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The Philosophy Of Time Travel

  
 
Quantum
14:11 / 29.09.06
So the thread in convo about Dan Simmons' essay got me thinking about time travel and paradox and the nature of time etc. and I thought it would be interesting to explore. Given that it's something which might be impossible there's more theory than fact of course, but the various theories depicted in fiction and physics have broad implications.

For example, if we could go back into our own body in the past with memory of the future and act differently (as in Replay) that would rely on the possession of free will and disprove determinism. Or if we can tinker with fate it defies God's will (I admit id's raccoon church dilemma is the seed for that one). Or we could adopt the Terminator stance, 'No fate' where possible futures only potentially exist and we have time streams with possible future like paths to choose in the woods, or the common sci-fi trope of collapsing timelines used in Donnie Darko and Star Trek where everything is reset once the time travelling device is destroyed, or where the worlds time travellers come from are seperate, parallel worlds to our own to conserve causation.

You get the idea. How do you think time works? Will time travel ever be possible? What about the grandfather paradox and the many other paradoxes involved? Does free will enter into it? I'll be back with some links and try to remember the Heinlein short story where he turns out to be his own mum.
 
 
Axolotl
14:34 / 29.09.06
The Heinlein story is "All you Zombies" though "By his Bootstraps" also deals with similar themes.
 
 
All Acting Regiment
13:44 / 19.10.06
Science people would need to help me with this, but does "time" actually exist? I'm sure I read somewhere (and certain experiences would seem to bear this out) that time-as-linear-path is an illusion. I know that's a rather trite little thing to contribute, but I think it's a basic question to ask...
 
 
Mirror
16:00 / 19.10.06
Here's an interesting quantum-mechanical take on time travel to the past. Abstract:


Quantum Theory Looks at Time Travel
Authors: Daniel M. Greenberger, Karl Svozil

We introduce a quantum mechanical model of time travel which includes two figurative beam splitters in order to induce feedback to earlier times. This leads to a unique solution to the paradox where one could kill one's grandfather in that once the future has unfolded, it cannot change the past, and so the past becomes deterministic. On the other hand, looking forwards towards the future is completely probabilistic. This resolves the classical paradox in a philosophically satisfying manner.
 
 
Quantum
23:24 / 19.10.06
Mmm. That's interesting, but it assumes that 'now' is a privileged time- that the future is probabilistic and the past determined, so people (including myself) in the future see our actions as determined and people in the past (including me) *were* determined in their actions and didn't have free choice. If I am looking back from 2012 at myself now, I am embedded in an unchangeable sequence of events.
If we allow causal determinism, what's happening now will shape future events so if you allow that the past becomes determined then doesn't that imply the future is predetermined? Sounds like having your tachyon cake and eating it.

does "time" actually exist? Legba Rex

In physics time is usually defined in terms of the increase of entropy or disorder, the arrow of time, because a lot of subatomic particles are moving backwards in time (antimatter). There are theories of multiple temporal dimensions that allow (for example) diagonal movement in time, and Einstein thought of space and time as one fabric, spacetime.

Philosophically, time is a dimension like 'up' and has pretty much the same sort of justification for existence (although existence is not a predicate).
Space and time are conceptual spectacles according to Kant, categories we impose on experience, and an ancient chinese philosopher whose name I forget said "Space and time are in your mind; your mind it is that creates space and time." It exists inasmuch as anything can be said to exist, but quantum physics says it is arbitrary.

It's tricky.
 
 
PatrickMM
02:20 / 20.10.06
Hopping off The Invisibles model, all of time already exists, but we're experiencing each moment fresh. So, even though the future exists, we still have free will because we make each choice, and thus determine the nature of the future.

That's a bit convoluted, but the basic point is, the future that exists is the creation of our present actions, our actions are not manipulated to create some arbitrary future. Obviously we can't know for sure, but I like that idea because I think it resolves the essential paradox of free will in a way that allows for both possibilities. Also, again sourcing Morrison, the closest we can get to being gods is to look at a piece of fiction. Here, the whole book's already written, but going through it, we don't know what's going to happen next, and at each moment, the characters are making their choice for the first time.

Looking at a piece of fiction, you can travel through time simply by turning a page, but you don't actually rewrite the universe by jumping into the future. I'd imagine it's the same for time travel, if you were going to make a change in the past, you're living in the world where that change already exists.
 
 
werwolf
06:58 / 20.10.06
i'm not very good at the technical and physical aspects of time but from my philosophical p.o.v. there is always the dilemma or rather constraint of perception.

meaning: a human (w/o gadgets or tools, like a 'time travelling device') can experience his/her/its individual past only as memories. memories are not very reliable and also of course subjective, so each individual will have a different memory even of the same moment or phase in the past shared with others. we have no intrinsic way of perceiving the future other than thought-projection, which again is a very subjective and unsure thing. as a matter of fact we can put it onto the same level as 'speculation'. all of this means that the only thing that we can know for sure about is our subjective present. to be simplistic: past and future do not exist.

now, if i were to assume that time travel is possible at all, i would think of it also as a very individual and subjective thing.

let's imagine time as a blanket of infinite dimension with an infinite amount of knots - similar to that vedic image of the net of... who was it? krsna, shiva? where each pearl reflects the others infinitely? nevermind. anyway, if time were of such nature each individual's presence would be one such knot in the blanket. other individuals will be located in the general area of your own 'knot' but cannot be on the same spot, because - as assumed before - they experience the present through their own sense and therefore different from you. some might be very close by, some far away, but none will occupy the exact same place on the blanket as you do at that moment. travelling back and forth in time would mean occupying a different point on this blanket, a different 'knot' of it.

i'd interpret it as like this: all possible futures do already exist, all possible past do also exist. we occupy only 1 possible moment in time. if time travel were possible it could allow us to visit moments in time that we did not previously inhabit. change is therefore an illusion since all future and past and present do already exist. free will is a must as we pass from one moment to another.
 
 
Quantum
13:45 / 20.10.06
So, even though the future exists, we still have free will because we make each choice, and thus determine the nature of the future. Patrick MM

I'm afraid that doesn't work- if the future exists and is fixed, we have no choice. Say the future is you wearing a panama hat, you don't have the choice to wear that beret. You only think you do.
at each moment, the characters are making their choice for the first time. Which gives them the illusion of free will, but no actual freedom.

werwolf- it's Indra's net. change is therefore an illusion since all future and past and present do already exist. free will is a must as we pass from one moment to another. Those two statements aren't compatible. If there is no change, how can I make decisions that affect the world?

Both those posts assume a god's eye view where our future is already mapped out like a railway line or a book or whatever. If that's true, our freedom is an illusion I'm afraid. Compatibility between free will and determinism is a bit trickier than proposing parallel worlds or asserting the primacy of the present moment.

I quite like the 'Time Traveller's Wife' model.
 
 
Mirror
14:14 / 20.10.06
It doesn't seem to me that there's any objective reason we should have free will, however much the notion that we do may be psychologically comforting.

It is telling that philosophers and physicists go to great length to preserve the possibility of free will when attempting to determine the nature of time.
 
 
Quantum
16:09 / 20.10.06
It is telling that philosophers and physicists go to great length to preserve the possibility of free will when attempting to determine the nature of time.

Maybe they have no choice... More seriously, we have a subjective experience of freedom of choice that has to be explained somehow, either by defying determinism or framing it as an illusion.
 
 
Tom Coates
16:12 / 20.10.06
This really really feels like it should be a Laboratory thread.
 
 
Lurid Archive
16:37 / 20.10.06
Science people would need to help me with this, but does "time" actually exist?

Someone with a better grasp of General Relativity should correct me here, but from I understand, "time" is a local phenomenon but not necessarily a universal one. That is, it isn't clear that time really makes sense over the universe, and it is possible (depending on the shape of the universe) that you could travel in time, by travelling in space (at sub-light speeds). From this point of view, time is a tricky beast.
 
 
Quantum
17:19 / 20.10.06
Well if you fly very fast time slows down (relative to an external observer) so by e.g. flying to another star and back you would effectively travel into the future. It's easier to freeze yourself Futurama style though I spose.

It's the travelling back in time that's the hard part. Now we're in the lab we can whip out the Einstein-Rosen bridges and such but I still think fictional sources are some of the best guides to time travel theory. Even Dr Who.
 
 
Lurid Archive
18:02 / 20.10.06
Yeah, sorry, I didn't make that clear as I definitely meant travelling back in time.
 
 
trantor2nd
18:20 / 21.10.06
Someone should search through Astounding, Asimov's, or somesuch science fiction magazines for articles on this topic. I read a science article they had about all forms of proposed mechanisms for FTL, so I'm sure they should have published a similar article on this SF theme.
 
 
Haloquin
19:03 / 21.10.06
I'm not altogether sure why free will isn't entirely compatible with the idea that the future is mapped out. What if the future is mapped out by our free choices?

I'm not sure that we have free will at all, but I don't think that is because the future is already destined (in some externally prescribed way) more that every experience programs us and so at any point we are programmed to choose a specific point. But, taking this out of the equation, say thats not the case...

Out of 2 choices, I can only choose one. (say buying a car, and not buying a car) I either do or I don't.
If I can choose freely, it doesn't change what happened when I look back at what choice I make.

(This is potentially too blurred to make much sense in the way I mean it, but its basically looking at the nature of time as a series of events we move through, and mostly thinking of the direction we look in as irrelevent, i.e. whether we look at the past or the future.)

Even though we will make choice x, doesn't mean we haven't made it freely. Does it?
Like, even just because God can see what we will do (for the sake of argument) it doesn't mean we haven't chosen what we're doing... just that ze knew we would.
Otherwise, couldn't this argument be stretched to say that if someone who knows us well knows what we will choose when faced with two choices, then when we choose what they thought we'd choose, we actually had no choice because they knew we would? (although it could be argued that in the case of a close friend, this isn't knowing, its assuming, guessing, making an informed judgement etc.)

Sorry, I've wandered off topic slightly, just trying to illustrate thoughts on free will and time/determinism.

We look at things in a particular way, a specific direction, and presumably theres a reason we don't see what will happen as clearly as what has happened, even if it is/was possible. (Someone that did see things this way could make an interesting character in a story...) but the choices we made weren't made less freely because we know what happened. Or is this irrelevent?

I guess part of what I'm wondering with this is what free will actually is... I'm assuming it means that we make the choice, not that someone makes it for us.

Sorry if this isn't coherent. Please feel free to poke holes in this if I'm upside down in my thinking.
 
 
Evil Scientist
09:19 / 25.10.06
This is something I've hacked off of an old email I sent to someone a ga-gillion years ago.

Time travel is possible in theory. Matter distorts space/time. So by getting something suitably massive, say a universe, rotating at the right speed you could feasibly twist space and time into a closed timelike curve that would allow you to walk along it in space and also move backwards or forwards in time.

Of course the energy requirements needed to rotate a universe kind of rule out doing this in the real world. But you don't have to use mass to bend space/time, not when you can use light.

A paper published back in 2001 suggested that a circulating laser creates a spacial vortex in the middle of it's circle. Now, shine a second laser in the opposite direction to the first and increase it's intensity enough and you've got a possible form of time travel. Again though, the energy needed would be impossible for human civilisation to produce. But there is an alternative to increasing the laser's intensity.

Slow the laser down.

The technology to slow light down has been available since 1999. I believe they managed to get it moving at about 38mph. The 2001 studies showed that slowed light distorts space/time as well without the need for ludicrous amounts of power. So, in theory, a practical time machine in the future is a definite possibility.

This kind of time machine would also answers the critic's question "If time travel exists, then why haven't we seen evidence of time travellers?". Because you would only be able to use it to travel back as far as the first point that the device was switched on.

So, if it gets switched on at 10.00, November 2nd, 2102. Then that's the first time the future would be able to make contact with it's past.

I believe this paper got published in Physics Letters, and I do recall reading about it in a feature in New Scientist that year. I kept that battered issue in my bag most of the way around Australia, then lost it when I got back to the UK. Typical really.

Still, pretty fun eh? I hope they intend to regulate how many travellers turn up during the first activation of the device, otherwise there'll be a lot of people in a rather cramped space.
 
 
Quantum
17:06 / 25.10.06
The technology to slow light down has been available since 1999. I believe they managed to get it moving at about 38mph

Eh? What?
 
 
Mirror
18:08 / 25.10.06
See article here

Light can been slowed down to just over 200 kilometres per hour using only simple desktop equipment at room temperature, US researchers have shown.

In a vacuum, light travels at 300,000 kilometres per second. It is slowed, for example, by a third when it passes through glass. But slowing light to 200 kph - a factor of 5.4 million - normally requires large and complex laboratory equipment and cryogenic cooling.

Light can even be stopped by causing it to interact with atoms of gas tuned with laser beams. These laboratory techniques use an effect called electromagnetically induced transparency (EIT) to alter the speed at which light travels through a material.

Journal reference: Physical Review Letters (DOI: 10.1103-1/PhysRevLett.90.113903)
 
  
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