Supernovas and Neutrino Speeds

Matt Strassler 9/20/11

[This was originally a blog post, written before the OPERA experiment’s announcement, but since it contains, at the end, some comments about how we get a limit on neutrino speeds from the 1987 supernova, I’ve included a copy of it in this section of the website.]

Supernovas are some of nature’s most common and powerful nuclear bombs.  They are also among the most useful for particle physicists and astrophysicists alike.

In “core-collapse” supernovas,  a huge number of protons are converted, by the absorption of electrons, into neutrons, with the consequent emission of neutrinos.  [Powering this process is one of the most important natural roles of the weak nuclear force.]  Somehow — and active research continues on this topic — the resulting shock waves (perhaps aided by something we don’t know about yet?) blow the star apart.

One of the most exciting events ever to occur in human astronomy was the 1987 explosion of a giant bluish star in the Milky Way galaxy’s largest satellite galaxy, the Large Magellanic Cloud, a luminous blob that is easily visible south of the equator.  Astronomers looking with the naked eye one night in February 1987 spotted a star in the Cloud that they were quite sure shouldn’t have been there.  This simple naked-eye observation began a great wave of astronomical activity that swept round the southern half of the earth, as every astronomer who could do so sought to take advantage of this once-in-a-lifetime opportunity.

A supernova is so bright that its light can briefly outshine the entire galaxy that contains it.  Yet only a small fraction of a supernova’s energy is emitted as light, or in other forms of energy that are eventually converted into light. Most of the energy of a supernova streams out, unseen, in the form of the above-mentioned neutrinos.

Here are some astounding numbers, quoting from a pedagogical website maintained by Steven T. Myers, Tenured Astronomer,  National Radio Astronomy Observatory, Socorro, NM : Almost all of the energy of the [1987] supernova came out in lightweight weakly-interacting neutrinos. About 10⁵⁸ neutrinos were produced in the core collapse. On Feb 24, 1987, about 10¹³ neutrinos passed through your body from the supernova! About a million people on the Earth had an “interaction” with a neutrino, of course with no noticeable effect.

That’s right: something like 10 trillion neutrinos passed through your body after the explosion of a star more than 160,000 light years away, several times further than the center of the Milky Way.  Amazing, this universe of ours.

Thousands of trillions of neutrinos passed through several neutrino detectors on earth, and of these a tiny handful — about two dozen — hit something on their way through.  These collisions were recorded over a period of about 13 seconds. No one was paying particular attention at the moment that this occurred, but after the supernova was noticed, experimental physicists went back and found this flurry of neutrino collisions in their data.  The flurry occurred about 20 hours before the first observation of the unexpected star in the Large Magellanic Cloud.  This discovery represented the birth of neutrino astronomy, now an active field of research.

Meanwhile, a look back at older photographs revealed one showing visible light from the supernova that was taken only 3 hours after the neutrinos had arrived on earth. Since the shock wave from the supernova blast had to make its way out of the exploding star before the debris could begin to shine, whereas the neutrinos from the explosion could sail right through the star unimpeded, a delay of a few hours between the arrival of the neutrinos and the arrival of the light was expected.

Now this story is wonderful and fascinating, but why am I mentioning it now? There are two reasons.

First, a relatively nearby supernova has been observed recently, and astronomers are very excited about it.  But press reports don’t seem to have a sense of scale.

• There have been numerous headlines saying “Brightest Supernova in 40 Years“, “Youngest and Closest in Decades“.  I do not think one needs a degree in physics or astronomy to calculate the time since the great 1987 supernova.
• The current supernova is over 20 million light years away, more than 100 times further away than the 1987 supernova.
• The current supernova can be seen by amateurs, but only by those armed with a good telescope, or maybe good binoculars in dark skies; the naked eye is insufficient.  [Its brightness has recently peaked; look soon!] The 1987 supernova was bright enough to be seen easily with the naked eye.

Why all the discrepancies?  Supernovas come in a few different types.  1987 saw a Type II supernova, in which the core of the star collapses and protons convert to neutrons as described above, with the ensuing neutrino blast. The current supernova is a Type Ia, which explodes through a different though not completely understood mechanism.  Type Ia supernovas are very important in astronomy, since they show considerable regularities that can be used to estimate their distance from earth, a fact that played a central role in the discovery that the universe’s cosmological constant, sometimes called “dark energy”, is not zero.  So astronomers are very excited to have an opportunity to study a Type Ia supernova in very great detail and with modern equipment, especially one that was discovered so soon after it exploded.

To sum up, this is the brightest and closest and most scientifically useful Type Ia supernova in several decades (ignoring one from 1986 that was not so easy to see or study), though it is not nearly as bright or close as the 1987 Type II supernova.

Second, there are rumors on the blogosphere of neutrinos being observed to travel faster than the speed of light. A beam of high-energy neutrinos from the CERN laboratory near Geneva (which also houses the Large Hadron Collider) is rumored to have arrived earlier than expected at the Gran Sasso laboratory in Italy, where a tiny fraction of the neutrinos are observed by the OPERA experiment. Keep in mind there’s no official announcement from OPERA yet, so this is just rumor-mongering at this point, quite possibly in error to a greater or lesser degree.

But in any case, one should treat any such claim, if indeed one is eventually made by OPERA in the near future, with considerable skepticism. This is partly because of the observations made in 1987 following the supernova.

As I mentioned, the neutrinos from the 1987 supernova arrived on earth within 13 seconds of each other, and were followed within at most 3 hours by the light from the supernova, the delay being roughly as expected.   These coincidences are considerable evidence that the neutrinos were traveling neither significantly slower nor faster than light, and that they were all traveling at almost the same speed.  Think about it: these neutrinos traveled for 168,000 years, about 5 trillion seconds, and arrived on earth within about 13 seconds of each other, and within 3 hours (about 10,000 seconds) of the light.  If the neutrinos had been traveling 1 part in a million faster than the speed of light, they would have arrived months before the light; a part in a million slower, and they would have arrived months later. And if they had traveled at different speeds by even one part in a billion, they would have arrived not in a 13 second burst but spread out over hours.

In short, there is evidence from this data that the neutrinos traveled at the speed of light to an extremely good approximation — to perhaps a few parts in a billion.

To measure an effect of a few parts per billion on the speed of neutrinos traveling from CERN to Gran Sasso — a distance of 730 kilometers, which light can travel in 1/400 of a second — would require measuring the travel time of these neutrinos to a small fraction of a nanosecond (a billionth of a second). Measuring anything to better than a nanosecond is tough; coordinating clocks 730 kilometers apart to this level of precision [probably not a good argument by itself] would be quite a feat.  [Several readers have already pounced on this paragraph, and they are right to do so; it is the weakest one in the whole post.   I didn’t say this feat was clearly impossible, just difficult.  But I do know this:  picosecond (trillionth of a second) timing measurements are rare in particle physics experiments; for instance typical timing at LHC experiments is at best at the 100 picosecond level, and usually a bit worse.  And I can’t think of a reason why OPERA, given its main task — the OPERA experiment has been designed to perform the most straightforward test of the phenomenon of neutrino oscillations, exploiting the CNGS high-intensity and high-energy beam of muon neutrinos produced at the CERN SPS in Geneva pointing towards the LNGS underground laboratory at Gran Sasso, 730 km away in central Italy — would have had clear cause to expend the money and effort to have picosecond-level timing.  (By the way it’s not just timing but also distance that requires an exceptional measurement.) So I suspect there is something crucial that I am unaware of.  This is the problem with rumors; we do not have enough details to make intelligent and wise comments.  Anyway, thank you readers for forcing me to clarify what I meant here, and let’s now just wait for the curtain to rise on OPERA. NOTE ADDED: After the experiment was announced and detailed, it became clear that the effect was, relatively speaking, an enormous one: 60 nanoseconds!]

There are loopholes in my argument.  Maybe neutrinos traveling through the earth behave differently from those traveling through space.  Maybe, of the three types of neutrinos, one type behaves very differently from the other two Loophole Closed.  [And oops! I had meant to mention a third loophole: Or the effect might depend on the neutrino’s energy, and the neutrinos from the supernova have a lower energy from those studied by OPERA.  Thanks to theorist Marc Sher for his comment  pointing out that this is specifically what was suggested by theorist John Ellis (along with Nicholas Harries, Anselmo Meregaglia, André Rubbia, and Alexander S. Sakharov) back in ’08, in a paper that was probably OPERA’s motivation for the current measurement.]  In these cases, perhaps one could imagine neutrinos traveling faster than the speed of light by a larger amount, one that would be easier for OPERA to observe.  But one should keep the supernova from 1987 in mind when evaluating the plausibility of any claim that high-energy neutrinos traveling 730 kilometers in 0.0024 seconds do so at a speed significantly different from that of light.

By the way, if you happen to read articles about this elsewhere, please don’t be confused by people suggesting that OPERA has a “6-sigma observation” of something new. Such language seems at first to imply a very clear sign of something novel and extraordinary — that the discrepancy observed in the data must be a real effect.   But all it really means is that the effect can’t be a statistical fluke, or due to a known systematic effect. It could still be due to a mistake, or something wrong with the apparatus, rather than a new physical phenomenon.

I should emphasize once again that until the OPERA experiment announces its results, we’re dealing in rumor, and rumors are often wrong.   But it’s no rumor that a star, in its final death aria,  can potentially teach us more than any earthbound measurement.