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Time travel; can we change the past?

Updated on April 26, 2015

Time Machine: DeLorean


Is affecting the past a coherent notion?

It seems natural to assume that the past is unchangable; it has already happened and the results it has caused are already in place. How then could it be possible to affect the past when it would result in a change in the present? If it is impossible to change the past then does that neccessitate that it is impossible to travel backwards in time? Lewis, Dummett, and Wharton all believe that affecting the past is not a coherent notion.

David Lewis argues that time travel is possible, that the problems and paradoxes of time travel are merely oddities and are not impossibilities. He describes a time traveler's journey as separation in time between departure and arrival which is not the same as the time the journey takes; the traveler arrives later than the journey suggests when moving forward and earlier when moving backwards in time. Lewis forcuses on time as one dimension of the four dimensions of the world in order not to complicate the idea. Despite believing time travel to be possibile, even if difficult to understand, Lewis still maintains that we cannot change the past. Things which lack temporal parts such as events which have already occurred are impossible to change. If possible to travel through time then the same two events must be separated by two unequal amounts of time. In order to understand this Lewis distinguishes external time from personal time. Personal time functions to the time traveler as external time functions to the non-time traveler; tracing the life from infantile to senile. This personal time is similar enough to time that temporal vocabulary can be applied to it. An event in a time travelers life may have more than one location in his personal time, separated by personal time but simultaneous in external time. What unites the stages of a time traveler is that the same mental continuity remains throughout but with only personal time taking the stages in order whereas usually external time would also trace the continuity. The connectedness and continuity are not accidental, they are explained by the fact that properties of each stage depend causally on those of the stages before in personal time. Traveling faster into the future does still seem to be a much easier idea to swallow though as traveling into the past requires reversed causation as there must be causal continuity in the time travelers personal time from earlier to later stages. Lewis's notion that a persons personal time could exist separate from external time though is a little uncertain. The idea is that one would not be affecting the past as the events happened simultaneously in external time. The one affecting the past would actually only have been affecting the present. However, the notion that one could have a personal causal chain leading to the person with the certain traits and qualities they have grown into and acquired without this person having had to necessarily experience external time in the same way as everyone else is hard to contemplate. How could a person be formed into the person of the qualities they have and go back to a moment which they have already been to because it has already happened and have any active role in how the event takes place? It seems natural to assume that the initial growing of a person in external time has to take place before they can travel back to a past event yet how can they have already traveled back to that event and already had an effect on it if they had yet to grow in external time. The direction of counterfactual dependence and causation is governed by the direction of the de facto asymmetries of time. Reversed causation and time travel are not excluded altogether but can only occur where there are local exceptions to these asymmetries. Lewis follows from this that if there are local causal reversals then it must also be possible to have causal loops; closed chains of events. Every event in the loop can be causally explained as being caused by other events on the loop, the loop as a whole though may lack explanation. A time traveler telling their younger self how to build a time machine for example, the events within the chain are all causally connected yet how the original knowledge of how to build a time machine came about is unexplainable. Lewis believes this to be strange but not impossible as we accept similarlly inexplicable things such as the big bang, or the entire infite past. However, these few examples do not seem to be adequate to justify the illogical idea of causal loops. Idea's such as the big bang being difficult to explain does not make them the same logical impossibility as a chain of events without a true beginning or cause. We do not know that what the causes of these other unexplained events are but they are not accepted as illogical, merely as still to be explained. These are also events where are understood to necessarily be, we have a form of understanding that they have occurred and so explanation is sought. Why is it that we should accept, without explanation or understanding, the idea of a causal loop when we have no evidence or reason to believe that it does actually happen?

Despite believing that time travel is possibile Lewis still agrees that changing the past is not possible; 'The events of a past moment could no more change than numbers could' (Lewis; 138). As the events of the past are not subdivisible into temporal parts they cannot change. It does though seem that a time traveler should be able to change the past if he to become a part of the moment. A time traveler visiting the past should in theory both be able and be unable to change the past. For example, if a time traveler were to go back to attempt to kill his own grandfather. We would like to say it is impossible if his grandfather survived beyond that moment in the time travelers memory and personal time. Surely he cannot be killed by the time traveler if the time traveler has lived a life in which he grandfather died much later. Yet the time traveler does have the skill, equipment, plan etc. and so is capable in theory of killing his gradfather. Assuming there is not a parallel universe of separate external time from the original then it must be impossible for the time traveler to succeed; one cannot change the actualised way the past turned out, only the unactualised way it would have been without the time traveler. Something commonplace and logical would naturally prevent the killing such as missing, distraction, loss of nerve... the time traveler doesn't and cannot kill his grandfather, even if he can in theory as he has the skill and ability. What he can do relative to one set of facts (skills, plan, tools etc.) he cannot do relative to another set of facts (this already did not happen). Despite this though Lewis maintains that time travel into the past is possible, yet changing it impossible. How then does the time traveler come to be in an event which has already occurred without altering it at all? Is the freewill to have traveled into the past in the first place not actually a freewill at all but a predetermined action over which he had no real control? It does seem that even if it was the time travelers choice to travel to the past, while there their actions are definitely limited to to those predetermined events which have actually already happened. If this is the case then is the time traveler still a causal agents or merely a history repeating itself? It seems that at some point surely the time traveler must have chosen to do as he does, but by the time his first memory of the event has passed, so has his freedom to do as he wishes at that moment. In external time then his freewill remains, yet in his personal time his freewill in moments which have already happened does not exist. For Lewis though this is the fatalists argument which he would disagree with. The fatalist, Lewis argues, takes irrelevant facts in saying what someone can do and disguises them to make them seem relavent and thereby argues that we ca do less than we think. A fatalist may even agrue that we cannot do anything that we do not, as though there is really only one outcome possible. Usually the fatalist argument is easier to exclude, but in the ideas of time travel, precognition and such where we do not know so much it may be easier to fall for the idea. I do not know how Lewis would like to maintain freewill and nondetermination in teim travel into the past while also claiming the changing the past is impossible. He was at least right that the fatalists agruments seem more convincing when talking of time travel.

Michael Dummett claims that the causal relations we experience have a temportal direction from earlier to later and that this is not just how we speak but is based also in the fact of the way things are in nature. Our understanding of causal laws though seems to stem from our being able to bring about the effects by utilising the cause; it is founded in our agent ability to cause. Dummett believes that we would find these causal laws even if were not agents but mere observers; the asymmetry of cause and the temporal direction would reveal itself. Mere observers would still view this world as a world of causality from earlier-to-later in temporal direction. However, observes could also be thought of in another world in which causality in the other direction could be more appropriate. The lack of agency means that there is little in terms of conceptual problems with this idea of backwards causation. The idea of backwards causation for agents though seems to fail as the notion that we would be doing something in order that something should have already happens seems absured. Thus, it must surely be that backwards causations it irrational and impossible in any world in which we act as agents. Agency itself seems to require a freedom of will which backwards causation would nulify. Dummett thus claims that we cannot change the past by our actions, it is nonsensical to believe that any action can alter it given that it is already happened. Dummett points out that Jewish theologians believe that retrospective prayer is blasphamous. God is not able to do anything logically impossible, like altering what has already happened, so to ask for Him to change something is asking for the impossible. It could though be possible that God has the foreknowledge to know that you will pray and so could act on the knowledge of your future actions. Foreknowledge itself is only a characteristic compatible with God. The future itself is not available to be known, its features have no truth values until it has happened. Events of the past though do have truth values and so cannot be actively altered. If this is the case though, that logically it is impossible to change the past then we must be able to convince people of this. Suppose a tribe in which the men hunt for 6 days, 2 days to travel there, two days to hunt, and two days to return. The chief of the tribe, while the men are away, performs a dance in order to encourage their bravery the whole time they are away. It seems natural to assume a greater absurdity in the idea that the chief is affecting the past when dancing for the last two days in which they travel back then in the idea of him affecting the future or present of men 2 days journey away. If it is a logical absurdity to dance once the men are on their return journey then we should be able to convince the chief of this. Perhaps though, because the chief does not know whether they have been brave or not it is worth continuing to dance. Perhaps a previous chief failed to dance while they were returning and it resulted in unbrave men. Or maybe the men have been brave, but they have only been brave because the chief is going to continue to dance, thus not changing yet still having an effect on the past. One could ask the chief to dance for the last two days when he knows that the men have already failed to be brave, this could prove that it is not the dancing which causes the bravery, assuming also that the chief's knowing how brave the men have been already does not have an effect. What if he cannot dance when the men have failed to be brave? Perhaps it would seem that the mens lack of bravery had prevented the chiefs ability to dance rather than the dancing causing bravery in the men. Or perhaps the chief would find out that the knowledge of the mens lack of bravery was a false account. The difference between past and future lies in the idea that we think that it is possible, in principle at least, to know whether an event in the past took place independent of present intentions. Future events however are impossible to know independent of our intentions.

Dummett, like Lewis, believes that fatalism is a threat to logical understanding. Fatalism is explained by Dummett as the belief that doing something in order to achieve a further result is pointless. He uses the example of the 'London war fatalist' which would argue that

'either you are going to be killed by a bomb or you are not. If you are then any precautions you take will be ineffective. If you are not, all precautions you take are superfluous. Therefore it is pointless to take precautions' (Dummett; 345).

This stems from the opening tautology that claims you are either going to be kills in the raid or you are not. This argument seems to be parallel to the argument that it is pointless to try to affect the past, only the tenses are different. Philosophers though have argued that two-valued logic such as this fails to apply to events of the future, however they may still apply to events of the past. The argument of the 'London war fatalist' continues as

'“If you are going to be killed in this raid, you will be killed whatever precautions you take" and "If you are not going to be killed in this raid, you will not be killed whatever precautions you neglect."' - 'if p, then if q then p' (Dummett; 346).

Thus, any precautions taken are of no use, if you are going to be killed then this will happen despite precautions, and if you are not going to be killed then you will survive regardless of whether you take precautions or not. The first of these, if you are going to die then the precautions are unnecessary, is understandable and acceptable to a degree. The second though, that any precautions taken are superfluous if you are going to survive, seem to be a little decieving. Surely it is correct to say that there are come precautions capable of preventing death in the raid if it is true that if they are taken, one should survive, and if they are not taken, one should be killed. The truth of these two statements are compatible with the truth of the statement also that if one does not take precaustions one will not be killed. Solely knowing the truth of this later statement is not sufficent for the claim that taking precautions will have no effect in preventing ones death for the options are not so limited. Dummett's method for overcoming the fatalist argument it to argue the case of the meaning of 'if'. It is impremissible to move from “if you do not take precautions, you will not be killed” to “Your taking precautions will not be effective in preventing your death”. For this step to be permissible the 'truth of “if you do not take precaustions, you will not be killed” would have to be incompatible with that of “if you do not take precautions, you will be killed”; but on the sense of 'if' on which the first step was justified, these would not be incompatible.' (Dummett; 348).

Ken Wharton's fictional tale 'Aloha' is the story of a man at the mid point of the universe in which there is a period of time changing direction described as the universe reversing its causal arrow. There were two sides of the universe heading towards one another moving in different time direction beginning at either the big crunch or the big bang. At the point where they meet everybody switches over to the other direction and becomes someone of the other universe. From that moment on though all their decisions will have already been made; 'soon he would be no more able to affect the future than he could affect the past' (Wharton). Felix's partner Hannah switched over before him, he was the last of his universe to change. The person Hannah changed into, Aloha, was moving through time in the opposite direction to Felix. He spoke to her once and was determined it would be the last time until Hannah implied to him that they had spoke many times before in her past and Felix's future. Thus, his freewill had vanished as if he didn't visit Hannah again then he would be changing her past and creating a pardox which he couldn't do. The illusion of freewill remained though as it was Aloha who visited him.
'Forced consistency was a tricky business for the universe. With two symmetric boundary conditions on its temporal extremes – one at the big bang and another at the big crunch – the universe was a classic case of an over constrained system. While this meant that the universe lost many of its degrees of freedom, this had usually been irrelevant to Felix, changing only the mircoscopic details of his quantum state, not the overall picture.
But now everything was different. Now the universe needed to control every detail, every decision he made, all in the name of paradox prevention.' (Wharton)

The universe itself was in a way predetermined entirely during this midpoint in which people were changing over as every event would have happened in someone elses past. Outwith this midpoint though it still seems that the universe had some degree of control as each person surviving until that point of change had to necessarily have an opposite person to change into. The story Aloha not only agree's that it is impossible to alter the past, but also inplies that the future may also have a somewhat determined state, even if less than the past. Either way though the past is a fixed event, altered in this story only by how the character has already behaved in someone else time line. Perhaps though his freewill did actually remain as he must have at some point chosen the actions he had already taken. Could it not be that Felix had already used his freewill? It is difficult to determine if there was truly a point in which his freewill was at play or not. For Felix it is impossible to alter his own future now though, the decisions have already been made, whether by him or not. Affecting Aloha's past is now impossible beyond what he is already predetermined to do/ what has already happened.

In conclusion, It seems that affecting the past after it has occurred it not a coherent notion as any change in the past would create a causal chain of events which would alter the future. In terms of time traveling this could mean that the time traveler never actually went back in time to do what was done. For example, if he was to kill his grandfather before he could have a child then the timetraveler would never have been born to go back and kill him so we would be left with a schrodingers cat type scenario in which the grandfather is both dead and alive. Dummett argues that the causal chain of events moves from earlier-to-later not only in speech but is actualised in nature. The notion of affecting the past for him is also an impossibility. The closest thing to affecting the past for Dummett would be a God like being able to see the future and thus affect the present based on that, at least for causal agents. In a world of mere observers then perhaps a backwards causal chain is posisble to understand. As we can have knowledge of the past though it seems illogical that we could affect it. Wharton's story of Felix and his lack of freewill and power seems to point out the impossibilities of altering the past, even if it is your future and only someone elses past. The events of the past have a truth value and this alone makes it seem unalterable. Both Dummett and Lewis do agree that the fatalist accounts should be feared in terms of time and causation. Though seemingly illogical in terms of future events, the fatalist account is more tempting when considering the past and the effect we could have on that. Freewill seems on all account to be removed when talking of being in the past. No action can be taken that was not already taken and the event exists in the past at least in someone else timeline. When this is the case the events cannot be changed, even when it could be the want-to-be agents present or future. The past has already been determined.


David Lewis, 'The Paradoxes of Time Travel' [on MyAberdeen].

Michael Dummett ‘Bringing About the Past’, The Philosophical Review, Vol. 73, No. 3 (Jul., 1964), pp. 338-359, available through the Library:

Ken Wharton, 'Aloha' Analog Science Fiction & Fact. New York: Jun 2003. Vol. 123, Iss. 6; pp. 92-99, available through the Library:


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    • m abdullah javed profile image

      muhammad abdullah javed 

      4 years ago

      Hi jades, you have touched an important topic of our time. Man has always be curious both of his past and the future. Since the prediction of future with far sightedness is possible as there is no truth value but affecting the past is impossible as it has some definite truth value. Excellent hub with credible views of scholars. Thanks for sharing.


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