Is time travel possible?

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**rubiksnoob**

Just as a warning: This thread is supposed to be a legitimate discussion on the topic of time travel as it relates to physics, philosophy mathematics, etc. Please don't be a troll and post ridiculous comments. I don't want to say "be serious" because this isn't the most serious subject in the world, but don't be ridiculous or goofy. Thanks.

In my astronomy class, we watched a video (an educational type video, not a science fiction movie) about time travel and discussed how it relates to physics and astronomy. It really grabbed my interest with all the possible paradoxes and all that cool stuff. I am nowhere near conversant with the mathematics and physics that deals with time travel and wormholes and all that, so I have to approach the topic from a philosophical standpoint. I just kinda have to think about it. Well, I was thinking about it and I came to the conclusion that time travel (at least into the future, not so sure about traveling into the past.) is fundamentally impossible.
Here's the way I see it:
-The future hasn't happened
-If he future hasn't happened then it does not exist
-If the future doesn't exist how can you travel to it?

Your thoughts/opinions?

**dogma**

If you're a determinist, then if its possible, it has happened.

If you're an indeterminist, then you can't walk out your front door.

**Nurglitch**

You really do need a solid grounding in tensor calculus to have any meaningful discussion about time travel and modern physics. Really, don't bother with the philosophy until you're familiar with the current state of the art, and therefore understand what motivates the various philosophical positions. The world has enough bad philosophy as it is.

**Dronze**

rubiksnoob wrote:Just as a warning: This thread is supposed to be a legitimate discussion on the topic of time travel as it relates to physics, philosophy mathematics, etc. Please don't be a troll and post ridiculous comments. I don't want to say "be serious" because this isn't the most serious subject in the world, but don't be ridiculous or goofy. Thanks.

In my astronomy class, we watched a video (an educational type video, not a science fiction movie) about time travel and discussed how it relates to physics and astronomy. It really grabbed my interest with all the possible paradoxes and all that cool stuff. I am nowhere near conversant with the mathematics and physics that deals with time travel and wormholes and all that, so I have to approach the topic from a philosophical standpoint. I just kinda have to think about it. Well, I was thinking about it and I came to the conclusion that time travel (at least into the future, not so sure about traveling into the past.) is fundamentally impossible.
Here's the way I see it:
-The future hasn't happened
-If he future hasn't happened then it does not exist
-If the future doesn't exist how can you travel to it?

Your thoughts/opinions?

If you follow einsteinian physics, then time travel is only possible in a forward direction, due to the fact that a negative speed is impossible, as speed is an absolute value of velocity.

If you follow the various other rules and theories (rooted in the laws of both macro and quantum physics), you'd only be able to go as far back as the last time the machine was initialized for operation, and as far forward as it was turned off following that point, meaning you'd be unable to give yourself plans for a time machine before one was completed. All that being said, the other implications of time travel would reveal weather or not free will actually existed, and would lead to a complete shift in the moral and ethical patterns of society.

**Flashman**

The big fundamental problem with time travel with regards to travelling forwards and backwards to another point in Earth's history/future is that Earth won't in the right place.

In Back to the Future they always arrive at the exact same location on Earth. In reality, the Delorean would emerge at the point in space where Earth was when they left. Earth at the point in history where they wanted to go would be somewhere else along it's orbital path. This is further complicated by the fact that the galaxy doesn't stay in one place either.

**Dronze**

Nurglitch wrote:You really do need a solid grounding in tensor calculus to have any meaningful discussion about time travel and modern physics. Really, don't bother with the philosophy until you're familiar with the current state of the art, and therefore understand what motivates the various philosophical positions. The world has enough bad philosophy as it is.

Actually, philosophy is almost a required part of any quantum physics discussion, as thought experiments are more often than not, the things that break the theories, concept wise... which allows for actual progression without having to make a new physical experiment to test with.

Automatically Appended Next Post:

Flashman wrote:The big fundamental problem with time travel with regards to travelling forwards and backwards to another point in Earth's history/future is that Earth won't in the right place.

In Back to the Future they always arrive at the exact same location on Earth. In reality, the Delorean would emerge at the point in space where Earth was when they left. Earth at the point in history where they wanted to go would be somewhere else along it's orbital path. This is further complicated by the fact that the galaxy doesn't stay in one place either.

except you could account for that with some rather basic calculations, compared to what you would need to actually make the temporal shift...

**dogma**

Nurglitch wrote:You really do need a solid grounding in tensor calculus to have any meaningful discussion about time travel and modern physics. Really, don't bother with the philosophy until you're familiar with the current state of the art, and therefore understand what motivates the various philosophical positions. The world has enough bad philosophy as it is.

The philosophical positions are self explanatory to anyone who has studied logic. Philosophy clarifies experimental thought, it doesn't advance it.

Also, *cough*string theory*cough*.

Not that I'm a philosopher of science. I'm merely a humble logician.

**jbunny**

There are only two possible conclusions.

1. Time travel is not possible, otherwise we would have heard at time travelers by now.
2. Time travel is possible, but the Earth was destroyed before we discovered how to do it.

**dogma**

jbunny wrote:
1. Time travel is not possible, otherwise we would have heard at time travelers by now.

We've heard of time travelers. They're simply not taken as credible. The really serious ones are in therapy.

**Nurglitch**

Dronze:

I wasn't saying that philosophical discussion wasn't necessary: it is. The problem is that before you can have a meaningful philosophical discussion you need to have a solid grounding in the super-structure of what you're discussing. That's why there's so much philosophy done in physics around quantum physics, because it comes down to an evaluation of the foundations one might choose to adopt, rather than any empirical observation (although it helps to avoid the "p" word around physicists).

Dogma:

Logic is barren without empirical data to fertilize it, and useless without the ability to compare the normative structure a particular logic might suggest with the empiricial results in a meaningful way.

But I was merely a humble philosopher of science, and definitely not a logician...

**rubiksnoob**

I remember a funny steven hawking qoute, something like, "if time travelers are here why would they only show themselves to crazies" or something like that. made me chuckle.

**Flashman**

jbunny wrote:There are only two possible conclusions.

1. Time travel is not possible, otherwise we would have heard at time travelers by now.
2. Time travel is possible, but the Earth was destroyed before we discovered how to do it.

3. Time travel is possible, but the tight bastards from the future are keeping it to themselves so they don't have to tell us what the lottery numbers are... or is that were?

**dogma**

Nurglitch wrote:
Dogma:

Logic is barren without empirical data to fertilize it, and useless without the ability to compare the normative structure a particular logic might suggest with the empiricial results within a meaningful way.

We're in general agreement. I was referring only to internal consistencies as defined by the philosophy in question; ie. the areas of inconsistency in the various theories of quantum gravity.

Though it now occurs to me that I know more about physics than the average philosopher.

Nurglitch wrote:
But I was merely a humble philosopher of science, and definitely not a logician...

I hated my philosophy of science professor. Damned Popper disciple.

**Nurglitch**

Nurglitch wrote:Time, time, time.

The fact of the matter seems to be that people like the idea of time travel. After all, it appeals to a sense that things might have been otherwise, that things might be different had the past been somehow different. "Would that things were otherwise." It seems pretty natural, on the face of it, to suppose that because we think of the future as being full of possibilities, that the past and the present were a path of actuality picked out of all the previous possibilities.

So we get into hypothetical discussions about stuff like "If you went back in time and killed your grandfather, would you still exist?" or "If you went back in time and accidentally killed a certain fish, would humanity still exist as we know it?" These questions, it seems, are basically all variations on the idea that the link between the past and present is loose enough we could make a reflexive move and have some effects act as causes.

The answers to these sorts of question range from the affirmative to the negative. In the affirmative it may be said: "Why yes, if you went back in time and killed your grandfather you would still exist." The specific reason would range from the notion that someone else would be your grandfather, to the notion that your existence would be fixed by a now non-actualized future which has become the past of the currently actualized future.

In the negative, reasons would range from the notion that you would disappear the instant that you disrupted the causal roots of your own existence, to the notion that you would actually be unable to kill your own grandfather because you already exist. The latter is interesting in that it does not deny anything about your existence, but just something about the timing of it. Denying that you can kill your own grandfather does not deny that you wouldn't exist under that counter-factual situation, but that you would never had existed under that situation. Had your grandfather been killed prior to initiating the chain of events that lead to you, you would not even have the opportunity to travel through time, let alone get a crack at him with a lead pipe.

Now all this buys into a particular notion of time as something that proceeds from past to present and onto the future, a line upon which events progress. But if you want to accurately measure trajectories on large scales, it seems that you need to consider time as being another spatial dimension. This notion of time, and its concurrent effects of dilating and expanding the relative progresses of time, is considered from the outside, or using techniques that have been successfully used to explore one to three dimensional spaces from the outside and generalized to a phenomena to which we are actually internal. So why relative time may seem to move slower for someone moving relatively very fast, it doesn't seem to go backwards (as I understand it), or allow causal chains to be reflexive. I have heard that certain particles seem to have a backwards causation, or teleological effect, but I'm not sure whether these are artifacts of the theories involved, or actual reversal of the 'direction' of causation.

I bring this up, despite my woolly understanding of the subject matter, because it has some bearing on the directionality of time, at least as we perceive it. If we understand time as being something like a spatial dimension, or at least insofar as tensors are concerned, then time as more directions than we commonly notice, and its shape may be either closed (like a sphere) or open (like a saddle-shape or hypersphere), or maybe even both. There may very well be teleological causes as well as the usual past-to-future ones (material, efficient, and formal).

If causation goes in the other direction, then it seems that it would be impossible, even hypothetically, to kill one's own grandfather because he could cause you, and you him, and your effect on him would be the opposite of killing him! It's rather like vicious circularity than recursion: you couldn't just appear, kill him, and then disappear as you appearing would necessitate him surviving. Killing him before he could cause you would be logically absurd.

But suppose that the logical absurdity gets filled in with another person, so that your grieving grandmother gets lucky on the rebound (there's a thought I bet you didn't want...) so that now you've been caused by another person and not the person you killed. Again, absurdity because you didn't kill your grandfather.

Finally, suppose that the grandfather you kill could have been your grandfather in a possible world, and that in killing him you de-actualize yourself in one world and re-actualize yourself in another (or move from one possible world to another, with the world you occupy being your actual world in terms of relative reference). After all, at his point in the causal chain the links between you and him are still only potential, despite being actual from your perspective, and that they'd simply re-actualize in another configuration rather than disappearing if you killed him. So the grandfather you kill is the right one, the one that potentially causes you, and it's not absurd to be able to kill him because while killing him de-actualizes the chain of events leading to you, they re-actualize you in another possible world where you haven't killed your grandfather.

But if that's the case then it's rather pointless, as well as sadistic and wasteful, to go back in time to kill your own grandfather, because you will dematerialize in the world with which you no longer have any causal connection, and materialize in another one unlike the first. Given the implications for changing the past so far explored, it's unlikely that this world will be any different from the one you started in. Really the only thing is accomplishes is hitting a reset button if you ever do something so collosally stupid as trying to kill your own grandfather. If this is the case, we can imagine why a time machine will never exist, and why we'll have to keep on moving through time in the old fashioned way and in the same old direction.

This doesn't just apply to killing your own grandfather, it seems. Any attempt at temporal recursion will run into these problems of changing its own causes, so it won't work the other way, like if you want to go into the future, find out the winning lottery numbers, and take them back with you. For the same reason that none of us would be going back into our pasts, our future would not be venturing into our present.

Now, having been a spoil-sport and ruined everyone's favourite movies, I would like to point to an interesting treatment of time travel that I have seen, an episode of BBC Wales' "Doctor Who" entitled "Blink" by Steven Moffat. In this episode the time-travelling Doctor is zapped back in time and must arrange things to that his time machine, the TARDIS, is sent back to him so that he can carry on his merry way. He does this by finding another victim of the same creatures that sent him back in time, and giving him a recording to hide on the DVDs of his eventual DVD business twenty years down the line. This recording has the neat effect of forming a coherent conversation with whoever is watching it, regardless of when they step in, pause it, and so on. It's not that the recording changes, but that it's always relevant to the conversation.

Quite aside from amusingly vague explanation he gives on the recording when a main character asks him how he's anticipating what she's saying, this episode shows how you can avoid much of what I've talked about here and instead feature the spookiness of the Doctor having recorded a single side for a conversation he'll have with several people. The ending of the episode reveals that these conversations, both sides, are recorded after the fact, compiled, and given to the Doctor in his own relative past.

Something similar is done in "How to Get Ahead in Advertising" starring Richard E. Grant, which appears to be the story of an advertising executive going mad until he manages to have the same conversation twice: the first time while recording a message to his wife, and the second time while happening upon his wife watching that recording. The voice that he speaks to the first time is his own voice the second time!

So obviously there's still plenty of interesting things that can be done with time travel without going into fantasy, but the whole recursive 'changing the past' is played out, and incoherent to boot.

Automatically Appended Next Post:
Incidentally, you should all watch "Primer"

Automatically Appended Next Post:
dogma:

Some inconsistencies or contradictions aren't as bad as others. Part of the importance of various quantum theories is that they expose weaknesses in wholly consistent logics, so speaking about the inconsistencies exposed when applying one of these logics is not to make any claim about any kind of "internal" consistency, but to make a claim about the consistency of an external metric.

Yeah, Popper sucks. Lakatos sucks. And part of the reason I'm no longer a philosopher of science is that I decided to hold that opinion around a professor that disagreed (although similar opinions that Searle sucked, Turing sucked, and the list goes on, certainly helped).

**dogma**

Excellent essay, Nurglitch. My only objections stems from the lack of explanation with respect to 'reverse causation'. It isn't necessarily a clear concept, and it would be helpful if you outlined what you mean by that comment.

**whitedragon**

Thanks Nurglitch, kill the fantasy why don't you.

Or did you already? Or didn't you? Or did you in reverse to my comment?

**Nurglitch**

dogma:

There's several ways of talking about "reverse causation", with the easiest way being a discussion of Aristotle's four causes (material, formal, efficient, and final).

So take a statue made of marble. The marble is the material cause of the statue. The formal cause of the statue is its shape. The efficient cause of the statue is the action of the sculptor on the marble to turn the block from a block into the shape of the statue. The final or teleological cause is the resulting combination of shape, work, and material which came together.

So while the first three causes follow the normal progression of time having a direction of past, present, future, the final cause follows a reverse progression of future, present, past. A better example might be that of an Oak tree, whereby the fruiting tree is the telos or final cause of its fruit, the acorn, or the point that the first three causes converge on.

This is pretty hard deterministic in the sense that the acorn has its formal case determined both genetically and epigenetically, its efficient cause being the genetic machinery of the Oak tree, and the material cause being the constituent elements that the acorn absorbs and turns into the constituents of the Oak tree. The Oak tree causes itself in the sense that if the other causes didn't result in an Oak tree, then you wouldn't have an Ork tree.

This is counter-intuitive if you buy into meta-theory or logic by which causation has a direction consistent with that of time, is "non-commutative". But Aristotle's logic of causation isn't a "well-founded" theory, and accepts a notion of circularity that would be vicious if you're trying to find a foundational or starting point that is absolute rather than an equilibrium.

In other words, if Russel's Paradox is a problem, and the set that contains all sets that do not contain themselves cannot either contain itself (and hence be a set that contains all sets that do not contain itself), or can contain itself (and hence cannot cannot contain all sets that do not contain themselves), then such causation implies a contradiction, if not simply an error in the grammar of the logical system you're applying. If Russel's Paradox is not a problem, then reverse causation is simply one half of a commutative function.

In other words, either causation is non-commutative, or it's commutative. If it's non-commutative, then the future is not only un-knowable, but also unpredictable or non-deterministic. If it's commutative, then the future is predictable or deterministic.

Notice, however, that Aristotle's causes combine the two notions of causation, as three causes are non-commutative, and one cause is commutative, and that the ratio is 3:1. So if you're going to accept that events in the future are going to effect those in the past, then you're going to have to change three causes in the past to effect one event in the future.

But then you get into a mess about defining what counts as a cause and what counts as an effect. Traditionally cause and effect is predicated one there being one cause for one effect, with the number of effects multiplying out with the number of interacting causes, with the exact magnitude being undefined.

Either way if future events are going to cause past events, you can redefine those future events as past events co-relative to the past events which are co-relatively in the future of those past events, in which case there is no "reverse-causation" worth the name, or you can say that there is no proper distinction of past/present/future in relation to events in which case any such "reverse causation" is grammatically suspect, like saying there's such a thing as a "left-handed coffin" (or, on the gripping hand, that any such notions as "reverse causation" are simply metaphorical rhetoric to get a point across)

The problem, as I understand it, is employing a three-valued logic of time (past/present/future) rather than a two-valued logic of time (before/after), but that was a friend's thesis and you'll have to look it up in the Canadian National Archives for the details.

**George Spiggott**

The clock is always ticking in San Dimas.

**Wrexasaur**

Another great post Nurglitch, that was an interesting read.

Nurglitch wrote:The problem, as I understand it, is employing a three-valued logic of time (past/present/future) rather than a two-valued logic of time (before/after), but that was a friend's thesis and you'll have to look it up in the Canadian National Archives for the details.

You could just as easily say that the present is composed of before/after, and not a complete description in itself. That may have been what your friend was talking about in basics though, I would have to find the thesis.

Oh, and to lighten the thread up just a bit... *Arnold voice*... "Sarah Connah"...

**MadEdric**

The way I see it is like this. Time travel does not yet exist, so we have no travelers from the future. Some time in the future time travel will be discovered. From that point onward time travel will exist and the past of that era will understand it.
We will not be that past, since we never were visited by people from the future, but there will be alternate pasts where people know that there are/were time travelers.

**Khornholio**

Flashman wrote:The big fundamental problem with time travel with regards to travelling forwards and backwards to another point in Earth's history/future is that Earth won't in the right place.

In Back to the Future they always arrive at the exact same location on Earth. In reality, the Delorean would emerge at the point in space where Earth was when they left. Earth at the point in history where they wanted to go would be somewhere else along it's orbital path. This is further complicated by the fact that the galaxy doesn't stay in one place either.

...or maybe you're not giving Doc Brown enough credit for his scientific genius for figuring that out.

I heard a quantum theory one time that there was only one electron in the universe and that is everywhere simultaneously traveling through infinite wormholes. How's that for time travel?

**Ketara**

I read an article in which a guy was claiming he could use four lasers to distort temporal space or something. However he said you could only go back to a a point in which the machines existed.

If that's the case, then the reason we haven't started getting time travellers yet is because we haven't built one yet. They won't start cropping up until we do.

**Albatross**

We are travelling through time right now, so yes, time travel is possible.

**dogma**

Nurglitch wrote:
dogma:

Some inconsistencies or contradictions aren't as bad as others. Part of the importance of various quantum theories is that they expose weaknesses in wholly consistent logics, so speaking about the inconsistencies exposed when applying one of these logics is not to make any claim about any kind of "internal" consistency, but to make a claim about the consistency of an external metric.

Clearly, I was speaking only to internal consistency, or consistency which is defined the explanatory metric of the theory in question.

Nurglitch wrote:
Yeah, Popper sucks. Lakatos sucks. And part of the reason I'm no longer a philosopher of science is that I decided to hold that opinion around a professor that disagreed (although similar opinions that Searle sucked, Turing sucked, and the list goes on, certainly helped).

Whoa now, no need to pull in Searle and Turing. And, from the 'suck' moniker I can see why you were put out by your professor.

**Nurglitch**

Actually that's another weird thing about time-travel. Movement is distance over time, so the notion of movement of time over time doesn't really make sense. Makes a great solution to the Chicken-Egg problem though.

**dogma**

Nurglitch wrote:
Either way if future events are going to cause past events, you can redefine those future events as past events co-relative to the past events which are co-relatively in the future of those past events, in which case there is no "reverse-causation" worth the name, or you can say that there is no proper distinction of past/present/future in relation to events in which case any such "reverse causation" is grammatically suspect, like saying there's such a thing as a "left-handed coffin" (or, on the gripping hand, that any such notions as "reverse causation" are simply metaphorical rhetoric to get a point across)

Yes, this is what I was getting at with my question.

**malfred**

Dronze wrote:
rubiksnoob wrote:Just as a warning: This thread is supposed to be a legitimate discussion on the topic of time travel as it relates to physics, philosophy mathematics, etc. Please don't be a troll and post ridiculous comments. I don't want to say "be serious" because this isn't the most serious subject in the world, but don't be ridiculous or goofy. Thanks.

In my astronomy class, we watched a video (an educational type video, not a science fiction movie) about time travel and discussed how it relates to physics and astronomy. It really grabbed my interest with all the possible paradoxes and all that cool stuff. I am nowhere near conversant with the mathematics and physics that deals with time travel and wormholes and all that, so I have to approach the topic from a philosophical standpoint. I just kinda have to think about it. Well, I was thinking about it and I came to the conclusion that time travel (at least into the future, not so sure about traveling into the past.) is fundamentally impossible.
Here's the way I see it:
-The future hasn't happened
-If he future hasn't happened then it does not exist
-If the future doesn't exist how can you travel to it?

Your thoughts/opinions?

If you follow einsteinian physics, then time travel is only possible in a forward direction, due to the fact that a negative speed is impossible, as speed is an absolute value of velocity.

If you follow the various other rules and theories (rooted in the laws of both macro and quantum physics), you'd only be able to go as far back as the last time the machine was initialized for operation, and as far forward as it was turned off following that point, meaning you'd be unable to give yourself plans for a time machine before one was completed. All that being said, the other implications of time travel would reveal weather or not free will actually existed, and would lead to a complete shift in the moral and ethical patterns of society.

Hehe, the big finish to the Planetary comic book series had a Time Machine. It was a
great way to end it all.

**Alpharius**

malfred wrote:
Hehe, the big finish to the Planetary comic book series had a Time Machine. It was a
great way to end it all.

One of my all time favorite comic series ever!

And, Nurglitch is making my head hurt.

And, apparently we don't discover Time Travel during my life time.

Or, we do, and I don't have access to it!

**ShumaGorath**

Watch these.

Conceptually there is no reason to assume that time does not flow both backwards and forewords simultaneously, or that it is in "motion" at all.

**Nurglitch**

dogma wrote:Clearly, I was speaking only to internal consistency, or consistency which is defined the explanatory metric of the theory in question.

Is there a forum rule that means I can't make my own comments about consistency unrelated to the point you were making?

dogma wrote:Whoa now, no need to pull in Searle and Turing. And, from the 'suck' moniker I can see why you were put out by your professor.

Well, I'm curious about why you think I was "put out by [my] professor". He wasn't my professor, he was my external reader and universally known for being a dick (unfortunately the other researcher in the philosophy of mind in the area who wasn't my advisor had decided to go find greener pastures, presumably without the dick in them). After my thesis defense the chairman of my department apologized to me for that external reader's behaviour. That apology didn't protect me from the agreement that was brokered where he and another idiot ghost-wrote the published version of my thesis, but at least someone had the balls to admit the guy was protecting his work rather than objectively evaluating student work. I'd pointed out what Turing had gotten wrong, reducing thinking to displays of intelligence, and how it allowed people like Searle to further cloud the problem of consciousness with nonsense like the Chinese Room Argument. I still have one of the idiot's journal articles; an author's proof copy handed to me so I could familiarize myself with what Turing really thought. I suppose it's an objective lesson in telling people what they want to hear, especially in scholarship, but what's the point of practicing philosophy if you aren't interested in the truth?

On topic:

The existence of free will is independent of time travel and the truth-status of determinism. There's plenty of theories of ethics that don't require agents to have powers of causation; indeed, it's usually the mark of a decent theory of ethics that it doesn't get bogged down in whether the world is one way or another, but works with what we have whatever that may be.