Time travel is a persistently tantalizing idea, and has been a recurring theme in literature and other cultural media since at least as far back as the Mahabarata. Today it lives at the very edge of scientific plausibility: never entirely ruled out, but subject to persuasive objections. One of the most problematic is the “grandfather paradox”, which asks what would happen if you were to go back in time and kill your own grandfather in his boyhood. He would, then, never have grown to adulthood, and would never have become a father — which means, of course, that you would never have existed. But then you wouldn’t be able to go back in time to do the dastardly deed in the first place! There seems no way out of the loop, and to many that means that time travel itself must be forbidden by the fundamental order of Nature. But this objection doesn’t cover ordinary, non-murderous tourism; perhaps it is simply in some way impossible to perform actions that disrupt the continuity of the time-stream. Or perhaps our consciousness can travel in some purely observational way that lacks causative power; after all, to physicists the very “flow” of time itself appears to be just some sort of illusion presented to our consciousness. (In relativistic physics time doesn’t “move”; it is, in a sense, all there at once: just one of the axes in the space-time framework.)
Stephen Hawking asked: if we were to become able in the future to learn how to travel to the past, where are all the tourists? Shouldn’t we see them here now? But perhaps they are hiding, or simply aren’t visiting, or perhaps they can only travel back to regions of time that lie after some point in the future where we learn to make suitable cosmological arrangements. And of course there have been various allegations of actual time travel; perhaps the most curious and interesting one, though almost certainly a hoax, is the strange story of “John Titor”, which you can read about here.
But lately the idea has been popping up again, and in rarefied circles. Some physicists, noting the oddly unproductive history of super-high-energy particle colliders — first the startling de-funding of the Superconducting Supercollider here in America (an “anti-miracle”, in one scientist’s words), and then the suprising breakdowns at CERN’s Large Hadron Collider — have suggested, and apparently seriously so, that operating these devices above a certain energy threshold may lead to such horrifying calamities that some sort of causal agency has actually reached backward in time to prevent them from getting up and running.
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