Beyond The Shadow of A…?

“Alright, Vinnie, what’s the rest of it?”

“The rest of what, Sy?”

“You wouldn’t have hauled that kid’s toy into Al’s shop here just to play spitballs with it. You’re building up to something and what is it?”

“My black hole hobby, Sy. The things’re just a few miles wide but pack more mass than the Sun. A couple of my magazines say they give off jets top and bottom because of how they spin. That just don’t fit. The stuff ought to come straight out to the sides like the paper wads did.”

“Well, umm… Ah. You know the planet Saturn.”

“Sure.”

“Are its rings part of the planet?”

“No, of course not, they go around it. I even seen an article about how the rings probably came from a couple of collided moons and how water from the Enceladus moon may be part of the outside ring. Only thing Saturn does for the rings is supply gravity to keep ’em there.”

“But our eyes see planet and rings together as a single dot of light in the sky. As far as the rest of the Solar System cares, Saturn consists of that big cloudy ball of hydrogen and the rings and all 82 of its moons, so far. Once you get a few light-seconds away, the whole collection acts as a simple point-source of gravitational attraction.”

“I see where you’re going. You’re gonna say a black hole’s more than just its event horizon and whatever it’s hiding inside there.”

“Yup. That ‘few miles wide’ — I could make a case that you’re off by trillions. A black hole’s a complicated beast when we look at it close up.”

“How can you look at a thing like that close up?”

“Math, mostly, but the observations are getting better. Have you seen the Event Horizon Telescope’s orange ring picture?”

“You mean the one that Al messed with and posted for Hallowe’en? It’s over there behind his cash register. What’s it about, anyway?”

“It’s an image of M87*, the super-massive black hole at the center of the M87 galaxy. Not the event horizon itself, of course, that’s black. The orange portion actually represents millimeter-radio waves that escape from the accretion disk circling the event horizon. The innermost part of the disk is rotating around the hole at near-lightspeed. The arc at the bottom is brighter because that’s the part coming toward us. The photons get a little extra boost from Special Relativity.”

Frames again?”

“With black holes it’s always frames. You’ll love this. From the shell’s perspective, it spits out the same number of photons per second in every direction. From our perspective, time is stretched on the side rotating away from us so there’s fewer photons per one of our seconds and it’s dimmer. In the same amount of our time the side coming toward us emits more photons so it’s brighter. Neat demonstration, eh?”

“Cute. So the inner black part’s the hole ’cause it can’t give off light, right?”

“Not quite. That’s a shadow. Not a shadow of the event horizon itself, mind you, but of the photon sphere. That’s a shell about 1½ times the width of the event horizon. Any photon that passes exactly tangent to the sphere is doomed to orbit there forever. If the photon’s path is the slightest bit inward from that, the poor particle heads inward towards whatever’s in the center. The remaining photons travel paths that look bent to a distant observer, but the point is that they keep going and eventually someone like us could see them.”

“The shadow and the accretion disk, that’s what the EHT saw?”

“Not exactly.”

“There’s more?”

“Yeah. M87* is a spinning black hole, which is more complicated than one that’s sitting still. Wrapped around the photon sphere there’s an ergosphere, as much as three times wider than the event horizon except it’s pumpkin-shaped. The ergosphere’s widest at the rotational equator, but it closes in to meet the event horizon at the two poles. Anything bigger than a photon that crosses its boundary is condemned to join the spin parade, forever rotating in sync with the object’s spin.”

“When are you gonna get to the jets, Sy?”

~~ Rich Olcott

Three Shades of Dark

The guy’s got class, I’ll give him that. Astronomer-in-training Jim and Physicist-in-training Newt met his challenges so Change-me Charlie amiably updates his sign.

But he’s not done. “If dark matter’s a thing, how’s it different from dark energy? Mass and energy are the same thing, right, so dark energy’s gotta be just another kind of dark matter. Maybe dark energy’s what happens when real matter that fell into a black hole gets squeezed so hard its energy turns inside out.”

Jim and Newt just look at each other. Even Cap’n Mike’s boggled. Someone has to start somewhere so I speak up. “You’re comparing apples, cabbages and fruitcake. Yeah, all three are food except maybe for fruitcake, but they’re grossly different. Same thing for black holes, dark matter and dark energy — we can’t see any of them directly but they’re grossly different.”

EHT's image of the black hole at the center of the Messier 87 galaxy
Black hole and accretion disk, image by the Event Horizon Telescope Collaboration

Vinnie’s been listening off to one side but black holes are one of his hobbies. “A black hole’s dark ’cause its singularity’s buried inside its event horizon. Whatever’s outside and somehow gets past the horizon is doomed to fall towards the singularity inside. The singularity itself might be burn-your-eyes bright but who knows, ’cause the photons’re trapped. The accretion disk is really the only lit-up thing showing in that new EHT picture. The black in the middle is the shadow of the horizon, not the hole.”

Jim picks up the tale. “Dark matter’s dark because it doesn’t care about electromagnetism and vice-versa. Light’s an electromagnetic wave — it starts when a charged particle wobbles and it finishes by wobbling another charged particle. Normal matter’s all charged particles — negative electrons and positive nuclei — so normal matter and light have a lot to say to each other. Dark matter, whatever it is, doesn’t have electrical charges so it doesn’t do light at all.”

“Couldn’t a black hole have dark matter in it?”

“From what little we know about dark matter or the inside of a black hole, I see no reason it couldn’t.”

“How about normal matter falls in and the squeezing cooks it, mashes the pluses and minuses together and that’s what makes dark matter?”

“Great idea with a few things wrong with it. The dark matter we’ve found mostly exists in enormous spherical shells surrounding normal-matter galaxies. Your compressed dark matter is in the wrong place. It can’t escape from the black hole’s gravity field, much less get all the way out to those shells. Even if it did escape, decompression would let it revert to normal matter. Besides, we know from element abundance data that there can’t ever have been enough normal matter in the Universe to account for all the dark matter.”

Newt’s been waiting for a chance to cut in. “Dark energy’s dark, too, but it works in the opposite direction from the other two. Gravity from normal matter, black holes or otherwise, pulls things together. So does gravity from dark matter which is how we even learned that it exists. Dark energy’s negative pressure pulls things apart.”

“Could dark energy pull apart a black hole or dark matter?”

Big Cap’n Mike barges in. “Depends on if dark matter’s particles. Particles are localized and if they’re small enough they do quantum stuff. If that’s what dark matter is, dark energy can move the particles apart. My theory is dark matter’s just ripples across large volumes of space so dark energy can change how dark matter’s spread around but it can’t break it into pieces.”

Vinnie stands up for his hobby. “Dark energy can move black holes around, heck it moves galaxies, but like Sy showed us with Old Reliable it’s way too weak to break up black holes. They’re here for the duration.”

Newt pops him one. “The duration of what?”

“Like, forever.”

“Sorry, Hawking showed that black holes evaporate. Really slowly and the big ones slower than the little ones and the temperature of the Universe has to cool down a bit more before that starts to get significant, but not even the black holes are forever.”

“How long we got?”

“Something like 10106 years.”

“That won’t be dark energy’s fault, though.”

~~ Rich Olcott

Dark Horizon

Charlie's table sign says "Dark Energy is bogus"

Change-me Charlie attacks his sign with a rag and a marker, rubbing out “Matter” and writing in “Energy.” Turns out his sign is a roll-up dry-erase display and he can update it on site. Cool. I guess with his rotating-topic strategy he needs that. “OK, maybe dark matter’s a thing, but dark energy ain’t. No evidence, someone just made that one up to get famous!”

And of course Physicist-in-training Newt comes back at him. “Lots of evidence. You know about the Universe expanding?”

“Prove it.” At least he’s consistent.

<sigh> “You know how no two snowflakes are exactly alike but they can come close? It applies to stars, too. Stars are fairly simple in a complicated way. If you tell me a star’s mass, age and how much iron it has, I can do a pretty good job of computing how bright it is, how hot it is, its past and future life history, all sort of things. As many stars as there are, we’re pretty much guaranteed that there’s a bunch of them with very similar fundamentals.”

“So?”

“So when a star undergoes a major change like becoming a white dwarf or a neutron star or switching from hydrogen fusion to burning something else, any other star that has the same fundamentals will behave pretty much the same way. They’d all flare with about the same luminosity, pulsate with about the same frequency —”

“Wait. Pulsate?”

“Yeah. You’ve seen campfires where one bit of flame coming out of a hotspot flares up and dies back and flares up and dies back and you get this pulsation —”

“Yeah. I figured that happens with a sappy log where the heat gasifies a little sap then the spot cools off when outside air gets pulled in then the cycle goes again.”

“That could be how it works, depending. Anyhow, a star in the verge of mode change can go through the same kind of process — burn one kind of atom in the core until heat expansion pushes fuel up out of the fusion zone; that cools things down until fuel floods back in and off we go again. The point is, that kind of behavior isn’t unique to a single star. We’ve known about variable stars for two centuries, but it wasn’t until 1908 that Henrietta Swan Leavitt told us how to determine a particular kind of variable star’s luminosity from its pulsation frequency.”

“Who cares?”

“Edwin Hubble cared. Brightness dies off with the distance squared. If you compare the star’s intrinsic luminosity with how bright the star appears here on Earth, it’s simple to calculate how far away the star is. Hubble did that for a couple dozen galaxies and showed they had to be far outside the Milky Way. He plotted red-shift velocity data against those distances and found that the farther away a galaxy is from us, the faster it’s flying away even further.”

“A couple dozen galaxies ain’t much.”

“That was for starters. Since the 1930s we’ve built a whole series of ‘standard candles,’ different kinds of objects whose luminosities we can convert to distances out to 400 million lightyears. They all agree that the Universe is expanding.”

“Well, you gotta expect that, everything going ballistic from the Big Bang.”

“They don’t go the steady speed you’re thinking. As we got better at making really long-distance measurements, we learned that the expansion is accelerating.”

“Wait. I remember my high-school physics. If there’s an acceleration, there’s gotta be a force pushing it. Especially if it’s fighting the force of gravity.”

“Well there you go. Energy is force times distance and you’ve just identified dark energy. But standard candles aren’t the only kind of evidence.”

“There’s more?”

“Sure — ‘standard sirens‘ and ‘standard rulers.’ The sirens are events that generate gravitational waves we pick up with LIGO facilities. The shape and amplitude of the LIGO signals tell us how far away the source was — and that information is completely immune to electromagnetic distortions.”

“And the rulers?”

“They’re objects, like spiral galaxies and intergalactic voids, that we have independent methods for connecting apparent size to distance.”

“And the candles and rulers and sirens all agree that acceleration and dark energy are real?”

“Yessir.”

~~ Rich Olcott

Dancing in The Dark

Change-me Charlie at his argument table

The impromptu seminar at Change-me Charlie’s “Change My Mind” table is still going strong, but it looks like Physicist-in-training Newt and Astronomer-in-training Jim have met his challenge. He’s switched from arguing that dark matter doesn’t exist to asking how it worked in the Bullet Cluster’s massive collision between two collections of galaxies with their clouds of plasma and dark matter. “OK, the individual galaxies are so spread out they slide past each other without slowing down but the plasma clouds obstruct each other by friction. Wouldn’t friction in the dark matter hold things back, too?”

Jim’s still standing in front of the table. “Now that’s an interesting question, so interesting that research groups have burned a bazillion computer cycles trying to answer it.”

“Interesting, yes, but that interesting?”

“For sure. What we know about dark matter is mostly what it doesn’t do. It doesn’t give off light, it doesn’t absorb light, it doesn’t seem to participate in the strong or weak nuclear forces or interact with normal matter by any means other than gravity, and no identifiable dark matter particles have been detected by bleeding-edge experiments like IceCube and the Large Hadron Collider. So people wonder, does dark matter even interact with itself? If we could answer that question one way or the other, that ought to tell us something about what dark matter is.”

“How’re we gonna do that?”

Newt’s still perched on Charlie’s oppo chair. “By using computers and every theory tool on the shelf to run what-if? simulations. From what we can tell, nearly everywhere in the Universe normal matter is embedded in a shell of dark matter. The Bullet Cluster and a few other objects out there appear to break that rule and give us a wonderful check on the theory work.”

The Bullet Cluster, 1E 0657-56 (NASA image)

“Like for instance.”

“Simple case. What would the collision would looked like if dark matter wasn’t involved? Some researchers built a simulation program and loaded it with a million pretend plasma particles in two cluster-sized regions moving towards each other from 13 million pretend lightyears apart. They also loaded in position and momentum data for the other stars and galaxies shown in the NASA image. The simulation tracked them all as pretend-time marched along stepwise. At each time-step the program applied known or assumed laws of physics to compute every object’s new pretend position and momentum since the prior step. Whenever two pretend-particles entered the same small region of pretend-space, the program calculated a pretend probability for their collision. The program’s output video marked each successful collision with a pink pixel so pinkness means proton-electron plasma. Here’s the video for this simulation.”

“Doesn’t look much like the NASA picture. The gas just spreads out, no arc or cone to the sides.”

“Sure not, which rules out virtually all models that don’t include dark matter. So now the team went to a more complicated model. They added a million dark matter particles that they positioned to match the observed excess gravity distribution. Those’re marked with blue pixels in the videos. Dark matter particles in the model were allowed to scatter each other, too, under control of a self-interaction parameter. The researchers ran the simulations with a whole range of parameter values, from no-friction zero up to about twice what other studies have estimated. Here’s the too-much case.”

“Things hold together better with all that additional gravity, but it’s not a good match either.”

“Right, and here’s the other end of the range — no friction between dark matter particles. Robertson, the video’s author/director, paused the simulation in the middle to insert NASA’s original image so we could compare.”

“Now we’re getting somewhere.”

“It’s not a perfect match. Here’s an image I created by subtracting a just-after-impact simulation frame from the NASA image, then amplifying the red. There’s too much left-over plasma at the outskirts, suggesting that maybe no-friction overstates the case and maybe dark matter particles interact, very slightly, beyond what a pure-gravity theory predicts.”

“Wait, if the particles don’t use gravity, electromagnetism or the nuclear forces on each other, maybe there’s a fifth force!”

“New Physics!”

A roar from Cap’n Mike — “Or they’re not particles!”

~~ Rich Olcott

Dark Passage

Change-me Charlie’s not giving up easily. “You said that NASA picture did three things, but you only told us two of them — that dark matter’s a thing and that it’s separate from normal matter. What’s the third thing? What exactly is in that picture? Does it tell us what dark matter is?”

The Bullet Cluster ( 1E 0657-56 )

Physicist-in-training Newt’s ready for him. “Not much of a clue about what dark matter is, but a good clue about how it behaves. As to what’s in the picture, we need some background information first.”

“Go ahead, it’s not dinner-time yet.”

“First, this isn’t two stars colliding. It’s not even two galaxies. It’s two clusters of galaxies, about 40 all together. The big one on the left probably has the mass of a couple quintillion Suns, the small one about 10% of that.”

“That’s a lot of stars.”

“Oh, most of it’s definitely not stars. Maybe only 1-2%. Those stars and the galaxies they form are embedded in ginormous clouds of proton-electron plasma that make up 5-20% of the mass. The rest is that dark matter you don’t like.”

“Quadrillions of stars are gonna make a super-super-nova when they collide!”

“Well, no. That doesn’t even happen when two galaxies collide. The average distance between neighboring stars in a galaxy is 200-300 times the diameter of a star so it’s unlikely that any two of them will come even close. Next level up, the average distance between galaxies in a cluster is about 60 galaxy diameters or more, depending. The galaxies will mostly just slide past each other. The real colliders are the spread-out stuff — the plasma clouds and of course the dark matter, whatever that is.”

Astronomer-in-training Jim cuts in. “Anyway, the collision has already happened. The light from this configuration took 3.7 billion years to reach us. The collision itself was longer ago than that because the bullet’s already passed through the big guy. From that scale-bar in the bottom corner I’d say the centers are about 2 parsecs apart. If I recall right, their relative velocity is about 3000 kilometers per second so…” <poking at his smartphone> “…the peak intersection was about 700 million years earlier than that. Call it 4.3 billion years ago.”

“So what’s with the cotton candy?”

Newt looks puzzled. “Cotton… oh, the pink pixels. They’re markers for where NASA’s Chandra telescope saw X-rays coming from.”

“What can make X-rays so far from star radiation that could set them going?”

“The electrons do it themselves. An electron emits radiation every time it collides with another charged particle and changes direction. When two plasma clouds interpenetrate you get twice as many particles per unit volume and four times the collision rate so the radiation intensity quadruples. There’s always some X-radiation in the plasma because the temperature in there is about 8400 K and particle collisions are really violent. The Chandra signal pink shows the excess over background.”

“The blue in the Jim’s picture is supposed to be what, extra gravity?”

“Basically, yeah. It’s not easy to see from the figure, but there are systematic distortions in the images of the background galaxies in the blue areas. Disks and ellipsoids appear to be bent, depending on where they sit relative to the clusters’ centers of mass. The researchers used Einstein’s equations and lots of computer time to work back from the distortions to the lensing mass distributions.”

“So what we’ve got is a mostly-not-from-stars gravity lump to the left, another one to the right, and a big cloud in the middle with high-density hot bits on its two sides. Something in the middle blew up and spread gas around mostly in the direction of those two clusters. What’s that tell us?”

“Sorry, that’s not what happened. If there’d been a central explosion the excess to the right would be arc-shaped, not a cone like you see. No, this really is the record of one galaxy cluster bursting through another one. Particle-particle friction within the plasma clouds held them back while the embedded galaxies and dark matter moved on.”

“OK, the galaxies aren’t close-set enough for them to slow each other down, but wouldn’t friction in the dark matter hold things back, too?”

“Now that’s an interesting question…”

~~ Rich Olcott

The Prints of Darkness

There’s a commotion in front of Al’s coffee shop. Perennial antiestablishmentarian Change-me Charlie’s set up his argument table there and this time the ‘establishment’ he’s taking on is Astrophysics. Charlie’s an accomplished chain-yanker and he’s working it hard. “There’s no evidence for dark matter, they’ve never found any of the stuff and there’s tons of no-dark-matter theories to explain the evidence.”

Big Cap’n Mike’s shouts from the back of the crowd. “What they’ve been looking for and haven’t found is particles. By my theory dark matter’s an aspect of gravity which ain’t particles so there’s no particles for them to find.”

Astronomer-in-training Jim spouts off right in Charlie’s face. “Dude, you can’t have it both ways. Either there’s no evidence to theorize about, or there’s evidence.”

Physicist-in-training Newt Barnes takes the oppo chair. “So what exactly are we talking about here?”

“That’s the thing, guy, no-one knows. It’s like that song, ‘Last night I saw upon the stair / A little man who wasn’t there. / He wasn’t there again today. / Oh how I wish he’d go away.‘ It’s just buzzwords about a bogosity. Nothin’ there.”

I gotta have my joke. “Oh, it’s past nothing, it’s a negative.”

“Come again?”

“The Universe is loaded with large rotating but stable structures — solar systems, stellar binaries, globular star clusters, galaxies, galaxy clusters, whatever. Newton’s Law of Gravity accounts nicely for the stability of the smallest ones. Their angular momentum would send them flying apart if it weren’t for the gravitational attraction between each component and the mass of the rest. Things as big as galaxies and galaxy clusters are another matter. You can calculate from its spin rate how much mass a galaxy must have in order to keep an outlying star from flying away. Subtract that from the observed mass of stars and gas. You get a negative number. Something like five times more negative than the mass you can account for.”

“Negative mass?”

“Uh-uh, missing positive mass to combine with the observed mass to account for the gravitational attraction holding the structure together. Zwicky and Rubin gave us the initial object-tracking evidence but many other astronomers have added to that particular stack since then. According to the equations, the unobserved mass seems to form a spherical shell surrounding a galaxy.”

“How about black holes and rogue planets?”

Newt’s thing is cosmology so he catches that one. “No dice. The current relative amounts of hydrogen, helium and photons say that the total amount of normal matter (including black holes) in the Universe is nowhere near enough to make up the difference.”

“So maybe Newton’s Law of Gravity doesn’t work when you get to big distances.”

“Biggest distance we’ve got is the edge of the observable Universe. Jim, show him that chart of the angular power distribution in the Planck satellite data for the Cosmological Microwave Background.” <Jim pulls out his smart-phone, pulls up an image.> “See the circled peak? If there were no dark matter that peak would be a valley.”

Charlie’s beginning to wilt a little. “Ahh, that’s all theory.”

The Bullet Cluster ( 1E 0657-56 )

<Jim pulls up another picture.> “Nope, we’ve got several kinds of direct evidence now. The most famous one is this image of the Bullet Cluster, actually two clusters caught in the act of colliding head-on. High-energy particle-particle collisions emit X-rays that NASA’s Chandra satellite picked up. That’s marked in pink. But on either side of the pink you have these blue-marked regions where images of further-away galaxies are stretched and twisted. We’ve known for a century how mass bends light so we can figure from the distortions how much lensing mass there is and where it is. This picture does three things — it confirms the existence of invisible mass by demonstrating its effect, and it shows that invisible mass and visible mass are separate phenomena. I’ve got no pictures but I just read a paper about two galaxies that don’t seem to be associated with dark matter at all. They rotate just as Newton would’ve expected from their visible mass alone. No surprise, they’re also a lot less dense without that five-fold greater mass squeezing them in.”

“You said three.”

“Gotcha hooked, huh?

~~ Rich Olcott

A Force-to-Force Meeting

The Crazy Theory contest is still going strong in the back room at Al’s coffee shop. I gather from the score board scribbles that Jim’s Mars idea (one mark-up says “2 possible 2 B crazy!“) is way behind Amanda’s “green blood” theory.  There’s some milling about, then a guy next to me says, “I got this, hold my coffee,” and steps up to the mic.  Big fellow, don’t recognize him but some of the Physics students do — “Hey, it’s Cap’n Mike at the mic.  Whatcha got for us this time?”

“I got the absence of a theory, how’s that?  It’s about the Four Forces.”

Someone in the crowd yells out, “Charm, Persuasiveness, Chaos and Bloody-mindedness.”

“Nah, Jennie, that’s Terry Pratchett’s Theory of Historical Narrative.  We’re doing Physics here.  The right answer is Weak and Strong Nuclear Forces, Electromagnetism, and Gravity, with me?  Question is, how do they compare?”

Another voice from the crowd. “Depends on distance!”

“Well yeah, but let’s look at cases.  Weak Nuclear Force first.  It works on the quarks that form massive particles like protons.  It’s a really short-range force because it depends on force-carrier particles that have very short lifetimes.  If a Weak Force carrier leaves its home particle even at the speed of light which they’re way too heavy to do, it can only fly a small fraction of a proton radius before it expires without affecting anything.  So, ineffective anywhere outside a massive particle.”

It’s a raucous crowd.  “How about the Strong Force, Mike?”

.  <chorus of “HOO-wah!”>

“Semper fi that.  OK, the carriers of the Strong Force —”

.  <“Naa-VY!  Naaa-VY!”>

.  <“Hush up, guys, let him finish.”>

“Thanks, Amanda.  The Strong Force carriers have no mass so they fly at lightspeed, but the force itself is short range, falls off rapidly beyond the nuclear radius.  It keeps each trio of quarks inside their own proton or neutron.  And it’s powerful enough to corral positively-charged particles within the nucleus.  That means it’s way stronger inside the nucleus than the Electromagnetic force that pushes positive charges away from each other.”

“How about outside the nucleus?”

“Out there it’s much weaker than Electromagnetism’s photons that go flying about —”

.  <“Air Force!”>

.  <“You guys!”>

“As I was saying…  OK, the Electromagnetic Force is like the nuclear forces because it’s carried by particles and quantum mechanics applies.  But it’s different from the nuclear forces because of its inverse-square distance dependence.  Its range is infinite if you’re willing to wait a while to sense it because light has finite speed.  The really different force is the fourth one, Gravity —”

.  <“Yo Army!  Ground-pounders rock!”>

“I was expecting that.  In some ways Gravity’s like Electromagnetism.  It travels at the same speed and has the same inverse-square distance law.  But at any given distance, Gravity’s a factor of 1038 punier and we’ve never been able to detect a force-carrier for it.  Worse, a century of math work hasn’t been able to forge an acceptable connection between the really good Relativity theory we have for Gravity and the really good Standard Model we have for the other three forces.  So here’s my Crazy Theory Number One — maybe there is no connection.”

.  <sudden dead silence>

“All the theory work I’ve seen — string theory, whatever — assumes that Gravity is somehow subject to quantum-based laws of some sort and our challenge is to tie Gravity’s quanta to the rules that govern the Standard Model.  That’s the way we’d like the Universe to work, but is there any firm evidence that Gravity actually is quantized?”

.  <more silence>

“Right.  So now for my Even Crazier Theories.  Maybe there’s a Fifth Force, also non-quantized, even weaker than Gravity, and not bound by the speed of light.  Something like that could explain entanglement and solve Einstein’s Bubble problem.”

.  <even more silence>

“OK, I’ll get crazier.  Many of us have had what I’ll call spooky experiences that known Physics can’t explain.  Maybe stupid-good gambling luck or ‘just knowing’ when someone died, stuff like that.  Maybe we’re using the Fifth Force in action.”

.  <complete pandemonium>
four forces plus 1

~ Rich Olcott


Note to my readers with connections to the US National Guard, Coast Guard, Merchant Marine and/or Public Health Service — Yeah, I know, but one can only stretch a metaphor so far.

A Three-dog Night Would Be So Cool

“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?”grebe messenger pairs“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

“Hot Jets, Captain Neutrino!”

“Hey, Cathleen, while we’re talking IceCube, could you ‘splain one other thing from that TV program?”

“Depends on the program, Al.”

“Oh, yeah, you weren’t here when we started on this.  So I was watching this program and they were talking about neutrinos and how there’s trillions of them going through like my thumbnail every second and then IceCube saw this one neutrino that they’re real excited about so what I’m wondering is, what’s so special about just that neutrino? How do they even tell it apart from all the others?”

“How about the direction it came from, Cathleen?  We get lotsa neutrinos from the Sun and this one shot in from somewhere else?”

SMBH jet and IceCube
Images from NASA and JPL-Caltech

“An interesting question, Vinnie.  The publicity did concern its direction, but the neutrino was already special.  It registered 290 tera-electron-volts.”

“Ter-what?”

“Sorry, scientific shorthand — tera is ten-to-the-twelfth.  A million electrons poised on a million-volt gap would constitute a Tera-eV of potential energy.  Our Big Guy had 290 times that much kinetic energy all by himself.”

“How’s that stack up against other neutrinos?”

“Depends on where they came from.  Neutrinos from a nuclear reactor’s uranium or plutonium fission carry only about 10 Mega-eV, wimpier by a factor of 30 million.  The Sun’s primary fusion process generates neutrinos peaking out at 0.4 MeV, 25 times weaker still.”

“How about from super-accelerators like the LHC?”

“Mmm, the LHC makes TeV-range protons but it’s not designed for neutrino production.  We’ve got others that have been pressed into service as neutrino-beamers. It’s a complicated process — you send protons crashing into a target.  It spews a splatter of pions and K-ons.  Those guys decay to produce neutrinos that mostly go in the direction you want.  You lose a lot of energy.  Last I looked the zippiest neutrinos we’ve gotten from accelerators are still a thousand times weaker than the Big Guy.”

I can see the question in Vinnie’s eyes so I fire up Old Reliable again.  Here it comes… “What’s the most eV’s it can possibly be?”  Good ol’ Vinnie, always goes for the extremes.

“You remember the equation for kinetic energy?”

“Sure, it’s E=½ m·v², learned that in high school.”

“And it stayed with you.  OK, and what’s the highest possible speed?”

“Speed o’ light, 186,000 miles per second.”

“Or 300 million meters per second, ’cause that’s Old Reliable’s default setting.  Suppose we’ve got a neutrino that’s going a gnat’s whisker slower than light.  Let’s apply that formula to the neutrino’s rest mass which is something less than 1.67×10-36 kilograms…”Speedy neutrino simple calculation“Half an eV?  That’s all?  So how come the Big Guy’s got gazillions of eV’s?”

“But the Big Guy’s not resting.  It’s going near lightspeed so we need to apply that relativistic correction to its mass…Speedy neutrino relativistic calculation“That infinity sign at the bottom means ‘as big as you want.’  So to answer your first question, there isn’t a maximum neutrino energy.  To make a more energetic neutrino, just goose it to go even closer to the speed of light.”

“Musta been one huge accelerator that spewed the Big Guy.”

“One of the biggest, Al.”  Cathleen again.  “That’s the exciting thing about what direction the particle came from.”

“Like the North Pole or something?”

“Much further away, much bigger and way more interesting.  As soon as IceCube caught that neutrino signal, it automatically sent out a “Look in THIS direction!” alert to conventional observatories all over the world.  And there it was — a blazar, 5.7 billion lightyears away!”

“Wait, Cathleen, what’s a blazar?”

“An incredibly brilliant but highly variable photon source, from radio frequencies all the way up to gamma rays and maybe cosmic rays.  We think the thousands we’ve catalogued are just a fraction of the ones within range.  We’re pretty sure that each of them depends on a super-massive black hole in the center of a galaxy.  The current theory is that those photons come from an astronomy-sized accelerator, a massive swirling jet that shoots out from the central source.  When the jet happens to point straight at us, flash-o!”

Duck!

“I wouldn’t worry about a neutrino flood.  The good news is IceCube’s signal alerted astronomers to check TXS 0506+056, a known blazar, early in a new flare cycle.”

“An astrophysical fire alarm!”

~~ 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.”

“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