For a ghostly type of particle, oblivious to even the massive bulk of a star or planet, neutrinos sure can generate a fuss. In the 1960s they created a stir by seemingly appearing from nuclear processes in our Sun’s core at a third of the anticipated rate – the so-called solar neutrino “problem“. In the 1980′s they seemed like they might offer a solution to the nature of cosmic “dark matter” – except they didn’t because their speedy nature would have erased too much of the small structure in a young universe, those annoyances like galaxies. And all along we’ve not quite managed to pin down their actual stationary mass, although it must be small, probably less than a millionth of the rest mass of an electron.
If any other recognizably intelligent life exists somewhere else in this vast cosmos one can only presume that they too have spent an inordinate amount of time scratching whatever part of their anatomy they scratch when puzzled. Neutrinos are unlikely to be any more readily accessible to alien beings composed of normal matter than they are to us. Last week neutrinos created a fuss again here on planet Earth. As reported by countless tweets, posts, reports, and sound-bites the neutrino detection experiment OPERA announced their analysis suggesting that high energy (17 giga-electron-volt, GeV) muon-neutrinos might be traversing 730 kilometers through the Earth’s crust at a speed slightly greater than the free-space speed of light by approximately 0.00025% (plus or minus about 0.00006%).
Exactly how fast would we have expected a (sub-luminal) neutrino to be moving? Well, just like if I asked this question of a tennis ball or a speeding train, it depends on its energy. If, for the sake of argument, the heaviest neutrino weighs in at about 3 eV (for comparison a normal electron has a mass of about 510,000 eV) and it’s moving ultra-relativistically so that its total energy is about 10 giga-electron volts, its speed will be less than the speed of light by a mere 0.00000000000000001 %. Theories proposing that neutrinos can take “shortcuts” through extra (large) dimensions – described as going “off-brane” – predict velocities that might differ from (and exceed) the speed of light by as much as 0.01% at similar particle energies.
If the OPERA result were correct it would point to some extraordinary things. We’ve all grown up understanding that the only sensible thing for matter to do is to never exceed (or indeed reach) the speed of light. It’s a fundamental piece of Einstein’s special theory of relativity, which is one of the best verified theories in modern physical science. I won’t go into the details here, but for matter with mass to exceed lightspeed there are a large number of headaches, not least of which are the most basic ideas of causality – at both the microscopic and macroscopic scales. So its critical to consider what’s next for neutrinos? Where do we go from here?
Two main avenues are obvious. The first is that others analyze and re-analyze the OPERA results, scrutinizing every aspect of the experiment to see if there is an explanation for their result that does not require super-luminal neutrinos – it might be mundane, it might be complex, and it could even be unexpected. The second is for someone to perform another experiment somewhere else that makes the same kind of measurement, but entirely independently.
Enter the particle hunters of Fermilab, or rather re-enter the particle hunters of Fermilab. Back in 2007 the MINOS experiment presented the intriguing results of a very similar test of the speed of high energy neutrinos. Much like OPERA, MINOS employs an artificial beam of neutrinos (at slightly lower energies of 3 giga-electron volts) that traverses a similar distance through the Earth – in this case traveling 734.2986 kilometers from just outside Chicago to a 5.4 thousand ton detector sitting 700 meters down in a former iron mine in northern Minnesota. Four years ago the MINOS collaboration published a constraint on the velocity difference between neutrinos and light of about 0.005% with an uncertainty of 0.003% at a 68% confidence level – in favor of super-luminal motion. It didn’t get the same fanfare with the public, but it definitely set tongues a-wagging in the particle physics community.
It now looks like the MINOS people are going to both go back in and re-analyze their data in an effort to reduce their level of uncertainty, and to try to perform some new experiments. This is clearly now critical, but it will take some time, at least 6 months for the analysis and perhaps a year or more to run further actual experiments. So, the good news is that the two obvious followups to the OPERA result seem to be underway, the bad news is that we are going to have to wait a little before we know the results.
In the meantime, as with the much discussed neutrino results of Supernova 1987a (if super-luminal like the OPERA claim, the neutrinos from stellar core-collapse would have preceded the light of the supernova by about 4 years, which they didn’t), we may want to cast our gaze at the natural world. If by some chance particles like neutrinos really can trip the light fantastic, are there other cosmic phenomena that would betray this behavior? We should definitely be on alert and, above all, try not to blink.