Worldlines
We all of us exist on an unlimited number of timelines. Elon Musk believes that the chance we're not already living in some sort of simulation is one in billions. He believes, in other words, that there's a high, extremely high, probability that we're all working on pre-determined code programmed by some computer from a species not so unlike us some millions of years more advanced than us. If this were the case, then the very concept of worldlines and time travel is simply explained away through glitches in the computer's clock and some ethereal awareness of other computers running simultaneous simulations. This is to say: life could be a game and we could all be figments of each other's imagination.
It's not so hard to reconcile once we consider how charmed some of our lives are, and how some things just seem to turn out the way we expect them to, but of course all of that is madness.
On another worldline, everything that ever could have happened is happening, on some other worldline. On some other worldline you became a King, in another a president, and in another you were a very violent criminal, while in still another you were some kind of Joan of Arc. All of the possibilities happened somewhere, or at least, this is the only sensible way that we can understand the chaotic nature of our universe and even our own being.
Frequently I find myself considering it all. I think the age of the blockchain provides a unique opportunity. It's an immutable ledger. Someone could prove they had time traveled if they could provide the hash of blocks in the near future. If they had come from the future, providing the hash of these blocks would be a trivial matter. But therein lies a problem: the worldline crossing problem. It's said, especially by the alleged time traveler John Titor, that when we travel through time we don't actually go to the same world we came from, but one very near to it.
Therefore in another worldline there may be no blockchain at all, or it's certainly very likely that the blockhash could be slightly different. Yet, perhaps not. If there are going to be a limitless number of copies of the universe operating at the same time, it would seem plausible that only a few things would be different in ones that were time traversible. It would seem that if the two realities diverged too much, then even geography would be different, and the ability to actually land and take off would be limited in some cases. In fact, there'd be no single worldline where geography was exactly the same. For while human beings have unlimited worldlines, so too do the animals. There'd also be a limitless range of evolutionary species. It's all so fascinating.
--
I've seen good arguments that the blockchain would be a good way to find evidence of timetravel, when it happens. The thing is that if it had happened and they had already come back this far, it's more likely we would know.