Of Particular Significance

Guess What?! Neutrinos Travel Just Below the Speed of Light

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 06/08/2012

Five out of five experiments agree: neutrinos do not travel faster than the speed limit.

Or more precisely: to within the uncertainties of current measurements, neutrino speed, for neutrinos with energies far larger than their masses, is experimentally indistinguishable from the speed of light in vacuum.  This is just as expected in standard Einsteinian special relativity, which would predict they move just below light speed, by an amount too small to measure with current experiments.


Based on data taken in May 2012 using a beam of neutrinos sent from the CERN laboratory to the Gran Sasso lab, the four experiments ICARUS, LVD, Borexino and even OPERA (the source of  all the excitement) find results consistent with the speed of light, with uncertainties (at one-standard-deviation) about 10 times smaller than OPERA’s original measured deviation of neutrino speed from the speed of light.  The new results are consistent with ICARUS’s result from 2011 data.  Moreover, OPERA’s mistaken result from September and November 2011 — a claimed six standard deviations away from the expected speed — has now been corrected, following their detective work presented in March.  Even MINOS, a U.S. experiment, has revised their older result, which was previously slightly discrepant from the speed of light by a small amount (two standard deviations), and they find now that their data too are quite consistent with neutrinos traveling with light speed, though with much less precision in the measurement.

And so with a final quintet, sung in unison, this melodramatic comic OPERA buffa comes to a close.  As with all classic operatic comedies, there’s been crisis, chaos, and a good bit of hilarity, all the while with wise voices speaking reason to no avail, but in the end the overzealous are chastened, the righteous are made whole, everyone (even the miscreant) is happy, and all is well with the world.

Curtain!! Applause!!  Science Triumphant!!

Favorable review to follow when time permits.

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29 Responses

  1. Professor Strassler,

    I think there is a typo in your answer to Oaktree: “If the Higgs field were non-zero, the electron would be massless.”

    Did you meant to type: “If the Higgs field were zero, the electron would be massless.”

      1. So if the Higgs Field were zero, the electron were massless, and it would travel at c. Is this only true for an electron in a vacuum? Would this massless electron travel at less than c through a medium?

        And to make sure I understand how this currently works for light with our non-zero Higgs Field, which is more correct:

        Light travels at c in a vacuum and slower through a medium, i.e. ≈ c/1.3 through water with the exact denominator depending on wavelength?


        Light always travels at c but when it’s traveling through water every so often it hits a water molecule and gets absorbed and then later emitted with the average time lost is these absorb/emit interactions ≈ c-c/1.3?

  2. dear proff,,,im really greatefull for your blog,,,you taught me the intricacies of particle physics,,,i knew of higgs/field,and basic stuctures,quarks etc,,,but you post toke me the next step,and for i fell a debt of gratitude….please understand when i get anxious or angry,its because,being taken seriously was a very steep hill for me,,,as it should be,because i didnt study att universiy…you helped me with that,,,you gave me the confidence of knowing,that i wasnt just kidding myself,,what i thought i knew,i did…and you have no idea how badly the self doubt was hurting me mentally…and you helped me fix that,,,and as a thank you,,,id like to ask you to reread my comment on the reverse parallax,,,if you look at it closely,,,it predicts the neutrino exactly as the experiment measured it,,,and therein lies the mistake..
    im not saying that i cant be wrong,,,i may very well be,,,what im saying is we are measuring in reverse,which is why you get a positive reading,,,please read through my comments again,,,its all there…and im telling you the finger will turn and point the other way….read through my comments again,,,you will see it,,,,i know this because i do…..and i can tell you,youll never find the boson,,,as long as youre neutrinio has a + next to it,,if you do make sure you let me know,,,and ill send you a letter of apology
    thank you for the post,i will continue to read it,,,but wont make further comments,,i think thats best…
    all the best john…

  3. The heaviest neutrino (15.5 Mev) seems to be heavier than the electron. Is there a good reason that neutrinos always seem to be travelling at almost the speed of light.

    Could there be lots of slow moving (15.5 Mev) neutrions around ?

    1. Where do you get 15.5 MeV from? I thought all three neutrino types are supposed to have roughly the same mass, which is estimated to be on the order of 1 eV or so at most.

      1. Tim — I do not know where you got 15.5 MeV from; I’m curious, what was your source? Is that just a direct experimental limit from tau decays or something? Certainly the muon neutrinos emitted in pion decay (which are the ones used at OPERA) are much, much lighter than that. I don’t have the limits in front of me but I believe direct experimental limits are a thousand times better than the number you quote.

        But there are even much stronger limits yet that come from indirect measurements. As Chris says, cosmological studies put limits on all three masses on the order of 1 eV, whose only loophole is if our understanding of cosmology is drastically wrong. It is believed most likely that all three neutrinos are well below 1 eV, but the three masses are not yet known… only the mass splittings are known, from neutrino oscillations. One possibility is that all three masses are almost identical and the masses are around 1 eV. Another is that the three masses are very different but all are well below 1 eV.

        Of course it is also possible that there are more than three neutrinos (but in that case the extra ones are different qualitatively from the first three.) This is a long story. It doesn’t change the fact that the OPERA neutrinos are too light for this to be an issue.

      2. Thanks Charles, Matt.

        Well my source was wikipedia 🙂 – http://en.wikipedia.org/wiki/List_of_particles which has 15.5 Mev as the upper limit to the tau.

        Wikipedia does reference the Particle Data Group, but I can’t see where the 15.5 Mev came from. This should teach me to beware of wikipedia ( or maybe I’ve just misread things ).

        1. Right — In tau lepton decay you get a limit of 15.5 MeV on the mass of any neutrino thereby emitted. BUT a limit does not tell you what the mass is, it just tells you that the mass has to be smaller than this.

          And this is not the strongest existing limit. Limits from cosmology are around an eV.

          So Wikipedia is correct (this time), but your conclusion, that the existence of a limit implies there actually is a neutrino whose mass lies at the limit, is logically invalid.

  4. This whole “speed of light in a vacuum” thing seems sketchy. Have you seen the inside of my vacuum? Of all the things in there, light doesn’t describe any of them. 😉

    1. if you can see the inside of a vacuum,then youre looking at the boson…light is how they are observed,,,have you ever seen the other side of light…?if all 5 agree,,and all share the constant,,,then its the constant,,,speed of light…you know the equations,do you know what they mean??or you just agree with them???

    2. why do we have two w particles,and only one z??or even simpler,,,whats gravity,,,not a description,,or what it does but what is it made up of,,,and where does it come from,,,if its the bending of spacetime,the why was the singularity at the big bang so dense,,,how was spacetime condensed to singularity,,when there was no spacetime beyond that singularity???if you know why,,,then i apologise,,,,please dont take offence,,but i cant agree,with what i know is wrong

      1. Regarding the W’s and Z that is built into the structure of the weak nuclear force, see http://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-apparently-elementary-particles/the-known-particles-if-the-higgs-field-were-zero/

        We do not know if gravity has a more fundamental description, nor do we know what happened at the beginning of the Big Bang. Any speculations about these questions are based on extrapolating the laws of nature that we know so far, or speculative ideas such as string theory, into a situation where we cannot be confident that they still entirely work; there may be other things we need to learn before we can answer such questions.

        1. what would you like to know about the big bang?…an event within absolute singularity,before singularity…like shroedingers cat,,,time was there before space, but neither existed yet….any singularity is absent in the absence of duality.;…you want the big bang,,,all you need to do is ask…i have a graphed illustration of it on my wall…not the easiest concept to express,,,but well founded,and supported….infact,,it is this that leaves me still unconvinced of the results on the neutrino…particles very similar to the wwz,,were the catalysts,and it all links in perfectly together with your current research,,,everything began one singularity of time…even time itself,,i have mentioned this earlier in one of my posts,,,dancingzerosandeternity,,,though i admit little general in expressing it,its not from lack of understanding,a concept not unfamiliar to physics….
          as far as being confident,,, anyone that can understands it would agree,,,the big bang was an event before any other,so there can only be one cause/reason…its actually simplicity at its best,,,thats what makes it so elusive…any complexity has to be assumed wrong.

  5. a1 a
    thinking of a parallax vector >d….,d1
    me(d) is determined by positions a and b,,,one of these being constant,say(b)..the ratio of Me(d)/me(d1),is same as the ratio of a/a1…as d is observed a can be any position determined by position and momentum of observer within system ,,,since b is constant,,d is predetermined by a,,and its velocity,and momentum (p) relative to vp of c(which is unchanging)
    that determines a system of positive mass e+positive vp,,,,
    massless system,,has an unobserved a,and therefore(a/b)=1,,can only be observed at the rate of constant,,therefore is never observed as =,or >1…because d,has no intersecting points of ab so line is infinite
    now if you reverse the parallax b1 a1….the system is still
    d 1< d11 relative to (b)….so while a and b determined the vector,,,and both a,and b are of same coordinates,,the velocity of a relative to b, is greater therefore (a) is observed as b,while the ratio of b1/c is 0…
    in this case a particle of mass at high speed >c…is measured at =,or .00000000000000000~1…..and observed as d1 where the ratio of b1/d1=ratio a1/b1…therefore a1=b1 d1>0o is due constant bbeing observed as the ratio of (b1>c)/c-(d1<c)….

  6. if takes me a while please be patient,,,youll be glad!!! is will was!and will was issed….!!clearer physics will follow as soon as i can put it on pc…

  7. 1. Particle-wave duality,
    2. Lightest fermions (electrons – neutrinos),
    3. Bosons – fermions interplay,
    4. Fermi spheres,
    5. Higgs field,
    6. Vacuum permittivity (ε0) – vacuum permeability (μ0)
    7. Quantum confinement.

    1. Professor, could you put all of the above in a paragraph or two to describe the source of oscillatory nature of electromagnetic radiation?

    2. A simple experiment … A hermetically sealed box, fill it with same diameter ball bearings (very precise spheres at maximum packed lattice with sufficient depth), emit collimated light at one end and monitor the light exiting at a certain radius area on the other end.
    2a) What do you think the pattern would be?
    2b) Is it valid to say that the ball bearings represent the fermions and the cavities between the ball bearings as the bosons?

    ” Quantum mechanics can be used to describe spacetime as being non-empty at extremely small scales, fluctuating and generating particle pairs that appear and disappear incredibly quickly. It has been suggested by some such as Paul Dirac or Robert B. Laughlin, that this quantum vacuum may be the equivalent in modern physics of a particulate aether. However, Dirac’s aether hypothesis was motivated by his dissatisfaction with quantum electrodynamics, and it never gained support by the mainstream scientific community.”

    … “particulate aether” … Has Dirac been proven wrong or is the jury still out?

    1. 1.) Afraid not. Each topic requires an article. But some of those articles are already present on this website.

      2a) The problem isn’t precisely enough specified, but in any case a small calculation would be necessary. Shouldn’t be too hard but it’s not one I’ve done before, and I certainly don’t have much time to spare at the moment.
      2b) No; that would not be accurate at all.
      Finally — the aether of the 19th century broke Lorentz invariance; in Einsteinian relativity there is no such thing. If you want to call the Lorentz-invariance-preserving vacuum of the 20th century an “aether” that’s a choice and you’re free to do it.

      1. e− + e+ → γ + γ

        ” (Paul Dirac, theorizing electron-positron
        annihilation said, “Such process is spontaneous, i.e. it
        occurs necessarily for any pair of electron and
        positron independently of their energy. The process
        does not need any previously existing radiation.”
        (Dirac, 1930, Heitler, 1954).) ”

        “If there is no previously existing radiation and no net linear momentum with the pair electronpositron, then what is the real rest energy of electron
        and positron without association of kinetic energy? ”

        Do physicist know how all this energy (electron) can be confined in such a small space? In other words if a fermion is a compressed spring (since the photons shoot out at the speed of light) then what created it and confined it with such stability?

        1. The question is somehow not the right one.

          For decades, starting right after the discovery of the electron, people tried to see whether they could come up with a dynamical picture of where the electron’s mass (and corresponding rest-energy, or E=mc^2 energy) comes from. It wasn’t the right way to think about the problem.

          Today we understand the electron’s mass as due to its interaction with the non-zero Higgs field. If the Higgs field were non-zero, the electron would be massless. The mass of the electron is more an effect of moving in a medium than it is the effect of somehow getting energy from outside and shoving it into a little container.

          Thinking of an electron as a little ball with energy compressed inside it will confuse you badly. There’s no sign of such a picture in the equations.

          I’m still thinking hard about the article I am going to have to write on particles and fields. Your questions help me see subtleties that I’ll have to explain, somehow…

      2. How about this …

        1. We live in a 3D universe because three are the minimum dimensions required to create massive particles.

        Why …

        2. I believe there is no such condition as a rest state (zero motion), every joules of energy is in motion (in transition). So when any point of energy expands (high density to low density or visa verse) whether it is the Big Bang or electron-positron annihilation, the process is not spontaneous, as Dirac theorized, put occurs in a finite time intervals, Planck’s time, tp, (or lower?). The energy-time uncertainty principle, Delta-E x Delta-t ~ h, also points to this time interval mechanism, i.e. the time in the energy-time uncertainty is the time interval the quantum state remains the same, unchanged. …

        So …

        If the time interval is not zero then there will be an expansion of “space”, Delta-X ~ Delta-t, i.e. motion and hence velocity. I speculate that c is the velocity that “came out of the wash” at the Big Bang, i.e. proportional to the initial conditions at t = 0. Hence if space (variable energy densities) is expanding at a finite velocity, c, then the distances between the isothermal “rays” will increase and hence create normal rays, the radiation is no longer collimated. It is this mechanism that creates vortices, “rotation” and hence oscillatory waves that lead to standing waves and “massive” particles.

        So …

        If I can further my speculation a bit more, I would say as we have seen so far through the SM, quanta, repetitive ans stable states of energy, will exist for the first fundamental “particle” created by the expansion, Higg’s boson.

        Hence, my question to you, Sir. Could the Higg’s boson = Dirac’s particulate aether?

  8. I do feel sorry for the guys who felt it necessary (were pushed?) to resign though. Its not right to be punished for not sitting on your data and it can only make others more scared to talk about any result that contradicts the commonly accepted truths in physics. Who knows, next time it could be a real effect that is measured and it gets missed or only re-discovered many years later due to fear. There has to be sufficient scientific openness to allow other groups to hear about and try and duplicate such effects.

    I know how much you preach caution about rushing to conclusions on the basis of “3 sigma bumps” and OK this seems to me to be a case of someone seriously underestimating their systematic errors so you may not agree?

    1. 1) The guys who resigned mismanaged the public relations issues badly, in my opinion and that of many (though not all) of my colleagues, and they thereby damaged the reputation of the experiment, their immediately colleagues, and even their host laboratories. I think it is appropriate that they stepped down. They are still practicing scientists with academic jobs, but they just aren’t running the experiment right now. They lost face, not pay; they lost power, not career. It seems an appropriate price to me.

      2) I don’t think one should view this as an issue of underestimating systematic uncertainties — it was a plain and simple goof.

      1. am i correct in assuming the mass of neutrino to be <.00000000001??
        and even that does not come with high degree of certainty…
        if that was correct then what is being measured is a fluctuating zero,,,which is exactly what something moving faster than speed of light would measure,,if its speed is not constant…wish i knew how to use a pc better.i could show a diagram..you will receive another reply re this shortly

      2. with all due respect ,i dont know the story behind this,but i know no scientist should be judged for mistakes,,sir isaac newton…galileo galilee…in the history of science,there has been no worse pr nightmares…scientists should not be valued for getting it right,,,rather for what they challenge..pr is for politics,not science.
        again i dont know the story,,but pr should be regadrded with the same importance as kitchen duties,,,mind you,,im sure there are exception

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