When astronomers first observed light from a supernova arriving 7.7 hours after the neutrinos from the same event, they ignored the evidence. Now one physicist says the speed of light must be slower than Einstein predicted and has developed a theory that explains why

In the early hours of the morning on 24 February 1987, a neutrino detector deep beneath Mont Blanc in northern Italy picked up a sudden burst of neutrinos. Three hours later, neutrino detectors at two other locations picked up a similar burst.

Some 4.7 hours after this, astronomers studying the Large Magellanic cloud that orbits our galaxy, noticed the tell-tale brightening of a blue supergiant star called Sanduleak -69 202, as it became a supernova. Since then, SN 1987a, as it was designated, has become one of the most widely studied supernovas in history (see animation above).

But even today, there is a significant mystery associated with this SN 1987a that astrophysicists have brushed under the carpet. The event consisted of two bursts of neutrinos separated by three hours followed by the first optical signals 4.7 hours later.

Neutrinos and photons both travel at the speed of light and should therefore arrive simultaneously, all else being equal. The mystery is what caused this huge delay of 7.7 hours between the first burst of neutrinos and the arrival of the optical photons.

Today, we get an answer thanks to the work of James Franson at the University of Maryland in Baltimore. Franson has used the laws of quantum mechanics to calculate the speed of light travelling through a gravitational potential related to the mass of the Milky Way.

Because all previous speed-of-light calculations have relied only on general relativity, they do not take into account the tiny effects of quantum mechanics. But these effects are significant over such long distances and through such a large mass as the Milky Way, says Franson. He says that quantum mechanical effects should slow down light in these kinds of circumstances and calculates that this more or less exactly accounts for the observed delay.

First, some background about the mechanism behind the supernova. A supernova begins with the collapse of a star’s core, generating both neutrinos and optical photons. However, the density of the core delays the emergence of the photons by about 3 hours. By contrast, the neutrinos interact less strongly with matter and so emerge unscathed more or less immediately.

Many astrophysicists believe that supernovas can also undergo a second collapse, generating an additional burst of neutrons. That’s why the detectors on Earth spotted two bursts.

But the timing is still a puzzle. The optical photons should have arrived about 3 hours after the first burst of neutrinos rather than 4.7 hours after the second burst

In the absence of any explanation, astrophysicists have simply ignored this burst, saying that it cannot have been associated with the supernova and must have been a flukish coincidence. That’s despite the chances of such a coincidence being something like 1 in 10,000.

Franson comes to this problem from a different angle as a leading thinker on interferometry and quantum mechanics. He points out that physicists have had to consider the effects of the gravitational potential on the quantum behaviour of atoms in matter interferometry experiments for some time. So it’s not such a stretch to think that the gravitational potential might also have an effect on photons.

His thinking goes like this. As a photon travels through space, there is a finite chance that it will form an electron-positron pair. This pair exists for only a brief period of time and then goes on to recombine creating another photon which continues along the same path. This is a well-known process called vacuum polarisation.

Franson’s idea is that the gravitational potential must influence the electron-positron pair because they have mass. “Roughly speaking, the gravitational potential changes the energy of a virtual electron-positron pair, which in turn produces a small change in the energy of a photon,” he says. “This results in a small correction to the angular frequency of a photon and thus its velocity.”

By contrast, neutrinos are not influenced in the same way. Their interaction with virtual particles comes about through the weak force and this is negligible in comparison. Therefore, neutrinos travel at the unperturbed speed of light. “The analogous effects for neutrinos involve the weak interaction and they are negligibly small in comparison,” he says.

Franson goes on to calculate the magnitude of this effect over the intergalactic distances between here and SN1987a. That involves including a term for the gravitational potential in the quantum electrodynamical description of the photons, a process that essentially combines quantum theory and general relativity.

The results are eye-opening. The calculations suggest that photons are delayed by a factor that is proportional to the fine structure constant. When putting the relevant numbers into the equations, Franson concludes that the new effect can easily account for the observed delay of 4.7 hours. “The predictions of this model are in reasonable agreement with the experimental observations from Supernova 1987a, in which the first neutrinos arrived 7.7 hours before the first photons,” he says.

That’s an interesting piece of work. A correction to the speed of light over these kinds of astrophysical distances is a big result. And it has other implications. Franson says that this kind of thinking would result in a small correction to the anomalous magnetic moment of the electron and to the decay rate of orthopositronium (the name for an electron and a positron orbiting each other). However, these corrections are some two orders of magnitude smaller than physicists are currently able to measure.

That should make for some interesting experiments in future. Franson has been pushing this idea for a few years and only recently published it in the New Journal of Physics. That should provide some momentum to push it forward. What he needs to back up his ideas is more evidence of the difference in the arrival times of neutrinos and photons from supernovas. That shouldn’t be too hard to gather but not easy either. Then there are the experiments to measure the anomalous magnetic moment of the electron in the decay rate of orthopositronium. If he’s right—that the speed of light is slower than Einstein predicted—that is hugely significant. We’ll be watching to see what happens next. : Apparent Correction to the Speed of Light in a Gravitational Potential