Comments on: Is the LHC colder than space?

By: Anne

Anne — Tue, 12 Aug 2008 08:34:40 +0000

Er, Sean Carrol, I have to argue with you on one point: not everything at thermal equilibrium puts out a blackbody spectrum. It puts out the same amount of power as it absorbs, but the spectrum can look rather different - that’s why you get emission lines. If you have enough gas - if it becomes “optically thick” - it will start to have a blackbody spectrum, but that’s a somewhat different issue.

In response to the original article, I’d like to point out that space has at least two “temperatures”: there is the temperature of the photons in some fixed volume, which is normally pretty much 2.7 K, but you also have the temperature of the particles, which is often *not* 2.7 K. Considering, say, a cubic AU of “space” as a whole, it doesn’t have a well-defined temperature because it’s not in thermodynamic equilibrium: the photons are not in equilibrium with the particles. If you put it in a perfectly insulating and reflecting container for long enough, the photons and the particles would randomly exchange energy and it would settle down to equilibrium; its temperature would probably be not too far from 2.7 K, since there’s a lot of energy in the CMB.

In fact, this issue of thermodynamic equilibrium is a tricky one: under most terrestrial conditions systems come to thermodynamic equilibrium very rapidly; it takes moderately heroic measures to move a system out of thermodynamic equilibrium (this is why it’s fairly difficult to build lasers). In “space”, though, it’s very common for systems to be out of thermodynamic equilibrium; in fact we see naturally-occurring masers and lasers from all sorts of strange places. And a system that’s not in thermodynamic equilibrium doesn’t have a well-defined temperature.

Practically speaking, saying that the temperature of space is 2.7 K may be accurate but is misleading for most purposes. It doesn’t carry away heat like a solid, liquid, or gas (of reasonable density) at 2.7 K does, so you don’t feel cold in the same way. You can still radiate heat, and for those purposes it behaves like a system at 2.7 K - except that there’s the Sun, which produces another radiation field which is much hotter. And if you put an object in space, it will be heated - a lot - by sunlight on one side. So thinking of space as “cold” is a bit misleading. (You can’t really think of it as “hot” either, since the object will lose a lot of heat from the non-illuminated side.)

By: andyo

andyo — Wed, 30 Jul 2008 00:21:25 +0000

Thanks Buzz, I was about to ask the same question in the more recent post about colors. Didn’t realize you answered here. I have read about those subjects a bit (just layman stuff), but I get it.

Thanks again.

By: Buzz Parsec

Buzz Parsec — Sun, 27 Jul 2008 06:56:18 +0000

andyo - A photon’s energy is directly proportional to its frequency (or equivalently, inversely proportional to its wavelength.) If it changes its frequency it will change its energy. But here’s the kicker, the *observed* frequency of a photon depends on the velocity of the observer with respect to the source (a blue shift or increased frequency if the observer is approaching the source and a red shift or decreased frequency if the observer is moving away from the source. Since the microwave background is receding from us a high velocity due to the expansion of the universe, the photons, originally representative of a very high temperature (ultraviolet) now appear as those of an extremely cold black body (microwaves.) If you want to know more about this, look up black body radiation, Planck’s constant, the Doppler effect and the good Dr. Einstein.

By: David Ratnasabapathy

David Ratnasabapathy — Sat, 26 Jul 2008 02:15:53 +0000

In Afterglow of Creation Marcus Chown mentions that the Cosmic Background Radiation warms gas clouds in deep space. Years before the CBR was discovered, astronomers measured the clouds’ temperature and wondered why they weren’t stone cold.

By: symmetry breaking » Blog Archive » How can the LHC be colder than space if space has no temperature?

Fri, 25 Jul 2008 10:53:55 +0000

[…] Bad Astronomy tackles that question in a most interesting way involving emptiness, photons and the Mini Marshmallow of Science: Sure, if there’s no there there, then there is no temperature. Of course, space isn’t really empty, it’s only mostly empty. Near the Earth, space actually has lots of subatomic particles per cubic centimeter. Even between galaxies, there are one or two particles per cc. But still, space is so empty that these hardly count when talking temperature. Right? […]

By: andyo

andyo — Fri, 25 Jul 2008 08:30:52 +0000

Can anyone explain something for me?

I thought all particles were the same and didn’t change without converting into another type of particle (or splitting, or merging, whatevah). How is it that photons can lose energy and remain photons?

By: Tim G

Tim G — Fri, 25 Jul 2008 06:16:34 +0000

Thanks Daran and John Phillips.

I am now convinced that you are correct.

I think my problem was that I took phenomenon such as reflection-absorbtion and radiation as stand-alone independent factors.

I will still have to mull this over when I’m refreshed until I understand all issues on a fundamental level.