Killing your Grandfather is the least of your problems
The Time Machine from the 1960 movie
The Time Machine
For example, the old time travel conundrum of going back and time and killing your Grandfather, and if you’ve killed your grandfather how were you born?. It is actually quite a soft version of how absurd from a common sense point of view time travel is. Other examples of bizarre paradoxes involved in time travel to the past are:
A) I build time machine go back 5 minutes before I activated time machine. I then kill myself before I entered time machine to go back to kill myself.
B) I burn a cord of wood in my fireplace until it is reduced to ash. I enter time machine go back to before cord of wood is burned. Take cord of wood go back to time just after I entered time machine. Burn cord of wood to ash, then go back in time etc., etc. forever burning same cord of wood over and over again.
C) I build time machine go, back 3 minutes, pick up myself, we both enter time machine, go forward, or back repeat until machine is filled to capacity with dozens, hundreds etc., of me.
D) I build time machine go back in time to take Mona Lisa, go backwards or forwards in time, to take Mona Lisa each time until I have hundreds of Mona Lisa's.
E), If there are two of you or of anything as a result of time travel you have created mass out of nothing. Thus for example I take a diamond with me and I go back in time before I went into the time machine and take the diamond so now I have two diamonds and I keep doing this until I have hundreds if not thousands of the same diamonds. So where did all the extra mass come from?
F) The very existence of you at the same time as you is a paradox because by definition there can only be one you at a time and again it involves the creation out of nothing of mass.
G), The never written book. I go back in time with a copy of Bleak House by Dickens. I give it to Dickens. Dickens publishes the book under his name. So no-one wrote the book. The above can be applied to anything made. Like say a piece of jewelry or the Mona Lisa.
If time travel is for real than the above MUST be possible. Time travel back in time seems to be unlikely due to the paradoxes created, especially the breaks in causality that would apparently inevitably result. Now I should mention that it appears that going back in time, time travel does seem to be allowed by at least some solutions to the theory of relativity.1
The Paradoxes are quite horrendous.
There are various ways of getting around these paradoxes. One of them is the so-called “many worlds” hypothesis2 which involves the idea that if you go back in time you automatically create a new timeline and do not change your past at all. Since in so doing you create a new timeline that is different from your own there are no paradoxes. This includes the paradox of being two of you at the same time. There can be two of you at the same time because each “you” is in fact not the same person. If a time traveller could modify the past after traveling there; and if they could travel into their own pasts, they should logically be able to modify it, the result would be two different "pasts" that would have really happened. The extreme example of killing himself is a problem for this type of time travel because i.e., what did happen? The travellers own past MUST have happened otherwise he could not have gone back to change it. The new past has just happened. If they are the same timeline then a paradox as occurred. In the many worlds hypothesis there is now one time line where he is dead and another where he continued to live, i.e., the one he remembered. Or a less extreme example a past where he did not go to Expo 67 versus one in which he did go to Expo 67. The result is however many times you go back and change the timeline you totally fail to ever change your actual past. It has nothing to do with being allowed, you simply can't do it. Your past remains what you remember.
Lets see you can only go back into "your" past but the moment you change it, it is no longer your past, and your actual past and new past exist. So it appears that you do have access to other "pasts" than your "past".
This works on the assumption (yes its an assumption) that you can't change your own past otherwise you get paradoxes, no matter what you do it remains the same so that when you go back it cannot be your past. I'm not talking about subjective either everything you remember that has happened to you remains the same because if it changed you would suddenly have a change of memory.
Of course one other solution to the time travel paradox is that yes you can go back into the past but no matter what you try to do you can't change anything that happened your just part of it. This is called Novikov consistency principle:
- Stated simply, the Novikov consistency principle asserts that if an event exists that would give rise to a paradox, or to any "change" to the past whatsoever, then the probability of that event is zero.3
Aside from the fact that this principle existed in some Science Fiction before Novikov thought of it. The TV show The Time Machine, for example, had our heroes constantly trying to change events and constantly failing. Further I find it hard to believe that if you get there you cannot change things. This principle has the problem that it violates the paradox of having two of something exist at the same time, one at least created out of “nothing” thus having two of exactly the same thing at the same time. Although has mentioned above it seems that this idea of back in time, time travel is allowed by various solutions to aspects of the theory of relativity it strikes me that it may violate the law of conservation of mass / energy, i.e., that energy / mass can be neither created nor destroyed merely modified, and hence absurd on the face of it.
Of course one could give a comic paradox of time travel. Steven Hawkings, world renown Physicist as said that if time travel was really possible into the past we would be, right now, be visited by hordes of tourists from the future.4
From H. G. Wells to today time travel as been a stable of fantasy and Science Fiction writing. It does appear that given the rather absurd paradoxes outlined above that real time travel is not possible.
Of the two solutions to the paradoxes listed above the “many worlds” idea seems to make the most sense because in effect with it you are NOT traveling back into your time but traveling into another time and thus the paradoxes are avoided.
The idea that if you travel into the past you simply can’t change anything although it resolves some paradoxes does not resolve some of them, i.e., two of you at the same time and assumes the assumption that you can’t change things. Well I would think you could anyway.
All in all a real mind bender.
1. Time Travel in Wikipedia Here.
3. Novikov consistency principle in Wikipedia Here.
4. Footnote 1.