ThinkerEmeritus wrote:It is difficult to hold a conversation about time travel without having any idea how the time travel would be accomplished. To attack this problem, I will propose general principles for eventually building a time travel machine.
[Tongue-in-cheek mode on] Physics actually contains a mechanism for travelling backwards in time, as first pointed out by Feynman. It does not allow the sci-fi version of jumping to a particular time, but it does allow reversing the direction you are moving in time. So you could in principle go backwards in time, but you would move at the same rate that you would otherwise move forward. Clearly this will limit the willingness of volunteers to remain in motion, but that helps explain the absence of observed time travellers.
If the energy of a particle is reversed in sign, it automatically moves backward in time. This reversal doesn't happen spontaneously, but it can be arranged if you can dispose of the large amount of energy that results. The particle moving backward in time is called an antiparticle, and examples are well-known. Once we have adequate knowledge of the human body, it should be possible to move all the particles in the body together into corresponding negative-energy states, and that body will procede to go back in time. I assume that since the human body generates only classical information it is essentially a classical collection, and if this is not true building the machine will be a great deal harder if not impossible. Moreover, the volunteer must be protected from collisions with all positive-energy matter, which would otherwise fall into the negative-energy states and corrode the volunteer. So if the volunteer is to accomplish anything useful, he/she [hereafter he for brevity] must be returned to a forward-moving body at some time in the past. Hence useful time-travel cannot extend to times before the invention of the time machine, which further explains why time travellers are not yet seen. All the mechanical laws of physics are essentially the same when time is reversed, so there should be no anomalies during the trip. It is not possible to predict how thermodynamics will work during the time travel, but we will assume that since we know the condition of the body at the later time and not at the earlier time, thermodynamics will also reverse for the time-travelling body.
To a body travelling forward in time, like us, the time-traveller will appear to be a body made of antimatter travelling with us in a positive-time direction. The return to forward time-travel will appear to be a person-antiperson pair creation event. I have not explored the possiblities very carefully, but I think that this fact will prevent any serious paradoxes.
So all that is left is for some reader to accumulate the knowledge necessary to build such a machine. In the meantime we should be able to determine in principle if the process is safe, i.e. if there are indeed no possible paradoxes.
If anyone thinks that the machine has already been made secretly, and that we should try to find out about it, I suggest a careful examination of the biographies of all twins.
Have fun. In particular, is the volunteer moving backward still the same person or just a copy? If not the same person, is participating in the experiment, which necessarily involves the disappearance of the original, forward-moving version, a case of suicide? [/tongue-in-cheek mode]
EDIT: I have spent some time thinking about the billiard-ball paradox upthread with this mechanism for time travel. I haven't spotted a rule that would forbid the paradox. Interesting. Could it be that the mechanism works only in quantum mechanics?
That would be like this: I gather a lot of energy, enough to make two older copies of myself, one of anti matter, another of matter. The matter one lives on and continues to age, the antimatter one lives backwards and gets younger and younger. Once he is a perfect copy of me, we anihilate.
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Trouble is that I would need the information of how my complete state would be in in the older form. The second law is turned upside down for the anti-me part. He rembers being me, he remembers the anihilation istant, and the anxiousness he (I?) felt rigth before it, and he remembers setting the whole thing up. The older matter-me remembers the whole thing including two rather similar anihilations.
But how do I produce an anti-me destined to "become" the exact counterpart to what I will become? I can't. I can't reverse the second law. It is possible that this sort of thing could just happen, that it was non of my doing, but it is roughly as likely as me tunneling through the door, since there is no reason this blob of anti-particles would have to be a me.
With wormholes and so, spacetime could be warped in a way to require that two different spacetime are dependend on each other in this way. Then the equations of motion would have to be solved self-consistently. This should certainly be possible. Trouble is that there would be no 2nd law in such a spacetime, in shorth such a space time would have to be a thermal equilibrium. Therefore I believe that we will never experience it, because anything in thermal equilibrium is not alive. All our paradoxes (paradoxi??) arise because we try to consider time travel while still having life, intelligence, memory, metabolism, "free will",... in short: The second law.