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.

Follow @just_shaun

Yet another voice in the neutrino OPERA

I feel just a little prophetic, given that only a week ago I wrote,

“... it is also useful to simply measure as many new things as possible. History is littered with moments where we didn't bother measuring something because we knew what the result would be, only to get a big surprise whenever someone finally did measure it. So nowadays if something is measurable, then someone, somewhere, is trying to measure it.”

Well, true to the claim, someone out there had the ridiculous idea that measuring the speed of neutrinos was a good idea, go figure. And, lo and behold, it seems they've found something! Maybe? Who knows!

Well, what sort of blog writer, who is a scientist (physicist even), would I be if I didn't join in with the neutrino cacophony? So, what follows is my attempt to say something interesting that hasn't already been said a million times already. I will focus on why this is such a surprising result and why this means nobody believes it.

This disbelief has nothing to do with the quality of the experiment. The measurement was recorded with impeccable accuracy and has been extremely carefully analysed. Neither is the disbelief because the result "proves Einstein wrong". It even has very little to do with the prospect of over-turning special relativity.

The disbelief is due to the fact that this observation brings into question the holy principle of causality – that is, the seemingly incontrovertible fact that cause must come before effect and not after

And, as my former PhD supervisor, said in the Guardian today:

“If we do not have causality, we are buggered."