Neutrinos From the Sky

It’s been hard to find time for blogging, but there’s one story I don’t want to let slip by before the end of the year: the observation by Ice Cube of neutrinos from beyond the Solar System.

It was my own bad sense of timing to blog about Ice Cube mere days before they announced this result — but just to mention the fun fact that they confirmed the existence of the Moon. And, like noticing the Moon, there’s a sense in which we shouldn’t be too surprised — we were pretty confident that neutrinos were in fact raining down upon us from the sky all the time. But that’s a bad attitude, because this is a big deal. It’s a new way of looking at the universe, and historically new ways of looking at the universe have always brought us surprises and new insights of one form or another.

The actual process by which Ice Cube determined that they had found cosmic neutrinos is a bit convoluted, so let’s go through it. For one thing, the detector doesn’t “see” neutrinos directly. It sees Cherenkov radiation, which is emitted when a charged particle moves through a medium at a speed faster than the velocity of light in that medium. (Nothing moves faster than light moves in vacuum, but the speed of light in ice is lower than in vacuum.) Neutrinos, you may have figured from the name, are neutral particles, not charged ones. So what you’re actually seeing are events where a neutrino bumps into one of the water molecules in the ice and creates some charged particles.

But most of the neutrinos you detect by this method are not really cosmic. They’re byproducts of cosmic rays — mostly charged particles flying through space at enormous energies, which smash into Earth’s atmosphere, creating neutrinos (and various other particles) along the way. So a cosmic ray interacts with the atmosphere, creating a neutrino, which then interacts with the ice to make charged particles we can observe. Ice Cube sees these “atmospheric neutrinos” all the time; indeed, it makes maps of them. And that’s great, and certainly helps teach us something about cosmic rays. But it would still be cool to find some neutrinos that have themselves made the long journey across the desolate cold of interstellar space. And that’s not easy; even if the detector finds some, they are likely to be swamped by the bountiful atmospheric beasts.

Enter Bert and Ernie.


Those are the colorful names given to two events observed over the last couple of years by Ice Cube. What makes them remarkable is their very high energies; over 30 trillion electron volts (TeV). (Francis Halzen, doyen of the experiment, “takes no responsibility” for the whimsical names.) That’s a lot more than you would expect from atmospheric neutrinos, but right in line for the most energetic cosmic neutrinos we predicted. But it’s only two events; the finding was announced earlier this year, but like good cautious scientists the collaboration didn’t quite say they were sure the events were cosmic in origin. (Note that a “cosmic neutrino” is one that traveled across the cosmos by itself, not one that was produced by a cosmic ray — sorry for the confusing nomenclature, it’s a cosmic world out there.)

Now we can do better. In November, right after my blog post about the Moon, Ice Cube announced that they had more data, and were able to identify another twenty-six events at very high energies. They put the confidence that these are truly cosmic neutrinos at four sigma — perhaps not quite the five-sigma gold standard we would like to reach, but pretty darn convincing (especially where anything astrophysical is concerned).

This result opens up a new era in astronomy. We can now look at the universe with neutrino eyes. Previously we had discovered neutrinos from the Sun, as well as the lucky few from Supernova 1987A, but now we apparently have a persistent source of these elusive particles from very far away. Perhaps from the center of our galaxy, or perhaps from hyper-energetic events in galaxies well outside our own. At the very least this kind of work should teach us something about the origin of cosmic rays themselves, and who knows what else.

I’m not sure whether to feel happy or sorry for Bert and Ernie themselves. Born in a cosmic cataclysm half a universe away, they sped through billions of miles of empty space, witnessing untold astronomical wonders, only to come crashing into the ice on a fairly run-of-the-mill planet. But at least they brought more than a little joy to the hearts of some curious scientists, which is more than most particles can say.

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21 Responses to Neutrinos From the Sky

  1. Gizelle Janine says:

    Simply put: Seriously awesome. Believe me, I’m just sitting on my couch reading for the rest of the year. *eats popcorn*

  2. FrankL says:

    So what would a neutrino-detecting satellite (to avoid atmospheric problems) with the same good sigmas look like?

  3. sohbet says:

    Thank for emoution.

  4. Navneeth says:

    “…only to come crashing into the ice on a fairly run-of-the-mill planet.”

    I can sense the attempt at dry humour, but seriously, the Earth is the most interesting planet we know*. As for the star around which this planet revolves…

    *And I’m certain you knew that. 😉

    The IceCube experiment’s ‘telescope’/’collider’, as it were, is made up of few cubic kilometres of Antarctic ice. I’m not sure how, at this point in time, one can replicate that in space.

    P.S.: I should also be able to use the tag in the comment.

  5. Ernest says:

    dear Sean,

    thank you so much for your site. I tremble in awe at your presentations of our discoveries, the true heritage of our generation to those that follow.

    is there any recent data on the observation that neutrinos accompanying solar flares (or some other particle) influence radioactivity?

    and may I ask a quick off-topic question? what is roughly the mass of a proton moving at 0.99 999 999 times the speed of light? in discussing the LHC and the Higgs mechanism (which I consider the intellectual achievement of my lifetime) this question is “emergent.”

    deep regards,


  6. Sean Carroll says:

    Ernest– I’m personally very skeptical that solar flares influence radioactivity. There’s no sensible mechanism for it to happen. About the proton, I like to say that the mass is just constant, but it’s the energy that changes. The protons moving in the LHC have energies of about 8 trillion electron volts, compared to the proton mass which is a bit less than one billion electron volts.

    FrankL– As Navneeth says, to detect neutrinos you need a giant amount of detector, since they interact so rarely. Space isn’t really the way to go.

  7. Platohagel says:

    Sean said,” So a cosmic ray interacts with the atmosphere, creating a neutrino, which then interacts with the ice to make charged particles we can observe.”

    This distinction was important in the mapping process for me, so I thank you for this clarification.

    See Also : AIRES Cosmic Ray Showers

  8. Ernest says:

    dear Sean,

    thank you. I’m a layman here, but I’m gobsmacked by the incandescent beauty of this human achievement, the confirmation of the keystone of the Standard Model. I need an analogy I can almost grasp, however coarse and lacking in accuracy and elegance you must see it.

    I’ve heard you say that the energy of the photons in the collider roughly match the energy of a freight train moving 100 mph. I’m trying to take this analogy to the granularity of single collisions.

    the constituents of a typical atom, carbon,weighs on the order of 12 billion electron volts. roughly 667 carbon atoms/masses/energies add to 8 trillion. so, by way of analogy, we’re smashing together two particles the size of protons with the mass/energy of fairly large Bucky-balls?
    I’m almost embarrassed to bug you again, but I’m doomed to need to know.

    and seasons greetings.

  9. Ernest says:

    my understanding is this is not too large a macroscopic object to demonstrate coherence and interfere with itself in a double slit experiment.

    the reason I was thinking about molecular (bonded) Bucky-balls …

  10. Ernest says:

    you win.

    there are no particles. just excitations on waves. the waves of the phalanges of my hand salute you.

  11. Marc Sher says:

    Sean—I like your last paragraph. But think of the CMB photons discovered by Penzias and Wilson. Produced in the fires of the big bang, traveling though billions of light years, through galaxies forming, superclusters forming, traveling unimpeded for all that time, only to end up in New Jersey.

  12. Ernest says:

    Marc, it can always be worse. I built a computer network in the Dominican Republic for two of our govt agencies.

    I’ve never seen the Lorentz factor in play and find deep mystery in the fact M(relatively fast)=LFxM(at rest). so the LHC Lorentz factor of 7,500 x 0.9383 GeV/c = approx. 7 TeV/c per proton. Given two of these slamming together, I wonder why “more” stuff doesn’t pop out.

    i’ll be out of your lab soon. but it was wonderful to visit. thanks for letting me sit in the back of the room and ask low-knowledge questions at the mike after the lecture is over.

  13. Navneeth says:

    Look on the positive side, someone somewhere ‘more special’ probably received a couple of photons in their TV static much earlier than ’64-65. 😀

  14. milkshake says:

    Where do you get your accent from?

  15. Ernest says:

    Dear Doctor Carroll,

    I apologize if I’ve been stumbling about in your lab. you are a national treasure, and your ability to render accessible what asymptotically approaches the incomprehensible is rarely paralleled in my world. i directly attribute to your presentations and writings one of my most profound “ah hah!” moments. i’ve really just entered your lab to say thank you. you have accepted an incredibly difficult task and i owe you gratitude.

    i’ve made my living at the level of electromagnetism. while i’ve been paid to be a computer network architect of fairly large networks, my hobby has always been building RF devices and my own little lab is full of them and the books and notes that represent my tiny comprehensions of what they are actually “doing.”

    but i’ve never successfully “dropped down” to the next level where i could comprehend the dance of photons that mediated the attraction of a refrigerator to a magnet, nor instantiated a mental model of how two interacting fields (electric and magnetic) could “conspire” to make the equivalent of a gravity well (complete with an inverse-square law) that would pull magnets together.

    i sometimes feel a bit like Moses, struggling at the edge of the promised land (quantum mechanics) but unable even to stand at the outer perimeter of the exquisite relations represented by the fundamental equations. I can accept that because it doesn’t lock me out of the mystery and poetry; I can feel the trembling awe.

    thank you for your efforts to pull all of us toward this knowledge, the true deep lovely incomprehensibly beautiful heritage of our species.


  16. Do Neutrinos always clump into spheres whilst photons always radiate away from spheres?

  17. Instead of your readers Disliking my last question, can someone please answer it? (How can you sensibly “dislike” a genuine request for information?) Even if it is stupid, I would like to know the reason for its stupidity.

  18. Eric says:

    @deeponics: The answer to your question is: “they don’t”. (That’s not “not always”; no, they simply don’t.)

    To be blunt, the question shows a misunderstanding of basic concepts of physics which can’t be remedied in a blog comment. If your request for information indeed is genuine, I’d suggest you pick up a few popular science books. Books by authors such as Carl Sagan, Sean Carroll (obviously), Lawrence Krauss, Brian Cox, and Neil DeGrasse Tyson. The Feynman Lectures on Physics are a bit old but still relevant; Penguin books has published them in a series of six or seven smaller books which are remarkably readable even for a layman.

  19. Pingback: Morsels For The Mind – 27/12/2013 › Six Incredible Things Before Breakfast

  20. @Eric Thank you. I really appreciate your interest. My understanding is that Neutrinos germinate from post-nuclear fusion, from past conflagration or collapsing stars or post nuclear explosions anywhere else; that Neutrinos are ubiquitous; that they travel near light speed and that they travel through everything in the cosmos. However, what I do not understand is the raison-d’etre as to why they seem not to travel backwards into sources of nuclear fission. I can understand that photons will never light up the Sun – they radiate out, hence my question, which could have been better constructed to read “Do Neutrinos, like photons, always travel towards cold increasing mass and never into fusing mass that is reducing?”
    By the way, I have always looked upon Blogs as being vehicles for two-way traffic; to develop ideas and conversation, so, hopefully someone will help me out of my dilemma by answering my re-constructed question.

  21. John Duffield says:

    Deeponics: re “Do Neutrinos, like photons, always travel towards cold increasing mass and never into fusing mass that is reducing?” The answer is no, and that photons don’t do this either. Generally speaking photons and neutrinos are emitted when some “subatomic change” occurs, and there’s a lot of this going on in the Sun. It takes a long time for a photon to get out of the Sun, but this isn’t true for neutrinos. Once they’re out of the Sun they move at pretty much c in pretty much a straight line. Other photons and neutrinos which didn’t start in the Sun do this too. Some will travel into another star, which is a “fusing mass that is reducing”. Some will travel to the Earth, such as into IceCube where they may cause some subatomic change.

    Blogs aren’t really a vehicle for two-way traffic I’m afraid. They’re primarily a vehicle for the blogger. You want a physics discussion forum for two-way traffic.