Heavenly Messengers

A gorgeous Fall day, a little bit cool-ish, perfect for a brisk walk in the park.  I’m striding along the lake-bound path when there’s a breathless shout behind me.  “Hey, Moire, wait up!  I got questions!”

“Hello, Mr Feder.  What’s the topic this time?  And keep up, please, I’ve got geese to watch.”

“I been reading in the business pages <puff, puff> about all the money different countries are putting into ‘multi-messenger astronomy.’  <puff>  What’s that about, anyway?  Who’s sending messages and ain’t the Internet good enough?”

“It’s not who, Mr Feder, it’s what — stars, galaxies, black holes, the Universe.  And the messages are generally either ‘Here I am‘ or ‘Something interesting just happened‘.  The Internet just doesn’t reach that far and besides, no kitten pictures.”

“Pretty simple-sounding messages, so why the big bucks for extra message-catchers?”

“Fair question.  It has to do with what kind of information each messenger carries.  Photons, for instance.”

“Yeah, light-waves, the rainbow.”

“Way more than the rainbow.  Equating light-waves to just the colors we see is like equating sound-waves to just the range from A4 through F4# on a piano.”

“Hey, that’s less than an octave.”

“Yup, and electromagnetism’s scale is hugely broader than that.  Most of the notes, or colors, are way out of our range.  A big tuba makes a deep, low-frequency note but a tiny piccolo makes a high note.  Photon characteristics also scale with the size of where they came from.  Roughly speaking, the shorter the light’s wave-length, the smaller the process it came out of and the smaller its target will be.  Visible light, for instance, is sent and received by loosely-held charge sloshing inside an atom or molecule.  Charge held tight to a nucleus gives rise to higher-energy photons, in the ultra-violet range or beyond.”

“Like how beyond?”

“X-rays can rip electrons right out of a molecule.  Gamma rays are even nastier and involve charge activity inside a nucleus, like during a nuclear reaction.”

“How about in the other direction?  Nothing?”

“Hardly.  Going that way is going to bigger scales.  Infra-red is about parts of molecules vibrating against each other, microwave is about whole molecules rotating.  When your size range gets out to feet-to-miles you’re looking at radio waves that probably originated from free electrons or ions slammed back and forth by electric or magnetic fields.”

“So these light ranges are like messengers that clue us in on what’s going on out there?  Different messengers, different kindsa clues?”

“You got the idea.  Add in that what happens to the light on the way here is also important.  Radio and microwave photons with their long wavelengths swerve around dust particles that block out shorter-wavelength ones.  Light that traversed Einstein-bent space lets us measure the masses of galaxies.  Absorption and polarization at specific wavelengths tell us what species are out there and what they’re doing.  Blue-shifts and red-shifts tell us how fast things are moving towards and away from us.  And of course, atmospheric distortions tell us we’ve got to put satellite observatories above the atmosphere to see better.”

“One messenger, lots of effects.”

“Indeed, but in the past few years we’ve added two more, really important messengers.  Photons are good, but they’re limited to just one of the four fundamental forces.”

“Hey, there’s gotta be more than that.  This is a complicated world.”

“True, but physicists can account for pretty much everything at the physical and chemical level with only four — electromagnetism, gravity, the strong force that holds nuclei together and the weak force that’s active in nuclear transformation processes.  Photons do electromagnetism and that’s all.”

“So you’re saying we’ve got a line on two of the others?”

“Exactly.  IceCube and its kin record the arrival of high-energy neutrinos.  In a sense they are to the weak force what photons are to electromagnetism.  We don’t know whether gravitation works through particles, but LIGO and company are sensitive to changes in the gravitational field that’s always with us.  Each gives us a new perspective on what’s happening out there.”

“So if you get a signal from one of the new messengers at the same time you get a photon signal…”

“Oh, look, the geese are coming in.”Heavenly messengers

~~ Rich Olcott


The Neapolitan Particle

“Welcome back, Jennie.  Why would anyone want to steer an ice cube?

“Thanks, Jeremy, it’s nice to be back..  And the subject’s not an ice cube, it’s IceCube, the big neutrino observatory in the Antarctic.”

“Then I’m with Al’s question.  Observatories have this big dome that rotates and inside there’s a lens or mirror or whatever that goes up and down to sight on the night’s target.  OK, the Hubble doesn’t have a dome and it uses gyros but even there you’ve got to point it.  How does IceCube point?”

“It doesn’t.  The targets point themselves.”


“Ever relayed a Web-page?”


“Guess what?  You don’t know where the page came from, you don’t know where it’s going to end up.  But it could carry a tracking bug to tell someone at some call-home server when and where the page had been opened.  IceCube works the same way, sort of.  It has a huge 3D array of detectors to record particles coming in from any direction.  A neutrino can come from above, below, any side, no problem — the detectors it touches will signal its path.”

IceCube architecture

Adapted from a work by Francis Halzen, Department of Physics, University of Wisconsin

“How huge?”

“Vastly huge.  The instrument is basically a cubic kilometer of ultra-clear Antarctic ice that’s ages old.  The equivalent of the tracking bugs is 5000 sensors in a honeycomb array more than a kilometer wide.  Every hexagon vertex marks a vertical string of sensors going down 2½ kilometers into the ice.  Each string has a couple of sensors near the surface but the rest of them are deeper than 1½ kilometers.  The sensors are looking for flashes of light.  Keep track of which sensor registered a flash when and you know the path a particle took through the array.”

icecube event 3“Why should there be flashes? I thought neutrinos didn’t interact with matter.”

“Make that, they rarely interact with matter.  Even that depends on what particle the neutrino encounters and what flavor neutrino it happens to be at the moment.”

That gets both Al and me interested.  His “Neutrinos come in flavors?” overlaps my “At the moment?”

“I thought that would get you into this, Sy.  Early experiments detected only 1/3 of the neutrinos we expected to come from the Sun.  Unwinding all that was worth four Nobel prizes and counting.  The upshot’s that there are three different neutrino flavors and they mutate.  The experiments caught only one.”

Vinnie’s standing behind us.  “You’re going to tell us the flavors, right?”

“Hoy, Vinnie, Jeremy’s question was first, and it bears on the others.  Jeremy, you know that blue glow you see around water-cooled nuclear fuel rods?”

“Yeah, looks spooky.  That’s neutrinos?”

“No, that’s mostly electrons, but it could be other charged particles.  It has to do with exceeding the speed of light in the medium.”

“Hey, me and Sy talked about that.  A lightwave makes local electrons wiggle, and how fast the wiggles move forward can be different from how fast the wave group moves.  Einstein’s speed-of-light thing was about the wave group’s speed, right, Sy?”

“That’s right, Vinnie.”

“So anyhow, Jeremy, a moving charged particle affects the local electromagnetic field.  If the particle moves faster than the surrounding atoms can adjust, that generates light, a conical electromagnetic wave with a continuous spectrum.  The light’s called Cherenkov radiation and it’s mostly in the ultra-violet, but enough leaks down to the visible range that we see it as blue.”

“But you said it takes a charged particle.  Neutrinos aren’t charged.  So how do the flashes happen in IceCube?”

“Suppose an incoming high-energy neutrino transfers some of its momentum to a charged particle in the ice — flash!  Even better, the flash pattern provides information for distinguishing between the neutrino flavors.  Muon neutrinos generate a more sharp-edged Cherenkov cone than electron neutrinos do.  Taus are so short-lived that IceCube doesn’t even see them.”Leptons

“I suppose muon and tau are flavors?”

“Indeed, Vinnie.  Any subatomic reaction that releases an electron also emits an electron-flavored neutrino.  If the reaction releases the electron’s heavier cousin, a muon, then you get a muon-flavored neutrino.  Taus are even heavier  and they’ve got their own associated neutrino.”

“And they mutate?”

“In a particularly weird way.”

~~ Rich Olcott

Bigger than you’d think

Al’s coffee shop, the usual mid-afternoon crowd of chatterers and laptop-tappers.  Al’s walking his refill rounds, but I notice he’s carrying a pitcher rather than his usual coffee pot.  “Hey, Al, what’s with the hardware?”

“Got iced coffee here, Sy.  It’s hot out, people want to cool down.  Besides, this is in honor of IceCube.”

“Didn’t realize you’re gangsta fan.”

“Nah, not the rapper, the cool experiment down in the Antarctic.  It was just in the news.”

“Oh?  What did they say about it?”

“It’s the biggest observatory in the world, set up to look for the tiniest particles we know of, and it uses a cubic mile of ice which I can’t think how you’d steer it.”

A new voice, or rather, a familiar one. “One doesn’t, Al.”
Neutrino swirl 1“Hello, Jennie.  Haven’t seen you for a while.”

“I flew home to England to see my folks.  Now I’m back here for the start of the Fall term.  I’ve already picked a research topic — neutrinos.  They’re weird.”

“Hey, Jennie, why are they so tiny?”

“It’s the other way to, Al.  They’re neutrinos because they’re so tiny.  Sy would say that for a long time they were simply an accounting gimmick to preserve the conservation laws.”

“I would?”

“Indeed.  People had noticed that when uranium atoms give off alpha particles to become thorium, the alpha particles always have about the same amount of energy.  The researchers accounted for that by supposing that each kind of nucleus has some certain quantized amount of internal energy.  When one kind downsizes to another, the alpha particle carries off the difference.”

“That worked well, did it?”

“Oh, yes, there are whole tables of nuclear binding energy for alpha radiation.  But when a carbon-14 atom emits a beta particle to become nitrogen-14, the particle can have pretty much any amount of energy up to a maximum.  It’s as though the nuclear quantum levels don’t exist for beta decay.  Physicists called it the continuous beta-spectrum problem and people brought out all sorts of bizarre theories to try to explain it.  Finally Pauli suggested maybe something we can’t see carries off energy and leaves less for the beta.  Something with no charge and undetectable mass and the opposite spin from what the beta has.”

“Yeah, that’d be an accounting gimmick, alright.  The mass disappears into the rounding error.”

“It might have done, but twenty years later they found a real particle.  Oh, I should mention that after Pauli made the suggestion Fermi came up with a serious theory to support it.  Being Italian, he gave the particle its neutrino name because it was neutral and small.”

“But how small?”

“We don’t really know, Al.  We know the neutrino’s mass has to be greater than zero because it doesn’t travel quite as fast as light does.  On the topside, though, it has to be lighter than than a hydrogen atom by at least a factor of a milliard.”


“Oh, sorry, I’m stateside, aren’t I?  I should have said a billion.  Ten-to-the-ninth, anyway.”

“That’s small.  I guess that’s why they can sneak past all the matter in Earth like the TV program said and never even notice.”

This gives me an idea.  I unholster Old Reliable and start to work.

“Be right with you… <pause> … Jennie, I noticed that you were being careful to say that neutrinos are light, rather than small.  Good careful, ’cause ‘size’ can get tricky at this scale.  In the early 1920s de Broglie wrote that every particle is associated with a wave whose wavelength depends on the particle’s momentum.  I used his formula, together with Jennie’s upper bound for the neutrino’s mass, to calculate a few wavelength lower bounds.Neutrino wavelength calcMomentum is velocity times mass.  These guys fly so close to lightspeed that for a long time scientists thought that neutrinos are massless like photons.  They’re not, so I used several different v/c ratios to see what the relativistic correction does.  Slow neutrinos are huge, by atom standards.  Even the fastest ones are hundreds of times wider than a nucleus.”

“With its neutrino-ness spread so thin, no wonder it’s so sneaky.”

“That may be part of it, Al.”

“But how do you steer IceCube?”

~~ Rich Olcott