If 'space' is defined as the absence of everything, then why can it carry heat?

Question

I apologize if this question is confusing, I will try to clarify a little... so, this is talking about 'space' in the form of space that exists outside of our atmosphere. Outer space.

I once read that the temperature of outer space varies, but is almost always above absolute zero with the theoretical exception of the eridanus supervoid (and relating voids), where the amount of heat sources present is few to none. But why is outer space above absolute zero in the first place?

If space is defined as the absence of everything, why is the ambient temperature of the absence of everything above absolute zero?

EDIT: For example, the region of space immediately surrounding a star (past the atmosphere). Why is the temperature of this space 'warm' even though there are no particles to heat up or excite?

Answer 1

The short answer is that heat is transmitted through space by radiation, but space has a temperature because it isn't truly empty. Or, better, it is not space that has a temperature at all, but the (very sparse) light and matter found in space.

Transmitting heat is not the same thing as having a temperature. You may remember learning in school about convection, conduction, and radiation as the three ways of transmitting heat. Convection, which is stuff that has heat moving around and carrying the heat to new places, does not work in perfectly empty space. Conduction, which is what you call it when rapidly jiggling atoms knock against their neighbors and make them jiggle faster, spreading the heat through the material, also doesn't work in empty space. But radiation, which basically means photons, can carry heat through perfectly empty space from one object to another. (By "perfectly empty", I mean "perfectly empty of matter", which I think is what you mean too.)

But you also ask a different question about how people can talk about the temperature of the space outside the atmosphere of a star. And the answer is that space is never completely empty, if you look on a large enough scale. Even between galaxies, there is estimated to be about an atom every cubic meter. And those atoms are moving. Since temperature is a measure of the energy of how much random motion there is, you can meaningfully assign a temperature to even the sparsest collection of atoms. You can also assign a temperature to the electromagnetic radiation, since photons have kinetic energy too.

Even though temperature has meaning in this case, it would do almost nothing to heat you up if you were out there. The interstellar medium can be immensely hot, but there is so little of it that the few atoms hitting you have essentially no effect, and you end up cooling off to near absolute zero by emitting radiation. This is similar to the way air at 450 degrees coming out of an oven is harmless to you, but you would burn yourself touching glass at the same temperature.

There are a lot of subtleties here. In a normal gas, atoms collide all the time, and so their speeds tend to get averaged out to be roughly similar. Some are slower and some are a good amount faster, but there's a formula for how many particles would be moving at a given speed that's different from the average. In space, they hardly collide at all, so those formulas don't apply. That changes the rules a bit, but people still find it useful to use the concept of temperature.

Answer 2

The concept of temperature started as a term in classic macroscopic physics. When dealing with steam engines, you transfer heat by steam convection and conduction through the walls of a heat-exchanger. These are the major mechanisms in a high pressure system.

Outer space is near vacuum. Convection and conduction do not play a significant role. It would be impossible to bring a sensor (which is a thermal reservoir of its own) into equilibrium with the few particles in outer space. The concept of temperature as mean kinetic energy of particles is meaningless in space.

Each body radiates "black body radiation" with a (Planck-) spectrum depending on its temperature. At the same time, it receives radiation from other bodies. This mechanism also establishes an equilibrium without the bodies being in contact or exchanging particles.

In space, if you are far from any star, not much of your field of view may be filled with matter to exchange radiation with. Most will be void. But even the cosmic background is not at absolute zero Kelvin; it has a radiation spectrum which can be translated into a temperature. This is the temperature of space. The radiation is not constant in all directions, but these inconsistencies don't relate to particles: a "cold spot" from earth's perspective in the cosmic background does not imply any particles in that direction are less thermally excited.

The question of background radiation is interesting for physicists and astronomers (including space-based telescopes). It it totally unimportant for space travel, because spacecraft never reach thermal equilibrium with the cosmic background.

Answer 3

Particles are heated up because of the presence of heat, but they are not the medium through which heat is necessarily transferred. Heat is radiation, which travels through vacuum just as well as (actually, better than) it does through matter.