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The Question

(Submitted March 01, 1998)

All I wanted to ask you is that if we put a thermometer in Space with no other light or heat source around and absolutely no background radiation there, what would it read? Would the temperature be really cold or what?

The Answer

Yes, it would be really cold. Temperature measures the energy per "degree of freedom" (i.e. way something can move) of whatever molecules happen to be around. So, it it becomes so cold that the molecules stop all together, then this is the "absolute zero" temperature. On the Celsius Temperature Scale (i.e. water freezes at 0, and boils at 100) this takes place at -273 degrees C.

We usually use the Kelvin temperature scale, where Zero Kelvin is this "absolute zero" temperature -- or -273 degrees C. Water freezes at +273 Kelvin and water boils at +373 Kelvin.

If we put a thermometer in darkest space, with absolutely nothing around, it would first have to cool off. This might take a very very long time. Once it cooled off, it would read 2.7 Kelvin. This is because of the "3 degree microwave background radiation." No matter where you go, you cannot escape it -- it is always there.

Jonathan Keohane
for Ask an Astrophysicist

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