Monday 2 January 2017

Picture Post #20 Olber's Paradox raising insoluble questions

'Because things don’t appear to be the known thing; they aren’t what they seemed to be neither will they become what they might appear to become.'

Posted by Martin Cohen and Tessa den Uyl

A NASA  image from the Hubble Telescope looking into the 'Deep Field'
This is a patch of BLACK sky - empty when initially seen - even through the largest earthbound telescopes. Yet, with the  Hubble space telescope and a long-enough exposure time, even the darkness of space soon comes to glowing life. The point is, every bit of sky is actually packed with light - not merely with stars but with uncountable distant galaxies.

Heinrich Olbers (1758–1840) was a Viennese doctor who only did astronomy in his spare time, but realised that there was a bit of a logical problem about the night sky. And ‘O’ is for ‘Olbers Paradox’*,  which can be summed up by saying that if the universe is really infinite in size, the the night sky should not only be bright – but should be infinitely bright. Put short, we should see stars everywhere we look. So why don't we and why isn't the night sky all lit up ?

The paradox touches upon profound issues in cosmology, or the study and theory of the origins of the universe. Simply saying that most of the stars are too far away to see is not enough. Certainly it is true that starlight, like any other kind of light, dims as a function of distance, but at the same time, the number of light sources in the ‘cone of vision’ increases – at exactly the same rate. In fact, on the mathematics of it, given an infinite universe, with galaxies and stars distributed uniformly, the whole night sky should appear to be not black, not speckled, but white!

Olbers’ paradox is a ‘thought experiment’ in the very good sense that most of the reasoning is done by hypotheticals. What if the universe is infinitely large? And infinitely old? If the stars and galaxies are (on average) spread out evenly?

Various possible explanations have been offered to explain the paradox. Such as that stars and galaxies are not distributed randomly, but rather clumped together leaving most of space completely empty. So, for example, there could be a lot of stars, but they hide behind one another. But in fact, observations reveal galaxies and stars to be quite evenly spread out.

What then, if perhaps the universe has only a finite number of stars and galaxies? Yet the number of stars, finite or not, is definitely still large enough to light up the entire sky…

Another idea is that there may be too much dust in space to see the distant stars? This seems tempting, but ignores known facts. Like that the dust would heat up too, and that space would have a much higher. The astronomers who took this image claim it shows some kind of spectral shift into the red specturm. Or is it only the dust? The questions are not really resolved, even yet.

So what is the best answer to Olbers’ riddle? The favoured explanation today is that although the universe may be infinitely large, it is not infinitely old, meaning that the galaxies beyond a certain distance will simply not have had enough time to send their light over to fill our night sky. If the universe is, say, 15 billion years old, then only stars and galaxies less than 15 billion light years away are going to be visible. Add to which, astronomers say that the phenomenon of red shift may mean some galaxies are receding from us so fast that their light has been ‘shifted’ beyond the visible spectrum.

After reading this, and then standing here on planet Earth and watching the night sky, one might feel a little trapped by the questions. Our sight is limited and it always will be but maybe this is our hope for we can continue to philosophise: afte rall, what are we thinking? The picture above might as well represent pieces of coloured glass, under water visions where fluorescent life flows in deep dark sees, a pattern for printed cloth. Our brain only represents what we think we see, not necessarily the reality in which we live? In the incredible immensity of space, mankind has always been aware of this, even if, once in a while, the tendency is to forget.

* Although the paradox carries Olbers’ name,  it can really be traced back to Johannes Kepler in 1610.  In Wittgenstein's Beetle and Other Classic Thought Experiments, Martin’s book, which talks a little more about all this, 


Keith said...

In the spirit of this curious thought experiment, I’d also like to experimentally add a few thoughts about Olbers’s paradox, too. As a starting point, it seems that a paradox holds up as a paradox only if ‘all’ the assumptions are true. Accordingly, my understanding is that Olbers’s paradox makes (at least) four assumptions: that the universe is infinitely old, that’s it’s static, that it’s infinitely large, and that it’s populated by an infinite number of stars.

As for the universe being ‘infinitely old’—the first assumption—cosmologists say, instead, that the Big Bang that gave rise to the universe from a point-like singularity occurred 13.8 billion years ago—the most-current refinement of the universe’s age, I believe. That is, old but not infinite.

As for the universe being ‘static’—the second assumption—cosmologists say that the universe is actually expanding, and doing so at a rapidly accelerating rate. Hubble has confirmed the accelerating rate of expansion; also, I recall that three scientists won the Nobel Prize in physics in 2011 for their work on this. As a consequence of the accelerating expansion, the stars’ light is ‘redshifting’, and dimming, as the stars get farther away.

As for the universe being ‘infinitely large’—the third assumption—cosmologists say that it’s only as large as the most-far-flung stars, given the universe’s finite age. Given light’s approximate speed of 300,000 kilometers per second, light from the most-distant stars wouldn’t have had time to reach us yet; there’s a significant delay.

As for the number of galaxies and stars in the universe being ‘infinite’—the fourth assumption—cosmologists estimate the number to be finite. There’s a mind-bogglingly large number of stars in the observable universe alone, and a still larger number (estimated in the many trillions) estimated for the universe as a whole; but the number supposedly is still finite. Stars, furthermore, have a lifespan depending on their type—they don’t live forever—instead dimming and dying at fairly predictable ages.

Are the paradox’s four assumptions suspect, then? And if so, how many of the paradox’s assumptions must fail to hold water in order for the paradox itself to break down as a whole? Is just one false assumption enough to do that? More? All? That is, what’s the threshold between the paradox being rock solid, or its being fragile? And has that threshold been reached given the knowledge acquired by cosmologists, astrophysicists, and others since Olbers’s time, two hundred-plus years ago?

Pursuing the thought experiment further, there may be two other questions, unrelated to the four specific assumptions per se, that might be germane to Olbers’s paradox. Again, just thinking out loud. First, how might the presence of ‘dark energy’—whose repulsive force counteracts attractive gravity and results in visible matter moving apart faster—possibly affect the paradox, such as canceling the stars’ effect upon the amount of light in the night sky?

And second, how might the role of entropy—which irreversibly trends toward disorder, and thermodynamic demise, in an isolated system like the universe—possibly affect the paradox in the much longer term, on the order of many billions (even trillions) of years?

(Perhaps the answer to both preceding what-if questions is ‘not at all’.)

docmartincohen said...

Thank you, Keith, for this very scientific response to the post. There is still the aesthetic aspect, to consider! My point for choosing the image was how an astronomical image can lead to deep hilosophical questions. I think, in this sense, it follows in the same spirit as your post.

As far as being a'paradox' goes, I think it is fair to use the term. The paradox is that given certain assumptions, x, y, z, we ought to see something - and yet what we see is different. As I understand it, even if the universe is only as old as people say (and the offical age is far from settled fact - it was recently adjusted up wasn't it?) the night sky should be far brighter than it is. Likewise, your points about how many stars there are (on the numbers believed under present theories, the sky shoudl be much brighter) and so on. So there are I think mysteries - but the point was not to promote any particular version of comsological theory but to raise some of the many fascinating aspects - which your comments have helped, as ever, to do as well.

Keith said...

The fuzzy points of light in this Hubble image remind us of the primordial detritus, spewed across the universe from supernovas, that provided the very elements essential to making us possible: From the stars, to the stars. As you suggest, Martin, that ‘philosophical wormhole’ leads to many places.

Tessa den Uyl said...

Yes, supernova's! I always like to think of us as star-stuff, actually we were struggling around with that phenomena for this post, but left it out. Another time we might touch that aspect...

Post a Comment