In the beginning of this SEP article is the discussion of contradictions a backward time travel may bring (e.g. the classical grandfather paradox) and possible ways to "eliminate" them. My question is: is the very simple act of just being in the past contradictory, let alone the act of killing your grandfather?

  • It depends on the rules upon which you consider backwards time travel. If you dismiss the scientific rules we know (e.g. as in cartoons or religions), logical/physical contradictions exist, but have no effect on existence. If you consider philosophical (causality, the subjectivity of time) and scientific (2nd law of thermodynamics, cosmic topology, etc.) rules, then, it is contradictory and binding (empirical/physical "contradictions" have a destructive effect: push and pull something at the same time: you lose energy, increase your entropy and get closer to dissipation).
    – RodolfoAP
    Commented May 27, 2022 at 7:48

6 Answers 6


It should be understood, to start, that any answer to this question will be entirely speculative — this is not a question of logic, but of physics, i.e. it is an empirical question (albeit about a phenomenon that no-one has ever knowingly observed and reported). Any answer we provide is merely a proposal which could possibly serve as a principle for the development of physical theory, in any theory of physics which possibly allows for time-travel. With that being understood —

The classic paradoxes of time-travel are not ones of logical consistency so much as they are ones of consistency of the historical record. There are different solutions to this, which are so well-worn that they are well-represented, for instance, in science-fiction literature.

  • Alternative worlds: our history is one of many possible histories, each of which are developing at the same time. Your presence in the past is a parameter which serves to partially describe these histories. When you travel back in time, you may not end up in the same historical line that you started off in: you end up in a subjectively different — but objectively equally real — timeline. Furthermore, if you travelled again in time, there's no particular reason to suppose that you wouldn't ever end up back in your original time-line, so long as your appearance were consistent with the history of the timeline.

    This requires that we can posit more worlds than we currently observe, and may prove unsatisfying for this reason. Of course, we haven't developed time-travel, either; and time-travel would be one way that we could attempt to determine the existence of co-evolving divergent historical timelines.

  • History as a fixed point of time-travel: one can answer your question "is the very simple act of just being in the past contradictory" with another question: how do you know you don't exist in the past already? There is no reason why you couldn't travel to the past, immediately be hit on the head by a falling object, and be a mysterious dead body discovered by the police or a wild animal. Or: you could be a John Doe in an asylum or burned at the stake for making the obviously crazy claim that you're a time-traveller from the future.

    Or: you could be wily and fit in to your society — possibly succeeding so well that there is little to no trace of anything strange in your having appeared there. So little trace that no-one suspects; and in particular, you never found out that you were already there in the past, before you decided to travel back in time.

    In fact, how can you be sure that you aren't already an ancestor of yours? The chances of your genes mixing with others over several generations to accidentally produce an exact clone are vanishingly small, of course — the key word being "vanishing", as in extremely small to start with and diminishing further over generations, but not actually impossible. Rather than a "grandfather paradox", you become a "grandfather fixed point": history and the chance meeting of your ancestors has developed in just such a way so that your birth and travel backwards in time are logically consistent with a single historical record, and — possibly — so that your later contribution to the genetic pool gives rise to yourself as if by accident.

    This may seem dissatisfying because it involves a circular pattern of cause and effect: but again, there is nothing actually logically inconsistent with such phenomena — as with time-travel, we just haven't observed any examples of it, and so have no particular reason yet to believe that they occur.

  • On the notion of a fixed history that already includes the influences of time travelers and is bound to be totally self-consistent, I wrote up a post on the sci fi stack exchange here which sketches a way in which such histories could be generated in an algorithmic way (given a sufficiently huge amount of computing power), perhaps making the idea a little more intuitive.
    – Hypnosifl
    Commented Sep 21, 2020 at 21:59
  • @Hypnosifl: The film "12 Monkeys" also does a good job at illustrating something of the fixed-point concept, albeit not quite in the form of being one's own ancestor. Commented Sep 21, 2020 at 22:08

Obviously this will largely be opinion, but my answer is yes. If we are referring to physically interacting with the past (even so much as breathing the air in an open field) then history will be changed from the one that spawned the original "you."

Simply breathing the air exposes the past to your "future" virii/bacteria. Even assuming no people are around, there are plenty of living things that may prove an effective host. If you actually are around people, infection rates go up wildly (think Small Pox vs the Native Americans). Even bacteria thought harmless to you could reek havoc on past populations.

Another example would be your natural bodily functions interacting with the environment. The food you eat while in the past is no longer there to be eaten. It seems like an inconsequential thing, but that means someone may now go hungry, might get food poisoning when they wouldn't have (or NOT get food poisoning), or have their daily activities significantly altered to procure food for you (or food for themselves to replace the food eaten by you). Even if you bring your own food, that is additional matter entering the environment (packaging, feces, et cetera) that wasn't there before, and could impact the past in a similar manner.

Physically travelling into the past is wrought with "butterfly effect" issues. No matter how careful you are, it is likely going to make the future you left different than the one you return to.

  • Though there should be "butterfly effects" to some extent, my question is not really about interacting with environment. To understand my question, consider the example: I sit in my room for 5 minutes, then enter my time machine (which is also in my room) and travel 5 minutes into the past. But it's a contradiction! When I travel 5 minutes into the past, I see my earlier self, and he sees me, but it is me 5 minutes ago. I didn't see myself 5 minutes ago in my room, though here I am! There should be in a sense only one past, the past.
    – user132181
    Commented Apr 15, 2014 at 16:04
  • I see, but it runs into the same problem on the micro scale. By encountering "You -5 minutes", you have drastically changed the past. Now, "You -5m" might decide to go for tacos instead of continuing to perpetuate the cycle. And what happens then? Will there be two of you from then on? Unless either of you hope back in one of your time machines, then there is no reason "You" and "You -5m" can't continue to coexist. So backward time travel necessitates a multi-dimensional view, because doing so will change the past. This will lead to a different future than the one you left. Commented Apr 15, 2014 at 16:28
  • I understand your point (and I even upvoted your answer), but you still haven't answered my question.
    – user132181
    Commented Apr 15, 2014 at 16:37
  • Yeah, I could have been more direct there. Yes, simply being in the past is a contradiction. You weren't there "originally", and now you are which changes the past (maybe slightly, maybe greatly) from the one that caused you to go back in time. Whether this contradiction means that time travel is impossible or leads to multiple dimensions is up for debate, but "You were there 5 minutes ago" is direct a contradiction with "You weren't there 5 minutes ago". Commented Apr 15, 2014 at 17:13

To conclude whether the fact that I was in the past as a result of time travel is contradictory, I need to decide who I am. Otherwise, why can't I say that the stone which lied along the road in 2000 was me? However, this question is not at all easy. Materially, a human is changing constantly. And, when some decide to consider these changes, in a supposition that these changes constitute what the human is, they find out that it is difficult to discern the changes that concern the human only from the changes that concern the rest of the material Universe…

So, the reduplication of me, which is what is meant by “time travel”, is possible only if I exist. But do I? To feel something consistently, it is not necessary that the thing to be felt exist…


I thought a little more on this. What's a scientific fact? Whenever I observe A, I observe B. Itself, a scientific fact has nothing causal in it. We can develop causal notions ourselves from this fact in order to use it. That way, we declare that it's A which depends on B or it's B which depends on A. It's the voltage that depends on the current in the specific case, or it's the other way around, if we believe the voltage to be fixed.

Now, the purpose is to find a scientific fact that connects some actions that I can do know (make the time travel happen) and existence of my other self in the past. The scientific fact that I have to find ought to concern matter only (my ideas on matter), that is it ought to be a physical fact. To find out whether the purpose has been achieved, I ought to establish the physical fact, and I should connect one of the observations with my two material selves. I need to conclude that I am both selves, so I should answer the question who I am, materially.

The first part is left to physicists. There can be no contradiction within this part, if the physics for something that can be interpreted as a time travel can be found. The second part is conventional, by itself it is free of contradictions, too. I may follow the convention that the cosmonaut who returns from the space travel to Sirius is myself who enters the spacecraft now, even though I shall have gone through so much experience that'll change me. Or I may not; anyway, the consistency of the world is what the physics has dealt with in the first part.

In short: there is no contradiction, just some vagueness. The past has to be constant, though, unless our physics becomes truly post-modern.


Yes you are very close to the answer in your question. The notion that backwards time travel and hence impossiblity of alteration of history is not limited to you being a human or a life form or anything intelligible.

For the universe you are collection of particles with some states they are in. The arrangement of these particles is what drives all events and happening in the universe including your thoughts.

If we assume that time travel is impossible then the universe forbids any arrangement of particles that would lead to any particle or anything to travel back in time. Hence no thoughts pertaining to building a successful time machine would ever be conceived by anyone since it would lead eventually to time travel which the universe forbids

  • I'm not sure I see how this answers the specific question asked. Can you make that more explicit in your answer?
    – E...
    Commented Jul 22, 2016 at 19:39

Well, if I already was in the past, and my existence in the past was scientifically explained by time travel, then I can predict my future… I mean, I can scientifically predict from the data that I can gather observing the past that in the future I will travel back in time… The question is, is it contradictory to anything established that my free will is so constrained by science that I can know what I will do in the future and not escape it in any way possible? If it is, then backward time travel is indeed contradictory.

  • 1
    Welcome to Philosophy.SE! As written, it seems that this post merely reflects your own view. Typically, we encourage answers to be objective, meaning you don't write in the form "P is true, because ..." but "Philosopher X would say P, because ... (see here: ...)". Can you edit your answer to improve it along those lines?
    – user2953
    Commented Oct 14, 2016 at 21:12

It depends what is meant by..."The past". Is the past one second ago or one millennium ago? If one was to ever travel back in time, exactly how far would he or she travel within the KNOWN timeline?-(that is to say, a conventionally acceptable timeline established by Historians, Archeologists and Physicists).

The past can range from the prior Nanno second, to the Big Bang. If we were to solely focus on traveling within the human past, are we talking about actually exploring and visiting our earliest known origins dating back 3 million plus years or are we primarily focusing on traveling within the last 6000 years-(the birth of systematic advancements in agriculture and the birth of writing).

Would you really want to actually witness your family line and how it truly developed over the years?...that is to say, meeting your parents or grandparents at a younger-(or even much younger age) than you. The bizarre, as well as the incomprehensible nature of such an experience would be too much for you, (as well as your younger parents and younger grandparents)....to handle.

Rod Serling's Twilight Zone addressed backward time travel in a few of his episodes. And in nearly every time travel episode, the results were negative and potentially destructive to the time traveler leading Rod Serling to coin a poignant phrase...."Stay in your own backyard". In other words, do not probe into and reorder the existence of time...it is best for the past-(and for time in general)....to remain as such.

("You can't step in the same river twice".....Heraclitus of Ephesus).

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