Why does the vacuum of space have a temperature at all? What is it that has the temperature?
In: Physics
Yes, deep space has a temperature of about 3K. Or -270°C. It’s not a vacuum though. It’s filled with light, specifically light left around from the big bang called the cosmic microwave background. The light, the gas made of photons, has a temperature. It’s not like a temperature, it is a temperature. It’s not the occasional atom that has the temperature, the light itself has this temperature.
How can light have a temperature? Same way as anything else, it’s a bunch of particles with energy. Does it mean an object in deep space would reach this? Yes, if an object was send out there, it would give off its heat as light. Everything emitts thermal radiation off. The amount if gives off would rapidly decrease, but would never lose all its heat. At 3K, the amount of CMB light that it absorbs would match the amount an object gives off. That is the definition of being at the same temperature, two things exchange an equal amount of energy.
What temperature is it within the solar system? Poorly defined, it’s not in equilibrium. Facing the sun near earth will cause rapid heating, and the side away from the sun will cool. It’s like being in the sunshine on earth, it gives a poor temperature reading.
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