a man, who somehow gets his hands on a time machine, travels back in time 30 years and sees himself as a baby. he thinks, "what would happen if i killed this baby? would i cease to exist?" unable to contain his curiosity, he takes out a gun and shoots himself as a baby.

does he exist anymore or not?

you see, he technically wouldn't exist since he died from a gunshot wound as a baby. but if he didn't exist, then he wouldn't have been able to come back from the future and shoot himself. so what's the answer?

here's another one.

a man travels into the future 10 years and carves his name into a tree. then he returns to the present and chops the tree down. but if the tree is chopped down, where did the tree which he carved his name into come from?

Dimensional splits. If you're disregarding that time is linear, then you're also disregarding the concept that there is only one "time". Quantum physics opens the possibility of multiple dimensions, what would happen would be a parallel universe would be created.

If it turns out that quantum physics and the multiverse don't work like that, then it's also possible that it's physically impossible to kill the baby or chop down the tree. I find Blib's scenario far more likely, though. I read about this fun stuff in physics books a couple years ago.

It's quite likely that if you go back in time you will have created a different space-time that split from the original at the point of your arrival. If that's the case then indeed you can kill the baby that was you since it will now only affect this new time line. I find this the easiest scenario for thinking exercises--simple rules and you have few paradox to deal with.

Er… Maybe it's just impossible to travel in time ?

Astra :D

Well I doubt he would be able to kill himself the gun wouldn't work someone would stop him etc... If he traveled ahead to the tree then he must have cut down a different tree.

I believe Blib probably would have the best answer :) lets drop to our knees and give the king of worms our respect :D

C'mon guys lets really think about this.

Each particle is connected to itself throughout time. The particles that make up our bodies are from the beginning of time.

An object like the Earth is actually a 4 dimensional string (imagine it leaving a trail as it moves through space).

If you yank a Hydrogen atom into another point in time you pull the 4D string which shifts the same atom throughout all of time. So at every moment the same atom would shift 5 years or whatever.

Now... if you take a human and drag him through time the particles that make him up would all travel, but six years ago he was composed of 50% different particles. As a baby he migh have been 100% different particles. So every version of the human through time would be ripped apart to some degree.

Massive objects (anything bigger than a subatomic particle) cannot time travel.

Dimensional splits. If you're disregarding that time is linear, then you're also disregarding the concept that there is only one "time". Quantum physics opens the possibility of multiple dimensions, what would happen would be a parallel universe would be created.

yeah, i kinda suspected that too. i got that stuff from this strange book that i found during geometry class, and it said something about that. supposedly there's a theory that parallel universes are created at every single instant, and each one contains a different combination of events which could have occured at that instant. kinda blows one's mind.

If you yank a Hydrogen atom into another point in time you pull the 4D string which shifts the same atom throughout all of time. So at every moment the same atom would shift 5 years or whatever.

is there actually a way to pull that string? the concept is still pretty cool anyway...

Presumably you need to travel faster than light to go back in time.

The theory of Relativity prohibits time travel because the whole Universe moves along the fourth dimension at the speed of light (time).

Any objects total speed can only be the speed of light so that's why when you spend velocity in other directions (length width or height) time slows down (you are moving that much more slowly through the time dimension). As you approach the speed of light time slows exponentially and the amount of energy required to propell yourself grows exponentially such that 99.9% the speed of light requires 1/10 of the energy as 99.99% the speed of light (or something like that). So to go from 99% to 99.9999% would require, like, a million times more energy.

The amount of energy required to actually reach the speed of light is infinite (read: impossible). And since time would stop, you'd never get there.

(read some high end physices books)

Originally posted by Ed Blank
The theory of Relativity prohibits time travel because the whole Universe moves along the fourth dimension at the speed of light (time).
Untrue. Relativity prevents reverse time travel, but we can go forward in time easily (easily from a relativistic standpoint, anyway). All one needs to do to go forward in time is to zip around really fast for a while. You will only have aged, say, a year, while the rest of the universe aged ten.

Originally posted by BorgHunter
Untrue. Relativity prevents reverse time travel, but we can go forward in time easily (easily from a relativistic standpoint, anyway). All one needs to do to go forward in time is to zip around really fast for a while. You will only have aged, say, a year, while the rest of the universe aged ten.

True.

I don't really consider that time travel in the "Time Machine" sense.

Even so, there are no paradoxes and no meeting one's future self with that (rather shabby) version of time travel.

Each particle is connected to itself throughout time. The particles that make up our bodies are from the beginning of time.
Quantum theory makes this meaningless. With the idea that every infinite possibility actually happens it means that this particle is not bound by time constraints. What I mean is that according to Feynman's formulas regarding the double slit experiment for a photon to reach it's destination it must travel all possible routes, simutaneously. That means that it has to go to the end of the universe and back to the spot at the same time it is going straight there. What happens is that the paths all "cancel each other out" until there is only one path. For the specifics of the experiment try looking up Thomas Young's experiment and about Feynman on it.

Backward time travel is possible by travelling faster than the speed of light -- I.E. and put in simple terms ...

A photon of light takes ~ 8 minutes to reach earth so when you see the first rays of sunshing in the morning, those rays left the sun ~ 8 minutes earlier ( that's how long it takes light to reach earth from the sun travelling at the speed of light)

Now -- if I leave the sun at the same time as those first rays and travel faster than the speed of light, I will reach earth before those first rays and it will still be dark (the rays haven't arrived yet) and therefore I would have travelled backward in time relative to all other earthlings.

Travelling forward in time is generally considered to be impossible as it is the opposite of travelling faster than light ... slower than light??? what the heck does that mean?

Normally I would restrict the discussion of time travel to classical physics and in particular the teachings of Einstein (and Newton) with no regard to quantum mechanics but I notice up-thread we have a few Quantum physics fans so I'll include it (and a Feynman fan -- my fav -- RIP Richard).

In order for QM (quantum mechanics) to exist, then the outcome of any sub-atomic experiment must and always will include each and every possibility -- with each possibility existing in it's own parallel universe.

Now, if that's true and it's also true that you can reach a parallel universe using the scenario that you would need to enter a black hole, traverse a worm hole and exit a white hole into the parallel universe then it stands to reason (at least for me) that if one could figure out how to reverse the order from white-to-worm-to-black hole, then you can not, with an open mind, rule out forward time travel.

And yes, your body would have to be at the sub-atomic level for this to even be possible but don't worry, the difference in gravity between your head and your feet as you approach the event horizon would well take care of that problem.

It's deep but it's out there for the curious.

ROTF

Just my 3 cents

Dop

Obviously he shot the wrong baby...:D

Relativity prevents reverse time travel, but we can go forward in time easily (easily from a relativistic standpoint, anyway) Have you ever read "Timequake" by vonegut (sp?) It was a book he didnt really want to write so its not his best but the premise is great. Time decides to go backwards because its never done such before. It does this be compressing the universe instead of expanding. But then decides it doesnt like going backwards so everyone has to relive the past/next 10 years of their life. "my ten years on automatic pilot" by kilgore trout.

Time Space is bent by gravity. They flew a clock at extremely high altitudes around the earth (for I cant remember how long) and it came back slightly behind. This opens a slight possibility in the area of space travel if they could overcome every single other problem with it that is. I wonder what the theory of anti-gravity means to space/time.

I remember seeing a thing on time travel on tv. To deal with this problem there were 2 solutions. One was that as previously stated, it would be impossible to kill the baby. The other is that you cant go back further than you started. Basically, the latter works by forbidding you to unmake the existance of something by not allowing you to go back before you decided to go back.

Borg, here is an interesting situation. Suppose you found a wormhole. This wormhole will lead you right into this universe. Getting past the problem of going inside of your own reality, suppose you put this wormhole on a spaceship. That spaceship went around at the speed of light for 10 years, preserving the time within the wormhole. If you stepped through, what would happen? You are technically going into the past, but being in the past, you wouldnt know the future. Wouldn't that just cause a forever repeating, parallel universe generating loop? There is the universe which you left, then there is the universe you went into, then there is the universe inside the universe which you went into. cosmic man.

the idea that me deciding to pick my nose creates infinite universes where I picked my nose, I didn't pick my nose, I started do but decided not to at the last minute etc.. is absolutely insipid.

I dont really see how it is insipid, i think it is very intriguing, and it is also very appealing. Less wondering about the other choices because they actually do exist. Everything is possible, and everything is accounted for.

Have you ever read "Timequake" by vonegut (sp?)

yeah, i love vonnegut. if he's written it, chances are i've read it. i thought it didn't really apply as much to time travel as it did to human nature, though. time travel was just a metaphor. the whole idea of having to relive the last 10 years of your life with no power to change anything that happened but fully aware of how it would turn out...is really really depressing if you think about it.

the idea that me deciding to pick my nose creates infinite universes where I picked my nose, I didn't pick my nose, I started do but decided not to at the last minute etc.. is absolutely insipid.

if you stopped and thought about how insipid everything really is, you wouldn't enjoy yourself very much. i've heard peope *coughjmancough* say that the notion that we evolved from apes is insipid, but that doesn't prove it false.

Everyone here DOES know what insipid means right?

Originally posted by Evil Homer
Everyone here DOES know what insipid means right?
I certainly hope so. Ed's comment that it's insipid is a tad confusing...perhaps he meant something like "counter-intuitive"?

"in·sip·id ( P ) Pronunciation Key (n-spd)
adj.
Lacking flavor or zest; not tasty.
Lacking qualities that excite, stimulate, or interest; dull."

Thank you for correcting me.

When I said it was an "insipid" idea, I didn't mean "dull" I meant "stupid", or "ridiculous".

How could there be enough energy in our Universe to spawn even one more universe, not to mention INFINITE universes. Not to mention INFINITE UNIVERSES for every move that every particle makes every instant. That is just fucking retarded.

Originally posted by Ed Blank
How could there be enough energy in our Universe to spawn even one more universe, not to mention INFINITE universes. Not to mention INFINITE UNIVERSES for every move that every particle makes every instant. That is just fucking retarded.
A lot of quantum theory is, as you so aptly put it, "fucking retarded", actually. Mini-wormholes, for instance, seem ridiculous, yet it's been proven they randomly pop up according to quantum mechanics. The fact is, we don't know on these things, and it's possible we never will. I certainly don't have much hope that we'll discover these things in my lifetime, or that of my grandkids...

Wouldnt the idea of an infinately large area seem retarded? Or how about a figure with an infinite number of sides? What about imaginary, or even negative numbers?

Wouldnt the idea of an infinately large area seem retarded? Or how about a figure with an infinite number of sides?
A circle is an infinite number of straight lines all connected to each other. :D

Originally posted by Evil Homer
Wouldnt the idea of an infinately large area seem retarded? Or how about a figure with an infinite number of sides? What about imaginary, or even negative numbers?

Exactly. There is no such thing as an infinitely large area or an imaginary number (in the case of the "imaginary" number it's pretty obvious). You can say "population growth is -1% per year", but you could just as easily say that it was "shrinking 1%".

These are conventions used by mathematicians for the purposes of calculating. They have no real life corelaries.

Humans are weird. We think because we can imagine a number that it has to "exist" out there somewhere in the Platonic realm of perfect form.

It's all in your head, buddy.

I know its not real, but we can still imagine it, it still makes sense. And we use these things to prove real things.

And also, a circle is real. Something extremely hard to comprehend (infinity duh) but it still exists. cosmic man.

Humans are weird. We think because we can imagine a number that it has to "exist" out there somewhere in the Platonic realm of perfect form.
If those calculations predict real occurances down to a very minute level of precision...

To try and say that all of those things are impossible and that they don't exist is to claim that every prediction based off of them is false. Which they aren't. For me it's rather easy to comprehend infinity. An infinite universe, simple to understand. A finite universe? To me, I just can't see how that is even possible. Could you try and explain?

Originally posted by Ed Blank
Exactly. There is no such thing as an infinitely large area or an imaginary number (in the case of the "imaginary" number it's pretty obvious). You can say "population growth is -1% per year", but you could just as easily say that it was "shrinking 1%".

These are conventions used by mathematicians for the purposes of calculating. They have no real life corelaries.

Humans are weird. We think because we can imagine a number that it has to "exist" out there somewhere in the Platonic realm of perfect form.

It's all in your head, buddy.
The number is, itself, an abstract concept. "One" doesn't exist anywhere, there being "one" of something is completely abstract. No math is concrete. So what's wrong with things like imaginary numbers (sqrt(-1)), infinity (lim x->inf. etc.)? Especially since they work in our mathematical system...and are quite important, in fact.

I am not discounting the importance of imaginary concepts like infinity and negative numbers.

Let's start over:

Quantum Physicist say that there are alternate universes because their mathematical calculations say so.

The solution to an equation being "infinity" proves nothing about the nature of the actual universe.

Many conventions of mathematics are completely imaginary like negative numbers, imaginary numbers, and infinity. They are useful for high end mathematics or debt management but they are simply concepts with no physical corelaries.

These concepts might very well help engineers build spaceships but the are no more real than 'words', which are also very helpful for our thought processes.

Time travel is not possible because it would take energy to "create" universes. There is not enough energy in a fart or the chirp of a cricket to create anything except the sound you are already familiar with.

You dont know how much energy there is. Hey, maybe there is something that will defy all of our laws and actually produce new energy.

I do know how much energy there is in our Universe:

1 Universe worth of energy.

Maybe something out there can actually create energy, and I'm sure it's waiting for me to wipe my ass so it can create a spereate universe where I didn't have time to wipe my ass and ended up with skidmarks.

You can't create energy nor can you destroy it -- you can only change it's form.

Dop

That's as much as we know at this point in time. Eventually that knowlege may change. I mean, all this energy had to come from somewhere right?

Originally posted by Lungdop Philing
You can't create energy nor can you destroy it -- you can only change it's form.

Dop
As far as we know. But we had to throw our ideas about gravity out the window when Einstein came along -- why not eventually about energy?

Originally posted by BorgHunter
As far as we know. But we had to throw our ideas about gravity out the window when Einstein came along -- why not eventually about energy?

Newton's gravity is not defunct, Einstein just refined our understanding (Newton thought it was a mysterious force, Einstein figured out what the "force" was: curved space).

It's not like we can bend gravity now because of the new detail which has been revealed.

You are, however, right. Our understanding of the Universe will change monumentally as time passes.