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A Critical Analysis of William Grey's Article, "Troubles with Time Travel"
By Michelle Karl

Is time travel possible – is a question long thought over by philosophers and science fiction authors. One prominent supporter of the possibility of time travel is David Lewis.[1] However, in William Grey’s “Trouble with Time Travel,” Grey disputes several of Lewis’ arguments. Grey believes that there are holes in Lewis’ explanation of time travel and time travelers, which give ground to the claim that time travel is not possible. However, I will argue that Grey’s claims do not actually show that time travel is impossible. Grey rather makes several assumptions in his arguments that do not necessarily stand as fact and therefore cannot be used to dispel the possibility of time travel. Do to limited space and time, I will only focus on four of Grey’s main arguments.

To begin, we need to understand the view of time being used to discuss these arguments. Grey holds a Heraclitean view of time, which states that only the present is real, therefore the future does not exist.[2] Where as, Lewis holds a Parmenidean view of time in which time is a ‘four dimensional manifold of events’ where ‘truth and reality are stable and eternal.’[3] For the purpose of refuting Lewis’ argument, Grey discusses time travel within the Parmenidean view. Because of this, I too will base my discussion within a Parmenidean view of time. 

The first argument that Grey presents against Lewis is the double occupancy problem. Grey writes

Tim, we supposed, stepped into the time machine on 1 January 2000, adjusted the dial, and precisely at noon set off into the past. For observers outside it seems that the machine simply vanishes when Tim presses the button. But how can the ‘movement’ into the past possibly get started? We know that at times later than noon there is no machine present, since it has disappeared into the past. But what of the moments just before noon? At those times there seem to be not one but two machines – one going backwards and the other forwards – each apparently occupying the same location![4]


Grey makes the further claim that the only way around this is for Tim-at-noon to be located somewhere else, however this would cause Tim to become a broken streak, which then threatens Lewis’ idea of personal identity.[5] When Grey focuses on the fact that Lewis’ uses a one-dimensional theory of time, he assumes that when one travels back in time he must take the same path and occupy the same space. Moreover, Grey claims that the one-dimensional view of time travel prevents Lewis’ railroad example [6] from being a true spatial analogy.

However, the broken streak will not occur because Grey’s original double occupancy problem does not exist nor does one-dimensional time rule out the use of Lewis’ railroad example. When Lewis refers to one-dimensional time, he is referring to it as a fourth dimension to the three spatial dimensions.[7] We can only visualize three dimensions, therefore we must conceptualize time as an additional axis or dimension. Height (Y), Length (X), and Width (Z) are each considered one dimension by themselves, but when combined they make three dimensions. Time, which is also only one-dimension, is then added to create a fourth dimension. As we move along each axis or dimension, we change our spatial location. In the same sense as we move back in time or along the time axis we can also be simultaneously moving along the X, Y, and Z axes, therefore we do not have to occupy the same space as Grey claims.

In Lewis’ railroad example, he still maintains one-dimensional time. Just as Grey says “doubling back is unproblematic because there is more than one dimension for the railway track to follow,”[8] there are also other dimensions for the time traveler to follow.

To further understand how it is possible to have one-dimensional time yet not occupy the same space, let’s look at the following example. A time machine has been invented in Australia in the year 2004. Sally decides that she wants to travel back in time to 1979, when she was born. However, she was not born in Australia, she was born in the United States. Not only does Sally want to travel back in time, she wants to travel around the world. The time machine then transports her through more dimensions than one. So the seconds before she is actually transported she is standing in the time machine, but as soon as the button is pressed she begins to move through space not only time. Therefore, Sally never occupies the same space.

As we can see, Grey’s double occupancy problem does not show that time travel is impossible, it only gives way to the idea that time machines will not only have to transport people in time but  also space, even if it is just across the room from where the they began their journey.

Another position Grey argues is that reverse causation is problematic.[9] Reverse causation occurs when the cause occurs later in time than the effect.

 Grey explains the problem with reverse causation by saying: 

If the cause were later than the effect, then once the effect has occurred then its cause would have to be unstoppable. However we also believe, in general, that we are able to intervene in the world to bring about or to prevent contingent happenings. If the cause of some event is located in the future then such interventions are subject to clear constraints, and in some cases will turn out to be impossible.[10]


However, in this explanation, Grey makes a common mistake made by many philosophers – he assumes a libertarian view of free will. The libertarian view holds that we have the capacity to do other than that which we do and we can also exercise this capacity. This view clearly conflicts with time travel in that our known capacities are sometimes limited. However, two other viewpoints can be taken which allow for time travel without contradictions. The first is the hard determinist view, which holds that all things are determined, therefore we can only do that which we actually do[11]. Lewis, however, takes the compatibilist view which states that we have the capacity to do other than what we in fact do although we do not exercise this capacity.[12]

Grey continues to hold the libertarian view when he discusses Lewis’ Grandfather Paradox.[13] He writes, “The facts of the story however do not square with the belief that we can choose efficaciously.”[14] All Grey has done is show that the libertarian views on capacities could conflict with reverse causation. If we are to accept the hard determinist or the compatibilist viewpoint, then our inability to intervene would not be seen as a contradiction.

When time travel and reverse causation are discussed, several paradox scenarios arise. Grey uses the example of Tim sending a device into the past, which will explode when a button is pressed. However, if the device explodes so will the button. Therefore pressing the button will destroy the button so it would have not existed to be pressed in the first place. This creates a contradiction. He then revises the scenario so that the button is located on a remote activator. It looks as though Tim can push the button as long as the activating device is not near the explosive. However, as soon as the device nears the explosive, Tim can no longer push the button. Grey states, “It is quite mystifying how mere proximity could be causally efficacious in this way.”[15]

However, there is nothing mystifying about this example. We can conceptualize about the scenario, but in reality if Tim pushes the button, we would not know that he would ever be incapable of pushing the button. It is not the case that Tim can push the button up until the point which the remote device nears the explosive. Tim either can or cannot push the button, but the button can only be pushed once because once it has been pushed the explosion would have occurred. If he can push the button, it is because in the past the remote device was not close enough to the explosive. If he cannot push the button, it is because the device was too close to the explosive. What keeps Tim from pushing the button? Lewis would argue that common place reasons, i.e. slipping on a banana peel, keep Tim from pressing the button.[16]

It could be argued that Tim cannot push the button because the remote devise is too close to the explosive, however as it moves away Tim suddenly finds that he can push the button resulting in Grey’s mystifying proximity. However, it could just be that the button was jammed or that the Tim answered the phone and missed the chance to push the button when it was close to the device. In any case, Tim would not push the button when the remote device was near the explosive.

There is also the argument that when time travel is possible, coincidences such as slipping on banana peels or being able to push a button depending on the location of a remote device will no longer be mystifying or rare. Rather they will be common occurrences. In his article “Banana’s Enough For Time Travel” Nicholas Smith, uses the example of Parramatta Road in Sydney.[17] It used to be mystifying that a tomato would be squashed if ten tomatoes were rolled across the road. However, it is now a coincidence if any of the tomatoes would make it across the road. In the same sense, even though right now it seems unlikely that pushing a button can depend on the location of remote device, it is possible that in a time where time travel exists this would no longer be a mystifying or improbable coincidence.

Grey’s use of this mystifying proximity does not show any problems with time travel. Lewis’ commonplace occurrences would still be able to prevent Tim from pushing the button. And in a world where Tim can send a message into the past, certain coincidences may no longer be mystifying or improbable.

Grey presents one last argument, which he calls the problem parent. It is based on the idea of someone growing up, traveling back in time and fathering himself. The question arises as to whether we can be our own parents or ancestors. Grey points out that the probability of genetic composition allowing this is so improbable that it can  be termed impossible[18]. For example if Tim were to travel back in time and father himself he would never be born to travel back in time, hence the paradox. However, unlike the grandfather paradox, this is not a problem for time travel and would not occur. Since Tim was indeed born, he had to have a father other than his older self. To claim that his older self could go back and father him, you would commit the second-time-around fallacy.[19] Since his conception occurs once, and we know that Tim was born, someone else must exist to be his father. Since we know that the past cannot be changed[20], his older self may have an affair with his mother, but he would not become his own father. This scenario differs from auto infanticide and the grandfather paradox because he is not trying to eliminate either of his parents, therefore they would still be able to give birth to him despite his older self’s interference in their life. His older self and his mother would just have a different child if she were to conceive a baby.

I have discussed four of Grey’s main arguments against the possibility of time travel. His first argument claiming that a time traveler would have to occupy the same space does not show time travel is impossible, rather it just shows that a time machine must not only transport someone in time but also space. His second argument concerning reverse causation and changing the past assume a libertarian view only showing that time travel may be incompatible with the libertarian view not that time travel is impossible. His claim that time travel would have to include mystifying occurrences also does not show the impossibility of time travel because there may be a time when these occurrences are not seen as mystifying. His last argument concerning the possibility of parenting ourselves is not a problem at all and would not occur. So, as we can see if there are troubles with time travel, they are not the ones that William Grey presents.

[1] For David Lewis’ explanation of the possibility of time travel see Lewis D. (1986) “The Paradoxes of Time Travel” in Philosophical Papers Vol II, New York: Oxford University Press, pp. 67 – 80.

[2] Grey pp. 56

[3] Ibid., pp. 56

[4] Ibid., pp. 60

[5] Lewis defines humans as streaks through time, which are made up of parts or stages. The time traveler is a zigzag streak. The parts that create a whole person must have causal, mental, and spatial continuity. A broken streak is one that travels either into the past or future instantaneously and therefore loses the spatial continuity.

[6] A place five miles down the tracks can be the same place as seven miles down the track. Just as this can occur for a train, five years from now can also be twenty years from now for a time traveler. (Lewis, pp.71)

[7] Lewis, pp. 68

[8] Grey, pp. 61

[9] Grey, pp. 63

[10] Ibid., pp. 63 - 64

[11] Brown, pp. 36

[12] Ibid., pp. 37

[13] Tim travels back in time to kill his grandfather, however, commonplace reasons prevent Tim from succeeding.

[14] Grey, pp. 66

[15] Grey, pp. 67

[16] Lewis, pp. 76

[17] Smith (1987), p. 386 -369

[18] Grey, pp. 67 - 68

[19] Time only occurs once. “To fail to see this point is to commit the second-time-around fallacy” Smith (1998), pp. 156

[20] Lewis, pp. 75



Brown B. (1992) “Defending Backwards Causation” Canadian Journal of Philosophy 22: 429 –  444.

Grey W. (1999) “Troubles with Time Travel” Philosophy 74: 55 – 70.

Lewis D. (1986) “The Paradoxes of Time Travel” in Philosophical Papers Vol II, New York: Oxford University Press, pp. 67 – 80.

Smith N. J.J. (1987) “Bananas Enough for Time Travel?” British Journal for the Philosophy of Science 48: 363 – 389.

Smith N. J.J. (1998) “The Problems of Backwards Time Travel” Endeavour Vol 22(4): 156 – 158.