Reverse the polarity of the neutron flow!

On 26 February 1972 the BBC broadcast the first installment of “The Sea Devils,” a six-part story in its science fiction series Doctor Who.

The Sea Devils of the title are an ancient race of intelligent reptilian humanoids. Like their terrestrial relatives, the Silurians, the Sea Devils had been hibernating for centuries. At the start of the story, the show’s main character, the Doctor, visits his nemesis, the Master, who is incarcerated on an island run by the Royal Navy. Reports that nearby ships have gone missing portend the awakening of a nearby colony of Sea Devils.

The plot unfolds as the Master not only escapes but succeeds in building a machine that would revive and control all of Earth’s colonies of Sea Devils. His goal, as usual, is world domination. The Master fails because the Doctor has sabotaged the machine by reversing the polarity of its neutron flow.

“Reverse the polarity of the neutron flow” or sometimes just “reverse the polarity” became a catchphrase for the Doctor, as played in his third incarnation by Jon Pertwee. Subsequent incarnations of the Doctor uttered the phrase as an intertextual in-joke that loyal viewers appreciated. YouTube user DW Supercuts has created a 35-second video that shows 14 instances of the phrase being used by several of the Doctor’s incarnations.

The original author of the phrase was scriptwriter Malcolm Hulke (1924–79). I’ve been unable to establish whether he had a background in physics. My hunch is no. A physicist would be more likely to have written “reverse the polarity of the neutron beam.” Still, it’s to Hulke’s credit that the phrase is physically plausible. Neutrons, being spin-½ fermions, can indeed be polarized with a magnetic field. Reversing the field’s direction reverses the neutrons’ polarity. What’s more, at neutron diffraction facilities around the world, physicists are creating and using polarized beams of neutrons.

The idea that spin-polarized neutrons might serve as a useful probe originated in a 1936 Physical Review paper by Felix Bloch. He developed the theory of the magnetic scattering of neutrons and then went on to propose that spin-polarized neutrons could be used in an “experimental study of the magnetizing electrons in ferromagnets.”

Thirteen years later, Clifford Shull and J. Samuel Smart put Bloch’s proposal into action when they proved that manganese oxide is an antiferromagnet, thereby vindicating the prediction that Louis Néel made in his 1932 PhD thesis that such a state could exist.

Bloch’s 79-year-old idea continues to pay dividends. Just last week, Marc Janoschek of Los Alamos National Laboratory and his collaborators published the result of their using neutrons from the Spallation Neutron Source to resolve a long-standing mystery: the absence of ferromagnetism in plutonium. It turns out that the magnetism of plutonium’s δ allotrope is not missing, but the fleeting result of virtual valence fluctuations within the element’s complex electronic ground state.

I know of at least one other case from Doctor Who in which the Doctor uttered a phrase that, while seeming nonsensical, in fact pertained to real (or at least real theoretical) physics. In “Blink,” which first aired on 9 June 2007, the Doctor’s Tenth incarnation, played by David Tennant, is shown delivering a video message to someone in his future. At one point, he says,

Although “wibbly wobbly” is not a technical term, it reminds me of the notion that the arrow of time points in one direction, thanks to quantum entanglement .

Updated (17 July 2015): The image caption has been changed to correctly identify the episode and year.