Star Traveler from Angelbert Metoyer's Dark Energy Splitting the Universe . Click to enlarge.At the end of the 19th Century century many physicists were still searching for the luminiferous aether - the mysterious, invisible substance that permeated the universe - the medium that (it was presumed) needed to exist in order to support light waves.
At that time, waves were understood to be mechanical disturbances in a medium - in effect, shapes that propagate through the medium transporting energy from source to receiver with a speed dependant on the medium itself . Water waves, waves on a string, sound waves in a column of air - all of these phenomena relied on matter to support these moving shapes.
The Aether was supposed to be the invisible stuff that somehow oscillated as the shapes of light waves passed from point A to point B. It had to be really odd stuff - invisible, for one thing, and also incredibly resilient, because it had to support an unbelievable speed - the speed of light.
Nevertheless, by this point the mechanical qualities of the aether had become more and more magical: it had to be a fluid in order to fill space, but one that was millions of times more rigid than steel in order to support the high frequencies of light waves. It also had to be massless and without viscosity, otherwise it would visibly affect the orbits of planets. Additionally it appeared it had to be completely transparent, non-dispersive, incompressible, and continuous at a very small scale. (wikipedia)
In other words, aether couldn’t be seen, weighed, touched or tasted, but it was stronger than anything known to man. And it was everywhere - in your shoes, hair, mouth, nose, and subway tunnels. A really strange thing for physicists to believe in.
The Michelson-Morely experiment in 1887 shattered the idea that the aether existed. M & M were looking for evidence of the aether, and truly expected to see this evidence in their carefully-designed and high-precision experiment. As landmark experiments go, this was earth shattering for a number of reasons, not the least of which was that it was a failed experiment - it yielded a "null result." It’s hard not to rhapsodize about an effort that failed so mightily that it revolutionized not just the theory of electromagnetic waves, but of the nature of the universe - ultimately leading to Einstein’s Special Theory of Relativity in 1905.
Why was the aether believed to exist with absolutely no direct evidence before M&M? This is a case of predictions being so successful that scientists come to totally believe the components of the model that is used for the prediction. For the aether, it was a fact that incredible success had been achieved by modeling waves as mechanical disturbances. So, if light was a wave, it needed to be supported by a mechanical medium, even if we couldn’t possibly tell that it was there.
This phenomenon of belief that a model is in fact the reality of a situation is certainly not the first time something like this has happened - just consider the flawed models of heat as a bubbling caloric, for example. But it may be the biggest case of a model requiring that something exist - something that by it’s very name is so aethereal that it can’t ever be sensed.
Which brings be to the reason I am writing this post in the first place. Jump ahead a century and we are now in an eerily similar place in our view of the physical universe. We believe in magical, mystical, mysterious stuff that, according to our models, are not just present (though invisible), but dominant in terms of amount and effect. Even crazier, there’s two different types of this nutty stuff. I am referring, of course, to Dark Matter, and Dark Energy.
The reason for belief in these mystery items appears similar at first to the luminiferous aether. In order to be able to use our current models of the universe, and physics in general, to explain experimental results, something extra and invisible has to be postulated to exist. Dark Matter naturally arises as a way to make sure that our standard theory of gravitation coincides with observations of galactic dynamics. Dark Energy arises as a way to explain observed acceleration in the rate of expansion of the universe..
Of the two, Dark Matter is a little bit easier to understand, once one gets beyond the fact that it is believed to comprise approx. 21% of the total matter & energy of the universe. The belief in the existence of Dark Matter, which by its definition can’t be observed visually, is high. A recent experiment claims`the first direct observational evidence of dark matter. (Read A Direct Empirical Proof of the Existence of Dark Matter, by a team of researchers using NASA’s Chandra X-ray Observatory.) The title of the paper makes it very clear that this finding is not direct observation - just what does "empirical proof" mean? It would seem then that the belief in dark matter is itself based on a model that may be as ontologically-challenged as the mechanical wave model of light that lead to the luminiferous aether.
Colliding galaxies showing spatial separation of baryonic and non-baryonic matter. Click to enlarge.Actually, the "proof" is an intriguing one, and brings in clear relief the difference between the aether of the late 19th century and dark matter today - it is the triumph of physical laws over model. The idea of the aether was dictated by the model of what a wave was - and, at that time, the model was clearly that of a mechanical wave. On the other hand, the idea for dark matter is dictated by the belief in the law of gravitation being a universal law. The dark matter is postulated in order to make sure that gravitation still works. In other words, we just have to believe that there is different stuff in the universe that obeys the law of gravitation, but, conveniently, can’t be seen. The proof is complicated, but does rely on showing that a change in the law of gravitation cannot explain the experimental results, and therefore dark matter must exist. Quite a stunning argument. (Not surprising, there is a vocal anti-dark matter contingent who don’t necessarily by the "proof," and who believe that gravitational theory must be tweaked. For a gentle introduction to the proof, as well as a description of the position of those who claim that gravity must be modified, and why they are probably incorrect, see Bedeviling Devil’s Advocate Cosmology.)
It’s still not clear what dark matter is made of - there are ideas out there that include MACHOS (Massive Compact Halo Objects), and WIMPS (weakly interacting massive particles) - which sound like rival gangs in a universal West Side Story - but it appears that dark matter is here to stay - even if we can only infer its existence.
Now on to Dark Energy. Suffice it to say that this is the most mysterious stuff in the universe, where I am using the word "stuff" deliberately to stress the equivalence between matter and energy. Why is is postulated to exist? Again, like Dark Matter, it is needed to preserve a physical law - in this case the conservation of energy (See the Scientific American piece on Dark Energy for full details.) The Conservation of Energy is a law that is so ingrained that any suggestion that we should tweak it seems absurd (Actually, some physicists did consider violations of conservation of energy before sanity was regained in the heady early days of Quantum Mechanics).
Energy distribution of the universe. Click to enlarge.But consider that Dark Energy is thought to be everywhere. Like the luminiferous aether it is everpresent in our shoes, hair, mouth, nose, and subway tunnels. There’s (supposedly) even more of it than Dark Matter, it represents 75% of the universe’s matter & energy. (If you are counting, then, dark matter + dark energy account for 96% of the entire universe!) It’s a very strange energy - a source of repelling force causing the entire universe to expand at an accelerating rate, but, like aether, we can’t see, weigh, touch or taste it.
Will Dark Energy fade into the dustbin of history to join the luminiferous aether? I don’t think so. I expect that we will someday, in the not-too-distant future, see an "empirical proof" of its existence.
You see, what’s really different about the late 19th century and now is that we are much more inclined to consider outlandish ideas - new states of matter and energy, each new one weirder than the last. This willingness to "think outside the galaxy" has been quite successful. Consider how quasars and pulsars have come to be accepted as matter-of-fact. The idea of a "black hole" is now so accepted that it is the universally agreed-upon reason/destination for missing socks in drawers.
Meanwhile the laws of gravitation, and conservation of energy keep on ticking, and we stay legal by inviting the most outrageous characters into our party and our models - The Dark Lords of Matter and Energy. Modify the models, not the law. Let the darkness in.
But darkness can illuminating. As Jesse Colin Young sang:
Darkness, Darkness, be my pillow, Take my head and let me sleep In the coolness of your shadow, In the silence of your deep Darkness, darkness, hide my yearning, For the things I cannot see Keep my mind from constant turning, To the things I cannot be Darkness, darkness, be my blanket, cover me with the endless night Take away the pain of knowing, fill the emptiness with light Emptiness with light now