The Temperature of a Vacuum
Most recent answer: 10/22/2007
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.
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.
(published on 10/22/2007)
Follow-Up #1: vacuum temperature
- jbeauty (age 25)
(published on 10/22/2007)
Follow-Up #2: vacuum temperature
- Olle (age 12)
(published on 01/20/2011)
Follow-Up #3: seeing IR in a vacuum
- Ryan (age 13)
(published on 05/18/2011)
Follow-Up #4: feel of vacuum temperature
- Ryan (age 13 )
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.
(published on 06/05/2011)
Follow-Up #5: temperature of vacuum
(published on 07/23/2011)
Follow-Up #6: too small for particles?
- joe (age 27)
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.
(published on 07/28/2011)
Follow-Up #7: defining vacuum temperature
- David (age 17)
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)
Follow-Up #8: exact vacuum temperature?
- sabin duwal
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.
(published on 09/08/2011)
Follow-Up #9: vacuum temperature measurement
- lawrence (age 25)
I don't see what the problem is.
(published on 04/13/2012)
Follow-Up #10: temperature of vacuum
- Charlene (age 16)
much higher temperature.
(published on 12/10/2012)
Follow-Up #11: heating in vacuum
- mike (age 25)
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.
(published on 01/02/2013)
Follow-Up #12: absolute zero and CMB
- Shane (age 22)
Linden, NJ, USA
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.
(published on 04/15/2013)
Follow-Up #13: temperature and fields in vacuums
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"? "
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.
(published on 06/14/2013)
Follow-Up #14: photons and temperature
- Z (age 25)
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.
(published on 11/21/2013)
Follow-Up #15: vacuum temperature
- Emily (age 16)
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.
(published on 09/15/2014)
Follow-Up #16: Cooling a computer in a vacuum
- 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).
(published on 10/16/2014)
Follow-Up #17: temperature of vacuum
- Boe Phil (age 15)
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.
posted without vetting until Lee returns from the Serengeti
(published on 11/06/2014)
Follow-Up #18: temperature drop as jar evacuated
- Simon (age 22)
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.
(published on 07/06/2015)
Follow-Up #19: temperature of space?
- Sukavanan (age 16)
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.
(published on 07/29/2015)
Follow-Up #20: particles and vacuum temperature
- Vaishnavi Gupta (age 17)
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.
(published on 07/24/2016)