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2. law of thermodynamics wrong afterall?

  1. Oct 17, 2009 #1
  2. jcsd
  3. Oct 17, 2009 #2

    berkeman

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  4. Oct 17, 2009 #3
    A lens and black paint can not decrease entropy. However a circular device that bends all light towards it's center even diffuse light and thereby concentrates it, is decreasing the entropy of the light. If you did that with infrared light you could create a heat gradient in a room without using any energy.
     
  5. Oct 17, 2009 #4
    You do realise that the sun is providing the energy dont you. And that if you consider the sun as part of the system entropy increases.

    2nd law, closed system only. Ringing any bells?
     
  6. Oct 17, 2009 #5

    Dale

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    The core converts the energy to heat which then radiates back out, the changing permittivity won't stop that. The entropy increases as normal.
     
  7. Oct 17, 2009 #6
    Let's say there is a closed and perfectly insulated room. In that room the temperature is exactly the same at every point. Then I put this device in there and I built this thing so that it works in the infrared range.
    Now if I understood the article correctly the temperature at the core of the device will be higher then in the rest of the room because all the IR photons in the room that hit the outside shell will be directed towards the core.
    So where did I misunderstand the article?
     
  8. Oct 17, 2009 #7
    There are two questions about this device

    1)Can it replace large mirrors as claimed

    2)Can it concentrate electromagnetic radiation

    The answer to (1) is fairly obviously no, there's no way that it can 'reach out and grab' light which doesn't hit it (nor do black holes outside 3/2 schwarzchild radii).

    (2) is more of a problem. DrZoidberg is right that no device can concentrate energy which is evenly distributed. I think that this means that the device couldn't work just by using optics. But a photoelectric cell could generate electricity from light and use it to light a bulb which was locally brighter that the incoming light. It's possible that the 'metamaterials' mentioned in the article might be able to do the same sort of thing. This doesn't break the 2nd law of thermodynamics, since the energy is not evenly distributed, the effective radiation temperature of the light is much higher than the ambient temperature.
     
  9. Oct 18, 2009 #8

    Vanadium 50

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    I just read the paper of Cheng and Cui, and the word "entropy" does not appear at all. They make no claim like this.

    The way a lens works is that you have a material of constant index of refraction and varying thickness. The way this device works is that you have a material with constant thickness and varying index. It's quite clever, but one should keep in mind that it reproduces something very familiar - a lens - in a novel way. It's no more a threat to the 2nd Law than an ordinary lens is.

    I would suggest that anyone who wants to claim this device violates the 2d Law to do the calculation. There's lots of handwavy arguments, but nothing quantitative. Define a closed system, calculate the entropy at one time, calculate it again at a later time, and show that the second value is smaller.
     
  10. Oct 18, 2009 #9

    Dale

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    First, the article is not a technical or scientific publication. It is "pop sci" journalism, so you have to avoid the "black hole" claims and pay attention to the description of the device itself. The device described is simply an interesting lens. There is simply no way for a lens to violate the 2nd law of thermo because for every ray that takes heat from the room to the core there is a ray that takes heat from the core to the room. If you put a lens in a situation of thermal equilibrium then it will remain in thermal equilibrium.
     
  11. Oct 18, 2009 #10
    Where can I read the paper published by Cheng and Cui?
    The question is whether this paper actually states that completely diffuse em radiation is concentrated by this "lens" without the use of energy (i.e. without increasing entropy somewhere else). Because that should be impossible.
    If you use a solar cell to convert diffuse light into electricity and then power a lamp with it, you effectively concentrated the light. However this only works if the temperature of the solar cell is lower then the temperature of the light(which depends on it's wave lenght) since then you have a heat gradient. Drawing energy from a heat gradient increases entropy. Electric energy is energy with very low entropy. That means it is necessary to increase the entropy of the rest of the world to create it.
    So the point is - since concentrating diffuse light decreases it's entropy it can only be done by increasing the entropy of something else. This "black hole lens" seems to violate that. But maybe the article was simply describing it incorrectly.
     
  12. Oct 18, 2009 #11

    Vanadium 50

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    Go to the New Scientist page describing the result, and at the very bottom it has a section called "Journal references:". That contains the journal references.

    Half of the rest of your message is devoted to what the journal article might say, and the other half explaining why that's wrong. I suggest you read the article. "I haven't read it, but I am sure it's wrong" is not an easily supportable position.
     
  13. Oct 18, 2009 #12
    Thanks.
    I found the paper on http://arxiv.org/abs/0910.2159v1
    and it confirmed what I was thinking. The photons get concentrated. Close to the core there is a higher concentration of photons then outside of the device.
    On page 5 it says
    So since all photons get directed towards the core independent of the direction they travel in, it should concentrate diffuse light.
    If you design this "black hole" to work for infrared light and you put it inside a closed, insulated room, what exactly will happen? What will be the temperature of the core? The same as the room?
     
  14. Oct 18, 2009 #13

    Vanadium 50

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    DrZoidberg, the only person who is claiming this violates the 2d law is you - certainly not the authors. I think it's time that you backed up this claim: Define a closed system, calculate the entropy at one time, calculate it again at a later time, and show that the second value is smaller.

    When you do this, you should pay particular attention to DaleSpam's observation that every path that goes from the outside to the inside also goes from the inside to the outside.

    By the way, did you miss the abstract of the paper when you found that quote? They call this black hole a "thermal emitting source". Emitting.
     
  15. Oct 19, 2009 #14
    Right. I'm claiming that this device - the way it's described in the paper - violates the 2nd law. And I'm wondering if anyone here can spot an error in my thoughts or in the paper.

    Why so complicated? That a photon density gradient possesses a lower entropy then uniformely distributed photons is obvious. The same is true for heat.
    All I was asking is if this device can really create a photon density gradient when hit by completely diffuse light without requiring any additional energy for the job because it shouldn't be able to do so.
    I did a google search and found this site

    [crackpot link deleted]

    It also deals with the concentration of diffuse light and mentions the violation of the 2nd law.
    It's hard to believe that it might be possible to circumvent such a fundametal law. However I just can't find any error on that website nor in my thoughts about this "black hole". So maybe you could tell me where exactly the error is? What did I overlook?

    Of course the amount of energy going in will be equal to the amount coming out. But that's not the point.
    In a battery the amount of electrons going in is equal to the amount coming out but there is still a potential gradient.
     
    Last edited by a moderator: Oct 19, 2009
  16. Oct 19, 2009 #15

    russ_watters

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    Do you understand why a simple convex lens does not violate the 2nd law? This device is not fundamentally different.

    Your posts are full of badly wrong logic, assertions and examples and read like they are intentionally dense/argumentative. If you are really interested in learning stop with the dismissive hand waving and start listening to what people are telling you.
     
  17. Oct 19, 2009 #16
    I searched for something which might help to illuminate the issue, but came across

    [crackpot link deleted]

    This describes a lens systems which concentrates diffuse light, and the author is quite clear about it - it breaks the second law of thermodynamics
     
    Last edited by a moderator: Oct 19, 2009
  18. Oct 19, 2009 #17

    Vanadium 50

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    That's because he's a crackpot.
     
  19. Oct 19, 2009 #18
    This device behaves very different from a normal lens.
    In my last post I already said that I would appreciate someone pointing out where exactly the error in my logic is. And your answer to that is simply that there are errors in my logic.
    How does that make sense?
     
  20. Oct 19, 2009 #19

    Vanadium 50

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    I'm closing this thread. The authors are making no claim anything like the one being brought up here, and now people are linking to crackpot sites in support of their views.

    It's over.
     
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