[thetada]

Hello,

I have a question about the temperature of "the universe" for thermodynamic calculations. In thermodynamics, we consider the system and it's surroundings, the latter being defined as the rest of the universe. Next, the entropy of the surroundings is calculated by dividing the heat absorbed by the surroundings by the temperature of the surroundings. This can be used to show why ice cannot freeze at 5oC. My question is this, in such calculations, surely 5oC is the temperature of the immediate surroundings only. If the surroundings are defined as the rest of the universe, why is the temperature of the surroundings not defined as the average temperature of the whole of the rest of the universe?

Thanks

[Enthalpy]

Of interest for a transformation is the surroundings that can interact with the object of interest.

On Earth it's often the atmosphere.

Cases on Earth where a mean Universe temperature is meaningful are rare. For a space probe, the Sun would be one part of the environment, a planet if near enough, and the Universe the rest; even for Pioneer at its present distance, whether the Universe has 0K, 2.7K or a bit more makes no difference and the Sun is more important.

One difficulty with a mean temperature is that the Universe isn't uniform, so it radiates at varied wavelengths. A perfect black body would feel some mean Universe temperature which wouldn't be meaningful for a normal body, which (is designed to) has varied absorptivity at different wavelengths.

At very sensitive telescopes and antennas, designers try to decouple the receiver from the warm and noisy Earth. This includes directivity so the receiver senses heat only from the target direction, and possibly cold baffles in other directions.

See the home antennas for satellite TV? Apart from the paraboloid, they have a "primary source" (often a horn) which is well below the dish's axis, at least at mid-latitudes where I am. One important purpose is that this primary source looks to the sky, so that the secondary lobes in its sensitivity diagram which miss the dish look to the cold sky, not to the warm Earth.

Even with such tricks, an antenna temperature (=the noise it picks) would be 20K...150K depending on the wavelength and the design, so a "mean universe temperature" would be hard to define. But tables exist for the background temperature of the sky depending on the wavelength.

[thetada]

Thanks, i've also just realised that I made a rookie error in my interpretation of the equations, as the entropy of the universe is the sum of the entropies of the system and the surroundings. I still think my question is valid on some level, but it needs to be better worded. Anyway, i take your point that the actual impact of the universe in our direct proximity is unlikely to significantly impact parochial processes.