Schrödinger's Cormorant

3.8 That’s how the light gets in

The energy-momentum relation predicts that photons have non-zero momentum, and this is supported by observations.

Even as you, Nefertiti and Cormorant are out, celebrating your success in coming to terms with special relativity, you notice that Nefertiti is looking a little tired. “You look a little tired, my dear,” you say. “Oh, I’m just feeling the pressure of the lights, this evening.” “She speaks figuratively, of course” murmers Cormorant. Nefertiti looks at him disdainfully: “Have you learned nothing? How could you even think that?” Cormorant is uneasy now. “But the photon is a massless particle!” he exclaims, “if it has no mass it has no momentum and if it has no momentum it can exert no force upon you. Except ….” Cormorant experiences a moment of revelation. See if you can share it with him:

Question 3-18

So there you have it – another remarkable consequence of relativity: a photon can indeed have momentum, even though it has no mass.

The momentum is related to the wavelength by: p = h / λ

Just how much momentum are we talking about?

Question 3-19

This Gedankenexperiment suggests that the momentum of a photon is not an enormous amount. But is there, nevertheless, any evidence for its existence, as special relativity predicts? Absolutely there is: the effect is called radiation pressure. To take but one example, which way do you think a comet’s tail points? The answer is that it always points away from the sun. Why does it do this – because it is caused by the tremendous flux of photons arriving from the sun and colliding with dust particles in the comet. Each collision transfers a tiny bit of momentum to a dust particle and gradually these particles get pushed backwards, away from the sun. Radiation pressure can also – as Nefertiti recalled back in Chapter 1 - be used to power spacecraft: the Japanese Ikaros craft flew to Venus powered with the aid of the momentum of solar photons.

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