Friday, March 16, 2012

At the neutrino OPERA, the fat lady is warming up.

If you weren't living under a rock last year, you will remember the neutrinos that seemed to be going faster than the speed of light. Everybody and their cat wrote about it, including me (though my cat only had editorial supervision of the final manuscript).

Earlier this year, OPERA (the experiment that apparently saw faster than light neutrinos) updated their press release informing us that they had spotted a few mistakes in their experiment. The mistake was an unfortunate one, but wasn't in OPERA's actual analysis or method itself. The mistake was basically a piece of faulty hardware. Normally you don't need to know things at a neutrino detector to within nanoseconds, so this fault hadn't been noticed before.

I don't hold OPERA to be at fault for going public with their initial findings. They had checked everything they could think of and still had this seemingly crazy result. If the result stood up to scrutiny and was repeated by other experiments, then they'd made one of the biggest discoveries for a long time, if not ever. Once they had checked everything they could think of, any lingering mistake is far more likely to be found by a group trying to repeat their experiment, than by someone in their collaboration.

I didn't write about this update, mostly because I was travelling, but also because nothing conclusive had yet occurred. OPERA made a mistake, fine, but it wasn't proven yet that this mistake was responsible (of course everyone, myself included, strongly suspected that it was – but we all strongly expected that some mistake had been made, and said so, from the start!).

Today, something genuinely new can be added to the story, due to ICARUS, another neutrino experiment. Ironically, both ICARUS and OPERA are at the same laboratory, at Gran Sasso, and in fact measure exactly the same neutrino beam from CERN. The new piece of the story is that ICARUS has measured the speed of these neutrinos and found them to be consistent with the speed of light, and crucially, inconsistent with the results from OPERA.

Here is the crucial figure from their article:

The neutrinos' arrival at ICARUS and OPERA. δt=0 corresponds to when light would arrive.

The purple bars show the scatter in time when neutrinos were measured to arrive at ICARUS. δt=0 corresponds to when light would arrive. Clearly the ICARUS neutrinos are arriving at the same time as light would. The bars on the right are what OPERA's previous measurements indicated.

So, given that (a) a lot of other (measured!) things in physics would be very difficult to reconcile with something going faster than light (b) We know OPERA have made a mistake that they need to fix and (c) ICARUS is measuring the exact same neutrino beam at the exactly same place and finding the neutrinos to be travelling at light speed, it seems that, in this opera, "the fat lady" might not just be warming up, but could very well be in the final stages of her aria.

But, as Matt Strassler pointed out in his own blog, there is a silver lining to all of this. 
The experimental particle physics community has learned how to make long-range distance and timing measurements that are more precise and more accurate than were ever possible before. Don’t be surprised if this knowledge turns out to be useful, in some unexpected way, in future experiments.
In fact, I'm sure this precision timing will be extremely useful in increasing the bit-rate of the neutrinos that have very recently been used to send digital messages through the ground, something that is impossible with any other known form of communication.

As a final passing comment I want to add that I am still in awe of the fact that both ICARUS and OPERA could make such a precise measurement of the speed of neutrinos. OPERA's mistake was unfortunate, but it doesn't change how impressive the measurement itself was. It just changes how impressive the result is.


  1. Light speed communication through solid rock... The internets would get a whole lot quicker. Just think of the bitrate possible for streaming funny cat videos! In High Definition!

  2. On a serious note though - how do you 'catch' something that travels through solid matter!?

    1. The thing is, while most of the neutrinos travel through solid matter, a very small fraction of them will interact. The vast, vast majority of the neutrinos made at the collider will travel right past the detector and out into space without being seen. But, if one of them happened to hit the rock outside the detector, then the detector will see the result of the collision and thus know that the neutrino has passed.

      If you send enough neutrinos then enough of them will interact (still a tiny proportion, but not a tiny number) and be seen. Then if you change the number being sent, you can detect this change, thus sending information with neutrinos.

      Unfortunately, the bit rate is ridiculously low because so few neutrinos interact. As a result, this will never become the preferred form of communication to anything that isn't completely embedded in rock, or water. To get a message from one side of the Earth to another it will always be quicker to send a message between satellites in orbit. While that message will travel further it will be quicker because the rare of information exchange is faster.

      However it has been suggested that neutrinos could be used to communicate with submerged submarines, which at the moment need to surface to communicate, making them vulnerable to attack. The process would be slow, but safer.

  3. I am surprised that no one seems to be impressed that neutrinos are travelling at the speed of light through SOLID ROCK while light can only achieve this speed in a vacuum! So , has anyone thought maybe neutrinos in a vacuum travel a wee faster???? ( and yes, I know about relativity supposedly precluding this ..but experimentation rules!) in any case the expression should be the 'speed of neutrino' rather than 'speed of light'

    1. Hi Anonymous, thanks for the comment.

      It is impressive that neutrinos do this through solid rock, it just isn't unexpected so no big deal was made of it. There's even hypothetical talk of using neutrinos to communicate with submarines because they will travel further through water than light will (there's a link in my comment above).

      Your suggestion regarding neutrinos possibly travelling faster than light is a good one and in fact one of the possibilities theorists were considering was the possibility that light does travel slower than the limiting speed of relativity (which could be accomplished for example, by giving the photon a mass). The problem with this is that it would introduce small changes to electrodynamics, which has been very precisely measured and found to match the predictions of relativity. As you say, "experimentation rules!" and any new model would not just have had to match the apparent OPERA results, but also all the precision electrodynamics measurements too (something nobody managed to achieve)!

      Neutrinos do have mass though, so unless relativity is wrong (and again, if it is, any replacement model will have to match all the experimental results that match relativity), they do travel ever so slightly slower than light, just not slow enough that OPERA would have detected it.

      I'm not sure if you're aware, but OPERA have determined that their faulty hardware did cause an error of approximately 60 nanosecond, so their result is now consistent with both ICARUS and relativity.


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