5 ms·

"Temperature" is a slippery word that has many different definitions depending on what area of physics you're working in. In this case it is perfectly fine to

"Temperature" is a slippery word that has many different definitions depending on what area of physics you're working in. In this case it is perfectly fine to refer to "the temperature of space" since you're explicitly using it as a cold sink. As evidence, see the paper (published in Nature no less!) that MengerSponge posted. The authors say "On the other hand, outer space, at a temperature of 3 K, provides a much colder heat sink."

Aperocky 6y ago
Here's something I don't understand though, if we put a blackbody in deep space, would it eventually be cooled/heated to the temperature of the cosmic background radiation? The space around us is filled with photon coming from Sun at ~5500K, but it doesn't heat anything to anywhere near that. So given the current density of cosmic background radiation at 411 photon cm -3, would it be sufficient to place a blackbody to the equilibrium temperature? I am not a physicist by training, and this is a genuine question.
- pdonis 6y ago
  > if we put a blackbody in deep space, would it eventually be cooled/heated to the temperature of the cosmic background radiation? No. As you note, in space in our vicinity there is a hot object, the Sun, in the sky, so the "temperature" of space in our vicinity is not really the 3K of the CMB, because an object placed in space in our vicinity will not come to thermal equilibrium at that temperature, but at some higher temperature. Exactly what higher temperature, if we assume the object itself is a black body, will depend on the object's surface area and its distance from the Sun. We could, of course, theoretically place an object outside our solar system, or even outside our galaxy, or even outside our local group of galaxies, or even outside the galaxy cluster of which our local group is a part. Doing those things would move the object farther and farther, on average, from high temperature radiation sources like the Sun. But there would still be radiation from those sources in the sky, meaning that the object would still reach thermal equilibrium at some temperature higher than the 3K CMB temperature. In short, an object would come to thermal equilibrium in space at the 3K CMB temperature only if the CMB were the only radiation source in the object's sky. But in actual fact it never will be.
  - heisenzombie 6y ago
    Nice. Seems related to: https://en.wikipedia.org/wiki/Planetary_equilibrium_temperature https://en.wikipedia.org/wiki/Planetary_equilibrium_temperat... The temperature of a perfect black body in earth’s orbit would be ~255K (-18C) (This is not the case for the technology under discussion because it’s very much not a black body) I will point out that even at modest (astronomically speaking) distances, the solid angle of a star gets preeeeetty small. For example, a black body at just 1 light year from the Sun (just a quarter of the way to Alpha Centauri) would come to an equilibrium temperature of ~1K (assuming an otherwise empty universe). So that’s already a smaller effect than the CMB. I think in intergalactic space, the thermal equilibrium of a black body would be absolutely dominated by the CMB.
    - pdonis 6y ago
      > even at modest (astronomically speaking) distances, the solid angle of a star gets preeeeetty small Yes, but you're not just being irradiated by a single star. There's a whole galaxy of them. They aren't all visible to the naked eye, but they're still there, and their radiation all adds up. However, I have not done detailed calculations, so I might be overestimating, for example, what the effective radiation temperature would be of galaxies in intergalactic space.
    - pdonis 6y ago
      > The temperature of a perfect black body in earth’s orbit would be ~255K (-18C) More precisely, of a perfect black body in Earth's orbit with the same surface area and shape as Earth.
      - heisenzombie 6y ago
        Yes, only true for black spheres. It’s size invariant though.
  - Aperocky 6y ago
    > In short, an object would come to thermal equilibrium in space at the 3K CMB temperature only if the CMB were the only radiation source in the object's sky. But in actual fact it never will be. I still don't see if this answer my question, assuming CMB is the only source, but the photon density has diminished significantly, would there be enough CMB photons to keep the object from going lower than 3K from emitting blackbody radiation? The gist of my question is: Will there ever be a time where there are not enough CMB to heat the blackbody? Just like solar photons at 5500K, there are not enough of them to heat stuff at Earth to the same temp.

Aperocky 6y ago

Here's something I don't understand though, if we put a blackbody in deep space, would it eventually be cooled/heated to the temperature of the cosmic background radiation? The space around us is filled with photon coming from Sun at ~5500K, but it doesn't heat anything to anywhere near that. So given the current density of cosmic background radiation at 411 photon cm -3, would it be sufficient to place a blackbody to the equilibrium temperature? I am not a physicist by training, and this is a genuine question.

pdonis 6y ago
> if we put a blackbody in deep space, would it eventually be cooled/heated to the temperature of the cosmic background radiation? No. As you note, in space in our vicinity there is a hot object, the Sun, in the sky, so the "temperature" of space in our vicinity is not really the 3K of the CMB, because an object placed in space in our vicinity will not come to thermal equilibrium at that temperature, but at some higher temperature. Exactly what higher temperature, if we assume the object itself is a black body, will depend on the object's surface area and its distance from the Sun. We could, of course, theoretically place an object outside our solar system, or even outside our galaxy, or even outside our local group of galaxies, or even outside the galaxy cluster of which our local group is a part. Doing those things would move the object farther and farther, on average, from high temperature radiation sources like the Sun. But there would still be radiation from those sources in the sky, meaning that the object would still reach thermal equilibrium at some temperature higher than the 3K CMB temperature. In short, an object would come to thermal equilibrium in space at the 3K CMB temperature only if the CMB were the only radiation source in the object's sky. But in actual fact it never will be.
- heisenzombie 6y ago
  Nice. Seems related to: https://en.wikipedia.org/wiki/Planetary_equilibrium_temperature https://en.wikipedia.org/wiki/Planetary_equilibrium_temperat... The temperature of a perfect black body in earth’s orbit would be ~255K (-18C) (This is not the case for the technology under discussion because it’s very much not a black body) I will point out that even at modest (astronomically speaking) distances, the solid angle of a star gets preeeeetty small. For example, a black body at just 1 light year from the Sun (just a quarter of the way to Alpha Centauri) would come to an equilibrium temperature of ~1K (assuming an otherwise empty universe). So that’s already a smaller effect than the CMB. I think in intergalactic space, the thermal equilibrium of a black body would be absolutely dominated by the CMB.
  - pdonis 6y ago
    > even at modest (astronomically speaking) distances, the solid angle of a star gets preeeeetty small Yes, but you're not just being irradiated by a single star. There's a whole galaxy of them. They aren't all visible to the naked eye, but they're still there, and their radiation all adds up. However, I have not done detailed calculations, so I might be overestimating, for example, what the effective radiation temperature would be of galaxies in intergalactic space.
  - pdonis 6y ago
    > The temperature of a perfect black body in earth’s orbit would be ~255K (-18C) More precisely, of a perfect black body in Earth's orbit with the same surface area and shape as Earth.
    - heisenzombie 6y ago
      Yes, only true for black spheres. It’s size invariant though.
- Aperocky 6y ago
  > In short, an object would come to thermal equilibrium in space at the 3K CMB temperature only if the CMB were the only radiation source in the object's sky. But in actual fact it never will be. I still don't see if this answer my question, assuming CMB is the only source, but the photon density has diminished significantly, would there be enough CMB photons to keep the object from going lower than 3K from emitting blackbody radiation? The gist of my question is: Will there ever be a time where there are not enough CMB to heat the blackbody? Just like solar photons at 5500K, there are not enough of them to heat stuff at Earth to the same temp.