Neutrinos

I’m sure you’ve all heard the news; the OPERA collaboration have taken measurements which seem to suggest that neutrinos emanating from the CERN Super Proton Synchrotron travelled the 455 miles through the Earth’s crust to the Gran Sasso Laboratory at very slightly more than the speed of light in vacuum.

For those not versed in the ways of the physics, a neutrino is a fundamental particle. It’s a lepton, the same family of particles as the electron. Unlike the electron, the neutrino has no electrical charge, and so can only interact via the weak nuclear force. That’s how they can travel hundreds of miles through the earth’s crust; they interact with the matter we’re more familiar with (atoms made of electrons, protons and neutrons) only very, very rarely.

That means to detect them you need huge, super-sensitive detectors, typically built deep underground to screen out the signal you would otherwise get from cosmic rays. One is the Super-Kamiokande detector in Japan, which contains 50,000 tons of water. When one of the rare interactions with a neutrino occurs,  the interaction generates a very small amount of light, which is detected and used to infer the properties of the neutrino interaction which caused it.

The OPERA experiment was designed to measure a phenomenon called neutrino oscillation, or neutrino mixing.

There are three types of neutrino: the electron neutrino, muon neutrino, and tau neutrino. The Sun produces a vast number of electron neutrinos as a by-product of the fusion reaction which powers it. When detectors were used to measure the neutrinos being emitted by the Sun, it was discovered that the number was less than would be expected. This became known as the “Solar Neutrino Problem”.

Despite claims to the contrary in certain elements of society and the media, when new evidence is discovered, the theory has to give way. Either the models of what was going on inside the Sun were wrong, e.g. the fusion yield of the Sun was lower that expected, or some aspect of neutrino physics was not properly understood. Cross-checks with other measurements of the Sun indicated support for the Solar models. So the problem was  with the neutrino physics.

It had been assumed that the mass of the neutrino was zero; all measurements made had indicated that it was, at the least, very close to zero. However, if the neutrino had even a very small amount of mass, it would undergo a very peculiar phenomenon due to a quirk of quantum mechanics, called neutrino mixing. Essentially, in the flight from the Sun to the Earth, some of the neutrinos would change flavour, from electron to muon, or tau neutrinos. The “missing” neutrinos were there all along; they just weren’t in the form of electron neutrinos that the detectors were capable of detecting.

The OPERA experiment is designed to more closely measure this process by generating a neutrino beam on demand in an accelerator, and then measuring the mixing that occurred while the beam was in flight.

In doing this, they have apparently detected, to a good degree of statistical significance, that their neutrinos travelled superluminally from the source to their detector. This is well-known to be forbidden by relativity, so if this is a true result, then it will require brand-new physics to explain, and could mark the start of a new era of post-Standard Model physics. It would be one of those fantastic moments where something amazing is discovered by people looking for something else entirely.

That said, it could also be a mistake in their methodology. Relativity has stood unmolested for a century; every experiment concurs with it.

When a supernova occurs, as well as a blinding flash, there is also an extremely intense neutrino pulse. So intense that the even with the rare interaction of a neutrino with the matter we’re made from, the pulse could give you a fatal radiation dose. Knowing how far away the supernova is, the lag time between the observation of the light pulse and the neutrino pulse, and just a dash of astrophysics, you can work out how fast the neutrinos must have travelled, and it comes out subluminal.

So the OPERA guys have done the sensible thing; checked everything they could, and published. It’s very probable that it will turn out to be a complex effect they hadn’t fully considered or was unknown at the time of designing the experiment which will explain the measurements.

The real trouble with these sorts of things is how to manage the media, and how to stop them getting over-excited at things that may well turn out to be nothing, c.f. the hints of the Higgs that melted away in the late Tevatron data.

I actually don’t have much of a point to make about all this, except that the relationship between science, the media, and society, means that there’s really great misunderstanding out there about what’s actually going on. Reading the comments on BBC News, especially the worst-rated ones (thankfully!) does demonstrate the mistrust of science and scientists, and a misplaced belief that science is about arrogance and certainty, when it is really more about doubt, and trusting the weight of the evidence. There’s also a certain group of people who seem to be fully unaware of just how well the world actually is understood these days.

It will certainly be interesting to see what happens if/when these neutrinos are shown to be subluminal!

Then, there’s the wackos, who take any new development as an excuse to just make weird and wacky stuff up. But they’re another story, really.

7 thoughts on “Neutrinos

  1. I’m thinking that however this turns out, it’s going to be interesting. Even if it turns out to be some error on their part, it’s looking like it won’t be something simple (else it’d have been called out already), so there will be something learned from this.

    I’m hoping for new physics, but not holding my breath :)

    1. Got to love a bit of good ol’ fashioned causality violation. Well, no, not really. The possibility of actual causality violations is actually kinda disturbing. It’s just wrong.

  2. If, however bizarrely, their measurements are correct, and we are looking at superluminal speeds, does this mean that Standard Model physics is *completely* overturned? Or is it plausible that we’re basically right but just need to tweak a few things and it turns out that things we thought were absolute were actually workable generalisations?

    This could well be totally not a sensible question given my knowledge of physics.

    1. The Newtonian mechanics you were taught at school is wrong. F is not equal to ma, and kinetic energy E is not equal to (1/2)mv^2, and we know this because of quantum mechanics and special relativity, the union and highest expression of which is the standard model.

      However, Newtonian mechanics is a really, really good approximation for anything that’s bigger than a few atoms across and travelling at much less than the speed of light. That’s why we discovered Newtonian mechanics first, because we didn’t have access to the conditions that would allow us to be able to distinguish Newtonian mechanics from the standard model, until the start of the 20th century.

      One of the things we know is that the Standard Model is wrong; it doesn’t include General Relativity, so it’s just a question of what comes along to overturn it.

      So that’s why big experiments like this OPERA experiment and the LHC are done; to probe the universe more deeply to find the ways that the real physics diverges from our current model.

      So in answer to your question, yeh, the Standard Model is good. Damn good, and it’s been tested to a phenomenal degree of accuracy. Where it’s applicable today it’s still always going to be applicable, just like you only use Newtonian mechanics to build a bridge, or a car; relativity would be overkill. Fundamentally though, physicists are interested in places where the standard model would be totally inapplicable, like right next to the singularity at the heart of a black hole.

      1. To my embarassment I typed my comment a while after having first read your post (which I did on my phone from whence commenting to your blog is too much like hard work) and without re-reading directly before posting. Presumably thanks to my poor memory what I really meant by Standard Model was actually Einstein-and-stuff.

        But then presumably the same thing holds – a model in which nothing can go faster than light will still work for the vast majority of things, even though (let’s imagine for a moment) it isn’t strictly true. So in a sense I have (I hope) answered my own question, or used your answer to get the answer I was looking for despite asking the wrong question – what we thought we knew will continue to be relevant, most of the time, just not 100% applicable.

        For future reference, best to assume I was taught no physics whatsoever. I can’t remember anything I was taught – after all it was eight years ago! – and everything I know about physics is basically everything I know about living in the world and not being surprised when I don’t fall onto my own ceiling.

        1. Is it just this blog or any blog? Disqus isn’t so great on mobile devices in my experience, but it’s so shiny on the desktop that I can’t resist its charms. 

          Yeh, the Standard Model is basically the end result of the synthesis of quantum mechanics and special relativity, along with a description of three of the four (known) fundamental forces. And the Higgs mechanism. So it’s Einstein circa 1905 + Quantum + A bit extra, really (funny how work which won about 3 Nobel prizes just fits under “a bit extra”)

          I think you’ve got the idea, anyway. Physics progresses in a pretty different way from, say, biology (although I haven’t done that since GCSE!). Hardly anything, once established, is found to just have been nonsense all along. Rather it’s found to just be an approximation to the truth; so you come up with a better approximation.

          The really big problem with super-luminal travel is that it really screws around with SR on a pretty fundamental level. One of the weird things about SR is that there is no universal moment of time; different observers will disagree about what events are simultaneous. Causality is only preserved because information must travel along what we call “time-like” trajectories; trajectories which pass from the past to the future, essentially. Normal objects travel along time-like paths. 

          A super-luminal particle, travelling along a space-like path throws a spanner into that whole thing. I’d have to go about drawing some diagrams to really prove to myself that it has wacky implications, though. So it potentially has complications that could shake the foundations, but by invoking bits of general relativity mixed with with fancy higher dimensions or whatever wacky idea the theorists come up with will probably save the causality day. Or it won’t, and the Universe will once again prove that it is stranger than we could possibly expect.

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