Tag: Physics: Quantum Mechanics

What Are Quantum Birds Made Of?

On By rolcottIn Physics: Quantum Mechanics, UncategorizedLeave a comment

“Do quantum thingies follow the same rules that birds do, Uncle Sy?”

“Mostly not, Teena.  Some quantum rules are simple, others are complicated and many are weird.”

“Tell me a simple one and a weird one.”

“Hm… the Principle of Correspondence is simple.  It says if you’ve got a lot of quantum things acting together, the whole mishmash acts by the same rules that a regular-sized thing that size would follow.  If all those birds flew in every direction there’s no flock to talk about, but if they fly by flock rules we can talk about how wind affects the flock’s motion.”

“It’s a murmuration, Uncle Sy.”

“Correction noted, Sweetie.”

“Now tell me a weird one.”

“There’s the rule that a quantum thing acts like it’s in a specific place when you look at it but it’s spread out when you’re not looking.”

“Kittie does that!  She’s never where you look for her.”

“Mm, that’s kind of in the other direction.  We see quantum particles in specific somewheres, not specific nowheres.  The rule is called wave-particle duality and people have been trying to figure out how it works for a hundred years.  Let’s try this.  Put your thumb and forefinger up to your eye and look between them at the blue sky.  Hold your fingers very close together but don’t let them touch.  What do you see?”

“Ooo, there’s stripes in between!  It looks like my finger’s going right into my thumb, but I can feel they’re not touching.  Hey, it works with my other fingers, too, but it hurts if I try it with my pinkie.”

“Then don’t do it with your pinkie, silly.  The stripes are called ‘interference’ and only waves do that.  You’ve watched how water waves go up and down, right?”

“Sure!”

“When the high part of one wave meets the low part of another wave, what happens?”

“I guess high and low make middle.”

“Good guess, that’s exactly right.  That little teeny space between your fingers lets through only certain waves.  You see light where the highs and lows are, dark where the waves middle out.”

“So light’s made out of waves, huh?”

“Well, except that scientists have done lots of experiments where light behaves like it’s made out of little particles called photons.  The funny thing is, light always acts like a wave when it’s traveling from one place to another, but at both ends of the trip it always acts like photons.  That’s the big mystery — how does it do that?”

“You know how it works, don’tcha, Uncle Sy?”

“Only kinda sorta, Teena.  I think it has to do with the idea of big things made out of little things made out of littler things.  Einstein — wait, you know who Einstein was, right?”

“He was the famous scientist with the big hair.”

“That’s right.  He and another scientist had a big debate over 80 years ago.  The other scientist said that when quantum things make patterns, like those stripes you’re looking at, the patterns are all we can know about them.  Einstein said that there has to be something deeper down that drives the patterns.”

“Who won the debate?”

“At the time most people thought that the other man had, but philosophies change.  Since that time lots of people have followed Einstein’s thinking.  Some of the theories are pretty silly, I think, but I’m betting on birds made out of birds.”

“That’s silly, too, Uncle Sy.”

“Maybe, maybe not, we’ll see some day.  It starts with what you might call ‘the smallness quantum,’ though it’s also called ‘the Planck length‘ after Mr Planck who helped invent quantum mechanics.  The Planck length is awesomely small.  It’s as much smaller than us as we are smaller than the whole universe.”

“But there’s lots of things bigger than we are.”

“Exactly.  We’re smaller than whales, they’re smaller than planets, planets are smaller than suns, and galaxies, and on up.  But we don’t know near as many size scales in the other direction – us and bacteria and atoms and protons and that’s about it.  I think there’s plenty of room down there for structures and chaos we’ve not thought of yet.”

“Like birds in murmurations.”

“Mm-hmm.”Bird made out of birds 1

~~ Rich Olcott

Teena And The Quantum Birds

On By rolcottIn Physics: Quantum Mechanics, UncategorizedLeave a comment

“Hey, Uncle Sy, what’s quantum?”

“That’s a big question for a small person, Teena.  Where’d you hear that word?”

“You and Mommy were talking and you said that something had to do with quantum mechanics.  I know car mechanics work on cars so I want to know what the quantum mechanics work on.”

“That’s a fun question, Sweetie, because there actually is a kind of car called a Quantum.  Not very many of them and they’re made in England so you don’t often see one here.  But the quantum mechanics we were talking about is completely different.  I’ll take it one word at a time, OK?”

<sigh> “OK, but let’s sit on the porch swing, I can tell this will take a while.”

“Oh, it’s not going to be that bad.  You know what mechanisms are, right?”

“Um.. they’re not like people or animals and they’re not like my tablet thingie…. They’ve got gears and things.”

“Good enough.  A big part of physics is thinking about how mechanisms work and that’s called ‘mechanics.’  There’s lots of different kinds of mechanisms.  Each kind has a different kind of mechanics, like ‘celestial mechanics’ which is thinking about how stars and planets move, and ‘fluid mechanics’ which is thinking about how liquids and gases move.  With me so far?”

“So quantum mechanics is thinking about how quantums move.  But what’s a quantum?”

“Quantum isn’t a thing, it’s a set of rules that add up to be a theory.  The first rule is, it only applies to things that are very, very small.  That’s what the word ‘quantum’ has come to mean — the smallest possible amount of something.  So quantum rules apply to quantum-sized things.”

“As small as my water bears?”

“Much smaller.  Things that are as small compared to a water bear as a water bear egg is small compared to you.  Things like molecules and atoms, and those are made of lots of parts that are even way smaller.”

“Ooo, that’s teeny.  How do you even see them?”

“Well, you don’t.  They’re far too small to see even with a microscope.  It’s worse — if you did try to see an atom’s parts, any light you could shine on them would move them around so they’re not where they were when you started to look.”

“Then how do the quantum mechanics people learn about them?”

“Umm…  Ah! See that flock of birds flying past?”

“Mommy says they’re starlings but I think they’re blackbirds.”

“Could be either or both, it’s hard to tell when they’re in the air like that.  Sometimes the two kinds flock together.  If it’s a flock of starlings, the flock is called a murmuration, which is one of my favorite words.”

“Oh, that’ll be one of my favorites now, too.  Murmuration, mmmurmuration, mmmm.  I love  ‘M‘ words.”

“Anyway, can you see what direction any one bird is flying?”

“No, there’s too many and they go back and forth and it’s too confusing and I like the shapes the whole murmuration makes.”

“But can you point to the middle of it and see how the pattern moves?”

“It’s right the— ooo, look, it did a spiral!”

“Murmurations are sorta like the kind of thing the quantum mechanics people work with.  They look at lots and lots of quantum-size things to see how the typical ones and the special ones behave.  Then they try to work out what the behavior rules are.  Sometimes the rules are really simple, like the rules the birds use.”

“Birds use rules?  I thought they could fly wherever they wanted to.”

“Sometimes they do, but if they’re flying in a murmuration they definitely follow rules.  Most of them.  Most of the time.  If I were one of those birds, I’d stay about the same distance from each of my neighbor birds, I’d usually fly in about the same direction as my neighbors are flying, and I’d also aim at about the middle of the flo— murmuration.  Scientists have found that just those three rules account for most of how a murmuration behaves.  Cool, huh?”

“Simple rules for bird brains, that’s funny!”

“But look at the beautiful shapes those simple rules make.”Murmuration 1

~~ Rich Olcott

Abstract Horses

On By rolcottIn Physics: Einstein, Physics: Newton and Gravity, Physics: Quantum Mechanics, UncategorizedLeave a comment

It was a young man’s knock, eager and a bit less hesitant than his first visit.

“C’mon in, Jeremy, the door’s open.”

“Hi, Mr Moire, it’s me, Jerem…  How did ..?  Never mind.  Ready for my black hole questions?”

“I’ll do what I can, Jeremy, but mind you, even the cosmologists are still having a hard time understanding them.  What’s your first question?”

“I read where nothing can escape a black hole, not even light, but Hawking radiation does come out because of virtual particles and what’s that about?”

“That’s a very lumpy question.  Let’s unwrap it one layer at a time.  What’s a particle?”

“A little teeny bit of something that floats in the air and you don’t want to breathe it because it can give you cancer or something.”

“That, too, but we’re talking physics here.  The physics notion of a particle came from Newton.  He invented it on the way to his Law of Gravity and calculating the Moon’s orbit around the Earth.  He realized that he didn’t need to know what the Moon is made of or what color it is.  Same thing for the Earth — he didn’t need to account for the Earth’s temperature or the length of its day.  He didn’t even need to worry about whether either body was spherical.  His results showed he could make valid predictions by pretending that the Earth and the Moon were simply massive points floating in space.”

“Accio abstractify!  So that’s what a physics particle is?”

“Yup, just something that has mass and location and maybe a velocity.  That’s all you need to know to do motion calculations, unless the distance between the objects is comparable to their sizes, or they’ve got an electrical charge, or they move near lightspeed, or they’re so small that quantum effects come into play.  All other properties are irrelevant.”

“So that’s why he said that the Moon was attracted to Earth like the apple that fell on his head was — in his mind they were both just particles.”

“You got it, except that apple probably didn’t exist.”

“Whatever.  But what about virtual particles?  Do they have anything to do with VR goggles and like that?”

“Very little.  The Laws of Physics are optional inside a computer-controlled ‘reality.’  Virtual people can fly, flow of virtual time is arbitrary, virtual electrical forces can be made weaker or stronger than virtual gravity, whatever the programmers decide will further the narrative.  But virtual particles are much stranger than that.”

“Aw, they can’t be stranger than Minecraft.  Have you seen those zombie and skeleton horses?”Horses

“Yeah, actually, I have.  My niece plays Minecraft.  But at least those horses hang around.  Virtual particles are now you might see them, now you probably don’t.  They’re part of why quantum mechanics gave Einstein the willies.”

“Quantum mechanics comes into it?  Cool!  But what was Einstein’s problem?  Didn’t he invent quantum theory in the first place?”

“Oh, he was definitely one of the early leaders, along with Bohr, Heisenberg, Schrödinger and that lot.  But he was uncomfortable with how the community interpreted Schrödinger’s wave equation.  His row with Bohr was particularly intense, and there’s reason to believe that Bohr never properly understood the point that Einstein was trying to make.”

“Sounds like me and my Dad.  So what was Einstein’s point?”

“Basically, it’s that the quantum equations are about particles in Newton’s sense.  They lead to extremely accurate predictions of experimental results, but there’s a lot of abstraction on the way to those concrete results.  In the same way that Newton reduced Earth and Moon to mathematical objects, physicists reduced electrons and atomic nuclei to mathematical objects.”

“So they leave out stuff like what the Earth and Moon are made of.  Kinda.”

“Exactly.  Bohr’s interpretation was that quantum equations are statistical, that they give averages and relative probabilities –”

“– Like Schrödinger’s cat being alive AND dead –”

“– right, and Einstein’s question was, ‘Averages of what?‘  He felt that quantum theory’s statistical waves summarize underlying goings-on like ocean waves summarize what water molecules do.  Maybe quantum theory’s underlying layer is more particles.”

“Are those the virtual particles?”

“We’re almost there, but I’ve got an appointment.  Bye.”

“Sure.  Uhh… bye.”

~~ Rich Olcott

Location, Location, Location

On By rolcottIn Physics: Quantum Mechanics, UncategorizedLeave a comment

“Hoy, Johnny, still got that particle inna box?”
“Sure do, Jessie.”
“So where’s hit in there?”
“Me Pap says hit’s spread-out like but hit’s mostly inna middle.”
“Why’s hit spread then?”
“The more I taps the box, the wider hit spreads. Sommat to do wiff energy.”

PIB0
Newton would have answered Jessie’s question by saying, sort of, “Pick a point anywhere in the box.  The probability that the particle is at that point is equal to the probability that it’s at any other point.” PIB stack

Quantum physicists take a different approach. They start by saying, “We know there’s zero probability that the particle is anywhere outside of the box, so there must be zero probability that it’s exactly at any wall.”

Now for a trick that we’re actually quite used to.  When you listen to an orchestra, you can usually pick out the notes being played by a particular instrument.  Someone blessed/cursed with perfect pitch can tell when a note is just a leetle bit flat, say an A being played at 438 cycles instead of 440. You can create any sound by mixing together the right frequencies in the right proportion. That’s how an MP3 recorder does it.

QM solutions use that strategy the other way round. They calculate probabilities by adding together sets of symmetric elementary shapes, all of which are zero at certain places, like the box walls. For instance, on average Johnnie’s particle will be near the middle of his box, so we start a set with an orange mound of probability right there. That mound is like our base frequency — it has no nodes, no non-wall places where the probability is zero.

Then we add a first overtone, the one-node yellow shape that represents equal probability on either side of a plane of zero probability.

Two nodal planes at right angles give us the four-peaked green shape. Further steps up have more and more nodal planes (cyan then blue, and so on). The video shows the running total up to 46 nodes.

.PIB sum
As we add more nodes, the cumulative shape gets smoother and broader.  After a huge number of steps, the sum will look pretty much like Newton’s (except for right at the walls, of course).

So if the classical and QM boxes wind up looking the same, why go to all that trouble?  Because those nodes don’t come for free.

Inverse tennisSuppose you’re playing goalie in an inverse tennis game.  There’s a player in each service box.  Your job is to run the net line using your rackets to prevent either player from getting a ball into the opposing half-court.  Basically, you want the ball’s locations to look like the single-node yellow shape up above.  You’ll have to work hard to do that.

Now suppose they give you a second, crosswise net (the green shape).  You’re going to have to work twice as hard.  Now add a third net, and so on … each additional nodal plane is going to be harder (cost more energy) to keep empty.  Not a problem if you have an infinite amount of energy.

Enter Planck and Einstein.  They showed there’s a limit for small systems like atoms and molecules.  Electrons dash about in atom- or molecule-shaped boxes, but the principle is the same.  The total probability distribution is still the sum of bounded elementary shapes.  However, you can’t use an infinite number of them.  Rather, you start with the cheapest shapes (the fewest nodes) and build upward.

Tally two electrons for each shape you use.  Why two?  Because that’s the rule, no arguments.

It’s important to realize that QM does NOT say that two specific electrons occupy one shape.  All the charge is spread out over all the shapes — we’re just keeping count.

When you run out of electrons the accumulated model shows everything we can know about the electronic configuration.  You won’t know where any particular electron is, but you’ll know where some electron spends some time.  For a chemist that’s the important thing — the peaks and nodes, the centers of negative and positive charge, are the most likely regions for chemical reactions to happen.

Johnnie’s energetic taps make his particle boldly go where no particle has gone before.

~~ Rich Olcott

Particles and Poetry

On By rolcottIn Physics: Quantum Mechanics, UncategorizedLeave a comment

“Hoy, Johnny, wotcher got inna box?”
“Hit’s a particle, Jessie.”
“Ooo, lovely for you.  Umm… wot’s a particle then?”
“Me Pap says hit’s sommat you calc’late about wiffout knowin’ wot ’tis.”

Pap’s right.  Newton was a particle guy all the way (he was a strong supporter of the idea that light is composed of particles).  One of his most important insights was that he could simplify gravitational calculations if he replaced an object with an equally massive “particle” located at the object’s center of mass.  Could be a planet, or a moon, or that apple — he could treat each of them as a “particle.”  That worked fine for his purposes, because the distances between his object centers were vastly larger than the object sizes.

Fleas
“Great fleas have little fleas upon their backs to bite ’em / And little fleas have lesser fleas and so on infinitum.” ~~ Augustus De Morgan

It took Roche to work out what happens when the distances get small.  Gravitational forces break the original “particles” into littler particles.  And when two of the little ones approach closely enough they break up, and then those break up…  You get the idea.  Take the process far enough and you get Saturn’s Rings, for instance.

But the analysis can keep going.  Consider one “particle” in Saturn’s A-ring.  It’s probably about 3″ across, made of ice, and contains something like 1024 particles that happen to be molecules of H2O.  Each molecule contains 3 nuclei (2 protons and one oxygen nucleus) and 10 electrons, all 13 of which merit “particle” status if you’re calculating molecules.  They’re all held together by a blizzard of photons carrying the electromagnetic forces between them.  The oxygen nucleus contains 16 nuclear particles, each of which contains 3 quarks.  The quark structures would fly apart except for a host of gluons that pass back and forth transmitting the nuclear strong force.  Hooboy, do we got particles.

“Particle” is a slippery word.  For Newton’s purposes, if an object is small relative to its distance from other objects, that was all he needed to know to treat it as a particle.

One dictionary specifies “a small localized object which has identifiable physical or chemical properties such as volume or mass.”  However, there are theoretical grounds to believe that the classic “particle of light,” the photon, has neither mass nor volume.  Physicists have had long arguments trying to devise a good working definition.  Nobelist (1999) Gerard ‘t Hooft ended one such discussion by saying, “A particle is fundamental when it’s useful to think of it as fundamental.”

It may seem a little strange for a physicist to argue for imprecision.  In fact, ‘t Hooft was arguing for a broad, even poetic but still precise understanding of the word.

Poets use metaphor to help us understand the world.  Part of their art is to pack as much meaning as they can into the minimum number of words.  In the same way, scientists use mathematics to pack observed relationships into a simile called an equation  — a brief bit of math may connect and illuminate many disparate phenomena.

Think of physics as metaphor, with numbers.

Newton’s Law of Gravity works for for galaxies roving through a cluster and for basketball-sized satellites orbiting Earth and for stars circling a black hole (if they don’t get too close).  Maxwell’s Equations, just 30 symbols including parentheses and equal signs, give the speed of light and describe the operation of electric motors.  The particle physicists’ Standard Model makes predictions that match experimental results to more than a dozen decimal places.

Good equations are so successful that Nobelist (1963) Eugene Wigner wrote an influential paper entitled The Unreasonable Effectiveness of Mathematics in the Natural Sciences.

We sometimes get into trouble by confusing metaphor with reality.  Poetic metaphors can be carried too far — Hamlet’s lungs were not in fact filling with water from his “sea of troubles.”

Mathematical models can also be carried too far.  Popular (and practitioner) discussion of quantum mechanics is rife with over-extended metaphors.  QM calculations yield only statistical results — an average position, say, plus or minus so much.  It’s an average, but of what?  The “many worlds” hypothesis is an unnecessarily long jump.  There are simpler, less extravagant ways to account for statistical uncertainty. les Etats Unis

~~ Rich Olcott

Perturbed? You’re not the only one

On By rolcottIn Physics: Fundamentals, Physics: Quantum Mechanics, UncategorizedLeave a comment
Dolls
Successive approximations
to a real girl, but still not there

It started with the Babylonians.  The Greeks abhorred the notion.  The Egyptians and Romans couldn’t have gotten along without it. Only 1600 years later did Newton gave final polishing to … The Method of Successive Approximations.

Stay with me, we’ll get to The Chicken soon.

Suppose for some weird reason you wanted to know the square root of 2701.  Any Babylonian could see immediately that 2701 is a bit less than 3600 = 602, so as a first approximation they’d guess ½(60 + (2701/60)) = 52.5.  They’d do the multiplication to check: 52.5×52.5 = 2756.25.

Well, 52.5 is closer than 60 but not close enough.  So they’d plug that number into the same formula to get the next successive approximation: ½(52.5 + 2701/52.5) = 51.97.  Check it: 51.97×51.97 = 2700.88.  That was probably good enough for government work in Babylonia, but if the boss wanted an even better estimate they could go around the loop again.

Scientists and engineers tackle a complex problem piecewise.  Start by looking for a simple problem you know how to solve. Adjust that solution little by little to account for the ways in which the real system differs from the simple case.  Successive Approximation is only one of many adjustment strategies invented over the centuries.

The most widely-used technique is called Perturbation Theory (which has nothing to do with the ways kids find to get on their parents’ nerves).  The strategy is to find some single parameter, maybe a ratio of two masses or the relative strength of a particle-particle interaction.  For a realistic solution, it’s important that the parameter’s value be small compared to other quantities in the problem.

Simplify the original problem by keeping that parameter in the equations but assume that it’s zero.  When you’ve found a solution to that problem, you “perturb” the solution — you see what happens to the model when you allow the parameter to be non-zero.

There’s an old story, famous among physicists and engineers, about an association of farmers who wanted to design an optimum chicken-raising operation.  Maybe with an optimal chicken house they could heat the place with the birds’ own body heat, things like that.  They called in an engineering consultant.  He looked around some running farms, took lots of measurements, and went away to compute.  A couple of weeks later he came back, with slides.  (I told you it’s an old story.)  He started to walk the group though his logic, but he lost them when he opened his pitch with, “Assume a spherical chicken…”

Fat chick bank
Henrietta
Fat Chicken Bank by Becky Zee

Now, he may actually have been on the right track.  It’s a known fact that many biological processes (digestion, metabolism, drug dosage, etc.) depend on an organism’s surface area.  A chicken’s surface area could be key to calculating her heat production.  But chickens (for example, our charming Henrietta) have a complicated shape with a poorly-defined surface area.  The engineer’s approximation strategy must have been to estimate each bird as a sphere with a tweakable perturbation parameter reflecting how spherical they aren’t.

Then, of course, he’d have to apply a second adjustment for feathers, but I digress.

Now here’s the thing.  In quantum mechanics there’s only a half-dozen generic systems with exact solutions qualifying them to be “simple” Perturbation Theory starters.  Johnny’s beloved Particle In A Box (coming next week) is one of them.  The others all depend in similar logic — the particle (there’s always only one of them) is confined to a region which contains places where the particle’s not allowed to be. (There’s one exception: the Free Particle has no boundaries and therefore is evenly smeared across the Universe.)

Virtually all other quantum-based results — multi-electron atoms, molecular structures, Feynman diagrams for sub-atomic physics, string theories, whatever — depend on Perturbation Theory.  (The exceptions are topology and group-theory techniques that generally attempt to produce qualitative rather quantitative predictions.)  They need those tweakable parameters.

In quantum-chemical calculations the perturbation parameters are generally reasonably small or at least controllable.  That’s not true for many of the other areas.  This issue is especially problematic for string theory.  In many of its proposed problem solutions no-one knows whether a first-, second- or higher-level approximation even exists, much less whether it would produce reasonable predictions.

I find that perturbing.

~~ Rich Olcott