> In space there's no temperature because there's no medium. So how
> is the temperature in space absolute zero?
Not quite. Although the statement that "there is no medium" in space is
incorrect (althougha vacuum, the particle density in space is not identically
zero everywhere), there are more fundamental reasons why your statement is
wrong.
Temperature is sort of an aggregate property of systems, including space.
Thermodynamically, temperature is defined as the change in internal energy of
a system with respect to a change in entropy, holding other properties
constant.
When you read temperature off some measuring device, like a thermometer,
you're really not measuring the temperature of the air (or whatever medium the
thermometer is in) - you're measuring the temperature of the thermometer when
the thermomenter has come into thermal equilibrium with its environment
(meaning it is giving off as much energy as it is getting in).
There are 3 ways that thermal energy can be exchanged: conduction, convection,
and radiation. In a dense medium like water, most of the energy being
transferred to the thermometer comes through direct contact of water molecules
- conduction. In space there are almost no molecules to carry out conduction.
But it is still possible for radiation to take place (thermal radiation,
mostly transfer of infrared photons, to be distingiushed from ionizing
radiation like that from nuclear decay.)
So if you threw a thermometer out into space it would continue to radiate its
energy away until the amount of energy it radiated was equal to the amount it
received. In deep space (away from stars and so on) it receives practically
nothing so it will continue to radiate until its atoms lose all thermal
energy. This state is the absolute zero of temperature. Though in truth the
temperature od space isn't quite absolute zero. It is actually about 4
degrees Kelvin above absolute zero (still VERY cold) - resulting from leftover
radiation from the Big Bang. Neat huh?
I dunno if I've explained this very well or not, and thermodynamics was never
one of my stronger points anyway. Ming, please feel free to correct any
inconsistencies.
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"Zu jeder Zeit, an jeder (sic) Ort, bleibt das Tun | Walter Amos
der Menschen das gleiche..." - Galactic Heroes II | amos@sedl.org