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2 unrelated questions

  1. Mar 1, 2006 #1
    i could be horribly off so if i am please excuse me

    A) what is the temperature of one atom?
    i mean temperature is the vibrating of atoms because they are hitting one another. But what if there is only one, is the temperature then the speed of the atom?

    B) this one i could be way off
    ok the way i see electromagnetic waves affecting a particle is kind of like a sea of waves and a bouy.
    the particle is the bouy and when a wave comes by, it sways the particle back and forth therefore transfering energy
    unfortunately my model doesnt work for neutrons, since they have no charge, they would be "invisible" to the electromagnetic waves.

    please tell me where i went horribly wrong
  2. jcsd
  3. Mar 1, 2006 #2


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    Can you tell me what is the average of one number, and if such an average makes any sense if you were looking for something that is supposed to follow a maxwell-boltzmann distribution?

    After you answer that, to see what is the connection to your question, look up the thermodynamic definition of "temperature", and what kind of a distribution that is used to define such a quantity.

  4. Mar 1, 2006 #3
    As you point out, your idea fails with neutral entities. Only particles which couple to the electromagnetic interaction will interact with photons. Unfortunately it seems as fact of quantum theories that your preconceived notions tend to end up tosh I'm afraid.
  5. Mar 1, 2006 #4
    ah but there are those who argue (in the field of laser-cooling) that a single atom *can* be said to have a temperature. For part of my master's project I showed that it is reasonable think of a single atom in a state of optical molasses as having a temperature. (it wasn't published but I can forward the relevant section if you're interested)

    The general argument is the following:

    we can view an atom in a state of optical molasses, as analogous to an atom in a state of Brownian motion (a system that certainly does have a temperature) [Reif F 1984, Fundamentals of statistical and thermal physics]. Both experience a resistive force slowing their motion, proportional to their velocity, and both experience random contributions to
    their kinetic energy, one through random photon absorption and emission, and the other through random collisions with other atoms.

    In such a scenario the equilibrium temperature of the atom is given by T_eq=hbar*(K^2 + 4*D^2)/(8*k_b*D) where k_b is boltzmanns constant, K is the linewidth of the transition involved and D is the detuning of the beam to that transition.

    We interpret the Maxwell-boltzmann distribution as giving the probability that the atom has a particular speed.

    So skywolf in answer to part A) I would say that you asked a good question and didnt go horribly wrong at all.

    In answer to part B): Essentially the problem is about degrees of freedom. An ocean wave affects objects with geometry and mass (like a buoy). Basically an Electro-Magnetic wave can only interact with charges or with magnetic dipoles. I don't think a neutron has either of those properties, but i'm not very sure, I remember one of my teachers at uni telling me about some research measuring some dipole in a neutron... I but may be getting confused
    Last edited: Mar 1, 2006
  6. Mar 1, 2006 #5


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    But in a laser cooling, and EXTERNAL field needs to be included into the whole system. There's no such thing in the OP. If you think that this is a relevant case for this particular thread, then you're reading a lot more into it than I did.

    Look, we can ALWAYS make many arguments that is the exception to anything by extending the question into areas in which even the orginator could not even imagine. I can easily drag this into the ridiculous 1-D spatial confinment and argue that the atom "fractionalizes" and go figure the "temperature" there.

    At what point do we stop and answer the question at the level that the OP can understand?

  7. Mar 1, 2006 #6
    Hmm if you read my post a bit more carefully Zz you´ll see that I cite a reference to a proof that a single atom in Brownian motion (i.e one of many atoms hitting one another as stated in the OP) does have a temperature... so my post was relevant I think.

    In fact I think I provided a direct answer to both his questions, (gave a Temp equation for a single atom and, well, no need to explain B further)

    I was attempting to answer the question in a full manner, although admittedly I may have gone too far (sorry if I have). The thing is I suspected skywolf might have ended up thinking that only a system with many atoms can be said to have a temperature, (which I think is untrue) and wanted to encourage what I thought was perhaps a good intuition, on his part, for physics. In other words I think the OP wil benefit to some extent from my post, perhaps. He can always ignore it :P

    Further I thought it was an interesting question about what "temperature" is. And I think its ok to discuss interesting questions in PF no? Is a thread only for the benefit of the OP?

    Respectfully, alfredblase
    Last edited: Mar 1, 2006
  8. Mar 1, 2006 #7


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    You cannot force something to run when that person is only learning how to crawl. When you have spent more time on a forum such as this, when the question is vague in terms of the level of knowledge of the person asking, you FIRST send out feelers on what that person actually knows.

    Think about it. If the originator does not understand the BASIC of statistical mechanics, how appropriate do you think is your answer? All of us here would love to dazzle someone else with our knowledge. However, this does nothing in trying to answer someone's question in an understandable manner. You can't tell someone about laser cooling if that person doesn't already understand statistical physics. That makes no sense. It is why I asked those questions first and see if the OP actually can answer such a thing. Only THEN can you build on top of something that person already knows.

  9. Mar 1, 2006 #8
    ok you are correct. Your way of answering was probably best. I apologize for butting in. I guess now I understand why people always say sorry for butting in. And yes I admit there was the "dazzling" factor involved, sorry about that too.
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