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From what I can tell from reading various articles about space, a metal bar on the side of the earth exposed to the sun will heat up to about +250 °F (120 °C) and on the shadowed side will cool down to about -250 °F (-160 °C). Well, that is a lot warmer than absolute zero which is about -460 °F. Why would the metal bar stay at -250 °F instead of cooling down to -450 °F or so?

Emilio Pisanty
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Ambrose Swasey
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1 Answers1

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Even on the shadowed face there are other sources of radiation that will control the temperature of the bar. For example, on the example that you read, the earth itself will radiate!

You may think that if you remove the Earth and other bodies floating around, including the Sun then the temperature will drop down to absolute zero (0 K). But even then the bar will stay warmer, at $T_{\rm CMB} = 2.72 ~{\rm K}$ (or $-454.76$ F), the reason being that the universe itself is permeated from a rather uniform bath of radiation coming from the Big Bang: the Cosmic Microwave Background (CMB)

caverac
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