Why Time Travel Won't Work (Sometimes)

[ADD Boy]

I was thinking about this the other day, and I'm very curious if anyone else has thought of this before me. Okay, here goes...

Say you have a person that uses a time machine to go back into time. What he does in the past is he kicks his father in the... family jewels, thus delaying him for a few minutes from arriving home to make love to his wife (okay, bear with me). After they make love, the wife conceives the time travelling person.

Therefore, assuming that sperm are randomly moving around inside of wherever they're stored inside your body prior to... well, you know (sorry, I don't remember much from biology :p ), that would mean that if there was a delay of several minutes between when you were supposed to be conceived, one sperm might come out closer to the egg that another sperm would. This would increase the chances of another sperm causing conception.

Therefore, if the baby that was going to be conceived that night was the person who went back in time, and the sperm that time traveller was conceived from does not make it to the egg, that would make the person never born. So therefore, that person would have never gone back in time to do that (because that person would not have existed) and so therefore, the person would exist, only it would have never happened!

Does this make sense to anyone else?

It seems to raise a lot of questions on free will... I mean, if you're not allowed to go back in time to kick your father in the nether regions, what else are we cosmically prevented from doing?

Furthermore, since there's still a chance that sperm might conceive the person, does that mean that we could only do things back in time sometimes? Like, there's a slight chance it would have never happened?

[Sundog]

Congratulations, you've discovered the causality loop, the basis for every time-travel story in the history of fiction.

No, no one else ever thought of this. You appear to be the first.

Just curious, can you factor polynomials?

[(S)]

This is essentially the classic time-traveler paradox, the time-traveler goes back in time and kills himself. If he's dead, he couldn't have killed himself. If he's not dead, he could go back in time and kill himself.

I myself prefer the 'no time-travelling tourists' proof. If time-travel is possible, it's not just possible a hundred years from now; it's already possible. Time travel wouldn't be invented and all of the sudden time-travelers start showing up; why weren't there time-travelers watching the Revolution or the atomic bomb tests. If even a few people each generation wanted to go back and see the Constitution signed, or hear Einstein's last words [which no one really knows], or whatever, 'extra people' would start piling up pretty quickly.

[ADD Boy]

Originally posted by Sundog

Just curious, can you factor polynomials?

Yep, really big ones :p

[ADD Boy]

Originally posted by (S)
I myself prefer the 'no time-travelling tourists' proof. If time-travel is possible, it's not just possible a hundred years from now; it's already possible. Time travel wouldn't be invented and all of the sudden time-travelers start showing up; why weren't there time-travelers watching the Revolution or the atomic bomb tests. If even a few people each generation wanted to go back and see the Constitution signed, or hear Einstein's last words [which no one really knows], or whatever, 'extra people' would start piling up pretty quickly.

True, although it may not be possible that we see time travellers tourists, due to the reasons I stated above. If we saw one, or if one bumped into us, there's always the chance it could affect history so drastically that they would have never gone back in time, or at least do

[TillEulenspiegel]

It has been thought of and explored very well, the premise is called a "Time Paradox". What that means is that you cannot go back in time and kill your grandfather because you would have never been born to go back...


Theres been an explanation by some of the worlds best Physicists have worked on , in terms of mathematics and the bottom line is that the universe does not allow certain behaviors such as faster then light travel and the grandfather paradox.

edit to add:
I know you didn't ask this but apparently Time flows only in one direction. That is forward, so the issue even considering a Time Paradox , means nothing since the real world does not allow us to travel backwards in time. That's all science fiction.

[Johnny Pneumatic]

Another question; how would travel to the past be possible?

[Rolfe]

Time travel seems to work just fine for me. I seem to be able to travel into the future at the rate of sixty minutes every hour, reliably and predictably. I could quite easily kill any number of relatives, if I chose.

Do I get the million? :D

(Apologies to C. S. Lewis, for shameless plagiarism of a passage in The Dark Tower.)

Rolfe.

[scribble]

I reada good book on this a while back... shame I can't remember the title. Author's name was uh... Rudy... uh, somthing. Okay, I read it in high school. Or maybe middle - that's not the point.

As I recally, Einstein's formulations make this sort of thing a *theoretical* possibility. If you can imagine an Einstein-Rosen bridge in space, then you can imagine the same in time, I've little doubt. After all, isn't it Einstein himself who equated the two?

Anyhow, enough rambling. Go play the new Prince of Persia game. If you like this kind of thinking, the plot of that game will leave you wondering for a good long time.

Or if you want something less involved, you could try Back to the Future I. Or was it II where they did this? Maybe it was III -- I forget.

I think I've heard some arguments that if you went back in time you couldn't possibly move fast enough to end up in a location you are familiar with space-wise. IE: if you went back to when your dad was alive you'd have to do some crazy stuff to even be in the vicinity of Earth.

Another argument has it that you'd be in an alternate timeline. I think this requires a funny interpretation of QM to be valid, however.

[Fendetestas]

How to build a time machine (http://www.sciam.com/article.cfm?articleID=0004226A-F77D-1D4A-90FB809EC5880000&sc=I100322)

[ADD Boy]

I know you didn't ask this but apparently Time flows only in one direction. That is forward, so the issue even considering a Time Paradox , means nothing since the real world does not allow us to travel backwards in time. That's all science fiction.

Oh I know :) I understand that we'd have to travel faster than light or make a wormhole through time to go back through time (although don't ask me to explain the specifics of those!)

[(S)]

Originally posted by Rolfe
I could quite easily kill any number of relatives, if I chose.

Do I get the million? :D

I'm sure one relative should be enough to qualify you, but they'll probably want you to off at least three for the million. Statistical chance, and all that.

[kittynh]

I can't remember which really big name scientists said that time travel isn't a possibility as we have never run into any time travellers in our history. What he was saying was that if it was possible, we would already have known about it.

Made sense to me.

[Archangel]

Originally posted by ADD Boy

Furthermore, since there's still a chance that sperm might conceive the person, does that mean that we could only do things back in time sometimes? Like, there's a slight chance it would have never happened?

Actually Parallel Universes would mean that there could be no paradox for instance:

In universe A) you build a time machine go back in time to the night of your conception kick your dad in the nuts etc. and go forward in time to find yourself not born (in your frame of reference) however what happened was that when you went back in time you actually exited Universe A) and entered Universe B).

So to inhabitants of a) it would seem that you were born, grew up, built a time machine and vanished never to be seen again, whilst in b) you were never born, suddenly appeared in one night of nut kicking insanity and then disappeared to reappear out of nowhere to live out your days.

As to time travelling tourists, given that they are of such an increased level of tech compared to us, how do we know that they dont have chameleon fields such as in Predator? we are already making strives towards active camouflage now, and if they are just viewing and not wanting to affect the time stream it makes sense to me (the assumption that they would visit different times with sentient beings and then make themselves visible seems odd to me).

Oh I know I understand that we'd have to travel faster than light or make a wormhole through time to go back through time (although don't ask me to explain the specifics of those!)

Another way suggested by a physicist on a show I saw recently, had him creating a tunnel of lasers which effectively bend the space around it creating a tunnel through which information would be able to be passed to the future or past (although not before the Machine was turned on or after it was turned off).

[TillEulenspiegel]

Yes the concepts of the E-R bridge and other constructs allow for so called "Time Travel" but it's all due to time dilation which means forward travel. The major scientists normally named are Hawking Thorne , Visser, Sagan,Gott and others.

Any time the discussion of time travel rears it's head theorists become silent because the general idea is viewed as crackpot or a science fiction escape mechanism at best. There is however a framework based on certain maths that describe time travel ( even backwards, according to Thorne, but later he discounted it ) is possible but the bottom line is causality.. You cannot travel back to a time before the machine you have built existed.

[shemp]

Tell you what kid, why don't you go kick your father in the nuts right now and see what happens?

[ADD Boy]

Originally posted by shemp
Tell you what kid, why don't you go kick your father in the nuts right now and see what happens?

I'd probably be hit so hard I'd go faster than light, and then you'd see me in paintings of the signing of the Declaration of Independence :D

Wouldn't that be cool... George Washington, Jefferson... and me, poking my head out from behind Ben Franklin :p

[EdipisReks]

i thought all you had to due was travel towards a star at a certain warp factor. i guess spock is a liar. but vulcans can't lie, right? :confused:

[Cecil]

I think the theory about "nature conspiring to make sure causality isn't broken" is a load of BS. It seems to presume some sort of supreme intelligence that watches everything all the time.

An idea I have found that resolves the issue in the OP is the following: The paradox apparently lies in the fact that by killing your father, you would never be born and therefore could not go back in time, and so could not have gone back in time. The solution comes by realizing that time is not like space. You do not have to have "left" sometime in order to "arrive" sometime else. Once you go back in time, you now exist in the past as an adult, never having been born. Here's an example:

Timeline A: Your father is born in 1950. You are born in 1980. In 2000 you invent a time machine and go back to 1960. At this point, time "starts over" from 1960, resulting in timeline B.

Timeline B: Your father is born in 1950. You spontaneously appear in 1960 in a time machine and kill a your father as a 10-year-old. Your father's child is therefore not born in 1980, and no time machine is invented in 2000. You die in your sleep in 2020, at the age of 80.

The paradox is gone. :D

Now, if you kill your father (in timeline B) and return to the year 2000 right away, we get timeline C.

Timeline C: Your father is born in 1950. "You" spontaneously appear in 1960, kill him, then disappear. Nothing relevant happens in 1980. In 2000, "you" spontaneously appear again in your time machine. You live to the ripe old age of 80 and die in 2060.

I realize that the idea of matter appearing out of nowhere and people/machines existing without ever having been born/invented is a strange concept to grasp, but it seems to work if you logically follow the timeline from start to end.

Oh, I just remembered. Here (http://www.mjyoung.net/time/index.htm) is an amazing site that analyzes all of this in depth and explains it better than I did. It's a fascinating read. :D

[iain]

In his book Flatterland, mathematician Ian Stewart goes into some detail about how, based on Hawking's work, it should be possible to construct a time machine by sticking black and white holes back to back. It's a bit mind bending, especially the bit about having a closed time loop where something just travels round and round without having existed in the past or ever existing in the future.

Stewart's take on the grandfather paradox is that it's not really worth worrying about unless time travel becomes possible, at which time the resolution will probably become obvious.

[davidhorman]

Perhaps going back and kicking dad in the nuts is exactly the event which caused you to be born in the first place, and not the other kid. I like to think that's it how it would work, anyway - that's the way things were and that's how they will always be. Even if you try and kill an ancestor, you would fail (for whatever reason). It does sound a bit like something conspiring to maintain the timeline, but perhaps it's only the same kind of thing that "conspires" to draw massive objects together.

Yes, I do have a Star Trek script in my head waiting to be written. Shame all the good ones have finished their run.

David

[richardm]

It seems likely to me that if a time-travel machine was ever invented, it would be (a) very expensive and (b) tightly controlled.

It could very well be that there were observers from the future at the atom bomb tests or various other important points in history, but they'd be well trained to blend in seamlessly. And you'd only need to do it once, or maybe twice.

Besides, for most historians it would be much less interesting to do that than to go and hang around ordinary people and ordinary events - those are the things for which records are sketchy, and it would be genuinely interesting to be able to "go back" and have a shufti. And again, you'd be one or two people nipping in and out of the past, and being well-trained. Not just your average tourist, in other words.

Certainly, it seems highly unlikely that anyone would be allowed to go back and start killing people! Although since that person would already have been killed in the history of the people who went back and killed him, then ...
... This is the point where smoke starts coming out of the back of the computer, and a metallic voice says "Does not compute! Does not compute!", right?

[Wile E. Coyote]

First, according to Star Trek, accidental time travel happens about once or twice a year. This kind of thing always gets the crew in some wacky situation.

Second, I find it odd that everyone seems to worry about people going back in time to kill ancestors. Sometimes I think they would be more likely to go back an knock off a couple of in-laws ...

I think the only backward time travel theory currently considered plausible is the one where a traveller can only go back in time as far the the point at which the time travel machine he is using was turned on. This would be some sort of wormhole thingy.

Of course, this is a horrible idea, because you would then have hundreds of people from the future streaming through the instant you turned it on.

[Mark]

Interesting psychological study: Why do all builders of time machines seem to hate their fathers and grandfathers so much?

What, suddenly killing your grandmother is off limits?

[kedo1981]

Here is an even simpler reason why time travel into the past is impossible; time does not exist, it is merely a label the human mind places on the order of events as they move through space.

[Sundog]

Originally posted by kedo1981
Here is an even simpler reason why time travel into the past is impossible; time does not exist, it is merely a label the human mind places on the order of events as they move through space.

Wow, that simplifies everything.

OK. How about order-of-event travel?

[TillEulenspiegel]

Cecil " I think the theory about "nature conspiring to make sure causality isn't broken" is a load of BS. It seems to presume some sort of supreme intelligence that watches everything all the time."
It's not conspiratorial at all, just one of the apparently unbreakable constraints of the universe we live in.

FTL is not conspiracy and requires no Omnipotent authority why do You think causality does?In fact the central thesis is accessible in plain English and is intuitive "Effect may not precede cause",not like QM which alto being counter-intuitive and having Gordian knots of logic, yet appears to describe the nature of our universe quite well.

[scribble]

Originally posted by kedo1981
Here is an even simpler reason why time travel into the past is impossible; time does not exist, it is merely a label the human mind places on the order of events as they move through space.

Hrm... interesting contention.

Would you also say it's impossible to build a "space machine" because space is merely a label the human mind places on the location of events as they move through time?

[Walter Wayne]

Originally posted by kittynh
I can't remember which really big name scientists said that time travel isn't a possibility as we have never run into any time travellers in our history. What he was saying was that if it was possible, we would already have known about it.

Made sense to me. Not that I think time-travel is necessarily possible, but there is one flaw with that argument.

What if the time machine must be at both the source and destination of the travel? How does a time travel machine get you to a point where the time travel doesn't exist. It could be that time travel is only possible back to the point when the time machine is invented.

Walt

[scribble]

Originally posted by Walter Wayne
What if the time machine must be at both the source and destination of the travel?

That seems like a big "what if" - I mean, when I drive to Detroit, I don't have to call 'em up first to be sure they've got a car, too, do I?

You're picturing something like a star-trek teleporter no doubt. Why couldn't a time machine be more like a car, hop in and go?

[Archangel]

Originally posted by Walter Wayne
What if the time machine must be at both the source and destination of the travel? How does a time travel machine get you to a point where the time travel doesn't exist. It could be that time travel is only possible back to the point when the time machine is invented.


Assuming that Time Travel is possible, it is probable that this will be the first(*) type that is invented, however once we have a time machine that works like this perfected of course we would try to build a T.A.R.D.I.S. style machine.

*Assuming that we dont want to count conventional rockets, sattelites and future spacecraft as time machines because of relativistic time travel.

[Mark]

Originally posted by scribble


Hrm... interesting contention.

Would you also say it's impossible to build a "space machine" because space is merely a label the human mind places on the location of events as they move through time?

But time as we perceive it is strictly a local phenomenon. At a quantum level, it doesn't exist at all...all "nows" are simultaneous.

[scribble]

Originally posted by Mark
But time as we perceive it is strictly a local phenomenon. At a quantum level, it doesn't exist at all...all "nows" are simultaneous.

It's not possible to differentiate T for T+1 when examining a quantum event? How would you know when the experiemnt was done?

[(S)]

Originally posted by Walter Wayne
Not that I think time-travel is necessarily possible, but there is one flaw with that argument.

What if the time machine must be at both the source and destination of the travel? How does a time travel machine get you to a point where the time travel doesn't exist. It could be that time travel is only possible back to the point when the time machine is invented.

Walt

This will make it readily apparent when you succeed in building a time machine: A gajillion people will suddenly burst through, wanting to buy all the stock they can.

[Mark]

Originally posted by scribble


It's not possible to differentiate T for T+1 when examining a quantum event? How would you know when the experiemnt was done?

Well, you are looking at 2 different things: at a quantum level, the experiment (if it ever happens) is already completed.

I was specifically addressing your---I think erroneous---assumption that time cannot be a local event/perception, because then macro "space" must be as well.

[CrossHair]

Originally posted by kedo1981
Here is an even simpler reason why time travel into the past is impossible; time does not exist, it is merely a label the human mind places on the order of events as they move through space.

I totally agree, however.... We observe time through our senses and by looking at distant gallaxies we percieve them as they were perhaps thousands or millions or billions of years ago. Therefore we are traveling back through time, not physically but still with one of our senses.

To physically travel through time would have to be in a space ship for this reason.... The earth and solar system are always in motion and we are not in the same location we used to be. To travel back in time from our present location would put us, most likely, into deep space because the earth and solar system are not here in the past. Therefore you had better be in a space ship to survive!

[Walter Wayne]

Originally posted by scribble


That seems like a big "what if" - I mean, when I drive to Detroit, I don't have to call 'em up first to be sure they've got a car, too, do I?

You're picturing something like a star-trek teleporter no doubt. Why couldn't a time machine be more like a car, hop in and go? Yes but you know that detroit has roads. And yes it is a big what if, but time travel is pretty much a big what if.

[Cecil]

Originally posted by TillEulenspiegel
Cecil " I think the theory about "nature conspiring to make sure causality isn't broken" is a load of BS. It seems to presume some sort of supreme intelligence that watches everything all the time."
It's not conspiratorial at all, just one of the apparently unbreakable constraints of the universe we live in.

FTL is not conspiracy and requires no Omnipotent authority why do You think causality does?In fact the central thesis is accessible in plain English and is intuitive "Effect may not precede cause",not like QM which alto being counter-intuitive and having Gordian knots of logic, yet appears to describe the nature of our universe quite well. Sorry for not being clear enough. The conspiratorial argument I was talking about was the one that goes something like, "I can travel back in time, but if I try to kill my grandfather the gun won't fire or I won't be able to get near him or he'll survive the 100-story fall, etc". I realize it's proposed because if you WERE able to kill him then you wouldn't have been born, so therefore your grandfather couldn't have died.

The flaw is that by travelling back to 1940 (eg) events which happened in 1980, like your birth, HAVE NOT HAPPENED anymore.

An analogy I like is that time is like a videotape that gets recorded as time passes. The tape is read-only before the current location and write-only after the current location. We have found the fastforward button (relativistic travel) but not the rewind button. If you jump back in time, it's like rewinding the videotape back and "starting over" from that point. Events which happened in the old timeline are not accessible anymore since the tape is write-only. If you then jump forward, you arrive where you started, but in the new timeline.

[ADD Boy]

Originally posted by CrossHair


I totally agree, however.... We observe time through our senses and by looking at distant gallaxies we percieve them as they were perhaps thousands or millions or billions of years ago. Therefore we are traveling back through time, not physically but still with one of our senses.

To physically travel through time would have to be in a space ship for this reason.... The earth and solar system are always in motion and we are not in the same location we used to be. To travel back in time from our present location would put us, most likely, into deep space because the earth and solar system are not here in the past. Therefore you had better be in a space ship to survive!

From what I read in "A Brief History of Time," I think a wormhole can tunnel through both space and time. So it's entirely possible to make the wormhole tunnel anywhere in time and space.

[iain]

Originally posted by ADD Boy


From what I read in "A Brief History of Time," I think a wormhole can tunnel through both space and time. So it's entirely possible to make the wormhole tunnel anywhere in time and space. Big jump there. Just because a wormhole can tunnel through time and space does not mean that it is possible to make one that does. Making a wormhole may require more energy and better technology than the human race is ever able to harness, not to mention getting through it in one piece could be a challenge.

[epepke]

Originally posted by TillEulenspiegel
I know you didn't ask this but apparently Time flows only in one direction. That is forward, so the issue even considering a Time Paradox , means nothing since the real world does not allow us to travel backwards in time. That's all science fiction.

This isn't so clear. With the exception of two kinds of meson (so far) the basic laws of physics appear to work just as well in both directions in time. This includes everything that is likely to matter to a human being who isn't studying mesons.

At the macroscopic level, the arrow of time appears to be a statistical property, described sometimes as an increase in entropy.

[davidhorman]

The flaw is that by travelling back to 1940 (eg) events which happened in 1980, like your birth, HAVE NOT HAPPENED anymore.

Why? Maybe your arrival in the 1940s was always one of the events that would conclude in 1980 with our birth.

Whether or not you could be your own grandpa is another matter.

David

[TillEulenspiegel]

Epepke
Well I did use qualifiers. There have been serious alternative takes on the time arrow. In QM in the many universe theory it has been postulated that by manipulating space-time ( warping it ) via a construct called a closed time-like curve (CTC) described by Gödel , it would be possible to "slide" sideways to an alternate universe at a time that occurred prior to the time in your normal universe, thereby excluding any grandfather paradox . Hawking believes that any CTC that exist would be extraordinarily short lived of would cease to be if one approached it

Richard Feynman theorized that in QED backward and forward time movement of particles and anti-particles is possible.

Tachyons are proposed particles that exhibit -t behavior . The original idea was evoked as a property of 10-dimensional string theory which has all but been dismissed altho there is a new reconsideration of this hypothetical beast as an adjunct of 11 dimensional string theory as an open string.

There are possibilities that these things may exist , but they appear to all be special cases and most physicists agree that any backward movement in time cannot transmit information ( and what is a human or a machine that got him there but information ) and that any construct used for time travel prohibits traveling to a point before the construct existed ( at least in the same universe).

Bottom line being for now special class HE physics can demonstrate -t behaviors, but the translation to macroscopic scales is something altogether different.


I'm sure I forgot to dot a few i's since this from memory.

[epepke]

Originally posted by TillEulenspiegel
Richard Feynman theorized that in QED backward and forward time movement of particles and anti-particles is possible.

Well, up until around 1958, if memory serves, it was believed that charge was a complete symmetry of the universe, which would mean that everything was reversible. Then weak beta decay was found to violate parity symmetry. Then sometime in the 60s (I think) k-mesons were discovered to violate time symmetry, so now we have to use CPT symmetry. This really bugged me for a long period of time, because I can't see how just one particle could do this. There's a sort of aesthetic to this stuff, and it always seemed to me that there should be an even number of them. Then someone, I forget who (maybe it was you), pointed out on this board that much more recently another meson has been found to violate T symmetry, which makes me feel a lot better.

CPT, or any group of three, forms a nice symmetry system, for some reason, and seems to crop up a lot.

Feynman's view was not precisely a belief, but an interpretation and popular description of CPT symmetry, that an antiparticle, which has CP reversed, in indistinguishable from its corresponding particle travelling backward through time. Of course, where one goes from there may involve beliefs. I find highly interesting his dodge for the backward photons from the Maxwell's equations, that essentually, every "backward" photon is involved in a virtual photon exchange with something in the past. Which got me speculating--it seems to imply that a photon has to hit something back in time, which is consistent with Big Bang ideas, because if you go back far enough in any direction, you hit something. So, does the fact that radio does work forward in time provide evidence that there will be no Big Crunch? Or is it a probabilistic thing, and at this time in the universe, the probabilities are just overwhelmingly forward? Some hard SF has been written assuming that it is probabilistic. Greg Egan, maybe, or Stephen Baxter. But I digress.

Tachyons are proposed particles that exhibit -t behavior . The original idea was evoked as a property of 10-dimensional string theory which has all but been dismissed altho there is a new reconsideration of this hypothetical beast as an adjunct of 11 dimensional string theory as an open string.

Tachyons are way older than that. Just take v greater than c and calculate gamma. Then all the annoying infinities go away, but you're left with imaginary numbers in all your mechanics equations. This is not as bad as it sounds, because standard Minkowski space used in relativity is just like plain space if you assume that the time coordinate is imaginary.

There are possibilities that these things may exist , but they appear to all be special cases and most physicists agree that any backward movement in time cannot transmit information ( and what is a human or a machine that got him there but information ) and that any construct used for time travel prohibits traveling to a point before the construct existed ( at least in the same universe).

The latter is true. Plus, these things tend to be made of completely uniform materials of infinite length, which we don't think are possible.

As to the former, it's my opinion that transmitting information back in time would violate relativity. But I'm not certain about that, and I've seen people who aren't entirely stupid disagree. It is a handy mathematical trick to assume that something like information is going back in time. But its usefulness is limited to quantum interactions, which seem not to be able to transmit information even superluminally, despite "spooky interactions over a distance." (This point completes Feynman's idea with Maxwell's equations.

Bottom line being for now special class HE physics can demonstrate -t behaviors, but the translation to macroscopic scales is something altogether different.

That's one of those quantum things. It just so happens that, at those temperatures, quantum behavior gets obviously big. As with, to take an earlier and simpler example, Einstein-Bose condensates.

Trouble is, I'm not sure where the microscopic ends and the macroscopic begins. People have been using time-reversal (call it a mathematical trick or reality, whatever) when drawing Feynman diagrams for decades. But there's always the caveat that, well, this is small, so I don't have to think about it when ordering a beer. There are plenty of macroscopic things that could not work in a classical universe, such as this computer and the eyeballs I use to see the screen. Light itself wouldn't work over distances of parsecs the way it does if it weren't for QED. But these are all indirect. The HE experiments just make a quantum operation directly large enough that it cannot be ignored.

Plus, there are about a half dozen mainstream interpretations of QM and about a bazillion others, none of which, so far, anyone has been able to convince me is in any way empirically distinguishable from the other. (Someone tried once; the best attempt I've seen, but it had a fatal flaw.) When I see this, I get an intuitive feeling that the fact that there are so many means that none of them is very good, and there might be some other way of looking at it.

I like to play a sort of ultra-skeptical game sometimes, wherein I make a distinction between what I really think I know, what I just kinda think I know, and what I just sorta assume I know. I really think I know some aspects of information theory, the whole Shannon schtick with irreversible computing engines, and that that it can be tied directly to thermodynamics. I really think I know that the best evidence for a macroscopic arrow of time is also thermodynamic and information theory in nature. I really think I know that I have a brain, and that this brain operates as a theremodynamically irreversible computing engine. I really think I know it because my head gets hot.

Put that together, and what I kinda think I know is that my brain operates according to the same set of statistical laws that provide the best evidence on a macroscopic scale for the arrow of time. So, I kinda think I know that my brain, being an entropy engine, is going to perceive other entropy engines, like the macroscopic universe, as going the same direction in time. My process of forming memories involves entropy-increasing operations, so the time when there was more entropy is in my memory, and the time when there will be less entropy is not in my memory.

However, I only sorta assume I know that this perception is an accurate perception of what's going on. If some demiurge or Maxwell's demon started playing tricks with the entropy of the universe, jogging it back and forth like the time slider on a PC DVD player, I wouldn't notice at all, because at all times my memories would always be of lower-entropy times. Again I digress.

But anyway, if I were writing an SF story, I'd either use a many-worlds interpretation, where the arrival of a time traveler in the universe he and/or she arrives in is just a big fat hairy quantum fluctuation with respect to that universe, or one wherein paradoxes are averted because of feedback both ways between the past and the future which will settle down to some stable configuration in which the paradox does not occur.

[davidhorman]

Greg Egan, maybe, or Stephen Baxter. But I digress.

There was a bit about detecting radio signals from the future in Stephen Baxter's Time.

David

[epepke]

Originally posted by davidhorman


There was a bit about detecting radio signals from the future in Stephen Baxter's Time.

David

That must've been it. Thanks.

[TillEulenspiegel]

IIRC one of Heinlein's last stories was about skipping thru parallel universes with some MIB chasing the protagonist. I thought he was loosing his marbles at the time ( think I was 12) but I wasn't aware of many worlds QM theory at the time.

[epepke]

Originally posted by TillEulenspiegel
IIRC one of Heinlein's last stories was about skipping thru parallel universes with some MIB chasing the protagonist. I thought he was loosing his marbles at the time ( think I was 12) but I wasn't aware of many worlds QM theory at the time.

There have been quite a few good stories about this. One, I think, was The Coming of the Quantum Cats by Frederik Pohl.

Of course, there's no evidence that any possible connection between parallel universes exists in the long term, and in many worlds it only has to exist long enough for the world where the electron goes through one slit and the world where it goes through another to interfere, or whatnot.

[Monster of Loch Ness]

A long time ago I read a cartoon in which the timetravellers - having travelled back in time - had to stay indoors because of rain. The reason for this was that every raindrop had already taken the route once and they would take the same route no matter what - therefore every raindrop would, in effect, be a bullet going through the time travellers.

I cannot recall the name of the cartoon. But the bearing idea was anyway that the past had already happened. Can anyone remeber whether there was any science behind this idea ?

[scribble]

Originally posted by Monster of Loch Ness
[B]A long time ago I read a cartoon in which the timetravellers - having travelled back in time - had to stay indoors because of rain. The reason for this was that every raindrop had already taken the route once and they would take the same route no matter what - therefore every raindrop would, in effect, be a bullet going through the time travellers.


I don't know - but it makes me wonder how they breathed.

[wipeout]

Originally posted by Archangel


Actually Parallel Universes would mean that there could be no paradox for instance:

In universe A) you build a time machine go back in time to the night of your conception kick your dad in the nuts etc. and go forward in time to find yourself not born (in your frame of reference) however what happened was that when you went back in time you actually exited Universe A) and entered Universe B).

So to inhabitants of a) it would seem that you were born, grew up, built a time machine and vanished never to be seen again, whilst in b) you were never born, suddenly appeared in one night of nut kicking insanity and then disappeared to reappear out of nowhere to live out your days.

Of course, in an exactly parallel universe, your parallel self would try to enter into our universe at the same time and place as you were entering into theirs, and so you end up with a whole new paradox if they could affect your background and you affect theirs. :D

[Monster of Loch Ness]

I don't know - but it makes me wonder how they breathed.

A commercial requirement for the story.:p
>>>>>>running

edit: typsos

[DrChinese]

Originally posted by CrossHair
To physically travel through time would have to be in a space ship for this reason.... The earth and solar system are always in motion and we are not in the same location we used to be. To travel back in time from our present location would put us, most likely, into deep space because the earth and solar system are not here in the past. Therefore you had better be in a space ship to survive!

Thank you! Of course this point is nearly always missed in discussions of time travel. Where would you go back TO?

Because we are orbiting large celestial objects, our path through space is not a straight line. We have no idea of our net velocity through space between any 2 points in time. Assuming we can go back in time and be anywhere we wanted: how would we know where to travel to?