Over the past week, I’ve engaged in some verbal gymnastics about the Grandfather paradox. The Grandfather paradox is one of those well-horned, logic puzzles. Could I go back in time and kill my grandfather before he reproduced? The obvious answer seems to be “No” since I would not have existed in the first place. Killing my grandfather before he could reproduced logically entails that I do not exist. It is logically impossible to maintain both propositions (1) that grandfather died before he reproduced and (2) that grandson killed him before he reproduced. If the grandson exists (2), it is contradictory to also maintain (1). Grandfather must have reproduced in order for grandson to exist so the murder of grandfather before he reproduced, no less than by the grandson himself, is an impossibility. And from there we go. Time travel is impossible because it entails logical absurdities. Thus, when it comes to defend or explicate the possibility of time travel ( as many do for philosophical and/or scientific reasons) one of the issues is to explain this paradox: to give an explanation that answers the grandfather paradox, to deflate it’s apparent force. And to do so, normally, without giving up some other assumptions, namely, that the time traveler is physically able to do what he is able in the present and there is only a singular causal line.
That last bit is important for what follows. It is my attempt to show that it is the conjunction of all three assumptions ( time travel, physical freedom, singular causal line) together that generate the paradox, a curious proposition, because it problematizes the straightforward inference “If grandfather died before he reproduced, I would not exist” that it generally begins with. But by doing so it undermines the general attempts at a solution to the paradox that attempt to vindicate time travel while precluding the murder of grandfather by grandson when they’re in the same physical space. These demonstrations are often meant to show that time travel and freewill are possible, or at the very least, that time travel is possible without constraints – you’re physically able to do in the past what you do in the present. You’re not physically able to do the logically impossible though, therefore, you cannot kill grandfather. This is the essence of the response that attempts to have time travel and the other assumptions together. From it, several escapes are constructed: all of them epistemic. Perhaps, when grandson goes to kill grandfather, the gun jams, he slips, or he kills the wrong man, etc. All valid escapes normally considered. There are plenty of reasons why things don’t happen but no one ever thinks that in direct confrontation with a person their semantic relation to them will have an impact on the physical conditions – this is the essence of the problem I bring up. The content of what is being pictured in these time travel scenarios contradict the straightforward entailment that is the focus. Ordinary concepts are used to picture the paradox but a retreat is made to more formal, esoteric concepts to solve it leaving the relation between the picture that generates the paradox and the solution to it tenuous.
So, let’s begin. The first thing we do is grant that time travel has occurred in order for the debate and mysteries to come up. Second, we carry over the auxiliary assumptions that one has traveled back in time and physically able to do what they’re able to in the present. Immediately, problems arise. Once it is stipulated that a person can travel back in time and be in the same physical space as their grandparent -akin to the physical space between me and my computer- Logic as such cannot prevent their death. Where we picture two physical beings in direct contact with each other, it is prime facie possible for one to strike the other fatally. Remember that in order to time travel, the grandson’s existence has been granted. The grandson’s birth and his traveling back in time, therefore, are two distinct events. How this second event [ traveling back in time] constrains causality across or within dimensions ( and hence, physically possible actions) is not immediately clear. If we grant time travel, what can prevent someone from going back in time and doing the same things they can do in the present – whether that’s committing a murder against anyone ( including grandfather) or having a cup of tea?
But it maybe responded ( and was responded ):
“Is it possible to do something in the past that you didn’t do in the past?”
The intuition here is that the past cannot be changed yet we don’t take this to constrain freewill. Here, it is not necessary to get too caught up on the thorny notion of freewill as this is just a way of indicating that the past being physically closed off doesn’t mean the present and future is. In an indeterminate world, that the past is already determined doesn’t tell us by itself what the future or present will bring. The inability to change the past does not indicate that every action that is possible now, is somehow not possible. In the same way, we can’t change the past in which grandfather reproduced ( as this guarantees our existence) and this should not really bother us. If I am alive then then neither I or anyone else killed grandfather before he reproduced. Q.E.D. But there is a bother here. This argument contains two different senses of “the past.” The past that is visited and the past that is not visited. The problem is that while it is obvious you cannot change the latter, when is it postulated that you can visit the former and walk around and be physically “free” in it, it becomes questionable. What I did do in the past “not visited” does not, in my view, entail that I must do the same in the “the past visited.” This is one of the major issues – whether the logical entailment expressed in the above “Is it possible to do something in the past that you didn’t do in the past?” pertains only “to the past not visited.” It is my suggestion that this is the case – the transference from “not visited” to “visited” is illicit. We’re trying to make two distinct events equivalent. That is, the counter position takes the past visited and the past not visited as equivalents and derives what you must do from the latter. Being in the same physical space as someone in a past time is is a different situation from them being in the same physical space in a future time. Yet the same laws of physics apply so why would it be impossible to commit murders just because you went back in time if it is postulated that you can be free in this past time? The only way to make the semantic argument is to equivocate “the past visited” and “the past not visited.” That is, make them identical so it is possible to derive all conclusions about what can happen from the latter while ignoring the content of what is postulated in the former – to suggest a person can do anything in the past when they time travel yet list of everything they will do simply because they existed in the past ( and can only do what they did prior) seems contradictory.
But there is only ONE actual past!
Here, again, we run into the same issues and where we begin to see more clearly that it is the conjunction of assumptions that generate the paradox and not so much the logical entailment that if grandfather is killed before reproducing, grandson would die. If there is a singular time-line, then this response seems appropriate but given what we’re imagining, we can’t simply rely on that assumption. The assumption begins to seem tenuous the moment we assume travel. Perhaps, there are multiple time-lines. Who knows? This is something that would have to be defended independently if we grant time travel.
That “There is only one actual past” is one of the assumptions that generate the paradox and as such can be questioned granted other things don’t make sense. To say they were ALWAYS in the past by inference from “there is only one past” is question begging from the perspective I’m bringing which is this: when we construct scenarios where people can literally time travel and be free to do what they want it’s not obvious why this “past” is not just like the potential future: utterly open to do whatever you want. The logical entailment does not obviously apply in both directions -the past is the past so long as I assume it gone and untraveled. However, what happens when I introduce a time traveler into the equation and maintain that they’re still free to do anything? Can I just assume that there is only one time-line? Can I just assume there is only one future as opposed to the ungodly number should everyone in existence travel back to the past?