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

To my knowledge, a vacuum will stop all transmission of heat except for radiation. Can this radiation account for all the loss of heat energy in outer space? It almost seems that there’s a sort of osmosis going on (since we have so much heat and space has so little).

I’m sure the answer is painfully obvious, but i’ll be darned if I can figure it out.

I don’t know the answer on any empirical basis, but it would appear to me that, if vacuum stops transmission of heat, then there would be no heat inside the vacuum. If there were, it would have had to have gotten there somewhere–and that, in turn, would have required transmission through vacuum.


Yes, all of it. On Earth, a vacuum can only be achieved in some sort of container, and the walls of this container will reflect some radiated energy back as well as radiate heat of its own (since it will be at ambient temperature), so the rate of loss will be much lower than it would in space, where all the energy is radiated away, and little is received (unless it’s in direct sunlight). A Thermos™ augments this by coating the walls of the vacuum flask with reflective metal.

head asplodes

This is nonsense; what does this even mean?

Heat is energy that is transferred between matter by direct contact. Since there is little matter in outer space, there is little for a large hot body to transfer its heat to in the first place, and that heat likewise has nowhere to go. It’s an insulator because it prevents heat from moving from one place to another. The reason it’s so cold is because it’s so good at being an insulator.
Radiation is the expulsion of energy via electro-magnetic waves/particles that move energy directly without intermediate transfers.

Hmm, interesting

I was always under the impression convection and conduction were much more substantial than radiation. Radiation loss always seemed naturally self limiting to me, sortof how a 20 milliamp powersupply would limit the power used in a low resistance circiut; there’s plenty of opportunity for engergy transmission, but the power suppply isn’t offering it.

I suppose that’s what it is, I always thought that radiation was something “offered” rather than something that could be drawn and that the amount being offered was very small in relation to what would be “kept” in a vaccum (conduction, convection).

yeah that too, I was thinking that conduction, convection and radiation each had their own energy reserves so to speak

I suppose that’s a simpleton’s view of thermodynamics; thanks for the clearup.

Someone who knows what they are talking about will be along soon.
In the meanwhile, consider that the Apollo spacecraft rotated to maintain temperature stability. When solar radiation hits something like a spacecraft, it heats up. In a vacumn, there is nothing to hit. The side of the spacecraft not in sunlight radiates to the vacumn and loses heat. The side of the spacecraft facing the sun gains heat.

We need to clarify a few things about heat, temperature, and vacuum.

Heat–in the statistical mechanics sense–is mass with random kinetic energy. Temperature is essentially an aggregate measure of the speed of that motion; a faster moving particle has a higher temperature. This may not make it feel “hot” however.

For instance, when the stiffled air on a still summer day feels “hot”, it’s because all of the gas molecules that make up air are bouncing back and forth in every which direction. The reason you feel warm–even when the ambient temperature is lower than your body temperature–is because you can’t reject heat from your own biological processes very well. When a breeze picks up, you can eliminate heat via convection, which is why it feels cool. When you sweat or pour cold water on your face, you lose heat via evaporation, and when you put your forehead against a cold piece of metal, you reject body heat via conduction. (The body’s ability to radiate away heat is very low, lacking, as we do, an efficient blackbody surface. Be thankful for this; good blackbody radiators are equally good blackbody absorbers.)

This random motion also occurs in liquids or solids, even though the molecules aren’t as free to move around. Because liquids and solids are much denser they feel (and are) much hotter–that is, they contain more energy–than gases at the same temperature. Some types of material, particularly metals, also conduct heat very readily, while others like ceramics tend to localize heat. Why this happens is complicated and requires delving into solid state physics, but it’s along similar lines to why some substances are good electrical conductors and others insulate well, and as a general rule, substances that conduct electricity well are also good conductors of heat and vice versa, though this isn’t universally true.

Space, and by this I assume you mean the vacuum of space, strictly speaking has no temperature at all insofar as it is made up of no matter to store kinetic energy. And because it lacks mass to serve as a conduit for the conduction or convection of heat energy, it tends to isolate heat sources pretty well unless they’re either very good radiators or they can cool themselves via evaporation of their own mass. Now, there is the cosmic microwave background which fills the universe with radiation at a blackbody distribution peaking at around 160GHz and giving the night sky an effective blackbody temperature of around 2.7 Kelvin. What this means is that from the perspective of the second law of thermodynamics, the cosmic background can act as an essentially infinite heat sink at 2.7K, which is cold enough to radiate heat to from virtuallly any body. Where did this CMB radiation come from? It’s an echo from the time shortly after when the nascent expanding universe first became transparent to electromagnetic radiation at ~10[sup]13[/sup] seconds. Before this, everything was packed so close together in a plasma of baryonic matter and extremely short wavelength photons that light had essentially no free mean path. As the universe expanded, the background “temperature”–that is, the mean energy density and wavelength–decreased, even though the actual photons are still moving, as they always do, at c.

Space isn’t cold; it’s just mostly…nothing. And nothing can’t conduct, convect, or absorb energy. A major problem with designing spacecraft is to reject enough heat to keep it functional, or in the case of a manned craft, livable. This is why the Space Shuttle always flies in orbit with the cargo bay doors open and facing away from the Sun as much as possible; the radiators are mounted to the inner surfaces of the doors.


You’re right that it’s nonsense, for there’s a typo in’t.

“Somewhere” was supposed to have been “from somewhere.”

Given that, I thinkthe idea is clear enough. Since we’re talking about “transmission” of heat, we’re talking about heat as something that can move around from place to place.* So, to illustrate my point, I’ll talk about something more concrete that can move around from place to place: a basketball.

Suppose there is a region of space in which, for whatever weird reason, it is impossible for basketballs to move.

Suppose also that no basketball ever originates in this region of space, and also that no basketball has existed within this region of space throughout the entire infinite past. (I didn’t make the analogous suppositions explicit in my first post. That may be what tripped you up, though “nonsense” seems a severe accusation in that case. Anyway, I think these suppositions are wrong, but I suspected otherwise at the time of my first post.)

Then if there’s a basketball in this region, it moved into that region from elsewhere. But something’s moving into a region entails its movement within that region. This contradicts the assumption that basketballs can’t move within this region.

Hence, no basketballs exist inside the region.


*I don’t mean to say we shouldn’t talk that way.

And herein lies the reason why it is so inappropriate to reason by analogy. Heat is not an object like a basketball; heat is a measure of the kinetic energy in a system that is exhibited in a distributed, undirected form. “Heat” can travel through vacuum quite readily in the form of an energetic gas, or a non-coherent group of photons, or any other grouping of energy which has the potential to transfer energy in such a way as to transfer thermal (motion) energy without any discrete information beyond a difference in reservoir temperature. A vacuum by itself can’t contain heat due to the fact that there is nothing–by definition–to move or vibrate in such a way as to display the properties of heat. Heat is stuff in motion arranged per a statistical distribution and directed only by a difference in temperature between regions. This is basic thermodynamics.


That is true, and is key to understanding your question. There are two ways heat gets from place to place: convection/conduction, where molecular motion is transmitted by collision of the particles, and radtiation, where radiated energy from excited particles excites other particles at a distance.

Here on earth, a lot of heat transfer happens through convection/conduction, and of course, molecular motion is a key measure of temperature. In fact, I’ve seen one temperature chart (I belive it was in The Atmosphere, by Lutjens) at various altitudes as measure by average molecular energy, and the area beyond the top reaches of the atmosphere registered a very high temperature. Those space particles are moving at quite a clip.

They are however, very rare (and the book had an asterisk on that part of the chart because of this). So there’s not a lot of convection or conduction going on. Lack of material is the definition of a vacuum. A vacuum bottle insulates well because the evacuated chamber has no particles to conduct/convect the heat away.

Space has a lot of radiation, of course, but it does not heat up space, only the occasional particle within it. We know it conducts radiated energy because the sun warms us here on Earth, despite 93 million miles of cold space in between.

I think I also might have been underestimating how much we rely on the ambient temperature to keep pumping warmth into us; I was looking at outer space as stealing heat from us, when all I suppose it’s doing is not keeping us warm.

Would the following statement be correct?:

In outer space, we would freeze to death because our bodies rely heavily on external sources to keep warm.

I think that’s the painfully obvious answer to the question.

I knew all that of course.

The basketballs are not analogous to anything like “particles of heat.” They are analogous simply to the presence of heat, whatever that means. While typing up the post, I was thinking that there was an easier way to get to my conclusion, since heat is, basically, matter moving around and if matter is moving around then you don’t have a vacuum. So if a basketball stands for matter moving around, then not only is it that in the region of space in my example, basketballs can’t move, rather, they simply can’t exist at all. I get to skip alot of steps that way. But I feared that doing it this way would lead to confusion over the question of whether particles surrounded by vacuum count as being “in” a vacuum or rather as simply delimiting the vacuum. (You alluded to this very issue in your last post.) To avoid this, I just decided to go with the logic of heat as a quasi-“substance” that can be moved from place to place. And though heat is not a substance, it can move in the sense that it can be transmitted, and merely from facts about what it means to be transmitted, you can show (as I did) that there can be no heat in a vacuum.

Notice that you and I have both given the same conclusion: That there can be no heat in a vacuum. I arrived at that conlcusion by reasoning that, since there can be no transmission of heat within a vacuum, and since, for there to be heat within a vacuum, the heat must have moved into it somehow, it follows that there can be no heat in a vacuum. This is a different way of arriving at the conclusion than yours is, but it still appears to me to be a sound way to arrive at that conclusion. (Given the assumptions I noted in my last post.)

I should be clear about something. I don’t insist my argument that there can be no heat in a vacuum was a good one. I suspect it contained some strong and questionable assumptions. Rather, I am just insisiting that the criticisms that have been brought to bear on it miss the mark.


There was a thunderstorm here last night, and now I find myself asking what sort of container held the lightning? :stuck_out_tongue:

Maybe this wasn’t clear from my posts. I wasn’t really trying to say anything substantive about heat. I was pointing out to the OP that his or her post already contained everything needed to give a sound argument for the conclusion the OP was inquiring into. It was an almost purely logical point.


Are you saying lightning can only exist in a vacuum or something? :confused:


Sigh. In all seriousness, this is why I have quit arguing with you about science. In every case, I have the same reaction as Q.E.D. to your assertions and analogies. They are pure nonsense. Gibberish, to use my term. I can’t help that you somehow think you are making sense inside your own head. To outsiders you are saying things that cannot be defended, even in the rare instances when anyone can decipher what you are saying. And then you continue to insist upon your correctness even when poster after poster proves you wrong.

This is a serious problem. I don’t know if you truly do not understand science at all or merely cannot express your understanding properly. I suspect the former. In either case, you need to take a long careful took at how you come across to others on the subject. It would save a lot of needless anger and frustration.

Please note: I absolutely will not argue with you over what I just said. It is friendly advice and nothing more sinister. Accept it or not.

What is the equation I’m thinking of? Something about radiation being proportional to DT[sup]4[/sup].

ETA: How do I make a delta?

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?


Please feel free to continue implementing this policy. :rolleyes:

Do see post number fifteen, though. As I say there, I’m not doing science in my first, second or third post in this thread, so the question of my understanding of science is not at all in play.

Also, a “sigh” is not “friendly.”