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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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