The Grandfather Paradox: Experimentally Resolved?
I am utterly astounded that I’d never seen this before today – an experiment that may have more profound implications for our worldview than… maybe any experiment since the Michelson-Morley experiment?
I’m going to assume here that everyone knows about the Grandfather Paradox. This is just the simple question “What happens if you have a time machine, and go back and kill your granddad so you can never be born?”, the staple of many TV science fiction shows.
Now the normal answer to that question is “You can’t, so don’t be daft”. But for physicists, that’s not good enough – apart from anything else, General Relativity allows for the existence of ‘closed timelike curves’. These are paths through space-time that act much like paths through space – you can go in at one end and pop out the other – except that the other end is somewhere else in time as well as space. So it’s theoretically possible that you *could* do that, and we’d quite like to know what would happen if you did before everyone’s granddad starts retroactively never-having-existed.
Now, the main hypothesis in physics up to now has been, in effect, that it doesn’t matter. David Deutsch, a quantum computing expert at Oxford University, demonstrated that in quantum-mechanical terms you could have an outcome that makes sense so long as you accepted the many-worlds version of reality. Essentially, the probability that you were ever born, and the probability that you killed your grandfather, would both be 1/2 – or in other words the ‘you’ in a universe where you were born would travel to a universe where you were never born, kill your grandfather there, then come back to one where you’d never killed your grandfather. Nice and simple.
However, Seth Lloyd, a quantum physicist at MIT, never liked the many-worlds hypothesis (for reasons which, I have to say, make no sense at all to me), and he and a team of colleagues came up with another, simpler, idea, which is just that if you go back in time and try to shoot your grandfather, something will stop you. Maybe the gun will misfire, maybe you’ll be arrested, maybe your grandma was having an affair with the milkman and you’re his biological grandchild – something will just make sure that you can’t do that, because it would be cheating.
Now, there are huge, huge, MASSIVE problems with this – it gets rid of causality, it allows information to come from nowhere, and it just seems like a gigantic handwave. It makes no sense at all, and just seems like a desperate attempt to try to get out of the obvious, blatant, truth that the Many-Worlds interpretation is the only one consistent with the experiments and maths. When I first read about it, I thought it was just a neat way of avoiding the truth.
Unfortunately, it appears to be true. What I hadn’t realised was that they’d *actually done the experiment*!
Lloyd and his colleagues came up with an ingenious experiment, which I’m not entirely sure I’m capable of explaining, as it’s not really sunk in yet. This will be a GROSS oversimplification, and is just designed to get the idea across – please don’t kill me for inaccuracies. The full description is in the linked PDF. This is what Pratchett, Stewart and Cohen call lies-to-adults – the story is right, but each individual fact is wrong.
Essentially, photons (light particles) can be polarised a couple of ways, and they’ll only go through stuff that’s polarised the same way. That’s why Polaroid sun-glasses work – they block all the photons that are polarised the wrong way, so only let some light through.
Now, until something detects it, a photon isn’t in any particular polarisation – it’s in all of the possible polarisations at once. But once something has detected what kind of polarisation a photon is in, it’s always been that way – quantum causality works both ways in time. So you can set up an experiment that only detects photons of one polarisation, and that way you can send a message back to the past, to the photon emitter (light source) saying “Only send photons of this type”. If you do this the right way, you can send a photon back in time (but you can’t look at the photon that’s been sent back in time until it’s come back to the time you sent it from, or the experiment can’t work). That might sound mad, but it’s the way things are – accept it for now.
Now, by doing this, you can set up a kind of quantum ‘gun’ – set it up so that the photon going back in time tries to cancel out itself coming forward in time – all you do is put something in the middle that tries to change the polarisation of the backwards-in-time photon to the opposite of the forwards-in-time one. Changing polarisation is easy, and works about 96% of the time.
It never worked on the backwards-in-time photons.
This means that if you went back in time and tried to kill your grandfather, the gun really *would* misfire! Every time.
Now, assuming their experimental design wasn’t flawed and their maths works – and it looks OK to me, but I’m not a quantum physicist – then that means a lot of things:
Firstly, it means the universe is completely deterministic. There’s no such thing as chance.
Secondly, it’s strong evidence *against* the many-worlds hypothesis – the first such evidence I’ve ever heard of. It almost certainly means there’s a single universe.
Most interestingly, it means we can say goodbye to cause-and-effect. Effects can cause their own cause. For science-fiction fans, we’re living in the universe of Bill & Ted, the Doctor Who story Blink, and By His Bootstraps, (EDIT or of this rather nice short-short story by Simon Bucher-Jones) rather than Back To The Future or Pyramids Of Mars.
This of course means that access to a closed timelike curve (something that has never been observed in the real universe, but is theoretically possible), gives you essentially godlike powers. Got a closed timelike curve and want a million quid? Just put two pence in the bank and say “tomorrow, if my account has two milion pounds or less in it, I’ll take half of the money out and bring it back today and stick it in the account.” So if tomorrow you’ve still got 2p, you’d go back and put an extra penny in, which means that actually tomorrow you’ve got 3p in, which means… and the only stable way that can work out (other than you dying or something over the next day) is for the million pounds just to appear in your bank account.
Want to write a bestselling novel? Decide to print out five hundred pages just covered with the letter “A” and send it to a publisher. If they publish it and it becomes a bestseller, you send that back to yourself. If they don’t, you print out all the letter “A” apart from one “B” at the end and send that back to yourself to try that, and repeat – the only stable outcome is that you have a novel arrive that you never actually wrote but that will be an instant bestseller. And so on.
The possibility of time-travel in a *single, consistent universe* has never been one that’s really been taken seriously before, because it was just so absurd. I’m still 90% sure that there must be a mistake somewhere – the many-worlds hypothesis, as odd as it may sound, is far, FAR less ridiculous than this. But this is one of those things where either in a few months we’ll have a very quiet paper by Lloyd saying “Oops, I was totally wrong about everything because I forgot to carry the one” or in a hundred years’ time we’ll have a totally new understanding of physics based around this paper. I really can’t see a middle ground here…