Heating a dark non-reflective body in space

[-Job-]

A relatively simple question: Suppose we have a dark non-reflective body at rest in space and hit it with fair amounts of radiation. This would cause the body to heat up because it absorbs most of the radiation. After some time would the body cool down? The first guess is that it would, but why would it cool down, where does the energy from the radiation go? If we start with a body at rest, put energy into it and end up with the initial scenario then this goes against conservation of energy, right?

 

[Danger]

Without getting into stuff that I don't understand myself, the heat will be re-radiated as infrared (or other EM depending upon the temperature). It should eventually equalize at ambient temperature (CMB?)

 

[-Job-]

So dark bodies can still be detected by analyzing their radiation emissions on different spectrums? If we have a dark, non-reflective object at rest in space, and push it so that it moves, assuming it doesn't come under any other forces or radiation, does this object now emit some radiation like its heated version (even if less)?

 

[Danger]

I'm afraid that you're getting out of my league here. I don't really know anything about black-body radiation and such. One thing I should mention is that once you heat a dark body, it isn't dark any more (using my own definition of 'dark', that is). As for simply pushing it, any mechanical push will involve some small generation of heat through conductance, compression and/or friction. Ignoring that, I think that a truly cold, dark body would only radiate gravity waves. Someone like Space Tiger really should handle this from now on. I'm plumb out of information.

 

[Tide]

I suggest that you provide a little more detail about the problem you are trying to solve.

It appears you have an object in space, you illuminate it briefly and want to know what happens to the energy. The absorbed energy heats the object. If the object was initially in thermal equilibrium with its surroundings (including the ambient radiation field) then it will eventually return to thermal equilibrium by emission of radiation. Energy is conserved.

 

[-Job-]

Tide, to be more direct, why is it that, when i focus a beam of radiation on a dark, non-reflective object at rest, the object heats up instead of moving in a given direction?
Suppose i only have some amount of energy (i have no matter), how would i apply this energy to the dark non-reflective object so as to cause it to move in a given direction?

 

[Tide]

-Job-,

Actually, you would have double the momentum transfer if you had a reflective surface instead of an absorbing one. In either case, you will need an extremely intense beam to cause a "macroscopic" object to move because the momentum of the photons is very small! It can be done but the conditions have to be designed accordingly.

You may be aware of the Crooke's radiometer which has small fins suspended and encased in a glass bulb. One side of each panel is black and the other side is white. Exposing the apparatus to light does cause fins to rotate -- but the reason for it is that light absorbed by the black heats the fin AND the air in contact with the surface. It is the hot expanding air that causes motion and not the momentum transfer.

 

[Danger]

Aye, the reflectivity is important for impetus. A laser sail spacecraft , if built, will use photonic pressure from an orbital laser to impart momentum to a (huge) silvered sail. Thanks for that explanation of the radiometer, Tide. I've seen one, but didn't know that's how they work.

 

[pallidin]

A relatively simple question: Suppose we have a dark non-reflective body at rest in space and hit it with fair amounts of radiation. This would cause the body to heat up because it absorbs most of the radiation. After some time would the body cool down? The first guess is that it would, but why would it cool down, where does the energy from the radiation go? If we start with a body at rest, put energy into it and end up with the initial scenario then this goes against conservation of energy, right?

Actually, this is very simple. A non-reflective black body placed into space and out of sunlight, will be in a very, very cold environment.

Let's look at this:
"...if you put a physical object into space, it will reach a
temperature that depends on how efficiently it absorbs and emits
radiation and on what heating sources are nearby. For example, an
object that both absorbs and emits perfectly, put at the Earth's
distance from the Sun, will reach a temperature of about 280 K or 7 C.
If it is shielded from the Sun but exposed to interplanetary and
interstellar radiation, it reaches about 5 K. If it were far from all
stars and galaxies, it would come into equilibrium with the microwave
background at about 2.7 K."
Source: http://www.faqs.org/faqs/astronomy/faq/part4/section-14.html

So you can see that space can be a very cold environment indeed.
Anyway, when you heat up the object and then shut off the heat source, what happens? The object will attempt to go back to a temperature state equal to it's environment. Typically this is accomplished by the spontaneous emmisions of infrared photons as the object seeks to be at the same temperature as the vacuum environment.

 

[Omegatron]

but the reason for it is that light absorbed by the black heats the fin AND the air in contact with the surface. It is the hot expanding air that causes motion and not the momentum transfer.

I thought they only worked in vacuum.

 

[Tide]

I thought they only worked in vacuum.

No. While the pressure inside a radiometer may be low it is certainly not a vacuum!