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Union University Department of Physics

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Science Guys > July 2003

July 2003

I’ve read that when the astronauts in space are out in the Sun, the temperature is +255 degrees and in the shadows the temperature is -255 degrees. Just what is the temperature of empty space?

Temperature has a precisely defined mathematical definition. For our purposes, we can say that temperature is proportional to the average kinetic energy of the random motion of a substance’s molecules. Kinetic energy is the energy something has by virtue of its motion. Molecules of all substances, even solids, have random motion and therefore an associated kinetic energy and temperature.

Air molecules in a 70°F room are in vigorous random motion and thus possess kinetic energy and an associated temperature. In fact, air molecules move at hundreds of meters per second! Because these molecules are randomly moving, as many particles move up as down, and as many move left as right, they do not disturb objects.

When we read that spacewalking astronauts experience certain temperature extremes, these extremes are the temperatures molecules in that location would achieve dependent upon the amount of solar radiation received. The astronaut’s space suit thus must withstand a temperature range of about -255 to +255 °F depending upon the amount of radiant energy absorbed from the sun.

Completely empty space would have no temperature since there are no molecules there - it would make no sense to discuss the temperature of nothingness. We wouldn’t even call it zero degrees. Technically, there must be matter present to have an associated temperature. Of course, even the emptiness of interstellar space has a few hydrogen nuclei, electrons, or neutrinos zipping through it and thus is not truly empty.

Even when there are no particles in a region of space, there is the cosmic microwave background (CMB) radiation. This ’radiation’ fills the entire Universe and is thought to be the afterglow of the Big Bang. The Universe was incredibly hot when the Big Bang initially occurred. There were tremendous amounts of radiation in the form of particle-like entities called photons - "particles of electromagnetic radiation." However, as the Universe expanded, space cooled and the radiation now is quite cold.

The photons that make up the CMB actually have a variety of energies. The range of CMB energies matches that which an object (say, a hunk of dark iron) at 2.7°C above absolute zero emits. Because of this, the CMB is sometimes said to have a temperature of 2.7 degrees Kelvin (that is 2.7 degrees above absolute zero, or about minus 450 °F!)

Therefore, the 2.7K temperature of the CMB may be quoted as the "temperature of space"- it is perhaps the best way to characterize the energy content of empty space. Of course, 2.7K is pretty cold! Basking in the CMB is no way to get a tan!