The Temperature of a Vacuum

What is a vacuum in physics? Does it have a temperature?
- Anonymous

Well, our idea of a vacuum is a bit of space with nothing in it. We don't know of any examples of a perfect vacuum, but know some bits of space that get pretty close. Space beyond the Earth's atmosphere isn't a bad approximation to a vacuum, but it is filled with solar wind particles, light from the sun, cosmic rays and cosmic microwave background radiation. It's probably also filled with dark matter which doesn't interact with other stuff (except gravitationally, and possibly only through the feeble weak interaction), as well as neutrinos.

If you manage to pump all the air out of a steel can, for example, you will have a vacuum in there, but there will be photons constantly radiated off of the walls and re-absorbed by them. This soup of photons will be in thermal equilibrium with the walls, and therefore will have a defined "temperature". In fact, even the deepest of deep space (outside the galaxy, for example), is in a radiation bath of temperature 3K, left over from the Big Bang. There may be other stuff, like the neutrinos, for example, which are not in thermal equilibrium with the 3K radiation because they don't interact with it, and so space may have two or more "temperatures".

But we said a vacuum is a region of space with nothing in it, and that means those photons have to go. Cooling the walls down to as close to absolute zero as you can get (and the limit here is that photons of energies that would be radiated by a wall of a cold temperature would have wavelengths longer than the size of the can -- that'll let you freeze out all of the photons) will give you a vacuum. You have to also shield it from outside sources of energy. There's little you can do about the neutrinos and dark matter -- they penetrate ordinary matter, but also don't really interact with it so to a good approximation you can neglect them.

Tom

p.s. So the answer really depends on what you mean by vacuum. If you mean what's left when all the atoms etc. are pumped out, yes it still has a temperature of electromagnetic radiation. If you want, though, you could choose to only call that a vacuum if the temperature is zero. By the way, the third law of thermodynamics says nothing can ever get to zero temperature, so by that definition there wouldn't be any vacuums.

Mike W.

(published on 10/22/2007)

why temperature go up in vacuum?
- jbeauty (age 25)
singapore

We didn’t say that temperature must go up in a vacuum. We just said that an ordinary vacuum can have a temperature due to the electromagnetic waves in it. That temperature will become the same as the temperature of the material surrounding the vacuum.

Mike W.

Lee H

(published on 10/22/2007)

What temperature is ther in vacuum? Or is there any heat at all?
- Olle (age 12)
Sweden

Hi Olle- There seems to be a glitch in our search engine, so that the terms "vacuum temperature" fail to turn up our previous answer to this question. Hence I'm posting your question as a follow-up. See the answers above.

Mike W.

(published on 01/20/2011)

So if you took an infrared camera to a vacuum would it show up as the temperature of the room or what?
- Ryan (age 13)

What will the IR camera see? It should be kept cold, so that it can more easily sense thermal IR. The IR that hits it will be largely absorbed. The time it takes to get an image is very long compared to the time it takes light to reach the camera from the opposite wall. So the image will depend not on the thermal equilibrium IR density (which depends only on temperature) but rather on the IR emissions from the wall. Those depend on both the temperature of the walls and their emissivity/absorptivity, i.e. whether they tend to absorb or reflect IR. The more absorptive, the more they will show up.

Mike W.

(published on 05/18/2011)

Sorry, let me rephrase my question. I was wondering more of what it would feel like. Let's just say hypothetically that I could stick my arm into a vacuum. What would it feel like because I just can't wrap my head around no temperature.
- Ryan (age 13 )
Canada

I think it wouldn't feel very hot or cold. Say you stick your arm into a vacuum room at some normal temperature, say 22°C. Your arm is at ~37°C internally,  somewhat cooler on the surface but still well above 22°C. So there's a net flow of radiant heat out from your arm to the room. If there were air in the room, there would also be some convective cooling of your arm by the air. So not having the air around should make your arm feel a bit warmer than it would otherwise. However, even without any cooling via sweat evaporation, heat is leaving your arm, so it will feel about like it would in a room that was warmer than 22°C but cooler than 37°C. 

If you sweat a little, it should evaporate easily (the relative humidity is zero) so it should feel cooler than our recent muggy weather here, with very high relative humidity and temperature around 34°C.

Of course the lack of pressure from the vacuum will feel weird. I'm not sure if it would pop blood vessels or something.  But that's another issue.

Mike W.

(published on 06/05/2011)

I still want to ask, what is the temperature of vacuum? The real one, with nothing in it. No elements, no radiation, no magnetic or other fields. I understand, that it's very difficult to get, but what is the temperature of NOTHING(vacuum in our case)?
- Sergei
Israel

If there is no electromagnetic radiation then the temperature would be absolute zero. The third law of thermodynamics says that's not reachable. So the vacuum you describe is not "the real one" but an unattainable ideal one. It's fair to say, however, that as the density of thermal radiation approaches zero the temperature (assuming the radiation can be described by a thermal spectrum) approaches 0 K.


Mike W.

(published on 07/23/2011)

I know there is a limit on how small a volume is that we can refer to, but still, if i look at a volume very small, smaller than the size of particles we know, wouldn't that volume be considered as vacuum, the "ideal" one you were talking about? And another thing troubles me, when we look at the position of matter as a probability function of space we find that a particle is spread all over the place, so in what sense are the walls of a vacuum cell not filling it up with themselves?
- joe (age 27)
israel

Many of our particles (electrons, muons, neutrinos, quarks,...) have no known size. That means that one could have their wave functions squashed in more and more, at least until things start to happen beyond our current laws of physics. Those would be things like the formation of tiny black holes for which the as yet undeveloped quantum theory of gravity would be needed. That's not the sort of event which would be called "nothing". So  I don't know of any way you can describe a piece of space so small that it has to be empty. Some of the books on string theory might be a good place to start looking at what people are trying to do about small-scale physics.

The walls of a box will indeed have some quantum spread, just like anything else. However, the magnitude of the quantum wave typically falls off exponentially with a characteristic distance scale of less than an angstrom, so you have effectively none in the bulk of the box.

Mike W.

(published on 07/28/2011)

Following Sergei's question regarding an "ideal" vacuum with absolutely no form of matter or energy (just plain space-fabric): I've learned that the temperature of something is the average kinetic energy of its particles, which is to say the total kinetic energy divided by the number of particles. Wouldn't by that definition the temperature of a vacuum be 0/0, and so undefined?
- David (age 17)
California

What you learned is wrong. There are certain familiar special cases where the "energy per particle" definition of absolute temperature isn't too bad. For example for an ideal monatomic gas, the average thermal energy per particle is (3/2)kT, where k is Boltzmann's constant.  However, in general that definition breaks down in serious ways. In a vacuum, if you choose to call photons particles, the very number of the particles depends on temperature, going as T³. (The average energy per photon does still go as T.) Since the number of photons only goes to 0 when T -> 0, if there are no particles of any type then T=0.

Mike W.

p.s. In many other cases, e.g. for crystalline solids, the formulation of T as energy per particle breaks down in very different ways. At low T, the thermal energy in a piece of diamond goes as T⁴, not T. The thermal energy per particle is then much less than kT at low T.

(published on 08/03/2011)

what is the exact temperature of vaccum?
- sabin duwal
nepal

If you want to insist that a vacuum have no electromagnetic radiation in it, then its temperature is 0 K. However, no such thing exists. If you want a vacuum that at least is empty of more conventional particles, then its temperature must be well under that needed to excite particle-hole pairs.  To be free of electron-positron pairs, that means T would be much less than 5x10⁹ K. To be free of neutrinos in equilibrium would require a lower temperature. Since the neutrino masses aren't known, I can't give much of a clear figure. In principle some neutrinos might be around in equilibrium at room temperature. However, neutrinos interact so slowly with ordinary matter that they won't reach equilibrium for an extremely long time.

Mike W.

(published on 09/08/2011)

to the first reply, i disagree that vacuum can have a temperature. for example, say in an enclosed vacuum, you can only measure the temperature of the measuring probe itself, caused by surrounding radiation. when we measure the temperature of something that exist, for example the human body, we are yet again trying to measure the temperature of the measuring probe, which temperature however, is in thermal equilibrium with our body. Hence i think it is pointless to say "the temperature of vacuum", although electromagnetic waves can travel through it, but that doesn't mean vacuum does indeed have a temperature.
- lawrence (age 25)
singapore

The definition of T is that 1/T= dS/dU |_V, where S is the entropy and U is the internal energy. The thermal radiation has U and it has S. When it's got a thermal spectrum, it has a defined T. We measure that T in various ways, but perhaps the most obvious is by measuring the spectrum. The temperature of remote space, for example, is 2.7255 +/- 0.0006 K.

 I don't see what the problem is.

Mike W.

(published on 04/13/2012)

Can a certain T be assigned to a vacuum?
- Charlene (age 16)
Philippines

Sometimes it can. If the distribution of different frequencies of radiation in the vacuum follows the right form, then there's a temperature. If it has a different form, then if you look at one frequency range you'd get one temperature and if you look at another frequency range you'd get another temperature.  For example, far from a star, the microwave radiation in space looks like it has a temperature of 2.72548 K. However, if you look in the visible range, the little bit of light would fit a
much higher temperature.

Mike W.

(published on 12/10/2012)

Is it possible to acheive a substantial increase in temperature of whatever is in the chamber compared to ambient temperature outsode the vacuum? Also the heat from a lightbulb, is that convection and conduction or is it thermal radiation?
- mike (age 25)
usa

Certainly you can get things very hot in vacuum chambers. That's routinely done in thermal evaporation systems to make, for example, metal films. 

The energy from an incandescent light bulb comes out both as electromagnetic radiation and as heating up of the nearby air. The radiation is similar to thermal radiation, but not necessarily with exactly a thermal spectrum. The air near the bulb gets heated largely by conduction through the glass. Once the air gets hot, the heat probably spreads more by convection than by simple conduction.

Mike W.

(published on 01/02/2013)

You said nothing can reach absolute zero because space is permeated by background radiation leftover from the big bang. Is the background radiation decreasing as the universe ages? Will it ever reach a maximum zero?
- Shane (age 22)
Linden, NJ, USA

Actually, the argument that one couldn't get things to absolute zero goes back to basic thermodynamics, long before anybody knew about the cosmic microwave background. Roughly, it's that the efficiency of heat pumps as coolers falls to zero as the temperature falls to zero. So with any heat leak, no matter how small, it would require an infinite amount of work to cool something to absolute zero.

On your main question, yes, the background temperature keeps falling as the universe expands. In principle, if nothing interesting happens, the infinite-time limit of this process will give T=0. It's possible that some sort of vacuum phase transition will interrupt that process. We don't know enough to predict the very long term with confidence.

Mike W.

(published on 04/15/2013)

So, when I hear "vacuum temperature," it means how much temperature "each" field (photon field, neutrino field, gluon field, etc.) has? and is this "field temperature" proportional to how much energy it contains per volume? Can physical concepts/measurements such as volume, pressure, temperature be used for measuring properties of "vacuum" or "fields"? for example, does vacuum or each field has "pressure"? And, if vacuum or field can never reach zero-temperature(zero energy?), does it mean the ground-state vacuum/field(w/ lowest possible energy allowed) always exerts some "pressure" no matter how cold it might get? Another puzzling thing is when photon is confined btw two very closely-placed walls, only extremely short wavelength can fit inside them. but in real experiments, I guess you do not observe such short wavelength(high-energy) photons. Why is this? or they exist there but in non-interactive form?(cannot exist in observable form??)
- Anonymous

Lots of questions, but mostly they can be answered.

1." So, when I hear "vacuum temperature," it means how much temperature "each" field (photon field, neutrino field, gluon field, etc.) has?"

That's the right idea, but except for photons, gluons, and neutrinos the known quantum fields represent quanta with so much rest mass that they are in effect at T=0 in vacuums at familiar temperatures. The neutrinos interact so weakly with everything else that they don't reach thermal equilibrium. The gluons don't form for reasons that I don't understand connected with how chromodynamics works. So it's really just the photons, until things get hot enough to start making some electron-positron pairs and so forth.

2. "and is this "field temperature" proportional to how much energy it contains per volume? "

No, definitely not. It's been understood since before 1900 that the thermal energy in the electromagnetic field goes as T⁴.

3. "Can physical concepts/measurements such as volume, pressure, temperature be used for measuring properties of "vacuum" or "fields"? for example, does vacuum or each field has "pressure"? "

Yes,

4. "And, if vacuum or field can never reach zero-temperature(zero energy?), does it mean the ground-state vacuum/field(w/ lowest possible energy allowed) always exerts some "pressure" no matter how cold it might get?  Another puzzling thing is when photon is confined btw two very closely-placed walls, only extremely short wavelength can fit inside them. but in real experiments, I guess you do not observe such short wavelength(high-energy) photons. Why is this? or they exist there but in non-interactive form?(cannot exist in observable form??)"

The effect of the spacing between, for example, conducting plates on the zero-temperature photon modes does exert a force on the plates. It's called the Casimir effect and it's been measured.

Mike W.

(published on 06/14/2013)

The "third law" implies there can be no vacuum. But, wouldn't that imply that there always has to be at least one particle within an arbitrarily defined box? Can there be empty space? Doesn't temperature require the presence of energy? I.e. the internal energy of something. Isn't temperature always the temperature of (a collection of) particles and not of space itself? If there are no particles, how can there be temperature?
- Z (age 25)
Sweden

If you count photons as particles (as we ordinarily do) then there are indeed particles in any otherwise perfect vacuum. This is a bit different from the situation you might be picturing, in which temperature is a property of some fixed collection of particles. The temperature here accounts for the existence of the particles. Cooling things down would leave fewer photons in the space.

Mike W.

(published on 11/21/2013)

If i were to create a simple vacuum in a jar by means of a vacuum pump, what would the temp. inside the vacuum be? Would it be equal to the surroundings(room temp.) or would it be lesser than that? And if i placed water in that vacuum would it boil or would i have to supply a little more heat?
- Emily (age 16)
Glasgow, Scotland

The jar may cool down a bit as you pump on it but after sitting a while trading heat with the room it should end up back at room temperature. 

The water will start to boil when the pressure gets low enough. That  will cool the water down until the boiling stops. If the pump is good it will get the pressure low enough for the remaining water to freeze before boiling stops. The water will continue to evaporate until it's all gone, with the pump sucking the water vapor out. When the water is all gone, the temperature will drift back up to room temperature.

Mike W.

(published on 09/15/2014)

If i created a partial vacuum (maybe 20 Torr) and placed a computer inside (therefore a heat source) and left the pump running would the vacuum help cool the computer? Or would the temperature increase just like it would in 1 atmosphere, perhaps even faster because of the lack of convection?
- Daniel Lewis (age 16)
York, Nebraska, United States

As you suggest, it would probably be harder to cool a computer in a partial vacuum. Most personal computers rely on a fan to cool the CPU, and with less air to carry off heat, the fan would be less effective. A separate problem is that hard disk drives (the kind with a moving arm, not solid state drives) rely on air pressure to keep the read/write head a few nanometers above the disk. They can't function in a vacuum.

If you want to cool a computer more effectively, air can only do so much. You can do better by using something with a higher heat capacity, like liquid water. In a liquid-cooled computer, water is pumped in through tubes so that it passes near the hot components of the computer. Then the hot water is pumped away, cooled with a fan, and re-circulated.

You can even cool a computer by completely immersing it in a cooling liquid. Water is conductive, so it would short out the electronics and ruin the computer—but mineral oil has similar cooling properties and does not conduct electricity. Apparently you can take pretty much a standard computer and  (although it seems like it would be messy if you needed to repair anything).

Rebecca Holmes

(published on 10/16/2014)

If temperature is the movement of particles, and absolute zero is zero movement, then if there are no particles to measure the movement of, is there any temperature at all? I don't think so, personally.
- Boe Phil (age 15)
monteal, canada

In school people sometimes say that "temperature is the movement of particles". Sometimes that's pretty close to a good definition, but in general it isn't. The general definition of temperature involves how much energy it takes to get to how many more quantum states. Zero temperature means that some system is in the quantum state with the lowest possible energy. 

As we said above, if you define a vacuum as having no particles in it at all, including particles of light, it can only exist at zero temperature. Nothing real, however, can reach zero temperature. So the more common definition of vacuum is space that has no particles like atoms and molecules. Space far from any galaxies is close to that. It does have some electromagnetic radiation in it, at a temperature of about 2.7K. 

Mike W.

 

posted without vetting until Lee returns from the Serengeti

(published on 11/06/2014)

Lets take a bell jar and put a thermometer under it. So before starting the evacuation, the pressure under the bell jar is the atmospheric pressure and the thermometer shows the current room temperature. When we now start the evacuation process an finish at a very good vacuum, which temperature will the thermometer show us? Does the temperature decreases slightly caused by flowing air out of the bell jar or is there a temperature drop caused by the evacuation which causes a pressure decrease?Hope you can help. Thanks in advance!
- Simon (age 22)
Germany

Really your two explanations sound like the same thing. The air leaves through the tube to the pump because the pressure inside is bigger than the pressure at the pump. So the air inside is doing work on the air as it flows out, because it's exerting a force in the direction of motion. The energy to do the work comes from the thermal motions of the air molecules, leaving them a little cooler.

With a typical lab vacuum, most ot the thermal energy near room temperatue will still be in the few remaining molecules, not in the thermal electromagnetic field.

Mike W.

(published on 07/06/2015)

So you say that there is some defined temperature inside a vacuum. So why is it not hot in space due to the radiation from the sun's rays and the temperature is below 0 degree Celsius?
- Sukavanan (age 16)
Chennai,India

We say that a vacuum can have a defined temperature, not that all vacuums do have defined temperatures. To have a defined temperature, the amounts of radiation at different frequencies have to follow a particular pattern known as a thermal black-body spectrum. Over a broad range of frequencies space far from any stars does follow such a pattern, with a temperature of 2.725K. The higher frequency part of the spectrum, including visible light, has far more energy from starlight than that 2.725 thermal spectrum would have, even in regions far from stars. So space doesn't have a well-defined temperature.

Near Earth, the total radiation energy density corresponds to a temperature of very roughly 300K. That's why the Earth is at roughly that temperature. The spectral distribution, however, is very far from the thermal pattern, with too much high frequency components, not enough low frequency components, and a non-uniform distribution of directions. So around here the vacuum isn't even close to having a defined temperature.

Mike W.

 

 

(published on 07/29/2015)

Ok ...... So you are saying that the temperature of vacuum night be because of small particles like quarks and muons which exhibit wave nature as well as particle nature so in this case what is the temperature a measure of in this case?????[since mass of these particles is so small we might neglect it]
- Vaishnavi Gupta (age 17)
Nagpur India

At the sorts of temperatures with which we usually deal, there won't be any significant number of quarks and muons in a vacuum in thermal equilibrium. There will only be electromagnetic waves (photons), and some neutrinos, unimportant because they interacts so weakly. In this case the temperature means the same thing that it means in all cases: 

1/T = dS/dU (at fixed V), where S is the entropy and U is the thermal energy.

Mike W.

(published on 07/24/2016)