Thread: if time travel were possible...

Originally Posted by Licity

Well, let's say you happen to have a time machine and are going to try just that. You don't remember a copy of yourself about to hit you in the head, do you? That means something foiled your attempt before you even got to your past self. Causality doesn't need to employ a new force to work, there is no supernatural power that forces cause and effect to behave. Remember that you trying to kill yourself already happened, so any more tries you make won't be in your memory. It's not that the past changes to accommodate your attempt to kill yourself, it's that the past has always included that attempt and you are just fulfilling continuity.

Lets say that I'm a billionaire with a finite but arbitrarily large stash of fusion bombs. Lets further suppose that I decide to send them back one at a time through my time machine in such a manner that they will appear directly behind my past self some arbitrarily small amount of time before detonating. Every one of them breaks down? what happens to them after they fail to explode? I suppose the answer to that is that I didn't decide to do that in the future but what prevents me from doing so?

I sort of agree with UM that this whole argument shows the implausibility of time travel into our own past but I still want to know why. Saying "because it would lead to paradox" just doesn't do it for me. Why can't the universe be paradoxical?

Originally Posted by PhilosopherStoned

Lets say that I'm a billionaire with a finite but arbitrarily large stash of fusion bombs. Lets further suppose that I decide to send them back one at a time through my time machine in such a manner that they will appear directly behind my past self some arbitrarily small amount of time before detonating. Every one of them breaks down? what happens to them after they fail to explode? I suppose the answer to that is that I didn't decide to do that in the future but what prevents me from doing so?

I sort of agree with UM that this whole argument shows the implausibility of time travel into our own past but I still want to know why. Saying "because it would lead to paradox" just doesn't do it for me. Why can't the universe be paradoxical?

The fact that you are alive long enough to send the bombs back in time would imply that every one of the bombs fails to detonate. Nothing happens after they fail to explode, because the bombs you would be sending back in time would come from the pile being fed by the very same time machine you are using. The fact that the the bombs are there for you to send back forces you to send them back in time without them detonating so that they will be around for you to send them back.

There's nothing completely concrete that says we can't have paradox in the universe. We are led to believe so because we cannot infer the results of the universe trying to support a paradox and because there seem to be no paradoxes of this type already in existence. The universe works based on a set of logical rules (i.e. laws of physics) and a paradox is a logical contradiction. When is it okay to be contradictory?

Originally Posted by Michael

I didn't mean that I was in the jet, but this explaination works.

I still don't understand how it's possible though, and I will have to research it. =P I know a little about this theory...

My belief as of now stays the same. That is that time is constant. Time can't be slowed down, sped up, or altared in any way. You can do things inside of time, and even bend space to change the amount of time for things, but you can't do anything to time. Just my opinion I guess.

There's not much more I can add, as Xei and PS are far more knowledgeable about physics than I am, so I'll just show you these two great videos that helped me understand Time Dilation
Two great videos...




And then here's a good documentary about the potentiality of time machines(where the second video came from, IIRC)
Part 1:

Part 2
Part 3
Part 4
Part 5

/end hijack

Originally Posted by Universal Mind

If you go to what appears to be "back in time" and kill what appears to be "yourself", you are neither back in time nor killing yourself. You are in a parallel universe, in a parallel past, killing a parallel person who mimics you. If you were not killed by yourself in your past, then you are not visiting your past when you do that to your replica. You are somewhere else.

I think the scenario we are discussing proves the impossibility of true time travel.

ah ha!

thats the answer ive been searching for.

thank you so much, that actually takes a lot of weight from me from thinking about it. because that makes so much sense, and i had already been told it was something to do with parrallel universes, so that explains it well.

and elis D, that first video, with carl sagan, that ive seen that video and its quite interesting, thats why i went into it in this thread. i dont have time to look at the other vids right now, but i will

Originally Posted by Licity

Nothing happens after they fail to explode, because the bombs you would be sending back in time would come from the pile being fed by the very same time machine you are using. The fact that the the bombs are there for you to send back forces you to send them back in time without them detonating so that they will be around for you to send them back.

I don't fully understand that but it seems to indicate that my past self would somehow fail to notice the bombs appearing. granted they're behind me but I do turn around from time to time.

The point that I'm trying to make is that this is sort of a god principle and its use leads us, while perhaps not to paradox, at least to highly non-plausible scenarios. Paradox avoidance is valid in mathematical proofs but IMO is sloppy in a physical context. As UM said, if anything, this family of arguments shows that time travel is impossible in the sense which we are discussing rather than placing any rules on the time travelers.

Originally Posted by Licity

The universe works based on a set of logical rules (i.e. laws of physics) and a paradox is a logical contradiction.

This is a good heuristic but I have a hard time accepting it as a fact. We should always keep our mind open. This is especially true in light of the fact that, as of yet, we haven't uncovered these laws. Quantum mechanics seems pretty paradoxical to me. Roger Penrose at least agrees and refers to what most physicists call the 'measurement problem' in QM as the 'measurement paradox'.

Time travel doesn't exist because time doesn't exist.

Originally Posted by Hercuflea

Time travel doesn't exist because time doesn't exist.

Mkay... Care to elabourate?

Originally Posted by PhilosopherStoned

I don't fully understand that but it seems to indicate that my past self would somehow fail to notice the bombs appearing. granted they're behind me but I do turn around from time to time.

The point that I'm trying to make is that this is sort of a god principle and its use leads us, while perhaps not to paradox, at least to highly non-plausible scenarios. Paradox avoidance is valid in mathematical proofs but IMO is sloppy in a physical context. As UM said, if anything, this family of arguments shows that time travel is impossible in the sense which we are discussing rather than placing any rules on the time travelers.



This is a good heuristic but I have a hard time accepting it as a fact. We should always keep our mind open. This is especially true in light of the fact that, as of yet, we haven't uncovered these laws. Quantum mechanics seems pretty paradoxical to me. Roger Penrose at least agrees and refers to what most physicists call the 'measurement problem' in QM as the 'measurement paradox'.

I'm trying to put the bomb thing in perspective... the bombs you're sending backwards are the undetonated ones you will be sending back soon.

I'm not sure why self-consistency is a god principle... the only way you can possibly travel is if you make it in one piece through whatever self-inflicted havoc you care to try.

I disagree that paradox avoidance is sloppy in a physical context, but I do agree that we really aren't sure whether or not paradox is permittable. Just because there are none we have seen doesn't mean we won't find one tomorrow, hm?

Originally Posted by Licity

I'm trying to put the bomb thing in perspective... the bombs you're sending backwards are the undetonated ones you will be sending back soon.

Ok, I thought that's what you might be doing. I specify that they go back to a time before I was a billionaire with a stash of nukes. Dance around that
The only way I see for it to work would be for the time machine to break down systematically if sending something into the past would change it. But then what can we send into the past that wouldn't change it?

Originally Posted by Licity

I disagree that paradox avoidance is sloppy in a physical context, but I do agree that we really aren't sure whether or not paradox is permittable. Just because there are none we have seen doesn't mean we won't find one tomorrow, hm?

Sloppy might be a little harsh but it doesn't lead to satisfying answers. The last sentence quoted is good reason to avoid it.

Originally Posted by Black_Eagle

I don't think it's a cop-out at all. If you went back in time to attempt killing your ten-year-old self that would mean when you were ten years old a future version of you also attempted to go back in time and kill you. There doesn't have to be a "mysterious force" involved. The fact that you were not killed by a future you says that he failed for whatever reason. Maybe he got caught by the security system, maybe your father shot him in the face, or maybe he even decided that it was impossible for him to kill you since he was not murdered by himself in his past. It's not that you can't kill a direct blood relative or yourself, it's that you won't due to circumstances that prevented you from being killed by yourself when you were younger. Even though they are basically the same thing in this case, they describe different mindsets.

Of course, it's not like this question will ever be relevant to our lives.

But it doesn't answer the paradox, because, as there is no mysterious force, there are a class of people to whom what you have just said applies, and then there a class of people caught in the paradox because they successfully killed themselves.

Originally Posted by slash112

Mkay... Care to elabourate?

Yeah...We as humans have created the concept of time because of two things: Movement, and Memory. We see things move, such as a car moving from point A to point B, and we immediately think that there is some force, or some facility called "time" that allowed that car to move from point A to point B. But let me ask you something. I am going to assume that time exists just for the purpose of this question. If all of the matter in the universe stopped moving, and froze, would time still be progressing? If everything is frozen, then it would be just the same as if time had stopped.

We also have created the measurement called "time" because we physically and mentally posess memories. We remember something that happened a while ago, and obviously it is not happening right now, so therefore there must have been some presence or facility if you will that fills up the gap in between those two actions, so we think. But my question is, what if there is no such thing as time? I think that there is movement, and there is animal memory, but there is no ever-present force that cannot be altered, called time. Think of it, every single human being that has ever lived is still residing right here on Earth, but the vast majority of all the people who lived in the past are all dust in the wind now. Their molecules are still here floating around somewhere, no matter has been created or destroyed ever since the Universe came into existence, whether by Intelligent Design or not, the fact remains.

this....

to put what hercuflea said simply...the past is only in our minds or history books. It no longer physically exists...thus we cannot travel to it. however, if there were a parallel universe of our universe in the past, that would be a way, would be cool.

Originally Posted by Hercuflea

Yeah...We as humans have created the concept of time because of two things: Movement, and Memory. We see things move, such as a car moving from point A to point B, and we immediately think that there is some force, or some facility called "time" that allowed that car to move from point A to point B. But let me ask you something. I am going to assume that time exists just for the purpose of this question. If all of the matter in the universe stopped moving, and froze, would time still be progressing? If everything is frozen, then it would be just the same as if time had stopped.

We also have created the measurement called "time" because we physically and mentally posess memories. We remember something that happened a while ago, and obviously it is not happening right now, so therefore there must have been some presence or facility if you will that fills up the gap in between those two actions, so we think. But my question is, what if there is no such thing as time? I think that there is movement, and there is animal memory, but there is no ever-present force that cannot be altered, called time. Think of it, every single human being that has ever lived is still residing right here on Earth, but the vast majority of all the people who lived in the past are all dust in the wind now. Their molecules are still here floating around somewhere, no matter has been created or destroyed ever since the Universe came into existence, whether by Intelligent Design or not, the fact remains.

hmmm... not sure i believe that

Originally Posted by tkdyo

this....

to put what hercuflea said simply...the past is only in our minds or history books. It no longer physically exists...thus we cannot travel to it. however, if there were a parallel universe of our universe in the past, that would be a way, would be cool.

i suppose this helps believe what he said. because maybe time is just infinite parallel universes working together

but still... i dunno

I'm not an expert at physics, but I think that time flows in only one direction, or at least we can only observe it flowing in one direction, because entropy always increases. So guess it's impossible to travel back in time while being in the system, i.e. the universe. You would have to be able to decrease entropy or something.

Maybe if you could break out of the system and you could exist outside of it, you're existence would perhaps no longer be attached to the universe's "timeline", even though another version of yourself would probably still exist within it.
But I think you just couldn't get out of this (probably closed) system, nor is it likely that you could re-enter it. Assuming you could re-enter at some point in the past and you killed your past-self, your future-self would stop to exist but you, who re-entered the system, would keep existing because your existence would then depend on the period of time from your re-entrance until now (when you just killed your past-self).

That's how I imagine it. Probably doesn't make much sense …

nope lol

Originally Posted by illidan

I'm not an expert at physics, but I think that time flows in only one direction, or at least we can only observe it flowing in one direction, because entropy always increases. So guess it's impossible to travel back in time while being in the system, i.e. the universe. You would have to be able to decrease entropy or something.

Maybe if you could break out of the system and you could exist outside of it, you're existence would perhaps no longer be attached to the universe's "timeline", even though another version of yourself would probably still exist within it.
But I think you just couldn't get out of this (probably closed) system, nor is it likely that you could re-enter it. Assuming you could re-enter at some point in the past and you killed your past-self, your future-self would stop to exist but you, who re-entered the system, would keep existing because your existence would then depend on the period of time from your re-entrance until now (when you just killed your past-self).

That's how I imagine it. Probably doesn't make much sense …

i think that is what some people would call the 6th dimention, or maybe the 7th, i cant remember.

but not that that matters because those people are the idiots who call time the 4th dimention

I'm not an expert at physics, but I think that time flows in only one direction, or at least we can only observe it flowing in one direction, because entropy always increases. So guess it's impossible to travel back in time while being in the system, i.e. the universe. You would have to be able to decrease entropy or something.

That's interesting... although I think it's true that increasing entropy isn't a 'law' as such; it's universally observed, but there's no fundamental reason for it; it's a statistical thing.

but not that that matters because those people are the idiots who call time the 4th dimention

Actually that has a firm basis, in Einstein's mathematical framework of General Relativity (his greatest achievement), time is essentially the '4th' dimension.

Originally Posted by Xei

Actually that has a firm basis, in Einstein's mathematical framework of General Relativity (his greatest achievement), time is essentially the '4th' dimension.

but i thought dimentions are split into two different groups and that time isnt the 4th of either of the groups.

Yes that's true, you can classify them as spacial and temporal dimensions, in which case there isn't a 4th of either.

However I do remember reading somewhere that in General Relativity the time dimension is identical to the spacial dimensions... however I don't understand that at all and the source could be apocryphal.

Originally Posted by illidan

I'm not an expert at physics, but I think that time flows in only one direction, or at least we can only observe it flowing in one direction, because entropy always increases.

This is very perceptive. While that's definitely not the way it's done in physics ATM, (the 'arrow of time' isn't explained) a lot of very smart people, not the least of them being roger penrose, agree with you. I do too. The second law just shows up in too many places, such as the horizon of black holes. It seems to be related to something very fundamental. check the unruh effect for example.

Originally Posted by Xei

Yes that's true, you can classify them as spacial and temporal dimensions, in which case there isn't a 4th of either.

However I do remember reading somewhere that in General Relativity the time dimension is identical to the spacial dimensions... however I don't understand that at all and the source could be apocryphal.

This is in special relativity as well actually. you can change the time axis that an observer is using as long as you do it with a lorentz transformation. lorentz transformations are just the change of coordinates from one observer to another. it's pretty cool.

That's bizarre... there really is total symmetry; nothing to distinguish the 4 dimensions?

it's not 'total symmetry', it's lorrentz symmetry. if you have a four vector (t, x, y z), then you can measure it's length using the spacetime metric I was starting to talk about on the other thread. So the length is:

sqrt(t^2 - x^2 - y^2 - z^2) = l

I'm assuming here that the unit of time is chosen so that light travels one space unit in one time unit. then if l is positive, the vector is 'timelike', if it's zero, the vector is 'lightlike' and if it's imaginary, the vector is 'spacelike'. One can only rotate the time axis onto a timelike vector. It's pretty bizarre though, being able to change time like that.

I used the (+, -, -, -) metric here. other people use the (-, +, +, +) metric so imaginary is timelike, positive is spacelike and zero is still lightlike.

So, is it not correct that the time dimension is essentially identical to the spacial dimensions in GR?

Originally Posted by Xei

So, is it not correct that the time dimension is essentially identical to the spacial dimensions in GR?

tough question to answer. One can only rotate the space axis onto spacelike vectors so they're constrained as well. The way to say it is that they are locally distinguishable from each other but not globally.

essentially, they're the same because I can rotate one onto a portion of spacetime that was previously part of the other. that is very informal though.