Sunday, September 25, 2011

A possible solution to the grandfather paradox?

"Reverse the polarity of the neutron flow!"
The grandfather paradox in time travel refers to any issue which would make travelling back in time logically impossible. Named after the possibility of travelling back in time and killing your own grandfather, thus preventing yourself being born, it has been extended to cove other issue through which time travel breaks the rules of logic.

In Doctor Who it is often stated that you can't go back to a time earlier in your own lifetime because of the risk of meeting yourself.  If you you break this rule you mustn't meet yourself or something nasty will happen.

It seems to me that in real life this is less of a problem than it appears. By the age of 25 there are very few (if any) of the atoms in your body that were there on the day you were born. In fact, at the time you were born most of your 25 year old atoms were in the bodies of other people or objects. Hmmm. This is often misunderstood because many body cells stay with you for your whole life, but those cells do regenerate and the atoms themselves will have swapped over many times.

If I, at the age of 44, go back and meet myself as a child the two people would be made up of completely different atoms. The only problem would be that my 44 year old atoms would exist in other people and objects. Rather than the two versions of me meeting and something nasty happening the likely outcome of travelling back in time would be that my atoms would take up their locations as they were at that point in time. As has often been said:

In every glass of drinking water there is at least one atom that passed through the bladder of Oliver Cromwell.

However, even given this problem the situation does raise an important existential question. If our bodies are not the same as 25 years ago then is our consciousness dependent on the physical hardware of the body?  The only logical answer is - no. Our personality and memories are much less connected to the  physical apparatus of our brains than we might like to think. This makes it entirely possible that human consciousness could exist in other atoms not directly linked to the body and therefore exist separately from the living body.


  1. Interesting but surely you would still need a parallel universe to be absolutely certain, because if you travel back to your own universe the moment you arrive you interfere with the entire history of your own universe and change history. Thus you might not exist the moment you arrive.

  2. Interesting but surely you would still need a parallel universe to be absolutely certain, because if you travel back to your own universe the moment you arrive you interfere with the entire history of your own universe and change history. Thus you might not exist the moment you arrive.

  3. Firstly it must be established that there are some things, which are impossible for a time traveler to do in order to propose that time travel and free will have any correlation at all. Suppose a world in which time travel is possible for the sake of argument, and this world is consistent with all the laws of nature that we know to exist in our world presently. Suppose then that a time traveler unhappy with his life, call him Eric, plans to travel into the past and murder his grandfather in order to prevent himself from ever have being born. He makes sure he travels to a time when he knows his grandfather is defenseless and alone, and takes a loaded gun with him to complete the deed. In fact Eric possesses the ability and the means necessary to kill his grandfather. The problem here is that if Eric’s grandfather is indeed killed by Eric at the time that Eric travels to, then Eric would have never have been born to be able to travel to the past and murder his grandfather. Thus it is easy to imagine a situation where Eric’s attempts to kill his ancestor doesn’t come to fruition due to a series of “coincidences”, i.e. his gun jams, he mistakenly kills the wrong person, he accidently slips over and so forth. It seems unlikely that repeated attempts of ancestral murder would result in repeated coincidental failure. Indeed it seems that there is some cosmic force outside the control of Eric, which affects his ability to murder his grandfather, thus Eric does not have free will.
    Contrary to this assertion of some sort of cosmic force, David Lewis (1976) defends the free will of the time traveler through proposing that in asking if Eric can travel to the past and murder his grandfather there are 2 different sets of facts that are relevant. The first being that Eric has the ability and means to carry out the murder. As we have seen in the above thought experiment he has both of these, and thus this set of facts is compatible with his ability to carry out his wishes. However the other set of facts include facts about the future, namely that Eric’s grandfather is alive at times which follow this point and thus does not die, exemplified by the presence of the Eric his descendant. Lewis rightly proposes that it is the second set of facts that give the appearance that Eric does not have free will. This argument is furthered through the assertion that if these sort of facts were known about the future of the present, it would appear that no one has free will at any time. For instance, suppose that Agnes drives to work every morning without fail, and (unbeknownst to her before the event) on the way to work on Monday she will crash her car into a wall whilst texting. To Agnes as she drives to work on Monday, she does not feel as though she doesn’t have free will as she doesn’t know that she will crash into a wall. In fact her decision, to text whilst she is driving causally affects the result of her crashing into a wall. Before the event it was determined that she would crash, but her free will is not compromised, as the fact that she will crash is not known. However if Agnes visited and oracle and it was revealed to her that she would crash, she would no longer think of herself as having free will, though the fact she was going to crash did not change. This is not to say that the future is fatalistic but rather predetermined based on a chain of causal links. If one could see this chain of causal links stretched out before them then it would appear as it does with backwards time travel, that there is no free will. Exemplifying how the present is no different to the past other than the absence of the second set of facts, and thus defends the free will of the time traveler despite the objection of the grandfather paradox.