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. The event consisted of two bursts of neutrinos separated by three hours followed by the first optical signals 4.7 hours later.
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.
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.