OPERA Blames the Blogosphere

April 20, 2012

It had to happen, OPERA has blamed its woes on the blogosphere, or perhaps not. Maybe it is just NewScientist trying to stir up trouble. This what they say in their editorial from 7th April

Perhaps the only way to have avoided the fuss would have been to keep the result under wraps for even longer than the three years it took to become public. Its flaws might have been uncovered more discreetly. But word was leaking into the blogosphere; OPERA physics coordinator Dario Autiero, who also stood down, says concealment was untenable

So it’s all our fault guys. If we had not leaked the story they could have kept it secret for a few more years until they found the loose cable and Autiero would be a hero.

In case anyone hasn’t noticed there are several flaws with this argument but not least is the observation that OPERA had planned its public presentation at CERN and already primed the press before the first hints of a rumour reached any blog, If NewScientist had checked the blogosphere they could have found a conveniently detailed timeline of the events here.

 


The Power of Theory

February 28, 2012

I couldn’t resist. No offense intended :)


OPERA fail to find error in Faster Than Light Measurement

November 18, 2011

The OPERA experiment has failed to find an error in their measurement of neutrino speeds that shows them travelling faster than light. The earlier result was most strongly criticized because of the statistical nature of the measurement which involved fitting the timing profile of many observed events at Gran Sasso to the known shape of long duration pulses of protons 730 km away at the source in CERN. This objection has now been quashed by using much shorter pulses so that the effect can be seen with just a few neutrinos. While the previous measurement used data gathered over three years, this new confirmation took just a few weeks.

The crucial new plot is this one

The timing of the neutrinos is spread over a 50 ns window but is still clearly different from the zero mark that would be consistent with travel at the speed of light. The spread could either be due to inaccuracies in the timing or differences in the speed of the neutrinos themselves, if the effect is real. An interesting question would be whether there is any correlation between the timing offset and the energy of the neutrinos and I don’t know if they have that data.

In fact this spread is the most exciting part of the new result. As far as I know it is bigger than the known systematic errors. If there were an unknown systematic error in the measurement of the distance between the experiments or in the timing, we would expect that to be constant. Here I am assuming that atomic clocks were used at each end to keep the timing stable rather than constant referral to GPS time which could vary. If this is the case then the spread actually rules out several other sources of systematic error.

Factoring in my prior probabilities from preconceived theoretical prejudices I can now say that the probability of the result being correct has increase from 1 in a million to one in 100 thousand (numbers are illustrative :) ). This is sufficient to convince most of the collaboration to sign the paper which may now go forward to peer-review. To convince more theorists they may need to do more checks on the result. The strongest criticisms will now fall on the use of GPS. To eliminate this they should check the timing and the distance calculation independently.  The timing could be checked by flying a portable atomic clock from CERN to OPERA and back at high-speed on a helicopter to calibrate the clocks at either end. Portable clocks can be stable to within a few nanoseconds over the course of a day so it should be possible to carry out this check  with sufficient accuracy and it would not be too expensive. The distance measurement also needs to be repeated, preferably using old-fashioned surveying techniques rather than GPS between the two locations.

If this is also fails to find an error then the probability of the result being correct goes up to one in ten thousand. The next most likely source of errors would be the timing measurement for the collisions that generate the neutrinos at CERN. This involves some electronics with a lag that may not be precisely known. To eliminate this they possibility need to build a near detector to catch neutrino events in the path of the beam near CERN. If the beam is everywhere deep underground this could be an expensive addition to the experiment, but it would be a very significant check taking the probability of the result being real up to one in 100 or better depending on what other possible sources of error might be left.

To really confirm that neutrinos are faster than light requires confirmation from other labs using measurements that could not be subject to correlated errors. Hopefully this will arrive next year.

For more details see TRF where this was reported two days ago based on a tachyonic version of twitter, or AQDS using conventional light-speed technology from arXiv. The official press release is here.

Update: People are telling me that the timing calibration has already been done. An update from Dorigo makes some interesting points including the fact that the timing depends on a 20 megahertz clock signal. This explains the spread of the measurements over 50 ns. In fact it means that the time offest must be very sharp which is not such good news. It makes a constant systematic error seem much more likely.

I think another essential upgrade to the experiment would be to record a timestamp for events with nanosecond accuracy.


BBC: Faster Than the Speed of Light?

October 20, 2011

Yesterday evening the BBC ran a documentary about the OPERA neutrino results. If you are in the UK and missed it you can watch repeats over the next few days or view it online here. Probably it will be available in other countries in some form soon.

The program was presented by mathematician and author Marcus du Sautoy who has become a familiar science host on the BBC in recent years. The tone of the show was skeptical but open-minded and I think this reflects the range of views that scientists have on the subject. Marcus described the results and surrounding debate as “a great example of science in action”. The show must have been put together very quickly but it follows clear logical steps and includes most of the relevant points that should be discussed at a popular programme level. I think they did a great job of bringing in the more exciting possibilities without hype. Here are a few highlight quotes from the guest scientists.

Marcus du Sautoy: “You can almost feel the shudder that passes through the entire scientific community when a result as strange as this comes out. Everybody’s talking about it. Is this the moment for a grand new theory to emerge that makes sense of all the mysteries that still pervade physics, or has there just been a mistake in the measurement?”

Marcus du Sautoy

Chiara Sirignano (OPERA): “On top of us we have 1400m of rock, the top of Grand Sasso mountain. Here the cosmic rays are very few because outside they are 200 per square meter per second and here it is just 1 per square meter per hour. This is a very huge shielding”

Chiara Sirignano

John Ellis: “If the speed of light turned out not to be absolute, we would just have to tear up all the textbooks and start all over again. On the other hand it would be nice if it were true.”

John Ellis

Fay Dowker: “For me it would mean that the direction of my own research was wrong, so it would be a revolution but to me it would also mean that nature is just playing tricks with us”

Fay Dowker

Jon Butterworth: I actually heard about this result in the coffee bar at CERN about two weeks before it came out, and I laughed. I have to say that was my thought, they have got something wrong haven’t they?

Jon Butterworth

Stefan Söldner-Rembold: “MINOS and T2K will both work very hard to get a similar measurement with a similar precision in the next few years, but it will take a few years I think”

Stefan Soldner-Rembold

Joao Magueijo: “Obviously this result contradicts what you find in textbooks, but if you are actually working in the frontier of physics, if you are really trying to find new theories this is not as tragic as you might think. It is a crisis, but we need a crisis because there are a lot of things in physics in those textbooks which don’t really make any sense.”

Joao Magueijo

Mike Duff: “Well, I have been working on the idea of extra dimensions for over 30 years so no one would be happier than I if the experimentalists were to find evidence for them. However, To be frank, although I like the idea of extra dimensions, this is not the way they are going to show up in my opinion. So I am not offering extra dimensions as an explanation for the phenomenon that the Italian physicists are reporting.”

Mike Duff

Tara Shears: “This could be one of those moments that turns our understanding on its head yet again, let’s us see further into the universe, let’s us understand more about how it ticks, how it sticks together, how things are related inside. If it does that, if we understand more, then it’s one of those magical moments that you get in the history of physics that just twists your understanding and brings the universe into focus, and if we are seeing the start of that now, and we are documenting it, then we are really, really, really privileged to be doing so.”

Tara Shears


Embargoes and Neutrinos

October 7, 2011

The Embargo Watch blog has revealed an interesting aspect of how the recent news of faster-than-light neutrinos was released, namely that information had been issued to the main-stream media news outlets before even the rumours started to spread on the blogs. Their report includes a statement by the CERN press officer James Gillies detailing how he thinks the news broke but it leaves out some important details. It is interesting to look back at what did happen because news of other discoveries may emerge in a similar way in the future, so for the record here is the timeline as I witnessed it.

12th September – A seminar was scheduled at CERN for 16th September with the title “Seminar DG”. I saw it posted on indico and I added it to the viXra event calendar. There was no indication of what it was about, but as we now know CNRS had asked CERN if they could report their results there. CERN does not operate OPERA, it just provides the neutrino beam.

13th September – According to Embargo Watch journalists were briefed about the results at about this time and asked not to publish yet.

15th September – An anonymous commenter reported on Resanaances that a 6.1 sigma effect was about to be reported by CERN but the seminar had been cancelled. I saw the comment and checked my link to the “seminar DG” to find that it had indeed disappeared. I posted a note on an earlier Seminar Watch post and twitter but was not sure if the rumour was genuine.

16th September – Anonymous posted comments on Resonaances, Not Even Wrong and Vixra to say that the report would be about faster than light neutrinos at OPERA and that the seminar had been rescheduled. I added a link to the new seminar to the Calendar.

19th September – Dorigo posted a report about the findings on Quantum Diaries Survivor. Posts quickly followed on viXra and The Reference Frame and other blogs. Dorigo then withdrew the post under pressure from his emplyer.

22nd September – Another Italian physicist gave an interview about it to an Italian paper. According to Gillies this is when CERN briefed some journalists with the intention that the news should be published the next day. Reuters and some others published immediately.

23rd September - An e-print appeared in arXiv in the morning and the news was widely reported in the media. The seminar was held later that day. The official press release was issued etc.

What do we learn from this? Firstly, a week is too long to contain a rumour about particle physics and if the rumour starts in Italy then it is far too long. If they had stuck to the original schedule the information would have emerged from the seminar as planned. Briefing the press and then delaying the seminar was not good. The original intention was to let the main stream media prepare the story before the blogs, but the result was that the news leaked onto the blogs while the press were under an agreement to stay silent, what a mess.

When Dorigo posted they should not have forced him to remove it. Other bloggers already knew what the news was and by all accounts it was being discussed widely by physicists. I for one was ready to post more at that time anyway.

The CERN press office and the DG give the strong impression that they do not like bloggers that they don’t have control over. As freelance bloggers we often get information in advance and contrary to what some people think we don’t always post it.  They need to stop working against us if they want that to continue.


Can Neutrinos be Superluminal? Ask OPERA!

September 19, 2011

Four days ago a rumour started circulating in the comments at Resonaances that some “6.1 sigma” signal of new physics had been seen at CERN. I reported it in an update on  the Seminar Watch post. There had been a seminar titled “Seminar DG” which was listed on indico and removed the day before it was due. The rumour confirmed that this meeting was rescheduled to Friday but as an update on OPERA, the neutrino experiment which a couple of years ago saw its first tau neutrino. The claim now is that they have measured the speed of muon neutrinos and got a result faster than the speed of light!

This is of course a crazy idea because if true it would violate everything we think we know about causality. Even if neutrinos are hard to detect it should be possible to use them to send information into the past if this result holds up. That does not sound very likely (but I am now setting up a neutrino beam to send the news back in time so that it was actually me who leaked the story) .

Hypothetical superluminal particles are known as tachyons and they always move faster than light because they have imaginary valued mass, but quantum field theories for tachyons have terrible problems. Aside from the causality issues, the vacuum becomes unstable because you can create neutrino pairs with negative energy out of nothing. You would need a very unconventional variation of relativistic quantum field theory to stop the universe degenerating into an instant burst of neutrinos, and we don’t have that.

However this is not the first time that superluminal neutrinos have been reported. Some people claimed that observations of neutrinos arriving before gamma rays from supernovae implied that they are superluminal see http://arxiv.org/abs/hep-ph/9712265 . Other people just say that the neutrinos were created before the gamma rays.  In fact some “crazy” people believed in superluminal neutrinos well before that. Early attempts to measure the squared mass of the neutrino in the 1990s always seemed to give negative results  I have not had time to look back at that old ideas but it may be time to do that.

Of course such extraordinary claims need very good evidence and for now the most likely explanation by far is a systematic error. The rumoured “6.1 sigma” significance is probably a statistical error and it will be important to consider any systematic sources of error before coming to conclusions.  For now we will need to wait for the official seminar at CERN on Friday to see what they have to say about that.

click for wallpaper

Update: It is of course worth recalling that the MINOS experiment also measured the neutrino speed and got a result faster than the speed of light at 1.7 sigma see http://arxiv.org/abs/0706.0437 If the OPERA measurements are consistent with this measurement it will have to be taken seriously. As far as I can tell no measurement of neutrino speed or mass refutes the claim that they are tachyons, it’s just the theory that’s a problem.

Measurements of mass-squared from beta decay in Tritium have tended to give negative value results with error bar consistent with zero or positive values. This plot from http://arxiv.org/abs/0909.2104 shows how the measurements have developed over time. The latest result I can find is -0.6 ± 2.2 (stat) ± 2.1 (syst) eV2 from http://arxiv.org/abs/hep-ex/0412056. These are measurements for the lectron anti-neutrino, not the muon neutrino that OPERA is looking at.

If you are wondering about theories that allow tachyonic neutrinos the least wacky one I can find is that neutrinos can take “shortcuts off the brane through large extra dimensions”

What about the Supernovae observations? The timing of neutrinos vs light from supernova 1987a constrains the speed of neutrinos to be within one part in 10-8 of the speed of light, while the MINOS measurement had a speed of about (v-c)/c = (5.1 ± 2.9) x 10-5 so this seems inconsistent, even taking into account any differences of energy. Since neutrinos oscillate between different flavours we can’t make the excuse that one case looks at electron neutrinos and the other muon neutrinos, can we?

That said, neutrino physics has many unknowns. Other experiments hint at sterile neutrinos and even differences in mass between neutrinos and their anti-particles, even though we don’t even know what kind of spinors they are yet. If the large extra dimension theory has any bearing they may only travel faster than light in the presence of a gravitational field. It all sounds too crazy to be true but I am reserving judgement until at least we have heard from OPERA to see what they are actually claiming and how confident they are.

Meanwhile we have other views from MotlStrassler and Kea.

Update 23-Sep-2011: The news is now officially out with a CERN press release and an arxiv submission at  http://arxiv.org/abs/1109.4897 The result they have obtained is that the neutrinos arrive ahead of time by an amount 60.7 ns ± 6.9 ns (statistical) ± 7.4 ns (systematic). On the face of it this is a pretty convincing result for faster than light travel, but such a conclusion is so radical that higher than usual standards of scrutiny are required.

The deviation for the speed of light in relative terms is (v-c)/c = (2.48 ± 0.28 ± 0.30) x 10-5 for neutrinos with an average energy of 28.1 GeV The neutrino  energy was in fact variable and they also split the sample into two bins for energies above and below 20 GeV to get two results.

13.9 GeV:  (v-c)/c = (2.16 ± 0.76 ± 0.30) x 10-5

42.9 GeV: (v-c)/c = (2.74 ± 0.74 ± 0.30) x 10-5

These can be compared with the independent result from MINOS, a similar experiment in the US with a baseline of almost exactly the same length but lower energy beams.

3 GeV: (v-c)/c = (5.1 ± 2.9) x 10-5

If we believe in a tachyonic theory, with neutrinos of imaginary mass the value of (v-c)/c would decrease in inverse square of the energy. This is inconsistent with the results above where the velocity excess is more consistent with a constant independent of energy, or a slower variation.

We also have a constraint from supernova SN1987A where measurement of neutrino arrival times compared to optical observation sets |v-c|/c < 2 x 10-9 for neutrino energies in the order of 10 MeV. For smaller energies we should expect a more significant anomaly so this is important, but perhaps the energy dependence is very different from this expectation.

So if this is a real effect it has to be something that does not affect the cosmic neutrinos in the same way. For example it may only happen over short distances or in the presence pf a gravitational field. It would still be a strong violation of Lorentz invariance of a type for which we do not really have an adequate theory.

So obviously there could be some error in the experiment, but where? The distances have been measured to 20cm accuracy and even earthquakes during the course of the experiment can only account for 7cm variations. The Earth moves about 1m round its axis in the time the neutrinos travel but this should not need to be taken into account in the reference frame fixed to Earth. The excess distances by which the neutrinos are ahead of where they should be is in the order of 20 meters, so distance measurements are unlikely to be a source of significant error.

Timing is more difficult. You might think that it is easy to synchronous clocks by sending radio waves back and forward and taking half the two way travel time to synchronise, but these experiments are underground and radio waves from the ground would have to bounce off the upper atmosphere or be relayed by a series of tranceivers. This is not a practical method. What about taking atomic clocks back and fourth between the two ends of the experiment? the best atomic clocks lose or gain about 20 pico seconds per day, but portable atomic clocks at best lose a few nanoseconds in the time it would take to get them from one end to the other. This could be a good check to carry out if a good atomic clock could be flown on a helicopter, but as far as I know this has not been done.

Instead the best way to synchronise clocks over such distances is to use GPS which sends signals from satellites in low earth orbit. Each satellite has four atomic clocks which are constantly checked with better groundbased clocks. The ground positions are measured very accurately with the same GPS and in this way a synchronisation of about 0.1 ns accuracy can be obtained at ground level. The communication between ground and experiment adds delay and uncertainty but this part has been checked several times over the course of the experiment with portable atomic clocks and is good to within a couple of nanoseconds. The largest timing uncertainties come from the electronic systems that are timing the pulses of neutrinos from the source at CERN. The overall systematic error is the quoted 6.9 ns, well within the 60 nanosecond deviations observed. Unless a really bad error has been made in the calculations these timings must be good enough.

The rest of the error is statistical so it is worth looking at the variations in timings to see if another error could be hidden there. Here is a plot from the paper of some of the timings over the years the experiment has run. The blue band shows the average delay relative to timing delays assuming travel at the speed of light that were calculated later to be 987.8 ns. I have added a green band at this time plus or minus the 6.9 ns systematic error so that we can see how cleanly the measurements are displaced.

It looks pretty consistent. I think the only conclusion we can draw at this point is that further independent results are required. Perhaps MINOS could upgrade their timing measurements to see if they get a similar result with increased precision. T2K might also be able to attempt a measurement but their baseline is 295km compared with 730km for OPERA and MINOS. Otherwise a new experiment with shorter neutrino pulses and superaccurate timers may be the only way to resolve it. OPERA could also remove possible systematic timing errors at the source end by installing a second (much smaller) neutrino detector much nearer to CERN.

Some more reports: arstechnica , BBC, and of course Dorigo whose earlier post was ordered off line by big cheeses at CERN. Look out for his repost of his  interesting review of where he thinks problems may lie.

Post-talk update: The webcast talk at CERN was very interesting with lots of good questions. The most striking thing for me was the lack of any energy dependence in the result, a confirmation of what I noted this morning. The energy of the neutrinos have a fairly wide spread. If these were massive particles or light being refracted by a medium there would be a very distinct dependence between the speed and the energy of the particles but no such dependency was observed. The speeker showed how the form of the pulse detected by OPERA matched very nicely the form measured at CERN. If there was any kind of spread in the speed of the neutrinos this shape would be blurred a little and this is not seen.

Most physical effects you could imagine would have an energy dependence of some sort. A weak energy dependence is possible in the data but that would still be hard to explain. On the other hand, any systematic error in the measurement of the time or distance would be distinguished by just such a lack of energy dependence.

The only physical idea that would correspond to a lack of energy dependence would be if the universe had two separate fixed speeds, one for neutrinos and one for photons. I don’t think such a theory could be made to work, and even if it did you would have to explain why the SN1987A neutrinos were not affected. I think the conclusion has to be that there is no new physical effect, just a systematic error that the collaboration needs to find.


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