My answer to the question ‘Would you like to know the time and cause of your death?’
Originaly Submitted by emil10001 on Mon, 11/22/2010 - 10:48
I might first ask if the knowledge of my death would have any impact on the time and cause. If it does have an impact, I might ask if I will live longer if I know the time and cause. Then, if it is in my best interest to know, then I would like to find out. If my knowledge did not have any impact on the time and cause, then I would like to know. If my questions could not be answered, I would like to know.
Here are a couple examples intended to illustrate why these might be pertinent questions.
This would be an issue regardless of whether or not we have free will. The information of my time and cause of death would only be known to the agent up until the time he interfered with me by giving me said information. Let’s assume that the agent is the Laplacian Demon, and we’ll also assume that determinism is true. In order for the Laplacian Demon to have perfect information about the universe, and be able to project from a given point all future events, the demon must be able to store that much information. Turing postulated that it is impossible for a system to have perfect information about itself, and it is impossible for a program to be able to know how long it will take to complete execution. See the Halting Problem. We can apply that knowledge to the Laplacian Demon to say that the demon can only have perfect knowledge of the universe, if it exists outside of the universe and does not interfere with it. It must exist outside the universe, because it needs more resources than the universe has to offer, and it cannot interfere and still have knowledge because of the halting problem.
In the situation described above, it would be the case that the information would only be good up until you were told of it. You could obviously then take steps to avoid getting yourself into that situation, and the Laplacian Demon would have no way of modeling that new information until it stopped interfering.
Alternatively, we could shift our view to that of a sci-fi paradigm. Let’s suppose that instead of a demon, the agent with said information is a time traveller. We will say, for the purposes of this story, that the time traveller is a physics and philosophy double major who is very interested in causality. Her university is testing out its time machine, the first ever, and she has gotten approval to run some experiments. She would like to know, once and for all, the rules about causality and time travel. She decides that a good test for this would be to go back in time and inform people of the time and cause of their death to see if they can avoid it. The time traveller would read my obituary in the paper in her current time, find out the pertinent details, and then travel back in time to tell me about it. At this point, she would not know whether or not my knowledge would have any impact on its coming true. Then, the time traveller would need to go back to her originating time to know whether or not it was still true that I died at that time. There are two possibilities, possibility A, in which I die at the same time before and after I have been given the information. Or, possibility B, in which I do not die at the prescribed time.
Possibility A would leave the time traveller with the option of trying this again, reading another persons obituary, going back, informing them, and returning to check its accuracy. In possibility A, the time traveller would not be able to be certain that she knew until this process was repeated enough times without fail. Once there is even a single chink in the chain, we fall over to possibility B. Possibility B would then prove to the traveller that knowledge of the information could change the outcome. She would never be able to posses any real knowledge about the time between her present and a given previous time if she were to interfere with it.
Notice that these examples preclude the option of knowing all possible futures, which I am always instantly skeptical of. You would need universes’ worth of storage and processing power to even attempt such a feat. Also, notice that neither of these scenarios requires Free Will. I’m sure that there are many other cases where some sort of magic is the source of knowledge which have different rules, but I think that my cases are still interesting.
See the original Reddit post here, posted by _Winters.