time travel showdown: past vs. future
A while ago, I wrote about how some of the recent, more exotic theories in cosmology and astrophysics have been making time travel seem like a plausible concept. Since being able to travel through time to see what really happened in the past, or get a glimpse of the far future, has been one of our fondest dreams, some are starting to wonder whether one day we could have honest to goodness time machines and popular science channels have been periodically showing scientists who say that maybe, possibly, conceptually, it might work one day if these theories make it past a set of equations on paper and become part of the standard model.
But when it comes to what would actually happen if we travel in time and whether we can change the past or shape the future, physicists are quick to raise serious doubts about our ability to be anything but observers in our new positions in time. If we end up in the future, there would be fewer paradoxes to deal with. Mainly, we won’t know what will happen next or whether we can do anything about it. A traveler into the future just couldn’t be in the position to actually disrupt the timeline. And she will have a much easier time getting there. Rather than having to create wormholes with mouths calibrated to have very precise rates of spin, she could just take advantage of special relativity. A seemingly short trip in a spacecraft traveling a big enough percentage of the speed of light would do the trick.
Going back to the past is a lot more labor intensive. To make time go backwards via space travel, you’d need to move faster than light and your only shot at doing that would involve trying to get a boost from a black hole without being vaporized in the process. Instead, as noted above, you’d need to make a shortcut in space and time, manipulating it to send you in the general vicinity of where you want to be but at a different time. What’s more, anything you do in the past should have no bearing on the future, especially if you try to change history. Otherwise we’d end up with countless paradoxes and a chaotic timeline in constant flux. How? Instead of the classic grandfather paradox, let’s consider a more straightforward example, one you’d find in countless sci-fi stories, novels and movies.
Let’s say you decide to travel back to the 1930s so you can find Adolf Hitler before he gained any power to kill him and prevent World War 2, erasing one of the most tragic, shameful and bloody chapters of our past. Your intent may be noble, but unfortunately, it has to fail in order for your trip to have any causal continuity. If you did track down and kill a young Adolf Hitler and stopped the rise of Nazi Germany, there would be no world war or Holocaust or tens of millions of deaths. But there would also be no reason to travel back in time. If there was never a Nazi Germany, there wouldn’t be anything to fix. So even if you did manage to go back in time and got close enough to assassinate one of humanity’s most despised specimen, you must’ve failed or you’d never have the motivation to try it.
As tempting as it seems to use future time travel technology to correct history’s darkest hours (or create them for personal gain and satisfaction if you want to be an evil mastermind), and as sure as we may be that being capable of free will would let us do it, the laws of physics would most likely either foil our attempts or leave us as clueless bystanders. After all, as much as we want to fly on our own power, we can’t do it on Earth because we didn’t evolve to fly and our planet’s gravity is far too strong. Just because we want to do something, nature isn’t obliged to grant our wishes and building a machine to counter causality sounds like a recipe for disaster. We might be much more successful in trying to travel into a future we know nothing about.