Field of Science

Superluminal claims require super evidence

Neutrinos, those mercurial smidgens of the particle world, travel faster than the speed of light. That's the claim the OPERA collaboration makes in a paper subtly titled: Measurement of the neutrino velocity with the OPERA detector in the CNGS beam. This is a big claim that could have implications for particle physics and time travel. It has made the news, news, news, news, but what does it all mean? Lets talk about neutrinos.
faster than the speed of light
First, let me say that if neutrinos do travel faster than the speed of light then physicists have a lot of explaining to do. The repercussions of faster that light travel for any particle (also known as superluminal travel) would be revolutionary. So revolutionary that most physicists I spoke to this past week at a conference did not take the news too seriously: it was too extraordinary to comment on without further thought and details. The OPERA collaboration is actually very brave for putting this paper out there (i.e. on the ArXiV) and asking for outside analysis. They don't even pretend to begin to consider the ramifications. The last line of the paper sums up their position:
We deliberately do not attempt any theoretical or phenomenological interpretation of the results.
So let me ignore the wild theoretical implications and discussions of tachyons and just talk about the experiment and an astrophysical constraint on the velocity of neutrinos.

Why are physicists so confident that neutrinos travel at the speed of light? Well, start with the fact that every piece of credible data ever taken has never seen anything—be it particle or information—travel faster than the speed of light. Given previous observations it is hard to understand how neutrinos could be any different. Of course neutrinos are very difficult to measure because they interact very weakly with regular matter. Consider that 60 billion neutrinos generated from the core of the sun pass through your pinky each second and none of them interact with you (nor do they interact with the Earth, they are passing through you day and night).

The creation and detection of neutrinos is complicated. The process begins for the OPERA experiment over at CERN where the Super Proton Synchrotron (SPS) creates high energy (400 GeV/c) protons that collide with a graphite target producing pions and kaons which decay into muons and muon neutrinos. The neutrinos coming out of SPS are almost pure muon type neutrinos with an average energy of 17 GeV. The neutrinos travel through the solid Earth in a straight path unimpeded into a cavern below a mountain, Gran Sasso, in Italy. The OPERA neutrino experiment was designed to look for the direct appearance of muon to tau neutrinos (νμ → ντ), but their anomalous findings on the velocity of neutrinos is much more interesting.

The OPERA experiment found that the velocity of neutrinos was about 0.00248% faster than the speed of light. This measurement was made by precisely measuring the distance traveled by neutrinos and the time of travel. The OPERA collaboration did a lot of work to measure both parameters precisely. They found this velocity by measuring that the time of arrival of neutrinos at their detector by using atomic clocks. Their measurement was precise to a few nanoseconds. Wow, that is quick. Light only travels about a foot in a single nanosecond.

In order to measure the distance between CERN and Gran Sasso the OPERA team used very precise GPS systems. For example, they noticed a 2009 earthquake in that area produced a sudden displacement of 7 centimeters. So the exact distance the neutrinos traveled was 730534.61±.20 meters (or about 2.44 light milliseconds), however some have suggested that the GPS based positioning they used has errors introduced by atmospheric refraction. Intriguing possibility.

In order to measure the time, what OPERA calls the time of flight measurement, they used atomic cesium clocks. But the 'time' cannot be precisely measured at the single interaction level since the protons from the SPS source have a 10.5 microsecond extraction window. They had to look at time distributions where the most likely time for a burst of neutrinos to be created was inferred to higher precision. Additionally, the actual moment where the meson produces a neutrino in the decay tunnel is unknown, but it introduces negligible inaccuracy in the time of flight measurement. So, these distance and time measurements are really important, but really subtle. I recommend reading the paper if you are a glutton for punishment.
There is a very interesting constraint on the speed of neutrinos that comes from astronomy. It was the neutrinos and photons released from the death of a star. Supernova 1987A (SN 1987A) exploded 168,000 years ago when fusion in the core of an old star ceased and the weight of the outer layers of the stars caused the core to collapse. The protons in the atoms of the core of the star merged with the electrons present and converted themselves into neutrinos and electron neutrinos. A mega amount of electron neutrinos, about 1058, were generated and they began their epic journey to Earth. Some of these neutrinos arrived on Earth one morning in February of 1987 in a burst lasting less than 13 seconds. Of those many neutrinos two dozen interacted with detectors on Earth.
Astronomers observed light from SN 1987A just three hours after the neutrinos arrived. Just such a delay is expected as the fireball of the supernova had to have some time to expand and become transparent to photons, whereas neutrinos could escape much sooner. The explosion occurred at a known distant out in the Large Magellenic Cloud. This distant explosion created photons and neutrinos in a timed race to the Earth. With the these measurements in hand (the distance to the supernova and the time of arrival of the photons compared to neutrinos) we can determine the speed of neutrinos from SN1987A.

The accuracy and precision of measurements from SN 1987A are actually much greater than measurements taken at Gran Sasso despite the three hour time window difference between neutrino and photon travel. It comes down to the fact that the relative distance between Earth and SN 1987A is about 1016 times larger than the distance between CERN and Gran Sasso. This means that time measurements from SN 1987A can be extremely imprecise and still be much more precise than the OPERA measurements.

If the neutrinos from supernova 1987A had been traveling as fast as the neutrinos detected at Gran Sasso they would have arrived about four years sooner than the light from SN 1987A.

This supernova constraint on the velocity of neutrinos is very nice, but it doesn't answer every question because the comparison may not be apples to apples. The OPERA neutrinos are tau type, not electron type. And they are traveling through the Earth, not empty space. And they were much higher energy. The neutrinos from SN 1987A were only about 10 MeV, about one hundred times lower energy than the neutrinos in this study. Some may argue that higher energy neutrinos travel faster than lower energy neutrinos. However, a velocity-energy dependence should have stretched out the 13 second arrival time of neutrinos. Further, part of the OPERA collaborations analysis involved splitting the data into two bins with mean energies of 13.9 and 42.9 GeV; a comparison between the two bins indicated no energy dependence on velocity. Thus, while it may still be true that GeV neutrinos move faster than MeV neutrinos, the theoretical wiggle room is shrinking.

This experiment may be a signal of new physics or a case of systematic errors. Yet, even physicists who have developed theories that allow for superluminal velocities are doubtful so I would not bet on proof of hidden extra dimensions or time travel to come from this experiment. Much more extraordinary evidence is necessary to confirm such an extraordinary claim as breaking the speed limit of our Universe.

1 comment:

  1. Only one remark: Opera has detected muon neutrino's time of flight. Actually they only detected one tau neutrino last year.

    ...Hope soon for new measurements from other experiment! :-)


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