Back to time travel... Having read “How to Travel Through Time” by Sean Carroll [Discover, 2010] I was pleased to find a new time travel novel by Willis [Blackout, see entry below]. Naturally, she deals with many of the same issues as Carroll. That is, can one change the past? If not, does this eliminate free will? Or can one go back in time and obviate an undesirable future? I made notes as I read Carroll's article, so I'll just go through and comment on them.
Carroll says: "If physics is not an obstacle[to time travel], however, the problem could still be constrained by logic. Do closed time like curves [physics talk for time travel] necessarily lead to paradoxes?"
The classic example is called the Grandfather Paradox. If your grandfather was an abusive brute, you might get angry enough at what he had done to your father when he was a child to get into a time machine, go back, and shoot your young grandfather before he met your grandmother. This ends the abuse before it happened. Only it would mean that you don’t exist. If you don't exist you can’t go back to kill your grandfather. That means that he will meet your grandmother and your father and you will be born which means you will go back and kill your young grandfather to be, which means...
Some resolve this by stating that if you go back in time, you are going to an alternate universe. Whatever you “change” does not affect your original universe. [This is a variation on Everett’s Many Worlds Hypothesis.] Others say this shows time travel is impossible. Some say that it proves that there is no free will and what must be must be and cannot be changed [que serra, serra]. I believe that all supposed paradoxes; from Zeno, Achilles, and the hare to time travel; are all misuses of language. Paradoxes are impossible, we think they might exist only due to our poor understanding of the facts.
Carroll later states: “In the usual way of thinking, the laws of physics function like a computer. You give as input the present state, and the laws return as output what the state will be... This ability vanishes as soon as someone builds a time machine and creates a closed time like curve.” in other words, the old anti freewill argument which was destroyed by the random nature of quantum physics. Random events, even at the quantum level mean that even knowing the exact state of every particle in the universe and the totality of all forces acting upon them, you cannot predict exactly what the universe will be like in the future. This drove Einstein to an obsession to disprove quantum physics. He and Neils Bohr spent decades debating the issue, with Bohr triumphing time after time.
Please be aware that I am skipping most of the article for the sake of focusing on the points which interest me, but what I am ignoring is also interesting. Carroll comes to the conclusion that if time travel exists, “We would therefore have to abandon the concept of determinism...We would also have to abandon free will...”
I disagree on both points. Determinism had to be abandoned when quantum uncertainty was discovered by Heisenberg. It is long dead. As for free will, this is what interests me the most. I have spent several decades gnawing at that problem.
I believe that the following scenario is much more realistic than the classic Grandfather Paradox:
Let’s assume time travel is possible. You do go back to kill your young grandfather to be. When you point the gun at his back from your ambush, you just can’t do it. You are not a murderer, not even of this awful man. You go back and nothing has changed.
Doesn’t this deny free will? You can go to the past, but you can’t change it. What happened has happened and must happen. This is a form of determinism, but one which applies only in the limited purview of time travel. But I still insist upon free will as an active force. Carroll insists upon logic as denying free will in such a case, I say he is limited to the narrow and constricted view of reality which appertained prior to Einstein's theory of relativity.
Let’s look at a classic and often repeated gedanken experiment, a thought experiment. There is a train traveling along. In the exact center of one car is a light bulb. At each end of the car is a sensor which, when the light is turned on, will open the door at that end of the car. There are two observers to this experiment. One is traveling along in the car, the other is standing still outside the car as it goes by. The light is turned on and the doors open. The observer in the car says both doors open at the same time as the light traveled the same distance and at the same speed. The observer outside the car insists that she is wrong, the door at the rear of the car opened first because it was moving toward the light beam, shortening the distance it had to travel. Since the speed of light is constant, there can be no other conclusion. Logically, using the classic Greek logic of Carroll, they cannot both be correct. The statements of fact are mutually contradictory.
But they are both correct, This has been proven in experiments. Impossible? Not in a relative universe. Each observer is correct, in his or her frame of reference! To the observer in the car the doors opened simultaneously, but to the observer outside the car, the rear door opened first. Reality depends on where you are standing relative to the experiment.
The same situation applies, I believe, to time travel. To the future observer, you are without free will, you must do what you have always done in his past. He sees your actions as completely and totally determined. You are only a robot doing what has been done and will be done and always was done [as it was in the beginning is now and ever shall be].
This is true for you only as long as you are in the future, but when you go to the past, your future now includes what you will do in the next few minutes, your now is September 7, 1956 or whenever you have traveled to. In other words, you have choices. You make them using your free will. From your framework this is true and as real as your lack of free will is to the future observer. This is logically contradictory only in a pre relativity world.
Well, took me along time to get to this but there is my answer, summed up in a very brief and simplistic summary.
I am feeling light headed from all this typing and editing. I am done for the day! Back to my sipper full of coke and The Mammoth Book of Golden Age Science Fiction!
Note: at least one physicist is attempting to create a time machine by using laser beams to create a twisting of space time [so far theoretically possible] in a manner similar to the "frame dragging" which occurs when water spins and swirls down a drain. In this case, of course, the frame is not the water but the space time continuum. Other physicists insist that the power to create such a distortion would requite a couple of cosmic strings, left overs from the big bang which are as thin as a proton but vastly long and heavier than neutronium. Said physicist intends only to create a communication device which can send a few photons into the past. But photons can contain information and the paradox remains.
PPS: If interested look into “light cones”. They get really messed up with time travel.