Friday, 31 August 2012

'If time-travel is possible, where are all the time-travellers?'


Philosophy Of... returns, after a short leave of absence! 

Let’s assume that time travel is both logically and physically possible (contrary to the conclusions drawn in my previous post on time travel). A common objection that is raised to the prospect is really just a common-sense question: if time-travel is possible, then where are all the time-travellers? If time travel is possible, then we might expect future time-travellers to have arrived in our time. Or, more formally:

  1. If time-travel is possible, then at some point in the future, humans will time-travel. 
  2. If humans will time-travel, they will travel back to our time. 
  3. If humans will travel back to our time, then we should have encountered them. 
  4. We have not encountered any time travellers. 
  5. Therefore, time travel is not possible.  

I don’t think this (the ‘future time-travellers argument’) is a very strong argument, so let’s evaluate it premise by premise. It appears that the argument is valid; that is, if all the premises are true, the conclusion cannot fail to be true (its truth logically follows from the truth of the premises). But is each premise true? I don't think so. This will be a common-sense investigation, rather than a technically philosophical one. 

Premise 1 
That humans would definitely make use of time-travel given its possibility can be easily questioned. We only have a finite time on the Earth (scientists tell us) before it is annihilated, and there doesn’t seem to me to be anything that guarantees we will have grasped time-travel by then. Perhaps it is too complicated to be grasped by humans even given 1000s of years. This certainly seems like a realistic possibility. 

Premise 2 
Let’s define ‘our time’ as the period between when human history records began and now. Even with such a general definition, this premise is also questionable. It might be, for example, that time-travel is developed, but only developed enough to allow random time-travelling, and not to specific locations in time which the future humans choose. If so, it seems perfectly possible that future humans, popping randomly into the past, never hit our time at all, and have to make do with some dinosaurs for company instead. After all, ‘our time’ thus defined is only a tiny region of the available pool space in which a time-traveller can plunge. 

Premise 3
Even if time-travellers did travel to our time, perhaps they might have reasons for not revealing themselves to us. This response is less plausible than my responses to premises 1 and 2, but nevertheless must be considered. Science fiction makes much of the fact that small changes in the past can lead to large changes in the future; perhaps it would be dangerous for future generations for time-travellers to reveal their identities to present-day folk. Maybe they were briefed about this by their leaders before setting off! 

Premise 4 
The truth of this premise depends on how we approach premise 3. It may be that if time-travellers have travelled to our time, they prefer to stay hidden. This makes it likely that some present day folk have encountered them; they just don’t know it. 

Conclusion 
Since there are good ways to challenge all four premises of the argument, I think we must conclude that this argument does not convincingly demonstrate that time travel is not possible. The discussion has strayed into the realms of science-fiction, and because of this, tongues must be kept in the vicinity of cheeks. However, there is a serious point to be made: if the above critique is reasonable, then a popular argument for the impossibility of time-travel is rebutted. This is to say nothing, of course, of the possibility of time-travel more generally. Considerations from physics may show that it cannot happen anyway. But at least if we do want to rule out the possibility of time-travel, we cannot do it by means of the ‘future time-travellers’ argument.

23 comments:

  1. We are all time travelers. We all travel through time on a daily basis. The reason why time travelers haven't come from the future is because that would create a paradox in the Universe thus rendering it impossible. If one would like to travel out of their time, only into their future would be possible by either a) travelling the speed of light for a short time or b) orbiting an extremely dense object in space-time.

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    1. Well, *there's* a productive counter-argument!

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    2. i need help with this too i need to go back in time

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  2. Don't forget the fact that the earth moves! A time traveler would die in the vacuum of space if they can't change their location.

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    1. But then the sun moves, and the galaxy moves and the local group moves and the supercluster moves. 'Moves' relative to what is the real question, why is the time traveller suddenly fixed to a specific reference frame and what reason prevents this frame being the Earths' rotational frame?

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    2. This comment has been removed by the author.

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    3. This reply was to the reply to you, but apparently I can't reply to a reply? *shrugs*

      Or rather, why are they fixed to any reference frame at all? They could easily end up in empty space or perhaps even inside a star or planet. That is, assuming time travel into the past is possible. I don't even think Time is really what we commonly say it is, so the whole argument is kind of moot honestly.

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  3. The only reason I can think of is the humanity dies out before it is capable of time travelling.

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    1. Well, now I'm depressed.
      Q: Why haven't we met any time travelers?
      A: Because everybody died.

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  4. What about the fact that Earth is always moving through space at n incredible speed? If I went back in time or whatever and ended up in the same location, I'd be in open, empty space.

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    1. I think any future civilisations that are scientifically advanced enough to discover time travel will be advanced enough to travel through space.

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  5. One theory I once heard about time travel is that it may be possible but would be limited to only go back in time to when it was invented. That is, if the first time machine is initially enabled on Jan 1, 2013, it would not be possible for someone in 2014 to travel back to 2013 but not possible for someone in 2013 to travel back to 2012.

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    1. I was thinking the same thing. It may be possible that the time travel device serves as the "homing beacon" to solve the whole problem of showing up in the middle of space.

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  6. Another civilization who mastered time travel say, 100 thousand years before mankind, could let us use their machine.

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  7. If we don't want time travel to lead to absurdity, we can suppose that the future that a time traveller came from is not the future of the time/space he travelled to. i.e at the moment the time traveller reached his destination, the future he came from ceases to exist/is inaccessible. This is kinda similar to the premise of the film 'Primer', if we ignore the two instances of the person being in same time for the duration of the travel (one being in the backwards travelling box and one travelling forward in the normal manner.)

    http://en.wikipedia.org/wiki/File:Time_Travel_Method-2.svg
    ..It's a really good film.

    kidna similar to/inspired by:
    http://en.wikipedia.org/wiki/Many-worlds_interpretation

    Also, the first part of what the anonymous above said is true, relativity says we are always travelling through spacetime at the speed of light, thus we can see that if we aren't moving in space then '60 seconds per minute' is the speed of light in the time 'direction'. Measuring the relative rate of time in different velocity frames is tricky, and the language gets kinda annoying/ambiguous, but I hope I made it clear.

    Nice blog by the way!

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  8. Time travel includes not only going back in time, but also going into the future. Therefore it very well could be that time travel to the past is impossible but time travel to the future is possible.
    Time travel to the future is actually proven by a linear accelerator. Particles with a life span of some certain time, when accelerated to close to the speed of light, have been shown to "appear" to live around 300% longer, which is not the case. They are actually experiencing time at a slower rate than us. Though this is the physics side of the argument.

    Stephen Hawking has a "simply test" involving throwing a "time travelers party" at noon that will last until 1. But he doesn't release the notice/invitations until after the party. (i.e creates a program that is on a timer to send out invitation.) Therefore he waits for the time travels to show up as the clock approaches 12. No one shows and it's a big disappointment. Of course this is a slight form of comedy he actually has a theory as to why we haven't seen any time travelers. And it has to do with something of paradoxes. His theory involves that the state of natural laws make it impossible for paradox to ever happen. (Ex of paradox: a man going back in time to kill himself. Who shot and who is dead/alive.)

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    1. Non so simply I presume. Need to code for a Party.

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  9. I need that clock. 0-0

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  10. Whatnever your O'clock is does it not matter to time travails. I premise. That they do, exists in travels to our times. Which we see them on days alltimes.

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    1. Alsos. Time travel back to the time, but also in the future not only to go. so, so well that time travel is impossible, but to the past for the future time travel is possible.

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  11. I don't buy your answer to premise 3. It supposes that ALL time travelers would reach the same conclusion about revealing themselves and none ever encountered situations where they'd be forced to reveal themselves even if they didn't want to.
    In other words people are different and if time travel were possible and achievable within our species' lifetime, we'd have to presume some would have less than honorable motivations.

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