If a vaccum is such a great insulator, why is space so cold?

[Squink]

@Sam Stone

Why does the black body temperature matter?

It tells you what equilibrium temperature the sun would heat you to if you absorbed all impinging light, and generated none of your own internally. The 125 or so watts a typical person puts out won’t make much of a difference when you add it to the 1366 W/m^2 solar constant near earth.
If you want to calculate a more accurate equilibrium temperature, you’ll have to find values for the reflectance and emissivity of your guy in space.

[Sam_Stone]

@Squink

The black body temperature for objects at earth’s distance from the sun is -2°F.
You wouldn’t be toasty even if you wrapped yourself in a black quilt.

I took that to mean you were saying that that’s what your body temperature would drop to, so you’d slowly freeze and die. I’m just pointing out that so long as you are alive, you’re generating a significant amount of heat, and therefore it’s not obvious that you would start getting colder. Sure, after you eventually die your body will cool down to its black body temperature. But that’s not really the question.

Maybe the best answer to the OP is that we perceive space as ‘cold’ because most of the stuff we’re talking about exists in a region of space that has relatively little energy to absorb. So most things just gradually cool down. You probably wouldn’t think space was particularly cold if you lived at the orbit of Mercury. Then you might be asking why space is so hot.

[Frylock]

@Stranger On A Train

The Aristotlian process of deducing the result of a real-world process via the application of an ontological framework is an old and timeworn tradition of almost invariably getting the wrong answer, or on rare occasions, the correct answer via a completely and utterly incorrect understanding of the underlying mechanics, which then had to be rectified, often at great personal cost and sometimes injury, by succeeding experimentalists and emperical theoreticians who were ill-regarded for challenging the “perfect” symmetry of Plato’s worldview. In the case of your original post, you seem to be confirming the o.p.'s presupposition that a vacuum is cold

As I’ve said, I was using the OP’s presuppositions intentionally in order to show that those presuppositions themselves provided material for an answer to the OP’s own question. As I said, mine was an almost purely logical point.

The task of educating the OP about the nature of heat etc. was not a task I was taking on. I intentionally left that to future posts, knowing that there are actual physicists, engineers, etc. here far betterprepared to take on that task.

before reasoning that “heat”–a quality you later deigned unworthy of actual definition–

What are you talking about here?

Later, you use an analogy of basketballs which are somehow embedded in space to represent heat, and subsquently reasoned that could therefore be no basketballs (heat) in space. Analogy and metaphor can be useful for the purposes of illustration

Indeed, and a cursory reading of my post shows I was using the “basketball” scenario as an illustration of the structure of the argument of my previous post. Illustration, as you say, is a useful explanatory technique.

Look. Forget basketballs. In my head I was just thinking ‘X,’ as in:

For all X and Y, if X can not move within Y, and if X can not originate within Y, then X can not exist within Y.

The OP used a word “heat” to stand for a concept which seemed to me to fill the same logical role as ‘X’ in the above. Now it may be that the OP’s term “heat” actually does not match the real-world phenomenon of heat. That was not my concern. My concern was just to draw out the logical consequences of what the OP itself said about this ‘X’ which it called “heat.”

Heat–that is, particles in energetic motion–have no difficulty whatsoever travelling through a vacuum; indeed, the lack of pressure guarantees that they’ll travel far. Similarly, heat energy that is radiated away from a body in the form of electromagnetic energy (infrared or otherwise) will transit unimpeded through a vacuum. However, the normal methods of heat transfer that we’re accustomed to on terra firma–convection, conduction, and evaporation–don’t occur in a pure vacuum owing to–by definition–the lack of other particles for your particles to bump up against and give away some of their energy. That’s why a vacuum is a good insulator for conductive and convective heat transfer. Vacuum itself has no temperature, again by definition; the background radiation of the cosmos has a very low temperature, thanks to the expansion of the universe, but in the close proximity to a flaming ball of gas, you’re going to absorb a lot of incoming solar radiations, which you’ll absorb and will make you hot, and then reradiate part of that energy on your shaded side, which will make that cold.

Okay, did you read my post:

To answer the OP again in simple terms:

Heat is stuff moving around. In a vacuum there is no stuff. Since there is no stuff in a vacuum, no heat can travel into it–for heat can only travel from one mess of stuff to another mess of stuff. That makes vacuum a “good insulator” as you called it. Meanwhile, since there’s no stuff in a vacuum, there’s no stuff moving around, and so the measure of heat in a vacuum is zero. If low measure of heat == cold, then that is why space is cold. However, if cold==a tendency to absorb heat, then your assumption that space is cold is incorrect.

To the OP: Makes sense?

To Stranger: Correct?

Could you say something more directly about what is right and wrong about the above? It would probably be helpful to the OP.

-FrL-

[Squink]

@Sam Stone

therefore it’s not obvious that you would start getting colder.

Have you ever put a beaker of water in a vacuum chamber?
As it boils, the water gets cold. Without a heat source, the heat of vaporization comes from the bulk liquid. That cools the water. With a good enough vacuum, the liquid will boil until it freezes.
While skin is a good barrier against vaporization, lungs are optimized for gas exchange, whether the gas be oxygen, or boiling water because you left the capsule without your spacesuit.
As you know, you can’t boil a lot of water with a 125 watt heat source, so if water is boiling from your lungs you will get colder.

[Sam_Stone]

@Squink

Have you ever put a beaker of water in a vacuum chamber?
As it boils, the water gets cold. Without a heat source, the heat of vaporization comes from the bulk liquid. That cools the water. With a good enough vacuum, the liquid will boil until it freezes.
While skin is a good barrier against vaporization, lungs are optimized for gas exchange, whether the gas be oxygen, or boiling water because you left the capsule without your spacesuit.
As you know, you can’t boil a lot of water with a 125 watt heat source, so if water is boiling from your lungs you will get colder.

Oh sure, now you get all logical about it…

I had kind of assumed that we were talking about a guy in a spacesuit. 'Cause if you’re not in one, losing heat through vaporization is probably a fair ways down on your list of problems.

[zut]

@Frylock

Could you say something more directly about what is right and wrong about the above? It would probably be helpful to the OP.

He did. You just have to match up the parts.

@Frylock

Heat is stuff moving around. In a vacuum there is no stuff. Since there is no stuff in a vacuum, no heat can travel into it–for heat can only travel from one mess of stuff to another mess of stuff.

@Stranger on a Train

Heat–that is, particles in energetic motion–have no difficulty whatsoever travelling through a vacuum; indeed, the lack of pressure guarantees that they’ll travel far. Similarly, heat energy that is radiated away from a body in the form of electromagnetic energy (infrared or otherwise) will transit unimpeded through a vacuum.

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@Frylock

That makes vacuum a “good insulator” as you called it.

@Stranger on a Train

However, the normal methods of heat transfer that we’re accustomed to on terra firma–convection, conduction, and evaporation–don’t occur in a pure vacuum owing to–by definition–the lack of other particles for your particles to bump up against and give away some of their energy. That’s why a vacuum is a good insulator for conductive and convective heat transfer.

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@Frylock

Meanwhile, since there’s no stuff in a vacuum, there’s no stuff moving around, and so the measure of heat in a vacuum is zero. If low measure of heat == cold, then that is why space is cold. However, if cold==a tendency to absorb heat, then your assumption that space is cold is incorrect.

@Stranger on a Train

Vacuum itself has no temperature, again by definition; the background radiation of the cosmos has a very low temperature, thanks to the expansion of the universe, but in the close proximity to a flaming ball of gas, you’re going to absorb a lot of incoming solar radiations, which you’ll absorb and will make you hot, and then reradiate part of that energy on your shaded side, which will make that cold.

[Frylock]

@zut

He did. You just have to match up the parts.

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Yes. I was asking him for something “more directly” responsive to my own post, as a favor to me.

-FrL-

[Frylock]

Never mind. I already know the answer. As a response to the OP, the post is basically correct.

-FrL-

[David_Simmons]

Aren’t those few particles that are in space actually pretty hot? I have been under the impression that the particles, for example in the solar wind, are traveling at a pretty good clip.

The total heat in space is small because the particle density is so low.

[Squink]

@David Simmons

Aren’t those few particles that are in space actually pretty hot?

Yes, the solar wind is running about 120,000°K today, at a pressure of 0.4 nanoPascals.

[David_Simmons]

@Sam Stone

I took that to mean you were saying that that’s what your body temperature would drop to, so you’d slowly freeze and die. I’m just pointing out that so long as you are alive, you’re generating a significant amount of heat, and therefore it’s not obvious that you would start getting colder. Sure, after you eventually die your body will cool down to its black body temperature. But that’s not really the question.

Actually, as Stranger pointed out a human would probably be too hot, at least on one side.

The solar constant at earth’s distance from the sun is 1370 w/m[sup]2[/sup]. Using the Stephan-Boltzman law and varying the percentage of the sunlight that is reflected the temperature of a human would go from 129 C at zero reflected to 16 C at 80% reflected. That’s why the suits are white and they still need cooling. The 125 watts produced by the human is added in.

Of course we are assuming a superman here who can really hold his breath.