By Kash Farooq
A sensational announcement was made recently by neutrino researchers at the Oscillation Project with Emulsion-tRacking Apparatus experiment (i.e. the OPERA experiment) – they think they have detected neutrinos that have broken nature’s speed limit: the speed of light.
The full interview is transcribed below.
Kash Farooq: Starting with the obvious question: what is a neutrino and why are they important?
Dr Ben Still: The neutrinos are one of the few fundamental building blocks of nature, which we call the fundamental particles. If you take nature and split it down until you could divide it up no more you’d find there was a very small subset of building blocks and these are the fundamental particles, and neutrinos are part of this set.
We think they are very important because we believe that they hold the key to a chapter in the creation story of the Universe. As we know physics today, when we turn energy into mass using Einstein’s equation in particle physics experiments all around the world, we always create equal amounts of the normal stuff around us, which we call matter, but also equal amounts of a mirror version, which is called antimatter. This would be a problem if this happened in the Universe in perfect balance because when matter and antimatter meet again, as they inevitably do as they are two opposites, then they form pure energy again. If this balance was perfect then the Universe would be nothing but a warm bath of energy. In fact, it would just be microwaves today. There wouldn’t be any structure because there wouldn’t be any raw materials to go into stars and planets.
We believe the neutrino holds the key to this imbalance between matter and antimatter. There are other building blocks called quarks; the imbalance between matter and antimatter has been measured in quarks and it does not quite account for enough of the imbalance that we see that has gone into the creation of all the raw material for the Universe as we see it today.
So we hope that by understanding more about these neutrinos we can learn more about this crucial chapter in the creation story of the Universe.
Kash: You’re involved with an experiment in Japan called T2K. Can you tell me about that? How it works? How you use it to study neutrinos?
Ben: Neutrinos are slippery customers, they are really ghostly and we are still at a stage where we are pinning down their exact characteristics. The neutrino sometimes exhibits a very bizarre phenomenon; it decides to change from one type of neutrino to another. There are three types of neutrino and they correspond to three charged particles: the electron and the electrons two heavier cousins – the muon and the tau.
At T2K we create a pure beam of muon type neutrinos, which are slightly heavier versions of the electron type neutrinos, and we fire them from the very east coast of Japan 300 km to the west coast of Japan and we look at them changing. We know that the majority of the muon neutrinos will change into the heavier tau type of neutrino.
What we are looking for in T2K for the first time is we’re hoping to see muon type neutrinos changing into electron type neutrinos. The reason we are interested in this is because this is the last type of this weird phenomenon , which we call neutrino oscillations, it is the last type that needs to be viewed before we can then start probing the difference between neutrinos and anti-neutrinos.
We create the neutrinos using a particle accelerator on the east coast of Japan by accelerating protons to high energies and smashing them into a steel target. This is much the same was cosmic rays hit the upper atmosphere. This creates showers of new particles called pions, which then rapidly decay into muons (the heavier version of the electron) plus muon neutrinos. We stop all of the particles apart from the neutrinos, which go sailing 300 km over to the west coast where we look for some of them disappearing and hopefully turning into electron neutrinos.
Super-Kamiokande is a neutrino observatory on the west coast of Japan. The neutrinos created in the T2K experiment over on the east coast are detected here. [Image credit: Kamioka Observatory, ICRR (Institute for Cosmic Ray Research), The University of Tokyo]
Kash: How does T2K compare to the OPERA experiment?
Ben: The OPERA experiment is using much higher energy neutrinos. They are using higher energy neutrinos because they are hoping that if you give a neutrino enough energy that they would be able to produce the heaviest type of charged particle, the tau. You need a lot of energy, 10s of GeV, to produce one of these massive tau particles. They are really huge; much heavier than the electron.
However they are using the same sort of beam; they have a pure muon neutrino beam that they are firing from CERN. They are a longer baseline though. At T2K we are firing from the east coast 300 km to the west coast of Japan. The distance from CERN to the Gran Sasso Laboratory where they are detecting the neutrinos is around 730 km.
So there are subtle differences; but essentially in the same way they are looking for these neutrino oscillations.
Kash: The announcement prompted lots of discussion on the Internet. Why is it such a big deal? Why is this so important?
Ben: If the neutrinos are travelling faster than the speed of light it does overturn pretty much most of our current theories of the Universe. So it is quite a phenomenal statement and it would require an almost entire rethink of a lot of the fundamentals of physics as we know it.
So, if this were proven it would totally overhaul physics.
That’s why it has been made out to be such a big deal.
Kash: You wrote a very quick blog post shortly after the announcement that was linked to on various websites…The Guardian linked to it, The Telegraph linked to it. Can you tell us about that?
Ben: I was talking to colleagues over coffee and we were just banding around numbers; Supernova 1987 came up as it always does because it is the first example of neutrino astronomy; the first example of seeing things in our Universe not using optical and other wavelengths of light.
We were just thinking what this time difference would mean if we were observing the neutrinos from the supernova.
I did a quick calculation. I took the fractional speed difference of the neutrinos to the speed of light that was quoted in the OPERA paper, which is around 2.5 x 10-5 and I just multiplied this by the distance to SN1987a, which was quoted again in a paper, as being about 167,000 light years. Just multiplying these numbers together you will get the time difference between the neutrinos arriving and the light arriving. I calculated that you would see neutrinos arrive about 4 years ± 1 year before the light arrived on Earth. What you would see essentially is four years before you even saw the supernova in the night sky you get this ultra energetic intense burst of neutrinos in neutrino detectors.
But what we actually saw was that SN1987a was seen optically and then people went back to the neutrino observatories such as Kamiokande-II and the IMB experiment. They looked at the historical data to see if they saw any neutrino detections. And they did see them – but only 3 hours before the optical signal was seen.
3 hours is a lot less than the 4 years.
And in fact the 3 hours can be taken into account because the light is actually slowed down when leaving the supernova through self-interaction. And that is what accounts for the neutrinos arriving faster than the light in that particular case.
Kash: There has been a lot of skepticism about the results. I guess you haven’t seen anything remotely similar to this at T2K?
Ben: At T2K we haven’t quite got the statistics that OPERA have and it will take us a good few years to gather the same kind of statistics.
There is quite a lot of skepticism around high accurate the systematic errors are in their paper. Talking to some experts on T2K, the guys working on the GPS feel that anything even around 25 ns would be a good resolution to get, but they have quoted a much higher resolution in the OPERA paper of around 1.7 ns.
So there is a lot of skepticism generally in the community as to how accurate, or how well estimated, the systematics are.
As far as T2K comes into it, it is going to be a couple of years before we are able to see anything really of the order of the accuracy that OPERA are stating at the moment.
Kash: We should probably point out that the OPERA team are being very cautious. They are basically asking for help, I guess….they want to know what everyone else thinks and what they may have done wrong.
Ben: Exactly. They’ve checked, double checked, triple checked their calculations and they’ve published a nice paper. It really is now at the point where the paper is out there in the general community and they are open for scrutiny and they want to obviously see if it stands up to rigorous testing. Really the whole community is looking at it; eyes are on it because it would be such an amazing and massive overhaul of our understanding of physics if it were true.
Everyone is very keen to figure out exactly where this discrepancy has come from; so it is very much being scrutinised at the moment.
Really this is a kind of new way of peer reviewing nowadays. It seems to be that if you’ve got these big results, this is the way to test it.
Kash: T2K is currently out of action due to the Japan earthquake so you won’t be able to start thinking about replicating this for a few years?
Ben: Even when we do get up and running it will take us a few years to build up the kind of statistics required to be able to get near this limit. But hopefully T2K will be back on its feet in January. We’re going to be talking a lot about this next week in our collaboration meeting; I’m out in Japan at the moment for the pre meetings. The OPERA result will be hotly debated I can guarantee.
We’ll also be talking about the timeline for getting back up and running and collecting data once more. I imagine we will be aiming towards looking at some sort of timeline to get a physics paper together to either confirm or refute the OPERA claim.
Kash: Finally, physicist Jim Al-Khalili tweeted “If the OPERA experiment proves to be correct and neutrinos have broken the speed of light I will eat my boxer shorts on live TV”. So, is there anything you can promise that you’ll do!?
Ben: I can promise to be there with him, with the ketchup, and I’ll eat my socks!
To be quite honest it’s such an amazing claim that obviously skepticism is the first port of call, but there is something inside of me that obviously would love this to be true because it does offer very many exciting opportunities for new physics. But at the same time we need proof, we need independent review of the systematics and we need independent results from other experiments.
As well as the T2K experiment there is the MINOS experiment in the U.S who previously have actually published results on this kind of thing; but they saw a much smaller probability of neutrinos being faster than the speed of light. They should, within a year or two, have the kind of statistics where they would be able to challenge the OPERA result too.
It is an exciting time. It is always good to find something you weren’t expecting.
That’s the beauty of science.
Neutrino Blog – Dr. Ben Still’s blog – there are several recent blog posts about the faster than light story.
The Thought Stash – Kash Farooq’s Science, skepticism and astronomy blog
Astronomy Twitter Journal Club meeting – the journal club’s most recent meeting discussed the OPERA paper.