the (im)possibilities of time travel [Archive]

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Skepticus Rex

5th August 2006, 04:29 PM

i'm not sure this is the right section to put this in, but i don't think it fits in science, and not in any of the others, so pseudoscience it is.
this isn't a study/debunking of people claiming to have travelled in time, or to have met people who have... it started as a project where me and a friend of mine tried to contemplate as many reasons as we could how time travel could be (im)possible. however there was more contemplating and less listing, so i can't remember half of them. i thought we did quite well though, because we didn't just show how time travel couldn't fit into our current understanding of the universe, we also followed through some of the changes that may make it possible, and showed how it was impossible in them.
anyway, i have thought further on the fundamentals since then, and want to see what people think of my conclusions below...

by 'time travel', i mean travelling backwards in time, since everything travels forwards in time all the time (and if you want to travel to the 'future' without experiencing the intervening time all you need is a way to remain in stasis for indefinite amounts of time, and a good timing mechanism), going backward is where problems arise.

the most important thing to remember about time travel (which would it really boring in sci-fi novels), is that even if you could travel back in time, you could never make changes to the timeline, since at the moment you depart the 'present', everything in the past has already happened, including you arriving in the past and doing whatever you think will change it.
many-worlds theory doesn't sort this out either, since alternative events don't create new universes, but rather there is another universe where everything has been the same up to the event in question. therefore if you wanted to be in a reality with some changes, you would have to somehow transfer yourself to the alternate reality where things are as you like it (which deserves it's own set of impossibilities).

now if you think about the above situation, it prompts an idea for the impossibility of time travel. namely: how do you define the 'present' (and therefore the past and future)?
any travelling back in time will shift your subjective present, however that won't cancel out your subjective past. your subjective past is already set in stone, including the time that you have travelled to, which is why you can't change anything. the moment you left would be the 'present', however, if any other people from further ahead travel back in time, then the moment they left would be the 'present', the latest point of departure would always be the 'present'. (since previous time journeys would be part of the past)
now, the invention of time travel at any point in time would enable time journeys from every point of time after that, because whatever enables the travelling will move forward in time like everything else. this means that there could only be one objective present: the very end of time (making the entirety of history set).
but in an infinite universe, there is no limit to time, therefore no end, and no objective present. therefore all time is subjective, making time travel impossible in nature. (except, theoretically, for one journey backward in time, with no return. however there are arguments against why time travel is impossible physically as well).

what do people think?

PS. i've had some ideas about how to explain why the universe must be infinite in time, if anyone wants to know.

wollery

6th August 2006, 12:46 PM

My favourite argument against time travel is that we haven't been inundated with tourists from the future! ;D

On a more serious note, the changing the time line problem is known as the grandfather paradox, and is put like this, "What would happen if you were to go back in time and kill your grandfather before your father was born?" The answer is of course that you yourself would never be born, so you couldn't go back in time to kill your grandfather, so you would be born, so you could go back in time to kil....

You get the idea.

Of all time travel films The Terminator is my favourite, for pecisely that reason.

Admin

6th August 2006, 07:15 PM

If I were to design a time machine and got it working in the year 2010 and then went back 100 years to 1910 could I do nothing or anything I liked?

Well it's 2006 now and so 1910 has already passed. Surely my existence in 1910 would already have happened. Even though I have yet to build my time machine, I should still have popped up in 1910.

I think it wouldn't matter what I did in 1910 as even if it changed history it would be history as we've known it since 1910. I think that holds true unless I do something to prevent myself being born; but then again, I couldn't have done that otherwise I wouldn't be here.

What if I went back to 1990 and met myself? :ponder:

Well that can't happen as I never met myself in 1990 (which I would have done if I'd come back from 2010). Of course I could have nipped to the shops and missed myself. ;D

Great these mind games, yes? :D

tkingdoll

6th August 2006, 10:20 PM

We can get round those issues by having time travel as an observer state, kind of like a one-way observation window to the past. You can't interact with, or affect anything, and you can't be seen, but you can view backwards through time rather than physically go there.

OK, so backwards in that context seems plausible, but what about forwards? If it hasn't happened yet, then you can't observe it.

vbloke

7th August 2006, 09:02 AM

tkingdoll

7th August 2006, 12:39 PM

My personal thoughts on this is that, since a lot of events at the subatomic level appear to have CP symmetry (meaning they can happen forwards or backwards and appear the same), then time travel is theoretically possible.

The fact that we're not overrun by time tourists could mean that we never develop the technology or means to time travel. It may simply be impossible at a macro-level.

Again, I guess it depends what kind of mechanism we're talking about. If it's something like teleportation into the past, then that's not possible simply because teleportation of a person isn't possible (not enough energy in the world, too much data storage needed, and possibly something about Heisenberg's uncertainty principle :-\). However, that's only physically moving a person through time. What about the sci I mentioned, in which you observe the past but don't actually go anywhere?

I'm struggling to work out what the physical mechanism would be that creates the window to the past, though. Help me out here!

Skepticus Rex

7th August 2006, 01:00 PM

i once thought up an idea (based on an amalgamation of several things from science fiction) of how you could view the past (although not physically viewing) using a computer model. it would require amounts of energy, data storage, processing power, mass/energy detection techniques and knowledge of the universe that are almost inconcievable considering our current situation, but i won't go into details (you can tell what it is from what i've said). alas, it falls down at heisenbergs uncertainty principle, since you'd have to observe and record the state of every single particle of mass and quanta of energy in the universe at one moment in time.

tkingdoll: you can actually (as in, without breaking the laws of physics) travel forwards in time in several ways. they arn't literally taking you out of time and putting you back in later, like most 'time machines', but they would work.
one that i mentioned in my first post is putting yourself in stasis (whether freezing, or some other method), and then reviving yourself in the 'future'. although it would be impossible to counter entropy completely, theoretically you could slow it down enough that only the equivalent of fractions of a second would have passed for you.

the other is using relativity to your advantage. this is actually one of the traditional sci-fi time travelling methods.
if you speed up to close to the speed of light, time would slow down for you (you being the entity, eg. ship, travelling at that speed), and then you could slow down, turn around, and come back. depending how fast and how far you go, you would arrive in the 'future'.

neither of those is likely to be possible any time soon though, because we have no idea how either would be done. also, you wouldn't be able to get back again.

Admin

25th July 2007, 08:56 PM

This is a restored post by CarlosNevaeh which was inadvertently deleted.

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666

25th July 2007, 10:02 PM

median

2nd August 2007, 08:30 AM

Caught this in the Daily Mail. http://www.dailymail.co.uk/pages/live/articles/technology/technology.html?in_page_id=1965&in_article_id=471359

Anyone here familiar with Ron Mallett and his ideas?