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Absolute Zero

  1. Oct 28, 2007 #1
    I am not sure if I should be posting this in another section, but my question is this:

    Does the uncertainty principle necessarily prevent a temperature of absolute zero occurring anywhere in the universe?

    I am no physics expert, just curious really.

  2. jcsd
  3. Oct 31, 2007 #2
    I don't know if it is the uncertainty principle, but nothing in the universe can achieve actually achieve absolute zero. You can get really close I believe the lowest temperature achieved in a laboratory was 2*10^-10 or something around there.
  4. Oct 31, 2007 #3
    why can't you reach absolute zero?
    Would time stop?
  5. Oct 31, 2007 #4


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    You can't reach absolute zero because heat flows from hot to cold. Since you can't completely insulate anything (while still allowing heat dissipation), you can't reach absolute zero.
  6. Nov 1, 2007 #5
    Would that mean that for something to reach absolute zero, it would have to be in a completely isolated system, or the entire universe would have to experience absolute zero?
  7. Nov 1, 2007 #6


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    In reality this is almost a meaningless question since "temperature" is simply not a well defined parameter at low temperatures (and strictly speaking temperature is not defined at all below 0.65K since this is the lowest point that is defined on the international temperature scale ITS-90, but that is another issue).

    I am of course not saying that you can't use the concept of concept of temperature at low temperatures. But one there is no single, well, defined parameter "T" any more.
    A good example is solid state systems where the temperature of the electrons is usually higher than the temperature of the lattice (the phonons); since the interaction times become so long at low T they can differ by several hundred mK.
    (As far as I remember the electon-phonon
    interaction time actually goes to infinity as temperature goes to zero, meaning you will never reach a true thermodynamic equilibrium).

    There are also plenty of room for confusion due to the fact that temperature is also often used as a measure of energy.
    When people talk about laser-cooled gases the temperature they refer to is in reality the "kinetic" temperature, i.e essentially the kinetic energy of the particles divided by Boltzmann's constant. However, the statistics of the gas is not given by a classical distribution so this is not a "true" thermodynamic temperature; it is just a measure of the average velocity of a (small) group of atoms.

    In some research fields people also talk about the "temperature" of radiation, this is only a "real" temperature if the radiation is coming from a black body but the word is still used even when this is not true; e.g. for a monochromatic source the "temperature" is just

    Also, the original meaning of the word "temperature" in statistical mechanics is only meaningful for ensembles; a single particle or small collection of particles can't (according to this definition) have a "temperature" which means that you can't -strictly speaking- talk about the temperature of e.g. ions in an ion trap.
  8. Nov 1, 2007 #7
    I would not necessarily jump to conclusions.

    If we were unaware of superconducters, would you believe that absolute 0 resistance was possible?
  9. Nov 1, 2007 #8


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    Superconductors don't have anything to do with absolute zero. The concepts are utterly different.
  10. Nov 1, 2007 #9
    I'm fully aware. But I'm simply stating that sometimes there is a point that we discover something that changes our thinking. I wasn't trying to relate the actual concepts.
  11. Nov 2, 2007 #10


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    The answer is yes. If an a piece of matter had a temperature of absolute zero then that would mean that all of it's atoms had zero velocity. But this is impossible because if we know with 100% that an atom has 0 velocity then according the UP we have no idea what its position is.
  12. Nov 2, 2007 #11


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    I don't agree. As I wrote above the main problem here is that "temperature" is a very fuzzy concept at low temperatures and is not really well defined. The answer would be "yes" if we defined temperature to simply be a measure of the kinetic energy of the ions in a lattice. However, the "classical" temperature is usually thought of as a measure of statistical properties of an ensemble (i.e. the how the velocities are disitributed. In this case we obviously need the full QM formalism, and the latter does not give results that agree with classical thermodynamics near 0K.

    A good example is quantum browninan motion (see e.g. Gardiner's book "Quantum Noise") which agrees well with classical theory at elevated temperatures. However, a particle will actually slowly move due to zero-point fluctuations even at zero temperature (but the effect would be masked unless the T<< 1e-16 K) but the motion does NOT follow the classical diffusion law and is obviously not caused by a "temperature" as such (since we explicitly have T=0 in the equations). Hence, the fact that particles are moving does not neccesarily mean that you have a non-zero temperature.
  13. Nov 2, 2007 #12
    No, (one answer given above was incorrect). The uncertainty principle implies that atoms in a finite volume cannot have a kinetic energy of zero. However, this only means that at T = 0 the system will be in the ground state in which the system still has some average kinetic energy.

    An extreme example of quantum effects leading to particles having very high energies at very low temperatures is degenerated matter. Electrons are so-called Fermions and have to occupy different quanum states. This means that the lowest possible energy state a system of electrons in a small volume can have is quite high. An example are conduction electrons in metals.

    If you equate the average energy of an electron at zero temperature to Boltzmann's constant times an effective temperature, you get an effective temperature of 80,000 Kelvin or so. Since even room temperature is negligible compared to this effective temperature the electrons behave at room temperature practically i the same way as at absolute zero. E.g., they practically do not contribute to the specific heat.
  14. Nov 2, 2007 #13


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    The "anything is possible" worldview sounds nice, but it isn't scientific. So just because one discovery is made that overturns our thinking about something, that doesn't automatically imply that it is going to happen for other things.

    The discovery/understanding of superconductivity results from an entire branch of physics that was missing until being discovered earlier in the 20th century. Thermodynamics, on the other hand, is a mature branch of physics.
    Last edited: Nov 2, 2007
  15. Nov 2, 2007 #14
    Aren't Bose-Einstein Condensates exactly what results when you combine the uncertainty principle with matter very near absolute zero?

    The conventional notion of absolute zero is based on extrapolating the behavior of normal matter under the action of Boyle's Law. As such, it's an oversimplification. Combine it with the UP and other phases of matter, and the notion of absolute zero no longer really applies. Is that a correct way of saying it?
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