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posted by janrinok on Thursday September 12 2019, @05:44PM   Printer-friendly
from the probably,-possibly,-maybe dept.

From WIRED, again. Sometimes they have good stuff.

In the early 1970s, people studying general relativity, our modern theory of gravity, noticed rough similarities between the properties of black holes and the laws of thermodynamics. Stephen Hawking proved that the area of a black hole's event horizon—the surface that marks its boundary—cannot decrease. That sounded suspiciously like the second law of thermodynamics, which says entropy—a measure of disorder—cannot decrease.

Yet at the time, Hawking and others emphasized that the laws of black holes only looked like thermodynamics on paper; they did not actually relate to thermodynamic concepts like temperature or entropy.

Then in quick succession, a pair of brilliant results—one by Hawking himself—suggested that the equations governing black holes were in fact actual expressions of the thermodynamic laws applied to black holes. In 1972, Jacob Bekenstein argued that a black hole's surface area was proportional to its entropy, and thus the second law similarity was a true identity. And in 1974, Hawking found that black holes appear to emit radiation—what we now call Hawking radiation—and this radiation would have exactly the same "temperature" in the thermodynamic analogy.

[...] But what if the connection between the two really is little more than a rough analogy, with little physical reality? What would that mean for the past decades of work in string theory, loop quantum gravity, and beyond? Craig Callender, a philosopher of science at the University of California, San Diego, argues that the notorious laws of black hole thermodynamics may be nothing more than a useful analogy stretched too far.

After what Hawking said about philosophy, I think that astrophysicists need a bit more perspective.


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  • (Score: 5, Interesting) by ikanreed on Thursday September 12 2019, @07:27PM (3 children)

    by ikanreed (3164) Subscriber Badge on Thursday September 12 2019, @07:27PM (#893282) Journal

    If we're talking about hard science we need to use the fucking hard science definition of entropy. Which isn't "disorder". It's a co-related concept to enthalpy that measures the distribution of enthalpy across possible states. The second law of thermodynamics describes the statistically true tendency of moving systems to distribute their energy more evenly, in the same way you don't expect a random cue ball shot into a billiard's table to rerack all the balls.

    But it also doesn't apply in a strictly intuitive way as energy changes forms. A extremely fast moving moon close in orbit to a planet plus a slightly heated planetary crust(from the tidal forces) seems like more concentrated energy than a moon in a much further orbit with higher gravitational potential energy. Having energy all in one place intuitively seems less chaotic. Less entropic. But it's not. There's more possible ways to re-arrange the energy of the wider orbiting state than the lower orbiting state.

    So by the same fucking token, things falling into a black hole decrease possible arrangements of their energy through the system they're part of. Quite substantially, in fact, because none of the possible arrangements afterwards have any part outside the event horizon.

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  • (Score: 2) by melikamp on Thursday September 12 2019, @11:24PM (1 child)

    by melikamp (1886) on Thursday September 12 2019, @11:24PM (#893418) Journal

    Can you please point to a precise (mathematical) definition of the entropy of a physical system, either classical or quantum or relativistic?

    Can you please also point to an effective procedure to measure what you defined above. If such procedure is not currently feasible, it should still be theoretically feasible to carry out in the future, with better instruments.

    This is a question for everyone, and I am really curious. I have a deep-seated suspicion that most people, even physicists, do not have a clear (mathematical) idea of what entropy is.

    • (Score: 2) by ikanreed on Friday September 13 2019, @02:10AM

      by ikanreed (3164) Subscriber Badge on Friday September 13 2019, @02:10AM (#893477) Journal

      I'm most familiar with it in the context of chemistry. This is a very limited context because it only really describes the entropy of gasses in terms of particle motion, but such descriptions are usually much easier to fully grok than some 30 million variable universal formula.

      dU = T*dS - p*dV

      U Interrnal Energy
      T Temperature
      S Entropy
      p Pressure
      V volume

      Now, that looks like a hellacious differential equation to solve generally for real world scenarios, but it's quite intuitive to visualize. As your "perfect environment containment box" expands in volume, if your energy and temperature and pressure remains constant, entropy goes up proportionately. The particles have more possible states that they're then scattering into as those physical spaces become available to fly into.

  • (Score: 0) by Anonymous Coward on Friday September 13 2019, @02:25AM

    by Anonymous Coward on Friday September 13 2019, @02:25AM (#893483)

    Were you there?