“So we’ve got three fundamentally different messengers from the stars, Mr Feder. The past couple of years have given us several encouraging instances of receiving two messengers from the same event. If we ever receive all three messengers from the same event, that might give us what we need to solve the biggest problem in modern physics.”
“That’s a pretty deep statement, Moire. Care to unpack it? The geese here would love to hear about it.”
“Lakeside is a good place for thoughts like this. The first messenger was photons. We’ve been observing starlight photons for tens of thousand of years. Tycho Brahe and Galileo took it to a new level a few centuries ago with their careful observation, precision measurements and Galileo’s telescope.”
“That’s done us pretty good, huh?”
“Oh sure, we’ve charted the heavens and how things move, what we can see of them. But our charts imply there’s much we can’t see. Photons only interact with electric charge. Except for flat-out getting absorbed if the wavelength is right, photons don’t care about electrically neutral material and especially they don’t care about dark matter.”
“So that’s why we’re interested in the other messengers.”
“Exactly. Even electrically neutral things have mass and interact with the gravitational field. You remember the big news a few years ago, when our brand-new LIGO instruments caught a gravitational wave signal from a couple of black holes in collision. Black holes don’t give off photons, so the gravitational wave messenger was our only way of learning about that event.”
“No lightwave signal at all?”
“Well, there was a report of a possible gamma-ray flare in that patch of sky, but it was borderline-detectable. No observatory using lower-energy light saw anything there. So, no.”
“You’re gonna tell me and the geese about some two-messenger event now, right?”
“That’s where I’m going, Mr Feder. Photons first. Astronomers have been wondering for decades about where short, high-energy gamma-ray bursts come from. They seem to happen randomly in time and space. About a year ago the Fermi satellite’s gamma-ray telescope detected one of those bursts and sent out an automated ‘Look HERE’ alert to other observatories. Unfortunately, Fermi‘s resolution isn’t wonderful so its email pointed to a pretty large patch of sky. Meanwhile back on Earth and within a couple of seconds of Fermi‘s moment, the LIGO instruments caught an unusual gravitational wave signal that ran about a hundred times slower than the black-hole signals they’d seen. Another automated ‘Look HERE’ alert went out. This one pointed to a small portion of that same patch of sky. Two messengers.”
“Did anyone find anything?”
“Seventy other observatories scrutinized the overlap region at every wavelength known to Man. They found a kilonova, an explosion of light and matter a thousand times brighter than typical novae. The gravitational wave evidence indicated a collision between two neutron stars, something that had never before been recorded. Photon evidence from the spewed-out cloud identified a dozen heavy elements theoreticians hadn’t been able to track to an origin. Timing details in the signals gave cosmologists an independent path to resolving a problem with the Hubble Constant. And now we know where those short gamma-ray bursts come from.”
“Pretty good for a two-messenger event. Got another story like that?”
“A good one. This one’s neutrinos and photons, and the neutrinos came in first. One neutrino.”
“One neutrino?”
“Yup, but it was a special one, a super-high-powered neutrino whose incoming path our IceCube observatory could get a good fix on. IceCube sent out its own automated ‘Look HERE’ alert. The Fermi team picked up the alert and got real excited because the alert’s coordinates matched the location of a known and studied gamma-ray source. Not a short-burster, but a flaring blazar. That neutrino’s extreme energy is evidence for blazars being one of the long-sought sources of cosmic rays.”
“Puzzle solved, maybe. Now what you said about a three-messenger signal?”
“Gravitational waves are relativity effects and neutrinos are quantum mechanical. Physicists have been struggling for a century to bridge those two domains. Evidence from a three-messenger event could provide the final clues.”
“I’ll bet the geese enjoyed hearing all that.”
“They’re grebes, Mr Feder.”
~~ Rich Olcott
“Wait, Mr Moire, we said that the event horizon’s just a mathematical construct, 
Their common experimental strategy sounds simple enough — compare two beams of light that had traveled along different paths
The second question is harder. The best the aLIGO team could do was point to a “banana-shaped region” (their words, not mine) that covers about 1% of the sky. The team marshaled a world-wide collaboration of observatories to scan that area (a huge search field by astronomical standards), looking for electromagnetic activities concurrent with the event they’d seen. Nobody saw any. That was part of the evidence that this collision involved two black holes. (If one or both of the objects had been something other than a black hole, the collision would have given off all kinds of photons.)
In contrast, a LIGO facility is (roughly speaking) omni-directional. When a LIGO installation senses a gravitational pulse, it could be coming down from the visible sky or up through the Earth from the other hemisphere — one signal doesn’t carry the “which way?” information. The diagram above shows that situation. (The “chevron” is an image of the LIGO in Hanford WA.) Models based on the signal from that pair of 4-km arms can narrow the source field to a “banana-shaped region,” but there’s still that 180o ambiguity.
The great “if only” is that the VIRGO installation in Italy was not recording data when the Hanford WA and Livingston LA saw that September signal. With three recordings to reconcile, the aLIGO+VIRGO combination would have had enough information to slice that banana and localize the event precisely.
We can investigate things that take longer than an instrument’s characteristic time by making repeated measurements, but we can’t use the instrument to resolve successive events that happen more quickly than that. We also can’t resolve events that take place much closer together than the instrument’s characteristic length.
Almost a century later, James Clerk Maxwell (the bearded fellow at left) wrote down his electromagnetism equations that explain how light works. Half a century later, Einstein did the same for gravity.
Gravitodynamics is completely unlike electrodynamics. Gravity’s transverse “force” doesn’t act to move a whole mass up and down like Maxwell’s picture at left. Instead, as shown by Einstein’s picture, gravitational waves stretch and compress while leaving the center of mass in place. I put “force” in quotes because what’s being stretched and compressed is space itself. See 
The experiment consists of shooting laser beams out along both arms, then comparing the returned beams.
Hard Science Ain't Hard 
