4 ms·

If you place a thermometer in space sufficiently far away from all heat sources, it will eventually reach 2.73 K, the temperature of the cosmic microwave backgr

If you place a thermometer in space sufficiently far away from all heat sources, it will eventually reach 2.73 K, the temperature of the cosmic microwave background. I am not sure about the correct terminology but at least in some sense this is the temperature of outer space.

mytailorisrich 7y ago
Vacuum has no temperature. What you describe is a black body spectrum 'temperature'.
- danbruc 7y ago
  That is why I said that I am not sure about the precise terminology. But it still does not change the fact that an object placed in the vacuum of outer space will eventually reach the temperature of the cosmic microwave background, i.e. the temperature of the electromagnetic radiation filling outer space. Also not that I am not talking about any vacuum, I am specifically talking about vacuum of outer space.
  - mytailorisrich 7y ago
    It does not change the fact that vacuum, any vacuum, has no temperature...
    - danbruc 7y ago
      Also no perfect vacuum exists which renders this rather irrelevant for discussing what happens in real situations. I also did some reading to figure out the correct terminology - it is certainly correct to say that a perfect vacuum has no thermodynamic temperature as it is defined by the state of motion of the particles making up the system you want to assign a temperature to, however I am still not convinced that it is wrong to associate a temperature with a vacuum based on the electromagnetic radiation filling it.
      - mytailorisrich 7y ago
        Yes, vacuum has no temperature. The energy of any radiation that may be going through a vacuum at a given instant is not a property of the vacuum (and is not an actual temperature). I'll leave it here as I think I have repeated the same thing enough though I am not sure it has been heard.
        danbruc 7y ago
        You keep repeating a point I never disagreed with, a perfect vacuum has no thermodynamic temperature. My issue with that is that I think it is a too narrow viewpoint for the comment I initially replied to. If one understands temperature as what a thermometer measures, then the temperature of a vacuum will be the temperature of its walls, it will not magically show »undefined« because you insist that temperature means thermodynamic temperature.
        mytailorisrich 7y ago
        You say you don't disagree but you keep disagreeing... I made a simple, yet fundamental statement. It's either true or false. There is no need to argue as if you had to out-do me in any way. Vacuum cannot have a temperature. This follows immediately from the definitions. This also means that the temperature of a vacuum's "walls", by which I'm guessing you mean particles in 'contact' with the vacuum, is just the temperature of these particles. It's not an indication of any "vacuum temperature". Have a nice day.
        danbruc 7y ago
        Try to look up the definition of temperature and you will see that it is not as simple as you think. One possible definition defines temperature as the partial derivative of the internal energy with respect to the entropy. Why wouldn't that be applicable to an ideal vacuum or a radiation-filled vacuum? Maybe it is not, I am neither a physicist nor particularly knowledgeable in thermodynamics and statistical mechanics, but it is - at least to me - not obvious that it is not applicable. There are also generalized definitions of temperature applicable to system of few particles, i.e. not relying on the statistics of many-particle systems, which sounds like it should be applicable to any real vacuum as any real vacuum will never be perfectly empty.

mytailorisrich 7y ago

Vacuum has no temperature. What you describe is a black body spectrum 'temperature'.

danbruc 7y ago
That is why I said that I am not sure about the precise terminology. But it still does not change the fact that an object placed in the vacuum of outer space will eventually reach the temperature of the cosmic microwave background, i.e. the temperature of the electromagnetic radiation filling outer space. Also not that I am not talking about any vacuum, I am specifically talking about vacuum of outer space.
- mytailorisrich 7y ago
  It does not change the fact that vacuum, any vacuum, has no temperature...
  - danbruc 7y ago
    Also no perfect vacuum exists which renders this rather irrelevant for discussing what happens in real situations. I also did some reading to figure out the correct terminology - it is certainly correct to say that a perfect vacuum has no thermodynamic temperature as it is defined by the state of motion of the particles making up the system you want to assign a temperature to, however I am still not convinced that it is wrong to associate a temperature with a vacuum based on the electromagnetic radiation filling it.
    - mytailorisrich 7y ago
      Yes, vacuum has no temperature. The energy of any radiation that may be going through a vacuum at a given instant is not a property of the vacuum (and is not an actual temperature). I'll leave it here as I think I have repeated the same thing enough though I am not sure it has been heard.
      - danbruc 7y ago
        You keep repeating a point I never disagreed with, a perfect vacuum has no thermodynamic temperature. My issue with that is that I think it is a too narrow viewpoint for the comment I initially replied to. If one understands temperature as what a thermometer measures, then the temperature of a vacuum will be the temperature of its walls, it will not magically show »undefined« because you insist that temperature means thermodynamic temperature.
        mytailorisrich 7y ago
        You say you don't disagree but you keep disagreeing... I made a simple, yet fundamental statement. It's either true or false. There is no need to argue as if you had to out-do me in any way. Vacuum cannot have a temperature. This follows immediately from the definitions. This also means that the temperature of a vacuum's "walls", by which I'm guessing you mean particles in 'contact' with the vacuum, is just the temperature of these particles. It's not an indication of any "vacuum temperature". Have a nice day.
        danbruc 7y ago
        Try to look up the definition of temperature and you will see that it is not as simple as you think. One possible definition defines temperature as the partial derivative of the internal energy with respect to the entropy. Why wouldn't that be applicable to an ideal vacuum or a radiation-filled vacuum? Maybe it is not, I am neither a physicist nor particularly knowledgeable in thermodynamics and statistical mechanics, but it is - at least to me - not obvious that it is not applicable. There are also generalized definitions of temperature applicable to system of few particles, i.e. not relying on the statistics of many-particle systems, which sounds like it should be applicable to any real vacuum as any real vacuum will never be perfectly empty.