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I am a science fiction writer.

I know how to time travel to the future (using black hole or speed). But I need to argue in my novel that time travel to the past might not be possible. Is there a way to do this without using a formula? And how basic would argument look like ?

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  • $\begingroup$ Look at this one my answer's to similar question. I have explained with example why time travel to the pas will not be possible physics.stackexchange.com/q/74181 $\endgroup$
    – Deiknymi
    Commented Sep 22, 2013 at 17:04
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    $\begingroup$ In your novel, who is making the argument? Does this person have the advantage of 30th century physics, or other information not available to the reader in the real world? $\endgroup$
    – Beta
    Commented Sep 23, 2013 at 2:54
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    $\begingroup$ The fact that we're not overrun by tourists from the future, while not a conclusive argument, might be a good intuitive argument for your readers. $\endgroup$
    – Olin Lathrop
    Commented Sep 23, 2013 at 12:32
  • $\begingroup$ You seem to be changing your profession to science-fact writer;-) Stick with Science fiction it's more fun and can become fact in the future. I'll even give you a plot. Now that the Higgs boson is popular you can have your scientist invent an anti-Higgs ray that endows objects with negative/imaginary mass and forces them to move faster than light and backwards in time. $\endgroup$
    – Jitter
    Commented Sep 23, 2013 at 12:33
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    $\begingroup$ possible duplicate of Is time travel possible? Is it possible to go back in time? $\endgroup$
    – user4552
    Commented Sep 23, 2013 at 21:16

6 Answers 6

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Time travel is difficult to prove or reject, even by using a formula (at this time there is no decisive proof for or against it). There are solutions of Einstein's equations which allow closed timelike curves, like Gödel's universe, Kerr black holes, wormholes, etc.

As mentioned in other answers, chronology protection conjecture, grandfather paradox and other causality paradoxes, and the absence of visitors from the future are the most used arguments against time travel. Each of them can be avoided, by creating alternative universes in which you don't exist because you killed your grandfather, or by protecting the past using Novikov's self-consistency principle, or a time patrol or similar solutions, etc.

Also, there are arguments against particular time travel methods. For instance, that you can't use wormholes, because they will become singular or/and self-destroy. There are theorems suggesting that in reasonable conditions closed timelike curves are accompanied by singularities, so if there are no singularities, there is no time travel.

Alternatively, you can make the characters discover fictional laws forbidding time travel.

I am very curious why a science fiction writer would want to reject time travel.

Update

If you are interested in various aspects of time travel (involving formulae), check this conference, The Time Machine Factory, 2012. The proceedings just appeared and are available online.

Update 2

The introduction of this paper by Seth Lloyd et. al. contains a brief review of many results concerning time travel, and references.

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    $\begingroup$ I am very curious why a science fiction writer would want to reject time travel. - Only in one direction. For one possibility, it creates "you can never return" internal conflicts, which can act as a metaphor for growing up. $\endgroup$
    – Izkata
    Commented Sep 23, 2013 at 1:52
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Well, the most trivial one is "Where the hell are all the time travellers from the future today?"

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  • $\begingroup$ What if Jesus was time travaller ? $\endgroup$
    – BioHazard
    Commented Sep 22, 2013 at 19:28
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    $\begingroup$ @BioHazard: Michael Moorcock en.wikipedia.org/wiki/Behold_the_Man_%28novel%29, and also Ilya Varshavsky has a short story, with the subject that Jesus Christ was time traveler. $\endgroup$
    – Cristi Stoica
    Commented Sep 22, 2013 at 20:34
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    $\begingroup$ How do you know those geniuses are not time travellers? They won't tell us that. $\endgroup$
    – Nobody
    Commented Sep 23, 2013 at 10:13
  • $\begingroup$ Because we lack the technology for them to come here. Once we have the technology to receive messages sent from the future things like this could go quick. Some experiments actually show very interesting results where a signal is received a fraction of a second before it is actually sent. But for a novel this would be a good explanation. $\endgroup$
    – Madmenyo
    Commented Nov 14, 2013 at 9:50
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One additional nice feature to consider in rejecting time travel is global entropy of the universe. As far as it is known today, in the past universe had lower entropy and it is impossible to reduce the entropy of the system without dumping the "excess entropy" (plus, whatever additional entropy created in the process) elsewhere.

The same consideration applies to why we can not precisely reconstruct the past (that is, perfect historical knowledge is not possible, no matter how hard we try). Entropy will always prevail and eat some of the precious information.

http://en.wikipedia.org/wiki/Entropy_(arrow_of_time)

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The most basic argument I can think of is the Chronology protection conjecture. In essence, any time machine allows a quantum feedback cycle that will result in the immediate destruction of the machine.

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One theory states that time travel requires cooperation from both sides, which explains why no time travelers have been seen yet. You can only travel as far back as the invention of the first time machine. And even then, it would be restricted to places and times where a "receiver" was set up.

That's of course very convenient for SF writers; it gives them a degree of freedom in deciding precisely where and when time travel fits the plot.

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Have a look at http://en.wikipedia.org/wiki/Grandfather_paradox. I think you can find a nice argument there. You can use the argument that past is unchangeable so it is pointless to go to the past if it can't be changed. Or you can use the parallel universe argument, meaning that somebody who goes in the past and alter something there ends up in a parallel universe where the effect of his alteration creates a completely different present than the present he left from. The parallel universe is different than ours where he can't return to ours.

For clarity, those are NOT scientific arguments. Those are arguments which were made to solve the Grandfather paradox.

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  • $\begingroup$ "it is pointless to go to the past if it can't be changed" — That is far, far from the only reason to visit the past. Just to pick one at random, who wouldn't want to spend their vacation riding an apatosaurus? $\endgroup$
    – jwodder
    Commented Sep 22, 2013 at 22:03
  • $\begingroup$ I didn't think of that! it is wonderful. But still that counts as changing. Just like butterfly effect $\endgroup$
    – Gotaquestion
    Commented Sep 22, 2013 at 22:20
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    $\begingroup$ It's not changing the past if you already rode the apatosaurus 150 mya, which you obviously did if you travel back then from now and ride it. $\endgroup$
    – jwodder
    Commented Sep 22, 2013 at 22:24
  • $\begingroup$ @jwodder I am really confused now. $\endgroup$
    – Lescai Ionel
    Commented Sep 23, 2013 at 15:02
  • $\begingroup$ @LescaiIonel right, this is the point of time paradox :) $\endgroup$
    – Ruslan
    Commented Sep 24, 2013 at 14:47

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