The coldest possible theoretical temperature is Absolute Zero, this is the point at which no further energy can be extracted from a system. How are physicists working to get as close as possible to this extreme cold?
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Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program. Visit Astronomy.Swin.edu.au for more information.
Faser Cain: Astronomy Cast Episode 383: Approaches to Absolute Zero.
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 Faser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of CosmoQuest.
Hey Pamela, how are you doing?
Dr. Pamela Gay: I’m doing well. How you are doing, Fraser?
Faser Cain: Good. So happy hiatus day. So this is the last episode that we’re going to do because it’s the end of June, beginning of July and we will now take a two month little break.
Dr. Pamela Gay: And to everyone who I said no, no, no, we have one more episode, I’m sorry I lied. I didn’t know I was lying until I found –
Faser Cain: Right, I made you a liar.
Dr. Pamela Gay: Yeah.
Faser Cain: But I already planned something next week. Yeah and so we’ll be taking a break and then we’ll be back after Labor Day. But I think I put it on the calendar what the next it is. So yeah. But you know what? We’re not actually going to be taking a break. We’re going to be taking a break from trying to be near a good internet connection many times a week, but both of us will be working on a ton of other projects and putting together some good ideas for the fall. So this is our chance to kind of recuperate and relax.
Dr. Pamela Gay: Now, this our time for catching up on other work.
Faser Cain: Catching up, yeah, checking of the email inbox, etc. So thanks everyone for joining us over the whole year and as always it’s been an honor and a pleasure to entertain and educate you about astronomy. So let’s get on with this week’s show.
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Faser Cain: So the coldest possible theoretical temperature is absolute zero. This is the point at which no further energy can be extracted from a system. So how are physicists working to get as close as possible to this extreme cold?
All right, Pamela, so we did this show I think on absolute zero, but now we’re going to talk really about the methods and the technologies and the physics involved to really get that close to absolute zero. So can you give sort of that nice physics explanation of what absolute zero is?
Dr. Pamela Gay: So absolute zero is something that we in theory can never actually get to. It’s that point where all of the atomic and subatomic vibrations and motions stop. And so at that point you have – well, first of all, broken the Heisenberg uncertainty principle because you know that something has stopped, therefore it has a position and no momentum, thus the theoretically can’t get there part. But beyond that, it’s that point where you’ve completely removed the energy from a system.
Faser Cain: And so it’s not that the particles have necessarily stopped, because they still – I mean even if there’s no energy at all, right, there’s still a little bit of motion isn’t it?
Dr. Pamela Gay: No. By definition that would mean that there was some form of kinetic energy. The second you have any motion, you have energy.
Faser Cain: Well, right, but the way we always describe it and the way I’ve had my wrist slapped many times is that I don’t say that it’s when the motions have stopped. It’s that when no further energy can be extracted from the system.
Dr. Pamela Gay: And that is the realistic point that we get to. So you have that theoretical can’t get there point, which is where all motions have stopped, then you have the reality, which is absolute zero in reality is that point where you can extract no further energy from the system unless you start tearing apart the atoms through fusion or fissions processes, but through lasers, through magnetic fields, through any other method you can’t get any more energy out of the system.
Faser Cain: And is it sort of like a similar counter to, I think about relativity and we think about moving trying to reach the speed of light and that as you get closer and closer to the speed of light, it takes more and more energy and eventually it would take more energy than is contained in the entire universe and you still couldn’t reach quite the speed of light. So is that same sort of idea that you’re really approaching infinity – the Heisenberg uncertainty principle. You’re approaching where quantum mechanics is really trying to stop you at all costs.
Dr. Pamela Gay: And it’s not even so much that quantum mechanics is trying to stop you at all costs. It’s not like there’s some limit in terms of, well, as you said, with relativity as you approach the speed of light. There just isn’t enough energy. With approaching the realistic form of absolute zero, the issue becomes more of – there’s just how do you get the energy out at that point. We don’t have the mechanisms. We just keep getting closer and closer as we find new ways to get up to that point. But it may actually be possible that we can hit the limit of no more energy can be removed from the system even if we can’t hit the point of the Heisenberg uncertainty principle being broken.
Faser Cain: So what is the temperature of absolute zero?
Dr. Pamela Gay: Well, by definition it’s zero.
Faser Cain: It’s zero kelvin.
Dr. Pamela Gay: It’s zero kelvin or there’s also Rankine’s, but that is the version of absolute zero that nobody uses where the spacing between degrees is the same as in Fahrenheit.
Faser Cain: Okay, we won’t use that one. We use kelvin.
Dr. Pamela Gay: But it’s important to note before we get the angry emails.
Faser Cain: Right. And they’ll come. Okay, great. So let’s talk about how cooling something works. How do we normally cool down some kind of system?
Dr. Pamela Gay: Well, when you’re dealing with your everyday refrigerator or even, heck, planetary nebula in the process of forming, cooling comes about by taking a system’s energy away somehow. Now, in general when you’re talking about the flow of temperature, you don’t actually say that. And the cold flows into the room. That whole notion that air conditioning works by bringing cold into a room. No, you don’t bring cold into a room. You remove the heat from a room.
One of the ways to remove heat from a room is to [inaudible] [00:07:39] bring in something that already doesn’t have heat and force the heat to move from an energy of high energy to an energy of low energy and equate the energy in the two systems.
So the way air-conditioning works isn’t that it brings in the cold. It’s that it forces the heat to heat up a larger volume. And it’s that larger volume that matters over and over. So in your refrigerator you have a gas that you go from compressing to expanding, from compressing to expanding and when you expand the gas it has a larger volume, the whole gas laws thing kicks in, volume goes up, temperature goes down, you flow that through your refrigerator, it absorbs the heat out because the energy in the refrigerator enters the colder stuff trying to bring the two into the same temperature. Heat is then carried back out of the refrigerator, heats up the space behind your refrigerator where the cat likes to attempt to get to, then compressor kicks in, thus rinse and repeat.
Faser Cain: Right. And so when we’re hearing that sound of the compressor kick on on our frig, the sound of the frig is it attempting to compress air down to a smaller volume which it can then –
Dr. Pamela Gay: With a refrigerator it’s a fluid actually, not an air.
Faser Cain: A fluid, okay, but then it’s going to then use that fluid to – runs it through all of those pipes on the back of the frig and then that pulls air out of the refrigerator itself, pulls the heat out of the refrigerator and then it gets rid of it and then it just keeps that compression going on.
And you sort of experienced that, too, right? You have a can of compressed air. Have you ever used one? And you blow it. You get ice forming around the can. The can gets cold and that’s just that same process, right, that you’re taking what was a smaller in a gas in a compressed space, you’re then expanding it and that sucks heat out of the environment to equalize the temperature.
Dr. Pamela Gay: And it’s always a matter of the energy is flowing from a high energy area, which is the same thing as high temperature and it’s going to try to equate the temperatures over multiple regions.
Faser Cain: Okay, so we’re looking at our scientific apparatus or in this case sort of our refrigerator, right? That only gets us so far, right, to a certain point. That’s about as – how cold can we get just using this method of using condensation and expansion of something that’s under compression to get to a colder temperature?
Dr. Pamela Gay: Well, condensation is a completely different process. We’re going to ignore it for this episode.
Faser Cain: Yeah, yeah, never mind. Expansion of a compressed substance to a larger volume.
Dr. Pamela Gay: So the universe has actually taken the Boomerang Nebula, which is a planetary nebula that’s in the process of forming. So the stellar core, that white dwarf that will be remaining eventually still hasn’t been exposed. It’s ultraviolet radiation still isn’t heating the nebular, but what you have is the outer layers at the star’s atmosphere rapidly expanding. And this rapid expansion has produced a nebula that is just one degree kelvin. So you can get all the way down to one degree kelvin just by expanding the tar out of a gas. And this requires solar system size volumes to be expanding into.
Faser Cain: And so that’s what they’ll do out in space. That’s what nature does. So can we get to a one kelvin with our instruments or –
Dr. Pamela Gay: Not quite simply because, well, we don’t have the capacity to quite expand and still measure things because when we look at things like the Boomerang Nebula, the densities are so low that we struggle to achieve densities that low here on earth. So when you go from stellar densities to densities we might call a vacuum, we kind of lose the ability to measure the temperature in that system because it’s just not a system we know how to measure.
That’s part of the problem in trying to measure the temperatures of things. We deal with temperature in terms of well, we get at something’s temperature by looking at the color of its black body radiation. That works for big things that you can measure the light from, like the Boomerang Nebula. We get at the temperature of things by measuring their motions. And when we go from being able to measure temperature via either your normal thermometer where we’re actually measuring the motions of the gas, which are expanding and contracting the fluid inside your thermometer to instead using some sort of a well, let’s measure the vibrations and how they give off different colors of light. That transition kind of gets hairy to measure temperatures.
You go from being able to deal an ensemble of particles to huh, now I have one thing. How do I measure this one thing? And it gets difficult to compare things in that range between the two processes.
Faser Cain: So we talked a bit about in the lab using refrigerators, cooling us down and the fact that nature has its own refrigerators, which is expanding gas coming out of a white dwarf in a planetary nebula. And you get down to that one kelvin. So if we want to go colder than that, what do we have to do?
Dr. Pamela Gay: We have to actually start finding ways to remove the motion of the individual electrons to dampen the motion of the actual atoms and just continually remove energy from the system by slowing the bits of the particle from moving.
Faser Cain: That’s mind bending.
Dr. Pamela Gay: Yes, it’s far.
Faser Cain: Yeah. But I can imagine, right? We’re imagining the atoms themselves, they’re jiggling and when it’s a gas it’s in a gaseous state and they’re all bouncing around and when they’re a liquid state they’re still moving, but they’re sort of able to sort of slide back and forth and when you’re in more of a solid state, things are still – they keep their place, but they’re kind of jiggling back and forth. How do you dampen this energy as a –?
Dr. Pamela Gay: Lasers. Lasers are the answer to many, many things in science.
Faser Cain: Always lasers. So how does a laser do this?
Dr. Pamela Gay: So it’s actually very counterintuitive. You don’t normally think of shooting something with a laser – not on a shark’s head. We haven’t done that yet. But you don’t normally think of shooting things with a laser as being a way to remove energy from the system, but it turns out that with an atom if you systematically shoot it with a color of laser that the electrons will absorb and readmit, so you’re shooting it with a color that matches the energy levels of the atom and you do this in the direction of motion. So you’re very carefully matching the motion of the atom, the Doppler shifting of the energy.
What ends up happening is the light that gets absorbed, when it gets remitted due to interference with the electric field and a bunch of other crazy, awesome effects, the energy that gets emitted in a completely random direction is actually a larger amount of energy than what was absorbed from the laser. And if you’re giving off more energy than you absorbed, that energy has to come from somewhere and where it comes from is the ensemble energy of that atom.
Faser Cain: And so with repeated pulses as you convince those electrons to give off those photons, that atom just gets colder and colder and colder. How far can you take that?
Dr. Pamela Gay: So far we are getting within a few hundred pico degrees – well degrees isn’t the right word – a few picokelvin. You have to remember with kelvin you don’t say degrees. Within a few picokelvin of that Heisenberg uncertainty reality based zero point.
Faser Cain: And when you cool down matter to this level very weird things start to happen, right?
Dr. Pamela Gay: Well, it’s even before you get to this point. The whole idea of a Bose-Einstein condensate is you take some sort of an ensemble of atoms and you cool them and cool them and cool them and as they begin to get all of their different electrons into their lowest energy spaces, as they reduce their energies and thus fit into a smaller and smaller volume, you reach this point where, thanks to the Pauli Exclusion Principle because we’re all about excluding things this week and last week – thanks to the Pauli Exclusion Principle which says that no two particles can have the same quantum numbers, things start changing their position and their quantum numbers to, well, prevent the Pauli Exclusion Principle from getting overrun and this moving out of the way of things with the same quantum number leads to, well, sometimes your Bose-Einstein condensate suddenly spreading itself out, which includes climbing up the sides of containers and other such silliness.
Faser Cain: But it’s very, very cold.
Dr. Pamela Gay: It’s very, very cold. Now, you don’t have to get –
Faser Cain: Don’t touch it.
Dr. Pamela Gay: Well, you don’t touch it, but you don’t have to get down to this couple hundred picokelvin of absolute zero to get Bose-Einstein condensates. So you can handle them at slightly warmer temperatures and involve fewer lasers in the process.
Faser Cain: But there’s no natural processes that we know of that will cool things down that far, right?
Dr. Pamela Gay: Well, at this moment in history is the thing. We have to look at the fullness of time and as our universe ages out, it’s not that the entire universe is going to someday become a Bose-Einstein condensate, but it’s going to achieve the kinds of overall temperatures where if atoms were close enough to care about the, well, quantum numbers of other atoms, you might worry about the entire universe becoming a Bose-Einstein condensate.
Right now we’re 13.8-ish billion years after the Big Bang and we’re already down to just over 4 kelvin for the temperature of the universe. As we continue to age, the temperature of the universe is going to get colder and colder and colder, starts are going to die off, atoms are going to spread out and yeah, we’re going to start hitting absolute zero over the fullness of time.
Faser Cain: Right. When we think about the degenerate age of the universe, say a 10 to the power of 100 years in the future, a google of years from now, right, where I mean, the size of the universe will be incomprehensible and you could have individual atoms that are separated by millions of light years, right?
Dr. Pamela Gay: Light years, yeah. And this is going to be the point at which if black holes evaporate because there won’t really be a cosmic microwave background to worry about, those black holes are going to evaporate. It’s going to be the point that if protons are unstable which I think we’re pretty close to saying with all of the experiments no, protons are stable. Get over yourself. But if protons are unstable this is the point at which all of the protons will have decided to decay. But at that point we will still be left with the fermions. We still will be left with the quirks. And they’re going to do their best to stop motion.
Faser Cain: That would be so strange, but I guess for what we know right now, laboratories on earth are the coldest places in the entire universe.
Dr. Pamela Gay: So far. And the thing is that the observations we’re making of the Boomerang Nebula as it forms. For instance, the process of a planetary nebula forming is very brief. It’s only about a thousand years. The fact that we caught one in the process of forming is kind of like winning the astronomical lottery. You just don’t expect it to happen. So it could be that there are other expansion processes like this that do get closer to absolute zero. But as far as we know, one degree kelvin is about as close as our universe naturally gets. And beyond that lasers and magnetic fields and ingenuity and millions and millions of dollars are what’s required to get even colder.
Faser Cain: Now, you talked a bit about the Pauli Exclusion Principle and just this idea that you can’t get electrons to be willing to experience the same kind of quantum state and then that’s sort of thing that’s holding us back. And so can you explain a bit about how this is stopping temperatures from getting closer and how are they related?
Dr. Pamela Gay: Well, it’s not so much with the Pauli Exclusion Principle that it is causing things to stay warmer, but rather it is causing things to hit degenerate stages as they get colder. Like we talked about in our last episode, a degenerate mass, a degenerate gas, a degenerate whatever the heck it is, is created when you have the pressures are so high, the densities are so high, in the case of white dwarves require great pressure to create it. In the case of your laboratory environment, you often require just a lot of cold to get to this high density. The density is so high that as the electrons would normally try and orbit their atoms, fitting themselves in wherever their energy level’s probability function wants them to fit in, you’re going to end up with two atoms side-by-side that say they’re both hydrogen while both of their electrons are going to try and be one of the same two spin states in that lowest energy level.
As they try and get colder and colder, they’re going to try and go to the lower energy of those two spin states, but if they’re right next to each other with overlapping probabilities, they can’t both have the same quantum number because two things with the same quantum number just can’t be together.
So they’re going to arrange their quantum numbers, shift their spin if it’s just two of them so that they’re different. Add in a third one, suddenly it’s like oh, okay, so spin change isn’t enough. And they end up adjusting the energy levels of each of the electrons to form this degenerate gas where the electrons are allowed to have probability functions in this extraordinarily dense cloud that as those probabilities overlap they have different energy levels, different quantum numbers, different spins, all of this adds up to different quantum numbers that create a degenerate gas that doesn’t break the Pauli Exclusion Principle.
Now, like I said, the doesn’t have the same quantum numbers can either come from changing the quantum numbers or, as sometimes happens with Bose-Einstein condensates, which aren’t under extraordinary pressure, confining them down to a tiny space, they just move out of the way of one another. It’s in white dwarves where you do have the significantly higher temperatures and can never cool as cold because you do have things in higher energy levels where it’s a completely different way of handling the degenerate gas problem.
Faser Cain: So, I mean, I guess you sort of, kind of answered my next question, which was with a white dwarf, with a neutron star, you’ve got this gravity that’s overcoming the Pauli Exclusion Principle and then you get the black hole.
Dr. Pamela Gay: It’s not overcoming the Pauli Exclusion Principle. It’s completely different physics. So the Pauli Exclusion Principle –
Faser Cain: No, I understand that. Yeah, so I was just saying you don’t have that same analogy. With a black hole, you’ve just pushed beyond where the atoms are willing to be and you’re into physics no man’s land, but with coldness you don’t have that force of gravity on top of it.
Dr. Pamela Gay: Right. So if you look at a neutron star, for instances, this is a degenerate gas of pure neutrons where the neutrons arranged themselves with quantum numbers, energy levels, all that sort of awesomeness so that they’re pushing one another apart with an atomic based gas pressure, degenerate pressure that is fighting against the gravity.
Now, if the gravity becomes greater than the neutron degenerate pressure, then the neutrons cease to be neutrons and they have to become something new that takes up a smaller volume, reduces its own pressure or not because we don’t really know what happens at this point. It could be it collapses down into quirk soup, it could be – we don’t know. But we do know that we’ve overcome the ability of the neutron degenerate gas to have a pressure sufficient to fight out gravity. Pauli Exclusion Principle happily, as far as physics knows, stays excluding. It’s just one of those things.
Faser Cain: So we talked about using refrigerators. We talked about lasers. Is there any other technologies that can theoretically take us closer and closer or are lasers the best that we will ever hope, just higher and higher power lasers and more carefully zapping?
Dr. Pamela Gay: Well, it’s the same idea if you can figure out how to do it with magnetic fields. The nice thing with a laser is you can very carefully control I am firing this direction with this exact energy and it’s going to confine this atom by getting absorbed by electrons that have a specific momentum that I have now matched, so I know which hemisphere of the atom with this motion is going to be – lots of easy to combine things as far as lasers go.
But that isn’t to say that someday we won’t figure out how to do this with some sort of very carefully pulsed magnetic fields and magnetic fields are involved in confining the atoms during these experiments. So it’s via the electromagnetic force that we’re doing all of this. So I think it’s safe to say that however we do this, we’re going to be playing with the electromagnetic force and be grateful to Maxwell and his equations.
Faser Cain: So I did a little Googling while we were talking. I was just looking at the temperatures of black holes. And so you would need a black hole with about the mass of the moon for it to be the same temperature as the cosmic microwave background radiation, the 2.7 kelvin and that’s when you would get it evaporating. But a stellar mass black hole would have a 100 nanokelvins. So you won’t get stellar mass black holes evaporating until the temperature of the universe gets below that 100 nanokelvins.
Dr. Pamela Gay: And to explain what this means in slightly easier language.
Faser Cain: I used the technical gobbledygook? Yeah, I guess I did.
Dr. Pamela Gay: Yeah, you totally did because I couldn’t figure out what you were talking about initially. So when you have a sufficiently large black hole it’s got a big event horizon and it’s happily absorbing photons from the cosmic microwave background as well as whatever dust, cosmic rays, whatever hits that event horizon. Now, an event horizon is trying to evaporate particles across it and with a large black hole, the rate at which it’s able to absorb photons from the cosmic microwave background as well as all the other detritus of the universe, is a higher rate than its evaporation rate. With itty bitty tiny microscopic black holes like you might form with a Large Hadron Collider, it’s event horizon is so, so tiny that the density of cosmic microwave background photons and other stuff hitting it isn’t sufficient to overcome the rate at which it evaporates. So it goes away almost instantly.
Now, as you bridge from microscopic black holes upwards through a whole bunch of masses that don’t seem to form black holes in any known way, you go from things that can evaporate to things that can’t evaporate until the cosmic microwave background has a significantly lower energy density to it.
So while microscopic black holes may have formed with the formation of the universe, they would have all evaporated by now. Here’s to hoping we figure out how to make them because that would be awesome. And then beyond that we’re not going to start seeing black holes until we have things that are three solar masses trying to collapse. So all of the masses that could currently evaporate, they don’t have black holes.
Faser Cain: But I love this idea of a 10 followed by 50 zeros or 1 followed by 50 zeros or 1 followed by a hundred zeros, the universe will be capable of making a temperature just because its spread out so far that is so cold that it’s colder than anything we can make in the laboratory. And if we’ve gotten better in the future, it doesn’t matter, the universe can just wait a few more 10 zeros on the end of it and the universe will get to a place that is colder than anything we can ever make. So our record won’t stand until the end of time.
Dr. Pamela Gay: And what’s intriguing to me is while it’s the fullness, the ensemble temperature of the universe at that point, there’s nothing to say. But if you capture an individual atom in a box kind of like a lightning bug that it won’t still have a higher temperature.
Faser Cain: Right.
Dr. Pamela Gay: It’s the cooling of the individual atoms that we struggle with in the lab.
Faser Cain: It’s such a mind-bending concept. I love it. Well, thank you very much, Pamela.
Dr. Pamela Gay: It’s my pleasure.
Faser Cain: All right. We’ll see you in a couple months.
Thanks for listening to Astronomy Cast, a nonprofit resource provided by Astrosphere New Media Association, Faser Cain and Dr. Pamela Gay.
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