The Physics Police

The Physics Police

Monday, May 16, 2016

Minute Physics hasn't solved the grandfather paradox

I like the channel Minute Physics a lot. Their videos tend to contain quick physics lessons which are fun and accessible to a lay audience. However, I was greatly disappointed by their latest video called Solution to the Grandfather Paradox which unfortunately contains pure pseudoscience in place of the advertised solution.

The video starts off with a great description of the Grandfather Paradox. In short, this paradox arises if you go back in time to kill your grandfather. Being quite dead, he never has grandchildren and so you were never born. This leads to a logical contradiction: if you were never born, who killed your grandfather?

The video goes on to propose a solution whereby you kill your grandfather in an alternate timeline, so he wasn't really your grandfather to begin with. But this solution is summarily dismissed.
"But that's boring because it just avoids the paradox." (0:34)
Fair enough. There's plenty of published work in the scientific literature on this topic that doesn't involve alternate timelines. Surely they're going to get to that science stuff, right? Wrong.
"I'm showing this as a linear series of events but really it’s two entangled histories happening in parallel." (1:02)
I don't know what it means for two histories to be "entangled" or to happen in "parallel". Here the video shows a Mobius strip with a timeline drawn on it, following the plight of our time traveler.

Mobius strip following actions to their logical consequences.
This shows a chain of causality which loops back on itself. This is a clever way of depicting the paradox but it doesn't "resolve" anything. It merely shows the circular reasoning which caused us to call this a paradox in the first place. This particular type of circular reasoning is called an infinite regress, and is generally considered illogical in science and philosophy.

This mistake is able to slip by due to selective inattention. Viewers are easily distracted by the pleasant visual stimulation (myself included, on first watching). As we try to follow along the Mobius strip with our eyes, we don't notice the false conclusion. This "argument from cool shapes" is something I'd expect from Spirit Science, but not from Minute Physics, which I've always taken as a serious science channel.

Next, to make this glaring fallacy more palatable, we're served a steaming pile of quantum woo-woo. Would you like physics jargon on the word salad? Just say when...
"Subatomic particles regularly do multiple different things in parallel – it’s called quantum superposition ... if the universe were to exist in a superposition of two states – your grandfather is alive and your grandfather is dead – then the natural result is a superposition of two states: you’re born and able to go back in time to kill your grandfather, and you’re not born. And the natural result of these is a superposition of two states – your grandfather is dead and your grandfather is alive – and so, at least from a logical perspective, this looping timeline is entirely consistent and there’s no paradox." (1:11)
When! When!

Let's notice the equivocation between the "parallel" lengths of paper in the Mobius strip and the use of the word with regard to subatomic particles being in different states at the same time. The implication is that what's true about the quantum world applies to our time traveler. This does not logically follow because people are macroscopic objects relatively unaffected by quantum effects like superposition due to decoherence.

Don't get me wrong. I'm cool with describing the quantum state of large objects, even the entire universe as in the Many Worlds interpretation of quantum mechanics. The problem is the superposition shown in the video makes no sense at all and doesn't show what it's claimed to show.

Sure it's science, see the math symbols? That makes it science!
We're told these states represent a model of the entire universe. I'll call this left-most state where your grandfather is alive |A>. We know this state has some representation of your grandfather being born. We can evolve it into the future to see him grow up, put on a bowler hat, marry, have son or daughter, etc. Keep evolving the model forward and eventually our time traveler is born, puts on a red hat, travels through a wormhole or whatever, and emerges on the other side to kill his own grandfather.

See the problem? We can't fully describe |A> without also including |B> in there, too. Because they aren't different possible states of the universe. They're all events in one and the same history! A history that's self-contradictory. Using math symbols from quantum mechanics doesn't make this problem go away.

Now, there's nothing wrong with trying to use the idea of superposition to solve the Grandfather Paradox. But to prematurely claim victory having done no work and having explained nothing is inexcusably poor physics. Which is sad, because many fascinating papers have been published on this topic. The three million Minute Physics subscribers could have learned about the foundational work done by David Deutsch, Seth Lloyd, and others.

Next the viewer is presented with a very telling tease.
"And a similar paradox-free solution can be obtained by viewing the problem as a steady-state solution to a Markov chain ..." (1:48)
When! When! Aside from the obvious attempt to stupefy the audience with big words, Minute Physics seems to have misinterpreted a quantum computing exercise for theoretical physics. In the comments section they cite a lecture and paper by Scott Aaronson. The lecture includes discussion of the Grandfather Paradox "in a computational form" which indeed uses a Markov chain.

This is not a resolution of the paradox per se, but a computer model which satisfies some of its properties. It's not part of any an actual scientific theory. It's just a toy model to teach computer science. Aaronson imposes upon this model the requirement that Deutsch calls "causal consistency". But that's boring because it just avoids the paradox! And it has nothing to do with quantum superposition. Because, as we've seen, quantum superposition can't solve the paradox.

So, having mentioned real science only once, teasingly, the video concludes with this moral lesson.
"The main point is sometimes we think a situation creates a paradox when it doesn’t, and really the only paradox is how our thinking can be twisted enough to dream up time-traveling murderous grandsons, but not twisted enough to think about twisting time." (2:09)
That's ironic because the only paradox is how such a terrible script got on Minute Physics.

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