Wed April 10, 2013
13.7: Cosmos And Culture

Our Dark Materials

The history of science is filled with obscure and bizarre substances. Despite all that we have learned in the past 400 years, the trend continues. Perhaps it's unavoidable, being the way we figure things out. We need to find some apparently weird stuff — playing a game of cat and mouse with Nature — in order to make sense of what's out there.

With time, most strange substances disappear as we understand what is going on. But, hard as we try, we always seem to be surrounded by some unknown material. It is a fog that doesn't ever seem to fully dissipate.

Philip Pullman used this to spectacular effect in his fictional trilogy His Dark Materials. Remarkably, it seems that reality is even weirder than fiction.

Some brief case studies may be useful. In 1667, the German alchemist and physician Johan Joachim Becher, trying to understand combustion, proposed that things burned because they liberated their "phlogiston"; without it, a substance wouldn't burn. Becher's hypothesis was doubted when experiments showed that certain metals gained weight when they burned, something hard to reconcile with losing stuff. Soon, speculations abounded. Perhaps phlogiston was lighter than air; or perhaps it had negative (!) weight.

This kind of wild hypothesizing is not rare in science: when an idea begins to fail, people try to save it with all that they've got. Who knows? Maybe a new law of Nature is hiding behind the conundrum? Only time and experimental exposure lead to the elimination or modification of the idea.

Combustion was finally understood in 1783 by the great chemist Antoine-Laurent Lavoisier. Through a series of brilliant experiments, Lavoisier demonstrated that combustion needed oxygen. Furthermore, the total mass of the reagents in any chemical reaction remains constant: "in all operations of art and Nature, nothing is created; an equal quantity of matter exists both before and after the experiment."

However, confused as to the nature of heat, Lavoisier proposed yet another strange substance: the "caloric." Things cooled down because caloric flowed from hot to cold. To obey his law of mass conservation, Lavoisier had to assume that caloric had no mass, being a kind of ether endowed with the ability to flow. Wrong, but quite useful as a temporary explanatory device. Only by the middle of the 19th century was the caloric was abandoned.

Heat was understood as a form of motion, an agitation of matter.

There are many more examples, such as the luminiferous aether, the medium that was to support the waving of light through space. That was discarded, not without much pain, after Einstein's theory of special relativity of 1905. There is also the Higgs field, the entity responsible for giving mass to all particles of matter but light itself, discovered last July at the European Organization for Nuclear Research. This one is probably going to stay with us. No one said that there aren't strange things in the cosmos.

We continue to live in a Universe filled with strange stuff. Recently, the cosmic recipe was revised, somewhat, with new data coming from the European Space Agency's Planck spacecraft.

The matter we are made of — you know, the chemical elements that appear in the periodic table, all made of protons, neutrons and electrons — is an absolute minority of the matter that makes up the Universe, coming in at only 4.8 percent of the total.

Of the rest, we know much less. There are two main components: dark matter (coming in at 25.6 percent of all matter) and dark energy (coming in at 69.9 percent). These numbers are from the Planck's team data, using a combined best fit with other observations.

His Dark Materials indeed. "Dark" here means that we can't see them. More precisely, they don't emit radiation in any frequency of the electromagnetic spectrum that we can probe. They are not made of anything we know. We detect their presence through their gravitational effects on "ordinary" matter, the stuff that we see in galaxies and clusters of galaxies. Dark energy pulls on the Universe as a whole. Actually, it pushes on the Universe as a whole, making it expand much faster than we had anticipated.

Last week, excitement mounted as the first results from the giant Alpha Magnetic Spectrometer (AMS), a detector mounted on the International space Station, were announced. Nobel-prize winner Sam Ting's brainchild, the AMS is a detector that collects rogue particles flying through space known as cosmic rays.

Theorists have speculated that an excess flux of positrons, the antimatter cousins of the electrons, could indicate that dark matter particles are annihilating each other. Well, it appears that exactly such extra flux, which had been hinted at by previous experiments, was indeed found by the AMS. According to Ting, "over the coming months, AMS will be able to tell us conclusively whether these positrons are a signal for dark matter, or whether they have some other origin."

A more mundane explanation for the extra flux of positrons would be rapidly-rotating neutron stars (pulsars) distributed across the galactic plane. We should find out soon, hopefully resolving a mystery that has been with us since the early 1930s. That's when the Swiss-American astronomer Fritz Zwicky first conjectured that a cloak of dark matter surrounded clusters of galaxies. We know that something does hover around galaxies since it causes a distortion in the geometry of space that we can see, a phenomenon known as gravitational lensing.

If AMS verifies that the positrons do come from the annihilation of dark matter particles, it would confirm the existence of a new kind of matter, one that contributes about five times as much as atoms to the cosmic recipe. If this is the case, it would be hard to discard dark matter as just another kind of phlogiston. Dark materials, it appears, exist and in luxurious abundance.

