[NOTE ADDED: A reader forwarded a message that IceCube did not see any neutrinos with energies above 1 TeV = 1000 GeV from this GRB. Maybe this is not quite the final word (there would still be sensitivity, with some effort, to neutrinos in the 100 GeV – 1000 GeV range) but clearly the neutrino signal isn’t striking, and it is probably not there at all. But as I’ve suggested below, even a non-observation might have significant implications for the science; the question is, how many neutrinos would the standard speculations about how GRB’s work have led you to expect at IceCube? If a reader can provide that info, I’d appreciate that.]
The very recent report of a powerful and long-lived gamma-ray burst (GRB), and questions and remarks by my readers (thank you!), have motivated me, both as a scientist and a blogger, to try to understand whether we should have observed neutrinos from this GRB. This is forcing me to catch up on the related subjects of GRB’s, searches for high-energy neutrinos, and the highest-energy cosmic rays. I’m certainly not caught up yet; there are decades of research out there, and I’m quite far behind on developments over the past three or four years. But here are some of the basics that I believe I understand. Still, be cautious with the content of this post, both because I’m not an expert and because this is a very active area of research in which some fraction of the more speculative stuff will surely turn out to be wrong. I will try to refine this post with a more detailed and corrected article sometime later, perhaps once we know whether neutrinos from this GRB were or were not observed.
GRBs that last more than a few seconds are widely believed to be associated with an exceptional form of Type II (or “core-collapse”) supernova, though this is not known for certain. In these types of GRBs, there are (at least) two sources of photons (everything from gamma-rays to visible light to radio waves) and two sources of neutrinos. It is important not to confuse the different sources! Continue reading
Reminder: New York, Saturday June 16th at 2pm, I’ll be giving a public lecture (click here for details): THE EINSTEIN OBSESSION: SCIENCE, MYTH AND PUBLIC PERCEPTION.
I’ve been doing a little work on my extra dimensions articles, adding one that describes how we know experimentally that the ordinary particles we’re made of (and most of the others we know about) can’t be moving in more than three spatial dimensions — more precisely, that any additional dimensions must be smaller in extent than 1/100th or so of the distance across a proton. The first half of the article is drafted; the second half, on what we know about dimensions in which no known particles can move but which are accessible to gravity and gravitons, will come soon, probably next week. Comments and questions welcome as always.
Meanwhile, following up on Friday’s post about the End of the OPERA Not-Faster-Than-Light Neutrino Story: a paper has appeared by the LVD and OPERA experiments explaining how they worked together to confirm that OPERA’s two known problems (a fiber-optic cable connection and a clock running off-speed) did in fact cause their faulty measurement of neutrino speeds. This information was made public (in large part) back in March and I wrote about it in detail here.
In case you haven’t yet heard (check my previous post from this morning), neutrinos traveling 730 kilometers from the CERN laboratory to the Gran Sasso laboratory do arrive at the time Einstein’s special relativity predicts they would.
Of course (as the press mostly seems to forget) we knew that. We knew it because
So the news from the Neutrino 2012 conference in Kyoto, on new data from May 2012 taken by OPERA and three nearby experiments, is no surprise to anyone who was paying attention back in March and early April; it’s exactly what we were expecting.
One thing that almost no one is reporting, as far as I can tell, is that CERN’s research director Sergio Bertolucci did not give the first talk on neutrino speeds in Kyoto. That talk was given by Marcos Dracos, of OPERA. Dracos presented both OPERA’s corrected 2011 results (with corrections based on the detailed investigation shown in March of the problems reported back in February) and also the new 2012 results, which were taken with a kind of short-pulse beams similar to that used in OPERA-2. (A short pulse beam allows for a neutrino speed measurement to be made rather easily and quickly, at the expense of OPERA’s neutrino oscillation studies, which were the main purpose of building the OPERA experiment.)
Following Dracos’ talk, Bertolucci spoke next, and reported the results of the neighboring Borexino, LVD and ICARUS experiments on the May 2012 data, which along with OPERA are all bathed in the same CERN-to-Gran Sasso neutrino beam, and collected their data simultaneously. All of the results are preliminary so the numbers below will change in detail. But they are not going to change very much. Here they are: neutrinos arrive at a time that differs from expectation by:
- Borexino: δt = 2.7 ± 1.2 (stat) ± 3 (sys) ns
- ICARUS: δt = 5.1 ± 1.1 (stat) ± 5.5 (sys) ns
- LVD: δt = 2.9 ± 0.6 (stat) ± 3 (sys) ns
- OPERA: δt = 1.6 ± 1.1 (stat) [+ 6.1, -3.7] (sys) ns
(Here “ns” means nanoseconds, and “stat” and “sys” mean statistical and systematic uncertainty.) The original OPERA result was an early arrival of about 60 nanoseconds, about six standard deviations away from expectations. You see that all the experiments are consistent with zero early/late arrival to about 1 standard deviation — almost too consistent, in fact, for four experiments.
So there is no longer any hint of any evidence whatsoever of a problem with the predictions of special relativity, and in particular with the existence of a universal speed limit.
A summing up is called for, but I want to write that carefully. So unless something else comes up, that’s all for today.
Five out of five experiments agree: neutrinos do not travel faster than the speed limit.
Or more precisely: to within the uncertainties of current measurements, neutrino speed, for neutrinos with energies far larger than their masses, is experimentally indistinguishable from the speed of light in vacuum. This is just as expected in standard Einsteinian special relativity, which would predict they move just below light speed, by an amount too small to measure with current experiments.
Based on data taken in May 2012 using a beam of neutrinos sent from the CERN laboratory to the Gran Sasso lab, the four experiments ICARUS, LVD, Borexino and even OPERA (the source of all the excitement) find results consistent with the speed of light, with uncertainties (at one-standard-deviation) about 10 times smaller than OPERA’s original measured deviation of neutrino speed from the speed of light. The new results are consistent with ICARUS’s result from 2011 data. Moreover, OPERA’s mistaken result from September and November 2011 — a claimed six standard deviations away from the expected speed — has now been corrected, following their detective work presented in March. Even MINOS, a U.S. experiment, has revised their older result, which was previously slightly discrepant from the speed of light by a small amount (two standard deviations), and they find now that their data too are quite consistent with neutrinos traveling with light speed, though with much less precision in the measurement.
And so with a final quintet, sung in unison, this melodramatic comic OPERA buffa comes to a close. As with all classic operatic comedies, there’s been crisis, chaos, and a good bit of hilarity, all the while with wise voices speaking reason to no avail, but in the end the overzealous are chastened, the righteous are made whole, everyone (even the miscreant) is happy, and all is well with the world.
Curtain!! Applause!! Science Triumphant!!
Favorable review to follow when time permits.
Almost all the news on neutrinos in the mainstream press this past few months was about the OPERA experiment, and a possible violation of Einstein’s foundational theory of relativity. That the experiment turned out to be wrong didn’t surprise experts. But one of the concerns that scientists have about how this story turned out and was reported in the press is that perhaps many non-experts may get the impression that science is so full of mistakes that you can’t trust it at all. That would be a very unhappy conclusion — not just unhappy but in fact a very dangerous conclusion, at least for anyone who would like to keep their economy strong, their planet well-treated and their nation well-defended.
So it is important to balance the OPERA mini-fiasco with another hot-off-the-presses neutrino story that illustrates why, even though mistakes in individual scientific experiments are common, collective mistakes in science are rare. A discipline such as physics has intrinsic checks and balances that significantly reduce the probability of errors going unrecognized for long. In the story I’m about to relate, one can recognize how and why scientists start to come to consensus. Though quite suspicious of any individual experiment, scientists generally take a different view of a group of experiments that buttress one another.
The context of this story, though much less revolutionary than a violation of Einstein’s speed limit, still represents a milestone in our understanding of neutrinos, which has been advancing very rapidly over the past fifteen years or so. When I was a starting graduate student in the late 1980s, almost all we knew about neutrinos was that there were at least three types and that they were much lighter than electrons, and perhaps massless. Today we know much, much more about neutrinos and how they behave. And in just the last few months and weeks and days, one of the missing entries in the Encyclopedia Neutrinica appears to have been filled in. Continue reading
So, many of you have probably been following, to a greater or lesser degree, the story of the OPERA experiment. This is the one that found that neutrinos sent from the CERN lab near Geneva, Switzerland to the Gran Sasso lab in Italy (where OPERA is located) arrived earlier than they expected. Of course there were, from the beginning, two natural explanations:
- Einstein was wrong and neutrinos travel faster than light, or
- OPERA made a mistake, and their expectations were off.
The news media made a huge deal out of the first possibility, while the vast majority of professional physicists assumed, for various reasons we can discuss, that the second possibility was almost certainly correct. It is now pretty clear that possibility #2 was right; first OPERA admitted it had found two mistakes which made its previous results invalid; then its competitor down the lab, ICARUS, announced it had seen neutrinos arriving just as expected from the same CERN neutrino beam; and finally OPERA itself revealed that it had managed to characterize its errors in detail and now, re-analyzing its data, finds (preliminarily) that neutrinos do in fact arrive as expected.
Now, with this backdrop, I would like to ask YOU a question or two. And by “you”, I mean non-scientists. I would like to know how seeing this episode unfold changed (or did not change) your view of science, or physics, or particle physics. Or of science journalism. What’s your perspective on all of this? What surprised you most? What annoyed you or turned you off or excited you? Are you disappointed in or pleased with the scientific process as you saw it unfold? Are you more suspicious of or less suspicious of scientists and/or of science now that you’ve seen this happen? I think these are things that many scientists would be curious to learn.
Granted, since you’re reading this blog, you’re a member of a non-representative sample of the public. But I still think it would be useful to hear what you have to say. So, please. Comment.
[p.s. As BBC reports today, the LHC now has stable data-quality proton-proton collisions at 8 TeV of energy per collision; data taking will start at slow collision rates and ramp up over the year. Here’s a post and a following article on why 8 TeV is better than last year’s 7 TeV. As usual, BBC says correctly that 2012 will be a crucial year for the search for the Higgs particle, but say incorrectly that this will be the year that the Higgs is found or not found; that statement is true only of the Standard Model Higgs particle, the simplest possible form of Higgs particle. For an overview of what I mean by this, read my guest post at the Cosmic Variance blog.]
On Friday I learned, and reported to you, that the OPERA experiment’s investigations into its early-arriving-neutrino anomaly (widely reported as `faster-than-light neutrinos’), performed with help from the nearby LVD experiment, have basically confirmed that a combination of (1) an optical fiber within the main timing system that was incorrectly screwed in, and (2) a timing drift in OPERA’s main synchronizing clock, together caused the observed 60 nanosecond early-arrival time. The fiber provides the main effect, with the clock drift playing a subsidiary role.
However, in Friday’s post I only gave you the main idea of how this was done. I have now finished an article that goes through the OPERA story in detail, to the extent I understand it, from the initial discovery and diagnosis of the two problems through the scientific investigation that demonstrated that the two problems probably caused all of the effect that OPERA observed. On the one hand, the solution of the mystery is a classic scientific detective story, instructive and interesting, and for its rather convincing and successful conclusion, the OPERA team deserves applause. On the other hand, it leaves one wondering if this whole episode could have been avoided; why didn’t some of these investigations, which don’t seem exceptionally subtle, happen before OPERA announced its results?
Be that as it may, preliminarily (which means unofficially in this context — OPERA still has more work to do before they can announce a result officially) the revised result from OPERA-2 (the short-pulse version of OPERA) agrees with Einstein’s prediction that neutrinos at these high energies should travel at a speed unmeasurably close to the speed of light. And thus it agrees with the ICARUS experiment’s recent result. So we now have two preliminary confirmations that the neutrinos coming to the Gran Sasso lab from CERN obey Einstein’s speed limit.
[QUICK UPDATE April 2: I’ve now finished an article giving more details of how OPERA, with LVD’s help, solved the mystery.]
[UPDATE March 31 2 a.m.: following study of the slides from a mini-workshop recording the results of investigations by OPERA and LVD, I now have the information to remove all the guesswork from my original post; you’ll see outdated information crossed out and newer and more precise information written in orange. I’ve also added figures from the talks.]
March 30 5:30 p.m. Two main scientists at OPERA, one leading the OPERA team as a whole and the other leading the neutrino speed measurement, resigned their leadership positions today. The suggestion from the press is that this is due to personal and scientific conflicts within the OPERA experiment, rather than due directly to the errors made in the neutrino speed experiment; but of course the way the measurement was publicized by OPERA caused serious internal conflicts at the time and are surely part of the issue. [Oh, and meanwhile, back over at the CERN lab, some good news: collisions at the Large Hadron Collider with 8 TeV of energy per collision were achieved this afternoon.]
The mystery surrounding OPERA, the Gran Sasso experiment which (apparently through a technical problem) measured that neutrinos sent from the CERN lab to the Gran Sasso lab in Italy arrived earlier than expected by 60 nanoseconds,
seems to be on the verge of being is resolved. Statements made by an OPERA scientist in the Italian language press, pointed out to me by commenters (Titus and A.K.), seem to imply that OPERA has more or less confirmed that the problematic fiber optic cable (along with the clock problem, to a lesser extent) was responsible for a 60 nanosecond (billionth-of-a-second) shift in the timing, creating the false result. We do not yet have official information from OPERA about this, but talks given at a mini-workshop a couple of days ago make clear that this is the case.
The way this was done
if I/we understand the Italian correctly is something like is the following with all details still very uncertain. Continue reading