Results 1 to 4 of 4

Thread: Can thermodynamical model be fundamental: reason not result?

  1. #1 Can thermodynamical model be fundamental: reason not result? 
    Forum Senior
    Join Date
    Jul 2008
    I always thought that thermodynamics/statistical physics is effective theory – statistical result of some fundamental physics below, but recently there became popular theories starting from ‘entropic force’ as fundamental (basing on holographic scenarios, like in ).
    For a simple mathematician like me it sounds like a nonsense – in fundamental theory describing evolution of everything there should be one concrete history of our Universe – there is no place for direct probabilities of scenarios required to define e.g. entropy.
    I was taught that to introduce effective local thermodynamical parameters to given concrete situation, for each point we average inside some ball around it to get for example local entropy or temperature, what allows to work on simplified statistically typical behavior.
    So I wanted to ask if someone could explain why we can even think about fundamental ‘entropic’ theories?

    To start the discussion I would like to briefly remind/discuss looking clear for me distinction between deterministic and stochastic/thermodynamical models:
    DETERMINISTIC models – the future is completely determined
    - evolution of gas in a tank is full dynamics of all its particles - for given valve opening there escaped concrete number of particles,
    - it's usually Lagrangian mechanics of some field – there is some scalar/vector/tensor/’behavior of functional'(QFT) in each point of our spacetime, such that ‘the action is optimized’ – each point is in equilibrum with its four-dimensional neighborhood (spacetime is kind of ‘4D jello’),
    - evolution equations (Euler-Lagrange) are HYPERBOLIC PDE - linearized behavior of coordinates in the eigenbase of the differential operator is
    d_tt x = - lambda x
    (0 < lambda = omega^2 )
    so in linear approximation we have superposition of rotation of coordinates – ‘unitary’ evolution – and so such PDE are called wavelike – the basic excitations on water surface, in EM, GR, Klein-Gordon are just waves,
    - the model has FULL INFORMATION – there is no place for direct probability/entropy in electromagnetism, general relativity, K-G etc. – the model has some TIME (CPT) SYMMETRY INVARIANCE (no 2nd law of thermodynamics – there is still unitary evolution in thermalized gas or a black hole)

    THERMODYNAMICAL/STOCHASTIC models – there is some probability distribution among possible futures
    - gas in a tank is usually seen as thermalized, what allows to describe it by a few statistical parameters like entropy (like sum of –p*lg(p) ) or temperature (average energy per degree of freedom) - for a specific valve opening, the number of escaped particles is given by a probability distribution among possible scenarios only,
    - it is used when we don’t have full information or want to simplify the picture – so we assume some mathematically universal STATISTICAL ENSEMBLE among POSSIBLE SCENARIOS (like particle arrangements ) – optimizing entropy (uniform distribution) or free energy (Boltzmann distribution),
    - thermodynamical/stochastic evolution is usually described by discussion-like: PARABOLIC PDE – linearized behavior of coordinates in the eigenbase of the
    differential operator is
    d_t x = - tau x
    (tau - ‘mean lifetime’ )
    so in linear approximation we have exponential decay (forgetting) of coordinates – evolution is called thermalization: in the limit there survive only ones with the smallest tau – we call it thermodynamical equilibrium and usually can be describe using just a few parameters,
    - these models don’t have time symmetry – we cannot fully trace the (unitary?) behavior so we have INFORMATION LOST – entropy growth – 2nd law of thermodynamics.

    Where I’m wrong in this distinction?
    I agree that ‘entropic force’ is extremely powerful, but still statistical result – for example if while random walk instead of maximizing entropy locally what leads to Brownian motion, we do it right: globally, we thermodynamically get going to the lowest quantum state probability density – single defects create macroscopic entropic barriers/wells/interactions:
    For me the problem with understanding quantum mechanics is that it’s between these pictures – we usually have unitary evolution, but sometimes entropy grows while wavefunction collapses – there is no mystical interpretation needed to understand it: entropy maximizing from mathematically universal uncertainty principle is just enough ( )

    What do you think about this distinction?
    Can thermodynamical models be not only effective (result), but fundamental (reason)?
    Can quantum mechanics alone be fundamental?

    Reply With Quote  


  3. #2  
    Forum Senior
    Join Date
    Jul 2008
    I see my intentions were wrongly understood - no I don't wanted to promote 'New Hypotheses and Ideas', but to warn about them and their alternative logic ...

    I wanted to remind what thermodynamics is - simplified effective picture in which we assume statistically typical behavior, like that when we completely don't know which scenario is happening, we should assume maximizing entropy uniform distribution
    Unfortunately in world of quantum mechanics which is generally believed to be impossible to understand but still fundamental - the logic of reason-result distinction is no longer binding...
    The belief that QM is fundamental leads to many worlds interpretation - that our spacetime is infinitely quickly branching tree of parallel universes ...

    ... while field theories we use on all scales (GR, EM, Klein-Gordon, QFT) are deterministic and clearly say what our spacetime is - in these theories we live in static 4D action optimizing solution - each point is in equilibrium with its 4D neighborhood - spacetime is kind of '4D jello'.
    They are deterministic and like QM mechanics have 'wavelike/unitary' evolution.

    So what's happening when we cannot fully trace the evolution? ... for example the behavior of a single particle...
    In such situations we have to use some thermodynamical model - assume some statistical ensemble among possible scenarios for example to maximize entropy - assume that the particle makes some random walk ...
    Maximizing entropy locally leads to Brownian motion in continuous limit - but when we do it right: assume global entropy maximum (like in models I advocate) - we get thermodynamical going to squares of coordinates of the dominant eigenvector of discrete Hamiltonian (and finally the real Hamiltonian while assuming Boltzmann distribution among trajectories).

    These new but fundamental stochastic models finally show what was missing - that in field theories on thermodynamical level: when we cannot fully trace the evolution, we should assume collapse to some local lowest quantum state.
    Living in specetime ('4D jello') leads to many nonintuitive 'quantum' consequences - like (confirmed) Wheeler's delayed choice experiment, that in models with limited information to translate what we are working on (amplitude) into the real probabilities - we should 'square' it against Bell's intuition, or allows for 'quantum' computations:

    Reply With Quote  

  4. #3  
    Forum Sophomore
    Join Date
    Aug 2009
    as a piece of advice, most people stop when they see 3 pages of text as your opening statement. i got bored after the second paragraph. try to make it shorter and more clear. it shouldnt look intimidating to read.
    Reply With Quote  

  5. #4  
    Forum Senior
    Join Date
    Jul 2008
    This topic was moved here, but it was rather intended to warn (and discuss) about alternative logics - to remind (and discuss) what thermodynamics is ...

    ... for example that the essence of thermodynamics is using mathematical theorems like maximum uncertainty principle.
    Standard random walk was successfully pretending to already do it - its continuous limit is enough to model diffusion in fluids, but from QM or for example recent STM pictures of electron stationary probability density on a surface of semiconductor, we clearly see that fixed structure of defects in condensed matter makes this approximated thermodynamical model inappropriate.
    But when we do it right - use the real Maximal Entropy Random Walk and generalized models, we get exactly what's needed - that we should get going to the square of coordinates of the dominant eigenvector of (discrete) Hamiltonian - that when we cannot trace unitary evolution, we should assume 'wavefunction collapse' - explaining the decorence.
    Here is new discussion about it:
    Reply With Quote  

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