temperature of a vacuum

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From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Sep 20 2000 - 11:30:46 PDT 

Message-Id: <l03110719b5eeb278c857@[192.174.2.173]>
Date: Wed, 20 Sep 2000 10:30:46 -0800
From: Paul Doherty <pauld@exploratorium.edu>
Subject: temperature of a vacuum

 Hi Professor!

I¥ve started to teach physics now, and I get all kinds of strange questions
from kids. Most of them I¥ve been able to find answers for, but this one I
need help with:

What temperature is there in a vaccuum? If there is nothing there that can be
hot or cold, how can you tell?

Can you give me a grade 7-level answer for this?

If you don¥t have the time to be my physics-oracle, just tell me, but I really
appreciate being able to ask you.

I really hope I¥ll see you and the Exploratorium next summer.

Per Sporrong,

Hi Per

A great question.

Let's define remperature as what is measured by a thermometer, a good 7'th
grade definition.

Put a thermometer in air and it comes to the same temperature as the air.
Same thing in water.

But what about a vacuum?
There is no matter in a vacuum.
However there are light and radiowaves in a vacuum.
Put a thermometer in sunlight and it gets warmer than the same thermometer
in the shade both in contact with the air. So light can change the
temperature of a thermometer as well as matter.

The vacuum of space far from a star is full of radio waves and dim light.
The radio waves are left over from the big bang. A sensitive thermometer
placed in a vacuum will record a temperature of just under -270 degrees
celcius. (Or 3 Kelvins)

Hope this helps

Paul D

Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld

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