typodupeerror

## New Model Solves Grandfather Paradox887

goldfishy writes "If you went back in time and met your teenage parents, you could not split them up and prevent your birth - even if you wanted to, a new quantum model has stated. Researchers speculate that time travel can occur within a kind of feedback loop where backwards movement is possible, but only in a way that is 'complementary' to the present. In theory, you could go back in time and meet your infant father but you could not kill him." From the article: "Quantum behaviour is governed by probabilities. Before something has actually been observed, there are a number of possibilities regarding its state. But once its state has been measured those possibilities shrink to one - uncertainty is eliminated."
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## New Model Solves Grandfather Paradox

• #### You insensitive clod! (Score:5, Funny)

by Anonymous Coward on Friday June 17, 2005 @07:19PM (#12847980)
I am my own grandfather!
• #### Re:You insensitive clod! (Score:5, Funny)

on Friday June 17, 2005 @07:40PM (#12848149)
Many, many years ago when I was 23
I was married to a Wider who was purty as can be
My father fell in love with her and soon they two were wed

For my daughter was my mother cause she was my father's wife
To complicate the matter even though it brought me joy
I soon became the father of a bouncing baby boy

I'm my own grampa,
I'm my own grampa
It sounds funny I know
But it really is so
I'm my own grampa

My little baby then became a brother-in-law to dad
For if he was my uncle then that also made him brother
Of the Wider's grown up daughter who of course was my step-mother

My father's wife then had a son who kept them on the run
And he became my granchild for he was my daughters son
My wife is now my mother's mother and it makes me blue
Because although she is my wife she's my grandmother too

I'm my own grampa,
I'm my own grampa
It sounds funny I know
But it really is so
I'm my own grampa

Oh if my wife is my grandmother then I'm her grandchild
And every time I think of it, it nearly drives me wild
For now I have become strangest case you ever saw
As husband of my own grandmother I'm my own grampa

I'm my own grampa,
I'm my own grampa
It sounds funny I know
But it really is so
I'm my own grampa
• #### Re:You insensitive clod! (Score:5, Funny)

on Friday June 17, 2005 @07:57PM (#12848279) Homepage Journal
And I thought compiler dependencies were tough!
• #### Re:You insensitive clod! (Score:4, Insightful)

<yoda AT etoyoc DOT com> on Saturday June 18, 2005 @12:08AM (#12849361) Homepage Journal
The next time someone talks about the difficulties of multiple inheritence, they may not be talking about OOP.
• #### Re:You insensitive clod! (Score:4, Funny)

on Friday June 17, 2005 @10:49PM (#12849114)
• #### Re:You insensitive clod! (Score:5, Informative)

on Saturday June 18, 2005 @12:49PM (#12851802)
At least give the authors of the song credit: Dwight Latham and Moe Jaffe.

• #### Futurama - Roswell that Ends Well (Score:5, Funny)

on Friday June 17, 2005 @07:19PM (#12847983) Homepage
Farnsworth: Oh, a lesson in not change history from Mr. I'm my own Grandpa!
• #### Re:Futurama - Roswell that Ends Well (Score:5, Informative)

on Friday June 17, 2005 @07:32PM (#12848080)
I think a more appropriate quote would be:
Farnsworth: "Don't do anything that affects anything. Unless it turns out you were supposed to do it, in which case for the love of God. Don't not do it!"
• #### Re:Futurama - Roswell that Ends Well (Score:5, Funny)

on Friday June 17, 2005 @08:39PM (#12848531)

One of my favourites, at the end of much messing about with causality, is:

"Oh screw history, let's get the hell out of here!"

• #### Novikov? (Score:5, Informative)

<full.infinity@NOSpAM.gmail.com> on Friday June 17, 2005 @07:21PM (#12847995) Journal
Sounds like the Novikov self-consistency principle [wikipedia.org] to me.
• #### Re:Novikov? (Score:5, Interesting)

on Friday June 17, 2005 @07:33PM (#12848096)
Yes, it sounds similar.

And this frustratingly vague article makes a meaningless argument.

It tries to use the fact that we observe no disappearing people, or other strange temporal modifications as an argument that such things don't happen, and are thus impossible. But if somebody actually changed the way a wave function collapsed at some time in the past, why on earth would we expect to remember things from the way it was "before" it had been changed, since the change by definition happened in our own past, and thus to us it always occurred the way it now occurs? This isn't a logical argument. And it explains part of the aesthetic appeal of the many-worlds interpretation.

In pure quantum mechanics, time is a special property because wave function collapse via quantum operators (i.e. "observation") is a privileged thing that moves in only one direction. In general relativity, time doesn't have a privileged status. I don't see how you are going to reconcile that basic difference without coming up with a more complete theory (i.e. quantum gravity, GUT, etc.), but then again, my undergrad physics major knowledge is a bit rusty five years later.
• #### Re: Novikov? (Score:4, Interesting)

on Friday June 17, 2005 @08:51PM (#12848593)

> It tries to use the fact that we observe no disappearing people, or other strange temporal modifications as an argument that such things don't happen, and are thus impossible.

IIRC it has been proven that no time machine could take you back before the time when the machine was created, so unless someone has already created on and kept it secret we shouldn't be seeing tamper effects or visitors from the future anyway.

• #### Re: Novikov? (Score:4, Informative)

<marcosdumay@ g m ail.com> on Friday June 17, 2005 @09:28PM (#12848765) Homepage Journal
Nobody has ever proved that. What happens is that everybody accepts this fact as a base, and build physics from there. If you assume that a consequence can never happen before the cause, you end up with a model where is impossible to build a time machine that goes to the past.
• #### Re: Novikov? (Score:5, Interesting)

on Saturday June 18, 2005 @02:49AM (#12849836) Homepage Journal
The reason I've always understood is that the only theoretical model for (backward) time travel thus far involves temporally dialating (which is in itself, forward time manipulation) one end of a wormhole by sending it around a big circle at near-c. You then step back through the wormhole, back in time across the temporally dialated wormhole, and come out... a few feet away, where you then walk over to the same end you walked through last time and step back across time again. But at some point the point in time you step back across will be when the other end of the wormhole is still on it's journey through space and thus you CAN'T just walk over to the stationary end and do it again. Thus, you can never travel back before the time machine was built, because there needs to be the temporally dialated wormholes (the time machine) to come out of. It's not temporal teleportation.

Of course, even that model may be incorrect [slashdot.org], and timetravel may be utterly impossible (unless by some other strange means).

Personally, after that thread I just linked, I'm leaning in favor of impossible. If it were possible though, I definitely go with the many-worlds interpretation. (Hell, I already go with the many-worlds intrepretation just of quantum physics. Wave collapse my ass).
• #### Re: Novikov? (Score:3, Interesting)

IIRC it has been proven that no time machine could take you back before the time when the machine was created, so unless someone has already created on and kept it secret we shouldn't be seeing tamper effects or visitors from the future anyway.

1. Never use the word "proven" around scientists. They'll kick your butt for it.

2. The theory of which you refer to is only applicable to using stable wormholes for time travel.

3. Stable wormholes are a thought experiment and have not been shown to exist. (In fact
• #### Re: Novikov? (Score:3)

There's evidence of time travel all around you, you're just not paying attention to it. Much of the inexplicable in the world is due to time travel. A type of time travel is responsible for the creation and re-creation of the universe itself.
• #### Re:Novikov? (Score:4, Interesting)

on Friday June 17, 2005 @09:44PM (#12848825) Homepage
In pure quantum mechanics, time is a special property because wave function collapse via quantum operators (i.e. "observation") is a privileged thing that moves in only one direction. In general relativity, time doesn't have a privileged status.

In nonrelativistic QM you mean. In the Dirac and Klein-Gordon equations, time is treated identically with the other dimensions, ie, the Dirac and Klein-Gordon equations are Lorentz invariant. So, yes, time-reversal is something that must be dealt with in relativistic QM. Unfortunately, these equations only describe a few very restricted situations, so they are not as generally applicable as the Schroedinger equation. Also, they only include special relativity, not general relativity, which seems to be where the time-travel is coming from in the as you said frustratingly vague article.

More bad science in the article:

Quantum behaviour is governed by probabilities. Before something has actually been observed, there are a number of possibilities regarding its state. But once its state has been measured those possibilities shrink to one - uncertainty is eliminated.

This is a common misconception. Prior to an observation, any system has a perfectly well defined state: its wave-function. This state, however, may or may not determine various properties of the system. In fact, for a given wave-function, or state, most properties (ie position, velocity, kinetic energy, etc) are restricted to a certain set of eigenvalues, and the wave-function merely determines the probability that a measurement of that property will yield any particular eigenvalue. Immediately after the measurement, the system will be in a state such that that particular property is exactly determined (ie the wave-function will have changed so that the probability of measuring that value for that property is 1, and the probability of measuring any other eigenvalue for that property is 0). This is called the collapse of the wave-function. However, other properties, some of which may have been uniquely determined by the previous wave-function, some of which were not, are now not uniquely determined.

In other words, what the article said was precisely wrong. Any system always has exactly one state which it is in, and after a measurement (or observation), whatever was measured is no longer uncertain, but most other properties are still uncertain.

IANA Physicist, yet. I have just finished my junior level undergrad physics courses, and am currently working for the summer at the Fermi National Accelerator Laboratory. Just to establish credentials.
• #### Verifying the Theory (Score:5, Funny)

<lawrenceperson@[ ]il.com ['gma' in gap]> on Friday June 17, 2005 @07:22PM (#12848001) Homepage Journal
I would be most interested in how they'll set up the experiment to verify the theory...

"OK, McFly, here's the gun. If you can kill your own father and thus erase yourself from existence, we'll know the theory was wrong."

• #### Re:Verifying the Theory (Score:4, Insightful)

on Friday June 17, 2005 @07:29PM (#12848060)
Um Doc... how about we test on a monkey first?
• #### Re:Verifying the Theory (Score:5, Funny)

on Friday June 17, 2005 @07:46PM (#12848194)
"OK, McFly, here's the gun. If you can kill your own father and thus erase yourself from existence, we'll know the theory was wrong."
-"Hey! I'm not that stupid! I wouldnt do that!"
-"Chicken!"
• #### Re:Verifying the Theory (Score:4, Interesting)

on Saturday June 18, 2005 @12:02AM (#12849342) Homepage
The physicist David Deutsch had a theory that McFly could kill his own father, but that this would just spawn off a new time line in which his father died young and didn't have a kid, in parallel with the time line in which there was no murder and McFly was born. So you could go back and time and do whatever you want, but it would not affect your own history.

It all falls out of the many-worlds interpretation of quantum mechanics; the closed time-like loops would not "really" be closed and paradoxes could not happen, but you could meet many copies of yourself. Or find out what might have happened had you made a different decision.

• #### sounds exactly like the move "the time machine" (Score:2)

this was a principle plot point in the move the time machine.
• #### Re:sounds exactly like the move "the time machine" (Score:2, Funny)

not to mention the best-selling novilization of the movie that followed shortly there after!
• #### Summary (Score:4, Insightful)

on Friday June 17, 2005 @07:22PM (#12848010)
In other words, even if you take a trip back in time with the specific intention of killing your father, so long as you know he is happily sitting in his chair when you leave him in the present, you can be sure that something will prevent you from murdering him in the past
• #### That's great! (Score:5, Insightful)

on Friday June 17, 2005 @07:22PM (#12848012)
This means that you cannot be killed when you go back in time, nor can you kill or destroy anything! That's just perfect!!

Go back in time and be able to observe, only... no ability to interact with anyone either... it should be kinda like ghosts... we go back in time and observe and be like ghosts in the sense that we cannot interact and change anything that has already happened but only observe!

Imagine the possibilities of history classes of the future... maybe there are already a lot of ghosts watching us right now... the future students studying history!!
• #### Re:That's great! (Score:2, Funny)

No sane history teacher would force his/her students to watch people post to /.
• #### Re:That's great! (Score:2)

rent Grand Tour: Disaster in Time [imdb.com] some time... Not ghosts, but world disaster time-travel tourists.
• #### Re:That's great! (Score:2)

From the summary of the film, I gather that this is the film adaptation of Vintage Season by Lawrence O'Donnell... an excellent story. I read it in Science Fiction Hall of Fame Volume 2A ed. Ben Bova. The point is that even though these time travellers could possibly change the events that they are witnessing, they are forbidden by the rules in the future where they come from because changing anything could potentially wipe out the future. Seeing how one of the characters (a woman who is not fond of follow
• #### Re:That's great! (Score:2, Funny)

"Imagine the possibilities of history classes of the future... maybe there are already a lot of ghosts watching us right now... the future students studying history!!"

Great. It's not like I wasn't uncomfortable enough having sex in front of the just the dog. I have to worry about porn technology from the future.

• #### Re:That's great! (Score:2, Interesting)

Actually, there's nothing that would prevent you from being killed on your journey to the past. You just couldn't kill your past self. Your present future is unknown, so you may not survive your trip to the past.
• #### Re:That's great! (Score:3, Insightful)

Sure you could. It would just make a branch of the timeline.

In one branch, someone killed you.

In the other branch, no one killed you.

The fork would be at the point before doing the killing. I just hope you know how to get back to the branch that you belonged in. :)

• #### Re:That's great! (Score:5, Insightful)

on Friday June 17, 2005 @07:43PM (#12848168) Homepage
Here's an interesting twist: I can change the past in a way that changes the future, if I do so in a way that I don't know now. I could, for example, buy some shares of a stock that I know will rise a hundred-fold in value, but make sure that I don't get the money until after the moment I depart current time. Since I don't know that I don't have an envelope worth millions of dollars hidden, with a mechanism that will inform me of it, say, a week after my time-travel tip, it does not contradict what is known about the present that I become a millionaire a week from now.
• #### Re:That's great! (Score:5, Informative)

on Friday June 17, 2005 @08:52PM (#12848604) Homepage
You are quite correct, in fact, this is the only way people make money in the stock market.
• #### Re:That's great! (Score:5, Funny)

on Friday June 17, 2005 @09:48PM (#12848837)
You are quite correct, in fact, this is the only way people make money in the stock market.

The fact that this was modded "Informative" can only be explained by time-travelers coming back in time to clue us in on how screwed the stock market is about to be.

....Huh?
• #### Re:Ugh... (Score:2, Insightful)

i think it means you're your own grandpa

or something
• #### No no no! (Score:5, Insightful)

on Friday June 17, 2005 @07:23PM (#12848020) Journal
"Clearly, the present never is changed by mischievous time-travellers: people don't suddenly fade into the ether because a rerun of events has prevented their births - that much is obvious."

That's not clear at all. If I went back in time and killed the baby George W Bush, it's like he would disappear in the middle of a speech. Rather the entire course of history branching from that moment would be changed, so that in the "present" no one would ever know GW had existed.

-Alex
• #### Re:No no no! (Score:2)

How could that possibly work though?

Would the people in the present suddenly vacuum to a moment where George Dubya is gone?

Your explanation does not make any more sense than what you quoted.

If you were to travel back in time two years, and were to murder my dog that I was walking at the time, what would happen? Would the leash just go empty, and or would I teleport back to a time before I knew the dog?

• #### Re:No no no! (Score:5, Funny)

on Friday June 17, 2005 @07:38PM (#12848138) Homepage Journal
You mean like when my friend went back in time and killed President Barnes when he was in first grade?
• #### Re:No no no! (Score:3, Insightful)

It's actually slight more complicated than that.

Basically, if time travel is actually possible, the instant that you travel back in time, you would create a fork in the past; you go back to 1978, and every single event prior to the time that you land in may be the same, but as soon as you land in 1978 you create a version of 1978 where you existed. Getting back to your own future would be really difficult, if not impossible.

The cool thing is, if you kill someone, in that timeline that person completely ce
• #### Re:No no no! (Score:5, Funny)

on Friday June 17, 2005 @08:35PM (#12848508) Homepage
The problem with this, of course, is that it only affects *that* timeline and any future forks created from that point onwards; it doesn't change the fact that back here, in our timeline, W became the president and launched another Gulf War.

I am sick of this democrat-republican warring permeating every aspect of society. Can't we become suspicious at other countries and invade them together, as Americans?

• #### Re:No no no! (Score:3, Insightful)

"...but as soon as you land in 1978 you create a version of 1978 where you existed."

What makes you think that you weren't there?
• #### What about... (Score:3, Interesting)

on Friday June 17, 2005 @07:23PM (#12848021) Homepage
Couldn't you go back in time to kill your grandfather, only to have him rematerialize out of quantum randomness 5 minutes later? It's not impossible, just really improbable... maybe that's the protection mechanism.
• #### Re: What about... (Score:3, Interesting)

> Couldn't you go back in time to kill your grandfather, only to have him rematerialize out of quantum randomness 5 minutes later? It's not impossible, just really improbable... maybe that's the protection mechanism.

The actual protection mechanism is that you discover your grandfather to still be a young stud rather than a cranky old man, and he gives you a good ass-beating before sending you back where you belong.

• #### This is just a cop-out (Score:2)

One great big unscientific cop-out;

"You go back to kill your father, but you'd arrive after he'd left the room, you wouldn't find him, or you'd change your mind," said Professor Greenberger.

Sounds more like 'faith' to me.
• #### Re:This is just a cop-out (Score:2)

Or, something will kill you before you can kill him. Deus ex machina. Definitely smells like faith-based fate.

= 9J =

• #### Re:This is just a cop-out (Score:4, Interesting)

on Friday June 17, 2005 @07:44PM (#12848185)
Nope. I can prove it to you.

Lets say that at t = 0 your father is alive. And you go back to t = -10 to kill him. Let's say, further, that you kill him. So at t = -10 your father is dead. Then at t = 0 your dad is dead. This is a contradiction by hypothesis. The logic here is valid, so some premise must fail.

So it is logically impossible to kill your own father, given a relatively naive understanding of causation and fatherhood. A more nuanced understanding of causation and space-time might include things like "branching universes" and the like. Which is perfectly fine. But then there's the philoshical issue whether the person killed is actually your father or "merely" your "parallel universe father."
• #### Re:This is just a cop-out (Score:3, Insightful)

Lets say that at t = 0 your father is alive. And you go back to t = -10 to kill him. Let's say, further, that you kill him. So at t = -10 your father is dead. Then at t = 0 your dad is dead. This is a contradiction by hypothesis. The logic here is valid, so some premise must fail.

There are numerous problems with your "proof". Here are some assumptions that you gloss over:

- You assume that t automatically progresses from -10 to 0, with all other values in tact. That is, you assume that if father is d

• #### H'uh? (Score:2)

"You go back to kill your father, but you'd arrive after he'd left the room, you wouldn't find him, or you'd change your mind," said Professor Greenberger.

Anyone else having difficulty imagining a scenario where it would be "impossible" to kill somebody?

I mean sure, there could be the above difficulties - but short of a divine miracle, what could possibly stop (for example) a determined psychopath from doing everything possible to kill his own father?
• #### Re:H'uh? (Score:5, Funny)

on Friday June 17, 2005 @07:34PM (#12848107)
"Anyone else having difficulty imagining a scenario where it would be "impossible" to kill somebody?"

• #### So basically... (Score:2)

If I went back in time, and killed my mum, I wouldn't have been born to go back in time to kill my mum?
So she wouldn't have died, leaving her to eventually give birth to me, for me to go back in time and kill her, preventing her from getting pregnant with me, meaning I would never have existed to go back and kill her?

Where's the paracetamol, mum?
• #### what is _wrong_ with this reporter?! (Score:4, Insightful)

on Friday June 17, 2005 @07:29PM (#12848058)
Clearly, the present never is changed by mischievous time-travellers: people don't suddenly fade into the ether because a rerun of events has prevented their births - that much is obvious.

So either time travel is not possible, or something is actually acting to prevent any backward movement from changing the present.

So let me get this straight, BBC reporter. Your proof that time can't be changed, is simply that you don't remember it happening?

There are just so many flaws in that reasoning. ;)

First, time changes could be happening everywhere, but perhaps you have not witnessed one. Wait! How about this? How about time changing, and altering your memory at the same time?

What's the matter with you? Do you believe that it is impossible for something to occur, without you being aware of it?

Is this a God complex?

What unmitigated self-importance, BBC reporter!

Now sure, I know this reporter was likely trying to parse some marlaky that they were told, but this has to be the worst use of logic I have seen.
• #### Terminator (Score:2)

So, I guess this makes the Terminator Series pretty much infinite then, since basically they've been trying the grandfather paradox against SkyNet all these years.

That and John Conner will apparently never die, at least from a time traveling robot.
• #### Stupid machines! (Score:2)

I knew those Terminator's would never kill John Connor or his mother.

Humans rule!
• #### Information cannot travel back in time? (Score:2)

I'm quite sure someone like Hawking will soon step in and say that though time travel of something is theoretically possible, that no intelligent being would be able to make the trip successfully because no information would be able to travel back in time.
• #### One sperm in a million (Score:5, Interesting)

<farrellj&gmail,com> on Friday June 17, 2005 @07:32PM (#12848081) Homepage
The one thing that always bothered me about those time travel movies (besides the ridiculous timetravel part) like "Back To the Future", is that you wouldn't have to go to extremes to prevent your birth. All you would have to do is bump into your Mom or Dad to delay them for 1 second; that slight change in the timeline would guarantee that it would be a different sperm that won the race to impregnate your mom.
• #### Re:One sperm in a million (Score:5, Funny)

on Friday June 17, 2005 @07:48PM (#12848211)
Hmm, how long have you been fantazing about bumping into your Mom so that a different sperm would impregnate her?
• #### Re:One sperm in a million (Score:5, Funny)

on Friday June 17, 2005 @07:50PM (#12848224) Homepage
I really could have gone without hearing the phrase "impregnate your mom" this evening.
• #### Sounds like double-talk to me. (Score:2)

"Before something has actually been observed, there are a number of possibilities regarding its state. But once its state has been measured those possibilities shrink to one - uncertainty is eliminated."

There must be more here than it sounds, as this sounds like "once something has occurred, it has occurred, not something else." Well, duh. I didn't know that.
But it really proves nothing. I mean, if you go back in time and kill your grandparents, then you would never have observed their existence, so

• #### So wait, that means... (Score:2)

That means, the Roadrunner is the Cayote's daddy, Tweety bird is Sylvester's pappa, and GI Joe and Cobra are the world's most incestous family?

Ryan Fenton
• #### Lame! (Score:5, Insightful)

<brandioch.conner@gmail.com> on Friday June 17, 2005 @07:32PM (#12848085)
Clearly, the present never is changed by mischievous time-travellers: people don't suddenly fade into the ether because a rerun of events has prevented their births - that much is obvious.
Ummm, if you prevented their births, they wouldn't exist to "fade into the ether".

It could be happening all the time and you wouldn't be aware of it (by definition).
And now a team of physicists from the US and Austria says this situation can only be the case if there are physical constraints acting to protect the present from changes in the past.
Sounds like bad fiction to me.
The researchers say these constraints exist because of the weird laws of quantum mechanics even though, traditionally, they don't account for a backwards movement in time.
Sounds like the techno-babble "justification" in the bad fiction.
So, if you know the present, you cannot change it.
And the easiest way to not change it is for time travel to be impossible.
If, for example, you know your father is alive today, the laws of the quantum universe state that there is no possibility of him being killed in the past.
If, for example, you knew a picture would be taken, you could reflect light from your body and appear in that picture, thereby altering the future.

So, travelling back in time, you cannot reflect light, and, by the same token, you cannot absorbe light.

And it just moves up from there for all other physical effects. Nothing touched, no air breathed, no light disturbed, nothing.

So, how would you even know you were in the past?
• #### Re:Lame! (Score:4, Informative)

on Friday June 17, 2005 @08:23PM (#12848443) Homepage
Look up "closed timelike loop."

Basically all this article is saying is that all time travel must consist of closed timelike loops. That is, you "fulfill" the present, rather than altering it. This isn't news - it's the only kind of timelike loop that can exist in GR anyway. The difference here is that quantum mechanics also forces them to be the only ones that exist.

Point of note, however: as far as I know, we don't actually have the math to deal with the formation of a topological change in a surface (i.e., the "alteration" of a timeline). This is very much akin to a wave crashing - fluid dynamics works up until the exact point when the top of the wave touches the rest of the ocean. After that point, the math breaks down. So it's a little difficult to say "X isn't possible, because the math won't allow it" when theorists are in fact only using math that won't allow it. So it's moderately circular. That's GR. In QM, we don't actually have the math that deals with the collapse of the wavefunction (the 'measurement'), and so again, it's moderately circular. If you instead suppose that the wavefunction doesn't actually collapse, then of course you can change the past - you just end up following a different course in probability the second time around.

Examples of closed timelike loops actually are more common than you think in modern scifi/fantasy. Harry Potter and the Prisoner of Azkaban had an excellent example of a CTL, and the timetravel used in Anne McCaffrey's Pern series are entirely CTLs. This leads to statements like "I know I can do this, because I've done it already."

The problem with CTLs is that they muck with certain people's belief in free will.

If, for example, you knew a picture would be taken, you could reflect light from your body and appear in that picture, thereby altering the future.

Assuming you didn't exist in the picture before you went back in time.

And it just moves up from there for all other physical effects. Nothing touched, no air breathed, no light disturbed, nothing.

Unless it was already disturbed to begin with.

Again, there's no real logical problem here. Just the fact that you would have to disassociate yourself from the fact that all of your future actions are possible.

I don't necessarily agree with time travel. A closed timelike loop is essentially the equivalent of a monkey popping out of thin air, and then disappearing a few seconds later. It seems idiotic, and completely counter to all natural laws. But that wouldn't be the first time nature did that to us.
• #### Re:Lame! (Score:3, Interesting)

Nah... it gets better than that. Unlike what we see in Doctor Who and such, time travel would be a VERY dangerous thing.

For example... if you try to assasinate your mother before you are conceived then since time is self consistent something will prevent you. That is what this theory is saying. Since you have observed the state everything in known in that state will be self consistent.

So... since we know that we can't change any part of the past that we know lets create a scenario.

You go into the past t
• #### Dupe!!! (Score:2)

Just kidding. I was just traveling back in time to say hi to gramps when I decided to see how the "old" internet was working. let me tell you about the future:
- still lots of spam
- geeks still not getting laid
- cmdrtaco promoted to admiral taco.
• #### Two simple rules... (Score:2, Interesting)

To avoid contradictions in time travel, two simple rules must apply:

1) You can observe, but not alter the past.
2) You can alter, but not observe the future.
• #### Time travel violates causality (Score:2)

By necessity, since traveling backwards in time causes effects to occur before their cause. The universe stops making sense when causality is violated because causality is what makes the universe make sense in the first place.

Anyway, I think the authors may have seen this movie [imdb.com]. The idea is certainly not new.
• #### Predicted by GURPS and Chrono Trigger (Score:3, Interesting)

on Friday June 17, 2005 @07:42PM (#12848161)
This is pretty nifty. It reminds me of the old GURPS [wikipedia.org] time-travel rules supplement. Under those rules, time travel was possible, but it was not possible to change the past in a way inconsistent with your knowledge of the future -- the Game Master was instructed to thwart any such attempts by any means necessary, however unlikely. So, an organization of bad guys might try to take over ancient, remote civilizations where doing so would leave no evidence surviving into the present, while the good guys would go around recording as much information about history as possible in order to fix it in place, protecting it from the bad guys.

If you saw your buddy killed before your eyes, you would leave the scene immediately, and avoid examining the body in any way. Instead, you'd go get a dummy that looks like your buddy, then return to the time just before your buddy died, rescue him, and leave the dummy behind to "fool" your past self. I was delighted later on to see that in the game Chrono Trigger [wikipedia.org] it was possible to use exactly this mechanism to save the life of one of the characters in spite of their onscreen "death".

• #### Laws of physics are time symmetric (Score:5, Interesting)

on Friday June 17, 2005 @07:49PM (#12848215)
Something I don't think a lot of people really grok is that the laws of physics are time symmetric (actually the full symmetry is CPT, charge+parity+time, an electron going back in time would be a positron for example) so the fundamental weirdness is why we perceive time to flow in one direction in the first place. That's why I've always loved Feynman's absorber theory and it's associated spin-off the transactional interpretation of quantum mechanics. Those theories don't discard the so-called "advanced" (that is, backwards in time)solutions and work out how in a universe with appropriate boundary conditions you get an arrow of time. The advanced solutions actually exist but because of the boundary conditions they cancel each other out except where they "count". So according to the theory, when you go to push an electron every other particle in the universe sends waves back in time in response to push back on the electron at the exact instant you push it! The advanced waves only manifest themselves as the normal radiation resistance we observe when accelerating charged particles. The transactional interpretation takes this line of thinking with regards to the collaspe of the wave function. When one particle of a two particle entangled system wave function collapses it sends an advance wave back in time to collapse the wave function of the other particle. So in the EPR experiment there is no instanteous "spooky action at a distance" but travel exactly at the speed of light but in the opposite direction in time.
• #### Discovered Simultaneously At All Times (Score:3, Interesting)

on Friday June 17, 2005 @08:02PM (#12848318)
The thing about time travel is that by nature, it has to be discovered simultaneously at all points in time. So, if we're not doing time travel right now, we never will. (At least random-access time travel. Someone might come up with a short term 'rewind' ala Superman or "Prince of Persia.")

The reason is...as soon as the first time machine is invented, then everyone from the future will jump back into the past and invent it first.
• #### Uva Jed (Score:4, Funny)

on Friday June 17, 2005 @08:38PM (#12848526) Homepage Journal
This story is a dup. The original will be published next week.
• #### Unintentional movement (Score:3, Interesting)

on Friday June 17, 2005 @10:58PM (#12849137)
If we arbitrarly set time as a 4th dimension, which encompasses some arbitrary number of 3 dimensional states, space, then can't probability be a 5th dimension that contains all the different possible timelines?

We unintentionally move forward through the 4th dimension of time right now. Let's say we can move through time freely with a time machine, but by doing so there is an unintentional movement 5th dimensionally through possibility.

We see no time travelers because in our timeline the time machine is never created, but we might eventually create one, but every time we go back in time with it, we travel unintentionally through probability and there's probably already a bunch of time travellers there, we can't ever go back to our own original histories.
• #### So? (Score:3, Insightful)

on Saturday June 18, 2005 @12:37AM (#12849471) Homepage
We, existing in this universe, are on a crash course toward the future.

There's no stopping it or slowing down of time, in the traditional sense. However, it might be possible (with the help of absolute zero) to stop all things in the area of the absolute zero. This would be akin to stopping time, as nothing could be happening within that area.

Unbaking a cake or uncracking an egg is a good example of going back in time. Hey, if you can take a fully-baked cake, reverse the steps, and make it back into cake-mix-egg-and-milk-inna-bowl, that's good enough time travel for me.

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