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

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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.

Fraser Cain: Now we’re at the third part of our trilogy on the human exploration and colonization of Mars. Humans living on Mars will inevitably tire of living underground. They will want to stretch their legs and fill their lungs with fresh air. One day they will contemplate what it will take to reshape Mars to suit human life. Is it even possible? What technologies would be used? And what is the best that colonists could hope for in changing Mars? 

Pamela, we talked about last week about how Mars is very hostile to life. The temperatures can be 150 degrees below zero at the poles. Even on your best hottest summer day you’re not going to get but a few degrees above freezing. The pressure is one percent of what we experience here on Earth. There’s radiation, essentially a long-term lethal dose of radiation. It’s cancer for everyone living out on the surface of Mars.

So, we’ve talked about people living underground in essentially pressurized space suits or in pressurized chambers where you can keep the pressure, the temperature, and protect them from radiation. That’s a really hard way to live. So inevitably the question is: Could we change Mars? How possible is it with the laws of physics to change Mars?

Dr. Pamela Gay: Well with the laws of physics, you can do it. The question is can you afford to and if you can’t afford to, should you? There are a lot of ethical questions to consider when you start thinking about completely changing the nature of another planet that just might harbor fossil life or actual life somewhere within its soils.

Fraser: Why don’t we deal with the ethical issues later? Imagine that you were some superpower human being and some advanced civilization with tremendous access to power and all of that and you want to sorta quickly modify Mars to make it more appropriate for human life. What are the things that you could do with a snap of your finger?

Pamela: [Laughter] If you want to do it quickly…..

Fraser: No, I don’t want to say do it quickly, but the point is if we could change Mars to be more appropriate for human life, what are the things that need to change on the planet?

Pamela: Well, there are basically three issues. First of all you have to give it a thicker atmosphere. Then you have to give it warmer temperatures. These two are actually coupled to one another. Third, you need to find a way to deal with the whole lack of a magnetic field and that one gets much trickier to deal with. Dealing with the first two, the low temperatures and the very thin atmosphere, can be dealt with by first releasing a lot of greenhouse gases into the atmosphere of Mars. Either by perhaps crash-landing rockets filled with chloro-flora carbons onto the surface of Mars or use some other mechanism that some chemical engineer still needs to imagine.
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By releasing all of these greenhouse gases onto the planet you can systematically increase the planet’s ability to retain its heat. Right now, sunlight hits Mars, it reflects off and a lot of this heat is lost back into Space as infra-red light.
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You can imagine being able to essentially wrap an atmospheric blanket around Mars such that sunlight hits the planet, starts to reflect off the soil, hits the atmosphere and then reflects back down. This heat stays with Mars and continues to build up and warm the planet over time. Sorta like heat that’s released by the coils in your oven is kept in by the walls of your oven allowing the temperature in your oven to continually increase.

Fraser: You could really see that when on your car. When you keep the windows up, even on a not very hot day, it’s amazing how hot the car will get inside. If you even crack the window just a little bit to let that heat escape, then the car will cool down quite a bit.

Pamela: And right now Mars’ atmosphere is so thin that it is essentially a convertible with the top completely removed. By simply dumping a bunch of chloroflorocarbons on to the planet, we can raise the roof of that convertible. That small change will allow more of the water ice on Mars to melt and add water vapor to the atmosphere of Mars. Water vapor itself is a greenhouse gas so here we’re starting to roll up the windows of our convertible.
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Over time as you build up more and more gases that have been previously trapped as various ices within the soils and at the north and south poles of Mars, you can convert these ices into atmospheric gases which will continue to heat the planet. Pretty soon we might be able to get something that at least in terms of temperature is suitable for human life.

Fraser: Right, so you would be able to put on a jacket, head outside, and the temperature wouldn’t be really awful. But, you’ve still got the atmospheric pressure so you’re saying that you really can’t thicken the atmosphere really too much?

Pamela: Well, you can thicken it but getting it thick enough that it is comfortable for a human being used to living at ground level here on the planet Earth is unclear. We don’t know if there are enough gases trapped within ices on Mars that you can, using just what is there, get that thick of an atmosphere on a useful time scale. The problem is these gases are continually escaping from Mars.
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It is a little planet with very little gravity, light elements that are released into the atmosphere just through normal particle to particle collisions they are accelerated to escape velocities. Then you have the Sun blasting the atmosphere of the planet and in turn blasting away bits and pieces of the atmosphere. So, you have to be able to release these gases quickly. Even then, they might only hang around for a thousand to a few thousand years without being restocked. You have to start asking where do you restock the gases from?
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You start considering ways to capture comets and otherwise bring oxygen, carbon dioxide, and nitrogen all to the atmosphere. There is also no natural buffer gas. Here on Earth we have an atmosphere rich in nitrogen that doesn’t totally get involved in building molecules on a regular basis.
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The types of gases that the Martian atmosphere would be rich in are things that like to form molecules. Thus, you need this buffer gas as well to make a nice happy atmosphere.

Fraser: So, the problem you’re getting here is that Mars has no magnetic field, right? The Earth’s magnetic field forms a protective bubble that buffets away the Solar Wind and keeps it from really interacting with our atmosphere. If we didn’t have our magnetic field, the Solar Wind would be blasting away our atmosphere just like it did to Mars.
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You would need some way to constantly replenish the atmosphere on Mars or you might have it incredibly thick in the beginning and then over time it would just be eroded again. You would no longer have reserves on the planet you could be using to rebuild it. Without that magnetic field it’s a lost cause. And also with a lower gravity, that sounds like a pretty big problem.

Pamela: Yeah. But it’s not something that if you throw enough energy at the problem isn’t unsolvable. As I said, you could conceivably go out and start capturing comets. Comets are rich in things like water that you can tear apart and turn into atmospheres.
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It’s not exactly a trivial problem to go out to the Quaker belt and capture yourself a whole bunch of large icy objects and tote them back to Mars. While we’re wandering out in the dreamland of terraforming, it’s possible to conceive that on the thousand year time scales things like this might be possible.

Fraser: But it almost sounds like with that problem and the lack of a magnetic field is going to come back to haunt us for another part as well. It might make sense to just get more mass on the planet, right? [Laughter] Smash Mars and you know all the Asteroid Belts gather. Crash that into….you know? [Laughter]

Pamela: There is actually an even more interesting solution that I read about. There weren’t any real descriptions of how you do this but one of the ideas I saw kicking around on the internet was you find a way to build a giant magnifying lens. The biggest magnifying lens ever and place it in an orbit such that it focuses the light of the Sun on Mars and melts Mars until it has a liquid core again. You wait for the planet to cool off enough that you can stand on the surface again.
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Now that you have a new version of the planet Mars with a new liquid core having destroyed everything else and probably lost all the atmospheric particles in the process of vaporizing the planet, you at least have magnetic fields now. It was a suggestion that needs to be thought out a bit better but it did amuse me.

Fraser: Right – it almost sounds like domes are your way to go, right?

Pamela: Yeah.

Fraser: You build a dome and then boom. You’ve got protection from the radiation; you have the temperature and pressure. You’ve got everything you need. Okay, so we’ve talked about the temperature and the pressure like rise in lock step. You give the impression that there is a limit to the pressure, right?

Pamela: There’s a limit to the pressure until you start bringing in things from other places. As you heat the planet up, you’re going to be melting ices and the gas from these ices slowly pop up the atmosphere. They add pressure to it by adding additional molecules.
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This is similar to taking a balloon and dropping in a piece of dry ice and tying a knot. As that dry ice changes from ice to gas the balloon will blow up. With Mars, gravity acts as the walls of the balloon helping hold that gas to the surface of Mars. As you melt the ice the atmosphere gets thicker and thicker.
 :
But, the thickness of the atmosphere is limited by how much material there is to melt. We’re still not fully sure of just what reserves there are within the soils that we can release. There is a limit – a limit defined by what ice is trapped within the soil on Mars and at this point in the studies I’ve found, it looks like you’re going to get maybe a tenth of the atmosphere of Earth tops.
 :
And that is standing at the bottom of the deepest chasms of Vales Marineris which I’m sure I’ve mispronounced thirty times in this series, standing down in the base of that chasm you might be able to start to get something that will at least allow you to stand outside and not have your skin bruised to badly as it is subjected to too low of a pressure.

Fraser: Right. Let’s talk about the third problem then, which is the lack of a magnetic field and the radiation that’s coming.

Pamela: Here is where we’re not sure how huge of a problem it is. We know that the radiation is such that you’re going to just blow right past the United States’ recommendations on radiation levels in the workplace. In a matter of days you will get the recommended level for a year. That’s bad.
 :
At what point it becomes cancer causing, we don’t know. That’s an experiment that we haven’t done and can’t do because it is slightly immoral you might say. There’s a radiation problem. You can probably walk around on the surface a few hours at a time in a space suit without worrying for your life, but you do need a way to protect yourself during your day-to-day activities.
 :
Burying yourself underground is sufficient. Air actually helps deter radiation but you’re starting to need a hundred meters or more of the atmosphere to get out (and that’s sea level what we have down here on earth atmospheric pressures) to start getting the radiation down to acceptable levels.

Fraser: Right, that’s what I was going to mention is that the atmosphere itself does help protect against the radiation. So that thickening of the atmosphere will really serve triple duty.

Pamela: If we can come up with a way to do what’s called either para terraforming or creating a world house, a giant greenhouse that combines a thick (defined as like a hundred meters) atmosphere beneath a lead-lined glass or leaded glass canopy that itself is fairly thick to help deter from radiation.
 :
There are different types of high-energy particles that can’t penetrate through glass. Through this sort of combined construction that allows sunlight through that stops things like alpha particles from coming through, beta particles from coming through, it may be possible to lower the risks to the point that they’re acceptable.

Fraser: What about plants? Would you be able to grow plants on the surface of Mars?

Pamela: This is one of the cool ideas. I highly recommend anyone who is interested in terraforming go read the excellent book “Red Mars” by Kim Stanley Robinson. There are different people who have hypothesized ways to basically take algaes that are perfectly happy to live on Mars today, release them and then slowly genetically engineer high altitude grasses, arctic grasses to get something that is both happy in the extremely frigid temperatures and in the extremely low pressures.
 :
You also start finding ways to suppress the different plants’ chemical responses to trauma, putting the plants essentially on “happy pills”.

Fraser: Like giving them some Prozac.

Pamela: Through a combination of “better living through chemistry and genetic engineering” it may be possible to create agricultural products that will live beyond domed protection on Mars.

Fraser: And that could start fairly soon, right? As you said, there are algaes here on Earth that are almost able to live in the Martian environment.

Pamela: And there are those who think that actually we probably could just dump some of the algaes found on Earth on Mars and they would happily go off and do its little algae thing.
 :
It’s somewhat frightening to imagine that just a cavalier accidental transportation of green slime from Earth to Mars could create new forms of life living on another planet. This is where the ethical considerations start to come in.
 :
It’s not impossible to imagine piggybacking life here to there and if it hasn’t already happened via rocks which is probably possible. We certainly send enough metal debris over to Mars with our own contaminations on it. We might have already started the terraforming process without realizing it.

Fraser: Sure, there’s no doubt that we have sent bacteria to Mars from Earth on the Rovers, on things that have crashed on the planet. I know that for example when bacteria were on one of the Lunar missions, the Surveyor mission was sitting on the surface of the moon. When the astronauts landed near it, they pulled off its camera and brought it back to Earth and scientists were able to find bacteria that had made the journey, sat perfectly dormant on the surface of the Moon for five years or more and then they brought it back to Earth and were able to get it going again. It survived no problem.
 :
So, who knows I guess if the kinds of bacteria that we have shipped off to Mars are the kind that can survive and even thrive in that kind of environment. So, I think you’re right we may have already begun that process.
 :
It is an interesting ethical debate. I think it’s not one that we have much of a conversation about on Earth. We do it and that’s that. Humans have modified the environment of Earth to suit our needs and I a lot of people are concerned about what impact we’re having on the environment with the species lost, temperature changes through global warming and all those kinds of things. At the end of the day, it’s not should we not at all build houses or cut down forests or that kind of thing but rather to what extent.
 :
It’s interesting to me to hear people say we shouldn’t modify Mars at all because we’re going to. It’s just inevitable. We will get to a point where Earth is full and colonists are living on Mars and they’re going to want to go and plant a plant outside. It’s just going to happen. [Laughter] I see it more.
 :
It’s like we have a whole Solar System here that we have existed and it is just inevitable to the point that we are going to reshape the Solar System to match human needs. So, on the one hand I think it’s an interesting ethical debate, but for me it feels inevitable that we will reshape the whole Solar System to meet human needs.
 :
There will come a day when we will control all of the energy output of the Sun. We will reshape the planets to provide as much habitable living space as possible. Ideally, we’ll keep some amount of green space and natural environments as we can but we will have torn apart the entire planet Earth, turned it into a ring or a sphere or who knows what it would be to collect every piece of sunlight from the Sun.
 :
It’s a matter of when, when along that time-line do we want to reshape and destroy every environment in the Solar System. [Laughter]

Pamela: I think that this falls into the same category of keeping an archaeologist on hand when you’re digging around, building a new building. It’s worthwhile as we look to take over new environmental niches within the Solar System to have the biologists on hand to go in first to dig through first to figure out what is it we’re about to destroy and let’s scrape some up and stick it in a bottle and try to understand what it is within the environments that was there originally.
 :
Let’s document the history of our Solar System before we remove the evidence for future generations. It’s a chance to be able to look back and see there used to be a dead Dodo bird on the planet Earth. We know there is nothing as exciting as the Dodo bird on the planet Mars unless it lives deep underground where the Rovers would fail to find it. Really I don’t think there is anything as exciting as the Dodo bird on Mars but I’ll always leave room for the amazing.
 :
As we go and explore Mars it would be good for us to keep track of the microbes before and the potential algaes and lichens. I don’t think we’re going to find anything that complex either. But if we do, it needs to go somewhere like one of these preserve all the DNA we can crypts that exist here on Earth already.

Fraser: I think the other possibility is that as our technology improves and changes we’re going to have more and more robotic, artificial life happening in the Solar System. It’s very possible that on the time scales, we’re looking at hundreds and hundreds and maybe thousands of years to terraform Mars.
 :
I would be very surprised if human beings were still the same hundreds of years from now that we really need to change Mars to suit our current form and that we may very well have a future form that is more integrated with technology and doesn’t need the planet to be changed so much. Our technology, the Mars Rovers are suited to live on Mars today. That might be what the future of life is going to look like so we have the old fleshy humans living here on Earth [Laughter] and the new robo-humans living on Mars and maybe the needs for terraforming aren’t so great.
 :
I think a lot of this stuff is very interesting. As the technology improves, as our science improves, it completely changes our understanding of what’s involved and what the ethical issues are. I’m looking forward to see how these debates go in the future but….

Pamela: Yeah, and you have a very creepy future living in your head with robo-humans living [Laughter] living on Mars.

Fraser: I know, just passing time until that last plucky black hole gives away to the future. [Laughter] Anyway, Mars is the most natural target that we would want to consider to terraform. But that’s not the only place in the Solar System we could have a go at. Venus strikes me as much closer. It has the mass, a thick atmosphere, what could we do to change Venus?

Pamela: The problem with Venus is it had a runaway greenhouse event at some point in its past where the atmosphere didn’t just trap some sunlight, it trapped all of it. In the process it boiled off any water that was in the soil to the point that plate techtonics can’t even exist anymore.
 :
The soil has been completely desiccated of moisture and all that moisture went into continuing a run away greenhouse effect and now we have a planet with a temperature on the order of 500 degrees Celsius and that’s just not a way for any planet to be.
 :
To fix that particular planet we need to strip the greenhouse gases out of its atmosphere finding some way to sequester all of the carbon-based molecules, the carbon monoxide, and carbon dioxide. Certainly we want to get that sulphuric acid out of the atmosphere as well while we’re at it.
 :
Exactly how to do all of this is something that is not a well-defined pathway. It is not as if we can plant redwoods on the surface of Venus that will happily absorb all of these toxins. The planet is 500 degrees.

Fraser: Yeah, we drop spacecraft made of metal onto it and it barely survives more than a couple hours before they die.

Pamela: They melt.

Fraser: In this case, our happy Mars Rovers would be burned alive if they were dropped on the surface of Venus. [Laughter] You need some mechanism for extracting and sequestering carbon out of the atmosphere of Venus. You would need to do a lot. The atmosphere on Venus is thicker than Earth, right?

Pamela: Yeah, so we would need to find a way to take these tens of atmosphere’s atmosphere and suck a lot of the gas out of it. There are different ways to imagine doing this using chemical reactions and catalysts where you basically rain through the atmosphere some sort of chemical that as it falls through the atmosphere, bonds with the different greenhouse gases.
 :
This has huge energy requirements. This has huge requirements of just taking resources from planet Earth or resources that we figure out how to mine out of asteroids or comets that make the mistake of getting in our clutches.
 :
Then we use those resources to rain chemical fear on the surface of Venus by changing everything. By changing its entire atmosphere, by transforming gases back into solids. That’s not an easy process. Again it’s something to keep the chemical engineers out there who like to dream big very busy.

Fraser: And I think I mentioned this before that in Venus if you can reach an altitude of about 50 kilometers, the temperature and pressure are the same as Earth. So, you could actually set up a floating balloon station on Venus and live there and go outside with just a breathing mask and feel perfectly normal.

Pamela: You can call yourself Lando Calrisian.

Fraser: Yeah exactly living in your cloud city. That would be awesome. [Laughter] Okay, so Venus you have to extract the carbons and maybe send it all to Mars. Pull out chunks of carbon in your cloud city and [Laughter] fire off rockets to Mars delivering fresh carbon to thicken Mars’ atmosphere.
 :
Is there anywhere else maybe in the Solar System that could use a little terraforming?

Pamela: Well, there is a discussion that perhaps you could take Europa with its liquid seas, but the problem with Europa is while it has water, it also has high radiation.
 :
A poor astronaut on the surface of Europa would have ten minutes to live before the radiation killed him. They probably wouldn’t want to be alive all of those ten minutes.

Fraser: Now this is coming from Jupiter, right?

Pamela: Yes, it’s the radiation field of Jupiter that unfortunately Europa has to deal with and any astronauts going there would have to deal with as well. There’s also discussion that maybe we could do something with Titan again. It’s a planet with a thick, organic rich atmosphere but orbiting Saturn, it’s kinda cold out there.

Fraser: Right, the temperatures are so low that hydrocarbons and methanes freeze out of the atmosphere and form lakes.

Pamela: It’s so cold out there and the Sun is so far away that Solar energy is no longer a particularly useful way of generating energy. Especially not once you get to the bottom of the big thick, fairly opaque atmosphere.
 :
As you start to look at Titan you have to ask how exactly to get a large enough nuclear power source out there with me. While you’ve moved far enough away that Solar radiation is less of a problem, you no longer have a good way to keep yourself warm and again it has very low gravity. Humans tend to like to have gravity. It makes our bodies happy.

Fraser: As we go forward though, in time as the Sun’s energy output increases and especially as it nears the end of its life, a lot of these worlds are going to suddenly become very habitable, right?

Pamela: The moons of Saturn are going to become very appetizing.

Fraser: Yeah, as the habitable zone of the Sun expands outward, suddenly they will melt and will have oceans and maybe even atmospheres?

Pamela: It’s going to be a brave new Solar System. What will be interesting to see is with these moons that are extremely rich in ice just how much moon to you have left once you heat them up. That will be an interesting question to sort the answer out to.

Fraser: Yeah, a lot of them are half ice, right?

Pamela: Yeah. So we’re going to be melting the outer Solar System, which is an interesting picture.

Fraser: And then in the far future as the Sun nears its very end and its habitable zone extends out it might even include like Pluto and some of the Quaker Belt objects?

Pamela: Yeah, the things that are going to melt.

Fraser: I know you can imagine some far future when people are sitting on the beaches of Pluto [Laughter] right, where the Sun is completely different and it could actually be briefly the only place to live.
 :
Once again there would be some terraforming involved to try and make it as human habitable as possible. But once again, if humans are the same six or seven billion years from now [laughter] we have a problem.
 :
I would be quite amazed. I think that goes through the terraforming argument. I know this is sort of a sacred cow for a lot of people that we must terraform Mars and reshape it to our will so I look forward to your e-mails.

Pamela: And go read Red Mars by Kim Stanley Robinson.

Fraser: Absolutely, Red Mars, Blue Mars and Green Mars, right?

Pamela: Yeah, the whole trilogy.

Fraser: There are also some interesting articles written. The Mars Society has done a lot of thinking on this. There is great stuff on Wikipedia and a lot of other interesting books have been put out. Even some thinking was done at NASA.
 :
We’ll have a whole bunch of links in our show notes that people can chase down. All right Pamela thanks again for a cool show. I think we’re done with Mars. We’ve given it six episodes so [Laughter] I think Mars’ Phoenix Lander can feel like we’ve given it a tribute and it will be awhile before we return back to Mars.

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

Fraser Cain: Hi Pamela.

Dr. Pamela Gay: Hey Fraser, how’s it going?

Fraser: Good. So you’re going to be off on another trip shortly another Astronomy conference?

Pamela: Well, the reality of it is by the time people are listening to this, I will be in Munich, Germany. And on Friday, I will be in Cambridge, England.

           In fact, if any of you are interested in going to a Pub with me, I will be at a Pub in Cambridge and the exact Pub will be listed on the BAUT Forum and on StarStryder.com.

Fraser: Okay. Once again, I wish I could make it but it’s kind of expensive to fly to Germany.

            [Laughter]

Pamela: Yeah.

Fraser: Okay, so this week, it’s going to be another rough one.

            [Laughter]

           I think we’ve been putting off this side of science, so let’s go. Have you ever heard that light behaves like both a particle and a wave? That is a crazy dual nature of light. What does that really mean? How did scientists figure it out?

           Today we’re going to get into it and try to explain one of the most complicated and non-intuitive concepts in science. Get ready for a little lesson in quantum physics. Can you explain how does light behave like a wave and how does it behave like a particle?

Pamela: Well, if you hit something with a photon of light it will move just like if you hit it with a soccer ball or something like that. This whole idea that you can hit something and it bounces and it moves just like you’d hit it with a particle is the particle nature of light.

Fraser: Right and that’s the action that is keeping stars ballooning out. The photons are bumping against each other and trying to get out of the star and that’s what keeps the star up.

           That’s how solar sails work. You take the sail and put it in space and just the pressure of the photons bouncing into it will move the sail through space. Are there any other examples where we use this?

Pamela: Well you can use it for imaging in fact. You can’t see something except for the fact that photons are coming from some light in the room and bouncing off of whatever you’re looking at and then going into your eye.

           So just that little thing, the fact that eye glasses work can explain the way that light reflects or refracts or the way rainbows are formed.

           All of this is explained initially by Newton through the particle idea of light.

Fraser: So then how is light like a wave?

Pamela: Now here’s where things get screwy. If you take a golf ball and you hit it the exact same way a thousand times, in a no wind environment, it will always fly the exact same way.

           If you take a thousand photons and you fling them through a slit a thousand times, they’re going to go all over the place. The pattern that they end up forming on the other side of the slit is the pattern that you would get from waves going through a slit.

Fraser: Right, I’ve seen this. You have a sea wall and your waves are bumping up against the sea wall and there is a hole in the sea wall.

           Where the wave hits the hole then you get another little wave that comes out the other side of the hole and continues to propagate through the water.

Pamela: And what’s really cool is you can have a perfectly straight wave front hitting this wall and the wave that comes out on the other side of the slit is round. So you end up with these round wave fronts radiating away.

Fraser: Even an individual photon will still make that shape?

Pamela: Well, not only that but if you have two slits, just like with two slits in a wave wall, you end up with interfering waves. In some places the waves get especially big and in some places they get especially small.

           With light when you have two waves that are interfering you can end up with places where you see no light. Where the light seems to cancel itself out and you end up with other places where the light seems to be especially bright.

           You get this interference pattern with only one photon going through at a time. So, how does one photon at a time manage to interfere with itself?

Fraser: Hold on a second. One photon interferes with itself? I don’t understand.

Pamela: No one does.

Fraser: Okay, then what do you see? What’s the experiment to make one photon interfere with itself?

Pamela: The experiment is actually kind of simple to set up. You just need to have a very precise light source. You can do this with a set of razor blades in fact. You take four razor blades and you very carefully set them up so that you end up with two razor-edged slits that the light can go through. Then you set up a screen on the far side of the room.

           Or if you want to do this with one foot on time, which you can’t see with your eyeball, you put a detector there, a CCD or something that will detect light. Then you take a laser and you put so many filters in front of it that all that’s getting through from that laser to hit that set of razor blades is one photon at a time.

           When you do this, the one photon goes out, through the slits, hits the detector on the other side and ends up leaving a single point of light on the detector. No big deal. But, if you let a second photon through, a third proton, and a few thousand photons through, it will build up a pattern of interfering waves.

           It’s in fact the exact same pattern that you get if you allow a few thousand photons to go through those two slits all at the same time and you can see the light and dark patterns with your eyeballs.

Fraser: If you only had the one photon, it would actually be creating an interference pattern. You just can’t see the whole pattern yet because you’ve only pushed one photon through. But the pattern is essentially there.

Pamela: What’s happening is when we detect that photon, it decides where it’s going to be, its wave function collapses. Its probability collapses and it picks a place and that’s the place it lands.

           All photons have the same set of probabilities. It’s most likely it will end up here. It’s least likely it will end up here. And just like if you roll the dice enough times you end up getting all the different numbers on the die.

           If you send enough photons through you end up building up the full pattern. When you send them through one at a time they each land where they land and the full pattern of all of them going through over time builds up this interference pattern.

Fraser: So, does that mean that each individual photon, because they are building up that interference pattern, each photon is going through both slits at the same time?

Pamela: This is where it gets philosophically confusing. You could either say there is a statistical probability that the photon is going to go through in all these different ways and the photon goes through in one of them.

           Or, you could say that the wave is passing through both slits interfering and the wave function is collapsing in a specific location on the screen.

Fraser: Right.

Pamela: So you can look at either statistically as it’s just picking a place and going through there just like you’re rolling the die and you can’t get one through six all at the same time. It picks one.

           Or you could say that when the observation is made, the wave function collapses and you have detection.

Fraser: Right, but aren’t there ways that you can split up or block the photon from going in through one of the slits and you can get a better sense of what’s actually an impossible thing seem to be happening?

Pamela: No. That’s the weird thing. There are some really cool experiments that have been done with this. If you block one of the slits, you get a completely different pattern for where the photons land. That’s one cool thing. You have to have both slits there in order for the photons to build up this distribution.

Fraser: Yes, you just don’t get the interference pattern until you have both slits there.

Pamela: Right. I saw the coolest demo a few years ago. One of the neat things about this is if you have two slits and you shine a laser through them you can actually see on the wall a diffraction pattern of bright spots separated out scattered across the wall.

           Now, if you put a lens in front of the slits, instead of getting the diffraction pattern, you can actually focus the two slits and get an image of the slits on the wall. This is one way that we say the lens forces the light to behave like particles instead of like waves.

           This is where we get into the whole “it’s both” argument. The slits make the photons interact with each other or perhaps with themselves and build a diffraction pattern, an interference pattern on the wall. The lenses force them to work like particles and you end up getting images of the slit on the wall.

           If before you put the lens in you very carefully take a grid of wires and you arrange the grids so that the wires are in the dark points on the diffraction pattern. Then when you’re looking at your screen all you are seeing is the diffraction pattern.

           When you’re looking at your experiment, you’re seeing laser shining on two slits and then this grid of wires that appears to do absolutely nothing. It’s just hanging out there. Then you see the pretty diffraction pattern.

           If you put that lens back in you just see these focused two slits. The wires in the experiment seem to have absolutely no affect on anything. You don’t see them at all.

           If you cover one of the two slits and the wires aren’t there, then the lens causes you to just see one image of one of the slits. But, with the wires there, if you cover up one of the slits, and you get rid of the interference, all of a sudden you can see the wires in the lens.

           It’s the creepiest thing ever to watch. There are videos of this.

           There’s a man in New England, his last name is Afshar, who has been doing this experiment. No one is quite sure they understand his interpretation of the experiment.

           But what it is showing is a photon is simultaneously a wave and a particle. It’s just cool.

Fraser: All right, now let’s go into the how we know what we know part. How on Earth did scientists figure this out? As I mentioned in the intro, this is completely [Laughter] non-intuitive to come up with the answer that light behaves as both a particle and a wave is about the last thing that you would rationally conclude.

           It’s not surprising that it took so long for scientists to get to the bottom of this or to the top of it. [Laughter] So, how did they even go down this road?

Pamela: We’ve been arguing over it for about 400 years. This is one of those things that made people feel queasy to their stomachs starting as early as Christiaan Huygens.

           He was playing with light and noticed this cool if you shine light through a slit, you get an interference pattern. If you shine light through two slits you get a different interference pattern.

           It’s just cool. It can only be explained with waves.

Fraser: Right, so I guess he said “that’s it, case closed, it’s waves.”

Pamela: Right. Except Newton working not that differently in time said “no. It must be a particle. Look light reflects.” It reflects light particles. You hit a mirror at a 45 degree angle. The light goes off at a 45 degree angle.

Fraser: Right, ocean waves don’t reflect in the same way particles do.

Pamela: Right. Because of this difference in how waves reflect and how light reflects, Newton said no, particle. He was able to go on and build beautiful mathematical explanations for reflection for refraction, for light going through prisms and forming rainbows all based on a particle understanding of light.

           We have Huygens explaining interference and diffraction patterns using a wave nature to light and we have Newton explaining lenses and reflection and rainbows using a particle theory of light. We have these two competing theories. Things continued and continued.

           Then finally, in the 1800s, there were two more scientists who did a bunch of experiments using interfering light again; Thomas Young and Augustin-Jean Fresnel.

           Fresnel is the person who came up with the perfectly flat, weirdly textured pieces of plastic that allow you to essentially magnify what is behind your motor home or something. Those funky little magnifying flat things use interference of waves.

           Young and Fresnel said “interference patterns, it’s a wave.” Maxwell came along and with his theories of electro-mechanics and his equations of electro-mechanics he built into all of this mathematics which explained electricity, magnetism and all the cool stuff of the day, he built in the idea of waves. The dominant scientific way of thinking was waves in the 1800s.

           Then things changed again. In the early 1900s, from about 1901-1905, we had people doing more experiments. This is where we keep getting into problems. One set of experiments says particles while another set of experiments says waves.

           The people then working in the early 1900s were saying weird. This now acts like particles, sort of. So we had Max Planck who was trying to explain the distribution of the colors of light that come off of heated objects. This is black body radiation.

           If you’ve ever seen an old generation episode of ‘Star Trek’, when Captain Kirk heats up a rock with his phaser and it glows red that’s black body radiation. Any of you who have ever used a kiln when you heat things up and they glow in the kiln, that color is directly related to the temperature of the kiln.

           Hot coils on your stove are again black body radiation. He was trying to explain mathematically why you get the distribution of light that is observed.

           The only way he could explain it was to say that it looks like the energy allowed must have specific values. It must have what we call quantized values. He came up with a model of the oscillators and the atoms having quantized energies.

           Today we understand this as atoms have different allowed energy levels and they release photons with specific energies related to those allowed energy levels. This was a particle idea.

           We also had Einstein come along with his photoelectric effect which is what he got for his first Nobel Prize. What he found with the photoelectric effect is that if you have a sheet of metal and you shine a blue light on it, you can often get it to conduct electricity by hitting the atoms with light that causes the electrons to leave the atoms.

           If you use pretty feeble blue light, the metal just sat there and went, ‘yeah, I don’t care”. But if you hit it with brighter blue light current would flow.

           If you used red light, you could blast it with as much red light as you wanted and nothing would happen. This seemed to indicate that different colors of light carried different individual energies.

           So that when you look at a light beam, the brightness that you see is related to how many particles of light there are. The energy in each individual particle is related to the color.

           What was happening in the red light versus blue light case was: If you have a power pitcher imagine with like cricket or baseball, and they’re throwing a fast ball, that fast ball is going to hurt when it hits and if in this case we have this powerful blue photon and it hits a piece of metal’s atom just right, it will fling off an electron and it’s going to fly somewhere.

Fraser: Like in your analogy, the ball is hitting so hard that a catcher lets it drop back out again. Or let’s something drop back out again.

Pamela: Exactly. [Laughter] The brightness is how many balls you have flying at one time.

Fraser: But if you have a little leaguer throwing that ball the catcher is not going to let go of it.

Pamela: Yeah, so it’s just going to hold on. You can have as many little leaguers as you want throwing baseballs and they’re probably not going to hurt anyone.

           But, if you have only one power pitcher out there, and there are 40 catchers, he may not be able to get past those 40.

           If you get ten power pitchers out there, they probably have a chance at to get at least one of those catchers to drop a ball.

           What’s happening is with the blue light, it is able to knock electrons out and the brightness just says how many pitchers you have going. The color is how powerful the throw and the brightness is how many throwers.

Fraser: So we’re back to particle.

Pamela: We’re back to particle. There is this terrible moment of “oh no, oh dear”. Max Planck has these great equations that say wave, wave, wave, wave. But Einstein has this great explanation that’s particle, particle, particle. Wow, it’s both.

Fraser: There must have been just some awful fights. [Laughter] Can you imagine the battles? Because they’re both right and so when you’re both right and the evidence supports you, the battles must have just been ferocious.

Pamela: Yes. That’s an understatement.

Fraser: Enemies must have been made. Funding must have been cut. [Laughter] Oh, it must have been awful.

Pamela: And it gets even worse. With light, yeah fine, it doesn’t have mass let’s let it be both a wave and a particle. Okay, it’s just this weird thing.

Then a guy by the name of de Broglie came around in 1924 and he says no, everything has a wavelength. You have a wavelength Fraser.

Fraser: So not just photons, but larger things like electrons or atoms…..

Pamela: Buckeyball.

Fraser: So, I’m both a particle and a wave. That’s what you’re saying. All of our listeners are both particles and waves or a collection of particles and a collection of waves.

Pamela: Yes. Isn’t that cool?

Fraser: Well then, I mean don’t [Laughter] get to the buckeyball yet. So you’re saying that like a proton….let’s say an atom, a hydrogen atom. How is that both a particle and a wave?

Pamela: It’s hard to explain. The way that we look at it is every object has this wavelength over which it exists. It is capable of interacting and things like that.

           This is in part defined by our uncertainty in being able to figure out where things are. The way I figure out where you are is I look at you which requires me to look at you with light. Light is coming from some source and hitting you and coming back to me.

           If I try looking at you in radio light which has huge wavelengths, I can only figure out where you are within the wavelength of that radio wave. But if I start looking at you with x-ray light, I’m probably going to give you cancer, but I can really figure out where you are because my uncertainty is the same size as that x-ray wavelength which is very tiny.

           Now if your wavelength is such that your specific uncertainty in you is something that is commensurate with the wavelength of how I’m trying to look at you. You can interfere with yourself.

           It gets to what are the spacings between the slits, what are the sizes of the slits, what is your wavelength.
Your wavelength is defined by well, this is the de Broglie wavelength. It’s defined by this constant called the Plancks constant divided by your momentum.

Fraser: Is it almost like an averaging out of all the particles in my body?

Pamela: It’s that everything has this intrinsic uncertainty in where’s it’s located. This intrinsic little jiggle is what we refer to as the de Broglie wavelength. The catch is that for you, you large human being you, this uncertainty is so small that it’s actually smaller than if I turned you into a black hole what your short shield radius would be.

           There’s basically no uncertainty in where you are because I can’t measure anything that tiny.

Fraser: But for single particles, for instance, haven’t scientists been able to make hydrogen atoms behave like a wave?

Pamela: We can make hydrogen atoms behave like waves, we can make actually the smallest bacteria if we wanted to we could make behave like a wave. We’ve actually, not me personally, but there was an experiment in 1999 in Vienna where a group of scientists took buckminsterfullerenes.

           These are molecules made up of 60 carbon atoms. They were able to make these buckminsterfullerenes behave in this wave-like interfering way and get this diffraction going on.

           This statistical uncertainty in position led to a distribution on how they were measuring where these things ended up on the other side of two slits. That’s just kind of cool.

           You have these 60 carbon atoms in this soccer ball like shape that are perfectly happy to interfere with each other. That’s a pretty big atom.

Fraser: Yeah. That’s amazing. So your ability to detect that or the ability to interfere or the precision with where you are gets harder to measure the more mass that you have, the more particles that you have. Right. Okay.

           So, what is the current research into this? Where are scientists trying to push the limits of this right now? Apart from making fullerenes behave like waves.

            [Laughter]

Pamela: It’s always fun to see what new and interesting things you can get to diffract. There’s also the question of how do we interpret this? One of the classic things that people say is: individual items are wave functions and they move through space in packets.
Wave packets is one way that we look at things and we can either know how fast that thing is moving or we can know where it’s located but we can’t know both and that when we make an observation, we’re collapsing a wave function. So you can never observe something as both a wave and a particle.

           People are trying to figure out if that is true. Can you really not observe the wave function and the particle at the same time? This is where the arguing over the experiment I explained earlier with the wires and the diffraction and the lens comes in. We’ll put some links to some stories on that.

           There’s also well … really the wave function only collapses into a particle when an observation is made or is it really a just a particle all the time and there is this statistical wierdness. There are still philosophical arguments going on as well. Then there’s the old are we really limited to you can either know where something is located or how fast it is going and not both.

           Is there any magical way using enough technology to get around what is called the Heisenberg Uncertainty Principle? That’s where you start to get into things like we can’t have Star Trek transporter beams until we figure out the uncertainty principle, because how do you put someone back together.

           Trying to figure out if these are actual limits or things that there is always somebody trying to do?

Fraser: Right, and there are some other aspects of this like entanglement and like that concept of Schrödinger’s cat, the uncertainty, so I think we’ll come back to this in future shows and have another go at it.

           I think we wanted to get across today just how to understand that when you hear that wave particle duality, what are we talking about and how do we figure it out and what were the experiments that brought that into play. I think that was a really good explanation.

           I’m sure we’ll get a lot of questions but there’s going to be more shows on this. I know this is an astronomy pod cast, but there is so much physics involved that there are many times where we have to bring this stuff in as well. So bear with us.

Pamela: Spectrographs wouldn’t work without light interfering. Stars wouldn’t support themselves without waves acting like particles. Everything that makes astronomy work goes back to the wave particle duality of light.

Fraser: Well there you go then, there’s our explanation.

Pamela: And it’s just really cool that the uncertainty in you and I is smaller than our short shield radius. I just think that’s cool.

Fraser: Right. That’s the event horizon of a black hole. If you took my mass, turned me into a black hole, the size where nothing, not even light could escape, that’s my uncertainty – smaller than that.

Pamela: And that’s just cool.

Fraser: We’ll talk to you next week Pamela. Have a good trip.

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

Fraser Cain: We’ve been so crazy following our own whims through the universe that we’ve neglected your questions. That ends today. It’s time to dig deep into our overflowing email box to retrieve the puzzling questions our listeners have sent in.
 
Let’s start with what I think is our best question ever.
 
[laughter]
 
This is it, this is the greatest question. Justin Craig sent in an awesome email. He says, “I was wondering, if you had enough anti-matter and you put it into a black hole with an equal mass, would the black hole disappear or just become twice as heavy?�
 
Now, before we go into the actual answer of question, let’s give the listeners background on anti-matter. I don’t think we’ve done a show on anti-matter, so what is it?

Dr. Pamela Gay: Anti-matter is basically the “Spock has a beard� universe of matter if you’ve watched old generation Star Trek. An electron of antimatter has the opposite charge. It has all the opposite physical characteristics that a regular electron has. So you have an electron and a positron, take one and turn it inside out in every way you can (except for the mass – you can never have negative mass, it’s always a positive quality), and it’s exactly the opposite. Opposites often annihilate one another.

Fraser: This isn’t just some crazy, calculated theory. This is real stuff – you can calculate it in the lab, you can smash it together, it annihilates and produces a gigantic amount of energy. This is real stuff.

Pamela: Yeah, we’ve produced positrons – I think they’ve even put together anti-matter hydrogen and helium anti-atoms in various laboratories. You’re just very, very careful to suspend them away from everything else while you’re working with them. But we can create these things.

Fraser: So this isn’t some theoretical concept. Astronomers see the presence of antimatter out in the universe, being produced naturally. In fact, it was recently announced there’s a cloud of antimatter in the Milky Way.

Pamela: There’s a bunch of natural processes that it’s just part of how energy settles out when it’s becoming matter. If you take energy and you say, “okay, let’s change it into matter� you’re going to get a regular matter particle and an antimatter particle. Everything is created in the yin and yang, in terms of you have to have a positive charge and a negative charge. All of these things have to balance out in these energy goes into matter reactions. In some cases it can actually create clouds of antimatter.
 
There’s a cloud here in the Milky Way that we detect because of the very specific gamma ray light it gives off, that has a colour that you pretty much only get when you have these matter/antimatter reactions. We think this is perhaps coming from low-mass x-ray binaries that are creating this cloud of antimatter.

Fraser: All right. We know there’s antimatter, but just creating clouds which are annihilating instantaneously. We’re not actually clumping together gigantic quantities – enough to say, create a black hole. But let’s say we could. We’ve pulled it all together and fashioned it into a ball of antimatter with exactly the same mass as our target black hole. Then we smash them together.

Pamela: Here’s where I said it’s really important that mass is always a positive quantity. If you take a pile of matter and a pile of antimatter, they’re both going to have the same gravitational effects on things, they’re both going to have the same style event horizons… they’re going to have the same everything.
 
The thing is, with a black hole, you can’t tell if it’s matter or antimatter.

Fraser: Whoa.

Pamela: Yeah.

Fraser: Right, of course. So an antimatter black hole would, in all cases, feel identical to a regular black hole, if you’re trying to orbit it or whatever.

Pamela: If you basically went around with some sort of antimatter vacuum cleaner and collected antimatter into a bigger and bigger and bigger pile until the pile had enough mass (regular mass with antimatter characteristics), that it condensed down to a black hole… it’s a black hole. When you take an antimatter black hole and a matter black hole and throw them together… we don’t know what’s going on within the event horizon. From the outside perspective, you just made one really large black hole. Which is kind of cool.

Fraser: Hold on, let’s break this down a little bit. Say we have our antimatter black hole and we’re streaming planets and asteroids at it, they’re just disappearing into the antimatter black hole. Now, there would’ve been an explosion going on as these asteroids are striking the antimatter, right?

Pamela: The problem is you’re starting from the assumption the matter inside a black hole is normal. It’s not – at least, we can’t think of any way that it’s normal. So the whole idea that you have electrons goes away. The whole idea that you have protons goes away.
 
What you have is some surreal quark soup that all the different bits and pieces that make up both matter and antimatter are slammed together and forced into these really small volumes. Things lose their identity in the process.

Fraser: We’ll get that question in a second – we’ve actually got another question on that identity and information. I guess my question – sorry to not let go here – let’s imagine you had your antimatter black hole and your regular matter black hole, wouldn’t your antimatter black hole be the exact same configuration as the regular black hole, just the antimatter version of that/
 
Say it’s some kind of soup of particles which no longer look like protons/electrons/whatever. Wouldn’t they just be anti-versions of whatever’s in the black hole, and wouldn’t that still create the explosion?

Pamela: How do we know, because it’s in the event horizon, that these particles are able to hold on to that level of identity? How do we know they haven’t turned into pure energy as they cram themselves in there?

Fraser: Let’s say we do know. Let’s say they do remain as a mere version of the particles that are in a black hole. What happens then?

Pamela: Well, it’s just energy being released, but that energy can’t get out because it’s a black hole.

Fraser: Well that’s the question isn’t it – the energy can’t get out because it’s a black hole which would stop even energy. So obviously you’re not going to have an explosion of chunks of things because they would just be sucked down. You’re not going to have radiation because it’s going to get sucked down. You’re not going to get sound…. Anything. There may very well be an explosion, but you wouldn’t know it happened. Is that right?

Pamela: Exactly. Basically, what goes in stays in and we can’t find out anything beyond that. Since the antimatter systems have positive mass, you just have a bigger black hole.

Fraser: Right. Wow.
 
All right. I think that’s it – I guess, that’s the question at the heart of it. The hope was maybe the two would cancel each other out and you’d be able to break past black hole-ness, right, and the whole thing would explode and turn into a release of energy. Even energy can’t escape this black hole, so even if there is a release of energy, no one’s the wiser.

Pamela: Exactly.

Fraser: Awesome question. Best question ever.
 
[laughter]
 
Let’s get on to some other best questions ever. Let’s go with that – continuing on the information.
 
We had a question from Maureen Egan, and she wants to know, “in terms of information not being able to escape the gravitational pull of a black hole, what exactly is information? When I imagine information I think of data like that stored on a floppy disk or CD.�
 
I think astronomers do use that term quite loosely – all the information is lost, so who knows what happens to it. But what is information?

Pamela: I’m not sure so much that we use the phrase loosely as we just throw it around a lot without ever telling anyone what it means.

Fraser: Oh, fine – yeah.

Pamela: Which is a little bit more evil on our part.

Fraser: Yeah, no – I just throw around, “information’s gone – moving on�
 
[laughter]
 
“Stop, what does it mean?� So really – what does it mean, information loss?

Pamela: It’s an idea that came out of quantum mechanics. It’s this whole idea that particles have varied states in them. If you take an atom – let’s talk about helium, which is nice and simple. In a helium atom, you can have two different electrons. One is going to have spin-up and the other will have spin-down if they’re both in their lowest energy state. This is something that comes from the poly-exclusion principle, which says you can’t have two electrons with the same spin in the same orbital.
 
The fact that one is spin-up is information. The different states particles take on, or the different wave functions, all of this is different types of information. It’s the quantum states that are tied up in particles that people try and figure out how to take advantage of in building the next generation of hard drives where we store information in the spins of electrons.
 
How do we figure out how to tap into this so that we can build atoms that store the genetic code of the human genome, or something crazy like that. I don’t think you can actually do that one.

Fraser: Maybe we could do an analogy in star trek, like where you hop in a transporter and you’re going to be teleported from where you are to the moon down below. You want to make sure that the teleporter can rebuild you, atom by atom, and for it to be able to do that it’s going to be able to put an atom here with this quantum state, an atom there with that quantum state, etc. It’s got to get it exactly right or you won’t be you anymore. You’ll be somebody else, or even just a mess.

Pamela: All of this can include information such as what the polarization of a photon, what is the orientation of the waving of the electric and magnetic fields of that photon as it passes through space.

Fraser: So there’s any number of ways you could measure an atom or a photon or a particle or anything and that’s the information that is thought to be destroyed when it goes into a black hole.

Pamela: It’s the most basic way of putting this is what are the quantum states of the particles – that’s the information the particles carry.

Fraser: So the thinking is that if you could somehow pull that stuff back out of the black hole, there would be no way to re-create that information. No way to ever know what its quantum state was.

Pamela: Yeah.

Fraser: Why is that bad?

Pamela: Well, we like to think that no information is ever lost. Ever particle in some way, holds its entire past inside of it. It couldn’t exist if a whole series of different things hadn’t existed. You take energy and that energy has to split into a positive and negative charge, different spins that are conserved… you have all these different things that have to get conserved in the creation of matter and reactions and there are very specific processes that are the only allowed atomic processes, as particles decay from one to another through time.
 
If this information could get lost, it’s sort of like erasing the history of the particle, which is kind of sad and also implies that there’s information about our universe that gets lost forever.

Fraser: Right, but astronomers aren’t boo-hooing about lost information. This information loss breaks something, right?

Pamela: Well, one of the tenets we start with is no information can ever be lost or destroyed. If black holes can lose or destroy information, there’s one of our basic tenets gone, and that makes people uncomfortable.

Fraser: Okay, I think we could talk about this all day. Hopefully that gives you the information you were looking for, Maureen, and we’ll come back around and talk about information in black holes… that’s a whole show, I’m sure.

Pamela: Yeah. The short answer is, as near as we can tell, black holes don’t eat information – it has ways of escaping. But that’s for another entire episode.

Fraser: Right, right. Okay. But when they’re talking about information, that’s what they’re talking about – the quantum state of the stuff that gets consumed.
 
Let’s move on and go to our next question. This is from Mark Maultby. “If gravity is the force of interaction between objects, what is the smallest object that could noticeably be said to have gravitational attraction?�
 
I just want to set some scale here. If I have the Earth, with the Sun… Here we are on the Earth, feeling its gravity. If I go across the universe, to the other side of the universe, I’m still feeling the effect of gravity from the Sun, right?

Pamela: Oh yeah.

Fraser: Now, not much, obviously.

Pamela: Not noticeably.

Fraser: It’s so miniscule you can’t even have numbers to describe it, but it is there. Every piece of matter in the whole universe is interacting gravitationally with every other piece of matter in the whole universe. That’s true?

Pamela: That is exactly true.

Fraser: Okay. All right, and then that doesn’t matter for any size – for a planet, a moon, a proton, an electron, a neutrino… anything, there’s still a gravitational force that’s being done across the universe. I guess the question is, is there some point where that doesn’t happen anymore?

Pamela: No. It’s either you have mass – and if you have mass, then you affect things with gravity. Or you have no mass, in which case you can fly across the universe at the speed of light.

Fraser: Is there a minimum amount of mass you can have/

Pamela: Nope.

Fraser: But, we had this conversation just a couple of weeks ago about the Higgs-boson. I know there’s the concept of gravitons. Is there some number where, if you’re smaller than the Higgs-boson, then you won’t have mass? Like, you need to have one Higgs-boson to have mass – I’m speaking gibberish, right?

Pamela: That’s one of the crazy things. Higgs-bosons have a fair amount of mass.

Fraser: Theoretically.

Pamela: Or at least, they have a fair amount of energy (and energy and mass are kind of interchangeable, which makes the way we talk kind of confusing). The real question comes down to what is the least massive particle that we know about? That’s probably quarks. Three quarks combine to make a proton.
 
I think the real question is what the particle is with the smallest mass out there. Here you have to start remembering there’s quarks, and they combine to create protons and basically have mass (in a sort of weird kind of way). Electrons have mass, neutrinos have mass. Then there’s this stuff called dark matter that we don’t know what the heck it is. It has mass. It gravitationally effects things.
 
I’m not sure we know, yet, at this point in time, exactly what the smallest particle out there is, because we’re still discovering particles. We’re still trying to figure out what this weird stuff called dark matter is. But it’s basically, when you start getting down to these… this is a single lepton, a single boson, a quark… these individual units have slightly different masses, but these are the smallest things (smaller than atoms), that are capable of gravitationally affecting other things in the cosmos.

Fraser: But that kind of feels like you’re not quite answering the question.
 
[laughter]
 
You’re kind of saying that these are the smallest particles that we know about – that we know exist for sure – and we know that those particles have mass, and therefore they can gravitationally attract. If you had one quark on one side of the universe and another on the other side of the universe, if they weren’t being expanded away from each other, they would eventually come together, over a long time.
 
But the question is, is there some theoretical limit where you just can’t have any less mass?

Pamela: When defining the smallest possible things, we often say, “there’s the Planck unit of time� (the smallest discernable unit of time), or there’s the Planck length… You’d think there’d also be a Planck mass, which would be a limit to how small mass could get. But there’s not.
 
As far as we know, there may not be a limit to how small something can get in terms of mass, but we’re still figuring out the particle world. We still haven’t found the Higgs-boson (if it exists). We still haven’t found the graviton (if it exists). There’s this whole realm (potentially) of different particles that don’t interact via the electromagnetic force like electrons and protons do, that are making up dark matter. For all we know, the least massive particle out there is also the most common particle out there and happens to be whatever it is that makes up dark matter.
 
We’re still learning. Particle physics, the standard model, these are things we’re still working to define. As far as we know, no – there is no mandated-by-the-cosmos boundary on how small a mass we can get. We’re still exploring.

Fraser: I guess the question will maybe help to be answered by upcoming work with the Large Hadron Collider.

Pamela: Yes.

Fraser: We don’t really know. That was a good question too.
 
[laughter]

Pamela: These are the ones that stump me.

Fraser: I know, I know. Let’s move on. Something I think is a little simpler – this comes from Sabre Rosewood and the question is: “since the tides on Earth are part of what causes our moon to slowly move away, what will happen once the oceans are gone? Will the Moon stop moving away from the Earth?�
 
We talked about this in our show “Where Does the Moon Come From?� and discussed that – the Moon is slowly moving away from the Earth. What’s the cause of that?

Pamela: It boils down to conservation of angular momentum. The Earth isn’t a perfect sphere: it has mountains, it deforms itself due to the gravitational pull of the Moon. As the planet rotates, it bulges out so that part of it bulges toward the Moon and part of it bulges in the opposite direction because the gravity is not so strong over there.
 
This deformity basically gives the Moon a gravitational handle to hold on to our planet and say, “no – don’t rotate past me, keep the bulge pointed this direction!� The rotation of the Earth is constantly trying to carry the bulge past the Moon. Gravity grabs that bulge and pulls it back. This pulling back on the bulge that’s trying to rotate past the Moon is slowly, slowly, slowly, slowing the rotation of the planet.

Fraser: You’re talking about a bulge that’s coming from mountains or one side of the Earth is a little more bulged than the rest of it. Oceans move huge distances – more than the mountains ever move. There’s a gigantic amount of ocean on the planet, so does that play a significant role in this?

Pamela: It plays a significant role, but it’s not the only role. So, 50 million years from now, when our oceans start to evaporate away, we’re still going to have these tidal effects. We’re still going to have this planetary flexing that prevents us from becoming a perfect sphere ever. This planetary flexing is going to continue to slow the rotation of the planet until eventually we’re completely locked so the same face of our world is always facing the same face of our moon.

Fraser: And the moon will stop moving away.

Pamela: It will stop moving away.

Fraser: Right, so the oceans are part of the bulge on the Earth, but they’re not the whole thing. Eventually, even when the oceans boil away, the Earth and the moon will still go through this dance until they figure it out – until the Earth and moon are facing the same side toward each other forever and always. Which I think would be longer than the lifetime of the Sun, right?

Pamela: Yeah, that’s what we think right now at least.

Fraser: It’ll be a red giant before it happens.
 
Okay, cool question. Let’s move on. Paul Barnett asks “Since the universe is expanding and we believe that matter cannot be created or destroyed but only changed from one form to another, I’m curious to know where the new matter comes from to occupy the new space that’s created. Is there new matter being spontaneously created?â€?
 
Now, let me try and rephrase the question, because I think he made a couple of mistakes there. We talk about the universe expanding, the expansion of the universe, both from the big bang but also from the additional dark energy that’s helping to push the universe apart. We’re getting more space in between the galaxies and galaxy clusters and interstellar space. But we’re not necessarily getting any space in between the galaxies because they’re held together.
 
I guess the question is, let’s look way out into the most unpopulated part of the universe where space is expanding apart and we can measure the density of how many atoms per cubic kilometre there are out there. As the space is expanding from dark energy, is there any more matter coming into existence?

Pamela: No, that’s the cool thing. The universe is basically diluting itself over time.

Fraser: So it’s like you’re pouring water into something that was quite thick, and it’s just making it more and more dilute – more and more thinned out.

Pamela: Or, the way I like to think about it, if you imagine blowing up a balloon, the balloon has very thick walls when it’s small, but the more and more you blow it up, the thinner those walls get, the fewer atoms there are per square centimetre of area on the surface of that balloon.

Fraser: Until it pops.

Pamela: Until it pops.

Fraser: Our universe isn’t going to pop, is it?

Pamela: No.

Fraser: Okay.

Pamela: But it’s going to get pretty empty.

Fraser: Right, and that’s it – there could be some point in the far, far future where everywhere you look, there’s no atoms around. Right now, I forget – did you mention how dense space is?

Pamela: It’s on the order of nothing per cubic meter?

Fraser: Right, okay. The occasional particle per cubic meter, but you could eventually get to the point where there’s one particle per light year.

Pamela: What’s weird though is this is true of atoms of normal matter. There’s this thing called dark energy, and near as we can tell, dark energy is constant at all times. When we look at how much energy there is per cubic meter of space, it works out to a few protons worth of energy at all points in time, even though the total volume of the universe has increased.
 
That means the amount of energy, the amount of dark energy in the entire universe, is somehow increasing as the universe gets larger, because its staying constant as a function of volume. This gets confusing.

Fraser: I’ve got a zinger for you now, then. We always talk about the fact that matter and energy are interchangeable. So, is dark energy interchangeable with matter? Could you freeze it into matter?

Pamela: As far as we know, no. Dark energy is this weird enigma, currently. We don’t know what causes it. As near as any theorist that I can understand has gotten, dark energy is basically a field of energy that permeates all of space and time. If you can imagine this mesh of energy that is everywhere, all at once, and not getting all metaphysical on me… if you can imagine this lowest possible energy state (that isn’t zero) it permeates everywhere. One of the fears is something will come along and trigger that wave, that field that permeates everywhere to crash down to zero and no one knows what will happen.

Fraser: Now you’re freaking people out here.
 
[laughter]

Pamela: Theorists do scary things with their mathematics sometimes. Like I said, we don’t really understand it right now. So, give us a few years.

Fraser: Okay. To summarize then, with the expansion of the big bang and the addition of dark energy, the universe is growing but the amount of matter in the universe isn’t changing, so it’s really just getting diluted. So, back to the question where’s the matter coming from, it’s not coming from anywhere – there’s no additional matter. All the matter in the entire universe was created in the big bang, and that’s all we’ve got.

Pamela: That’s all we’ve got.

Fraser: All right. Speaking of all we’ve got, there’s one more question. This is a good one too, in fact this is a question I was going to ask you about and I never got around to it.
 
This one comes from the forum, the Bad Astronomy & Universe Today forum. “I can’t wrap my head around the physics of gravity assist. Why does travel in the same direction of an object’s orbit speed something up, while travel in the opposite direction slows it down? I keep thinking the approach push and depart pull would cancel each other out either way and not change the speed at all.�
 
This is great – I’ve thought about it too. You’ve got a spaceship going toward Jupiter and it’s going to get a gravitational assist to pick up velocity and go much faster. As it approaches Jupiter, Jupiter is speeding it up. I get that – it’s velocity might be changing as it’s falling into Jupiter’s gravity well. As it does its fly past, and starts to move away from Jupiter again, now Jupiter’s pulling back on it. It should be slowing back down. Shouldn’t you just end up with the same velocity? It’s like going down a hill and then back up it on the other side, shouldn’t you end up going the same speed you were going before?

Pamela: That would be exactly right if the object you were having the gravity assist from wasn’t moving.
 
The key is you are gravitationally falling into the gravity hole of some object in motion. If it’s not in motion, you go in, come back out and your energy hasn’t changed at all. If you imagine a completely frictionless, gently curved valley in the road. You go down a hill, up a hill, no friction occurs so you’re going the same speed on both sides of the hill even though you speed up going in and slow down going up… it all cancels out in the end.
 
The catch is, if the object’s moving, the amount of time that it is either able to gravitationally pull on you to speed you up or gravitationally pull on you to slow you down changes. If you’re moving in the same direction as the object that’s giving you the gravitational assist, as you’re moving toward it, it’s saying “yes! Catch up with me!� and pulling on you to get you to catch up to it. So the whole time, you’re approaching it, it’s running away from you. As it’s running away, it’s pulling on you to help you catch up.
 
Once you catch up to it, you’ve gained all this velocity, so you’re able to zip away from it with extra velocity you didn’t have before, because the extra time you had catching up with it lead to you getting some of its velocity and spending extra time falling in and not extra time falling out.

Fraser: So, you’re slowing down Jupiter by a teeny-tiny little bit, to slow it down in its orbit, and its speeding you up to pull you up to its speed.

Pamela: Yes.

Fraser: Right. So the amount that you get falling into it and the moving back away from it do cancel out, but it’s that process where it’s pulling you up to its speed in the orbit which is what adds to your velocity.

Pamela: If you’re going in opposite directions, then you end up putting the extra effort into slowing down to meet its speed. Then you end up going slower on the other side. Same thing.

Fraser: Right, and I know the MESSENGER space craft is using that method to be able to go into orbit around Mercury.

Pamela: Yes.

Fraser: They’re using this process to be able to slow themselves down, as well – if you just go the opposite direction, you slow yourself down.
 
That totally makes sense. I honestly didn’t have it thought through, so thank you.

Pamela: It’s a really cool affect.

Fraser: You know what, I said that was the last one, but we’ve enough time for one last, quick little question. I think this is a quick one. Rich from New York wants to know “if the Sun were to suddenly vanish, would we feel the effects of gravity instantaneously, or would it take approximately 8 minutes, just like light?�

Pamela: It would take 8 minutes, just like light. Fast enough?

Fraser: The speed of gravity is the speed of light. If the Sun disappeared, we would see the light disappear and we’d also suddenly feel the gravity disappear.

Pamela: It would appear as if all of a sudden all the stars became visible and they’re moving in the wrong way. That’s kind of cool.

Fraser: Yeah.
 
And that effect works the same for us moving around the Milky Way, the Moon going around the Earth… it waits for the speed of gravity. Cool.
 
I think that plays into our recent show about gravity waves, that’s what the whole trick is about. You’re watching as waves of gravity are released from objects as they wash over the planet. That’s it – that was quick.

Pamela: Cool.

Fraser: Perfect. I think that covers everything.