What’s the temperature of deep space? Can it get any colder than that? How do we even define temperature when there’s nothing around? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

folks, it's really, really, really cold out there. I mean, really cold. Don't forget your jacket kind of cold here on the surface of the earth, though, it's warm, and we can think three sources of heat for that warmth. One we have this on you may have noticed comes around every single day. The sun is a giant thermonuclear bomb going off all the time, releasing quite a bit of energy. Some of that energy intersects the surface of the earth and keeps us warm. But that's not the only thing like you can't have volcanoes or plate tectonics with just solar heating. You need something else. We have heat internal to the planet Earth itself. Some of that heat, about half of our own internal heat, comes from our own formation of the planet. Earth right now is a solid ball of rock that is relatively compact and does not take up a lot of space. But it wasn't always like that, uh, billions of years ago, all the little particles and elements and molecules that now make up the earth were scattered around inside of a giant protoplanetary disc, a big nebula and all that material to make the earth had to squeeze down into a very tiny volume to become the earth.

And as they did that all those little molecules rubbed up against each other and struck each other and that generated heat and the solid body of the earth retains that heat even today, billions of years later, smaller planets like Mars have cooled off. They've lost their heat of formation, while bigger planets like Venus and Earth still have warm core. But the second source of internal heat that we can thank for keeping our surface warm has to do with radioactivity. You know, the Earth is made of more than, uh, silicon and oxygen and carbon. There's radioactive elements. These were present in the nebula that eventually led to the solar system. And so every planet gets its own share of radioactive elements simply because radioactive elements are part of the universe. They're part of the mixture. You, you, they're they're part of the deal. Radioactive elements like uranium and plutonium and different isotopes of potassium, like every banana you eat has a little bit of radioactive potassium inside of it.

And as these elements decay, they release heat and they help keep us warm. So about half of our internal heat comes from our own formation. The other half comes from radioactive elements. Plus, we get some heat from the sun and we have this bonus effect here on the surface of the earth of the greenhouse effect because we have a relatively thick atmosphere. Uh, Mars and Mercury are not getting a greenhouse effect, but we are. This thick atmosphere acts as a blanket. It traps heat. So you have all this sunlight coming in, blasting in from outer space and hitting the surface. You also have a bunch of heat coming up from the ground deep from the core, welling up from the surface. You have all these sources of warmth and our planet Earth, like all warm things, likes to emit radiation. Now, this radiation, if we didn't have an atmosphere, would just blast off into space back on its own, and everything would be balanced in balance. And we would have a relatively cooler temperature where all the incoming radiation was balanced by all the outgoing radiation.

And there's a temperature you need to make everything fit perfectly. But because of the atmosphere, our own air blocks and absorbs some of that light coming from the surface and then re emits it back onto the ground. It also traps some of the sunlight coming in and then emits on its own back to the ground. There's the all these complex feedback loops, and you you get warm. A little greenhouse is a good thing. It's about 40 to 60 degrees. Warmer Fahrenheit or Celsius doesn't matter here, 40 to 60 degrees warmer on the surface of the earth, with the greenhouse effect thanks to our thin atmosphere than it would be without an atmosphere. And that makes things relatively pleasant here, plus the atmosphere in this greenhouse effect. It gives us a blanket at night to smooth things out. So we're not boiling during the day and then freezing at night. Now, of, of course, a little greenhouse is a good thing, and too much of a greenhouse is a bad thing, just as Venus or our current levels of industrial carbon output.

So the earth is a pretty warm place. It's an exceptionally warm place. There are definitely hotter places in the solar system, and hotter places in the universe like Venus is hotter than us you can melt lead on its surface. Mercury is pretty hot because it's so close to the sun sun. The sun itself has a surface temperature of 10,000 Kelvin and a core temperature of the 2.5 million Kelvin. That's that's pretty hot. There's some random hot places. The interiors of the giant planets are very hot. IO, the the first moon of Jupiter is is hot because of all the gravitational interactions, There are definitely hot places and hotter places in the universe than the Earth. But hot is not the norm in general. It's very, very, very cold out there. The default state of the universe is very cold, and it takes extra special circumstances like strong gravity to form a planet. A nuclear power in the core of a star, various and sundry uh, explosions here and there.

Supernova are kind of hot, but you need these special circumstances for special little pockets in the universe to become not cold. The universe on average, is extremely cold. But before I get into the actual temperature of space, I want to describe what it would be like to actually experience the coldness of space, knowing that outer space is is generally very cold. What would it feel like? Well, let's say it's your unlucky day and your crewmates have decided to space you they're gonna shove you out of the airlock into the vacuum of deep space without the protection of a spacesuit. What would you feel? Well, to be honest, at first, you wouldn't feel much. Yes, it it would be cold, and you would have the sensation of cold. Don't get me wrong, but our bodily sensation of temperature depends on more than well, well, temperature. Let me explain. Do an experiment for me. Get up. Go to your fridge. You Presumably, your fridge is pretty cold in there. If it's not, you should call a mechanic.

Put your hand on, Say, I don't know a can of soda and then put your other hand on I don't know, a block of cheese. The can of soda in the block of cheese will be at the same temperature because they're both inside of the fridge and they have become in equilibrium with the temperature of the fridge. Whatever you set that temperature to the if they're in there long enough, then they're at that same temperature. That's the entire point of a fridge. But I guarantee that the can will feel colder because the metal of the can is more efficient at transferring heat away from your body and into the soda inside of it. We physicists like to say that the aluminum on the surface of the can has a higher thermal conductivity than, say, a block of cheese. It's more efficient at pulling heat away from you and into the can. I remember. I don't know how many times as a kid being so disappointed you reach in, grab a can of soda, and it feels cold. Yeah. Oh, great. Finally, a refreshing beverage. That's not exactly my thought process as a kid, but you get the idea, and then you crack it open and you take a drink.

And it's lukewarm at best. Even though it feels cold, it feels cold because metal is really good at pulling heat away from your body. The sensation of temperature, our sensation of the temperature, doesn't just depend on the raw temperature of the object we're touching. It also depends on how quickly heat is leaving our body, which depends on how efficiently that object can draw heat away from us. Here's another example of why you wouldn't feel that cold in space, and that is convection moving. Fluids are much better at pulling heat away from an object than a stationary fluid. You imagine you're just sitting there perfectly still and think of the layer of air that surrounds your skin right now from head to toe, that centimeter that inch of air that surrounds you imagine you just walked into a room and sat down, and then that air around you is very, very cold. Well, you heat it up through a couple of mechanisms. You're you're radiating energy, you're emitting radiation just like all other warm objects in the universe, and that radiation is impacting the air molecules and heating them up.

Also, the air molecules are literally right next to your skin. And so all the tiny little molecules of your skin are constantly vibrating and wiggling, and they're they're hitting the air and warming it up. But then, once you warm up that inch of air around you, or that centimeter of air around you, or 2.54 centimeters, if you want to be precise with the conversion. You're done. Yeah, it costs you a little bit of energy to warm up that little layer of air around you, but then that's it. Now you in the air are at the same temperature. But if the air is moving, you expend all this energy to warm up the pocket, the layer of air around you, and then a breeze comes in and takes away all that warm air. Or that air simply rises up to the ceiling of the room and gets replaced with fresh, cool air. And you have to pour more energy into heating that layer of air, and then it gets removed and the cycle keeps going. And before you know it, you've frozen to death. You can actually sit in relatively cold weather with nothing more than a light jacket, as long as the wind isn't blowing.

But if the wind is blowing well, there's a reason that wind chill in the winter is such a big deal. As another example. I love scuba diving. The coldest dive I've ever done was in Lake Tahoe, on the border of Nevada and California. The water that I went in was just a couple of degrees above freezing. But I had a dry suit on which inflates a layer of air between me and the water, and I was pretty toasty. I was pretty fine. It wasn't the the greatest thing in the world, but I was totally fine. The coldest, however I've ever felt in a dive was in Puget Sound outside of Seattle. That water was 20 degrees warmer than the water in Lake Tahoe, But I was wearing a wet suit that did not trap a layer of air around me and said water could circulate around my skin, and that was that was definitely not fun. In the vacuum of space, there is no conduction because there's no material that you're touching. There's also no convection. There's no wind or water to circulate heat away from you.

So what do you feel? Well, initially, you will feel a burst of cold, and that's because of evaporation. As soon as you step out into the vacuum, all the oils and sweat on the surface of your skin are gonna evaporate instantly. That will pull heat away from your body that takes energy to do it. They're gone there. Bye bye. It's like stepping out of the shower and instantly feeling way too cold, even though the room is warm. That's because the heat of your body turns the droplets of water on your skin into steam, and then the steam leaves that pulls heat out of you, making you feel cold. Stepping out into vacuum without a spacesuit is like stepping out of the ultimate shower into a cold room. All of it's gone, so I instantly you feel that sensation of cold. But then that goes away. You only have so much sweat and oils on your skin. I hope you bathed recently. After that, you won't feel very cold. Yes, it will feel chilly, I promise you, but it won't feel as cold as it should, given the temperature around you.

That's because the only thing left to actually pull heat out of your body and give you that sensation of coldness is radiation. Like I said, all objects emit radiation constantly all the time, and the amount in the wavelength of the radiation depends on how hot that object is. Our sun is kind of hot surface temperature of 10,000 Kelvin. It is emitting visible light, extreme objects like white dwarves are hot enough to emit x-ray radiation Supernova. Can he make gamma rays? They're really stinking on things like people with our body temperature. We emit infrared radiation, which is exactly how night vision works, because you can see the infrared light emitted by human beings. We're always emitting this radiation. It's one of the ways our bodies cool down and in space in the vacuum of space. This is all you get. You don't get conduction. You don't get convection.

You get a little bit of evaporation right away, but then it's gone. You are only left with radiation to give you that sensation of losing heat, and radiation is super duper slow. It's by far the worst way to transport heat, especially at low temperatures. You're gonna feel a little bit chilly, but you're not gonna freeze to death. It's true. You'll, uh, don't worry. You'll you'll die. You'll die. You'll die of lack of oxygen in a handful of minutes, and eventually you might freeze to death. You actually won't. If you're close enough to the sun and you're receiving enough radiation from the sun, it's gonna keep your body warm and prevent you from ever freezing. If you're far enough away from the sun or in a shadow, then yeah, you will eventually freeze. Your body will turn into a meat popsicle, but it will take hours. Yeah, that it's a very rough approximation. Depends on your body mass. The surface area. How much you're flailing around as you're doing this, but you can think hours to freeze in space. You know, if if I sat you in a room and I said, Man, it's gonna take you a few hours before you die.

That's not gonna feel very cold to you. And I'm talking about tossing you out of the air lock. Not just for fun, because I have a reason for this because when we ask how cold in space, this is a useful way to measure it. It's not just how you feel in space. I want to know what temperature your body will eventually reach. If I were to throw you out of the airlock, what temperature would you eventually acquire? Yes, I could have just used a thermometer, But using your corpse is so much more fun. I want to disentangle the sensation of temperature from the measurement of temperature. And one way to measure the temperature is to stick a thermometer in you like a Thanksgiving turkey. Throw your body out the airlock and then see what temperature it eventually reaches. This is how I can measure the temperature of something in a fridge. I can put something in a fridge, walk away for a few hours, come back and then measure the temperature of the object that tells me the temperature of the fridge. I'm gonna use this. I'm using your body as a measurement device. I hope you don't mind. This show is brought to you by better health.

One of my favorite things about doing ask a spaceman is how much I get to learn about my own field. It's either stuff that I forgot from graduate school or I'm learning brand new because it's simply not a part of my training and expertise. And getting to learn new things and learn about yourself is so powerful. One way to do that is through therapy. Therapy doesn't solve all your problems, but that's not the point. It solves some of your problems, and that makes it worthwhile. You get to continuously learn about yourself through my own therapy. I learn a lot about myself, my relationship with others, how the world works. It's pretty powerful. If you are thinking of starting therapy, give better help. A try. It's online. It's convenient, flexible, and it can be suited to your schedule. Discover your potential with better help. Visit better help dot com slash spaceman today and get 10% off your first month. That's better. Help EEP dot com slash spaceman.

Of course, when we talk about the temperature of space, we want to talk about the temperature of deep intergalactic space so we don't have to deal with all the pesky radiation from nearby stars and Galaxies. And what would happen to your body temperature? It will just drop and drop and drop as you continue to emit radiation in a perfect vacuum. With absolutely nothing else around you, your temperature would slowly approach absolute zero, which is for those of you keeping score at home negative 273.15 Celsius or negative 459.67 F or zero Calvi Doesn't matter how you want to measure it. You'll actually never reach that temperature. Nothing can reach absolute zero. But what you would do is your body temperature would slowly approach it. First, you'd be negative. 273 Celsius, then minus 2 73.1 then minus 2 73.14 then minus 2 73.149. Then we minus 2 73.1499999. And it will just keep going and going and going slowly approaching absolute zero, but never reaching it.

But space is not a perfect vacuum. Yeah, there's random Starlight. There's a few stray hydrogen atoms, some passing cosmic rays and neutrinos. But those aren't going to significantly affect your temperature. They're too spread out Too thin. Yeah, the some cosmic ray traveling at 99.99%. The speed of light might hit you, but it only happens once a year. It's not gonna change your temperature. What is going to affect you? However, what's going to keep you just a tiny bit warm is the warm, fuzzy feeling you get from donating to Patreon. That's patreon dot com slash PM Sutter. I greatly appreciate all your contributions for those of you who don't know. If you join patreon, you get instant access to me. I will answer your questions via text all the chat app on patreon and you get early release episodes and you get ad free versions of this podcast, not the patreon. You still have to hear the patreon ad, which is kind of ironic, but the other ads you don't have to hear that's patreon dot com slash PM Sutter, Thank you so much.

Now what keeps you a tiny bit warm is a flood of radiation left over from the earliest days of the Big Bang. I'm talking, of course, about everyone's favorite microwave background. The cosmic microwave background you see a long time ago, 13.77 billion years ago, our universe was about a million times smaller than it is today. And with all that matter crammed together into such a tiny volume, it had a very high temperature high enough to become a plasma. But eventually the universe expanded and cooled, and that plasma cooled off to become a neutral gas where all those literal wandering electrons got glued onto atoms for the first time. This left a flood of radiation and initially in the plasma state, the radiation, the photons were in equilibrium with all the matter. They all had the same temperature. But then, boom. The matter becomes neutral matter. Stop talking to radiation in bulk.

You know, still answer a text message here and there, but largely started ignoring radiation. And initially, that radiation had the same temperature as the matter, which was around 10,000. Kelvin. It was initially white hot, but then the universe evolved. The matter went on to do its thing, you know, create stars and Galaxies, large scale structure of the universe, you know, whatever. And it left behind this ball of radiation that flooded the universe. Now this ball of radiation living in an expanding universe is gonna cool off. One is gonna get more diluted because the universe is getting bigger and there's not more radiation popping into existence, so it gets more diluted. That cools it off. Plus, the expansion of the universe itself stretches the wavelength of the light, which causes it to lose energy. 13.77 billion years ago, this ball of radiation that flooded the universe had a temperature of a few 1000 Kelvin. Nowadays it has a temperature of about three kelvin, three degrees above absolute zero.

So when you sit alone in empty space, you're actually sitting in a bath of this radiation. In fact, the cosmic microwave background is responsible for something like 99.9999% of all photons in the universe. You can't escape it no matter where you go. And it's been here since the earliest days of the Big Bang, so and it's not going anywhere. So as you're sitting there as your corpse is floating through space, these photons a very low energy microwave energy at three Kelvin, three degrees above absolute zero are constantly hitting you, and they're hitting you enough to raise your temperature. Eventually, after enough time, just like going inside of a fridge, you will become an equilibrium. You will reach the same temperature as the cosmic microwave background. Your body will have a temperature of three degrees, and that's how we can say what the temperature of outer space is in a perfect vacuum. It'd be zero, but it's not a perfect vacuum.

It's actually flooded with cosmic microwave background light, and so if you throw a random corpse in there. That corpse will eventually reach three Kelvin and yes, in the far, far, far future as the universe continues to expand, that temperature will continue to drop. But that's not today's problem. But can it get even colder? Why, yes, it can. Good thing I asked. Meet the Boomerang Nebula. The Boomerang Nebula is a planetary nebula about 5000 light years away, And observations in the 19 nineties revealed something strange about this nebula. When we went to make our observations, as we typically do in astronomy of the Gas in this nebula, we saw that the nebula itself was absorbing the cosmic microwave background radiation from behind it. You know, the the CMB, the cosmic microwave background is everywhere. It completely floods the universe. And here was a nebula that was absorbing that radiation was blocking that radiation from coming behind it.

The only way that can work is if the nebula is colder than the CMB itself, because if it's hotter, then the nebula itself will be glowing at a higher temperature. But if it's blocking something at three Kelvin, that means it has to have a lower temperature than three Kelvin and we estimate the temperature at half a kelvin 0.5 Kelvin. It is the coldest, natural known place in the universe. We can make colder inside of our laboratories, but nature made a place that is only half a kelvin half a degree above absolute zero colder than the CMB itself. This means if I were to put you inside of the Boomerang Nebula, that temperature. If I put the thermometer inside your body and waited a while, I would register half a Kelvin. I would register colder than the CMB itself. Now, this is a very temporary situation. Of course it works because of expansion. This nebula is rapidly expanding something like 100 and 64 kilometers per second.

That's that's really fast and expanding gasses. Cool off. Check it out. You can do this experiment. It's so fun, you know, open your mouth, notice some water like this, and then blow out some air. Do it really fast, too, as fast as you can. If you put your hand in front of your mouth, I'm making myself out of breath. You can feel the warm air coming out of your lungs, and now you realize you should probably brush your teeth, but if you purse your lips and you make a little small hole and you blow that over your hand, the air feels cold. Even if you go super slow. I'm doing it right now. It feels cold, and if I open my lips, it feels warm. It doesn't matter how fast or how much air I'm blowing out of my mouth. What matters is the size of my lips. What's going on here is expansion by Ping your lips by making a really, really small hole and then forcing air out of that as soon as it comes out of your mouth. It was compressed in a tiny little volume, and now it rapidly expands and that rapid expansion causes it to cool off.

But if I keep my mouth wide open, there's no compression of the air as it passes through my lips. There's no expansion at the end of it. It stays the same temperature nice and warm. So we have here a situation where we have a cloud of gas that is rapidly expanding and that has dropped its temperature to below that of the CMB. Eventually, the CMB will win this expansion will slow down. All those trillions of CMB photons will warm up this nebula back to three, Kelvin. But for now, it is the coldest known place in the universe. This has been going on for somewhere between 3000 years, which is not very long. And it's not gonna last very long. We suspect that this rapid expansion is not due to the normal mechanism of generating a protoplanetary disc, which is just a star like our son dying. We suspect there may have been a collision. This may have been a binary star and that in its death rose, it swallowed the other star, led to a violent explosion that is then pushed like a wind in a sail that pushed this nebula out.

It won't last long, but right now, that is the coldest place in the universe. Surely there are other places like this in the universe, but they probably don't last very long, obviously. And so this is the only place that we know about That's like this. So that's it. How cold is space? Well, we can't rely on our sensation of temperature. The coldest place in the universe is most of the universe. Having a warm spot to warm your hands is the exception, not the rule. Almost all of space is at this temperature of the cosmic microwave background, three degrees above absolute zero. Maybe a little bit colder, depending on where you are. Maybe a little bit hotter, depending on where you are, but on average, three degrees above absolute zero. And we know that because we can shove your body into the vacuum of space and measure it. Like I said, you should bring a jacket, thanks to James on email and Javier L on email for the questions that led to today's episode and thank you to all my top top top tippy top patreon contributors.

That's patreon dot com slash PM Sutter. For today's episode, we have Justin G, Chris L Barbeque, Duncan M, Corey D, Justin Z Nalla Scott M, Rob H, Justin Lewis, M John W, Alexis Gilbert, M Joshua, John S, Thomas D, Simon G and Aaron J. Those are the top contributors this month. Please join them at patreon dot com slash PM. Sutter. Send me more questions. Ask us spaceman at gmail dot com At Paul. Matt Sutter on all social channels website is ask us spaceman dot com That's a website has all the show archives and everything you need to know, and I will see you next time for more complete knowledge of time and space.

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