Category: Physics

Bigger than you’d think

On By rolcottIn Astronomy: Techniques, Cosmology, Physics: Black Holes and Relativity, Physics: Light and Relativity, Physics: Quantum Mechanics, UncategorizedLeave a comment

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

Rhythm Method

On By rolcottIn Astronomy: Multi-messenger, Astronomy: Stellar Science, Cosmology, Physics: Black Holes and Relativity, Physics: Light and Relativity, Physics: Newton and Gravity, UncategorizedLeave a comment

A warm Summer day.  I’m under a shady tree by the lake, watching the geese and doing some math on Old Reliable.  Suddenly a text-message window opens up on its screen.  The header bar says 710-555-1701.  Old Reliable has never held a messaging app, that’s not what I use it for.  The whole thing doesn’t add up.  I type in, Hello?

Hello, Mr Moire.  Remember me?

Suddenly I do.  That sultry knowing stare, those pointed ears.  It’s been a year.  Hello, Ms Baird.  What can I do for you?

Another tip for you, Mr Moire.  One of my favorite star systems — the view as you approach it at near-lightspeed is so ... meaningful.  Your astronomers call it PSR J0337+1715.

So of course I head over to Al’s coffee shop after erasing everything but that astronomical designation.  As I hoped, Cathleen and a few of her astronomy students are on their mid-morning break.  Cathleen winces a little when she sees me coming.  “Now what, Sy?  You’re going to ask about blazars and neutrinos?”

I show her Old Reliable’s screen.  “Afraid not, Cathleen, I’ll have to save that for later.  I just got a message about this star system.  Recognize it?”

“Why, Sy, is that a clue or something?  And why is the lettering in orange?”

“Long story.  But what can you tell me about this star system?”

“Well, it’s probably one of the most compact multi-component systems we’re ever going to run across.  You know what compact objects are?”

“Sure.  When a star the size of our Sun exhausts most of its hydrogen fuel, gravity wins its battle against heat.  The star collapses down to a white dwarf, a Sun-full of mass packed into a planet-size body.  If the star’s a bit bigger it collapses even further, down to a neutron star just a few miles across.  The next step would be a black hole, but that’s not really a star, is it?”

“No, it’s not.  Jim, why not?”

“Because by definition a black hole doesn’t emit light.  A black hole’s accretion disk or polar jets might, but not the object itself.”

“Mm-hm.  Sy, your ‘object’ is actually three compact objects orbiting  around each other.  There’s a neutron star with a white dwarf going around it, and another white dwarf swinging around the pair of them.  Vivian, does that sound familiar?”

“That’s a three-body system, like the Moon going around the Earth and both going around the Sun.  Mmm, except really both white dwarfs would go around the neutron star because it’s heaviest and we can calculate the motion like we do the Solar System.”

“Not quite.  We can treat the Sun as motionless because it has 99% of the mass.  J0337+1715’s neutron star doesn’t dominate its system as much as the Sun does ours.  That outermost dwarf has 20% of its system’s mass.  Phil, what does that suggest to you?”

“It’d be like Pluto and Charon.  Charon’s got 10% of their combined mass and so Pluto and Charon both orbit a point 10% of the way out from Pluto.  From Earth we see Pluto wobbling side to side around that point.  So the neutron star must wobble around the point 20% outward towards the heavy dwarf.  Hey, star-wobble is how we find exoplanets.  Is that what this is about, Mr Moire?  Did someone measure its red-shift behavior?”PSR J0337+1715Cathleen saves me from answering.  “Not quite.  The study Sy’s chasing is actually a cute variation on red-shift measurements.  That ‘PSR‘ designation means the neutron star is a pulsar.  Those things emit electromagnetic radiation pulses with astounding precision, generally regular within a few dozen nanoseconds.  If we receive slowed-down pulses then the object’s going away; sped-up and it’s approaching, just like with red-shifting.  The researchers  derived orbital parameters for all three bodies from the between-pulse durations.  The heavy dwarf is 200 times further out than the light one, for instance.  Not an easy experiment, but it yielded an important result.”

My ears perk up.  “Which was…?”

“The gravitational force between the pulsar and each dwarf was within six parts per million of what Newton’s Laws prescribe.  That observation rules out whole classes of theories that tried to explain galaxies and galaxy clusters without invoking dark matter.”

Cool, huh?

Uh-huh.

~~ Rich Olcott

Moby Divergence

On By rolcottIn Physics: Electromagnetism, Physics: Fundamentals, UncategorizedLeave a comment

Stepping into Pizza Eddie’s I see Jeremy at his post behind the gelato stand, an impressively thick book in front of him.  “Hi, Jeremy, one chocolate-hazelnut combo, please.  What’re you reading there?”

“Hi, Mr Moire.  It’s Moby Dick, for English class.”

“Ah, one of my favorites.  Melville was a 19th-century techie, did for whaling what Tom Clancy did for submarines.”

“You’re here at just the right time, Mr Moire.  I’m reading the part where something called ‘the corpusants’ are making lights glow around the Pequod.  Sometimes he calls them lightning, but they don’t seem to come down from the sky like real lightning.  Umm, here it is, he says. ‘All the yard-arms were tipped with pallid fire, and touched at each tri-pointed lightning-rod-end with three tapering white flames, each of the three tall masts was silently burning in that sulphurous air, like three gigantic wax tapers before an altar.’  What’s that about?”St Elmos fire

“That glow is also called ‘St Elmo’s Fire‘ among other things.  It’s often associated with a lightning storm but it’s a completely different phenomenon.  Strictly speaking it’s a concentrated coronal discharge.”

“That doesn’t explain much, sir.”

“Take it one word at a time.  If you pump a lot of electrons into a confined space, they repel each other and sooner or later they’ll find ways to leak away.  That’s literally dis-charging.”

“How do you ‘pump electrons’?”

“Oh, lots of ways.  The ancient Greeks did it by rubbing amber with fur, Volta did it chemically with metals and acid,  Van de Graaff did it with a conveyor belt, Earth does it with winds that transport air between atmospheric layers.  You do it every time you shuffle across a carpet and get shocked when you put your finger near a water pipe or a light switch.”

“That only happens in the wintertime.”

“Actually, carpet-shuffle electron-pumping happens all the time.  In the summer you discharge as quickly as you gain charge because the air’s humidity gives the electrons an easy pathway away from you.  In the winter you’re better insulated and retain the charge until it’s too late.”

“Hm.  Next word.”

“Corona, like ‘halo.’  A coronal discharge is the glow you see around an object that gets charged-up past a certain threshold.  In air the glow can be blue or purple, but you can get different colors from other gases.  Basically, the electric field is so intense that it overwhelms the electronic structure of the surrounding atoms and molecules.  The glow is electrons radiating as they return to their normal confined chaos after having been pulled into some stretched-out configuration.”

“But this picture of the corpusants has them just at the mast-heads and yard-arms, not all over the boat.”

“That’s where the ‘concentrated’ word come in.  I puzzled over that, too, when I first looked into the phenomenon.  Made no sense.”

“Yeah.  If the electrons are repelling each other they ought to spread out as much as possible.  So why do they seem pour out of the pointy parts?”

“That was a mystery until the 1880s when Heaviside cleaned up Maxwell’s original set of equations.  The clarified math showed that the key is the electric field’s spread-out-ness, technically known as divergence.”

DivergenceWith my finger I draw in the frost on his gelato cabinet.  “Imagine this is a brass ball, except I’ve pulled one side of it out to a cone.  Someone’s loaded it up with extra electrons so it’s carrying a high negative charge.”

“The electrons have spread themselves evenly over the metal surface, right, including at the pointy part?”

“Yup, that’s why I’m doing my best to make all these electric field arrows the same distance apart at their base.  They’re also supposed to be perpendicular to the surface.  What part of that field will put the most rip-apart stress on the local air molecules?”

“Oh, at the tip, where the field spreads out most abruptly.”

“Bingo.  What makes the glow isn’t the average field strength, it’s how drastically the field varies from one side of a molecule to the other.  That’s what rips them apart.  And you get the greatest divergence at the pointy parts like at the Pequod’s mast-head.”

“And Ahab’s harpoon.”

~~ Rich Olcott

A Recourse to Pastry

On By rolcottIn Astronomy: Planetary Science, Astronomy: Techniques, Physics: Fundamentals, UncategorizedLeave a comment

There’s something wrong about the displays laid out on Al’s pastry counter — no symmetry.  One covered platter holds eight pinwheels in a ring about a central one, but the other platter’s central pinwheel has only a five-pinwheel ring around it.  I yell over to him.  “What’s with the pastries, Al?  You usually balance things up.”

“Ya noticed, hey, Sy?  It’s a tribute to the Juno spacecraft.  She went into orbit around Jupiter on the 5th of July 2016 so I’m celebrating her anniversary.”

“Well, that’s nice, but what do pinwheels have to do with the spacecraft?”

“Haven’t you seen the polar pictures she sent back?  Got a new poster behind the cash register.  Ain’t they gorgeous?”Jupiter both poles“They’re certainly eye-catching, but I thought Jupiter’s all baby-blue and salmon-colored.”

Astronomer Cathleen’s behind me in line.  “It is, Sy, but only in photographs using visible sunlight.  These are infrared images, right, Al?”

“Yeah, from … lemme look at the caption … Juno‘s JIRAM instrument.”

“Right, the infrared mapper.  It sees heat-generated light that comes from inside Jupiter.  It’s the same principle as using blackbody radiation to take a star’s temperature, but here we’re looking at a planet.  Jupiter’s way colder than a star so the wavelengths are longer, but on the other hand it’s close-up so we don’t have to reckon with relativistic wavelength stretching.  At any rate, infrared wavelengths are too long for our eyes to see but they penetrate clouds of particulate matter like interstellar dust or the frigid clouds of Jupiter.”

Jupiter south pole 1
NASA mosaic view of Jupiter’s south pole by visible light

“So this red hell isn’t what the poles actually look like?”

“No, Al,  the visible light colors are in the tops of clouds and they’re all blues and white.  These infrared images show us temperature variation within the clouds.  Come to think of it, that Hell’s frozen over — if I recall correctly, the temperature range in those clouds runs from about –10°C to –80°C.  In Fahrenheit that’d be from near zero to crazy cold.”

“Those aren’t just photographs in Al’s poster?”

“Oh, no, Sy, there’s a lot of computer processing in between Juno‘s wavelength numbers and what the public sees.  The first step is to recode all the infrared wavelengths to visible colors.  In that north pole image I’d say that they coded red-to-black as warm down to white as cool.  The south pole image looks like warmest is yellow-to-white, coolest is red.”

“How’d you figure that?”

“The programs fake the apparent heights.  The warmest areas are where we can see most deeply into the atmosphere, which would be at the center or edge of a vortex.  The cooler areas would be upper-level material.  The techs use that logic to generate the perspective projection that we interpret as a 3-D view.”

Vinnie’s behind us in line and getting impatient.  “I suppose there’s Science in those pretty pictures?”

“Tons of it, and a few mysteries.  JIRAM by itself is telling the researchers a lot about where and how much water and other small molecules reside in Jupiter’s atmosphere.  But Juno has eight other sensors.  Scientists expect to harvest important information from each of them.  Correlations between the data streams will give us exponentially more.”

He’s still antsy.  “Such as?”

“Like how Jupiter’s off-axis magnetic field is related to its lumpy gravitational field.  When we figure that out we’ll know a lot more about how Jupiter works, and that’ll help us understand Saturn and gas-giant exoplanets.”GRS core

Al breaks in.  “What about the mysteries, Cathleen?”

“Those storms, for instance.  They look like Earth-style hurricanes, driven by upwelling warm air.  They even go in the right direction.  But why are they crammed together so and how can they stay stable like that?  Adjacent gears have to rotate in opposite directions, but these guys all go in the same direction.  I can’t imagine what the winds between them must be like.”

“And how come there’s eight in the north pole ring but only five at the other pole?”

“Who knows, Vinnie?  The only guess I have is that Jupiter’s so big that one end doesn’t know what the other end’s doing.”

“Someone’s gonna have to do better than that.”

“Give ’em time.”

~~ Rich Olcott

Zwicky Too Soon

On By rolcottIn Astronomy: Techniques, Cosmology, Uncategorized, Vera RubinLeave a comment

Big Vinnie barrels into the office, again. “Hey, Sy, word is you been short-changing Fritz Zwicky. What’s the story?”

“Hey, I never even met the guy.  He died in 1974.  How could I do him a bad deal?”

“Not giving him full credit.  I read an article about him.  He talked about ‘dark matter’ almost fifty years before Vera Rubin.”

“You’ve got a point there.  Like Vera Rubin he had a political problem, but his was quite different than hers.”

“Political?  I thought all you had to do was be right.”

“No, you have to be right and you have to have people willing to spend time validating or refuting your claims.  Rubin wasn’t a self-advertiser, so it took a while for people to realize why her results were important.  They did look at them, though, and they did give her credit.  Zwicky’s was a different story.”

“Wasn’t he right?”

“Sometimes right, often wrong.  Thing was, he generated too many ideas for people to cope with.  Worse, he was one of those wide-ranging intellects who adds one plus one to make two.  Trouble was, Zwicky got his ones from different specialties that don’t normally interact.  When people didn’t immediately run with one of his claims he took it personally and lashed out, publicly called ’em fools or worse.  Never a good tactic.”

“Gimme a f’rinstance.”

“OK.  Early 1930’s, Zwicky’s out in the still-raw wilds of California, practically nothing out there but movie studios and oil wells, using a manual blink-comparator like the one Clyde Tombaugh used about the same time to find Pluto.  He’s scanning images taken with Palomar’s new wide-angle telescope to search out novae, stars that suddenly get brighter.  He’s finding dozens of them but a few somehow get orders of magnitude brighter than the rest.  He and his buddy Walter Baade call the special ones ‘supernovae.'”

“Ain’t that novas?”

“Novae — we’re being proper astronomers here and it’s a Latin word.  Anyway, Zwiky’s trying to figure out where a supernova’s enormous luminosity comes from.  He got his start in solid-state physics and he still keeps up on both Physics and Astronomy.  Just a year earlier, James Cavendish over in atomic physics had announced the discovery of the neutron.  Zwicky sees that neutrons are the solution to his problem — gravity can pack together no-charge neutrons to a much higher density than it can pack positive-charge protons.  He proposes that a supernova happens when a big-enough star uses up its fuel and collapses to the smallest possible object, a neutron star.  Furthermore, he says that the collapse releases so much gravitational energy that supernovae give off cosmic rays, the super-high-energy photons that were one of the Big Questions of the day.”

“Sounds reasonable, I suppose.”

“Well, yeah, now.  But back then most astronomers had never heard of neutrons.  To solve at a stroke both cosmic rays and supernovae, using this weird new thing called a neutron, and with the proposal coming from somewhere other than Europe or Ivy League academia — well, it was all too outlandish to take seriously.  No-one did, for decades.”

“He didn’t like that, huh?”Zwicky inspecting dark matter

“No, he did not.  And he railed about it, not only in private conversations but in papers and in the preface to one of the two galaxy catalogs he published.  Same thing with galaxy clusters.”

“Wait, you wrote that Rubin found clusters.”

“I did and she did.  Actually, I wrote that she confirmed clustering.  We knew for 150 years that galaxies bunch together in our 2-D sky, but it took Zwicky’s measurements to group the Coma Cluster galaxies in 3-D.  Problem was, they were moving too fast.  If star gravity were the only thing holding them together they should have scattered ages ago.”

“Dark matter, huh?”

“Yup, Zwicky claimed invisible extra mass bound the cluster together.  More Zwicky outlandishness and once again his work was ignored for years.”

“Even though he was right.”

“Mm-hm.  But he could be wrong, too.  He didn’t like Hubble’s expanding Universe idea so he came up with a ‘tired light’ theory to explain the red-shifts.  He touted that idea heavily but there was too much evidence against it.”

“One of those angry ‘lone wolf’ scientists.”

“And bitter.”

~~ Rich Olcott

Symphony for Rubber Ruler

On By rolcottIn Astronomy: Planetary Science, Astronomy: Techniques, Cosmology, Uncategorized, Vera RubinLeave a comment

“But Mr Moire, first Vera Rubin shows that galaxies don’t spread out like sand grains on a beach…”

“That’s right, Maria.”

“And then she shows that galaxy streams flow like rivers through the Universe…”

“Yes.”

“And then she finds evidence for dark matter!  She changed how we see the Universe and still they don’t give her the Nobel Prize??!?”

“All true, but there’s a place on Mars that’s named for her and it’ll be famous forever.”

“Really?  I didn’t know about that.  Where is it and why did they give it her name?”

“What do you know about dark matter?”

Rubin inspecting dark matter“Not much.  We can’t see it, and they say there is much more of it than the matter we can see.  If we can’t see it, how did she find it?  That’s a thing I don’t understand, what I came to your office to ask.”

“It all has to do with gravity.  Rubin’s studies of dozens of galaxies showed that they really shouldn’t exist, at least on the basis of the physics we knew about at the time.  She’d scan across a galaxy’s image, measuring how its red-shifted spectrum changed from the coming-toward-us side to the going-away-from-us side.  The red-shift translates to velocity.  The variation she found amazed the people she showed it to.”Pinwheel Galaxy NGC 5457 reduced

“What was amazing about it?”

“It was a flat line.  Look at the galaxy poster on my wall over there.”

“Oh, la galaxia del Molinete.  It’s one of my favorites.”

“We call it the Pinwheel Galaxy.  Where would you expect the stars to be moving fastest?”

“Near the center, of course, and they must move slower in those trailing arms.”

“That’s exactly what Rubin didn’t find.  From a couple of reasonable assumptions you can show that a star’s speed in a rotating galaxy composed only of other stars should be proportional to 1/√R, where R is its distance from the center.  If you pick two stars, one twice as far out as the other, you’d expect the outermost star to be going 1/√2 or only about 70% as fast as the other one.”

“And she found…?”

“Both stars have the same speed.”

“Truly the same?”

“Yes!  It gets better.  Most galaxies are embedded in a ball of neutral hydrogen atoms.  With a different spectroscopic technique Rubin showed that each hydrogen ball around her galaxies rotates at the same speed its galaxy does,  even 50% further out than the outermost stars.  Everything away from the center is traveling faster than it should be if gravity from the stars and gas were the only thing holding the galaxy together.  Her galaxies should have dispersed long ago.”

“Could electrical charge be holding things together?”

“Good idea — electromagnetic forces can be stronger than gravity.  But not here.  Suppose the galaxy has negative charge at its center and the stars are all positive.  That’d draw the stars inward, sure, but star-to-star repulsion would push them apart.  Supposing that neighboring stars have opposite charges doesn’t work, either.  And neutral hydrogen atoms don’t care about charge, anyway.  The only way Rubin and her co-workers could make the galaxy be stable is to assume it’s surrounded by an invisible spherical halo with ten times as much mass as the matter they could account for.”

“Mass that doesn’t shine.  She found ‘dark matter’ with gravity!”

“Exactly.”

“What about planets and dust?  Couldn’t they add up to the missing mass?”

“Nowhere near enough.  In out Solar System, for instance, all the planets add up to only 0.1% of the Sun’s mass.”

“Ah, ‘planets’ reminds me.  Why is Vera Rubin’s name on Mars?”

“Well, it’s not strictly speaking on Mars, yet, but it’s on our maps of Mars.  You know the Curiosity rover we have running around up there?”

“Oh yes, it’s looking for minerals that deposit from water.”

“Mm-hm.  One of those minerals is an iron oxide called hematite.  Sometimes it’s in volcanic lava but most of the time it’s laid down in a watery environment.  And get this — it’s often black or dark gray.  Curiosity found a whole hill of the stuff.”

Vera Rubin Ridge labeled
Adopted from a Curiosity Mastcam image from NASA

“Yes, so…?”

“What else would the researchers name an important geologic feature made of darkish matter?”

~~ Rich Olcott

Intermezzo for Rubber Ruler

On By rolcottIn Cosmology, Uncategorized, Vera RubinLeave a comment

“¡Dios mio!  Vera Rubin confirms that galaxies cluster and no-one thinks that’s important?”

“That was in the 1950s, Maria.  Her report was just a degree thesis and a minor paper.  Her advisor, who should have pushed her case but didn’t, was a cosmologist instead of an observational astronomer.  At the time, many considered cosmology to be just barely not metaphysics.  What she reported didn’t bear on what the astronomers of the day considered the Big Questions, like how do stars work and is the expansion of the Universe accelerating.”

“That’s political, ¿no?”

“That’s part of how science works — if observations  look important, other people work to invalidate them.  If results look important, other people work to rebut them.  The claims that are validated and can’t be rebutted survive.  But the verifiers and rebutters only work on what their colleagues consider to be important.  Deciding what’s important is a political process.  The history of science is littered with claims that everyone dismissed as unimportant until decades later when they suddenly gained the spotlight.  Galaxy clustering is one of those cases.  All things considered, I think clustering’s initial obscurity had more to do with the current state of the science than with her being a woman.”

“So how did Vera Rubin react to the nada?”

“She went back to her observing, which is what she was happiest doing anyway.  Especially when computers came along and her long-time colleague Kent Ford built a spiffy electronic spectrograph.  No more gear-calculating all day for a single number, no more peering down that measuring engine microscope tube.  Results came more quickly and she could look at larger assemblies out there in the Universe.  Which led to her next breakthrough.”

Rubin inspecting metagalaxy
“Dark matter, yes?”

“No, that came later.  This one was about streams.”

“Of water?”

“Of galaxies.  At the time, most astronomers thought that galaxy motion was a solved problem.  You know about Hubble Flow?”

“No.  Is that the streaming?”

“It’s the background for streaming.  Hubble Flow is the overall expansion of the Universe, all the galaxies moving away from each other.  But it’s not uniform motion.  We know, for instance, that the Andromeda and Milky Way galaxies are going to collide in about five billion years.  Think of galaxies like gas molecules in an expanding balloon.  On the average every molecule gets further away from its neighbors, but if you watched an individual molecule you’d see it bouncing back and forth.  Astronomers call that extra movement ‘peculiar motion.'”

“‘Peculiar’ like ‘odd?'”

“It’s an old-fashioned use of the word — ‘peculiar’ like ‘distinctive’ or ‘unique.’  Anyway, the community’s general notion was you could account for galaxy movement as a simple random motion laid on top of the Hubble Flow.”

“Again Occam’s Razor cuts too close?”

“For sure.  Rubin and Ford looked at data for almost a hundred distant galaxies all over the sky.  Not just any galaxies.  They carefully picked a set of one kind of galaxy, known in the trade as ScI, all of which have about the same ratio of absolute brightness to diameter.  Measure the diameter, you get the absolute brightness.  A distant light appears dimmer as the square of its distance.  Measure the brightness we see on Earth, make a few corrections, and the inverse square law lets you calculate how far the galaxy is from here.  Then Hubble’s distance-speed law tells you how fast you expect the galaxy to be receding.  That’s half of it.”

“OK…?”

“The other half is how fast the galaxies are really moving.  For that Rubin and Ford turned to spectroscopy.  From the red/blue-shift of each galaxy they had an independent measure of its speed relative to us.  Guess what?  They didn’t match the Hubble Flow speeds.”

Galactic velocity anisotropy
Adapted from
Astronomical Journal 81, 719-37 (1976).

“Faster or slower?”

“Both!  In one half the sky these distant galaxies appear to be fleeing faster than the Hubble Flow, and in the other half they’re going slower.  The simplest explanation is that our entire Local Group is streaming towards the ‘slowest’ part of the sky.  Rubin and company had discovered a large-scale, third kind of galactic motion — rivers of galaxies streaming through the Universe.”

“Did the people get excited?”

“Not for a while, of course.”

~~ Rich Olcott

Concerto for Rubber Ruler

On By rolcottIn Astronomy: Techniques, Cosmology, Uncategorized, Vera RubinLeave a comment

An unfamiliar knock at my office door — more of a tap than a knock. “C’mon in, the door’s open.”

“¿Está ocupado?”

“Hi, Maria. No, I’m not busy, just taking care of odds and ends. What can I do for you?”

“I’m doing a paper on Vera Rubin for la profesora. I have the biographical things, like she was usually the only woman in her Astronomy classes and she had to make her own baño at Palomar Observatory because they didn’t have one for señoras, and she never got the Nobel Prize she deserved for discovering dark matter.“

“Wait, you have all negatives there.  Her life had positives, too.  What about her many scientific breakthroughs?”

“That’s why I’m here, for the science parts I don’t understand.”

“I’ll do what I can. What’s the first one?”

“In her thesis she showed that galaxies are ‘clumped.’  What is that?”

“It means that the galaxies aren’t spread out evenly.  Astronomers at the time believed, I guess on the basis of Occam’s Razor, that galaxies were all the same distance from their neighbors.”

“Occam’s Razor?  Ah, la navaja de Okcam.  Yes, we study that in school — do not assume more than you have to.  But why would evenly be a better assumption than clumpy?”

“At the time she wrote her thesis the dominant idea was that the Big Bang’s initial push would be ‘random’ — every spot in the Universe would have an equal chance of hosting a galaxy.  But she found clusters and voids.  That made astronomers uncomfortable because they couldn’t come up with a mechanism that would make things look that way.  It took twenty years before her observations were accepted.  I’ve long thought part of her problem was that her thesis advisor was George Gamow.  He was a high-powered physicist but not an observational astronomer.  For some people that was sufficient excuse to ignore Rubin’s work.”

“Another excuse.”

“Yes, that, too.”

“But why did she have to discover the clumpy?  You can just look up in the sky and see things that are close to each other.”

“Things that appear to be close together in the sky aren’t necessarily close together in the Universe.  Look out my window.  See the goose flying there?”

“Mmm…  Yes!  I see it.”

“There’s an airplane coming towards it, looks about the same size.  Think they’ll collide?”

“Of course no.  The airplane looks small because it’s far away.”

“But when their paths cross, we see them at the same point in our sky, right?”

“The same height up, yes, and the same compass direction, but they have different distances from us.”

“Mm-hm.  Geometry is why it’s hard to tell whether or not galaxies are clustered.  Two galaxy images might be separated by arc-seconds or less.  The objects themselves could be nearest neighbors or separated by half-a-billion lightyears.  Determining distance is one of the toughest problems in observational astronomy.”

“That’s what Vera Rubin did?  How?”

“In theory, the same way we do today.  In practice, by a lot of painstaking manual work.  She did her work back in the early 1950s, when ‘computer’ was a job title, not a device.  No automation — electronic data recording was a leading-edge research topic.  She had to work with images of spectra spread out on glass plates, several for each galaxy she studied.  Her primary tool, at least in the early days, was a glorified microscope called a measuring engine.  Here’s a picture of her using one.” Vera Rubin

“She looks through the eyepiece and then what?”

“She rotates those vernier wheels to move each glass-plate feature on the microscope stage to the eyepiece’s crosshairs.  The verniers give the feature’s x– and y-coordinates to a fraction of a millimeter.  She uses a gear-driven calculating machine to turn galaxy coordinates into sky angles and spectrum coordinates into wavelengths.  The wavelengths, Hubble’s law and more arithmetic give her the galaxy’s distance from us.  More calculations convert her angle-angle-distance coordinates to galactic x–y-z-coordinates.  Finally she calculates distances between that galaxy and all the others she’s already done.  After processing a few hundred galaxies, she sees groups of short-distance galaxies in reportable clusters.”

“Wouldn’t a 3-D graphic show them?”

“Not for another 50 years.”

~~ Rich Olcott

Terzetto for Rubber Ruler

On By rolcottIn Astronomy: Techniques, Cosmology, Physics: Electromagnetism, Physics: Fundamentals, Physics: Light and Relativity, UncategorizedLeave a comment

ruler and sodium lines“So you’re telling me, Cathleen, that you can tell how hot a star is by looking at its color?”

“That’s right, Vinnie.  For most stars their continuous spectrum is pretty close to the blackbody equation tying peak wavelength to temperature.”

“But you can’t do that with far-away stars, right, because the further they are, the more stretched-out their lightwaves get.  Won’t that mess up the peak wavelength?”

“The key is Kirchhoff’s other kinds of spectrum.”

“You’re talking the bright-line and dark-line kinds.”

“Exactly.  Each kind of spectrum comes from a different process — each is affected differently by the object in question and the environment it’s embedded in.  A continuous spectrum is all about charged particles moving randomly in response to the heat energy they’re surrounded by.  It doesn’t matter what kind of particles they are or even whether they’re positive or negative.  Whenever a particle changes direction, it twitches the electromagnetic field and gives off a wave.”

“Right — the higher the temperature the less time between twitches; the wave can’t move as far before things change so the wavelength’s shorter; any speed’s possible so you can turn that dial wherever; I got all that.  So what’s different with the bright-line and dark-line spectrums?”

Cathleen and I both blurt out, “Spectra!” at the same time and give each other a look.  We’re grown-ups now.  We don’t say, “Jinx!” to each other any more.

“Alright, spectra.  But how’re they different?”

I pick up the story.  “Like Cathleen said, continuous spectra from same–temperature stuff look identical no matter what kind of stuff’s involved because heat is motion and each particle moves as a unit  The other kinds of spectrum are about transitions within particles so they’re all about which kind of stuff.  A given kind of atom can only absorb certain wavelengths of light and it can only relax by giving off exactly the same wavelengths.  There’s no in-betweens.”

She cuts in.  “Sodium, for instance.  It has two strong lines in the yellow, at 588.995 and 589.592 nanometers.  Whether in a star or a meteor or fireworks, sodium gives off exactly those colors.  Conversely, in an interstellar cloud or in a star’s outermost layers sodium absorbs exactly those colors from any continuous-spectrum light passing through.”

I’m back in.  “And there’s the key to your unmixing question, Vinnie.  We’ve talked about frames, remember?  Your far-away star’s light-generating layers emit a continuous spectrum that describes its temperature.  If we were right next to it, that’s the spectrum we’d see.  But as you say, we’re a long way away and in our frame the light’s been stretched.  It still looks like the black-body curve but it’s red-shifted because of our relative motion.”

Cathleen’s turn.  “But if there are sodium atoms in the star’s upper layers, their absorptions will cut a pair of notches in that emitted spectrum.  It won’t be a smooth curve, there’ll be two sharp dips in it, close together, with the blue-side one twice as strong as the other one.  Easy to recognize and measure the redshift.  The blackbody peak is redshifted by exactly the same amount so with some arithmetic you’ve got the peak’s original wavelength and the star’s temperature.”

Mine.  “See, because we know what the sodium wavelengths were in the star’s frame, we can divide the dip wavelengths we measure by the rest-frame numbers we know about.  The ratios give us the star’s redshift.”

Spectrum with only blackbody and sodium Cathleen turns to her laptop and starts tapping keys.  “Let’s do an example.  Suppose we’re looking at a star’s broadband spectrogram.  The blackbody curve peaks at 720 picometers.  There’s an absorption doublet with just the right relative intensity profile in the near infra-red at 1,060,190 and 1,061,265 picometers.  They’re 1,075 picometers apart.  In the lab, the sodium doublet’s split by 597 picometers.  If the star’s absorption peaks are indeed the sodium doublet then the spectrum has been stretched by a factor of 1075/597=1.80.  Working backward, in the star’s frame its blackbody peak must be at 720/1.80=400 picometers, which corresponds to a temperature of about 6,500 K.”

“Old Reliable calculates from that stretch factor and the Hubble Constant the star’s about ten billion lightyears away and fleeing at 240,000 km/s.”

“All that from three peaks.  Spectroscopy’s pretty powerful, huh?”

Cathleen and me: “For sure!    Jinx!”

~~ Rich Olcott

Zarzuela for Rubber Ruler

On By rolcottIn Astronomy: Techniques, Physics: Electromagnetism, Physics: Light and Relativity, UncategorizedLeave a comment

“Hey, Cathleen, if the expansion of the Universe stretches light’s wavelengths, how do you know when you see a color in a star what you’re looking at?”

“Excuse me, Professor, but your office-mate said you’d be here at the coffee shop and I have a homework question.”

“Good heavens, look at the time!  It’s my office hours, I should be over there.  Oh well, you’re here, Maria, what’s the question?”

“You showed us this chart and asked us to write an essay on it.  I don’t know where to begin.”Temp and BB peak

“Ah.  Hang on, Vinnie, this bears on your question, too.  OK, Maria, what can you tell me about the chart?”

“Well, there are five peaked curves, labeled with different temperatures.  Can I assume the green curve peaks, too, not continuing straight up?”

“Yes.  What else?”

“The horizontal axis, sorry I don’t know the word —”

“abscissa”

“Oh, we have almost the same word in Spanish!  Anyhow, the abscisa says it shows wavelengths.  It goes from a tenth of a nanometer to maybe 10 micrometers.  The chart must have to do with light, because sound waves can’t get that short.  The … ordinada…?”

“Ordinate”

“Thank you.  The ordinate says ‘Intensity’ so the chart must show light spectra at different temperatures.  But there’s only one peak at each temperature.”

“Is that Kirchhoff’s ‘continuous spectrum,’ Cathleen?”

“Right, Vinnie, a smoothly-varying cascade of every wavelength, photons arising from heat-generated motion of charged particles.”

“Ah, ya lo veo — this is blackbody spectra given off by hot objects.  You showed us one in class and here we have several.”

“Good, Maria.  Now —”

“But all the peaks look exactly the same, Cathleen.  The hot objects ought to be brighter.  A really hot flame, you can’t even look at it.  Something’s phony.”

“Good eye, Vinnie.  I divided each curve in the graph by its peak height to put them all on an even footing.  That’s why the axis is labeled ‘Intensity profile‘ instead of ‘Intensity.'”

“I’ve got a different issue, Cathleen.  Hot objects have more energy to play with.  Shouldn’t the hotter peaks spread over a wider wavelength range?  These are all the same width.”

“I think I know the answer to that one, Mr Moire.  In class la profesora showed us how the blackbody curve’s equation has two factors, like B=W*X.  The W factor depends only on wavelength and grows bigger as the wavelength gets smaller.  That’s the ‘ultraviolet catastrophe,’ right, ma’am?”

“Mm-hm.  Go on, Maria.”

“But the X factor gets small real fast as the wavelength gets small.  In fact, it gets small so fast that it overpowers W‘s growth — the W*X product gets small, too.  Do you have that movie you showed us on your laptop there, ma’am?”

“Sure.  Here it is…”Blackbody peaks 1

“OK, the blue line is that W factor.  Oh, by the way, the ordinate scale here is logarithmic, so the value at the left end of the blue line is 1027/106 or about 1021 times bigger than it is at the right end even though it looks like a straight line.  The green line is that temperature-dependent factor.  See how it pulls down the orange lines’ values for cold objects, but practically goes away for very hot objects?”

“Yeah, that shows it real good, right, Sy?  That orange peak moves to the left just like Cathleen’s picture shows.  It answers your question, too.”

“It does, Vinnie?  How so?”

“‘Cause the peaks get broader as they get higher.  It’s like the intensity at the, umm, microwave end hardly changes at all and the whole rest of the curve swings up and out from there.”

“Keep in mind, guys, that we’re talking really large numbers here.  Vinnie’s ‘hardly changes at all’ is actually a factor of 40,000 or so.  Those pretty peaks in my homework chart are only pretty because the spread-out tails are so small relative to the peaks.”

“Alright, Cathleen, but how does Maria’s question tie in with mine?”

“They both hinge on wavelength.  The blackbody equation lets us measure a star’s temperature by looking at its color.  Do you have enough to start on that essay, Maria?”

“Yes, ma’am.  Gracias.”

“De nada.  Now run along and get to work on it.”

~~ Rich Olcott