“If my theory of relativity is proven successful, Germany will claim me as a German and France will declare that I am a citizen of the world. Should my theory prove untrue, France will say that I am a German and Germany will declare that I am a Jew.” –Albert Einstein
One of the most famous scientific developments in the 20th century was the revelation that the Universe had a speed limit: the speed of light.
Clocking in at exactly 299,792,458 m/s, Einstein’s theory of relativity states that any particle with mass can only approach — but can never reach — this maximal speed.
This was tested most accurately for the neutrino (fun facts here), the lowest mass particle ever discovered — over a million times lighter than a single electron — to still have mass.
In 1987, a supernova from 168,000 light years away went off, with the first neutrinos and the first light (above) reaching our eyes within just a few hours of one another! This tells us that the speed of these neutrinos differs from the speed of light by, at most, two parts per billion!
Which is just one reason why last month’s experimental claims that neutrinos move significantly faster than light have come under heavy fire.
Since the OPERA collaboration announced that they had created ultra-relativistic neutrinos 732 kilometers away from their detector, and after a 2.43 millisecond flight, the neutrinos were detected an average of 60 nanoseconds too early, the main question on physicists’ minds has been to discover exactly what’s going on here.
Did they account for all of the effects of relativity? Was there a goof between the atomic clocks? Many others weigh in (e.g., the comments here), and arguments are currently raging across the world between those defending the experiment and their detractors.
From CERN, huge numbers of protons — something like 1020 of them — are smashed into a target beneath the ground, creating a myriad of particles, including the elusive muon neutrino. This beam is directed underground towards the giant scientific research station beneath the mountain of Gran Sasso. While the other particles collide with the intervening Earth, the neutrinos hardly interact at all, and fly freely towards their target, hundreds of kilometers away.
In the meantime, a lesser-known experiment, ICARUS, also capable of detecting these muon neutrinos, was also taking data deep beneath the mountain in Gran Sasso, where this neutrino beam was aimed in the first place.
And if these neutrinos are really moving at speeds greater than the speed of light, a second experiment should be able to confirm or refute whether this is what’s really going on!
Using a liquid-argon detector, ICARUS is able to detect muon neutrinos that interact with the atoms inside, producing a characteristic signature that’s dependent on the energy of the muon that gets created by these interactions.
By measuring the angle shown in the image above, they can reconstruct the energy of the muon that created this signal. Over the past year or so, they’ve observed just over 100 events; not a statistical goldmine, to be sure, but enough that they can compare their data (blue dots, below) with the expected event distribution (red line) from Monte-Carlo simulations.
Based on this information, they can go ahead and reconstruct the information about the muon neutrinos that caused these events, including what their energy distribution is, and compare it with the expected theoretical distribution.
The match is pretty good, for as much as you can determine with barely 100 events.
But, of course, this is assuming standard physics, with neutrinos that move slower (even if just barely) than the speed of light. We have all sorts of experimental and observational constraints on how much faster than light neutrinos are allowed to move, but many of these results take place at different energies for the incoming neutrino. Matt Strassler has compiled a very nice set of data showing this on a single graph.
What’s perhaps even more interesting is that — just as particles moving faster than light in a medium emit light known as Čerenkov radiation — if neutrinos are moving faster than the maximum speed of electrons/positrons, they should emit an analogous type of radiation! A recent paper showed this, and these electron-positron pairs that would be radiated by a superluminal neutrino are now known as Cohen-Glashow radiation. The hoops one has to jump through to explain the OPERA result are incredibly dissatisfying.
Here’s the thing, though. ICARUS has the same source for its neutrinos as OPERA does, but is a completely different experiment.
Based on Cohen-Glashow radiation — which, remember, is just standard, known physics combined with neutrinos moving at OPERA’s claimed speeds — ICARUS has gone and calculated what they would see if these neutrinos really were moving faster than light, and they don’t see that! (Tommaso Dorigo has the first analysis up, here.)
I’ve taken the liberty of taking the ICARUS graphs and superimposing what they would have seen if these neutrinos had been moving faster than light.
First off, the incoming neutrinos (although this is an extrapolated, not an observed, quantity) would have had their energies cut off at somewhere between 12 and 20 GeV. (See Strassler for a nuanced discussion of why.)
If your incoming muon neutrinos are lower in energy, it stands to reason that the muons they produce will be lower in energy, too. What would ICARUS have seen?
This is a huge difference! Were there actually a cutoff in neutrino energy due to this Cohen-Glashow radiation, we would see a very different energy distribution (purple line) than was actually seen by ICARUS!
In fact, as Tommaso so correctly notes:
They find that the energy spectrum of the detected neutrino interactions in ICARUS shows a very nice agreement with the expectation for well-behaved light-speed-moving neutrinos. A very dramatic distortion of that spectrum would instead be expected for the speed measured by OPERA, such that indeed ICARUS can place a very tight constraint on the superluminal speed of the CERN neutrinos: consistent with the speed of light, and not larger than that by more than four part in ten billionths.
If you are unfamiliar with millionths and billionths, I can make it easier for you: the ICARUS result says that the difference between the speed of neutrinos and the speed of light cannot be as large as that seen by OPERA, and is certainly smaller than that by three orders of magnitude, and compatible with zero.
Based on the ICARUS data, I’ve edited Strassler’s chart to include the ICARUS constraints:
So there you have it: while physicists are still arguing over just where the OPERA collaboration’s mistake is, another experiment seeking to check their result has found wild inconsistencies with OPERA’s claims. So congratulations to ICARUS on a very clever use of their apparatus, and for those of you who aren’t afraid of details, check out the ICARUS paper for yourself!
There might yet be some clever way to break the speed of light, but if there is, we haven’t found it yet, and OPERA’s claims to the contrary look to have just suffered a fatal blow!
(This article is dedicated to the memory of Lewis & Clark student Isaac Clark, who was always game to learn more about the workings of the Universe, and who really would have loved to know the latest on the faster-than-light neutrino saga. Isaac sadly passed away yesterday, and will be missed.)