Not so fast, neutrinos

MIT physics professors examine the subatomic speed limit controversy

On Sept. 23, European scientists announced that they had observed neutrinos, a class of subatomic particles, traveling faster than the speed of light — the universe’s fundamental “speed limit.” The experiment, OPERA (Oscillation Project with Emulsion-tRacking Apparatus), was a collaboration between the Italian Gran Sasso National Laboratory (LNGS) and Europe’s high-energy physics laboratory CERN. Since the announcement of this anomaly, the scientific community has been hotly debating its validity, as well as the possibilities that could arise from such results.

MIT Physics Professor Scott A. Hughes said, “Carl Sagan had this saying, that extraordinary claims require extraordinary evidence. This is not extraordinary evidence.”

Hughes pointed out that the OPERA experiment was not originally designed for measuring the speed of neutrinos. The main goal was to transmute — or convert — one type of neutrino, called a muon neutrino, into a tau neutrino, a heavier type of particle.

Hughes conceded that the researchers “have done as good a job as they can, but this is extremely hard to measure … there’s this table of systematic errors in their measurements, and one error tends to dominate. If they combined their errors in a different way, their results could have been within the error bars.”

“I don’t think the paper is outlandish,” added Physics Professor Janet Conrad.

However, she and Physics Professor Frank Wilczek — a Nobel laureate — both said that the main evidence that contradicts OPERA’s result is the data set from Supernova 1987A, when neutrinos produced from the star explosion arrived only a few hours before the light did (neutrinos leave the dying star before visible light from the explosion).

Every supernova is accompanied by production and emission of a massive quantity of neutrinos. Physicists can calculate the relative time between when the neutrinos are emitted and when the light is emitted from the explosion. If OPERA’s results are correct, however, the neutrinos should have travelled faster and arrived a few years before the light.

Hughes and Wilczek both guess that scientists will most likely approach the neutrino announcement with a variety of new experiments and do tests with different baselines.

“I would say there’s a 98 percent chance this is a systematic error,” Hughes said.

Scientists note that other predictions of special relativity are valid, and that these neutrinos might be something “special and weird.”

“Within the theoretical framework, we have been very successful in other parts, which is why it’s hard to isolate this neutrino ‘disease’ in this small sector,” Wilczek said.

But no one is claiming that the European researchers were careless. On the contrary, Wilczek believes that all of the scientists are “competent, professional experimenters who have been wrestling with this for months and can’t make this go away.”

“I’m not claiming they’ve done it wrong, I’m saying that it needs looking at very carefully,” added Hughes. “There’s a big difference between precision and accuracy — you can measure with precision a very inaccurate result.”

What if it’s true?

If it is true that neutrinos can travel faster than the speed of light, fascinating new lines of inquiry could open. One theory is that these speedy neutrinos could be a crack in the universe that reveals extra dimensions in high energies.

“If this were correct, our GPS wouldn’t work,” Hughes said of the navigation technology that relies on relativistic principles.

“It’s premature, to say the least, to speculate wildly about the implications, either theoretically or technologically,” said Wilczek.

Talk in the media about traveling in time or having causal loops were a misunderstanding of relativity. “If there’s any limiting speed, even if it’s not the speed of light, one would not be able to close the loop from the future to the past,” Wilczek added.

Although he is doubtful of the results, Hughes does not have any criticism of OPERA’s report itself. “The paper is very clear — they say they’re throwing [this discovery] out there for further testing. Their paper is very careful and pretty conservative.”

He found it “irresponsible,” however, that the authors held a press conference immediately after their accidental discovery. In fact, some of the researchers who were part of OPERA actually removed their names from the paper because they found the analysis to be too preliminary to be able to release the results in such a manner.

“This is one of the few things that reveals the tension that was going on within the experiment,” Hughes pointed out.

Public reaction

The fact that this story has made a huge appearance in headlines over the past few weeks does not surprise Hughes, but he is worried that after it dies down, any future and possibly contradictory discoveries will not have as large an impact in the media.

“No headline will say, ‘Oops, we goofed.’ … My concern is that this potentially big splash will not be compensated for by correction,” said Hughes.

Physics student Asher C. Kaboth G also noted that “It’s harder for the general public to understand the little details … it’s difficult to explain.” Those who do not know much about special relativity might have misconceptions about what these results mean and their possible implications, he said.

Conrad did not like the way some physicists reacted to the news. “Way up there in the responses I don’t like [is], ‘If it doesn’t fit my theory, it must be wrong.’ That’s not okay to tell people.”

She said that if neutrinos really do travel faster than the speed of light, it breaks current theories and scientists will have to construct new ones.

“There’s a difference between us saying what nature will do and nature telling us what it does,” Conrad said. “When you find a violation, you have to find a way to put it in the perspective of other data.”

Despite concerns, the neutrino results have been a great teaching opportunity for many professors. In Hughes’s 8.033 (Relativity) class, he discussed the concept of the experiment in lecture and asked students to think carefully about whether they believed that neutrinos could travel faster than the speed of light.

“In the hands of someone who can discuss this well, and the ears of students open to listening, it’s a great topic,” Hughes said.

Conrad also brought up the subject in her 8.02 (Electricity and Magnetism) class and asked her students to answer questions like “If this result is proven wrong, what does this say about science? Does science ever get it right?” Her opinion is that things go wrong in science all the time, but the beauty is that one discovery leads to the next, and we continue to change what we know about the world.

Wilczek agreed that on the whole, it’s a good thing that people are noticing current research in physics, and that there is exposure of the scientific process. “There’s something about Einstein and space that even after all these years has a certain magic because it’s so profound and unexplained,” he said.

Jim Burrill almost 12 years ago

The OPERA results and Einstein's relativity can both be correct as long as the definition of "c" is re-evaluated. When light travels through air or water it travels slower than "c". So to measure "c", the speed of light was measured "in a vacuum". Why, until now, hasn't anyone said "Wait a minute, according to quantum mechanics, there's no such thing as a vacuum, so that measurement must really be slower than 'c'"?

In a vacuum, a quantum soup of particles is constantly coming into and out of existance. Light interacts with these particles. Neutrinos interact with matter alot less, so the speed of nutrinos in the earth's crust could actually be closer to "c" (but still a bit slower). The speed of neutrinos in a vacuum would begin to approach the real value of "c".

In addition to these temporary particles, a vacuum may also contain a Higgs field, which could be another factor in slowing the speed of light below the real value of "c".

Jim Burrill

jburrill at gmail dot com

Deepak N. almost 12 years ago

I am from Pharmacy background, reading the awesome article and interesting comment and professor A. Huges sentence "You can measure with precision a very inaccurate results!" I thought God! why I didnt go for Physics instead? Good Going guys you people made highly inactive 'Neutrinos' highly reactive to our neurons in brain and feel them in Mumbai, India. Thanks!

David Price almost 12 years ago

Since light travels a "curved" path near a massive object (the Earth in this case), perhaps neutrinos do not feel this gravitational curvature and traveled a "straighter" path to the detector, arriving slightly faster than a photon would. In the case of the supernova, the neutrinos and the photons would have traveled similar straight paths, and so their speed would be the same.

Daniel L. Burnstein almost 12 years ago

I personally believe that the SN1987A neutrinos observations do not contradict the OPERA results. I hypothesize that the difference between the speed of neutrinos measured by the OPERA group and the speed of light corresponds to the speed of the Earth along the CERN-Gran Sasso axis relative to quantum-geometrical space (that is space assumed to be discrete rather than continuous.

If space is quantum-geometrical, then the speed of the OPERA neutrinos relative to it would be the same as the SN1987A neutrinos, hence the measurement would agree.

Daniel L. B.

Jevon almost 12 years ago

I was thinking of the curved path as well but I'm also thinking of "light" as having characteristics of both particle and wave. If light moves as a waveform is there a chance that neutrinos take the straight flat line approach. Waves have a distance from peak to peak that (totally just theorizing) a particle may follow and that must take a specific time to traverse all points on the waveform. Neutrinos take the baseline hot-rod approach from a to b. I absolutely hope OPERA's results prove to be true.

Ashish Manohar Urkude almost 12 years ago

Taconic speed (speed c) is possible both theoretically and practically if we check evidence from past scripts in India. May be today we don't have technology or knowledge of that but it is possible. 100 years ago faster than speed of sound was thought to be impossible, today we think faster than speed of light is impossible. However, everything is possible. If someone is interested, kindly, search for ancient scripts in India, they'll get evidence of it. In fact, aeroplane moving faster than speed of sound existed during even Ramayana/Mahabharata/Vedic era...I believe in place of discussing, let's help scientists develop spaceships moving faster than speed of light, to go to places in multi-verse and establish human civilisations there.

In fact, we must try for space ship moving as fast as our thoughts travel.

Anonymous almost 12 years ago

all these comments are wrong.

1. There currently is no combined theory of relativity and quantum mechanics, so you have to think of it one way or the other.

2. Even traveling at light speed neutrinos don't interact with things. So the chance of one hurting your brain is zero.

3. Photons have no mass, neutrinos do. Therefore gravity will affect the neutrino more. Hence the neutrino would take the more curve path.

4. I don't think you understand the SN1987A result.

5. I know you're just theorizing, but the speed of light is not related to it's peaks and troughs. This represents a changing electric field. There is also a perpendicular changing magnetic field usually not represented.

Ashish Manohar Urkude almost 12 years ago

Please Read: "Speed Greater than "C" (more than light)", the greater sign didn't appear.

Daniel L. Burnstein almost 12 years ago


I don't think 'you' understand the SN1987A results. See argument at

Lokendra almost 12 years ago

Disclaimer: I am no physicist so in all probability my comment is going to be totally useless fiction.

Real Comment: Just imagine fellows, won't it be awesome to know that it is really possible to break the speed of light barrier. Also, maybe some scientists could tell me whether it is possible that old calculations of the speed barrier were slightly off, let's say by 7.4 km per sec. So maybe the speed of these neutrinos is the real limit of speed and not the old number. Maybe that won't hurt the existing school of thoughts other than the fact that they working with a wrong constant (i.e. speed of light) so far, and that constant just got calibrated to its real value (the new speed barrier) which is around 0.0025 more than the value accepted previously. Maybe it's possible to have made such a small error.

11 almost 12 years ago

Nutrinous can travel faster

ex0du5 almost 12 years ago

This is extremely weird reasoning by someone who actually teaches physics. First, these effects of neutrinos having a measured speed faster than light have a lengthy theoretical background, concerning branal propagation along nonrestricted modes. These do not "violate relativity" - in fact, like QED and other relativistic quantum theories, the branal theories are fully relativistic in their extension to higher dimensions. These are just worldpaths with smaller metrics than their projection to 3 space dimensions. This is completely isomorphic to the use of wormholes to "violate relativity" in a completely relativistic way, except now it is not space-time but branal propagation. Additionally, it is very possible that such effects may be distance-sensitive, particularly since the world paths are both observed space and "inner dimensions of the theory". Extending out worldpaths to cover intergalactic distances may find that the projection of allowed worldpaths is slower, with asymptotic c. Although not isomorphic, this is similar to the way that the "spontaneous lorentz breaking" theories predict that c may be violated over small distances but average out in the long range.

Saying that this could have "systematic error" is just lazy. Obviously any measurement likely does have error, and they've done a fair job of describing that in their paper. It's sad to see such idle speculation from a professor (an MIT professor, nonetheless!) without any discussion of the actual reasoning for the speculation or any commentary on the lengthy theoretical work that could support it. And the comment on violations of relativity are rather ignorant.

Viswanathan almost 12 years ago

Not sure if the observations in this test accounted for the discrepancies that could be due to floating material like dust,which could have had charged particles and have sped up the neutrinos , while hindering the smaller wavelength of the light spectrum from going thru?( the sun is orange at sunset for this very reason).

Maybe a pure vacuum test would be the only confirmation of this puzzle.


M Workman almost 12 years ago

I'm not a physicist, but I'm wondering why it's assumed, in the case of the older OPERA results, that the neutrinos would necessarily travel at the same speed the entire way. I'm not saying they travelled faster than light, but that I don't understand why their "late" arrival disproves superluminal speed. Is there a logic here that evades me?

oisin almost 12 years ago

I wonder if what has been observed is not some special characteristic of particles but instead some previously unknown characteristic of gravity and space/time. Not all particles travel at the same speed in all places apparently and some special characteristic of this particular location in space time has the particles appear to arrive sooner than we thought they should.

John Fitzpatrick almost 12 years ago

Bartender: "We don't serve faster than light particles here".

A neutrino walks into a bar.

Floyd Mills almost 12 years ago

I am wondering what the basis of neutrinos leave a super nova sooner than photons. Is it because they get here first that they must leave before photons? How much before?

Nathan Moore almost 12 years ago

I wonder why no one has mentioned the possibility that neutrinos traveling faster through dense rock than through the near vacuum of space.

General relativity says that gravity is curves in sometime due to mass. We experience this curvature as a force. Perhaps neutrinos moving through rock experience other forces (curvatures in sometime) that made their journey shorter.

If so an experiment measuring the angle of refraction of neutrinos through dense material should tell us.

Joshua almost 12 years ago

not suprising.. as neutrino being the most elusive particle on earth should be plying altogether on a different law.... much different than which can be comprhended through quantum and classical physics.... what i am saying is .... to study neutrino we need a new law......

M.A. Padmanabha Rao, PhD almost 12 years ago

My views here focus on the following comment. However, she and Physics Professor Frank Wilczek a Nobel laureate both said that the main evidence that contradicts OPERAs result is the data set from Supernova 1987A, when neutrinos produced from the star explosion arrived only a few hours before the light did (neutrinos leave the dying star before visible light from the explosion). Every supernova is accompanied by production and emission of a massive quantity of neutrinos. Physicists can calculate the relative time between when the neutrinos are emitted and when the light is emitted from the explosion. If OPERAs results are correct, however, the neutrinos should have travelled faster and arrived a few years before the light.

The following reaction provides a key: neu_xe - neu_x e, where neu_x is a neutrino of any flavor given in . The reason could be something else, since the relative time between emissions of neutrinos and the light may not be a few hours. As the scattered electron causing Cerenkov light travels with the velocity greater than the phase velocity of light, neutrino which triggers the reaction is expected to travel at superluminal velocity.


The famous Einstein's mass-energy equivalence widely used for sub-atomic particles such as electron having a mass cannot be directly applied to neutrino with near-zero rest-mass or for gamma or X-ray having zero rest mass. Since gamma, X-ray or light photon are said to have no mass, all these photons can be assumed to have negligible but equal mass. When m is given a value 1, E equals to the square of light velocity C. On replacing C with V, which represents relative velocity, energy E of a gamma, X-ray or light photon equals to square of Velocity (V).

If this modified formula is really true, 40 keV X-ray photon goes 100 times faster than 4 eV light photon. Similarly EUV photon can go faster than an infrared photon. Neutrino's near-zero rest-mass somewhat closer to X-ray's zero rest mass facilitates neutrinos travelling faster than light. Using the above formula, neutrinos superluminal velocity can be estimated from its energy. Please examine how far I am correct.

Anonymous almost 12 years ago

Are neutrinos capable of acceleration while travelling through matter? Is there some inherent quality within matter which may foster such acceleration? Is this the question or is it a matter of space.time? Ie space is curved in the vicinity of mass, is it also contracted by mass? So the "distance" of travel within mass is less than you might think?

Anonymous almost 12 years ago

These particles are slower than what I thought. The vedas provide a clear picture that there is a 10-74 m dia particle which is the actual building block and is faster than these particles. GPS technology is different in dynamics.

David Brown almost 12 years ago

Is the OPERA neutrino anomaly within the range predicted by the Raada-Milgrom effect? In 1983 Professor Milgrom challenged conventional theories of gravitation and/or Newtonian dynamics. MOND

Is there overwhelming evidence in favor of MOND? The MOND pages (McGaugh) Pavel Kroupa: Dark Matter, Cosmology and Progress

I claim that the -1/2 in the standard form of Einsteins field equations should be replaced by -1/2 sqrt((6010)/4) 10-5 but this is merely part of my theory. (See Dark matter: why should Raada and Milgrom win the Nobel prize at nks forum applied nks.)

Anonymous almost 12 years ago

Speed of neutrino.

The OPERA experiment probably confirms conclusion what results of considerations in connection with Theory of everything: speed of light c increases in system with great density of energy (this presumption connects with mass of nucleons).

The neutrino contains unclosed mass wave and moves always by defined speed near to speed of light vc (considerations in connection with Theory of everything).

Conclusion: the speed near to speed of light of neutrino can be greater than speed of light in vacuum in system with great density of energy (body of Earth is system with great density of energy).