Vacuum temperture

[Jordan14]

I seem to be talking about vacuums constantly, but when facing another problem I hit this.

 

In order to stop a box holding a vacuum collasping inwards you could freeze the box to stop particle activity.

 

My point is what is the temperture inside the box (which is the vacuum) because you need particles to have a temperture however if there are no particle is there no temperture, which I find very hard to imagine.

[Klaynos]

This happened on IRC:

 

[19:11] <Jordan14> i posted in on the forum but is there a temperture in a vacuum

[19:11] <RICHARDBATTY> should be near absolute zero

[19:12] *** Jordan14 Quit (Quit: Jordan14)

[19:12] <Klaynos> dedends, yes there is, but only because we can't create a proper vacuum, if we could create a proper vacumm, ignoring the uncertainty principle, there could be no atoms, therefore no kinetic energy/internal energy, therefore no temperature

[19:12] <Tesseract> no

[19:13] <RICHARDBATTY> no energy or method of condution in a perfect vacum

[19:13] <Klaynos> no such thing as a perfect vacuum that we can create though :s

[19:13] <RICHARDBATTY> true

[19:13] <Klaynos> and even in vacumm there is constant creation and descrtuction of subatomic particles :S

[19:13] <RICHARDBATTY> quantum yes

[19:14] <RICHARDBATTY> agreed:)

[[Tycho?]]

Consider this. The walls of the box will be radiating photons, probably in the infrared spectrum (that of "heat"). So I think you can have a temperature just from the photons that are in the box, in the same way the cosmic background radiation has a temperature only slightly above 0 kelvin for even deep space.

 

Uh, I think.

[Martin]

...

My point is what is the temperture inside the box (which is the vacuum) because you need particles to have a temperture however if there are no particle is there no temperture' date=' which I find very hard to imagine.[/quote']

 

jordan AFAIK it is impossible to achieve absolute zero, one can get down to nanokelvins but not zero

 

so think of a box containing the best possible vacuum

Just for definiteness let's imagine that the walls of the box are temperature 273 Kelvin.

If the walls must be SOME positive non-zero temperature then they might as well be 273 Kelvin (everything I have to say is qualitatively the same at all non-zero temperature, hotter just means more)

 

by the planck radiation law, the walls radiate photons in proportion to the fourth power of the temperature, even very cold walls radiate

these walls, at 273 Kelvin, radiate infrared light

 

the vacuum will be full of infrared cavity radiation

 

it is in principle always possible to determine the temperature of thermal radiation. You can sample the radiation from a hot object and from that tell the temperature of the object.

 

You can sample the cavity radiation in a box and (even if there is nothing else there) from that tell the temperature of the box.

 

the temperature of the (typically infrared) light inside the box tells the temperature of the box

 

 

Even a very cold box contains cavity radiation. It may be so weak that no instrument on earth could detect it, but it is still there and if one could detect it this would allow the temperature to be defined

 

 

As a rule, the void inside the box is in equilibrium with the walls----the walls are emitting as much thermal radiation out into the space as they are absorbing from the empty space. Since they are in equilibrium they are the same temperature. So, as a rule, the void inside the box is the same temperature as the walls.

[swansont]

']Consider this. The walls of the box will be radiating photons' date=' probably in the infrared spectrum (that of "heat"). So I think you can have a temperature just from the photons that are in the box, in the same way the cosmic background radiation has a temperature only slightly above 0 kelvin for even deep space.

 

Uh, I think.[/quote']

 

 

Infrared isn't heat. As Martin stated/implied, things that radiate near room temperature will do so strongly in the IR, but you can heat things with microwaves and with visible light, too.