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Gravity .vs. Entropy

  1. Jan 2, 2009 #1
    I'm trying to understand why the formation of a new star from the surrounding space/dust does not violate the 2nd Law of Thermodynamics.

    From Wiki...2nd Law of Thermodynamics...states that the total entropy of any isolated thermodynamic system tends to increase over time, approaching a maximum value

    So, why doesn't the gravitationally induced formation of a star (or planet/moon for that matter), which would sure seem to decrease the entropy of the system not violate this law.

    Again from Wiki...Entropy is the only quantity in the physical sciences that "picks" a particular direction for time, sometimes called an arrow of time. As we go "forward" in time, the Second Law of Thermodynamics tells us that the entropy of an isolated system tends to increase or remain the same; it will not decrease. Hence, from one perspective, entropy measurement is thought of as a kind of clock.

    Thanks in advance.
     
  2. jcsd
  3. Jan 2, 2009 #2
    Sure, gravity might decrease the spatial disorder amongst dust particles (assuming they started out in an improbable state of low-velocity and homogeneity), but it also increases the disorder in momentum (things speed up as they fall). Entropy is constant.

    And then, the only way that gravitational collapse will asymmetrically proceed further is if the system can radiate heat to (increase the chaos of) its cooler surroundings. Entropy only decreases locally, not in total.
     
  4. Jan 10, 2009 #3
    Example of 'Uphill Energy Flow'

    Thanks cesiumfrog, however, I would like someone to address the 'uphill flow of energy' that is demonstrated by star formation from dust clouds under the force of gravity (or curved space if you prefer).

    It should be clear that the system has gone from one which would be very difficult to extract energy from, to a system which has 'concentrated energy' and is now actively radiating mass quantities. In other words, there has been an 'uphill flow of energy'...from the dispersed to the condensed, with no external energy input other than the force of gravity.

    Exactly where in physics is this 'uphill flow of energy' described, that allows for the formation of systems 'which concentrate 'energy' from the high entropy, unusable, dispersed state to the low entropy, readily tapped, condensed state?

    To say that they will balance eventually, via radiation, ignores the condensation that has occurred.

    Here is a quote from Roger Penrose....
    “We seem to have lost those most crucial conservation laws of physics, the laws of conservation of energy and momentum!” [Penrose then adds the Killing symmetry arbitrarily, to get conservation again, when the Killing vector applies and gravity is separated.]. “These conservation laws hold only in a spacetime for which there is the appropriate symmetry, given by the Killing vector ĸ…. [These considerations] do not really help us in understanding what the fate of the conservation laws will be when gravity itself becomes an active player. We still have not regained our missing conservation laws of energy and momentum, when gravity enters the picture. ... This awkward-seeming fact has, since the early days of general relativity, evoked some of the strongest objections to that theory, and reasons for unease with it, as expressed by numerous physicists over the years. … in fact Einstein’s theory takes account of energy-momentum conservation in a rather sophisticated way – at least in those circumstances where such a conservation law is most needed. …Whatever energy there is in the gravitational field itself is to be excluded from having any representation…”

    From... Roger Penrose, The Road to Reality, Alfred A. Knopf, New York, 2005, p. 457-458.
     
  5. Jan 10, 2009 #4
    Well, there is actually an increase in the microstates/entropy of the system. The matter is originally rigid dust particles and so forth (fewer microstates). From the agglomeration and associating interactions, there's an increase in molecular kinetic energy. A star is pretty "hot." I suppose that's a simple way of looking at it.
     
  6. Jan 11, 2009 #5
    So, Higher Entropy = MORE Energetic, in this case

    I've always understood the Laws of Thermodynamics to mean that a 'closed system', (in this case the dust clouds) could not self-assemble itself into a 'more energetic' system'. In other words, I equated increased entropy with decreased energy concentration.

    I think many others make this same assumption, and in fact, I think this is very strongly implied in the Laws of Thermodynamics, if not explicitly stated.

    I would say...there are NO closed systems in the universe, since Gravity and the quantum vacuum permeate it all, and in this case, clearly shows that a 'more energetic system' CAN self-assemble under the force of gravity (or curved-space of gravity, if you prefer). If there are no closed systems, what actual validity do the Laws of Thermodynamics have?

    Show me a closed system that is NOT exposed to the forces of gravity and the fluctuations of the quantum vacuum.

    WHERE is a closed system ?
     
  7. Jan 11, 2009 #6
    Re: So, Higher Entropy = MORE Energetic, in this case

    Well, you also have to remember that it's referring to a naturally-occurring spontaneous process. A closed system is one in which mass can be exchanged but not energy; an isolated system is a closed system that does not exchange energy with the surroundings.

    Increasing entropy is really not related to "energy concentration."
     
    Last edited: Jan 11, 2009
  8. Jan 11, 2009 #7
    Closed, Isolated....Neither Actually Exists

    From Wiki...
    CLOSED SYSTEM
    A closed system is a system in the state of being isolated from its surrounding environment. It is often used to refer to a theoretical scenario where perfect closure is an assumption, however in practice no system can be completely closed; there are only varying degrees of closure.

    ISOLATED SYSTEM
    In the natural sciences an isolated system, as contrasted with a open system, is a physical system that does not interact with its surroundings. It obeys a number of conservation laws: its total energy and mass stay constant. They cannot enter or exit, but can only move around inside. An example is in the study of spacetime, where it is assumed that asymptotically flat spacetimes exist.

    Truly isolated physical systems do not exist in reality (except for the universe as a whole), because, for example, there is always gravity between a system with mass and masses elsewhere. However, real systems may behave nearly as an isolated system for finite (possibly very long) times.

    Language is important, especially in science, and when these laws are predicated on definitions that do not exist in the real world, and used as examples of the 'absolutes' of physics, one has to question their preciseness. I will grant that they may be valid, within their own 'unreal' constraints, but the fact remains...Gravity produces highly energetic, self-assembling concentrations of mass/energy (stars), which for all intents and purposes are 'perpetual motion' machines within the confines of our lifetimes.

    To say that one cannot 'violate the laws of thermodynamics' when they are predicated on systems that DO NOT PHYSICALLY EXIST, is not really a 'scientific' statement about the nature of the REAL world.
     
  9. Jan 11, 2009 #8

    Mapes

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    Re: Example of 'Uphill Energy Flow'

    Magical, read cesiumfrog's post again. The system is not isolated by any approximation. It is radiating heat, which increases considerably as compression occurs. This increase in entropy more than balances the entropy reduction from compressing matter.

    You're misinterpreting cesiumfrog's post. Entropy flows don't "balance eventually." Total entropy increases constantly due to entropy production during the emission and absorption of radiation. Otherwise the process wouldn't be spontaneous.
     
  10. Jan 11, 2009 #9
    Here's how Brian Greene explains it (FABRIC OF THE COSMOS, PG 172+)

     
  11. Jan 11, 2009 #10
    Re: Closed, Isolated....Neither Actually Exists

    You can define the system however you want: the dust cloud particles along with the space-time manifold inherent to their existence and location and all associating properties of inertia. You can certainly include all the properties of inertia and consider them in the "isolated" system. In which case, entropy does increase as the formation of a star occurs. The thermal energy is greatly increased which is directly related to the change in entropy.
     
  12. Jan 11, 2009 #11
    being squeezed does not equate to lower entropy.

    Just think about ice, when frozen, it expands. So, does it mean ice has more entropy?

    Also, people seem to neglect what entropy really is and instead just talk about a general idea of "orderness".

    Indeed, there is an equation that describes how entropy changes,
    [tex]\frac{dS}{dt}=\frac{\delta Q}{dt}=\frac{1}{T}\frac{d U}{d t}+\frac{P}{T}\frac{dV}{dt}-\frac{\mu}{T}\frac{dN}{dt}[/tex]

    so, even if dV/dt is negative, dU/dt, the kinetic energy of the molecules increase dramatically, making dS/dt ≥ 0. Most of these so called violations of second law can be easily refuted by just looking at the actual equation describing entropy.
     
    Last edited: Jan 11, 2009
  13. Jan 11, 2009 #12
    Entropy .vs Energy

    Here is the key point I'm trying to understand.

    The Laws of Thermodynamics are most often cited as the reason that a 'perpetual motion machine' (PPM) cannot be created, but at the same time the laws are predicated on the 'closed' and 'isolated' terms used to 'qualify' their range of applicability.

    'Increasing Entropy' is cited as the reason that a PPM can't be created, however, it is clear from 'reading the fine print', that:
    1) Isolated and closed systems do not exist
    2) The concept of 'Increasing Entropy' actually does not preclude the formation of systems with an Increased Energy Concentration (GRADIENT), specifically under the force of gravity, and possibly by extraction of the kinetic energy of the quantum vacuum.

    The way in which these laws are written means they flat out, do not apply to the formation of a star under gravity, and would also not apply to a system that extracted energy from the vacuum.

    Therefore, because of the definition of closed and isolated systems, which do not exist in the real world, it is invalid to cite the Laws of Thermodynamics as the reason why a PPM can't be created, when nature is effectively doing it via star formation, to which these laws do not apply.

    The universe is a Dynamically Balanced System, which oscillates between concentration and dispersion, via the balanced forces of gravity and radiation. It is not a 'one way' system, as the laws of thermodynamics imply with qualified implications that it is becoming more random.
     
  14. Jan 11, 2009 #13
    Here's a general statement of the Second Law:

    The total entropy of any system plus that of its environment increases as a result of any natural process.

    It is up to you to determine and specify all the participants in the system. For practical purposes, the time-scale also helps us to classify something as "isolated" or "closed."
     
    Last edited: Jan 11, 2009
  15. Jan 11, 2009 #14
    Magical, I do not know where you get your "fine prints", but saying how thermodynamics is impractical just because some fine print says so is rather silly. Perhaps you can show us directly which equation (you think) breaks down and which assumptions are wrong? Also, thermodynamics applies to isolated system, closed system and open system just as well.


    Thermodynamics only assumes some very mild statements about the systems (Ergodic hypothesis, energy conservations, continuous change in the system and some other things). Entropy is defined as klnΩ (non-quantum mechanical), where Ω is the total number of possible configurations given certain constrains. This can certainly be defined in a system with gravity. Also, gravity does not extract energy from the vacuum, it extracts energy from the gravitational potential. I suggest you read some standard textbooks on thermal physics before making claims that seem to come from nowhere. The laws of thermodynamics come from logical deductions (from some basic assumptions) and are not just some kind of quotes or speculations.
     
    Last edited: Jan 11, 2009
  16. Jan 11, 2009 #15
    Its the definitions and extrapolations

    What I am pointing out is ---> the conclusion that the laws of thermodynamics preclude the existance of a PPM is invalid because the laws (as spefically written) do not account for the existance of either gravity or quantum fluctuations.


    You misquoted me...I never said, 'gravity extracts energy from the vacuum'. What I said was, 'and would also not apply to a system that extracts energy from the vacuum.'

    I never said the laws were untrue within the confines of their limited and non-physical reality definitions. I said 'because of the qualifications on the extent of their application to closed or isolated systems, the laws do not apply outside of that realm.'

    I suggest you show me exactly where and how the laws of thermodynamics prove that a PPM cannot exist, in light of the fact there are no closed or isolated systems. And don't try and say a star is not perpetual, because that is a distinction without a difference, when they last for billions of years.
     
  17. Jan 11, 2009 #16
    Re: Its the definitions and extrapolations

    Did you miss his statement?

     
  18. Jan 11, 2009 #17
    Re: Its the definitions and extrapolations

    Your confusion seems to be stemming from a lack of understanding of more thermodynamics. Basically, no engine can be 100% efficient because thermal energy is lost which cannot be used for useful work.
     
  19. Jan 11, 2009 #18
    Not QUITE

    I understand perfectly well how there are always losses...oops, even that isn't true...a superconducting wire will circulate the current essentially indefinitely...a PPM, since electrons have mass.

    I think your confusion, if you have any, stems from the fact that you do not make the distinction between a closed/isolated system and an open system.

    I think I've stated my case clearly enough for anyone who is listening to understand exactly what I am saying:

    The Laws of Thermodynamics do not preclude the aggregation of energy and creation of a gradient via external sources (gravity, quantum fluctuations) and once a gradient is created, energy can then flow, and work can be done. Simple as that...happens everywhere in nature...SPONTANEOUSLY. Its called cycles.

    Even rocks are making voltage...look at petrovoltaics....all it takes is a little natural rectifier...We must be smarter than rocks... :smile:
     
  20. Jan 11, 2009 #19

    Mapes

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    This is turning into an agenda thread. Your original question was on the spontaneity of star formation and the Second Law. You've gotten several answers and a literature reference, but instead of addressing these points, you're on to superconductors (which are not perpetual motion machines as these machines are commonly defined) and cranky pseudoscience. You're moving the discussion away from the purpose of these forums, which is discussing consensus physics.
     
  21. Jan 11, 2009 #20
    Bull

    Who's practicing 'Cranky Pseudoscience' ? That is a blatant false accusation and a personal attack.

    Show me ONE FALSE statement I made.

    But, I do get the message loud and clear from the fact you practice 'consensus physics' and I will not post anymore.
     
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