Monday, 3 October 2011

Indecisive neutrinos

Update, one year on from this original post, sience has moved on.  See the two links at the end of the post for the answer to the mystery.

The 'neutrino problem' keeps coming up in conversations, wherever I go.

For the few readers who don't know which problem this is, I am referring to the recent reports that a European experiment to observe oscillations in neutrinos has accidentally noticed that the neutrinos appear to travel faster than light.   That seems to break one of the fundamental and best established 'laws of physics'.  You might know that neutrinos come in three different flavours and they tend to change flavour sometimes.  Indecisive little blighters!  This is what they were trying to study.

If it turned out to be true that the light barrier has been broken it would be really exciting.  Physicists love a good mystery.  Imagine how much more we could learn about the universe if we found something that we did not fully understand and therefore were forced to look at the universe a little harder.  It might appear to non-scientists as though this is a threat to science, whereas in fact it is more of an opportunity.

Sadly it seems very unlikely that these observations will turn out to be true.  Almost everyone expects that a flaw in the measurement will be found when it is adequately reviewed.

Besides that, we already know how fast neutrinos travel.  Perhaps the most sensitive measurement was made when a supernova was observed in 1987.  The event was known as SN1987A.  After the optical observations were made, astronomers asked neutrino observatories around the world whether they had seen an increase in the number of neutrinos detected, and the exciting news was that they had.

Neutrinos are detected by the track of Cerenkov radiation from a bath of water - meaning that the neutrinos are originally travelling faster than light could travel in the same medium and they cause light to be emitted as they slow down.  The tracks give a clear indication of the direction of the source. Amazingly the source was in the correct direction to be from the supernova.  To confirm the observation, the energies observed were also consistent with current physical models of supernovae.  (Reference publication here, albeit not a formal peer reviewed publication in the classical way.)

These neutrinos did not arrive at exactly the right moment, but after traveling across the galaxy [see correction below] for 168 thousand years they arrived just 3 hours before the light.  If the new CERN measurements were true they might have arrived many weeks earlier.  (The small discrepancy of 3 hours is explained by some physics that I don't understand by the way.  It is something to do with the fact that the neutrinos do not interact much with the surrounding matter, but that the light does.)

A famous scientist's recent offer to eat his boxer shorts on live TV if the CERN observations turn out to be true seems very cautious in the circumstances.  He could have offered something much more unpleasant in the confidence that he will not have to do it.

Let's hope that the neutrinos are not undecided about how fast to go too.

[Small correction added 2011-10-10: SN1987A wasn't actually in our galaxy but in the Large Magellanic cloud, which is a sort of satellite galaxy to ours.]

You might also like "How many neutrinos does it take to change a light bulb?"

Update 2nd October 2012, someone kindly pointed out that this had been solved, not realising that this post was a year old.  So here are the links to two later posts on the same topic.

Not so fast with the neutrino mystery! 

Enforcing the cosmic speed limit


Anonymous said...

ah these neutrinos, they're clever, maybe they've simply speeded up since that supernova... and by the way the reason they arrived 3 hours before the light did from that supernova was because light had to get through the remnant of the supernova first...whereas neutrinos...well they just zoomed straight through...allegedly!

Plasma Engineer said...

That's a good analogy - thank you. And it is consistent with what we know in our natural world. Light bounces off things and we can see them. Neutrinos don't bounce very often and they arrive earlier. (All analogies are false but this is an instructive one.) Thanks.