# Black body, white noise **URL:** \ **Category:** Factual Questions\ **Created:** [January 22, 2000, 12:53pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496 "2000-01-22T12:53:00Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![pqlier](https://avatars.discourse-cdn.com/v4/letter/p/439d5e/32.png) [@pqlier](https://boards.straightdope.com/u/pqlier)\ **Post date:** [January 22, 2000, 12:53pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/1 "2000-01-22T12:53:00Z")
Surely, you jest. ---
**Author:** ![JillGat](https://avatars.discourse-cdn.com/v4/letter/j/9fc29f/32.png) [@JillGat](https://boards.straightdope.com/u/JillGat)\ **Post date:** [January 22, 2000, 7:50pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/2 "2000-01-22T19:50:00Z")
And your point is…? ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [January 24, 2000, 6:13am UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/3 "2000-01-24T06:13:00Z")
A black body has two meanings: First, in general, a black body is just an unknown “item” which might change something from state “A” to state “B”, but how it works inside is inconsequential. For example, your computer is a black box to most: garbage in, garbage out. In physics, in brief, a black body can also mean a perfect radiator. A black body is assumed to reflect no light which means it absorbs 100% of all light incident upon it. Likewise, a black body will emit off 100% of all radiation it has absorbed. White noise refers to the fact that you are receiving all wavelengths - typically in the radiowave end of the elctromagnetic spectrum. Similar to how the presence of all visible light combines to form “white light”, the various radio frequencies are said to combine, in a sense, to yield “white noise”. Well, that’s it in a nut-shell. ---
**Author:** ![JillGat](https://avatars.discourse-cdn.com/v4/letter/j/9fc29f/32.png) [@JillGat](https://boards.straightdope.com/u/JillGat)\ **Post date:** [January 23, 2000, 9:53pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/4 "2000-01-23T21:53:00Z")
Does this relate to a column by Cecil, or should I move it to General Questions? Jill ---
**Author:** ![system](https://global.discourse-cdn.com/straightdope/original/2X/e/e489c3b7d8fce19c4b355dd4fc3f88cc39c34b87.png) [@system](https://boards.straightdope.com/u/system)\ **Post date:** [January 24, 2000, 4:17pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/5 "2000-01-24T16:17:00Z")
Uhhh…jillgat this is GQ * * * The most Invisible poster in the history of the boards. Posting invisibly since sept 1999. ---
**Author:** ![JillGat](https://avatars.discourse-cdn.com/v4/letter/j/9fc29f/32.png) [@JillGat](https://boards.straightdope.com/u/JillGat)\ **Post date:** [January 24, 2000, 5:32pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/6 "2000-01-24T17:32:00Z")
> [@](#): > > Originally posted by Banks: > \*\*Uhhh…jillgat > > this is GQ > > \*\* Uh, Banks, I just moved it here from Cecil’s Columns. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [January 24, 2000, 9:08pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/7 "2000-01-24T21:08:00Z")
There’s something I don’t understand about black bodies (in the thermal sense). I can easily see that a black object absorbs more light than a white object, causing it to get hotter. But supposedly a black object _cools off_, or radiates heat away, faster than a white object. Why should that be? ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [January 24, 2000, 9:35pm UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/8 "2000-01-24T21:35:00Z")
> [@](#): > > Originally posted by Lumpy: > **There’s something I don’t understand about black bodies (in the thermal sense). I can easily see that a black object absorbs more light than a white object, causing it to get hotter. But supposedly a black object _cools off_, or radiates heat away, faster than a white object. Why should that be?** ## A very deep question… I’m not sure if I can do it justice. Briefly, the more strongly an object is coupled to the electromagnetic field, the more efficiently it can absorb light, but also the more efficiently a hot object can turn it’s heat into radiation. It’s called reciprocity. _It is too clear, and so it is hard to see._ ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [January 25, 2000, 3:31am UTC](https://boards.straightdope.com/t/black-body-white-noise/6496/9 "2000-01-25T03:31:00Z")
First of all, ‘blackbody’ in physics is one word, not ‘black body.’ ZemBeam is correct. To illustrate it a bit better - think about an object in a box. Say the box is under vacuum and the object is sitting on an insulated stand so there is no heat conduction or convection. Now leave them for a while. What happens? The object and the box reach the same temperature, because if one is hotter, heat would flow to the other by radiation. That’s what the definition of temperature is. Now, consider two such systems, but one with a black object inside and the other a white object. If the box temperatures are the same, the object temperatures are also equal. They are in identical boxes, so the radiation falling on them is the same, but the black object absorbs more of the radiation. But the system is stable, so each object must be emitting the same amount of energy it absorbs. Which means that the black object is emitting more radiation than the white one. This is called the Kirchhoff’s Law. Remember, white objects are white not because they emit radiation, but because they reflect radiation. As for white noise, it is analogougs to white light. White light is a mix of all wavelengths, or colors, of light. Similarly, white noise is a noise that is made up of a wide range of frequencies (wavelengths). Specifically, it refers to noise where every frequency is present in more or less the same amount. There are other types of noise such as 1/f noise, which contains more noise at lower frequencies. Also called “pink” noise, since pink has more red (low frequency light) than blue.