Imaginary Neutrinos

A new paper proposes that neutrinos may be tachyons.¹ This is being presented in the press as a claim that neutrinos move faster than light, but that’s not the focus of the paper. Instead the paper argues that the electron neutrino could have an imaginary mass.Tachyons are hypothetical particles proposed in the context of special relativity. One aspect of relativity is usually stated as “nothing can move faster than light,” but technically what relativity says is that nothing can be accelerated to the speed of light. So if a particle is initially moving slower than light, it can never travel faster than light. But what if there were particles initially traveling faster than light? According to relativity they could never slow down to light speed. Hence the idea of tachyons.

If you assume tachyons exist, then special relativity also requires that they have imaginary mass. Tachyons were originally proposed before we had a solid understanding of quantum theory. The mathematics of quantum theory is now very clear, so it is relatively straightforward to plug an imaginary mass into the equations to see what happens. When you do that, it looks like a description of a faster-than-light particle, but when you look closely what you find is that the “particle” aspect of the quantum field actually travels slower than light. So what started as an idea for particles to move faster than light turns out to be something that moves slower than light.

Hypothetical quantum particles with imaginary mass are often referred to as tachyons, even though they wouldn’t travel faster than light. Hypothetical particles that actually move faster than light are also called tachyons, but they would violate both special relativity and particle physics. Strangely, the author claims the two are the same, and that imaginary mass neutrinos would in principle travel faster than light, which just isn’t the case.

But the main focus of the paper is a demonstration that an electron neutrino with imaginary mass is consistent with current observations, including cosmic expansion and the cosmic microwave background. While that seems to be true, the data is also consistent with neutrinos having regular positive mass. Another weakness of the paper is that the author focuses only on electron neutrino mass, when we know experimentally that the different flavors of neutrinos have indefinite masses. So it doesn’t make much sense to talk about the mass of one type of neutrino.

Overall the paper is a fairly weak argument for a rather fringe idea. Just because one aspect of a model can be made to fit one set of data, that isn’t very convincing. Models need to account for a range of evidence, and the evidence points to neutrinos having regular masses like other particles.