People tell me that space isn’t cold.
But cold isn’t even a thing. It’s a privative. It’s defined by the absence of heat. You can’t increase coldness, you can only make something less hot, and in common parlance that is called “colder”.
It’s also relative. A cold day to you or me might be unbearably hot to a polar bear or a penguin.
So if space doesn’t have any heat, why is it wrong to call it “cold”?
In thermodynamics, one should put two systems in contact, and see which way the heat flows. If there is a true vacuum, then there is nothing, no particles or radiation, at least classically. You might note phenomena such as, you are on the Moon, and if you are exposed to the Sun you start cooking, while in shadow you start freezing, even though you are surrounded by space— so you need to think about, why does it not seem like the space has a fixed temperature. If it were “cold”, it should absorb some of that heat coming from the Sun, perhaps?
The conceptual package of the word “cold” or “coldness” doesn’t just refer to an absence of heat, but reversed heat flow. That is why a cold object feels cold, or measures as cold on a thermometer. The heat from the sensor flows into the substance.
In a vacuum, this doesn’t happen. Space does not feel cold. It doesn’t measure as cold, because there’s so little matter to reduce the temperature of the thermometer.
OTOH, there’s also nothing to impede radiative heating or cooling, so you warm up in direct sunlight without the buffering of air to absorb some of the infrared radiative heating, and you radiate heat away as infrared energy in the shade.
Space is neither hot nor cold, but space also doesn’t interfere with matter becoming hot or cold via radiation.
Another maybe shorter way to say what’s rightly said in the posts above is that colloquial “cold” is actually the capacity to absorb heat. Which is why 33F water feels colder than 33F steel which feels much colder than 33F plastic. The near vacuum of space has almost no such capacity.
Said yet another way, the very concept of temperature, be it a large or small value, is simply inapplicable to a Classical vacuum. Quantum stuff adds reams of quantum caveats, all of which are ignorable at the level of detail that matters here. Classical physics is close enough for our purposes. Asking the temperature of a classical vacuum is like asking what’s the square root of green.
The way I would think about it as that the temperature of something is the average energy of the atoms and molecules that it is made of. The vacuum of outer space doesn’t have atoms and molecules. (OK, it does, but so few that it doesn’t count). So a vacuum can’t have a temperature, and so it can’t be hot or cold.
When food comes out of the oven or fryer it is said to be “hot.” Take a bite, it will transfer heat into your mouth.
Let the food sit around for a while. Come back to it an hour later. It has achieved equilibrium with the surrounding air. Take a bite. It will not transfer heat into your mouth, It won’t absorb it either.
You can still say that the food is cold.
Space is defined by the absence of matter. In effect, it doesn’t exist. How can something that doesn’t exist be hot or cold?
And in so doing we are using the word “cold” in a very much not-physics way.
What someone in that situation is actually saying when expressed in detail is “the temperature of the food is less warm than is my preference.”
Which is the same reason we might refer to a room temperature soda as warm or hot, despite being the same measured number of Kelvins as the food we just described as “cold”.
Said another way, in colloquial use, “cold” and “hot” can actually be shorthand forms for “colder” or “hotter” . “Cold” can mean “colder than desired”.
The words can be comments about some absolute value, or some value relative to another value. Be that other value explicit or implicit.
Like most English words they have a lot of distinct meanings that are all collectively sorta about the same thing, just different aspects and flavors of it.
One of the huge advances of science between say 1750 & 1950 was to use a lot more math and a lot less words to describe phenomena. Because that strips off a lot of the inherent fuzziness of words and their ever-shifting definitions.
“It didn’t burn me” isn’t an adequate measurement of heat transfer. Room temperature food is going to receive your body heat until it reaches equilibrium, just not so fast as to trip your “wow, that’s cold” sensors…
ETA: as has been said better above, you are applying colloquial speech to a concept in physics, and it just doesn’t work well enough in this context to describe the underlying mechanism and get to the how and why regarding actual heat transfer.
“Hot” and “cold” are only meaningful terms where there are particles/atoms/molecules to interact with each other. An absolute perfect vacuum is neither hot or cold. If you start adding matter in space, the matter can collectively be called “hot” or “cold”, largely related to its distance from a star that is outputting energy in the form of photons that can collide with matter and become kenetic energy. Space near Mercury is neither hot or cold, but the collective molecules (and planet) there can be pretty hot. Spece near Pluto is neither hot or cold, but the collective molecules (and planetish ball) there can be pretty cold.
The thin “air” in a near-vacuum can be very hot but someone still freeze to death in it. For instance, the matter at the solar system heliopause is around 50,000 to 90,000 degrees F. But if you were there it would feel damn cold.
A lit match and a campfire may be the exact same temperature, but there’s a huge difference in their heat
Which is pretty much the entire point.
And can you talk about “cold” in a physics way at all? Cold isn’t a thing. You can’t measure how cold something is.
I thought the temperature of something was the average kinetic energy of its particles. If that’s about right, then the temperature of space is like a division by zero error.
ETA: Regarding your last post, if you’re saying you can’t say something is cold (in a physics sense), just how hot it is, doesn’t that negate the question in this thread?
A more complete definition would be something like “Cold is the absence of heat in matter”. Since there is no matter in a vacuum, this definition of cold would not pertain to it.
No, because I think that coldness is conceptual.
I always did find it odd in dramatizations of “Goldilocks and the Three Bears” when Goldilocks reacts to Mama Bear’s porridge as if she’s just stepped into an ice bath.
Space is not an absence of everything. Particles do exist, although typically far from one another. And space is pervaded by the Cosmic Microwave Background (CMB), which gives it an virtually identical temperature everywhere of 2.7 K (-459°F/-270°C).
The background radiation isn’t of help to radiating heat, though.
That’s why even though space doesn’t have a temperature people often still call it cold. The measurable aspect is near absolute zero.
Oh, cool, @Pasta is weighing in. I love how you explain physics questions!
Unfortunately, coldness is not conceptual to physicists, and in the context of your question in your OP, it’s definitely not conceptual.
Don’t set the bar too high! I have to pause my post, though. Work meetings loom. Will get back to the boards this afternoon.