39.34 metres per second, Marty!

This cat is a time traveller. From Wildstray on Flickr. CC by-nc-nd 2.0

Finally, after nearly two months, I get to write a post on my actual subject of study: physics!

The Telegraph today gives us the startling headline “Quantum time machine ‘allows paradox-free time travel’“. There were precisely two pictures that could have illustrated this article: the TARDIS and the Delorean. This time, the article is headed with a still of Doc Brown and Marty McFly climbing into their car.

The “time machine” described effectively allows you mess with quantum probabilities, choosing or “postselecting” the history you want to have happened by making it as likely as possible.

Now I’m torn on this one. On the one hand, the article is about a complicated topic which is hard to explain in layman’s terms, and they have tried to find academics to help analyse it and the central point of the paper – that quantum postselection won’t let you kill your granddad (or indeed become him) – still stands clear. On the other hand, the article is so sensationalist that the actual science behind it is a little lost behind all the “Whoosh! Back to the Future!”. Let’s go through the article point by point.

First things first: this technology would not allow you to go back in time. Instead, it lets you ‘pick’ a history of a given particle. You stay where you are, history changes around you. Imagine if Schrödinger, before opening his cat’s box, could teleport the cat across the lab into another box – but choose to teleport only the living cat from the superposition of dead and alive cats. When he opens the box, the cat has only ever been alive. The cat has only ever travelled forwards in time at the classic one second per second, but the path it travelled along was chosen from the future.

Sounds awesome, but there’s a few fairly major limitations. As far as I can tell from the paper, the experiment, like most things in quantum physics, only works if the observer (in this case, the time-meddler) is not entangled with the system (in this case, whatever you are trying to meddle with). So Schrödinger’s box must remain closed. If Schrödinger opens the box and finds the cat dead, he cannot then rewrite history so that the cat is alive. To use the classic alternative history example, you could make it so the Nazis win World War II… but only if you’re Switzerland.* This is why quantum time travel is supposedly paradox-free. You are entangled with your past, so you are completely unable to change it. Your grandfather is entangled with you – classical quantum mechanics will therefore not allow any possible history where your grandfather dies while you still exist, and therefore you can’t postselect for it.

Another, slightly more minor issue I have with this is that the paper has been published only on arXiv. For those of you who aren’t scientists, arXiv is a site for people to post scientific papers in physics, maths or computing. Now, arXiv is a very interesting and very useful site within its limits, but there’s one thing it isn’t: peer reviewed. All you need to do to post a paper to arXiv is register with the site (in its own words, it is a “pre-print” site); rather like Wikipedia, it’s useful for giving you an overview of the latest happenings in a field, but not a definitive source. You could get an interesting 50 word column to the effect of “A group from MIT claims to have invented a method for changing the past without paradoxes“, but a 560 word article based on an unreviewed paper is pushing it a little.

The Telegraph seems to have picked the story up from MIT’s Technology Review – a slightly more reasonable place for it, since it’s a) clearly marked as being from the non-peer reviewed arXiv servers, b) targeted at people who are more likely to understand the importance of peer review, and c) involves MIT researchers.

This isn’t the first time an arXiv paper has been picked up and sensationalised. The most famous example is the Exceptionally Simple Theory of Everything, the paper written by, as the papers put it, the “laid-back surfer dude” (who was actually a Ph.D. theoretical physicist) which purported to replace the Standard Model with an 8 dimensional algebra that connects particle physics to gravitation. The paper has never been submitted for peer review however, and the scientific community is rather indifferent to it, since it’s not easily testable, and it goes against the more widely accepted supersymmetry theory.

So this is where I’d really like some input from some non-scientists. Sure, a physicist can poke holes in the article, but are they really major enough for me to complain about. It won’t actually let you time-travel, and its history changing abilities are limited to such a degree that they are essentially useless outside of quantum computers, but do the liberties they’ve taken sound fair? After all, I have no objection to the countless articles about invisibility cloaks (which only make objects the size of pinheads invisible to low power microwaves) because there is, amongst the exaggeration, solid scientific fact. And heaven knows an article about quantum physics which didn’t handwave it to some extent would be unreadable to anyone – even a physicist. Did you find the article interesting even though it does play fast and loose with the facts at times?

* And don’t read the papers. And are completely unaware of the war situation. And absolutely nothing in your life is affected by WWII. And if you have never even breathed in an air molecule once exhaled by a German or British soldier. Yeah, it’s rather limited indeed.


  1. #1 by ukenagashi on Thursday, 22nd July 2010 - 21:34 UTC

    My failure at science finally comes in handy!

    Well, it doesn’t really.

    It does mention that you can’t go back and change history, and it specifically says that this is all ~physics stuff~ and not for people going back to have wacky historical adventures, so I’m not that bothered by it? Bearing in mind that I’m not good at this kind of thing anyway.

    What did annoy me the most was the line “However, because of the probabilistic nature of quantum mechanics, Prof Lloyd’s method seems to avoid this. Anything caused by the time travel must have had a finite probability of happening anyway, so paradoxical impossibilities are out.” but I assume that’s my own personal beef with the infinite convenience of “quantum says, so it must be true!” and not the article.

    Seriously, that is the thing I hate most about quantum. There’s no WHY. There’s just BECAUSE. Quantum physics is the Mary Sue of the sciences!

    So that was completely unhelpful!

  2. #2 by wickedday on Thursday, 22nd July 2010 - 22:27 UTC

    I read it twice, and it’s . . . opaque. The bit about particles “and in theory people” exploiting this technique seems downright wrong – at the very least wildly misleading, because surely a Schrodinger-style time machine would require an operator, and if the operator knew about the time-traveller, bam, entanglement and quantum fail.

    I think it does go out of its way to obscure that this is a teeny, tiny, mostly useless and difficult-to-produce effect. On the other hand, I can’t entirely blame them.

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