Why can’t I kill my grandpa? (Time travel, part I)

So, this is the first post that we will dedicate to the question of time-travel in physics. We’ll start easy, but things may get pretty confusing soon, so behold!

Of course, we’re all time traveling, right now. We’re traveling towards the future, at a rate of one second per second. Strange speeds in our time travel appear as early as the Mahabharata, when king Kakudmi visits lord Brahma for some chat and, when he returns, many years have gone by. Yet, travel to the past appears later in stories, and mostly for the pleasure of anachronism. The time machine appears by the end of the XIX century in a short story from a Spanish writer, Enrique Gaspar y Rimbau, el “Anacronópete”, where the theory is exposed that it is the atmosphere causing the flow of time, as can be checked by the conservation of food in hermetic cans… His machine travels to the past much like Superman, flying against the rotation of the Earth.

The first story to deal with the problems of time travel to the past seems to be
Tourmalin’s Time Cheques, by Thomas A. Guthrie, under pseudonym in 1891, which I can’t discuss yet… (it’s in my reading list).

To the best of my knowledge, the first story which shows the problems and paradoxes of time travel to the past in its full glory is By his own bootstraps, by Robert A. Heinlein, in 1941. If you enjoy discussion about these topics, you really should read that short story.

The first and foremost paradox of time-travel is the grandfather murder case. I travel 50 years back in time and kill my grandfather before he meets my grandmother… so I can’t be born, and can’t kill my grandfather. So, if I do A, I force not-A, which forces A… what is the way out? Somehow, something should prevent you from killing your grandfather, so that history remains coherent.

We physicists love to give a name to everything, so we’ve baptized it as the Novikov principle. History should be coherent. Perhaps, after all, I do not have free will, I can’t choose to kill my grandpa… You see, the paradox with people gets somehow out of focus. Let us put it up simply will balls. This way, we call it Polchinski’s paradox:

We have a time-machine which has an input slot, an output slot and one dial. If you put something in the input slot, it will come out of the output slot some time before given by the mark in the dial. OK. Now, we put the dial to “1 second” and throw a ball towards the input slot. The same ball will come out of the output slot 1 second before the original one hits the input slot, OK? Now we can fix the geometry so that the second ball hits the first and puts it out of the way. So the output ball will prevent the input ball from entering the machine and, therefore… where did the second ball come from?

Polchinski's paradox

There are ways to overcome this paradox. Can you think of any?

It's my turn on the time machine!!!!!

It's my turn on the time machine!!!!


18 thoughts on “Why can’t I kill my grandpa? (Time travel, part I)

  1. Well, personally i don’t think time-travel is posible. The only way one can tavel in time is “letting” his own time flow as he “goes back” in the main time current. Theres no way one can be “outside” of the time current. if we take Time as reference frame, we can’t move within it from “inside”, as we can’t interact with a 4th dimension from the 3rd. We can “viasualize” tesseracts, but we can’t build one anyway. The same way, we can “visualize” time-travel, but just that.

  2. That’s the funny part, Erynus. We believe in the possibility of time travel. The reason: general relativity has been tested to an amazing precision. General relativity predicts time travel in certain cases, which I will discuss soon. Therefore…

    The fact that you can’t visualize it is irrelevant to mother Nature… :)

  3. Erynus is right, time travel is impossible, as these paradoxes show. Tachyons are considered impossible becos they lead to paradoxes, like the loss of causality, no?

  4. Hypazia, this is the very question. Is this paradox impossible to solve? There are some schemes to avoid it, I will explain them soon and you will be able to decide by yourself… But there are also important reasons to believe in the possibility of time travel. Mostly, the success of general relativity.

  5. why the universe should keep track of all possible paradoxes and fix all of them by “dark means” when It can just forbid the posibility of paradox?
    If time-travel is posible, then the paradox must occur.

  6. Then, Erynus and Hypazia, do you propose that general relativity must fail? I do not propose dark means to overcome the paradox. In fact, the mechanism might be pretty simple…

  7. Yeah, of course. But simpler than cut the problems at the root?
    What i mean is that the universe don’t want any paradoxes, the simpler way to avoid it is to forbid the travel in time. On the other hand if the universe want/permits paradoxes they will occur whatever you try to avoid it. There’s no middle point.

  8. Hm… again, I describe the problem. General relativity explains amazingly well many observations in astrophysics. No other theory has such an amazing predictive power in this area (so far). General relativity predicts that time travel is possible, under certain circumstances. Hm… I’ll prepare a new post on the proposed ideas to overcome the paradoxes.

  9. If general relativity is correct and consistent, and it predicts time travel, it should give the answer to the paradoxes, right? In other terms: if the theory contains paradoxes, then it is not consistent, and still you may get results from it under certain approximations, like all time-like geodesics being open… (i.e.: no time-travel)

  10. So, do you think GR is somehow like QM, that needs special care of some physical phenomena – i.e.: measurements? Hm… I don’t think so, I still think time travel is feasible and that no inconsistencies can arise when you read GR well…

  11. But you enjoy challenges, right? :) I think it is a foreign language to us all (right, hypazia?) And, in a way, it is not. Was latin a foreign language to Petrarca, Erasmus or Newton? Only we have to make the British and American stop using it so that it becomes a dead tongue we scientists can use without remorse XD

  12. It’s hard to put in simple words complex thoughts. But put in simple foreign words complex thoughts is a titanic effort.
    Anyway, we have two scenarios. If paradoxes really exists they will occur every time. If paradoxes are just apparent, they are not paradoxes at all. So assuming that a paradox is a real paradox, if you travel in time you’ll cause paradoxes.

  13. You cannot evade a paradox, cause if the paradox don’t happen, then is not a paradox. You can’t have half a paradox. So if time-travel implies a paradox there’s no way you can have time-travel without paradoxes. Either you’re cattish or not, you can’t have the good part without the bad one.

  14. Pingback: Time travel from classical to quantum mechanics « Physics Napkins

  15. Time is an illusion.

    Here are the rules:
    1. The Universe is Debris of change in Possibility
    2. Time is continulus change in Possibility
    3. Singularity is the moment of change in possibility
    4. Only life can create change in Possibility, and only from outside the Universe.

    That Means that String THeory invalidates Religion and Evolution.

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