Some Nice Physics Blogging: Sean Carroll/Simple Questions dept.

Good New Year’s Eve fare (in that it bears, at least sort of, on why we should expect next year to be more or less the same as this one, only slightly less usefully energetic…) from Sean Carroll over at Cosmic Variance’s still more or less new and quite spiffy home at

Sean’s writing about Boltzmann’s argument that the apparent order of the universe as observed, with a low entropy past and a unidirectional arrow of time pointing towards future states with higher entropy cannot be simply a statistical fluctuation within a larger construct spending most of its time in thermal equilibrium.

Read the post — Sean goes into the history of the argument, and introduces some of what makes this a profound observation cum insight.

What made me smile on reading it, though, was not simply the content of this particular deep bit of thinking that comes from a delightfully (and deceptively) simple pair of questions — how is it that the universe display observable order everywhere we look; why does time flow in just one direction — but that there are a lot of such questions.  From one point of view, that is, physics is a very simple field.*

To give one more example, one that entranced me back in the late 80s when I was just beginning a decade and a half-long dance with Albert Einstein, consider Olber’s paradox.  In essence, the question asked here is “why is the sky dark at night?”

Shakespeare nailed that one, of course, in As You Like It, act III scene 2, when Corin the shepherd informs Touchstone the clown that he knows “that a great cause of the night is lack of the Sun.”

But the problem gets a little more complex when you make the assumption that we inhabit an infinite, static universe.  In such a place, would not the fact that for every spot on the sky there would be at some distance a star, whose light, given the infinitude of time, would ultimately reach the earth.  If so — the entire nightsky should glow with starlight, and the fact that it is not suggests some problem in the conception.

The paradox was proposed by a German astronomer, Heinrich Wilhelm Olbers, and was published as such in 1826, though it was anticipated by Kepler, among others. The solution to the apparent paradox encompassed this observation  by Edgar Allen Poe — yes, the Raven guy in his prose poem Eureka, published in 1848:

Were the succession of stars endless, then the background of the sky would present us an uniform luminosity, like that displayed by the Galaxy -–since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.

But while this served as an explanation for a dark sky at a given time, it does not preclude a star-lit sky in the future.  That loophole could only be closed with 20th century physics, especially the cosmological extensions of General Relativity that underpin suggest a finite age to the universe (at least our subassembly within a presumptive multiverse) and the expansion predicted within the Big Bang concept, and observered by Messrs. Hubble, Humason and a cast of thousands since (and not just by the Hubble wars vets either).

Why is the sky dark at night?

The answer to the question in detail is enormously technically complicated, and draws on ideas and technology that require years of specialized training to master.  But both the question itself and its qualitative answer can be understood by a child of eight.  (That is a factual claim, not a rhetorical flourish.  My son is eight, and I tried this on him.;)  This is what I call fun.

Image:  Vincent Van Gogh, Starry Night Over the Rhone, 1888.

Explore posts in the same categories: astronomy, physics

5 Comments on “Some Nice Physics Blogging: Sean Carroll/Simple Questions dept.”

  1. […] By meichenl Tom Levenson at the Inverse Square Blog posts a discussion of Olber’s Paradox. The gist is that if we lived in an infinite, static, homogeneous universe, there would be light […]

  2. meichenl Says:


    Thanks for pointing this out. I just wrote a response (pingback above), but the punch line is that Olber’s paradox shouldn’t be “why isn’t the sky as bright as a star everywhere?”, but “why isn’t the sky infinitely bright everywhere?”.

    That is, if in any direction you look and see a star, then if you look further you’ll see another star, and if you look further than that another star, etc. The universe wouldn’t just be bright everywhere. It would be infinitely bright. (details with some math at my post


  3. BlackGriffen Says:

    But the night sky does present us with a constant luminous background, it’s just at a different wavelength that you would expect from the simple argument. I am, of course, bringing up the CMB. Of course that doesn’t answer the paradox unless you can somehow find a way for the light to thermalize down in wavelength over about 13 billion years, but it’s still worth thinking about.

  4. Spiv Says:

    Unfortunately this is easier to explain to an 8 year old than it is to a college+ aged person, as far as I can tell.

    It’s a fantastic question to start fights over with physics people though. Friendly, science-y fights, of course.

  5. Kaleberg Says:

    There’s a great book on this riddle which includes Olbers and Poe. It’s called Darkness at Night by Harrison, and it is extremely well written and takes you into all sorts of neat little corners.

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