I love stories of time travelers. From H. G. Well’s The Time Machine to Back to the Future to 12 Monkeys and many others, the paradoxes, the excitement of the dramatic irony of the characters knowing secrets that others can’t possibly understand, the intricate relationships, are all fascinating to watch unwind.
But it can’t really be that way. Though physicists tell us that time travel is theoretically possible under certain extreme conditions, living beings would be destroyed by the energies involved. Ignoring that (NOTE – this is a BIG ignore), there is still at least one other logical reason why these stories can’t happen.
We assume that a time traveler would carry personal history into the past. This is what we mean by “now” in our progression through our lives. So our fictional traveler, though moving backwards in time, still has an experience of personal history moving forward. There are thermodynamic reasons why this couldn’t be the case. There is entropy associated with knowledge.
This is subtle. Entropy can be loosely defined as the degree of organization of the universe or some part of it. It’s a negative measure. Something with less organization has more entropy. When we say entropy is always increasing, we mean that the universe is always becoming less organized. A coffee cup has a certain amount of entropy. It’s relatively low because the cup is carefully organized into a useful and attractive shape. If you drop the cup on the tile floor and it shatters, it’s less organized, so its entropy has increased.
But what if you glue it back together? Doesn’t that decrease its entropy again? Yes, but the energy that you use to do that, and the energy used to manufacture, package, and ship the glue, all together turns out to increase entropy more than you decrease it by repairing the cup. It’s that way for all actions. This is stated by the second law of thermodynamics, which says, basically, in any action, entropy always increases.
Here’s where it gets tricky.
Knowledge is organization. Organization of what? Thinking about this is part of information theory, which can get pretty deep, but it can be shown that more knowledge implies less entropy. The second law guarantees that this knowledge is gained at the expense of an increase in entropy overall.
As we age, getting wiser (hopefully), we are burning calories and creating greenhouse gasses that “pay” thermodynamically for this wisdom. Taking this knowledge into the past implies a violation of conservation of energy (the 1st law) unless there is a corresponding increase of entropy. But where is this increase and when?
The universe has to obey the law of conservation of energy and the law of entropy everywhere all the time. So the entropy increase has to happen at every point in time through which the time traveler moves in order to offset the entropy decrease represented by the knowledge the traveler carries that didn’t exist before at that point in time.
Remember that entropy is negative. Increasing entropy is a loss of organization, which is sometimes a sudden release of energy. Would the time traveler heat up the then explode upon reaching some point in the past?! Other possibilities are that the personal timeline would also reverse, and the traveler would lose the knowledge from the future, or that the “past” destination, when reached, would be so changed by the traveler’s arrival as to make that knowledge useless. An exploding traveler is only one of countless possible changes.
Time travel without the knowledge of the future (the past in your personal timeline) is not time travel in a subjective sense. You would perceive your journey as new, the future. It would be a new world, and you might theorize that you are in a world of the past, but you would have no personal evidence of that. All memories of your “past” would be lost.
If you retained your knowledge, then the past would have to be different because of your presence in it. Your knowledge would be of no use because things that you remembered happening wouldn’t actually happen.
Either way, you’d be starting over.
The standard paradoxes, like meeting yourself or marrying your grandmother and being your own grandpa, would, I guess, be possible in a linear timeline, but quantum uncertainty makes it unlikely that you would actually be in your own past anyway. All the decisions that you would make would create a different future in which you, as you are now, don’t exist.
I see no paradox. A person’s timeline lies in a plane lying in a space lying in a hyperspace (at least). The experience of this clueless time traveler would be along a different timeline from anything in a forgotten past.
It’s all speculation, but you can’t get around the 2nd law. Something would change to make the traveler’s experience nothing at all like in the stories.