Quantum mechanics adds a new spin to the whole time-travel game. For a start, it may open up a way to create a wormhole. On the very tiny length scale of the quantum world, known as the Planck length (around 10-35 metres), spacetime is thought to be a quantum foam - a perpetually changing mass of tiny wormholes. Quantum foam is a kind of time machine. Time is slopping around inside the foam like spindrift bobbing on the ocean waves. You just have to harness it. An advanced civilisation might be able to use gravitational manipulators to grab a quantum wormhole and enlarge it to macroscopic size.
Quantum mechanics also sheds light, or possibly dark, on the paradoxes of time travel. Quantum mechanics is indeterminate - many events, such as the decay of a radioactive atom, are random. One way to make this indeterminacy mathematically respectable is the `many worlds' interpretation of Hugh Everett III. This view of the universe is very familiar to readers of SF: our world is just one of an infinite family of `parallel worlds' in which every combination of possibilities occurs. This is a dramatic way to describe quantum superposition of states, in which an electron spin can be both up and down at the same time, and (allegedly) a cat can be both alive and dead.'
It's OK for electrons and probably nonsense for cats. See Greebo's cameo appearance in The Science of Discworld.
In 1991 David Deutsch argued that, thanks to the many worlds interpretation, quantum mechanical time travel poses no obstacles to free will. The grandfather paradox ceases to be paradoxical, because grandad will be (or will have been) killed in a parallel world, not in the original one.
We find this a bit of a cheat. Yes, it resolves the paradox, but only by showing that it wasn't really time travel at all. It was travel to a parallel world. Fun, but not the same. We also agree with a number of physicists, among them Roger Penrose, who accept that the `many worlds' interpretation of quantum theory is an effective mathematical description, but deny that the parallel worlds involved are in any sense real. Here's an analogy. Using a mathematical technique called Fourier analysis you can resolve any periodic sound, such as the note played by a clarinet, into a superposition of `pure' sounds that involve only one vibrational frequency. In a sense, the pure sounds form a serious of `parallel notes', which together create the real note. But you don't find anyone asserting that there must therefore exist a corresponding set of parallel clarinets, each producing one of the pure notes. The mathematical decomposition need not have a literal physical analogue.
What about paradoxes of genuine time travel, no faffing about with parallel worlds? In the relativistic setting, which is where such questions most naturally arise, there is an interesting resolution. If you set up a situation with paradoxical possibilities, it automatically leads to consistent behaviour.
A typical thought-experiment here is to send a billiard ball through a wormhole, so that it emerges in its own past. With care, you can send it in so that when it comes (came) out it bashes into its earlier incarnation, deflecting it so that it never enters the wormhole in the first place. This is the grandfather paradox in less violent form. The question for a physicist is: can you actually set such paradoxical states up? You have to do so before the time machine is built, then build it, and see what physical behaviour actually occurs.
It turns out that, at least in the simplest mathematical formulation of this question, the usual physical laws select a unique, logically consistent behaviour. You can't suddenly plonk a billiard ball down inside a pre-existing system - that act involves human intervention, `free will', and its relation to the laws of physics is moot. If you leave it up to the billiard ball, it follows a path that does not introduce logical inconsistencies. It is not yet known whether similar results hold in more general circumstances, but they may well do.
This is all very well, but it does beg the `free will' question. It's a deterministic explanation, valid for idealised physical systems like billiard balls. Now, it is possible that the human mind is actually a deterministic system (ignoring quantum effects to keep the discussion within bounds). What we like to think of as making a free choice may actually be what it feels like when a deterministic brain works its way towards the only decision that it can actually reach. Free will may be the 'quale' of decision-making - the vivid feeling we get, like the vivid sense of colour we get when we look at a red flower.' Physics does not yet explain. how these feelings arise. So it is usual to dismiss effects of free will when discussing possible temporal paradoxes.
This sounds reasonable, but there's a catch. The whole discussion of time machines, in physics terms, is about the possibility of people constructing the various warped spacetimes that are involved. `Get a black hole, join it to a white one ...' Specifically, it is about people choosing or deciding to construct such a device. In a deterministic world, either they are bound to construct it from the beginning, in which case `construct' isn't a very appropriate word, or the thing just puts itself together, and you find out what sort of
[1] See Ian Stewart, Jack Cohen, Figments of Reality: the origins of the curious mind (Cambridge University Press, universe you are in. It's just like Godel's rotating universe: either you're in it, or you're not, and you don't get to change anything. You can't bring a time machine into being unless it was already implicit in the unfolding of that universe anyway.
The standard physics viewpoint really only makes sense in a world where people have free will and can choose to build, or not to build, as they see fit. So physics, not for the first time, has adopted inconsistent viewpoints for different aspects of the same question, and has got its philosophical knickers in a twist as a result.
For all the clever theorising, the dreadful truth is that we do not yet have the foggiest idea how to make a practical time machine. The clumsy and energy-wasteful devices of real physics are a pale shadow of the elegant machine of Wells's Time Traveller, whose prototype was described as `a glittering metallic framework, scarcely larger than a small clock, very delicately made. There was ivory in it, and some transparent crystalline substance.'
There's still some R&D needed.
Probably this is a Good Thing.
AVOIDING MADEIRA
THE JOINER WAS AMAZED, As he told his mates in the pub after work
- so I was just finishing, and this feller comes down the ladder and says beggin' your pardon, sir, but I'd just like to check that bulkhead, please. Nothing wrong with it, says I, it's as sound as a bell. Then he says, right, right, of course, but I've just got to check something. He pulls this piece of paper out of his pocket and reads it careful, and says he's got to check that the new timber hasn't got a rare tropical worm that'll leave it looking like good wood but weaken it so much that the ship will take in too much water and will have to put in to Madeira for repairs, or something, possibly. I'll soon see about that, says I and whacks it with my hammer and, blow me, it cracks in half. I'd have sworn it was prime timber, too. Little worms everywhere!'
`Funny you should say that,' said the man opposite. `One of 'em came up when I was working and asked if he could look at the copper nails I was usin'. Well, he takes out a knife, scrapes away at one, it's a bit of rubbish iron under a skin 'o copper! Had to do half a day's work again! Beats me how he knew. Tom said the chandler swore they were all copper when he supplied 'em.'
`Hah,' said a third man, `one came up to me and said what would I do if a giant squid pulled the ship under. I told him I'd do nothing, being as I live in Portsmouth.' He drained his mug. `Damned thorough, these inspectors.'
`Yeah,' said the first man, reflectively. `They think of everything. ..'
`A goose is an inconvenient bird, I've always thought,' said Mustrum Ridcully, carving it. Just a bit too much for one but not quite enough for two.' He extended a fork. `Anyone else want some? Rincewind, just get the man to send up some more oysters, will you? What do you say, gentlemen? Another six dozen? Let's push the boat out, eh? Hahah ...'