Karen’s piece on the temperature of the universe ( http://www.straightdope.com/mailbag/mspacetemp.html ) is… interesting. But the question started with the temperature of space, & she didn’t completely answer this. Space is not cold, contrary to popular belief; vacuum has no temperature. Just ask the astronaut who almost baked on the first extravehicular spacewalk because early spacesuits weren’t reflecting enough of the sun’s light. The temperature of an object in space depends on the radiation hitting that object & how it’s reflected or absorbed.
Just how perfect is the vacuum of space, anyhow?
Depends on where you are. According to this site, interstellar space can vary from 10[sup]4[/sup] molecules/cm[sup]3[/sup] inside a molecular cloud to 10[sup]-4[/sup] molecules/cm[sup]3[/sup] if you’re in a coronal gas region. Both are a far cry from the 10[sup]19[/sup] molecules/cm[sup]3[/sup] in the air we breathe. I don’t know what the densities in interplanetary space are, though.
I just want to know if I can marry Karen. She’s like, intellectually HOT
Hubba hubba hubba, baby…I love a woman with a mind!
As to the OP, sure, a perfect vacuum with no electromagnetic radiation in it doesn’t have a well-defined temperature. But (a) there’s gas & such present in minute quantities even in interstellar space, as my link describes; and (b) even without that, there’s the cosmic microwave background to contend with.
The question is probably best formulated as follows: if I was sitting in a spacesuit in interstellar space and I allowed myself to come to thermal equilibrium with my surroundings, what temperature would I attain? This is the question that Karen (correctly, I think) answered. Of course, if you change the assumptions (say, if I was tethered to a spacecraft orbiting the third terrestrial planet from a G-class star), then I would attain a different equilibrium temperature.
So yeah, certainly if you change your location space might not be cold, just like you could get too hot if you stood too close to the fireplace in an otherwise cold & drafty cabin. That doesn’t change the fact that most of the cabin is cold & drafty.
Great article. I have nothing else constructive to add, except to quote an expert on the subject, Khan Noonian Singh: “It is very cold… in space.”
Emphasis mine.
Has no one commented on the fact that she made this unbelievable, rookie mistake*?!
*Kelvins aren’t measured in degrees, they are a real unit of energy.
So, MikeS, when someone asks a question like this–
–don’t you agree that calls for an explanation of how hot it can get in space, in the Solar System? I think it calls for an explanation of near-vacuum dynamics, where radiation counts for more relative to convection than it does in our atmosphere.
And Ant-hailer, I hope you’re kidding. “Degrees Kelvin” is an old usage, used around laymen for clarity, if nothing else.
foolsguinea I disagree with your assumption that a vacuum cannot have a temperature. An object (say, a thermometer) in a vacuum will come into thermal equilibrium with the vacuum at different temperatures depending on the amount of radiation present. If there is no radiation present, that just means you have a vacuum at 0 Kelvin.
Here’s a Wikipedia entry that talks about this a bit.
The article should be changed to use just “Kelvins” without the degrees. That is the correct usage and has been for quite a while. The Kelvin scale is already explained in the next sentence, so layman should be able to figure it out.
If you put an object into a vacuum with no other particles/waves, it will eventually radiate way all of its thermal energy. So a perfect vacuum has a temperature of 0 K. Or, another way of looking at it is: how many states does a perfect vacuum have? Exactly one, and since the temperature is proportional to the logarithm of the number of states available, the temperature must be zero (log 1 == 0).
Not necessarily. The primary thrust of the question was whether the Universe has a well-defined temperature “in the middle of nowhere”, away from the Solar System; and the questioner already seemed to understand that the presence of the sun would throw things off considerably.
Radiative cooling in a vacuum is interesting, I’ll grant you that, but it wasn’t the primary thing the question asked about and there’s certainly a bonus to keeping Staff Reports from being long-winded. Maybe you can convince Karen to do a follow-up column on it. 
The entropy of a perfect vacuum is zero by this argument, yes, since that’s what’s proportional to the number of states available to the system at a given energy, S(E[sub]0[/sub]) = k (log N)|[sub]E=E[sub]0[/sub][/sub]. The temperature, though, is still undefined: normally the temperature is defined via 1/T = dS/dE, but S isn’t a well-defined function of E and thus doesn’t have a derivative.
The kelvin scale is commonly (though not absolutely accurately) referred to as “degrees”. People also refer (incorrectly) to “photoshopping”. There is a tradeoff to be made between pedagogy and pedantry.
Mind you, you yourself are making an “unbelievable” mistake in confounding energy and temperature. Remove the beam.
Oops! You are right.
Kelvin is a measurement of temperature. Temperature is a manifestation (result) of energy, not a measure of energy itself. You measure energy in joules (metric) or BTUs (English) as conventionally convenient units of choice. Saying Kelvin is a measurement of energy is like saying something like my oven generates 350F/hr …it just doesn’t quite work that way! - Jinx
And some of us are old enough to have been (correctly) taught “degrees Kelvin”. Lopping off the “degree” is a relatively recent innovation.
Not quite. While temperature is not the same as energy, once can compare them using Boltzman’s constant as a conversion factor. There is, however, no natural way to convert between degrees/hr and degrees.
However, regardless of which is standard, the distinction between “Kelvins” and “degrees Kelvin” is nothing more than pedantry, as either term unambiguously refers to the same thing.
… except, of course, if I torture/interrogate you with a red-hot poker (manufactured by Kelvin Company), that would be giving you the third degree for several hours, and you wind up with first degree burns, then that would have converted … um… nemmind.
But the pedants are right!
The Master wrote the prequel: Would a glass of water in space freeze or boil?, which I assumed everyone was familiar with.
My purpose was to describe the CMB and hype the COBE/WMAP results (no affiliation, I just think they are cool.) Also to elevate people’s understanding of temperature beyond “how fast molecules move”, namely measuring temperature using the concept of Black Body Radiation even in the absence of molecules and other objects. The concept of “the temperature of space” is very different than the concept of “the temperature of objects in space”.
Secondly, science geeks concern themselves with science concepts. Non-science geeks concern themselves with terminology. Hey, I say “microns” hundreds of times a day. Sue me.
Thirdly, DirkGntly : Let’s talk.
I’m not sure what y’all decided. Is it cold or not?
According to my brother-in-law who works at NASA in the space suit division and has been there for 20+years, the short answer is…it is very cold.
NO temperature? Theoretically maybe.