eli5: Why isn’t outer space hot (or at least not freezing)?

Question

The sun warms up our planet but space is cold. If I lit a candle and stood 20 feet away, I could see the light but not feel the warmth. So, why do we feel warmth from the sun but space isn’t warmed by it?

In: 512

Answer 1

What is heat? It is the energy of particles vibrating. When something is hot the particles are vibrating fast, when something is cold the particles are vibrating slow (or standing still if it is 0 kelvin).

So what is empty space? No particles, so is space hot or cold? Neither, there are no particles so you can’t say if they are fast or slow as they just aren’t there. Therefore space has no temperature. (what speed does a car that doesn’t exist have? None, there is no speed as there is no car)

The idea that space is cold is kind of wrong, I mean it’s less wrong than saying it’s warm as space does not contain any thermal energy but saying it has no temperature is more correct.

The way something heats up is particles hitting particles so that they start vibrating, either because the particles are close to each other (your hand touching a hot thing) or by particles emitted (infrared for example) by that source of heat that can travel and hit something to make it vibrate. (you sitting 5 ft. from a fire, still feeling it’s heat but not touching it)

The sun emits photons that can hit particles and make them move. However space is no particles at all so the photons move through the space without hitting anything so the particles don’t “hit” space. (which means they don’t lose any speed or energy) Once the particles reach the earth they start hitting particles and making them move, and thus they start to heat up whatever they hit on earth.

In practice the space in between earth and the sun isn’t completely empty but if a few photons hit some stray atoms on the way to earth there is still more than enough energy left.

Also, if you would be more sensitive, you would feel the candle from 20ft. away as that candle is emitting some infrared too, just not enough to be noticeable by a human.

In theory if you would probably feel more of its heat if there was no air in between you and the candle as the particles traveling from the candle to you wouldn’t be bumping into all of the air first and lose most of their energy.