OPERA’s Paper: My Initial Reaction

Matt Strassler 9/22/11

Well, every now and again an experiment reports a result that forces scientists to go back to a long-established principle, to check whether it needs revision, extension or adjustment, or perhaps even replacement. Most times it eventually turns out that the experiment is wrong, though often in some subtle and non-obvious way, and the principle survives. But of course there are the rare occasions when it is the other way round. So a scientist must go into such a situation with an open, though skeptical, mind.

Is there a more famous principle from 20th century physics than Einstein’s principle that nature has a speed limit? We call this the “speed of light”.

We call it that. However, let’s be a bit careful.

There are actually two principles at work here. 

  • The first is that a speed limit exists, due, in Einstein’s way of thinking, to the geometry of space and time themselves.
  • The second is that all massless particles, including light, travel at the speed limit — though only in completely empty space, i.e. in “the vacuum’‘.   (In water, for instance, light travels slower than the speed limit, and electrons in water can actually travel faster than light in water!  [But not faster than the speed limit!] A fact which is used in many particle detectors.)
  • Actually the second principle is more general: it further states that any particle whose motion-energy is far, far greater than its mass-energy (m c-squared) should travel (in vacuum) very, very close to (but just a tiny, tiny bit below) the speed of light.

This evening an experimental collaboration called OPERA put out a technical document.  In it, they claim to find that pulses of high-energy neutrinos, sent from the CERN laboratory near Geneva, Switzerland, arrive 730 kilometers away, in the underground Gran Sasso laboratory in Italy, just a bit earlier than expected. According to the principles I just stated, these neutrinos should travel at just about the speed of light. But apparently they are traveling slightly faster. To deduce their speed requires measuring the 730 kilometer straight-line distance, from the point where the neutrino beam pulses are created to the location of the OPERA experiment, to an accuracy of 20 centimeters. The excess in the speed is 25 parts per million of the speed of light, which translates into an early arrival of about 60 nanoseconds (billionths of a second.) This is not a simple measurement to be made with a stopwatch and a ruler! Clearly the experimenters have worked very hard.

If this claim were right, it would be revolutionary. At minimum, it would force physicists to modify at least one principle that has been taken almost (but not entirely) for granted for many decades.

But is it right?

Only a critical and thorough review by the high-energy physics community, and repetition of the experiment (or measurements using other experimental techniques), will tell us whether this result is correct. We certainly have to consider other measurements of neutrino speeds. As I described in my previous post, neutrinos from the 1987 supernova seem to have traveled over 160,000 light years at a speed within a few parts per billion of the speed of light. What OPERA sees is a several-thousand-times larger effect, which might seem inconsistent. However, one can’t use the supernova measurement to argue that OPERA’s measurement must be wrong. The supernova neutrinos had lower energy, by a factor of a few hundred, compared to those that are in OPERA’s neutrino beam. There are other possible differences too. And perhaps these differences matter. So we probably can’t reason away this result.   Particle physicists will have to check it, repeat it, and try other, related measurements.

Whether the result is right or wrong, theorists and experimentalists in high-energy physics will do some serious thinking about it.   We’ll ask questions such as: What mistakes might the OPERA team have made? Could there be any subtle way in which this result would cause a conflict with other previous experiments? What experiments might be done that would check the result? If the result were true, what other related phenomena might be present in nature, and could we do experiments to look for them?  At worst, even if time tosses OPERA’s claim into our very big garbage pile of false discoveries, we’ll learn something by thinking things through, and will be clearer about what we know and how we know it.

There will be much more to be said, but that’s enough for the moment. High-energy physicists need to read carefully through the OPERA “preprint” (the pre-publication paper, 22 pages long,  technical and subtle), listen carefully to what the OPERA experimentalists have to say, and examine carefully the papers written by the expert theorists who have considered carefully how Einstein’s principles could be modified. Carefully, carefully, carefully; this is a complex and subtle subject, and it is easy to make wrong statements and draw wrong conclusions. As the dust settles, and as I learn enough on my own and from my peers that I am confident I can explain more of the issues both clearly and correctly, I’ll write more about it.

A last remark for the night: think about what it is like to be an experimentalist making a revolutionary statement of this magnitude.  Talk about sticking your neck out! This result either means a Nobel Prize or international embarrassment — perhaps even ridicule if a serious mistake was made; there’s no middle ground.  The combination of excitement, hope, and terror must be unlike anything most of us will ever experience.   I cannot imagine how any of them have slept for days; I cannot imagine that they will sleep well for months, until a second experiment reports, “We have measured the speed of neutrinos, and we confirm…”

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