time. The macroscopic result is the appearance that time moves in one direction, our past "causes" our future.

But this cannot be rigorously demonstrated at the quantum level. There is nothing about the laws of physics which says there could not be matter which is moving through space-time backwards relative to our perception of time.

Hypothetically, for example, when the big bang occurred, half the energy of the newly created universe could have been sent in the other time direction from the portion of the universe we occupy. There is nothing in our understanding of physics which prevents this from being a possibility.

[Mark]

Doesn't Quantum Theory show that if such a thing happened, at that point, a new reality would be formed based on the alternation in time? But the old reality would also continue to exist. In other words, a new universe would be formed without harming the old one. Since there are thought to be (possibly) an infinite number of universes anyway, maybe this sort of thing has already happened over and over again.

Originally posted by kourama


Yep, which means, as far as I can tell, that the only way to travel in time is to be lucky enough to simply appear in the universe, fully formed with all the illusion of memory in tact, a la Bertrand Russel (that was his idea, right?)

Which unfortunately violates the Second Law of Thermodynamics, it seems to me.

[shanek]

My favorite time travel paradox:

The pockets in a pool table are wormholes. The wormhole in the left corner pocket is linked to the wormhole in the right corner pocket by a two second offset. If you dunk a ball in the right corner pocket, it pops out of the left corner pocket two seconds later. If you dunk a ball in the left corner pocket, it pops out of the right corner pocket two seconds earlier.

Now, you line up your shot and hit a ball into the left corner pocket, so two seconds earlier it pops out of the right corner pocket and knocks itself out of the way, causing it to miss the pocket, and hence, it could never have come out of the right corner pocket to knock itself out of the way.

[Mark]

Now, you line up your shot and hit a ball into the left corner pocket, so two seconds earlier it pops out of the right corner pocket and knocks itself out of the way, causing it to miss the pocket, and hence, it could never have come out of the right corner pocket to knock itself out of the way.

"Metaphysics gives me a headache." ---Nick Danger to Rocky Roccoco.

Originally posted by Mark
Now, you line up your shot and hit a ball into the left corner pocket, so two seconds earlier it pops out of the right corner pocket and knocks itself out of the way, causing it to miss the pocket, and hence, it could never have come out of the right corner pocket to knock itself out of the way.

"Metaphysics gives me a headache." ---Nick Danger to Rocky Roccoco.

Ooh, favorite time travel paradoxes? :D

I have two. One is an Anson MacDonald (Robert Heinlein) and I forget who wrote the other.

The Heinlein one is called "By His Bootstraps" and is a complicated maze of paradoxes. It finally turns out that most of the characters in the story are one person at different times in his life. Very entertaining, classic time-travel story.

I'm not sure of the other one's name, even. It involved an archaeologist who invented a time travel machine, went into the future and came back, dying immediately but bringing with him a mysterious knife made of an unknown metal. A museum is built in his memory and the knife placed in it. Years later his grandson tries to reproduce his trip into the future and does, following his grandfather's footsteps into a bombed-out building and ending at an empty display case with the dusty outline of the knife. The grandson looks up to discover, of course, that he's in the museum built to honor his grandfather.

[Mark]

My favorite was a short story by Robert Silverberg (possibly just edited by him; I don't remember) in which time travel is highly illegal and every time any traveler is caught, he faces a judge who always convicts them and sends them to a prison on the moon. Somehow, the judge ends up trying out a time device and ends up facing himself and is sent to prison.

[kedo1981]

Time travel isn’t possible because time doesn’t exist

[Mark]

Time travel isn’t possible because time doesn’t exist

You've obviously never had to spend time with a life insurance salesman. :D

[shanek]

Originally posted by Mark
My favorite was a short story by Robert Silverberg (possibly just edited by him; I don't remember) in which time travel is highly illegal and every time any traveler is caught, he faces a judge who always convicts them and sends them to a prison on the moon. Somehow, the judge ends up trying out a time device and ends up facing himself and is sent to prison.

I like the penalty for misusing time machines in The Hitchhikers Guide to the Galaxy: You get left back in the stone age and are told to evolve into a more responsible life form. :D

[Cecil]

There was a book I read when I was a kid called The Tale of Time City. Anyone else heard of this?

It was pretty confusing, but had a very interesting premise.

Originally posted by shanek


I like the penalty for misusing time machines in The Hitchhikers Guide to the Galaxy: You get left back in the stone age and are told to evolve into a more responsible life form. :D

That was the penalty for "drunk in charge of a time ship", as I recall.

OK, close enough. :D

[kourama]

Originally posted by sundog


Which unfortunately violates the Second Law of Thermodynamics, it seems to me.

Not necessarily (sp?), as long as some other part of the universe is increasing in entropy proportionately. :D

Favourite time-travel story: Bill & Ted's excellent adventure. Back to the future came out at the same time, but the logic was a bit broken. Bill & Ted's future was showing up in their past and all the pieces fit together (as far as I could tell). Also, being a HUGE Dr. Who fan, I totally dug the phone booth idea.

[BobM]

If time travel is possible, or will ever be possible, then it's already happening, and any consequences of such have already happened. However I'm not aware of any evidence that someone from the future is paying us visits.

Not that this thread is at all serious anymore.. but you're assuming here that time travel would be along a single timeline. Ie, we go back into our own past.

There's also the multiple time-line idea of time travel, where going back and changing something creates a new branch.


A B
-----------------------------------------------------------------


A C B
------------------------,. . . . . . . . . . . . . . . . . . . . . . . . .
\
\
\ D

A: the start of the time line.
B: the point you jumped from.
C: the point you jumped to.
D: the new "present", B is longer accesible to you.

It's more a way to travel through multiple universes than strict time travel.

[edit] wow. that did not work at all.

Nice link on the subject in re science fiction:

http://www.towson.edu/~flynn/timetv.html

And a really good article on the subject:

http://www.wikipedia.org/wiki/Time_travel

[shanek]

Originally posted by kourama
Favourite time-travel story: Bill & Ted's excellent adventure. Back to the future came out at the same time, but the logic was a bit broken. Bill & Ted's future was showing up in their past and all the pieces fit together (as far as I could tell). Also, being a HUGE Dr. Who fan, I totally dug the phone booth idea.

I think BTTF was at least internally consistent. I mean, it's not like anyone actually knows how this stuff would work. For example, Marty McFly going back and inadvertently stopping his parents from meeting. He didn't immediately fade out; the ripple effect slowly took place until the prevention of his birth would have been a certainty. That's why the older kids faded from the photo first; it kept delaying their time of meeting, getting married, and having kids. Had Marty failed, it would have meant that the alternate future Marty stuck around just long enough to prevent them from meeting. I thought that was an interesting way around the paradox.

I think the producer said once that Carl Sagan told him that BTTF was the most accurate time travel movie done to that point. Whether or not that's true I don't know.

[shanek]

Originally posted by Oso
If time travel is possible, or will ever be possible, then it's already happening, and any consequences of such have already happened. However I'm not aware of any evidence that someone from the future is paying us visits.

Come to think of it, I remember some big-brained theoretical physicist (don't remember the name, sorry) saying that if you build a time machine, you can go backwards in time, but only to the point where you built the machine, because you need the time machine to set up your entry point into the past. So that gets around the question of why aren't we being visited by historians from the future.

[DrMatt]

Originally posted by DrChinese
Time travel is a normal part of the quantum world. In the quantum world, particles can travel forward or backward in time.

Accordingly, it would be difficult to categorize this as a paradox. It is real.


On the scale of individual particle-particle collisions.
:rolleyes:

[gnome]

Originally posted by kourama

Not necessarily (sp?), as long as some other part of the universe is increasing in entropy proportionately. :D


One of the earliest "Trek" novels, "The Entropy Effect," has this happen exactly. Spock notices that due to someone's constant mucking about in time, that the entropy of the universe is increasing so fast that there's only about a century left.

An idea obviously abandoned by future authors and script writers....

[Acrimonious]

The quick and dirty reason I think time travel is not possible:

Everyone is taught that matter is Impenetrable (yes that is the term for it): no two pieces of matter can occupy the same place at the same time.

I draw a simple corollary; no one piece of matter can occupy 2 different places at the same time.

At any point in time, the matter that you are made of only exists in one place at that exact moment.

Going back in time would force that matter to exist in 2 places at once. The matter from the "current" you, winning lotto numbers in hand, and the matter of the "previous" you, a week before the drawing. It's the same matter, but in 2 different places at the exact same time. That's a no can do.

If you want to go back to before you were born, you're still out of luck. The disassociated compounds that would eventually be ingested and formed into you still existed in the universe before they ever got anywhere near your parents.

[Mark]

Everyone is taught that matter is Impenetrable (yes that is the term for it): no two pieces of matter can occupy the same place at the same time.

But, according to quantum theory, isn't that EXACTLY what happens in our universe(s)?

Originally posted by Acrimonious
The quick and dirty reason I think time travel is not possible:

Everyone is taught that matter is Impenetrable (yes that is the term for it): no two pieces of matter can occupy the same place at the same time.

I draw a simple corollary; no one piece of matter can occupy 2 different places at the same time.


Well, there are a lot of problems with that. It's not a corollary, because it isn't proved by arguments already given; it's a rearrangement of a sentence, with no logical grounds for its truth or falsity. It's not a valid logical transformation to just switch the clauses around.

This sort of argument often invokes the Law of Conservation of Matter and Energy more successfully.

[slimshady2357]

Originally posted by Oso
If time travel is possible, or will ever be possible, then it's already happening, and any consequences of such have already happened. However I'm not aware of any evidence that someone from the future is paying us visits.


Hogwash. :p

You say 'or ever will be possible'.

Well if time travel is invented and soley used in centuries beyond ours, we wouldn't have any evidence that someone from the future has come paid us visits, because no one would have! You assume that someone with time travel would attempt to change history prior to now, maybe they don't want to.

So there! :p

Adam

[phobos]

Originally posted by slimshady2357

You assume that someone with time travel would attempt to change history prior to now, maybe they don't want to.


If I was in the far future facing down the heat death of the Universe, and I had a time machine, I know what I'd do. Start shipping hot matter from the Big Bang, and run a heat engine between that and the cold, dead cosmos of the ultimate future. Whoopee - usable energy! Of course this totally undermines the universe from the word go, but what else are you going to do, in the end?

[Oso]

Originally posted by slimshady2357


Hogwash. :p

You say 'or ever will be possible'.

Well if time travel is invented and soley used in centuries beyond ours, we wouldn't have any evidence that someone from the future has come paid us visits, because no one would have! I quote from sundog's link: (thanks sundog, a good laymans discussion)
In fact, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel.Perhaps my statement was a little stronger but I'll accept Stephen Hawking's qualified support.:p

Well bye for now, gotta head offshore. Be back in a couple of weeks (I hope).

[slimshady2357]

Originally posted by Oso
I quote from sundog's link: (thanks sundog, a good laymans discussion)
Perhaps my statement was a little stronger but I'll accept Stephen Hawking's qualified support.:p

In fact, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel.

Well bye for now, gotta head offshore. Be back in a couple of weeks (I hope).

bah! All that proves is that you and Hawking were equally short sighted on this matter! :D

Good luck offshore and hopefully we'll see you soon. :)

Adam

[Psi Baba]

Originally posted by phobos
but what else are you going to do, in the end?
Build a restaurant!

[Keneke]

I partially agree with Acrimonious, and shall add my own observations.

Time, as I heard it described once, is merely the measurable quality between states of an object. This point in time is later than 5 minutes ago because of the fundamental changes in position, energy state, etc. of every piece of matter in existence.

Now then, this may present problem with time travel. Going backwards in time is not only a reversal of entropy, but also a rearranging of existence exactly as it was.

The reason the "no-further-back-than-inception" theory works is because of this theory. The theory operates like this: Take a wormhole through space. Have one end (A) stationary, have the other end (B) moving as fast as light, or near that. This means that local conditions at B, using relativity, are almost stopped in time *when observed by A*. Theoretically, a person could fly into A in the year 3030 and emerge from B in a little over 3000, if the B exit from the wormhole had been circling at the speed of light for 30 years.

So, that's that. I don't really believe in the possibility of true time travel, because to do so you would have to re-form the position and energy state of every atom in the universe at whatever time you wanted to go to. This, of course, is excluding the infinte universes theory, but I ain't touching that with a ten foot pole.

[Mark]

This, of course, is excluding the infinte universes theory, but I ain't touching that with a ten foot pole.

You can't touch it anyway; I believe quantum mechanics tells us that nothing can ever really touch anything. :D

Originally posted by Mark
This, of course, is excluding the infinte universes theory, but I ain't touching that with a ten foot pole.

You can't touch it anyway; I believe quantum mechanics tells us that nothing can ever really touch anything. :D

Oh good, now I can stop washing my hands after I pee.

[Mark]

Oh good, now I can stop washing my hands after I pee.

I respect your decision, but am going to decline any attempt to shake on it. :D

[rwald]

If anyone is seriously concerned about the future-tourism argument, I can tell the example which proves it to be fallacious. But I get the impression that you guys are saying it as a joke.

My thoughts are that if time travel is possible, than either there is no free will, or (more likely) we live in a quantum multiverse (MWI). I tend to believe the latter more, but since all time machines suggested so far are effectively impossible to build, we will likely never know.

Oh well. It sure would have given a whole new meaning to that song, "I'm my own grandpa."

[Kimpatsu]

Originally posted by rwald
My thoughts are that if time travel is possible, than either there is no free will...
This is the block universe theory. Everything in the universe has already happened somewhere, such as the incident of your birth is 30 light years away from Earth 30 years after you were born, so paradoxes are impossible. The flip side, however, is that free will is therefore also impossible.

[rwald]

Exactly. However, quantum uncertainty makes the block universe look extremely unlikely, to say the least. That's why I hold to the many worlds interpretation (MWI).

[Kimpatsu]

Originally posted by rwald
Exactly. However, quantum uncertainty makes the block universe look extremely unlikely, to say the least. That's why I hold to the many worlds interpretation (MWI).
Absolutely right. So do I, actually, but I thought I'd throw in the block universe theory just to stir the pot. Personally, though, I find Hugh Everett's multiverse theories to be quite persuasive.
Best,

[gnome]

Originally posted by phobos


If I was in the far future facing down the heat death of the Universe, and I had a time machine, I know what I'd do. Start shipping hot matter from the Big Bang, and run a heat engine between that and the cold, dead cosmos of the ultimate future. Whoopee - usable energy! Of course this totally undermines the universe from the word go, but what else are you going to do, in the end?

Ahh but what do you do when those rotten wastrel bastards from your future start using THEIR time machine to suck energy out of YOUR project? :)

[Mark]

Ahh but what do you do when those rotten wastrel bastards from your future start using THEIR time machine to suck energy out of YOUR project?

Send Zaphod to use a Kill-O-Zap on them.

[Kevin_Lowe]

Originally posted by gnome


Ahh but what do you do when those rotten wastrel bastards from your future start using THEIR time machine to suck energy out of YOUR project? :)

I think we've figured out where the universe's "missing mass" has gone...

:D

[Kimpatsu]

Originally posted by Kevin_Lowe
I think we've figured out where the universe's "missing mass" has gone...
Yeah, just think of the empty space in the heads of all the woo-woos out there. :D