Reverse Time Could Explain Dark Matter

idot writes "According to Lawrence Schulman of Clarkson University, who will get his work published in the Physical Review Letters, the universe could contain reverse-time regions. The article from New Scientist says that this phenomenon could explain the yet mysterious dark matter. " The reverse time regions can help explain the "dark matter" problem because, as potential relics from the future, stars could have re-ignited under The Big Crunch and while we wouldn't see them, we would feel their gravity. Needless to say, more details will be needed then this small article.

Comments

New Scientist Link

[headshrinker]

Seeing as there wasn't a link - for more info, look at http://www.newscientist.com /ns/19991127/newsstory3.html

a bit vacuous?

[Noer]

This seems kind of vacuous... it's what I hate about New Scientist... it often tries to avoid any details that might ruin a sensational piece. No mechanism or evidence or theoretical reasoning is proposed here.... just the laws as we currently understand them don't prohibit this sort of thing.

That said... could this reverse time be like a reflection of a wave in time (rather than in space) off the big crunch? does all time just reverse, and thus there'd be a big crunch even if omega >= 1 and the universe is flat or open? Hmmm...

This is interesting.

[Millennium]

Does this mean that, the problems with getting there notwithstanding, I could hop into one of these reverse-time zones, hang around for a while, and come out earlier than when I went in? The implications of that are hardly trivial...

Re:This is interesting.

[Millennium]

Ah, except that, if I'm reading this right, you would still percieve time as going forward in a reverse-time zone. The only difference is that time flows in the opposite "direction."

You know, I just thought of something. "Normal" time starts from the beginning and works forward. Reverse time, it would seem, must start from the end and work back, flowing at the same rate.

What happens when two adjacent but opposite-flow time zones hit the exact same moment, then? They have to at some point, after all; it's a case of meeting in the middle.

mean this does what?

[pen]

I have often thought about time travel, but I have come to the conclusion that even if it were possible, it wouldn't lead anywhere. Here's an example:

1. I know how to travel through time.
2. My friend gets hit by a car.
3. I travel back in time and save him.
4. My friend doesn't get hit by a car.
5. I don't travel back and save him.
6. My friend gets hit by a car.

Re:mean this does what?

[mmontour]

A couple of books to read:

Kip Thorne's "Black Holes and Time Warps", which simplifies the discussion by removing the issue of free will. He describes situations where a billiard ball is sent back in time through a wormhole, coming out on a trajectory where it deflects itself from entering the wormhole in the first place. This is a simple enough problem that it can be solved mathematically.

Robert L. Forward's "Timemaster ", which (without discussion of its literary merits) contains an interesting bit where the protagonist figures out a way to save his family from harm by filtering the information going back in time.

The key seems to be that whatever happens, must be self-consistent. So, Forward's version of your scenario might go like:

1. I know how to travel through time
2. A third person says "your friend and a car were involved in an incident at [place] and [time]"
3. I travel back to that place and time, and pull my friend back just before the car hits him, but it still runs over his shopping bag.
4. The third person reports that "your friend and a car were involved in an incident at [place] and [time]".
5. I travel back [....]

In this scenario, I do not change any known [to me] facts about the universe. However, I do ensure that any ambiguous situations are resolved in my favour.

Re:mean this does what?

[ralphclark]

This is the "many worlds" or "many histories" interpretation of quantum mechanics.

If this interpretation is correct, (and if travel backwards in time is possible) then all it means is that the time traveller is then forced to live out the rest of his life in the parallel reality ensuing from his intervention.

The reality he comes from, however, still exists as do all other possible realities. So the time traveller is unable to actually change anything; all he can do is pick a different timeline for himself to live in. And even then, other versions of himself will take different actions with varying degrees of success.

The many histories theory quite effectively disposes with the entire concept of free will because in that theory each individual chooses every possible option at each instant.

Consciousness is not what it thinks it is
Thought exists only as an abstraction

Re:mean this does what?

[plunge]

I always wondered about Dean R. Koontz's "Lightning." In it, a Nazi travels FORWARD in time, then comes back with a bunch of future weapons and knowledge. This actually seems much more plausible. Any ideas? Complaints?

Re:mean this does what?

[PurpleBob]

You know, that causes a similar paradox. For time travel to be invented, there had to be a time before that (since the Beginning, as you put it) when time travel hasn't been invented. Finally, someone invents time travel, and the invention leaks out into the past, so suddenly everyone can travel through time, and have been able to since the Beginning (define Beginning here as the first time that there's people worth teaching how to travel through time), so there's no reason to invent time travel...

There are two ways I can think of that this paradox would be resolved. The boring one is that time travel is impossible, and therefore time paradoxes don't happen.

The other is the "time threads" theory that other posters here have mentioned. Someone invents time travel, someone goes to the past to give time travel machines to everyone, and that causes a huge fork in the timeline. In the timeline where time travel was invented, life goes on, and it goes on forward. Meanwhile (if such a word applies), in the timeline where the past was given time travel, the people start splitting off all kinds of timelines and things get weird.

There probably wouldn't be too many splits in the "original" timeline. The fact that anyone who traveled through time never came back would be enough to put people off of time travel.

--

Pointer to Phys Rev article

[StupendousMan]

You can find a postscript version of the article on the astro-ph preprint server:

http://xxx.lanl.gov/abs/cond-mat/9911101

I don't understand the physics ... but I find the idea that local dark matter (yes, within our own galaxy) may be remnants of stars from a far-future universe very unlikely.

gniht hcus oN

[PhilHibbs]

.emit esrever ni eveileb t'nod I !hsibbur fo daol a tahW

Let me see...

[jd]

1. Time Travel is theoretically possible, BUT has some severe limitations (such as not being able to preceed itself).
2. An antiparticle travelling forwards in time is mathematically similar to a particle travelling backwards.
3. NEVER, EVER add more entities than is needed to explain a phenomina, completely. Even if you're a physicist.
4. Any interaction between a forward-moving and backward-moving "universe" would be instantaneous as far as the occupants were concerned, and therefore completely impossible to detect.
5. String Theory largely negates the need for any "Dark Matter", which was only a fudge factor anyway.
6. A fudge can never justify a fudge. Unless it's the edible kind, in which case, fudge always justifies as much fudge as is present.

Re:Let me see...

[jd]

Let's see if I can explain the basis behind Occam's Theory.

Ok, try this. Say you notice that apples fall off trees. Now, let's say there are three theories as to why this happens. Let's call these the "Murple" theory, the "Fzoom" theory and the "Quirp" theory.

Now, according to the "Murple" theory, Apples are influenced by the mass of spaghetti in nearby houses, the phase of the moon, and the sunlight, moonlight and starlight pushing down on it.

The "Fzoom" theory, on the other hand, speculates on the existance of a force that pulls the Apple down off the tree, emanating from the planet as a whole.

Finally, the "Quirp" theory predicts that Apples are affected by the same force as predicted by the "Fzoom" theory, but also adds in the effect of Maple Syrup and roast marshmallows.

Let's start with the first two theories. Occam's Razor predicts that the simpler theory that explains the phenonima is the most likely to be correct.

Yes, it's possible to do experiments to filter out the various sources of light, but let's say that we can't do that, for some reason. (Not all theories can be tested that way, such as the reverse time one.) Where does that get us?

Well, according to Occam's Razor, the second theory adequately explains the phenomina, in a simpler form, and so is the more likely to be true. Note that it does NOT say "it IS true". It is an expression of liklihood, not certainty.

Now, let's take the second and third cases. This reflects the case of multiplying entities. Here, the mysterious force accounts for all the behaviour of the apple. The effects of the Maple Syrup and roast marshmallows are, therefore, zero and can be omitted.

This is how additional entities can often be detected. The more you observe them, the smaller they get, until they vanish entirely. In the meantime, they've obscured important relationships and hidden details. That's why they should never be added, until you're certain there's something that simply CAN'T be accounted for with the existing model.

(A case in point is the theory of gravity, where the fact that the strength of a gravitational field at a distance N from a mass is proportional to the area of a sphere of radius N is not shown in the classic equations. Constants have been randomly folded into each other, in a hodge-podge that would win an award for obscure logic today.)

Re:Let me see...

[Otto]

Amazingly enough, you're still wrong, but it's not your fault.

Occam's razor doesn't mean quite what you think it does.

Imagine an apple fell on your head. Theory A is that the Earth as a mass created some force which acted on the apple. Theory B suggests that the direction in which the apple grew and the force with which the wind was blowing cause it to happen.

You're not understanding the fundamental concept. Occam's razor is to pick the simpler "method" in all cases. The whole "entities" thing seems to be confusing you.

Given your example, the easiest explanation is "things fall down". But why do things fall down? Well, loads of possibilities there..

Which is simpler? "A force exists that makes two object attract each other," or "thousands of invisible fairies are pushing the apple really hard"?

The force idea assumes that there's something fundamental going on that you don't understand. There's plenty of evidence for that. The fairy idea assumes that there's a hell of a lot more going on than you understand, and probably more than you wanted to know about as well. :-) Now instead of explaining gravity, you must explain where all these damn fairies came from.

Occam's razor says, basically, to never explain a phenomena with another phenomena for which you also have no explanation. That is what is meant by "Don't unnecessarily multiply entities."


---

Re:Let me see...

[Otto]

OK, we're getting somewehre. But what do those two statements have to do with each other? The first sentence makes sense. The second however, may sometimes be an outcome, yet it certainly should not be mentioned as part of the rule.

Again, you make no sense. What two statements are you talking about? Use quotes, man...

I guess, you state that the second sentence you quoted should not be part of the rule. I fail to understand you. That IS the rule.

Lets use a quote here.. Hmm.. Quick web search reveals.. well, quite a lot really.. Ahh, this one's good:

Occam's (or Ockham's) razor is a principle attributed to the 14th century logician and Franciscan friar; William of Occam. Ockham was the village in the English county of Surrey where he was born.
The principle states that "Entities should not be multiplied unnecessarily." Sometimes it is quoted in one of its original Latin forms to give it an air of authenticity.
"Pluralitas non est ponenda sine neccesitate"
"Frustra fit per plura quod potest fieri per pauciora"
"Entia non sunt multiplicanda praeter necessitatem"
In fact, only the first two of these forms appear in his surviving works and the third was written by a later scholar. William used the principle to justify many conclusions including the statement that "God's existence can not be deduced by reason alone." That one didn't make him very popular with the Pope.

Okay, I misquoted Occam's razor a bit in my original post. Still, not bad.

Anyway, be a bit more descriptive in your questions, and I'll try to be more helpful in my answers. :-)

---

Re:Let me see...

[Otto]

The "Don't unnecessarily multiply entities." is what I disagree with. If it truly is uneccesary, then the statement is obvious. Otherwise, it only makes sense when it is common sense. Or at least that is how I understood your original explanation.

Okay, sure then.

Occam's razor IS "Don't unnecessarily multiply entities." And it cannot be used for a proof.

It is a guideline, not a rule as such. The basic meaning behind it says that given two arguements, the simpler one usually is correct. The "usually" is the key. It may not be correct. Gravity may indeed be caused by invisible fairies. Who knows?

Occam's razor is a pretty good argument against Creationism, but not a great one. God created the Universe. Who created god? Nobody, he's always existed. Then why do you need god to explain the existance of the Universe? Why can't the Universe have always existed? The thing Occam's razor shows is that god is unneccesary to explain the existance of the universe. It says nothing about whether god really exists or not. Mainly, I've used this argument to shut up those idiots who try to force the burden of proof of the nonexistance of god onto me. Usually, I can force them to try to prove god exists (you can't, it's not possible) since they claim more "entities" than I do. I claim the Universe exists. They claim that the Universe exists and god created it. Thus they have the burden of proof.

Anyway, it's all that type of thing. Occam's razor, in a sense, _IS_ common sense, but if there's one thing I know to be fact in this entire world, it's that common sense is anything but common. I refer you to my favorite Heinlein quote in my sig...

---

Re:Let me see...

[Otto]

I disagree that you cannot prove that there is a god in the world. I believe that I can prove it. It does border, however, on what your understanding of time is.

Bunk. Of course you cannot prove god exists. God does not interact with the universe in any sense of the word interact. If I were to witness a miracle, it would not prove that god exists, it would tell me that I don't know everything there is to know about reality (which is true). Even if you could point out god, how could you prove that that _IS_ god, and not some other figure? Could be some Hindu deity in another form. I mean, really.


---

But will there be a big crunch ??

[Manifest]

As said in the article(New Scientist),the reverse time region needs an opposite of the Big Bang called the Big Crunch. But will a Big Crunch occur ?? Out of the 3 models by of non-static Universe, Big Crunch is the end-effect of just one of the models, the other being Ever Expanding and just exanding enough to avoid a Big Crunch.

So questions arises, will there be a Big Crunch ??

Manifest

Re:But will there be a big crunch ??

[Wah]

Yes, on Oct 7, 15,456,239,683 at approximately 7:45 in the a.m.

Re:But will there be a big crunch ??

[Robert Link]

A "Big Crunch" seems unlikely given our current knowledge of astrophysics. There are several factors that would seem to argue against an eventual recollapse of the universe. The first is that if we total up all of the matter that we "see", including dark matter detected only through its gravitational effect, we only find about 20% of the critical mass density needed to close the universe. It would be a bit surprising to find that there was another 80% of critical density lurking, undetected even through its gravitational effects.

Another factor that argues against a recollapse is that the time scale test heavily favors open (i.e. expanding forever) models. Basically, for a fixed Hubble constant, closed models are younger than open models. Since the age of the universe derived from the expansion is just barely consistent with globular cluster ages for open models, closed models are real losers unless there is some serious revision in the ages of globular clusters.

Finally, recent measurements suggest that there is a nonzero "cosmological constant", an extra term in the General Relativity field equations that acts as a repulsive counter to gravity. What's more, the measurements seem to indicate that the cosmological constant is not only nonzero, but in fact dominant at the current epoch. If true, and if the cosmological "constant" really is constant (it is typically assumed to be constant with time, but that is not a theoretical necessity) then this would preclude a recollapse, irrespective of the mass density in the universe. (Recollapsing models with positive cosmological constant are possible, but only if the big crunch occurs before the cosmological constant term becomes dominant.)

Re:Cool, so now I can go back in time?

[Lord Kano]

1) Splurge on surveillance equipment and tail Bill Clinton for 18 months circa 1990.

2) Swing past 1992 and warn the "1992 me" not to go out with that bigtime whore that I dated back then.(Yes Jamie, I'm talking about YOU)

3) Win every lottery in every state from 1993-Present.

4) Buy 10000 high capacity magazines for popular firarms before the 1994 ban.

5) Sell those magazines for 1000% profit and dump the proceeds plus my billions from the lottery winnings into the RedHat IPO.

6) Sell my RedHat stock for a 1000% profit and buy 51% of M$ stock and FIRE EVERYBODY, and release the source code to every M$ app ever made ON THE DAY that M$ is found to be a monopoly.

7) Give a copy of the current kernel source to Linus back in 1993.

8) Give a copy of the Colt 1911 and Browning High Power to John Browning in 1890.

I'd die of old age before I finished doing the things that I think should be done to improve things.

LK

Incarnation of Immortality

[Wah]

form Piers Anthony has a character than does this.

"Bearing an Hourglass" IIRC.

Basically all the gods of yore are roles taken over by mortals of various means (Death, War, Nature, Time, Fate).

While the Time guy does have the power to stop time he has the curse of having to live backwards, effectively starting at his current age and continuing until he turns into a baby then disappears. The book actually had Satan lead him to an anti-matter field where he could live his life "normally". Some light fiction.

Regardless, I think it quite likely that such matter exists. Remember the Universe is a very big, very old place, all sort of strange things have happened (cough*life*cough) and most anything is possible, so...

Pun Foul!

[SEWilco]

"more details will be needed then this small article"

Please use time puns which do not resemble spelling errors. Then we'll know when to complain about that there than then confusion which thou throw.

Get that cheese to sick-bay!

[jabber]

You know, when science starts sounding like Star Trek, it's time to re-evaluate your assumptions.

I'm all for science in sci-fi, but this sounds like too much fi in the sci... Only Star Trek resorts to the time-travel deus ex machina to make for an interesting show.

Yes, we live in a wonderous and amazing Universe. Yes, technology sufficiently advanced is indistinguishable from magic. But trying to explain a speculative theory with an even more speculative theory is unscientific in the extreme.

Transmeta using alien tech makes for a great joke. Matter traveling backwards thru time?? Please!

Braneworld better explanation

[scaryjohn]

Just based on the relative simplicities (and i mean relative), i think the darkmatter theory de jour of the "Manyfold Univerise Theory" preprint (posted: Roblimo, 19-Nov) had a much better explanation.

Refresher: that there are other dimensions that don't interact with ours, but whose matter still has mass and generates gravity that we can feel, thus causing the dark matter phenomenon

Even if they figured out that reverse time won't mathematically / quantumly cause the hypothetical annilhation of the universe, there are just too many doors it opens. (now just watch it proved right...) We also can't disregard more conventional theories for dark matter: Black Holes and the possible mass of neutrinos.

With regard to all the discussion of this explaining antimatter:
Antimatter, at least at a surface level is matter that has an opposite spin and charge of its corresponding normal particle... there was nothing in the article to make me think that reverse time has anything to do with it.

from the article:
Although Schulman has shown that a reverse-time region is not destroyed by interactions with a region of normal time,

Matter and antimatter is anhillated to their relativistic particle energies (E=mc^2 and all) when it hits its a particle of its counter-type (e.g. electron and positron).

All the same, it's an interesting read. Just wish I had the time and the physics bkgd to read the final article when it came out.
__

alt.geek

Re:Solution

[irqzero]

Another problem with time machines that no one seems to think of, is the motion in the universe. I.E. 1. Friend gets hit by a car. 2. You travel back in time with the good intention of saving him. 3. You're both dead. He's a smear on the road, and you are in the vacuum of space, because the earth is not where you left it when you went back in time.

New physics and the dark matter problem.

[Robert Link]

You make an good point. It is always possible to invent new physics to explain any new phenomenon you happen to encounter. However, this is equivalent to admitting that we know nothing about how the universe works, and that we never can know anything about how it works, since you never know what new phenomenon may crop up next; thus, your theories are incapable of making any predictions. It seems clear that whenever possible we should explain new phenomena in terms of old physics.

In the case of the dark matter problem, however, it seems likely that new physics will be required no matter what we do. The reason why is that if there is a whole passel of dark matter lurking in the haloes of galaxies, it can't be made up of baryons (i.e. protons and neutrons). In order for primordial nucleosynthesis to produce the present-day abundances of light elements, there can be no more than about 10% of critical density in baryons; whereas, the amount of dark matter required to explain the observations is around 20%. In fact, all of the types of matter that we know to exist are unsuitable, for one reason or another, as candidates for dark matter. Thus, if the dark matter exists it has to be some sort of "exotic" matter, which is a little troubling.

So, the question is, which sort of new physics that can explain is the least odious. Besides the dark matter, the contenders are modified Newtonian dynamics ("MOND", in essence a small correction to Newtonian gravitation at low densities), and (I guess) this time-reversed theory. Most physicists find exotic matter to be the lesser of the evils. I don't think this paper will change that. The new theory is sufficiently implausible that it will have to make some pretty strong predictions and have them borne out before anyone will (or, IMO, should) take it seriously. Interestingly enough, MOND has gained a small following because it has made some interesting predictions that have been borne out by observations. Unfortunately I can't recall off the top of my head exactly what they were (a speaker mentioned them as a throwaway comment in a talk here a few weeks ago), but they were the sort of predictions on which the standard model is mute; that is, it neither predicts nor forbids the phenomena that were observed. Unfortunately, nobody has been able to come up with a MOND prediction that would be forbidden by the standard model, so as yet the theory is purely speculative.

If I had to weigh in on the matter I'd say that dark matter is still the best game in town. There are several high-energy physics theories extant that predict an assortment of exotic matter, so there is at least some precedent for dark matter, which is more than the alternatives can say. The smart money is usually on the new theories that bear some resemblance to--or, better still, are incremental refinements of--the old theories; although, that's not to say we shouldn't reserve a small wager for the oddball theories; just don't stake the rent money on it.

-r

patent it

[/]

Clearly what this physicist should do is file a patent for the "reignition of stellar clouds during massive space-time collapse". Since intellectual property refuses to die, it will likely still apply when the Big Crunch occurs, and he'll file for a federal injunction. The result will be retroactive, and dark matter will cease to bother us in our own time. Hooray.

The source leaves a lot to be desired

[Masloki]

Other articles from the current New Scientist:

Sex is good for athletes before the game
An ice age is coming to Europe
Bringing dinosaurs back to life
The Pill may lead to gum disease
Stress may protect your from loud noises.

I would not consider this publication a valid source for scientific news. In addition, they are publishing an article that has not been subjected to peer review. "Schulman's calculations will appear in a forthcoming issue of Physical Review Letters."

If anyone can find collaborating evidence, I would like to see.

My 2cints

Time Travel's Information Barrier

[Effugas]

People like to think of going back in time, mainly because of the psychologically tragic reality of 20/20 hindsight.

The problem is the complexity a backwards traversable universe creates.

For example, if one can travel back to 1950, that implies that, somewhere, somehow embedded in either the fabric of the unvierse or in the structure of subatomic particles is the exact memory of where and when everything and everyone was. Perhaps an infinitely growing thread(imagine a pencil leaving a trail as it moves over a paper...of course, some small chunk of the pencil is removed with each motion), or perhaps some kind of structural memory, but somewhere, the State Of What Was has to be preserved.

Sounds rather deterministic, in a universe that seems to have blurriness built into its very design.

Yours Truly,

Dan Kaminsky
DoxPara Research
http://www.doxpara.com

Re:Time Travel's Information Barrier

[Effugas]

There's no problem of "memory" when you go from your front door to your mailbox and back...that is, there's no reason to think the universe has trouble "remembering" the door is there, or the mailbox is there.

Problem: You just created an entirely deterministic universe. Everything that was(at least) or will ever be(at most) has already been predetermined in your picture of things.

Welcome to the land of PseudoEntropy, where particles appear to follow undefined patterns but in fact are all going according to plan?

A particle with intrinsic randomness can act without memory. A particle without intrinsic randomness has absolute memory, since all effects are determinable...but guess which one physics (and Heisenberg, who is abused on an daily basis) prefers. ;-)

Yours Truly,

Dan Kaminsky
DoxPara Research
http://www.doxpara.com

The big crunch

[heroine]

But isn't the universe that is reborn after the big crunch going to be exactly the same as ours and aren't we merely tied to the molecules of our brains, forever repeating our lives in the same point in time at the same place every time the universe restarts?

Scheduled for PRL Dec 27 issue

[apsmith]

See:

Opposite thermodynamic arrows of time
for the abstract of this paper about to be published. More articles scheduled for the same issue are available here.

Some interesting problems

[Ektanoor]

So this means that patents and copyrights laws are relatively useless. At least at a Universum dimension...

Ooooooh. I see the future claimers
Copyright 1999 Ektanoor All rights reserved for Past, Present and Future.

Re:Terminator and Skynet

[David Gould]

Wow, I never realized that was how SkyNet got built.

Not to flame you too hard, but how could you possibly have missed that? It was almost the entire point of the movie. Or did you only see Terminator and not Terminator 2: Judgement Day?

As I recall, it wasn't stated in the first movie -- it wasn't even known to the characters at the time, except for it being "some kind of defense computer network gone wild", and the entire goal was to save Sarah Connor's life, so John could be born and lead humanity to win the war against the robots, while the Terminator tried to kill her and prevent the above.

The second movie wouldn't have been very interesting, though, if it had just been a repeat of the same "one time traveler protects key person from other time traveler who is trying to change history" plot. It was much more, though: they tried to prevent SkyNet from ever existing, by taking out Dyson (its inventor). He reveals that they were making breakthroughs that they "would never have thought of" based on analyzing a brain chip salvaged from the first Terminator, so they go to blow up his entire company, destroying the chip, the lab, the research, etc.

To the other replies:

Try not to think about it too much. That's the best advice I have.

Come on! That's the most interesting part! Even if it makes your head hurt, you've got to want to think about these things.

It is only alluded to but if true does create a really foul time paradox.

I thought it was much more than "alluded to". I'll grant that they didn't really think through the paradoxes, but the fact that SkyNet was made possible by the brain chip was central to the reasons why they had to blow up the lab, and why Schwartzenegger had to be fried at the end (to destroy the last existing brain chip).

It's true that the paradoxes get pretty nasty: first, the premise has the problem that, if SkyNet's invention was only made possible by analysis of the Terminator's brain chip, then where did the technology "come from" in "the first place", i.e., how can a technology exist without ever having been invented?

Then, if you grant that it is somehow possible for SkyNet and the Terminators to have "created themselves" spontaneously, you get another problem: if they destroyed all the chips, preventing SkyNet from being created, then none of it could ever have happened -- Sarah Connor's life should just go back to the way it was before, with Kyle Reese and the first Terminator never even showing up (and, incidentally, John never being born). But then, if SkyNet is not actively prevented from creating itself, wouldn't it do so again...?

The thing is, these paradoxes sort of cancel each other out: if you reject the idea of SkyNet being a figment of its own imagination, i.e., conclude that, despite what Dyson and the second Terminator said[1], it would have been invented anyway as a result of good ole' human ingenuity, then their efforts to prevent it would have been in vain and everything would still happen exactly as Kyle Reese described it. That seems to be the only way for them to have their memories of the events, or for the events to have occurred at all. This is the "Red Queen's Race" (an Asimov story that refers to the bit in Alice in Wonderland where you run as fast as you can just to stay where you are) view on time travel, which is also the theme of 12 Monkeys -- you can't change history through time travel because anything you do "already happened", and was thus "taken into account", making your version of history a self-fulfilling prophecy.

Another problem is that their trying to prevent it all may not have been a very good idea, given how unpredictable the effects would be. In short, you don't mess with a winner. At least in Kyle Reese's version of the story, humanity won in the end, but if they changed things the wrong way, the war night have still happened but without the happy ending.

Or, for a sort of eerie reality tie-in, you could use the multiple time-lines view, where, up to the end, the movie occurred in a time-line where they succeeded in keeping John Connor alive, but failed to prevent the war, and that time-line branched off when they destroyed the chips, allowing our time-line (the real world) to exist with no Terminators at all.

--
[1] Maybe Skynet created false records indicating that Dyson was the inventor, when in fact it was someone else working independently, so that they would blow up the wrong lab.

David Gould

A little bit about time

[tomservo3000]

I'm reposting this so that hopefully it'll get seen a little better. Besides, it's not completely a direct response, mostly a discussion of some questions that come up about time travel. I made an addition also.

"For those interested it is the easiest way of getting time travel. One day you come home and find all the plans for a time machine - you build it and then return to leave the plans for yourself (the exact pieces of paper)"

Ok, first of all, I believe that this can't happen, because, quite frankly, knowledge can't create knowledge. You can't say that we learned time travel because our future selves told how to do it. How did they learn it? Let's say that they discovered it, say around 2500, and then went back in time and gave the blueprints to us, in 1999. This would certainly change what is the present for the time travelers (2500), since they had time travel capabilities since 1999, and didn't need to discover it in 2500. By giving us the blueprints, they affected their timeline, begining in 1999, all the way up to their present (2500).

But that's one possibility. If there is only 1 universe (no parallel universes), and our changing of the past affects the future, then we have serious problems. Many, many paradoxs can arise, such as the going-back-in-time-and-shooting-myself paradox that everyone loves.

Richard Feynman addressed this issue once by implying that one simply cannot change the past. The example he gives is of a time travler (or chrononaut? is that a word) who goes back 5 years in time and attemps to shoot her past self. However, she misses the heart, and the bullet hits her younger self in the shoulder. Why did she miss? Because her aim was affected by her shoulder - the time traveler was shot in the shoulder 5 years ago.

The other option is that there are parallel universes, and that possibly they are spawned for every possible action in the universe at any given time. If this were the case, and you went back in time and shot yourself, you simply would be dead in THAT universe. You're still alive and well in your own, even when you returned to your own time.

Just one more issue to address - one may say, "Hey, if you go back in time, say to when you were 10 (and you are 20), then aren't there two of you in the same universe? If you met your past self, wouldn't you faint or destroy the universe like in Back to the Future 2?" Well, i'd have to say no, you probably'd no neither. I don't think anything would happen, except for the fact that the present you would be staring into a 10 year young mirror. Also, keep in mind, you can't be in 2 places at once. Even as you are there, staring at your younger self, you are NOT where should should be (in the future). You are present in the past, yet missing in the future, even for only a split second, so it evens out. And remember, time is all relative (I hate time). So don't worry that your extra mass (that is in the universe at the time you're visiting your younger self) is going to cause a sudden cosmic crunch :-)

Btw, the above assuptions (the 20 year-old visiting his 10 year-old self) are made with the assumption that we're dealing with 1 universe here, as opposed to parallel universes.