Faster-than-light neutrinos:

The reports of their death may be greatly exaggerated.

In a new paper submitted to Physics Letters, physicist Robert Ehrlich, recently retired from George Mason University, makes the claim that the neutrino, or more precisely its electron flavor, is likely to be a tachyon or hypothetical faster-than-light particle. Haven’t we heard this one before? There have indeed been many such claims, the last being in 2011 when the OPERA collaboration measuring the speed of neutrinos created by the CERN accelerator claimed they travelled a tiny amount (0.002%) faster than light en route to a detector in the Italian Alps. However, when the speed of neutrinos was measured again after fixing a loose cable, their speed that was found to be consistent with that of light.

Einstein’s Theory of Relativity was still safe, but of course, the negative result of such experiments cannot rule out much smaller departures from light speed than they could measure. In fact, Ehrlich’s new claim of faster-than-light neutrinos is based on a completely different, and much more sensitive, method than measuring their speed directly. If his claim is correct, then OPERA would have needed an impossible accuracy equivalent of a 100 kilometer ultra-marathoner beating out her nearest competitor by one billionth the diameter of an atom in order to see the effect. In addition, despite worries about Relativity being overturned if such particles are ever found to exist, Einstein never actually ruled out the existence of particles that always travel faster than light from the moment of their creation in particle reactions.

The Ehrlich test relies on an idea first proposed in 1985 by physicists Alan Chodos, Avi Hauser, and Alan Kostelecky that neutrinos are tachyons. In that event, according to the so-called reinterpretation principle, neutrinos could reverse their direction in time or equivalently the sign of their energy when viewed from some moving frames of reference. In effect, the emission of a tachyonic neutrino would to a second sufficiently rapidly moving observer appear to be an absorption, that is the time-reversed situation, and there is no way to say who was right.

Due to this time reversal property, if neutrinos are tachyons, the process known as proton beta decay should occur when the proton travels at sufficiently high speed towards us. Normally, this process is considered forbidden because it could not conserve energy. However, that changes if neutrinos are tachyons, whose energy can be negative in certain reference frames. This idea led to a 1999 Ehrlich paper that attributed the “knee,” or change in slope, of the energy spectrum of cosmic rays to the electron neutrino being a tachyon. In this paper Ehrlich predicted that one should find a peak in the cosmic ray spectrum at a specific energy of about 4.5 PeV, i.e. 4.5 million billion electron volts. Such a peak would be created by the proton being allowed to beta decay above this energy into a neutron, neutrino and positron. In a follow-up 1999 publication, Ehrlich claimed evidence for just such a peak based on observations of the microquasar Cygnus X-3 , a result which most physicists dismissed as implausible.

Now , in his new paper, “Evidence for a cosmic ray peak at 5.86 PeV,” Ehrlich presents additional evidence using data taken by the Tunka Collaboration, an international cosmic ray group based in Russia. Essentially, Ehrlich looks at the number of “candidate sources” for cosmic rays having various energies, where this phrase denotes small regions of the sky having anomalously large excesses in cosmic rays apparently emanating from them. He finds significantly more candidate sources than chance would predict when the cosmic rays have energies of around 5.86 PeV, but not at other energies, which is consistent with a peak at 4.5 PeV (the earlier claim), given the uncertainty in that claim, i.e., + 2.2 PeV. Ehrlich estimates the chances of the excess number of candidate sources arising by chance to be no more than one in about 4 million. Equally striking to the anomalously large number of candidate sources is their locations in the sky, which appear to lie very close to a series of straight lines. This strange pattern could be evidence that the cosmic rays at this energy originate from “cosmic strings,” which are hypothetical entities left over from the very early universe. Even though cosmic strings are predicted to have a thickness much less than an atom, they would have immense density, and so a cosmic string about a kilometer in length would be more massive than the Earth. Of course, Ehrlich’s results will require independent verification to see if it is reproducible.

One interesting implication of tachyons that has made many physicists dubious they exist is that they violate the principle of causality. This means that they erase any absolute distinction between which of two events is the cause and which the effect. For example, there would be no absolute distinction between the emission of a tachyon from a transmitter at point A and its reception at a receiver at point B, and the reverse, that is its emission from a transmitter at B and a receiver at A. Different observers will disagree which device was the source and which the receiver. This seems completely counterintuitive and even at variance with our ideas of free will. After all, receiving a signal is a passive act while sending one requires our intention to do so. In the meantime, until the result is confirmed, it may be premature to start trying to build a tachyon telephone to send messages back in time to your deceased loved ones, or even your earlier self. Although such devices makes for great science fiction, they may be impossible to build even if neutrinos are tachyons. Here you can find a physics colloquium presentation describing the results. It is best viewed in “slideshow” mode to see the animations.

Some illustrations

Locations of the 68 candidate sources in celestial latitude and longitude, with dotted lines showing how they cluster about straight lines. The sizes of the circles for each source shows the degree of spreading of the cosmic rays from that source on reaching Earth.

An artist's concept of cosmic rays hitting Earth's upper atmosphere and creating showers of charged particles that are detected based on the light they emit. Credit: Simon Swordy, University of Chicago.

Frame from a Youtube video: “Einstein on faster-than-light speeds?” imagining what Einstein might have thought about faster-than-light particles.

Cartoon of someone confused by a message from their future self, by Goeff Elkins, used with permission.