# DUMB HUMOR [and heat radiation in space]

Sun, 5 Dec 1999 20:54 PST

Spike pointed out to me that I sent the following response to him (rather than to the list) and that it was educational enough for general consumption. [My mail program (Pine) is victmizing me since I get the reply responses mixed up often enough that my email sometimes get misdirected.] Clearly we need a T-shirt at Extro5 "I am a victim of Email agent abuse!" (on the front) and "Bygones." (on the back).

At any any rate, the repost is:

On Sat, 4 Dec 1999, Spike Jones wrote:

Spike, I think you need another cup of coffee.
>
> The way I interpreted that is: in a near vacuum, it doesnt matter much
> what the temperature is. The heat transfer from or to the few particles
> present is very low compared to the heat transfer by radiation.

This is correct.

> If one were in deep space with a pressure of 1 picotorr, the temperature
> could be anything you wanted, but you wouldnt know the difference.
> You would still radiate heat away. Doug Jones explained it in his post.

I think you may have slipped on something wet and slimy here. You *will* radiate heat away, but the temperature cannot be "anything you want". If you are generating a fixed quantity of heat (e.g. a metabolically active human) and may be absorbing heat (if you are in the sun), then your heat disposal (by radiation) is controlled by 3 things -- the surface area of your suit, the thermal emissivity of your suit *AND* your temperature. This is known as the Stefan Boltzman law, expressed as

X = k * temp-diff^4 * e
where X is power radiated in Watts / m^2, k is the Stefan Boltzman constant, temp-diff is the difference between your temperature and that of your surroundings and e is your emissivity.

If, your emissivity is to low, or the power you are producing internally (or absorbing) is too high, your temperature *will* rise until the equation (Power-in = Power-out) balances. If you are producing too much heat internally relative to your radiative surface area and emissivity, you *will* melt (or combust). Thats why cars have radiators and people have bodies (to increase the surface area to get rid of the heat that the brain generates).

This funny little fact of thermodynamics, that radiation scales with temperature to the 4th power is perhaps the prime determining constraint on MBrain architectures (which as you know I've only devoted a small amount of time to thinking about... :-)).

>
> To get a feel for what I am saying, take a grinder and grind some
> steel. One can put ones bare hand in the shower of sparks and yet
> feel no discomfort, even tho the sparks are hotter'n hell. Its because
> there is so little mass there to conduct heat into ones skin. spike
>
Partially correct, the reasons I believe are:

1. The particles are very small and have a very low heat "capacity" so they lose their heat very rapidly. I believe heat should move through a solid material at the speed of sound in that material (through the transfer of vibrational energy between atoms). Small particles rapidly conduct the heat to the surface where it may be lost (through conduction & radiation).
2. Skin is a relatively poor conductor of heat (relative to steel), so the transfer of heat into your skin is slow.
3. The area of the particle exposed to the air is much greater than that exposed to your skin (so it loses most of its heat by conduction & radiation into the air).

Since both conduction & radiation occur simultaneously but radiation scales with T^4, the particles intially lose much of their heat by radiation (thats why they glow), then gradually transition to conduction, as the air particles impatting the steel particle (or your skin) absorb the remaining heat.

So, I'm curious about the example. Is this something you consider to be an entertaining activity when there is nothing on TV?

Robert