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Thread: Temperature of a vacuum?

  1. #1 Temperature of a vacuum? 
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    Does a vacuum have a temperature as radiation can pass through? If so, what is the temperature of a vacuum?Thanks!


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    Brassica oleracea Strange's Avatar
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    Temperature is a measure of the thermal (kinetic) energy of the atoms in matter. So, no, I don't think a vacuum can have a temperature.


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    Quote Originally Posted by Strange View Post
    Temperature is a measure of the thermal (kinetic) energy of the atoms in matter. So, no, I don't think a vacuum can have a temperature.
    Yes, as Strange stated a theoretical pure vacuum can have no temperature.

    The best vacuum on Earth that can be created, however, will have some gas remaining within it (atoms/ molecules). Since according to present understandings radiation can travel through a complete vacuum so that any temperature measurement within such a vacuum will be positive. Also such a vacuum would need to be also shielded from all possible radiation and cosmic rays, which would necessarily add some temperature to it. A vacuum is also known to contain a minimum amount of vacuum energy that cannot be removed. This is called Zero Point Energy. This theoretically could also add to the energy of atoms within the vacuum, and conceivably could contribute to the temperature. In the vacuum of galactic and intergalactic space such a minimum but positive temperature will exist. In galactic space there is much starlight to create some radiation and temperature. In intergalactic space the minimum temperature conceivably produced by radiation from galactic starlight is also thought to contain radiation from the aftermath of an original Big Bang.

    Atoms and molecules can be cooled by lasers very close to absolute zero Kelvin (temperature), and some assert that even absolute zero (no temperature) can be achieved. Even a negative Kelvin temperature has assertedly been achieved (link below). But in a "pure vacuum" this would not seem possible

    A temperature below absolute zero | News | R&D Magazine
    Last edited by forrest noble; February 5th, 2013 at 03:09 AM.
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    Forum Freshman WaterWalker's Avatar
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    Ok, I get that this so-called negative temperature is actually hot temperature. Or, at least, that's how I understand it. But besides that, if they really have done this, I mean, create a machine that has 100% + efficiency, would that not border on a perpetual motion machine? If it has an efficiency of more than 100%, would that not mean that there is no energy loss?
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    Quote Originally Posted by WaterWalker View Post
    Ok, I get that this so-called negative temperature is actually hot temperature. Or, at least, that's how I understand it. But besides that, if they really have done this, I mean, create a machine that has 100% + efficiency, would that not border on a perpetual motion machine? If it has an efficiency of more than 100%, would that not mean that there is no energy loss?
    From the link:

    Matter at negative absolute temperature has a whole range of astounding consequences: with its help, one could create heat engines such as combustion engines with an efficiency of more than 100%. This does not mean, however, that the law of energy conservation is violated. Instead, the engine could not only absorb energy from the hotter medium, and thus do work, but, in contrast to the usual case, from the colder medium as well.

    At purely positive temperatures, the colder medium inevitably heats up in contrast, therefore absorbing a portion of the energy of the hot medium and thereby limits the efficiency. If the hot medium has a negative temperature, it is possible to absorb energy from both media simultaneously. The work performed by the engine is therefore greater than the energy taken from the hotter medium alone – the efficiency is over 100%.
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    Just a note about some aspects of very low or, even negative, temperatures. There is a method of cooling molecules in a gas to temperatures just above absolute zero by entraining them in what is referred to as a "supersonic jet". The supersonic jet is simply a jet of a gas which is released into a vacuum as a pulse and it is referred to as supersonic because the atoms/molecules in the jet can achieve speeds above the speed of sound at the prevailing pressure. However, the cooling is achieved not by expansion, as one might expect, but by virtue of the fact that the spread in speeds of atoms/molecules in the gas becomes very narrow corresponding to a statistical distribution at a low temperature. Temperature is not simply related to the kinetic energy of the molecules, but to the distribution of molecular velocities. Molecules moving very quickly can be at a very low temperature provided that are all moving at around the same speed and in the same direction.

    This is related to the state of "negative temperature" which is sometimes mentioned in connection with the state of population inversion which occurs in lasers. Statistically, in any system at equilibrium there will be more atoms/molecules in lower energy states than in higher energy states at any positive temperature. In a state of population inversion, as occur in laser amplifying media, there can be more atoms in an excited state than in the ground state. In terms of statistical distributions, this can only be described by inserting a negative value of temperature in the distribution function. Personally, I find this way of looking at things misleading and unnecessary as amplifying media in a state of population inversion are not in equilibrium states.
    Last edited by JonG; February 5th, 2013 at 05:55 AM.
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    has lost interest seagypsy's Avatar
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    Quote Originally Posted by Strange View Post
    Temperature is a measure of the thermal (kinetic) energy of the atoms in matter. So, no, I don't think a vacuum can have a temperature.
    I have no idea what you just said, but is it the same as saying it is the measure of heat? and since in order to have heat you have to have matter that is in an excited state? So wouldn't a vaccuum then have an absolutely absence of heat? So would therefore have a temperature of absolute zero which is like a whole lot of degrees below zero F?

    (disclaimer: I have no idea what I am talking about since I have no background in, which subforum is this thread in?)
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    Quote Originally Posted by seagypsy View Post
    I have no idea what you just said, but is it the same as saying it is the measure of heat? and since in order to have heat you have to have matter that is in an excited state? So wouldn't a vaccuum then have an absolutely absence of heat? So would therefore have a temperature of absolute zero which is like a whole lot of degrees below zero F?

    (disclaimer: I have no idea what I am talking about since I have no background in, which subforum is this thread in?)
    Temperature is just the macroscopic effect of a bunch of atoms jiggling around. Higher temperature means they jiggle more (on average) and the opposite for lower temperature. So if you had absolutely no atoms to jiggle then it's kind of meaningless to prescribe a temperature for that system.
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    Brassica oleracea Strange's Avatar
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    What he said.

    Plus ...

    Heat is a measure of energy. The same amount of heat can cause a different temperature change in different materials.
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    Forum Bachelors Degree Kerling's Avatar
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    Well, a vacuum, with boundary conditions can impose limitations of the light inside the vacuum. Since Temperature is defined by heat and antropy. And heat can be considered in optical forms. Then yes optical vacuum cavities can be considered to have some temperature. It isn't hard to realize that a fuller cavity can naturally do more work. Also at every reflection light absorbs twice it's momentum from the vacuum door walls. So there is most certainly an exchange of heat between the optical vacuum and its surroundings.
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