Show Notes

• Doppler Effect — U of Illinois
• Doppler Effect flash animation
• How sound travels though different media — GSU
• Video:  Seeing sound waves
• Wavelength shift for moving objects — University of Oregon
• Doppler shift to measure how fast an object is moving — Virginia U
• Redshift and the expansion of the Universe — U of Illinois
• Using Doppler Spectroscopy to find extrasolar planets — HowStuffWorks
• Super-Earth
• National Radio Astronomy Observatory
• Christian Doppler
• Doppler’s original paper on binary stars (bid on it at Christie’s!)

Transcript: Doppler Effect

Download the transcript

Fraser: Astronomy Cast Episode 165 for Monday November 23, 2009, Doppler Effect. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville. Hello Pamela.

Pamela: Hey, Fraser, how’s it going?

Fraser: It’s good. It’s very cold here. We’re having a very unusual cold snap, so, normally I work upstairs in the winter where it’s nice and warm at the kitchen table… you can see the sun… but, I’m downstairs in my recording studio and got about eight layers of clothing on… gloves… and I wanna get out of here… So let’s get this show over with really quick!

Pamela: Sounds good.

Fraser: So you know how a police siren changes sound when it passes by you… that’s the Doppler effect. It works for sound waves and it works for light waves. Astronomers use the Doppler effect to study the motion of objects across the universe, from nearby extrasolar planets, to the expansion of distant galaxies. Ok, Pamela, so let’s use that concept of you standing there and some object is speeding towards you to explain the Doppler effect.

Pamela: Well, the best way to think of it is that you’re changing the rate at which things arrive simply by shortening the distance they have to travel. Sound is made up of a series of waves. So when you’re listening to a constant-pitch noise… when you’re listening to the constant noise of a trumpet player playing a single note…

Fraser: Right, or like a car engine… like a really loud car engine… Actually, if anyone’s every seen a car race… like a Formula One going past you… oh, you really hear it.

Pamela: So, that constant noise is actually made up of a whole series of compression waves through the air where the air molecules get a little bit packed together, and each of these packed-together lumps of air, as they hit your eardrum, they cause it to vibrate and the faster your ear’s getting hit by the vibrations, the higher pitched you hear the noise. Whereas if the compression waves are coming with more time between them, we hear that as a lower-pitched noise.

Fraser: Right, ok… so the pitch of the sound that we hear is purely how often these compression waves are bonking into our eardrum. If they’re hitting really quickly, then we hear a high-pitched noise, and if they’re very slowly or not as often, then we hear a lower-pitched noise.

Pamela: And the rate at which these waves hit your ear can be affected by how you’re moving, or how the object emitting the sound is moving. So you can imagine that you have a bucket of tennis balls and for some crazy reason you’re running toward a target, throwing tennis balls at the target and every tennis ball you throw, you throw at the exact same velocity. You throw them one per second, but because you’re moving toward the target, each tennis ball has a little bit less distance that it has to travel. So the target that’s getting hit by the tennis balls, it’s receiving tennis balls more frequently than once per second, even though you’re throwing the tennis balls at once per second. It’s that difference in distance that each ball has to travel that’s causing the tennis balls to arrive faster as you run faster toward the target.

Fraser: So, right, just to be clear then… this has nothing to do with the velocity of the car moving towards you… it’s not like the sound waves coming off the car are being added by the velocity of the car moving forward, right?

Pamela: Right, it’s all about the distance that the waves end up having to travel. The waves are always going to move at the same rate through a given medium.

Fraser: The speed of sound, right?

Pamela: The speed of sound… the speed of light… and each medium has its own speed of sound and its own speed of light. This is the rate at which waves travel. But, what we’re mucking about with is how far each of these waves have to travel. By causing the waves to either get packed together because the source emitting them is moving towards you, or to get spread apart because if you’re running away from the target, throwing a ball once per second, now each ball has to travel a little bit further. So, you’re spreading out the amount of time that they end up hitting successively the target at.

Fraser: Right, right, right… so once again, the actual speed of the waves moving through the air is still the speed of sound. It’s just that you’re putting more distance as you go, and so those waves are having to travel a little more distance and so it feels like they’re hitting at a slower rate… hitting your eardrum… and so you get the lower sound.

Pamela: Exactly.

Fraser: And so that’s why you get that… as the race car is moving towards you, you hear the high-pitched whine. Then as it goes past, then you hear… it sort of gets lower again. So… I’m going to try it… [Fraser’s race car sound effect] Like that!

Pamela: Exactly!

Fraser: Yeah, and that is the Doppler effect. So how does that translate to the universe with light?

Pamela: In day to day life, the way that it affects us is objects that are moving towards us… well, each successive wave doesn’t has to travel as far as the one before it. So, we see colors get bluer. Or if it’s an approaching vehicle, we hear the sound at a higher pitch. Now what’s kinda neat is there’s this weird misconception, especially with sound, that an object right near you is going to have its light shifted or its sound shifted a whole lot more. But the reality is that the shift is caused strictly by velocity. So if you have a train coming straight at you at a constant velocity, because it’s coming straight at you, you’re going to hear the pitch always exactly the same.

Fraser: Right.

Pamela: Now the reason you hear pitch change, is if you’re standing on a sidewalk and something is about to race past you, that moment that it’s directly in front of you… it’s not moving towards you… it’s not moving away from you… so you hear its sound as if it wasn’t shifted at all. And when it’s very far away, that’s when its sound is shifted the most because the rate at which it’s moving towards you or away from you is the greatest. But, as it gets closer and closer, more of its velocity is in front of you rather than toward or away from you. This is where horizontal velocity and perpendicular velocity start to matter. But, the least amount of shifting occurs when something is moving from left to right. The most amount of shifting comes when something is moving towards you or away from you. Distance doesn’t matter… all that matters is the velocity that something is moving at.

Fraser: That’s a really good point, I guess. So, with sound waves, we hear an increase or decrease in the pitch so it sounds like it’s moving faster or moving slower… it’s that change in pitch. So the effect on light waves is the change in color.

Pamela: Yes. So, this is the most typical example that people think about. If you were racing towards a stoplight…. as you race toward the light, the red color is getting shifted bluer and bluer and bluer. And along the way to blue, you shift through green. In fact, if you were able to get your vehicle going at a good fraction of the speed of light, you could actually shift the color of that stop light, making it a go light.

Fraser: You should totally use that with the cops. If you get pulled over for speeding or running a red light… yeah, I was driving so fast that it shifted to green!

Pamela: Yeah, except I’d much rather get a ticket for blowing a stop light than for going that recklessly high of a speed.

Fraser: A fraction of the speed of light, yeah… Ok, so as something is moving towards you, its light is shifted towards the blue. So red becomes green becomes blue… and if something’s moving away from you, it gets shifted towards the red. So, blue becomes green becomes red.

Pamela: And this is where things that are in the ultraviolet that we couldn’t normally see here on Earth, because our atmosphere tends to block a lot of the ultraviolet, the most distant galaxies in the universe… they’re moving away from us so fast that that ultraviolet light has gone into visible light and into infrared light.

Fraser: Yeah, and this is where we talk about the implications of the Doppler effect. So then what are some ways… and we’ve talked about Edwin Hubble, so let’s talk about some of the big ways that the Doppler shift is used in astronomy.

Pamela: Well, there’s three main areas that we use it scientifically. One is you can use it the most simplistic way to measure temperature. You can use it to measure how fast things are moving on their orbital paths. You can also use it to measure the expansion of the universe.

Fraser: Ok, so temperature… how do they use it to measure temperature?

Pamela: Well, if you have a gas, the molecules… the atoms in the gas… are going to be moving at a rate that is determined by the temperature of the gas. So if the gas is very hot, the gas has a lot of energy and the particles in it are moving very quickly. Now, some of those particles are going to be moving towards you, and some of them are going to be moving away from you. If the particles are excited enough that they’re giving off light, those individual particles’ motions are going to cause the light that’s being emitted to not be just seen as a single color, but seen as a whole variety of colors because some of the atom’s light will be blue-shifted, some of the atom’s light will be red-shifted… So you end up seeing the color emitted instead of as a single wavelength, as a spread across several wavelengths. A large-velocity gas will end up with a larger spread in the emitted colors and light that’s cold, that has kind of slow-moving particles, in that case you’re going to end up with a very, very narrow line.

Fraser: So, then for example, let’s say that you had this big ball of gas and it was rotating at some velocity…

Pamela: Maybe sort of call it the sun…

Fraser: Sure, this ball of gas we like to call the sun… we would then see, for example, the side of the sun that’s rotating away from us changing… so it’s moving away… and so it gets shifted towards the red… so it gets redder. And then the side of the sun that’s moving towards us would be shifted towards the blue. Then we would be able to calculate how quickly it’s rotating?

Pamela: Yes, and we refer to this as line broadening. The faster the object is rotating, the broader the spectral lines are going to be. So it gets very complicated, though, sorting out… ok, this amount of broadening is due to the velocities, and this amount of broadening is due to the rotation of the object, and gravity can also affect line broadening, as well. But at the end of the day, we can start to get at stellar rotation rates by looking at how broad the spectral lines are in the stars.

Fraser: And so astronomers can… I’m sure they have methods that they can distinguish between the star is careening towards us, and which is its rotation.

Pamela: Right. So, the careening towards us shifts all the lines… their centers go all the way to the red if it’s moving away, all the way to the blue if it’s careening towards us. So all of the lines’ center points get moved. Now if something’s rotating quickly, then each line gets fatter and fatter the faster and faster the object’s rotating. So if you have a star that isn’t moving relative to the planet Earth… it’s not coming towards us, it’s not going away from us, and it’s rotating very fast. You’ll see these nice, fat, spectral lines.

Fraser: And how precise is this?

Pamela: This is where is starts to get complicated by things like the gravity of the star. You can get order of… I remember making measurements… order of tens of kilometers per second. Some stars are easier than others. It all depends on the surface gravity.

Fraser: Right, and just to give you another piece of information… like the Earth… if you’re standing on the equator, you’re going about 1600 kilometers per second hour. So, for you to know within tens is pretty precise.

Pamela: And like I said, it all depends on what type of star we’re looking at. But being able to make precise measurements is actually really important because… well, we have the technology to make measurements as precise as one meter per second. That’s the rate at which a human being can walk if they have long legs and they’re moving fairly quickly.

Fraser: Wow…

Pamela: So we can make that accurate of a measurement. And because we can make measurements that accurate, we can start to look at the back and forth tugs that stars experience due to having planets orbiting around them.

Fraser: Right. And this is that next, second big use…. is using the Doppler effect to measure the motion of entire objects—planets, pulsars, you name it, right?

Pamela: Right. And with the Doppler effect we’ve gone from the extremes of starting to find Neptune-sized worlds around alien stars, to also finding systems that have a neutron star and a black hole orbiting around each other. It’s all the same science. It’s all looking at how the Doppler shift affects how we receive light.

Fraser: And so then how is this different from… we’re detecting how excited particles are releasing photons… in this case we’re looking at how an entire object is moving towards us or away from us….

Pamela: It’s a matter of looking at aggregate behavior. So if we’re looking at random blob of gas… it’s going to have a central velocity… this is all the particles are off-set one way or another. And then relative to that off-set, the particles might have an individual velocity. So you can imagine the bulk motion of a star moving away from us. It might have a bulk velocity of 100s of kilometers per second. Whereas the rate it’s rotating… that has a much smaller effect on top of that that broadens the lines out. So the centers of the lines define the velocity towards or away from us. The width of the line defines the rotation of the object.

Fraser: Ok, so then how is this used for discovering planets?

Pamela: Well, with the planets what we look at is how is that central point in the line behaving? We know that it’s going to have a general motion towards or away from us, but the way stars that are being orbited by planets move is sort of like watching people square dance where they’re do-si-do-ing while they’re going around a large circle. So you have individuals that as they lock arms, they rotate around each other. But as they go from person to person in the square dance, the contra dance, the choose you’re regional dance of choice, they go round and round individuals while going around a larger circle at the same time. Well, as we look at planetary motion and motion of stars as they orbit around the galaxy, we have the planets basically do-si-do-ing with their star, so the star will appear to move on a very small scale a little bit toward us, a little bit away from us, a little bit toward us, a little bit away from us; while at the same time consistently moving at a much different scale on average always towards us or always away from us. So you might see it moving towards us at, and I’m just making up numbers here, an average rate of 100 kilometers per second. But, some days it’s 100 kilometers per second, minus one or two meters per second. Other days it might be 100 kilometers per second toward us, and then that little extra few meters per second is in the opposite direction.

Fraser: Yeah, and this is the gravitational wobbling coming from the gravity of the planet yanking the star back and forth.

Pamela: And so what we’re doing is we’re very carefully using Doppler shifting to measure the centers of the lines to see what is first of all the average distance… subtract off the average… then you look at the residuals and look for that do-si-do… that little bit forward, little bit away, little bit forward, little bit away that comes from a planet orbiting another star.

Fraser: And this is what has turned up planets in the last couple of decades, and I guess the precision of this technique has gotten to the point now where we’re right on the cusp of being able to find super-Earths… planets with several multiples of the size of the earth… of the mass of the earth… orbiting a nearby star. You can imagine that if they keep refining this technique, we’ll start turning up Earth-sized worlds around other stars.

Pamela: One of the neat side ways that this can also get used is it’s not always the individual wave lengths that we look to have shifted. With pulsar planets, we have a neutron star that’s rotating very quickly, and as it rotates, it beams toward and away from us a hot spot… a bright spot that we see as a pulsing on and off. Well, with pulsars, you can look at them and in a few cases they have had planets discovered around them because as the entire neutron star gets slightly yanked around, the time that we receive the pulses gets changed as well. So this is where you can actually start listening to the Doppler shift using radio telescopes.

Fraser: Right, right. And so the different wavelengths, in the case of radio telescopes, you can build a great big worldwide telescope array and really take advantage of the size of that wavelength to detect these motions very carefully. But it only works if you’ve got a bright radio source, right? It doesn’t work in the case of a star that isn’t very bright in the radio spectrum.

Pamela: Right, so this is something that you do with pulsars, and it’s the same science but instead of looking at individual colors of light, you’re looking at the timing change in the pulse of light that’s coming from the hot spot on the neutron star.

Fraser: But that’s very similar to the sound waves bouncing against your eardrum. The pulsar is releasing these radio bursts in a very predictable pattern, then as it’s moving towards you, you’re then seeing them more often. I guess if they wanted they could also measure the wavelength of the radio waves and also detect the motion that way, right?

Pamela: That starts to get a little bit more complicated simply because radio waves are so darn big.

Fraser: Right, that’s what I’m saying… you have the two ways you could get at it, but one is so much easier to do.

Pamela: Right.

Fraser: The timing with your atomic clocks and stuff… Ok, so we talked about the two ways, right? We’ve got the motions of balls of gas, like a star. We’ve got the motions of orbits of planets going around stars and planets going around neutron stars, neutron stars going around black holes… so the third way is the movement of galaxies. This is probably one of the most important discoveries in human history.

Pamela: So, as we talked about in our last episode where we discussed Hubble, we live in an expanding universe. And because of the way it’s expanding, things that are nearby… there’s not as much stuff to expand, so we see them moving away fairly slowly. But as you get a little bit further away, there’s now a little more stuff expanding so that object appears to be moving away faster. As you add more and more stuff that’s all expanding, things get to be moving close to the speed of light. We perceive this as a change in the color of the objects that we’re looking at. So we see ultraviolet light that gets shifted into the visible and to the infrared. We start to be able to see wavelengths that normally we’d never be able to see just because our atmosphere doesn’t allow them. But, because of the shifting, they get shifted into visible parts of the spectrum that we can see here on the planet Earth. This simple process has allowed us to do all sorts of different types of science. On one hand, by combining distance measurements using standard candles like supernovae… we had an entire show on that… we’re able to say, ok… this chunk of universe is moving at this speed away from us. This chunk of universe is moving at this other speed away from us. Putting those pieces together, we’re able to say… oh no… insert a few expletives… the universe is accelerating apart. And we know the universe is accelerating apart because of the Doppler shift. Now we can also use this to figure out… oh, there must be a blob of matter over there that we can’t see because we see this flow of material toward a hidden great attractor that lies somewhere beyond the dusty edge of the Milky Way galaxy. We’re able to say… oh, over in this part of space there’s not that much stuff, because the stuff over there… well, it’s not getting held onto tightly enough so it appears to be moving away.

Fraser: That’s pretty amazing. It’s almost like you could imagine there’s like a whole bunch of cars all around you, moving away from you. Yet you can kind of tell that two cars off on the right are driving a little off in a funny direction because the pitch of their engines is a little off. It’s just amazing that astronomers have been able to figure out this kind of stuff.

Pamela: And this is one of the ways that we paint in very broad brush strokes the distribution of matter in the universe. When we want to get to finer grains, we do other things. Now the one thing that we sort of left out when we were talking about rotations is actually one of the neatest parts of galactic astronomy to crop up in the last ten years, and that’s the discovery of black holes in the hearts of galaxies. We know that we have a black hole in the heart of our own Milky Way because we can see objects whipping around it at ungodly speeds, and it’s absolutely amazing. Andrea Ghez has some really awesome animations that she’s put together. But we can’t resolve individual stars orbiting in the cores of other galaxies. But what we can do, using instruments like Hubble STIS, is take spectra across the cores of other galaxies, and then look to see how one side of that core is racing towards us at high speeds, and the other side of that core is racing away from us at high speeds. Then, we calculate how much mass has to be inside that core to cause these really high-velocity orbits.

Fraser: And aren’t they orbiting at essentially the limits that relativity predicts? They’re essentially moving at close to the speed of light… as fast as possible.

Pamela: As we start looking at the gas particles down in the center accretion disks, we have to use every ounce of relativity that we know to start to explain all of these different velocities. And what’s amazing is that they’re actually able to start looking at some cases at the cores of galaxies and going ah ha… we know that the super-massive black hole in the center must be rotating because of how we see the material in the accretion disk moving.

Fraser: Well, thanks, Einstein.

Pamela: This actually moves even beyond Einstein because Einstein, Schwarzschild, they all liked to work in a system where black holes didn’t rotate, and rotating black holes just make the whole universe a lot more complicated.

Fraser: Perfect. One last question for you… where does the term Doppler come from?

Pamela: From a dead white dude.

Fraser: Ah, so it’s named after somebody.

Pamela: Back in 1842, a now-dead white dude by the name of Doppler wrote a treatise on how he thought that this probably ended up working. You know, if you come up with a really good idea and you happen to be the first person to publish on it, sometimes you get lucky. This particular paper had a really long name that’s German, and I’m not going to humiliate myself by trying to say it. He was actually looking at binary stars, and he was making predictions about how the orbits of binary stars would affect the perceived color of the light.

Fraser: Really? So that was thought about more in terms of light than of sound?

Pamela: It initially came out of looking at light. Now, think about it, we really didn’t have that many high-velocity moving things…

Fraser: Right, we had an ox cart moving towards you and then away from you…

Pamela: We had a steam cars back then, but to really start worrying about sound a lot, you need race cars.

Fraser: That’s really interesting. Alright, well thanks, Pamela.

Pamela: It’s been my pleasure, Fraser.

Fraser: Alright, we’ll talk to you next time.

Pamela: Ok, bye-bye.

Shownotes

• Ernest Rutherford — Nobel Prize page
• Plum Pudding model — Wiki
• J.J. Thomson, discovers the electron — Chemical Achievers
• How we saw the atom 100 years ago –American Institute of Physics
• Atomic theory; the early days – Vision Learning
• Atom Tutorial – Learn Chem
• The Structure of the Atom — Purdue
• Proton
• Neutron
• Electron
• Quantized energy — Kent Chemistry
• Pauli Exclusion Principle — GSU
• new spectroscopy technique to take “snapshots” of complex molecules – RSC
• Atomic mass is determined by the number of neutrons and protons that are present in the nucleus
• How Carbon-14 dating works — All About Archaeology
• Bohr Model — UTK
• Quarks — GSU
• More about quarks — Stanford
• Ep. #106:  Search for the Theory of Everything
• Ep. #138:  Quantum Mechanics
• Lambda particles — GSU
• What is the largest possible atom? Argonne National Lab
• 1995 press release about the discovery of the Top Quark — Fermilab

Transcript: Inside the Atom

Download the transcript

Fraser: Astronomy Cast Episode 164 for Monday November 16, 2009, Inside the Atom. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville. Hey, Pamela.

Pamela: Hey, Fraser, how’s it going?

Fraser: It’s going very well. Very wet… but that’s the west coast in the winter.

Pamela: Yeah, we’re having the same thing here. Our wet is just a lot less wet than your wet.

Fraser: Yeah, oh, it’s crushing, it’s so wet… anyway, but, you know… it woud be nice to have a little bit of sun. I think my vitamin D level is low. So, we’ve talked about the biggest of the big, and now let’s focus in on the smallest of the small. Let’s see what’s inside that most basic building block of matter, the atom. Now, you probably know the basics, but with ever more powerful particle accelerators, physicists are revealing particles within particles, announcing new discoveries all the time. Alright, Pamela, so let’s come up with the simple, simple structure of the atom for starters and how we learned that. So, I think most of us know that atoms are the smallest particles that were first discovered that made up all of matter. We’re all made of atoms; molecules are made up of atoms. So what is the beginning structure of an atom?

Pamela: Well, initially we had it wrong. But sometimes it helps to start with what we did wrong to understand how we actually understand it today. So it all started with Rutherford, who came up with this idea for the model that said… ok, we have an atom; it’s made of a medley of electrons, of protons, of neutrons… we didn’t know all of those bits yet, but all this stuff is thrown in together. This was called the plum pudding model. J.J. Thompson, who was the discoverer of the electron in 1897, he decided he was going to test this plum pudding model. The idea was you take a very thin piece of gold sheet, and you fire electrons at it. If you get a plum pudding model, if you have this even smattering of everything put together, then you have all throughout it random scattering events. The electron flies in, it hits a proton, it flies off. It goes in, it hits a another electron, it flies off.

Fraser: Or it goes through and doesn’t hit anything.

Pamela: Or it goes through and doesn’t hit anything. But because you have this random mashing of everything together, the probability that it’s actually going to hit something is pretty high. It’s sort of like if you take 15 of your friends, and you tell them scatter yourselves about the driveway, and then you launch yourself on rollerskates down the driveway, you’re probably going to nail one of your friends. And they’re going to arrest you for crashing into the driveway. Now, the reality is, he started firing away at the gold foil, and most of the electrons went straight on through. And this was kind of confusing… because he had the wrong model. In reality, atoms have most of their mass—all of the neutrons, all of the protons—crammed down in the center. This is more of the case where you get your friends, you pile them in the driveway, and you tell them to get as close together as they possibly can… to stand in a really tight-knit huddle. Now in this case when you fire yourself down the driveway on your roller skates, you’re probably going to miss them and keep going and crash into whatever’s beyond your driveway. Well in this case when he fired, most of the stuff passed straight on through. But when he did occasionally make a solid connection, he got big scattering events and angles that indicated most of the mass was down in the center and most of the reactions were down in the center. It was a really neat experiment that forced him and everyone else to change how we looked at the atom.

Fraser: Right… as I recall it was an incredibly thin sheet of gold foil and you could imagine this constant beam of electrons firing out of this gun and them almost all going through, and then every now and then they were scattering back out, hitting detectors around the room.

Pamela: And it was Rutherford who interpreted this 1909 experiment to come up with our modern understanding of the atom. It’s just amazing to me to think that it was only 100 years ago, now that we’re recording this in 2009, it was only 100 years ago that we finally figured out what the model of the atom actually should look like.

Fraser: And so then we had the plum pudding model, so then that first model then was a tight bunch of protons and neutrons and then nothing…

Pamela: And then pretty much nothing. You have your electrons in a shell outside of this. It was actually the Bohr model a few years ago that started to really understand what the electrons were doing in their shell.

Fraser: But they first just thought that they were electrons going around the nucleus, kind of like a solar system, right?

Pamela: Right. And in fact, when you start looking at the equations that explain the electromagnetic force, that explain the pull between the electrons and protons, that force looks mathematically very, very similar to how gravity looks. And so it was easy to imagine that just as the planets orbit the sun, the electrons orbit the center of the atom… orbit the nucleus. Now the problem is, I can take a planet and throw it anywhere in the solar system, and as long as I throw it there with the right velocity, it’s happily going to orbit. But in atoms, it’s a little bit different. Instead we end up with these swarming clouds of electrons that are only allowed to orbit at said distances because the energies are quantized. Most of the atom is actually empty. One of the ways of looking at it that I adore is if you take a little green pea, and you toss it in the very center of a football field, the nearest electrons to that little pea in the center of the football field are going to be out at about the first row of seats in your standard stadium. That’s a whole lot of empty space.

Fraser: So the pea is the nucleus… those are the protons and the neutrons… and then those first electrons are the first row of seats.

Pamela: Exactly. Now it turns out that the electrons are sort of like people who hold tickets that are kind of open-ended. The electrons are limited to having certain energies, which means they have to orbit at certain types of distances away from the center of the nucleii. When we model this we use crazy shapes… what we’re actually explaining is clouds of stuff that exist at different distances. So, we have the inner-most orbits… these are the S1 orbits. Then we have a pair of orbits beyond that… each orbital level is restricted to having a very set number of electrons due to what’s called the Pauli Exclusion Principle, which we talked about in other shows.

Fraser: Right, but they’re like slots that can be filled and once they’re filled, then no more electrons are able to orbit in that shell.

Pamela: So, just as ticket holders fill up the rows of seats in a stadium, and your ticket says I’m restricted to a specific seat, the electrons are restricted to what slot… what energies they are allowed in the atom.

Fraser: And so if an electron gets out of one slot, then a different electron can go back into that slot, but the room has to be made first, right?

Pamela: Exactly. And it’s the specific slots that end up leading us to having the spectra of light that we see that lead to neon “Open” signs glowing in the particular shade of red they glow, and other neon signs in bars having the specific greens and yellows that we see.

Fraser: This is when electrons are going up and down levels, right?

Pamela: Exactly. And so here to continue our football analogy, you can almost imagine it takes energy to climb to the further energy levels because you have to trudge up the stairs, plant yourself in the seat higher up. Now for an electron to jump from one energy level to another, it has to absorb energy somehow… either through a collision or through the absorption of a photon… and it has to get just the right amount of energy so that when it’s done moving, it’s sitting in a new slot. It’s sitting in a new seat that already exists. It does it no good to get most of the way to the seat, because then it has to go back to where it started because it didn’t get all the way to the seat. And it doesn’t do it any good to get past the seat because now there’s no seat to sit in where you are now. So, the electrons have to be given just the right amount of energy to make the jump from one energy level to another, from one slot to another.

Fraser: So… sorry… so then what happens, right… If you have an electron that gets hit by a photon, it’s going to try to make the jump but it’s not enough energy to make it jump… what happens? Does it have new energy, or what happens?

Pamela: Well, you can end up just having a scattering event, where that photon comes in and it scatters off. You can have no interaction whatsoever… the photon just keeps going… this can go into collisional energy, it can go into kinetic energy. What’s amazing is atoms are a bundle of all sorts of different types of energies. It’s only when you add together the mass, add together the motion, add together what energy level within the atom everything is sitting in that you can get at the total energy of the system.

Fraser: Ok, so we’ve got this model, and I remember we drew them in chemistry class, right? You could draw the orbitals and that would help you figure out whether two atoms would combine together, right?

Pamela: Right. And, in fact, what you need in order to get a bonding… what you need in order to get two atoms to form a molecule is for there to be places that the electrons can be shared between the two atoms. Or, ways that the two atoms can fit together to fill electron energy levels together. So, in carbon, it has lots of empty slots just sitting there waiting for electrons to come in and fill the seats. Now, the neutral atom, the atom that doesn’t have any excess positive or negative electric charge, it’s going to sit there balanced out with these empty seats. Now, if another atom comes along, it can end up overlapping so that its filled seats line up on top of the empty seats of the other atom. This is a bit strange to think about. One way to think of it is if you take two egg cartons, it’s possible to fill one completely with eggs, and the other one half-way up with eggs, and then overlap the two cartons so that six eggs of one carton are filled, and then those little egg cardboard holders fill the empty holders on the other cardboard egg holder. Now you’ve bonded those egg boards together, and molecules can bond together in a very similar way.

Fraser: Right, and so one of the side-effects of sharing an orbital with an electron is that the two atoms will actually be bonded together, and this is what forms a molecule, right?

Pamela: Right. And so by sharing electrons, by holding everything together, we can get increasingly complex molecules. Building complex molecules is the easy part. What gets hard is when we try to build increasingly large atoms, instead.

Fraser: So now what really defines an atom with being a certain kind of element? Why is hydrogen hydrogen and gold gold?

Pamela: It’s all about the number of protons in the nucleus. If I have an atom and I strip every single electron out of the atom, I now have a very charged particle. But it’s still the same type of atom. I look in the core and I go… ah, you have 2 protons… you’re helium. Or, whatever the number of protons. What gets complicated is… ok, so gold can exist in many different forms depending on how many different neutrons it has in the center. We can get the same thing with carbon. In fact there are versions of carbon that are radioactive. What makes them radioactive is they’re not particularly stable because they have the wrong number of neutrons in the center. A nice happy carbon atom… it’s going to have six protons down in that core, and then it’s going to have another six neutrons. Now change those numbers and you start to get something that’s a whole lot less stable.

Fraser: Right, but if you add or remove protons, they become completely different elements.

Pamela: Yes.

Fraser: If you add or remove neutrons, you’re just changing how stable it is as an atom. And if you add or remove electrons, you just change its charge and how readily it’s going to bond to some other atom to create a molecule.

Pamela: So here we have Carbon 12 is nice, healthy and stable. Carbon 14 with just two extra neutrons… the numbers we use… that’s the sum of the protons and neutrons… Carbon 14… one atom… it will happily sit on a shelf for 5715 years, and then it just might decide it’s going to radiate away. It’s going to have that neutron decay… and in the process give off a bit of radiation. But more importantly, it’s the reason that we can use carbon dating. We can figure out when was something created and look at how much decay has happened and start to date archaeological sites all over the world.

Fraser: Now you mentioned the Bohr model as sort of the next more accurate understanding of the structure of the atom where the… where you’ve got these orbitals and it’s not like a little solar system. Is that still sort of the most accurate description? Because now we’ve got quantum mechanics to make things even more complicated.

Pamela: Well, Bohr was one of the people who started us down the quantum mechanics path. He was one of the ones who got us thinking… ok, energy is quantized… what does this mean? What are the implications? How does this relate to… well, we’ve got electrons going in circles, that means that there must be constant acceleration… what are the implications of this? And once we put all the pieces together, we had an entire new field of physics with which to torture undergraduates. And it’s that beginning model that Bohr created that has been fleshed out mathematically, but still remains the core of our modern model of how atoms work.

Fraser: Right. Now, let’s just draw this line in the sand, because the traditional understanding was proton, neutron, electron. That is the atom. But, scientists are never happy, and have been using particle accelerators to smash these things together to see what breaks… to see what comes out… So what was the next particle that was discovered inside the atom?

Pamela: The big kicker was we went from thinking first of all that atoms were as small as you could make things and then… no, no, no… totally wrong…. Now we have protons, neutrons, and electrons. We did all sorts of mean, awful, nasty things to electrons, and you really can’t harm an electron. Electrons are the smallest discrete piece of energy and mass and other characteristics that you can make that fit the electron description. You can’t break an electron into anything else… it can get absorbed… it can get turned into other things via processes, but the electron itself doesn’t have smaller things inside of it. Not so true for the proton and the neutron. This is where, as we started to smash and destroy and do evil, awful, nasty things to the protons and the neutrons… we realized that the protons and the neutrons were made of something different. In fact, they’re made of what we call quarks. This is a fairly new idea… it comes from the 1960s. It was proposed separately by a couple of different scientists… Murray Gell-Man and George Zweig… It was a way of trying to better understand the whole way of looking at particle physics in terms of… well, we have leptons… these are the electrons, the muons, the tau particles. We have bosons… these are the things that carry force, which are also fundamental and can’t be broken apart… So here we have the photon, the gluon, the weak force—which are the z and w bosons. Now how do you fit into this the proton and the neutron? Mathematically the way you fit into it is you come up with a family of six different quarks. Up and down are the ones that make up everything you and I deal with everyday. They’re the ones that make up protons and neutrons. But then there’s also charm and strange and top and bottom. It’s been a hunt that only ended in 1995 that we’ve been desperately trying to find in reality these particles that were proposed in the1960s.

Fraser: Now, I’ve got a million questions… let me see if I can put them into a kind of rational order… How many quarks are inside a proton or a neutron?

Pamela: Every proton and neutron is made up of three different quarks. So, if I have a hydrogen atom… just a straight old boring hydrogen with one proton and one electron. If I were to break it apart into as many possible pieces as I could break it apart, it would be made up of one down quark, two up quarks, and an electron… once I broke it all apart.

Fraser: Right, and now the down quark…. oh, and an electron?

Pamela: Right… because I’m breaking up the whole hydrogen atom.

Fraser: Right, right, right… of course. It’s got the electron already there. So now the up quark and the down quark doesn’t mean anything… it doesn’t mean it’s doing anything up or it’s doing anything down…

Pamela: They’re just names…

Fraser: They’re just names. How can they tell an up quark from a down quark?

Pamela: Well, when you add all of the pieces together, they end up carrying charge with them. And the different flavors… they actually have different masses. So the ups and the downs are the lightest weight of the quarks. The top quark… the one that was the most annoying to try to find.. this is the heaviest of the quarks. Now, a lot of times we tend to discuss the mass of things that are really, really tiny in terms of… well how much energy do I have to generate in order to get that particular amount of mass. The lightest weight is the up quark, and it takes 2.4 mega electron volts (MeV) to come up with an up quark. To get a top quark, that’s 171.2 giga electron volts (GeV). So there’s a huge difference between these two things.

Fraser: Right, and I think it’s important that people to listen to… we’ve done a bunch of shows about the search for the Theory of Everything and the search for the Higgs Boson… and trying to talk about particle colliders. That’s a whole show in itself… is how these particle colliders are using kinetic energy to freeze out mass through these collisions. I don’t think we want to go into that again in depth.

Pamela: Right.

Fraser: So, more energy… it takes more energy to create these heavier particles, with the top one being the heaviest and most difficult to find. So, really you crack open an atom… crack open a proton… three quarks are going to spill out?

Pamela: Three quarks are going to spill out.

Fraser: Always three. You’re going to weigh those three quarks, and that’s going to tell you what kind of quark they are. It’s almost like they’ll be quantized as well. They’ll always be one mass or a different mass or a different mass. And there’s six different… But they also come in groups, right? If you get one, you’re going to get the others, right?

Pamela: So you can build all sorts of different particle just by combining all the different types of quarks. We come up with tables of possible combinations… there’s six particles… you can combine them in all sorts of different ways. We know protons—up, up, down. We know neutrons—up, down, down. Then we start making stuff up. There’s lambda particles… these actually exist. This is a combination of an up, down, and strange. There’s sigma particles, which are two ups and a strange. And lots and lots of other combinations… But the one thing that holds always true is that the only ones that are stable are the ones made of ups and downs.

Fraser: Oh… so it’s kind of like molecules, right? You can mix and match your quarks and start making new particles… just whatever you can imagine. But, they just fall apart again in moments.

Pamela: Right. So we’re limited in what reality allows to exist. Reality periodically says ok… nice try… I appreciate the three quarks, but we’re done now, and it can say that in fragments of a second. The stable time for things like lambda particles are 2 x 10-10 of a second. You can’t even think about blinking that fast.

Fraser: So, then do any other of these particles exist in nature or are they only created in our labs?

Pamela: Well, some of the particles exist for passing moments in nature. We know that lambda and sigma and chi particles…. these regularly come out of different high energy events. Lots of times explosive, high power, collisional events… they create transitory things. They release huge amounts of energy and as that energy converts itself out to something else, it can pass through being… ah, I’m going to be this unstable particle… I’m going to be that unstable particle… before settling down to the combination of stable particles and light that fits what started best.

Fraser: Ok, then I guess my last big question is… have particle physicists been able to crack open quarks yet?

Pamela: As far as we know, it’s kind of like the electron… you can beat them all you want but they stay the same thing. The next big question for us is just how big can you make an atom? And this is where some of the particle accelerators have been doing really interesting work bombarding atoms with neutrons because the neutrons will eventually decay down into protons, and you can end up growing the center of an atom by bombarding something with huge numbers of neutrons and waiting to see what it decays into. There are mathematically various very, very large atoms that if you get just the right combination of protons and neutrons might be temporarily stable. We haven’t gotten there yet, but we’re working on it.

Fraser: Well, I know that we’re in the low hundred, right, in terms of the number of protons that you’re able to squish together. And for these Californium and Einsteinium and, you know, they last for just fractions of a second, and then they disappear again. I know the most heavy atom was created just a couple of years ago, now. And that record will continue to be broken. So you’re saying that they might hit some point where they’re stable again?

Pamela: Well, not so much stable, but they last slightly longer.

Fraser: Stabler…

Pamela: Right. So the highest atom that we’ve gotten to so far is, as near as I can tell, unpronounceable…. it’s Ununoctium. It’s abbreviation is Uuo. It’a a noble gas, and when you count up all of its protons and neutrons it has 294 crammed down in its center. So we’re getting to some hellaciously big atoms already, but there just might be some other ones up there that are waiting to be discovered mathematically.

Fraser: Alright, Pamela, I think that gives us a really good idea of the structure of the atom. It sounds so simple.

Pamela: It sounds so simple but it requires quarks and the strong force, which we have an entire show dedicated to… it requires neutrons to decay and electrons to jump levels. It’s a wonderful combination of all the different parts of physics.

Fraser: But what an accomplishment, I mean, over the last 100 years, essentially, they’ve gone from thinking atoms are little billiard balls to unraveling all the structure inside of it and really understanding how it all comes together. Some of the most basic research has been done in the last couple of decades. When was the top quark discovered?

Pamela: It was discovered in 1995. I remember it clearly because I wasn’t yet 21, and I was at Michigan State when the discovery was made. One of the senior faculty handed me champagne and said you’re going to drink. I had faculty-induced illegal drinking to celebrate the top quark.

Fraser: Yeah, and that’s only less than 15 years ago… so that’s amazing. Alright, Pamela, well thanks a lot, and we’ll talk to you next week.

Pamela: Ok, thank you very much. I’ll talk to you later.

Shownotes

• Quantum Entanglement – Centre for Quantum Computation
• Quantum Entanglement (explained with no math!) — David Jarvis
• Quantum Weirdness -- several posts by James A. Tabb
• Google Video: Fun and Games with Quantum Entanglement — from the 2005 Chaos  Communication Congress.  See also related videos from the site.
• Photon pair production — Wiki
• “How Not to Collapse the Wave Function” — from Science in Society
• Schroedinger’s Cat — Cornell U.
• Copenhagen Interpretation — Wiki
• Does quantum entanglement mean faster than light communication? – Cornell U.
• Patent Application:  Communications method and apparatus using quantum entanglement
• How Quantum Computers Work — HowStuffWorks.com
• Book:  “Entanglement” by Amir Aczel
• Fraser’s book review of “Entanglement” – Universe Today
• Book:  “The Age of Entanglement; When Quantum Physics was Reborn” by Louisa Gilder

Transcript: Entanglement

Download the transcript

Fraser Cain: During our last episode you scrambled for shelter [laughter] from an approaching tornado storm, but everything was fine, right?

Dr. Pamela Gay: Everything was just completely fine although we did get 6 inches of water in the basement.

Fraser: This happens like a few times every year right?

Pamela: It’s the middle of the U.S. we get tornadoes. It’s part of living here. [Laughter] I know, it’s very surreal but we don’t have hurricanes and we don’t have well we do have earthquakes. We don’t have forest fires just all other manner of natural disasters.

Fraser: I sometimes get emails from readers saying: “did you feel that earthquake?” Apparently we get pretty big earthquakes here on the west coast. There’s nowhere to live where you’re not going to have some natural disaster of some type. Once again, the universe is trying to kill us.

One of the most amazing aspects of quantum mechanics is quantum entanglement. This is the strange behavior where particles can become entangled so they’re somehow connected to one another no matter the distance between them.

Interact with one particle and the other reacts instantly even if they’re separated by billions of light years. How on Earth did the concept of quantum entanglement even get discovered?

Pamela: This basically originates from the fact that there are a lot of different processes that we think are conserved. When you have two particles created they’ll generally have matter and antimatter. That one we’re fairly familiar with from talking about in the show.

There are other properties as well. Things like spin where if you have a positron and an electron flying in opposite directions we’ll say that one has spin up and the other has spin down. One will have positive charge and the other will have negative charge.

It is in the process of conserving all of these different properties that we end up with these weird spooky often referred to as actions at a distance where we don’t actually know which particle has which property until we start to measure one. Once we measure them we realize that all of these properties line up.

Fraser: Give me kind of a classic example of entanglement going on. What sort of starts the process and how would you then interact? What kinds of properties would you see? What starts the process of entanglement?

Pamela: When you create two particles at the same time. This can be for instance two protons that might share the exact same polarization, the exact same spinning of their electromagnetic wave as they propagate through space.

Fraser: What would actually generate two photons at the same time?

Pamela: It could be some sort of a nuclear process generating light. All sorts of things generate light. You could have some sort of a light source and this is something you can actually do in a laboratory. You set up a light source where you have all of the light going through some sort of a crystal. Calcium crystals of different types can do this for instance.

As the light passes through it, only light that is rotating in one particular way will be able to make it all the way through the crystal. If you get things that are rotating in one direction and then you measure how much they have wavelengths going up and down, left and right you can get the light to behave in all sorts of crazy ways where the photons are in lock-step changing their orientation as they pass through these crystals and these filters. It’s just kind of creepy.

Fraser: You’re taking regular light, shining it through this crystal and you’re forcing it into a very specific pattern?

Pamela: Yes.

Fraser: The pattern of the photons that is coming out of it is these photons entangled? Because they’re kind of coming in sequentially right? You’re getting one then another then another. They’re not going in at the exact same moment.

Pamela: It’s similar physics. This is a very complicated idea and we usually build up how we explain it in class by first freaking people out with polarized light where you show a beam of light going through different polarizers; the way you can block and change light as it goes through first at a 45 degree angle then horizontally then vertically.

Then we work on photon pair production where we look at so we’ve figured out how to generate two photons at the same time they have the same polarization, let’s measure them when they’re far enough away from each other that there’s no way they’re communicating back and forth. We can see that they have the same polarization.

Then we start looking at wow we can start doing this with particles as well. This is where it really starts to get creepy. You can have all sorts of different reactions that will generate as part of the reaction an electron and a positron. This is a matter/antimatter pair.

They have conservation of momentum so they’ll fly off in opposite directions. They have conservation of charge built in so one is negative one is positive. They have conservation of spin so we have spin up and spin down. We don’t know which has what in terms of the spin until we measure it.

One of the weird things about the Copenhagen interpretation of quantum mechanics – this is only one of the ways of looking at quantum mechanics but it is the one that most people follow right now – is one of the things that it says is everything can be described by wave function. This includes you and me. We’re just waves. We’re really complicated waves that appear to be completely solid entities but we’re waves.

Part of being a wave is everything about us is probabilistic. Because we’re really big complicated waves we behave in a deterministic manner. You put your hand on the table you can feel the table. Your hand doesn’t pass through the table.

When you start looking at individual particles you have a stacking of all the different wave functions of what the particle could be. As the particle travels through space it simultaneously spins up and spin down.

It doesn’t know which one of these two things it is when you collapse that wave function until you actually observe it and that wave function goes: “Oops I have to be in one place right now. I have to behave like a particle right now. I need to decide which spin I am.”

Fraser: So whether a particle is spin up or spin down has no affect on the universe until it bumps into something, right? Until it somehow interacts with an observer or bonks into some other matter or somehow interacts.

If it’s just floating through the vacuum of space, it’s not doing anything. If it’s not touching anything there’s no need for it to choose a spin direction.

Pamela: Right and what’s cool about experiments like this is people have been trying to say no sorry this is just too creepy. There has to be a better explanation.

Einstein was one of the people who said this is creepy there has to be a better – he used much more complicated and philosophical longer sentences – but basically to paraphrase it’s creepy. This is wrong. We need a better explanation.

Fraser: I guess what people find creepy is a particle moving through space. It exists it is real and yet one of its characteristics has been un-chosen. It can be either way, one or the other and so far it does not have to decide.

It’s like waiting in line for your ice cream and thinking you’ll get vanilla, or chocolate [laughter]. I won’t decide until the person at the counter asks me to choose.

Pamela: So you live in a super positioning of I want chocolate, I want vanilla, I want bubble gum, I want raspberry sorbet you have the supposition of many different possibilities.

Some of these have a higher probability than others. It may be that while the bubble gum ice cream is tempting you because who doesn’t want to have 24 balls of bubble gum in their mouth at the same time, you also know it’s a pain to eat.

But the possibility is there, but chocolate is awesome and Rocky Road is even better because then you have the chunks of stuff.

Fraser: You have an ice cream problem don’t you?

Pamela: Well, I’m allergic to it so I spend all my time wanting it and not being able to have it.

Fraser: Oh, no! That sucks. [Laughter]

Pamela: There are all these different probabilities and in quantum mechanics we say that the probability is related to the square of the amplitude of the wave function.

This is sort of like saying make a graph of how much you want something. Then take the square root of where your peak is on that graph and that square root is related to how likely it is that it will actually happen.

Fraser: It’s like there’s a bell curve, like a percentage right? There’s a percentage chance that it’s going to go this and a percentage chance that it’s going to go that.

Pamela: All the probabilities add up to one because something will happen.

Fraser: Right.

Pamela: It could be that you get up and when the friendly person behind the counter says which flavor would you like? If this experiment happens 10,000 times you’re going to end up answering the question in a way that exactly matches the probabilities.

At any given moment anything could happen. It’s just what’s most likely is guided by the probabilities.

Fraser: Right. This is where you got this particle that so far doesn’t need to decide, however it will most likely make a decision based on the probabilities.

Pamela: Imagine that you and a friend have made this pact that anytime they get chocolate ice cream you’re going to get mint chip ice cream because you’re going to steal bits of ice cream from each other. Whichever one you get the other person will get the other thing. If he gets mint chip you get chocolate; if he gets chocolate you get mint chip.

This is the way you’ve decided your universe will always function. It could be that he goes to New York and you go to Los Angeles and neither of you have cell phones and you can’t yell at each other across the entire United States. Even though you’re not going to be stealing ice cream from each other you still have this pact. He gets chocolate and you get mint chip.

According to quantum mechanics if you two were entangled particles he could order chocolate chip in New York and you would know you have to order mint chip. This is where it gets weird is how is it that two things that can’t communicate to each other are able to know what they’re supposed to be when they’re separated by such a great distance.

Fraser: Just to be clear here the communication is going instantaneously regardless of distance. It’s not going at the speed of light. You could have these two particles – these ice cream orderers on opposite sides of the universe separated by billions of light years and one decision gets made and the other decision gets made perfectly with no communication happening in-between.

Pamela: When we talk about this scientifically we often refer to it as the Einstein – I’m going to mispronounce this and I’m sorry – Podolsky Rosen paradox.

Based on this paradox of how is it that two things at a distance can communicate instantaneously which shouldn’t happen according to relativity.

Fraser: Yeah Einstein how is that possible?

Pamela: Well and he sat there going no quantum mechanics, no. That was Einstein’s kind of up until he died he did not like all of this stuff.

There is what is called Bell test experiments. These are experiments that work very hard to figure out what the heck is happening. There have been postulations that there are particles that are somehow communicating between the two events. Experiments are devised to try and eliminate the possibility for these particles.

So far every experiment that people have done trying to either say no our understanding is wrong or no our understanding is right. All the experiments, no matter what their goal was has come out and said no quantum mechanics actually works. We are getting things behaving exactly the way they’re supposed to.

They’ve done experiments where they’ve sent particles kilometers and kilometers apart, made measurements and in the time that it takes to make the measurement you can’t send things at the speed of light between the two sites of the experiment.

This is one of the things that people have said is it takes time to conduct the experiment and while the experiment is taking place particles are flying back and forth and communicating so that everything is kept in sync.

We’re fine with invisible particles we don’t understand. This is how we explain things like mass and gravity. We were able to eliminate the idea of particles flying back and forth at the speed of light communicating what’s going on in the two experiments by conducting the experiments so far away from the place where the two particles were produced that the measurements can be completed before anything traveling at the speed of light can travel between the two locations.

Fraser: So then the scientists at the speed of light are then communicating afterwards and saying I got this, did you get that? It’s always turning out correct.

Pamela: Right.

Fraser: But the experiment is happening where the measurement is happening too fast for the speed of light to be able to communicate – for the particles to be somehow communicating. It really is instantaneous regards to distance.

Assuming you could extrapolate it would work if it was across the solar system, across the universe. The moment you do one the other one goes off. Wow.

So, why is this happening? I understand Einstein went to his grave trying [laughter] to puzzle this out but what is sort of the current thinking of about why this works?

Pamela: One of the things about quantum mechanics is in many, many cases we have understandings, for instance the Copenhagen interpretation of quantum mechanics. These interpretations state things like well everything is a wave function. The description is essentially probabilistic.

We have uncertainty principles that say we can never know exactly where something is and how fast it is going. When you put all the pieces together you’re kind of left with if it’s a probabilistic wave function moving in a given direction at a given velocity, I can’t tell you what all of those wave functions are going to collapse down to until I measure it.

It’s all these different pieces of it’s a wave function, it has a bunch of different probabilities, I haven’t observed the sucker yet that lead to this uncomfortable – well it’s all the things at once until you observe it. The math works. You start from the philosophical statement, build the math around it and the math works.

Fraser: Right but you’ve got this is the way it needs to behave and the math works but that still doesn’t explain why it works.

Pamela: And quantum mechanics doesn’t always explain why. That’s one of the uncomfortable parts about it.

Fraser: [Laughter]. Right. Quantum mechanics does not care that you need an explanation that feels right.

Pamela: [Laughter]. Quantum mechanics does not care how uncomfortable I am in trying to deal with it. It just does things and predicts things and says these things can happen and then they do.

Fraser: But isn’t sort of a part of the thinking is that when you get a wave function for a particle that you’re essentially getting that particle existing across the entire universe? It’s bunched up probability-wise in the spot where the particles most likely is, and then think of it as like a peak, a really tall mountain.

Then with the most likely position of the particle at the peak of that mountain. But then a really steep side all the way up and then a gently sloping side forever across the universe so the particle could be across the universe but it’s not in 99.999% of the time.

Pamela: This is part of the idea of collapsing wave function. With things like photons it’s even more convenient. The neat thing about photons is because they’re traveling at the speed of light they don’t experience time. As far as a photon is concerned, and I love the way James Tab explains this. He explains it as for a photon everything is goes flat.

They’re created, they go somewhere, and they are destroyed. That’s the entire existence of a photon is “oh, I’m created, oh, I’m destroyed.” There’s no time between those two different activities. Time stops when you’re moving at the speed of light which is what life does.

For them it’s sort of like they’re created in one place and then the next thing they know they’re destroyed so they don’t have a time to not know that they’re not next to each other. It hurts, it totally hurts.

Fraser: But isn’t that kind of the thinking that you’ve got the wave function can extend across the entire universe? That’s how they can somehow be interacting is because you’re going to get these overlapping wave functions, right?

Pamela: Not everyone goes that direction. There’s no evidence to prove that is the thing. There are places where you read people desperately trying to come up with ways to explain this. Like the wave functions fill up the entire universe up until the moment they’re observed.

We don’t have evidence. What we know is there are all these probabilistic outcomes. When we make the measurements on particle does one thing the other particle knows what it’s supposed to do and does the other. That’s all we know for certain.

Fraser: A couple of questions. How far can you scale this up? We talk about photons, electrons, how far can you go with this?

Pamela: Right now when we deal with entangled particles we only worry about the entanglement of things that when they’re created have to have conserved properties. You have to have conserved spin, conserved charge, conserved matter/antimatter things like that.

Here we’re talking strictly about particle physics. Things like electrons, neutrinos, individual particles not full out atoms, not full out molecules. Quantum mechanics really doesn’t care how big things are if you manage to entangle them.

One of the really neat thought experiments that was put forward as a way of basically trying to mock quantum mechanics and I love all these times in physics where a scientist just tries to say “oh what you’re doing is so stupid.” Then the way that they articulate it ends up being the way we think about the idea forever.

Fraser: Right, we should call this the “Big Bang”.

Pamela: Right and you know what? The name stuck. “We should call this dark matter.” Well, yeah, let’s move forward with that idea. For quantum mechanics it’s Schrödinger’s cat.

The idea here is you can actually entangle large systems where you can create the super positioning of wave functions simply by coupling a particle activity with a many atoms, many molecules more physical easy to see/touch tangible situations.

Fraser: Well why don’t you explain the thought experiment?

Pamela: Schrödinger’s way of running this was to say: look, let’s take a cat, stick the cat in a steel box. You have no way of observing the cat. We simply know there’s a cat in the box.

Put with the cat a small amount of radioactive material and a Geiger counter. Make it so that the amount material in the box is such that there’s a finite non-zero probability that within an hour a particle will decay and you’ll have a bit of radiation that will trigger the Geiger counter.

At the same time there’s a similar probability that nothing is going to decay and nothing is going to trigger the Geiger counter. Then attach to that Geiger counter a vial of poison so that if one of these wave function quantum mechanics probabilities plays out and you do have a particle decaying and releasing radiation, the Geiger counter will trigger and the cat will die.

They way Schrödinger looked at this was that means that until we observe if it decayed or not, until we observe if the cat is alive or not, the cat in the box is in the state of both alive and dead.

Fraser: Then its fate is entangled with the potential of that particle decaying.

Pamela: Up until we, the observers, open the box the cat is both alive and dead. It is us in opening the box that collapses the wave function and determines the outcome for the cat.

Fraser: Now no cats were harmed in the making of this thought experiment?

Pamela: Schrödinger made it very clear we should never run this thought experiment. This is a bad idea.

Fraser: I guess then because I know that you can entangle other particles in this kind of a way right? Can’t physicists take one particle and sort of interact two particles together and cause them to be entangled and then do it again?

Pamela: No not so much because it is primarily in when we created entangled particles it is through decays, through collisions, through systems where you’re creating two things that have to conserve traits.

Fraser: Okay.

Pamela: The thing with Schrödinger’s cat was, the cat is its own observer of its own fate. Its wave functions are fully collapsed already and its fate is fully decided by the quantum mechanics of the system.

The second that particle does or does not decay as observed by the Geiger counter, the cat is very much dead or not if it doesn’t decay.

Fraser: It’s almost like the Geiger counter is what’s doing the observing.

Pamela: Another way of looking at this experiment is for us the observer we don’t know what the cat observed. We don’t know if the cat did live or die. In our reality the cat is both alive and dead until we open the box.

According to another interpretation of quantum mechanics – which I think is going to get left for another show at this point – there’s the idea that every possibility that could happens. Thus that cat that was alive or dead up until we opened the box and look at it in some other parallel reality when we open the box it has the other state.

So, if I open the box and find a happy living tabby cat there’s another universe where I forget to open the box and the experiment goes on until the cat has to be dead. There’s another universe where I open the box and the cat is dead from a heart attack.

There’s a reality where the cat is dead due to quantum mechanics. Everything that possibly could or could not happen to that cat to the point of it randomly had kittens in the box, will occur in some universe. This is a manyverse way of looking at probabilities.

Fraser: I know that the listeners have a burning question in their mind right now and they’re dying for me to ask it.

Pamela: Okay.

Fraser: So I will: couldn’t you use entanglement as a way to have instantaneous communication? Couldn’t I across the universe collapse particles and then you would see them collapsing and we’d somehow be able to communicate?

Pamela: The problem is, how do I create entangled particles where I’m encoding “aha, this particle is going to be spin up; this one is going to be spin down and I’m able to send a set of information.” All I’m really able to do is create two entangled particles.

When I observe mine that causes yours to do something else but I can’t set what yours is going to do and the send information. I can send you a stream of particles and your stream of particles is going to be the mirror of the stream of particles I send to myself. But I have no way of encoding information in this yet.

There are people who are using quantum entanglement to try and figure out how to build quantum computers. This is a new field of research where they’re looking at ways to encrypt information where you actually use quantum entanglement to decrypt the way information is stored.

I have to admit I don’t fully understand quantum computers. I think there are probably two people on the planet who fully understand quantum computers. I’m nowhere near as smart as either of them.

Fraser: The gist of them is that because the particles collapse one of the great things about the quantum computer is that you can detect if the message has been interrupted. That’s one of the advantages of it.

As I understand with not being able to use entanglement as a communication device that the problem is that you still have to somehow communicate with one another at regular light speed to say I got this, what did you get?

Pamela: The thing with quantum entanglement is the particles are still moving at the speed of light or less. You might as well have used a pulsed laser beam.

Fraser: No, no but you take a box like we generate a bunch of entangled particles and I carry my box off to Andromeda and you hang on to your box here and they’re both entangled together.

Then I start collapsing them in some wave trying to communicate with you. The problem is that it will always be random, right?

Pamela: Yeah.

Fraser: Then all I’m doing is randomly collapsing these particles but until we actually communicate on the back end and say this is what I got, what did you get there’s no way to communicate.

Pamela: Right and I can’t even know which particles you have collapsed or not because when I make my measurement the only thing I know for certain is I made my measurement so yours has to be the opposite of mine.

Fraser: That’s all you know.

Pamela: Yeah, not very useful.

Fraser: You could have figured that out beforehand. [Laughter]. It can’t be used as a communication system.

Pamela: Nope.

Fraser: Nope you still have to communicate at the regular old light speed.

Pamela: Yeah, it’s one of those sad realities.

Fraser: That’s great but it’s a really interesting field. There are some good books on it. There’s one called ‘Entanglement’ – I read that a couple of years ago. It’s is amazing the experiments that have been done.

That’s the part that I think where quantum mechanics really delivers the goods. Some of these experiments as you said where you separate the particles and the do the experiment, get your measurement before the particles can communicate or before you can communicate and see the results.

There are just some astonishing experiments that are being done and being done right now and being done in the next decades that are just going to keep pushing this wide open.

Thanks Pamela. I think we’ve pretty much wrapped up quantum mechanics for now. We may come back around again later on but I think we’ve got some Earth science stuff we want to tackle next.

This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.

Shownotes

Spectra — an excellent overview by James B. Kaler

Doppler Shift — UCLA

Continuum Radiation – Physics Myths

The sun is green!  from The Color of the Sun – Scientific Blogging

Blackbody radiation — GSU

Elemental composition of the Sun — About.com

Deuterium — Wiki

Grism is a combination of a prism and grating to create a dispersed spectrum

Spectroscopy and Astronomy — UCLA

Slit Spectroscopy – Gemini Telescope

Balmer Series — Internet Encyclopedia of Science

Hydrogen Alpha Explained — Solar Observing

H-alpha spectral line is at 656.3 nanometers

Using Laser Combs to Find Exoplanets -- Universe Today

Hydrogen Spin Flip Transition – Wolfram

Zeeman Spectral Line Splitting – NOAO

Transcript: Energy Levels and Spectra

Download the transcript

Fraser Cain: Pamela, ready to hurt people’s brains again?

Dr. Pamela Gay: Oh, it’s so much fun. [Laughter]

Fraser: It’s so much fun and it’s so easy. This week is going to be a little easier on the mind and a little more connected to astronomy which will be great. Last week we took a peek into the tiny world of quantum mechanics and its unintuitive but very accurate mathematical predictions.

Although we all appreciate the physics lessons you’re probably wondering what all of this has to do with astronomy. Today we bring it all home and explain how quantum mechanics has given astronomers one of the most powerful tools they have to study the nature of the cosmos.

Okay so we’ve got quantum mechanics, it is the probabilistic nature that electrons and particles work at the tiniest levels. What on Earth does this have to do with astronomy?

Pamela: [Laughter] Everything. It has to do with everything, most centrally in trying to understand this whole red shift thing that we’ve talked about so much, the Doppler shifting of light. We wouldn’t be able to measure that Doppler shifting if it wasn’t for the specifics of spectra.

If you see just one photon you don’t know what’s happened to that photon. You don’t know if it is red shifted, blue shifted or anything else. If you have however a whole family of photons of a variety of different colors that were all emitted from one object that family of photons, that spectrum is likely to have distinctive patterns that are characteristic of the temperature of whatever gave off the light and of the composition of whatever gave off the light.

Both that fingerprint due to the temperature, the black body spectra, the curve of where does most of the light come out is a function of wavelength. That has to do with one part of quantum mechanics.

The fingerprint of all of the different atomic and molecular lines, the specific colors at which a different atom absorbs and emits photons also has to do with quantum mechanics. Everything we understand about what things are made of where they are and where they’re going all comes from our understanding of quantum mechanics.

Fraser: Let’s just give an example of the range of spectra that might be coming off of an object. Take the sun for example. What if we could look at the sun at all wavelengths simultaneously, what would we see?

Pamela: We’d see two different physical features superimposed on one another. The first thing that we see is what we call the continuum radiation. This is a curve of light where we can see very quickly that the sun gives off light in the radio but not a lot. It gives off light in the x-ray but not a lot except in occasional short flarey bursts.

But ignoring the short flarey bursts the sun in general doesn’t give off a lot of energy in the extremes. If you go extreme enough it gives off no energy. As you work you way to the middle we see that the amount of energy being given off, the number of photons given off at any given color increases as we work our way inwards until we finally hit on a specific shade of green.

At that color the sun is giving off the most of its light. The shape of this curve which is much steeper in the blue than it is in the red is entirely defined through quantum mechanics.

Fraser: Would you say that’s sort of almost like the sun’s fingerprint? Is it a unique or mostly unique set of wavelengths that are only coming from say our sun or very similar stars?

Pamela: We’re not quite there yet. This is just the temperature. If I heat a rock up to the exact same temperature of the sun, it’s going to have the exact same black body radiation.

Fraser: Ah, okay so any object of any temperature is going to give off the exact same temperature signature in this black body radiation.

Pamela: Right. The basic definition of black body is something that is a perfect absorber and emitter of light. It gives things off strictly as a function of temperature and there’s this very distinctive curve. The hotter something is the more steeply curved towards the blue the curve is.

So something that is 5,000 degrees Kelvin is going to give off a lot more energy in short wavelengths. Wavelengths that are just bordering on not being able to be seen with your eye than an object that’s say 3,000 degrees which is going to start giving off most of its light in wavelengths longer than you can easily see with your eye.

Fraser: Then the color of a black body object is the averaging out of the photons that we’re seeing.

Pamela: Right our eye handles that for us. There is of course integrated your eye says oh, I can’t see these colors I see these colors so there is a lot of faking going on in terms of if you do a complete map you suspect the sun should be green. You look at the sun and you actually see it as white if you ignore the atmosphere.

Fraser: I guess I was getting ahead of us, right? So, what is the part then that changes the black body to give the sun its specific fingerprint?

Pamela: This is where the atoms in the atmosphere of the sun get involved. Down in the core of the sun we have nuclear reactions going on that are emitting light all the time. They’re actually emitting extremely high energy light.

These high energy particles, not so much particles but these high energy photons are getting absorbed and readmitted as they randomly walk their way out toward the surface of the sun.

Eventually they end up hitting a zone where they just start heating stuff up. That heated material convectively rises and sinks through the outer layers of the sun until finally they burble to the surface and you start getting the thermal emission.

As this emission tries to escape it passes through gas that has a varied composition. The sun is mostly hydrogen. It also has a fair amount of helium. It also has things like iron, titanium and strontium and all sorts of complicated atoms. Each of these different atoms has electrons in complicated shells that are able to absorb and then reemit that light.

The majority of light from the sun is trying to get out through the surface. If I start absorbing light in one specific color and then redirecting that light out in a randomized direction, not as much of that specific color is going to go out as it would if I wasn’t absorbing and reemitting in random directions.

It’s like if you suddenly throw tennis balls at me. If I wasn’t there the tennis balls would all hit the wall behind me. If instead you’re throwing at me and I’m catching them and then randomly throwing them in all different directions tennis balls are going to go everywhere. The wall behind me isn’t going to get as many tennis balls as it would if I wasn’t there.

Fraser: So we’ve got elements in the sun’s upper atmosphere that are absorbing and changing the nature of the light. I guess what exactly is going on with those elements and I guess that’s where quantum mechanics comes in, right?

Pamela: This is exactly where quantum mechanics comes in. What ends up happening is the electrons in any particular atom are only allowed to be in certain what we call shells. You might have learned these as orbitals in high school but that tends to make people confused because they think of orbits like planets.

Planets can be anywhere in the solar system. Electrons can only be in very specific places in any given atom. Those places that they’re allowed to be have specific energies. The allowed energies are determined by how much mass is there down in the center of the atom.

This means that even though hydrogen and deuterium are both really the same thing they’re both an atom that has one proton. The deuterium has extra mass down in the center. It has a neutron in the center.

These two almost identical atoms actually have slightly different spectra; slightly different fingerprints of light that they’re able to absorb and emit because of this difference in mass in the center.

The spectra are also determined by how much charge is there in the system. If I start removing electrons from an atom it is going to have a different spectrum than an atom that had its full contingency of electrons.

All these things work together to define the big picture spectra. The big electron jumps where an electron for instance in a hydrogen atom might jump from its lowest energy orbital up to its next orbital up to its next orbital.

I can define this very specific fingerprint just by looking at what is the mass of the system, what is the charge in the system. It gets a little bit complicated because of all the constants and there are crazy powers but it is something I can calculate for the hydrogen atom with only reasonable amounts of pain.

[Laughter] It starts getting really hairy once you start getting to think like even a helium atom will make your typical fourth year physics undergrad cry.

Fraser: Okay, then what’s the process then that an astronomer uses to measure the spectra?

Pamela: This is the easy part. You go out and we use what are typically called grizoms where it slits to spread the light out in different ways. It’s usually actually a combination of these. You take the light, you take only a slits’ worth of the light though.

You can imagine taking two razor blades holding them up in front of the sun so that they’re just barely not touching each other. They’re only a hair’s width apart. Passing the sunlight through this pair of razor blades and then after it goes through the razor blades reflecting it off of what is called a grizom. It is basically a mirror that has a bunch of very fine – we’ll find a picture of one – they basically look like they have a bunch of little saw teeth on them.

This has the same effect that a prism has; the same effect that that carefully cut piece of glass that you might have hanging up in a window has of taking a light and spreading it out into a rainbow.

Then we measure that rainbow. We look specifically at what colors do and don’t have a lot of photons in them. We count the photons in each and every color event and that’s our spectra.

Fraser: You’re like measuring the thickness of the rainbow. You’re measuring like how wide is the green part and how wide is the yellow part comparatively.

Pamela: More than that we’re saying that at this specific shade of red I have this many photons. At this very slightly different shade of red I have this other different number of photons that tells me this system has hydrogen emission and hydrogen absorption.

I look for that emission and absorption above the black body an amount of light that I can’t account for strictly by how hot the object is. That’s where the spectra lines are.

Fraser: I guess if you were specifically looking for hydrogen emission for example, how would that look in the spectra?

Pamela: There is a distinct pattern of lines that you get depending on what the energy level of the electrons is that are jumping back and forth. The series that we talk about most is the bomber series.

This is actually where we start having not the lowest energy level transitions but transitions in and out of the second energy level of the atom. This is going from the third level to the second is what we call hydrogen alpha. This is a nice bright easy to see red line. It’s at 656.3 nanometers.

There’s actually a road in New Mexico that goes up to Apache Point Observatory that I’ve heard – I haven’t confirmed this – actually has that number as the road number.

There’s hydrogen beta – we have foreign names for these lines – it’s a blue-green line. We can calculate what wavelengths are each of these lights going to come out for. Then we look for lines at those separations.

It’s the separations that actually matter because if the light source, if the star if the galaxy that’s emitting the light or absorbing the light is coming towards us or flying away from us which is much more common, we’ll see this entire pattern shifted.

Fraser: Right, so it is pretty complicated and I don’t mean to keep dwelling and asking my stupid questions but

Pamela: I had to take three semesters of this in college. It’s really complicated. [Laughter]

Fraser: Okay but I can imagine then you’ve got your big spectra displayed out on a wall and you are measuring the number of photons in every wavelength throughout the entire spectra and you get to what was the number six hundred and

Pamela: Six hundred fifty-six point three nanometers for hydrogen output.

Fraser: Okay so you’re going to see an unusual number more for example of photons in that exact point along the spectra. That tells you okay great we’ve got hydrogen absorption. What does that mean?

Pamela: If you have extra it’s actually emission. This means that you have excited gas. If you heat gas up just the right way and it is a pure gas like hydrogen it is going to start emitting that light.

We get emission lines like this if I have a hot star off to the right shining its light on a nebula straight in front of me. The nebula is going to absorb all the light that is coming from that star off to the right. The light that might otherwise want to keep going through my field of view never once hitting me the hydrogen gas in that nebula is going to absorb that 656.3 nanometer light.

Excited electrons don’t stay excited. In this case the really excited electrons don’t stay excited. They instead collapse back down to a lower energy level. When they do this they give their light off in random directions. We’re able to see that light that’s being given off in a random direction.

Light that was meant to go from the star on the right off to somewhere on the left is instead going to hit a cloud of gas in front of me. The gas absorbs that light in that one color because it is hydrogen gas, reemits it randomly and I happen to be in one of the directions that that random light is being emitted so I see it.

If I happen to be off to the left trying to look at that star through the cloud of gas what I’m going to see instead is hydrogen lines are absorbed. The light that should have gone straight through if I was looking through the cloud and it wasn’t actually there which sounds kind of silly; I’m not going to get to see it because of that cloud. Instead someone off in another direction gets to see that light.

Fraser: This is one of the most used examples with the hydrogen emission and absorption but this is essentially the same technique that astronomers have used for everything.

Pamela: For [laughter] absolutely everything.

Fraser: Right they’re like well we’re seeing iron or we’re seeing all kinds of things in the atmosphere of the star or in this cloud of gas or what have you.

Pamela: Yes and it is an amazingly powerful tool beyond the Doppler shift that allows us to measure red shift which gets us a distance for galaxies. Beyond being able to say wow the sun has titanium in it. I just am constantly amused with the idea of our sun having titanium for no logical reason.

Beyond all of that we can get very accurate measures of temperature. Certain atomic transitions only occur at very specific temperatures. If I know this transition occurs between 4800 and 5200 degrees and I know this other transition occurs between 5000 and 5400 degrees I can start narrowing down that this star must be somewhere between 5000 and 5200 degrees.

Wait there’s this other transition line that allows me to narrow this further. Anytime you see someone quoting a specific temperature on a star, it’s because they’ve looked at all these different transitions and said okay, based on what I know about what temperatures allow these different transitions I can accurately measure the temperature of an object that’s a hundred thousand light years away. That’s just cool.

Fraser: You’ve mentioned this a couple of times now the other really useful tool for spectra is to be able to measure Doppler shift. How does that work exactly?

Pamela: This comes down to knowing what the fingerprints of the light looks like. With hydrogen I know that I have this series of lines that if I generate them in a lab – and we reproduce most of this stuff in a lab as a sanity check – I know that in the lab the light is at 656.3, 486.1, 434.1, 410.2.

I have this whole series of very specifically known lines that have a very specific set of separations. If I just look at a black and white image I can go aha, that set of separations, that for certain is hydrogen. If my camera is taking an image in the colors of light that my eye sees, I see exactly what my laboratory sees.

If instead my camera is off taking an image in the infrared of a very distant galaxy I’ll get the exact same pattern but it’s now shifted to the infrared. To understand where it shifted to what we often do is take a picture of a lamp.

Basically it is the very precise scientific version of that ‘Open’ sign made out of specific gases that you see at your local bar. We have our own versions of those. We take images of them to measure the fingerprints of the atomic spectra with our telescope with a non-moving source. Then we take a picture using the exact same set-up – nothing moves – of some distant object.

We know what the fingerprint looks like in the observatory. We measure the fingerprint of this distant object. We look at how that fingerprint has shifted in wavelength. It’s that change where what I usually see with my eyes is now shifted to where I see it with my infrared telescope that allows us to measure that red shift, that velocity of the object moving that causes its color to change.

All we’re doing is measuring the shift of an entire pattern of lines. Can’t do it with one. You need the separations of the entire pattern to say this is hydrogen. Then you measure the movement of the entire pattern because the object is moving through the sky.

Fraser: How much of a shift would you see? I can imagine that if it is sort of square in the red and you were looking at some galaxy that was billions of light years away and traveling away from us, how far of a red shift would that galaxy have?

Pamela: We can shift the initial transitions of hydrogen, the Lyman series of hydrogen which is normally so blue that we can’t see it with our eyes. It lurks out in the ultraviolet.

Objects that are about halfway back in the universe, those lines start to creep in to where we can start seeing them with our visual telescopes.

Fraser: So you would be able to see the spectra or as you wouldn’t be able to see it with Hubble for example. I guess Hubble can see in the ultraviolet a bit.

Pamela: Yeah, the Hubble is happy at many wavelengths.

Fraser: But an Earth-based telescope would start to be able to see the stuff even though it shouldn’t if it was right beside us.

Pamela: Exactly.

Fraser: Wow, that’s pretty cool. I think the other amazing thing about this technique is astronomers use it to find planets.

Pamela: Right, what’s really amazing is we can use this technique to do so many different things. We can measure currently motions of stars that are about the same rate at which a normal person walks down a hallway, one meter a second.

You can imagine tall guy one stride per second. We can measure a star moving at that speed using atomic spectra.

Fraser: You could measure my spectra walking towards the telescope and then turning around and walking away and know the difference just from the color of the light that I was giving off.

Pamela: Yeah. You’re a bit difficult but if I handed you that ‘Open’ sign from the bar. [Laughter]

Fraser: That neon sign and walk towards the telescope and then stopped and then walked backwards away from the telescope it would be able to sense the difference.

Pamela: It would be able to sense the difference. It’s not just atomic lines. This is the cool thing is spectra come from all sorts of different types of crazy transitions. We have the electrons jumping between energy levels. We also have electrons flipping. It’s called a spin flip.

You have an atom that is oriented one way relative to the proton in the center and that has one energy. If you flip the electron over so that it has a different orientation relative to the proton in the center, that’s a different energy. We actually get very specific spectra that we can see in the radia. This is the 21 centimeter line of hydrogen that’s due to this flipping.

With molecules you have bonds that bind the hydrogen and the oxygen and water together. These bonds can vibrate. So we’ll get entire bands in the spectra that are caused by the water vibrating a little, vibrating a whole lot depending on how much energy is in the water.

Molecular bands are evil because they take up huge swaths of the spectra you can’t otherwise see through. We like to build telescopes where there’s not a lot of water in the atmosphere above them. We can get at all these molecular transitions. We can understand the flipping of electrons and atoms.

We can even start to see how magnetic fields affect the electrons and atoms. This is called Zeeman line splitting. If you have atoms that are under an extremely large magnetic field you’ll actually have some of the energy levels that you see the transitions will just start to split into two slightly different energy levels.

This depends again on different attributes of the electron as it makes the jump. It’s really complicated; it’s really scary. They had to make up entire new fields of math to deal with this.

Fraser: Then I guess it is safe to say then, I think, tell me if I’m wrong, that all the light we see is being emitted by particles not atoms.

Pamela: Yes.

Fraser: And all the light that we see that those emissions are happening when the energy level in the atom is changing and a electron is changing states and getting rid of a photon to make that change.

Pamela: Most of the light. We’re almost there, there are always exceptions. Quantum mechanics likes to throw out ah, but this is another way to do it. The majority of the light that we see comes from electrons moving, molecules vibrating and light getting given off as a result.

You can also have a neutron deciding, ah, I’ve changed my mind. I want to be a proton and an electron again. As the neutron falls apart after you’ve set it on the shelf for 15 minutes because I have to say that once per show, so after you’ve done that during that decay process you also get a bit of light being given off.

During radioactive decay you get bits of light given off as well in many cases. Atoms falling apart, neutrons decaying all these sorts of things also give off bits of light. But the vast, vast majority is coming from electrons flipping, transitioning, and moving around and molecules vibrating.

Fraser: And photons being released.

Pamela: And photons being released.

Fraser: That’s like when we see the light coming from the sun that is just I guess the heat of the sun, the temperature of the hydrogen in the sun’s upper atmosphere and those photons being released. It’s amazing.

Alright, we’re not done with quantum mechanics and how it relates to astronomy or just how it is interesting in general. We’ve got a few more topics that I think we want to cover in the next couple of shows. Stay tuned for that. Thanks Pamela.

Pamela: It’s been my pleasure Fraser.

This transcript is not an exact match to the audio file. It has been edited for clarity. Transcription and editing by Cindy Leonard.