Killing Hitler is harder than it looks

Sorry, not available for killing
A week or so ago there was a piece on one of the Guardian science blogs on the downside of going back in time and killing Hitler. Dean Burnett points out that this ever-popular time travel fiction theme is not necessarily a good idea. The main arguments against it seem to be the difficulty of deciding when to kill him - not as obvious as you might think (perhaps the best idea would be to make sure he's never born) and the unintended consequences argument. After all, even if taking Hitler out of the historical equation made sure that the Nazi atrocities didn't happen (not of itself a certainty), it could result in a chain of events where, say, an all-out nuclear war breaks out in the 1960s. Would you be prepared to take that risk?

But the post misses the most obvious objection to going back and killing Hitler, which is that it's not possible.

Now, this might seem a silly complaint. After all, almost all science fiction features technology that isn't possible now - and many SF plots require the laws of physics as we know them to be seriously bent or outright broken. But the frustrating thing with time travel is that it is perfectly possible. There is nothing in the laws of physics that prevents it. (It's difficult, admittedly, but it's just an engineering problem.) However, real time travel is very different from time travel in fiction.

Firstly it's a lot harder to go back than forwards, where all fictional time machines seem to make no distinction. But more importantly, any backward time travel based on general relativity (which is the only means we know to make it happen to any significant extent) has a huge limitation. It doesn't involve reversing time - it's more about moving into a space where time has run more slowly and getting to the past that way. And that makes it impossible to time travel to a point before the time machine was first constructed.

So, at a stroke, killing Hitler is out of the window. It doesn't mean there might not be similar dilemmas for those who want to travel into the past once a machine has been created, to stop a future dictator before (s)he gets started. But Hitler is out of bounds.

Sorry, Dean, but killing Hitler is so last century.

Image from Wikipedia