Notices
Results 1 to 53 of 53

Thread: Stopping black body radiation

  1. #1 Stopping black body radiation 
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    Is there any theoretical way to prevent an object from radiating heat? The only thing I can think of would be to artificially cool the surface to the ambient temperature and store the heat generated from this cooling in a huge heat sink inside the object. This would be a temporary solution, obviously. Are there any permanent solutions? Some way to store heat indefinitely without it escaping as EM radiation? It doesn't seem to violate any thermodynamic laws that I can think of, but it also doesn't seem possible.


    Reply With Quote  
     

  2.  
     

  3. #2  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    The dewar vessel is pretty much adiabatic

    it is a double vacuumed flask with a silver lining


    everything is mathematical.
    Reply With Quote  
     

  4. #3  
    Forum Isotope Bunbury's Avatar
    Join Date
    Sep 2007
    Location
    Colorado
    Posts
    2,590
    It rather depends on the details - how big is the object, how hot and does it generate heat internally. What kind of object do you have in mind?
    Reply With Quote  
     

  5. #4  
    Forum Ph.D.
    Join Date
    Jan 2009
    Posts
    951
    Any object above Odeg K emitts radiation. therefore cool your object to 0 k and it will quit. Otherwise blocking it is no problem simply put something that absorbs in front of it. Heat is relative,anything colder than it is will "sense" it. so the asnswer to your question is NO.
    Reply With Quote  
     

  6. #5  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    Quote Originally Posted by Bunbury
    It rather depends on the details - how big is the object, how hot and does it generate heat internally. What kind of object do you have in mind?
    Any object at all, above 0 Kelvin. All objects radiate light based on how hot they are. It's why you can feel a bonfire's heat only when you're within eyeshot of it, but not if someone else is blocking your view. It's why stars give off light. Colder objects, like people, radiate in the infrared. But all objects radiate.

    Quote Originally Posted by fizzlooney
    Any object above Odeg K emitts radiation. therefore cool your object to 0 k and it will quit. Otherwise blocking it is no problem simply put something that absorbs in front of it. Heat is relative,anything colder than it is will "sense" it. so the asnswer to your question is NO.
    Blocking the light with something else defeats the purpose. I'm thinking in terms of a "heat battery". Something an advanced civilization could build to store energy over long periods of time. But even if you vacuum seal the thing, it's still going to radiate heat. So I don't see how to build such an object. But it also doesn't violate any of the thermodynamic laws I'm aware of.

    Is there some way to theoretically position the atoms of an object such that the heat either only radiates back inside the object, or does so extremely preferentially? Or use something other than atoms? Would any exotic particles work? The only requirement is that you have to be able to retrieve the heat later for use in a heat engine.
    Reply With Quote  
     

  7. #6  
    Forum Ph.D.
    Join Date
    Jan 2009
    Posts
    951
    NO
    Reply With Quote  
     

  8. #7  
    Forum Ph.D.
    Join Date
    Jan 2009
    Posts
    951
    NO , you can move heat from hot to cold ,thats it
    Reply With Quote  
     

  9. #8  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    NO , you can move heat from hot to cold ,thats it
    not strictly true, you can put work into the system to make heat flow from a cold source to a hot source, but i guess that is moot with this discussion.
    everything is mathematical.
    Reply With Quote  
     

  10. #9  
    Forum Junior c186282's Avatar
    Join Date
    Dec 2008
    Posts
    208
    Let me generalize the question:
    Is it possible to place an object of temperature in a bath of temperature where and prevent the exchange of heat?
    I do not think the mechanism of the heat exchange effects the answer.
    One can certainly slow the heat exchange but I not not think it is possible to stop it. It is obvious for the conduction and convection cases because the "hot" molecules will hit and give energy to the "cold" molecules. Because I think your question is fundamentally a thermodynamical question and the answer is obvious with two of the heat transfer mechanisms I would suggest that it is also true with radiative cooling.

    The visualization of black body radiation has always bothered me. What is the action of the event of a photon being radiated? The derivation of black body radiation comes from solving Maxwell's equation in a "box" then quantizing the energy levels (not quantizing the light) and applying some Boltzmann statistics. The derivation is an uncomfortable combination of quantum mechanics and classical statistical mechanics called semi-classical, but it matches experiment. There is no way to stop an object at a given temperature from radiating and absorbing. This process will continue and ultimately the object and the surrounding bath will come to the same temperature where the object will radiate the same amount of energy as it absorbs.
    Reply With Quote  
     

  11. #10  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    It would seem to be impossible, but I can't think of any named law or principle saying as much. It certainly doesn't violate any fundamental physical law, other than that there's nothing that does it.

    Hmm, let's take it from another direction. Is there any way to produce a substance of extremely high specific heat? Like orders of magnitude above and beyond water? Something you could construct (somehow) at very low temperatures (a few Kelvin maybe) and use as an effectively infinite heat sink? Such a sink would allow you to extract significant energy from just room temperature.
    Reply With Quote  
     

  12. #11  
    Administrator KALSTER's Avatar
    Join Date
    Sep 2007
    Location
    South Africa
    Posts
    8,245
    If the object is, perhaps, a spaceship, then all it would have to do for camouflage would be to be the same temperature as the surrounding space, I think.

    Anyway, maybe the way to go would be to employ a contained black hole. Pump the heat away from the hull and let it radiate into the black hole? The pumping can be done actively using the power source of the vessel. Is it possible to extract energy from a black hole?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
    Reply With Quote  
     

  13. #12  
    Forum Junior c186282's Avatar
    Join Date
    Dec 2008
    Posts
    208
    An objects blackbody radiation budget is only a function of its temperature and its Emissivity. A fudge factor between just above zero and 1. Here is a list of emissivity data. Notice the lowest emissivity is for polished pure silver 0.020-0.032. So maybe the best way to prevent heat lost is to place your object in a vacuum and make it out of polished pure silver. So now I'm thinking why does silver have such a low emissivity? Because it is a good conductor. So if we took a super conductor and placed it in a vacuum where the ambient temperature is below the normal temperature for the super conductor then maybe there would be not heat exchange between the object and the environment.

    Well I after I typed the stuff above I looked around and even super conductors have non-zero emissivity. Looks like we can not hold on to the heat forever.

    KALSTER: Yes you can get energy from a black hole or any other not so exciting gravitational thing. Build a Dyson sphere around it and drop in your garbage in buckets on a rope that spin a fly wheel.
    Reply With Quote  
     

  14. #13  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    I'm 99% sure that the non-existence of a perfect insulator (which is what you're asking for) is a consequence of the laws of thermodynamics, but I couldn't say why ATM. Still, like many other things, I think you can get arbitrarily close.
    Reply With Quote  
     

  15. #14  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    My thinking is that black body radiation is still light in some sense. Is it not? Couldn't you make something that's the equivalent of a black body mirror? Something that does for black body light what a mirror does for visible light?

    Or something equivalent to a light opaque surface? These usually absorb visible light and emit black body, right? What if you something could absorb black body, and emit an even lower frequency? If your frequencies get low enough, it could start to get very difficult for anybody to detect them.

    Anyways, I'm totally just guessing.
    Reply With Quote  
     

  16. #15  
    Forum Senior Booms's Avatar
    Join Date
    Sep 2008
    Location
    The perceptual schematic known as earth
    Posts
    361
    Initially I would have to say a resounding NO, it doesn't defy any laws really but it is just not feasable with current human knowledge or technology, Theoretically a superdense material could contain a heat source indefinately, if you think about heat as it's basic structure, heat is just the movement of atomic particles it's transfered by atoms colliding and moving their kinetic force around

    So by this knowledge to 'contain' heat all you need is something so solid it will not move, that no amount of force from colliding atoms will transfer enough energy to move the atoms of said substance, Now taking out the collision theory we actually have a method to do this, Absolute Zero is a voidness of heat and thus motion, the problem being Absolute Zero is so cold it will actually derive heat from the background radiation of space meaning it would instantly absorb any heatsource we attempt to contain within it



    Any true substance will never indefinately contain heat, assuming we have a megadense element with an atomic mass 5 thousand times greater than lead, it would sure as hell hold a hotter source for a very long time, but eventually the source would transfer enough energy to some of the atoms to give the element heat and thus losing the containment





    In theory I suppose a black hole contains heat, (then again black holes are an excuse for anything nowadays) if nothing escapes black holes then neither does heat, also black holes are substances denser than matter and thus probably not bound by laws of matter like thermodynamics and Conservation of energy
    It's not how many questions you ask, but the answers you get - Booms

    This is the Acadamy of Science! we don't need to 'prove' anything!
    Reply With Quote  
     

  17. #16  
    Forum Masters Degree organic god's Avatar
    Join Date
    Feb 2008
    Location
    London
    Posts
    567
    heat is just the movement of atomic particles it's transfered by atoms colliding and moving their kinetic force around
    this is one mode of heat transfer known as conduction but other modes exist, such as radiation which is heat in the form of waves.
    everything is mathematical.
    Reply With Quote  
     

  18. #17  
    Forum Junior c186282's Avatar
    Join Date
    Dec 2008
    Posts
    208
    Here are two blackbody plots. I have rescaled the y axis such that the peak stays at the same height. Normally the area under the curve should scale with Stefan–Boltzmann law


    Blackbody as a function of frequency. The ball is close to the color an at that temperature would look.


    Blackbody as a function of wavelength.


    OK so much for the entertainment. As you can see from the plot above as an objects gets cooler the peak wavelength gets longer. What will happen if the temperature of the object is so low and the size of the object is so small that the peak of the blackbody curve is at a wavelength that is longer than the size of the object? Will it be able to radiate at wavelengths that are longer than the size of the object?
    Reply With Quote  
     

  19. #18  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    @ KALSTER: Stealthy spaceship would maybe be a application, but mostly I was thinking about how an advanced civilization would build technology which could still function in a thousand years. Which led me to think about a "heat battery" for a heat engine, and if such a thing was possible.

    Quote Originally Posted by c186282
    OK so much for the entertainment. As you can see from the plot above as an objects gets cooler the peak wavelength gets longer. What will happen if the temperature of the object is so low and the size of the object is so small that the peak of the blackbody curve is at a wavelength that is longer than the size of the object? Will it be able to radiate at wavelengths that are longer than the size of the object?
    That's definitely an interesting question

    Good replies everyone. Emissivity is the buzz word. How do we get it really, really low?
    Reply With Quote  
     

  20. #19  
    Forum Junior c186282's Avatar
    Join Date
    Dec 2008
    Posts
    208
    I found a paper that talks about blackbody radiation from small spheres. The abstract says that a particle of diameter can not radiate at wavelengths above . The full article pdf costs $31 and I can't find it elsewhere on the web so I guess I will not be reading the details of this work.
    Reply With Quote  
     

  21. #20  
    Forum Junior c186282's Avatar
    Join Date
    Dec 2008
    Posts
    208
    Well there is more to the story. I found a rebuttal to the paper I linked to earlier that lays out a strong argument. I like the consistence of the argument in the paper linked to from this post. The arguments are based of our old standard of thermal dynamics and the long trusted Mie solutions to Maxwell's equations. (Mie scattering) Basically Mie scattering is diffraction from small spheres where the circumference of the sphere is about the same size as the wavelength.

    I have found some links for Mie scattering but the mathematics is not for the faint of heart. I could not find Mie scattering in my Jackson E&M but it is in my Born and Wolf but it is a bit much for today.

    Bottom line: the cross sections for absorption and radiation of blackbody radiation is effected by the size of the particles as described by the Mie solutions to Maxwell's equations.
    Reply With Quote  
     

  22. #21  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    So, what about a heat mirror? Is that possible? Are there materials with sufficiently high/different refractive indices that they can entirely reflect black body radiation?


    I'm just asking because of the stealthy space ship problem. If you have the ability to reflect the black body radiation into whatever direction you want, then you can concentrate all of your emissions onto a smaller area of the sky without losing any net ability to cool your ship.
    Reply With Quote  
     

  23. #22  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    Blackbody radiation is light. It's light of many different wavelengths. To reflect all blackbody radiation would require a perfect mirror, which I think is also an impossibility (100% perfect, at least, especially across all wavelengths). Also, the mirror itself would have a temperature, and thus blackbody radiation of its own.
    Reply With Quote  
     

  24. #23  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    So, as you approach that limit, does its rate of absorption (vs. reflection) get smaller and smaller? If it gets small enough, then maybe the cost of cooling it would be small enough to make it practical to use anyway, even though you can't ever reach absolute perfection.
    Reply With Quote  
     

  25. #24  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    The problem with that is that the only way to cool something is to move the heat elsewhere, adding even more waste heat in the process.
    Reply With Quote  
     

  26. #25  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    Yeah, this is true. You have to move the heat off of the mirror itself. I'm assuming that the space ship is still going to radiate a lot of heat into space, but for stealth purposes, it might be possible to radiate it all into the same part of the sky if you have access to mirrors that display a very low rate of absorption.


    Basically, diminishing the amount of area you radiate the heat into dramatically limits your ability to cool yourself. ............ at least it would normally.

    Using highly reflective mirrors, you might find that focusing all of the radiation in one direction didn't limit your ability to cool yourself much at all. It depends on how hard the mirrors are to keep cool. How much heat are they absorbing (and re-emitting, if you don't cool them) vs. how much are they reflecting?
    Reply With Quote  
     

  27. #26  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    As MagiMaster points out, it's hard/impossible to construct a mirror which reflects light of all wavelengths. A bathroom mirror reflects visible light but sure doesn't reflect x rays

    I find it interesting that the emissivity of an object and its reflection ability seem related (based on that emissivity table from the Wikipedia article linked earlier). Is it possible that polishing a material to make it reflective organizes the atoms/molecules in such a way as to also inhibit the material from radiating? If you had a perfect mirror, would it also be a perfect thermal insulator in a vacuum? Or are the properties totally different?
    Reply With Quote  
     

  28. #27  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    I'm pretty sure they're related by the conservation laws. All the energy hitting the mirror must be reflected, refracted or absorbed, so if more is being reflected, less is being absorbed, so less is being emitted. Of course, I don't know enough about this to say that it's a direct relation, but there's at least an indirect one.
    Reply With Quote  
     

  29. #28  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    If I understand right, you create reflection by putting two materials together, against each other, that have different refractive indexes. If the difference is great enough, the light won't be able to pass between them and gets reflected back.

    One efficiency problem is the fact that most materials that are transparent to light are not totally transparent, so some of it is certainly absorbed instead of allowed to pass through (and consequently absorbed instead of being reflected).

    I'm not sure how much the effect depends on the wavelength of the light.
    Reply With Quote  
     

  30. #29  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    If I might, once you actually look down to what heat is, I think you will find that it is impossible to prevent heat from being radfiated.

    Heat, you see, comes from random collision between particles. Theese particles are mainly electrons, and will radiate energy when in motion, which the existence of heat implies that they must be in. Consequently, these electrons will radiate photons, so unless you can stop electrons from moving (thereby stopping heat), it is practically impossible to prevent an object from radiating heat.

    This is just an answer to Numsgil's original post in this thread.
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  31. #30  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    Quote Originally Posted by Liongold
    If I might, once you actually look down to what heat is, I think you will find that it is impossible to prevent heat from being radfiated.

    Heat, you see, comes from random collision between particles. Theese particles are mainly electrons, and will radiate energy when in motion, which the existence of heat implies that they must be in. Consequently, these electrons will radiate photons, so unless you can stop electrons from moving (thereby stopping heat), it is practically impossible to prevent an object from radiating heat.

    This is just an answer to Numsgil's original post in this thread.
    Right, but suppose you found some way to preferentially have the emitted photons be redirected back into the object. The object would be thermodynamically isolated from the rest of the universe, with no meaningful interaction.

    Actually, that makes me see that such an object would indeed have to be perfectly shiny to prevent heat from the universe from flowing in to it and super heating it and violating all sorts of thermodynamic laws.
    Reply With Quote  
     

  32. #31  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    The trouble is if you keep generating more and more heat internally. You'll have to radiate it outward at some point, or you'll vaporize.
    Reply With Quote  
     

  33. #32  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    Where would more internal heat come from? Assuming that an object was perfectly shiny and therefore not receiving any energy from the universe, I can't see how it'd heat up. Instead, it should either maintain a constant temperature, thus no radiation, or it'd quickly fall towards absolute zero, thus less and less radiation.
    Reply With Quote  
     

  34. #33  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    Right, but suppose you found some way to preferentially have the emitted photons be redirected back into the object. The object would be thermodynamically isolated from the rest of the universe, with no meaningful interaction.
    I highly doubt that would be possible. For one thing, we'd first have to construct a perfect mirror which reflects all light falling on it; impossible by any standard whatsoever, because some light is almost always absorbed.

    Further, to do this, you would have to either cause the photons to redirect their path back into the object. A black hole is able to do this; however, since it must have entropy and consequently must have a temperature, it must radiate energy too. Provided the gravity is strong enough, I envision a black hole is a perfect radiation stopper.

    However, Hawking radiation does prevent this, so not even a black hole can stop black-body radiation.


    Where would more internal heat come from? Assuming that an object was perfectly shiny and therefore not receiving any energy from the universe, I can't see how it'd heat up. Instead, it should either maintain a constant temperature, thus no radiation, or it'd quickly fall towards absolute zero, thus less and less radiation.
    It won't fall towards absolute zero, as that implies loss of energy, and consequently radiation of heat. I believe it will maintain a constant temperature in such a case, as you mention.
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  35. #34  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    Quote Originally Posted by Liongold
    Right, but suppose you found some way to preferentially have the emitted photons be redirected back into the object. The object would be thermodynamically isolated from the rest of the universe, with no meaningful interaction.
    I highly doubt that would be possible. For one thing, we'd first have to construct a perfect mirror which reflects all light falling on it; impossible by any standard whatsoever, because some light is almost always absorbed.
    I'm just trying to see if it's impossible from an engineering standpoint only, or if there's a fundamental law it would violate. From what I can tell it's just an engineering problem. There aren't any fundamental physical laws it would break.

    Further, to do this, you would have to either cause the photons to redirect their path back into the object. A black hole is able to do this; however, since it must have entropy and consequently must have a temperature, it must radiate energy too. Provided the gravity is strong enough, I envision a black hole is a perfect radiation stopper.

    However, Hawking radiation does prevent this, so not even a black hole can stop black-body radiation.
    A black hole is an interesting case, because it's not thermodynamically isolated from the rest of the universe. It doesn't directly emit radiation, but it certainly absorbs it. Actually in a limited thermodynamic sense a black hole is at absolute zero since heat would always flow into a black hole, no matter what temperature another object is. This ignores Hawking radiation of course...

    My understanding of Hawking radiation is limited. Would Hawking radiation prevent a perfectly shiny 0 emissivity object from remaining thermodynamically isolated?
    Reply With Quote  
     

  36. #35  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    Would Hawking radiation prevent a perfectly shiny 0 emissivity object from remaining thermodynamically isolated?
    If the object is not a singularity, Hawking radiation does not apply. Hawking radiation works only for immense gravitational fields, or, by the equivalence principle, as an object undergoes immense acceleration. The gravitational field of a perfect mirror, let us assume, is minuscule.

    Consequently, a perfectly reflective non-singularity-forming surface would not be bound by Hawking radiation, so, no, a perfectly shiny object can remain thermodynamically isolated.

    Actually in a limited thermodynamic sense a black hole is at absolute zero since heat would always flow into a black hole, no matter what temperature another object is.
    Absolute zero is impossible to achieve. The heat that flows in will always remain within the singularity, constantly increasing the temperature. As long as Hawking radiation remains below the amount of heat sent within the singularity, the black hole's temperature, at least from an object's point of view within the black hole, will grow. From the point of view of someone outside the black hole, of course, we will consider the black hole's temperature to be decreasing.

    I'm just trying to see if it's impossible from an engineering standpoint only, or if there's a fundamental law it would violate. From what I can tell it's just an engineering problem. There aren't any fundamental physical laws it would break.
    You would be correct, then; no fundamental law prevents us from redirecting the photons back into the object. From an engineering standpoint, however, this is impossible, as all substances will absorb at least some light, and secondly, what would be the point? That is, however, an unrelated quesiton, so never mind it.
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  37. #36  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    Quote Originally Posted by Liongold
    I'm just trying to see if it's impossible from an engineering standpoint only, or if there's a fundamental law it would violate. From what I can tell it's just an engineering problem. There aren't any fundamental physical laws it would break.
    You would be correct, then; no fundamental law prevents us from redirecting the photons back into the object. From an engineering standpoint, however, this is impossible, as all substances will absorb at least some light, and secondly, what would be the point? That is, however, an unrelated quesiton, so never mind it.
    Well, the fundamental limitation that prevents perfect reflection is the way reflection happens.

    Even in the most efficient kind of reflection ("total internal reflection") 2 objects of very different refractive indexes are put together, and if the first substance has a higher refractive index than the second, and it strikes it a certain "critical angle", none of the light will be able to cross the barrier and it all gets reflected back. Even in this case, the light is passing through the first substance, and so unless that substance is infinity transparent, some energy will be absorbed.

    For other kinds of reflection, there's always a portion of the light that is allowed to pass through unreflected, and something behind the mirror has to absorb that light.

    So, as an engineering problem, the trouble is trying to find a substance that is very nearly perfectly transparent, and then finding another substance with a lower refractive index to pair it with. (Because if it's close perfectly transparent, then it probably already has a very low refractive index)
    Reply With Quote  
     

  38. #37  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    So, as an engineering problem, the trouble is trying to find a substance that is very nearly perfectly transparent, and then finding another substance with a lower refractive index to pair it with. (Because if it's close perfectly transparent, then it probably already has a very low refractive index)
    Good point, kojax. Even then, however, the most we can do is prevent as much heat as possible from escaping; it appears from this that preventing all of the heat from escaping is impossible, unless we can construct a material whose particles do not absorb light.

    Perhaps a material made entirely of neutrons, bound together by the strong force? Of course, Im not sure if the light wi'll ever interact with the neutrons then...
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  39. #38  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    Quote Originally Posted by Liongold
    Actually in a limited thermodynamic sense a black hole is at absolute zero since heat would always flow into a black hole, no matter what temperature another object is.
    Absolute zero is impossible to achieve. The heat that flows in will always remain within the singularity, constantly increasing the temperature. As long as Hawking radiation remains below the amount of heat sent within the singularity, the black hole's temperature, at least from an object's point of view within the black hole, will grow. From the point of view of someone outside the black hole, of course, we will consider the black hole's temperature to be decreasing.
    Ignoring Hawking radiation for a moment, let's say I have a normal object at 3K and a black hole. The 3K object will radiate heat from it's black body radiation into the black hole, but the black hole can't radiate heat back out. If we define absolute zero as a state where, no matter how "cold" another object is, heat will always flow from the "cold" object to the object at absolute 0, a black hole can be thought of as absolute zero.

    Bringing hawking radiation back into the equation probably change things, I dunno.

    what would be the point?
    Suppose you're an advanced alien civilization and you want your technology to be usable thousands or millions of years in the future, because it's a scifi trope and you're a good alien civilization and play by the stereotypes. Most Human technology is extremely fragile. A hard drive has an operational lifespan of maybe a few years. A decade or two at the most. More "primitive" technology tends to be more durable for longer periods of time. A well made hammer will be usable for hundreds of years, until the wood handle starts to rot. A solid metal hammer could last for thousands of years. (Would be a pain to use though. Lots of that hammering energy would be transferred to your hand).

    So if I'm an alien civilization and I specifically want my technology to last indefinitely after I'm gone, I'm going to rely on more primitive methods. So I was thinking about a "heat battery" to store heat for a heat engine indefinitely.
    Reply With Quote  
     

  40. #39  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    Ignoring Hawking radiation for a moment, let's say I have a normal object at 3K and a black hole. The 3K object will radiate heat from it's black body radiation into the black hole, but the black hole can't radiate heat back out. If we define absolute zero as a state where, no matter how "cold" another object is, heat will always flow from the "cold" object to the object at absolute 0, a black hole can be thought of as absolute zero.

    Bringing hawking radiation back into the equation probably change things, I dunno.
    Hawking radiation will change things, certainly. For one thing, if the black hole is extremely small, it will either explode or begin to radiate huge amounts of energy. If it is large, it may take years for the absorbed radiation to be radiated out from the black hole.

    If we define absolute zero as a state where, no matter how "cold" another object is, heat will always flow from the "cold" object to the object at absolute 0, a black hole can be thought of as absolute zero.
    A better definition for this would be the lowest possible enegy state excluding zero. But your definition is pracvtically equivalent to this, so I will take it for now.

    Even then, I don't think a black hole can be thought to be at absolute zero. Here, the mechanism for heat transfer doesn't obey conventional rules; the object becomes part of the black hole, you see, and does not pass on its heat to the black hole; it becomes a part of the heat of the black hole.

    Equivalently, a black hole is obviously not at the lowest possible energy state.

    So, no, not even a black hole can be thought of as at absolute zero.

    Suppose you're an advanced alien civilization and you want your technology to be usable thousands or millions of years in the future, because it's a scifi trope and you're a good alien civilization and play by the stereotypes. Most Human technology is extremely fragile. A hard drive has an operational lifespan of maybe a few years. A decade or two at the most. More "primitive" technology tends to be more durable for longer periods of time. A well made hammer will be usable for hundreds of years, until the wood handle starts to rot. A solid metal hammer could last for thousands of years. (Would be a pain to use though. Lots of that hammering energy would be transferred to your hand).

    So if I'm an alien civilization and I specifically want my technology to last indefinitely after I'm gone, I'm going to rely on more primitive methods. So I was thinking about a "heat battery" to store heat for a heat engine indefinitely.
    A heat battery? Have you thought of using a capacitator instead? It'll store electrical energy for as long a period you wish, and you can use to gain heat for your devices. The only thing is you will have to possess a constant supply of energy after you turn it on, but an advanced civilisation should be able to do that.

    I'm not very good at electronics, however, so I might be wrong.
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  41. #40  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    Quote Originally Posted by Liongold
    A heat battery? Have you thought of using a capacitator instead? It'll store electrical energy for as long a period you wish, and you can use to gain heat for your devices. The only thing is you will have to possess a constant supply of energy after you turn it on, but an advanced civilisation should be able to do that.

    I'm not very good at electronics, however, so I might be wrong.
    Sure, a capacitor works too. I was just idly thinking of a super advanced steam punk type civilization. Something where electronics hadn't ever been invented or caught on for whatever reason. Maybe the aliens evolved on a planet where solar storms are common.
    Reply With Quote  
     

  42. #41  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    Sure, a capacitor works too. I was just idly thinking of a super advanced steam punk type civilization. Something where electronics hadn't ever been invented or caught on for whatever reason. Maybe the aliens evolved on a planet where solar storms are common.
    Odd concept. Then again, the Victorians did rely more on steam and heat than on electricity, so it isn't that odd. Although I wonder if a civilisation can ever become advanced based only on its study of heat.

    Of course, this 'heat battery' would only work to store existing heat. You will never be able to either leech out the heat within or put in any heat, so this really isn't practical anyway.
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  43. #42  
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    You'd have to find some way to crack the shell. Actually you'd have to find some way to prevent conduction, so you'd have to levitate your heat battery in a vacuum somehow, unless the magical substance can also prevent conduction. A substance with 0 emissivity and a perfect thermal insulator to boot.

    But getting at the heat would be just like getting at an orange. You mechanically rip a small hole in the shell and squeeze out the heat. Or at least that's how I eat an orange :P
    Reply With Quote  
     

  44. #43  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    You'd have to find some way to crack the shell. Actually you'd have to find some way to prevent conduction, so you'd have to levitate your heat battery in a vacuum somehow, unless the magical substance can also prevent conduction. A substance with 0 emissivity and a perfect thermal insulator to boot.
    Lol. It most probably is.

    Let us assume it doesn't prevent conduction. Then, using magnetic fields (assuming the substance is magnetic), we should be able to isolate it, and placing it in contact with a good conductor, we should be able to draw out the heat.

    Of course, since such a substance is impossible in the first place, there really is no reason to discuss this.
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  45. #44  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    Well, good scifi is about breaking as few rules as possible as carefully as possible, so it's still worth discussing.
    Reply With Quote  
     

  46. #45 Re: Stopping black body radiation 
    Suspended
    Join Date
    Mar 2009
    Location
    Plutonia
    Posts
    398
    Quote Originally Posted by Numsgil
    Is there any theoretical way to prevent an object from radiating heat? The only thing I can think of would be to artificially cool the surface to the ambient temperature and store the heat generated from this cooling in a huge heat sink inside the object. This would be a temporary solution, obviously. Are there any permanent solutions? Some way to store heat indefinitely without it escaping as EM radiation? It doesn't seem to violate any thermodynamic laws that I can think of, but it also doesn't seem possible.
    No, not really. Even at an absolute minus temperature, you still find an energy that is basically referred to as the zero-point energy field. It's absolute temperature is around , and even though one would expect everything [every quanta of area] to be essentially frozen, you are still left paradoxically half of the given energy, so for some reason, we can never properly restrict black body thermal radiation from a vacuum; now we might be able to with black holes, but that's very speculative.
    Reply With Quote  
     

  47. #46 Re: Stopping black body radiation 
    Forum Masters Degree Numsgil's Avatar
    Join Date
    Jan 2009
    Posts
    708
    Quote Originally Posted by Manynames
    Quote Originally Posted by Numsgil
    Is there any theoretical way to prevent an object from radiating heat? The only thing I can think of would be to artificially cool the surface to the ambient temperature and store the heat generated from this cooling in a huge heat sink inside the object. This would be a temporary solution, obviously. Are there any permanent solutions? Some way to store heat indefinitely without it escaping as EM radiation? It doesn't seem to violate any thermodynamic laws that I can think of, but it also doesn't seem possible.
    No, not really. Even at an absolute minus temperature, you still find an energy that is basically referred to as the zero-point energy field. It's absolute temperature is around , and even though one would expect everything [every quanta of area] to be essentially frozen, you are still left paradoxically half of the given energy, so for some reason, we can never properly restrict black body thermal radiation from a vacuum; now we might be able to with black holes, but that's very speculative.
    Are you talking about negative temperatures in this sense? I'm not sure I see how it's relevant to black body radiation.
    Reply With Quote  
     

  48. #47  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    Posts
    3,440
    That looks like a typo to me. I'm pretty sure it should have been in degrees Celsius. (That'd make sense anyway.)
    Reply With Quote  
     

  49. #48  
    Suspended
    Join Date
    Mar 2009
    Location
    Plutonia
    Posts
    398
    quite right :-D
    Reply With Quote  
     

  50. #49 Re: Stopping black body radiation 
    Forum Freshman
    Join Date
    Dec 2008
    Posts
    43
    Quote Originally Posted by Numsgil
    Is there any theoretical way to prevent an object from radiating heat? The only thing I can think of would be to artificially cool the surface to the ambient temperature and store the heat generated from this cooling in a huge heat sink inside the object. This would be a temporary solution, obviously. Are there any permanent solutions? Some way to store heat indefinitely without it escaping as EM radiation? It doesn't seem to violate any thermodynamic laws that I can think of, but it also doesn't seem possible.
    Theoretical...

    I suppose you could create some sort of time dilation on the surface of the object and have that act as a heat sink. Or you could use extremely powerful electromagnets to keep the radiation from eminating from the surface of the object, but that wouldn't limit the heat flow indefinately.
    Reply With Quote  
     

  51. #50  
    Time Lord
    Join Date
    Mar 2007
    Posts
    8,035
    Basically, hot and cold can be thought of as being similar to the positive and negative potentials in electricity. Neutral is negative, compared to a strong positive charge, just like how room temperature is cold compared to the surface of the sun.

    It's all about having contrast. You're trying to keep something hotter than the ambient environment without any of the heat escaping.

    Quote Originally Posted by Numsgil
    You'd have to find some way to crack the shell. Actually you'd have to find some way to prevent conduction, so you'd have to levitate your heat battery in a vacuum somehow, unless the magical substance can also prevent conduction. A substance with 0 emissivity and a perfect thermal insulator to boot.

    But getting at the heat would be just like getting at an orange. You mechanically rip a small hole in the shell and squeeze out the heat. Or at least that's how I eat an orange :P
    If the planet has solar energy of any kind. I mean if it has a sun, then you can always add some heat back into the system from that source.

    So, you only need to prevent leaking out more heat than what the sun can add back in, if you want the battery to last forever. And... there are other renewable sources of energy as well.
    Reply With Quote  
     

  52. #51 Re: Stopping black body radiation 
    New Member
    Join Date
    Mar 2009
    Posts
    2
    Quote Originally Posted by Numsgil
    Is there any theoretical way to prevent an object from radiating heat? The only thing I can think of would be to artificially cool the surface to the ambient temperature and store the heat generated from this cooling in a huge heat sink inside the object. This would be a temporary solution, obviously. Are there any permanent solutions? Some way to store heat indefinitely without it escaping as EM radiation? It doesn't seem to violate any thermodynamic laws that I can think of, but it also doesn't seem possible.
    I suppose if you could devise an enclosure around the radiating body for which all light incident would experience a total internal reflection. This may not be geometrically possible.
    Reply With Quote  
     

  53. #52  
    Forum Masters Degree
    Join Date
    Dec 2008
    Posts
    625
    I suppose you could create some sort of time dilation on the surface of the object and have that act as a heat sink. Or you could use extremely powerful electromagnets to keep the radiation from eminating from the surface of the object, but that wouldn't limit the heat flow indefinately.
    Time dilation wouldn't really change a thing. Electrons are already moving close to the speed of light anyway, so it wouldn't actually matter. And light, well, it isn't bound by special relativity, especially when it comes to time dilation, owing to it having no inertial reference frame.

    And how would powerful electromagnets affect the direction of the radiaiton?
    In control lies inordinate freedom; in freedom lies inordinate control.
    Reply With Quote  
     

  54. #53 Containing heat / stopping black body radiation 
    Forum Freshman vistotutti's Avatar
    Join Date
    Apr 2009
    Location
    Australia/Adelaide
    Posts
    18
    To prevent a heated body radiating it's heat away, one needs a super thermos flask, that has no leakage at all.

    What could we possibly use as such a flask?
    How about a finite universe. You may already have one of these laying around.

    Simply place heated body into finite universe; any heat radiated will (eventually) be curved back to where it came from, and so maintain the heat of the body.
    --
    There is that which is known.That which is known to be Unknown. The unknown that is unknown to be unknown... and that which is thought to be known, but is actually unknown.
    Reply With Quote  
     

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •