Archive for August, 2008

The Rarity of Old Fossils
August 29, 2008

Don Lindsay is a computer scientist who likes to debate creationists and Scientologists. He has a knack for saying things in the simplest possible way. Here he explains why we don’t have many really old fossils.

We have lots of seashells. We’re very short of jellyfish fossils. That’s not too surprising.

We have a few T. Rex fossils, but we’re short on small, fragile creatures. This is easy to explain. First, it’s just easier to find the great big fossils. Second, fragile skeletons are, well, fragile. They are more likely to be scavenged or crushed before they can form a fossil.

But there is another pattern, which is that there just aren’t very many really old fossils. Why?

There are at least four reasons. For one, the earth’s surface has been rebuilt many times. Regions have been uplifted and then eroded away. Erosion destroys rock, and destroys any fossils in that rock. The new rock that forms contains new fossils. So, much of the earth’s surface is recent, compared to the age of the planet itself. Old rocks are rare, so of course old fossils are rare too.

The second reason is that many old rocks have spent time buried. While buried, they experienced great heat and/or pressure, and are now metamorphic rocks. Their fossils have turned to smudges.

Worse again is that the best fossils are found in ocean-bottom sediments. But as the continents move, they ride over the ocean floor. Old floor is sucked down towards the center of the earth at subduction zones, never to be seen again. (Places like the North Atlantic Ridge are creating new ocean floor to replace the old.)

Continents travel at about an inch a year. So, if you look at the size of an ocean, and do some simple arithmetic, you will see that most of the world’s ocean floor should be less than 200-300 million years old. But dating methods say that animal life arose 800 million to 1000 million years ago, and it moved onto the land about 400 million years ago. So, this is a frustrating situation. The oceans have been repaved since the really interesting stuff happened. We have to make do with the very few old ocean rockbeds that escaped destruction.

And the fourth reason is that the first creatures didn’t have skeletons, and they were tiny, too. We can tell in two ways. First, we’ve been lucky, and found a few very old deposits that preserved soft things. And secondly, we’ve found tracks.

Why didn’t they have skeletons? Well, because skeletons had to be invented at some point, and that point was about 600 million years ago.

Visit Don’s website.

What If Gravity Was Weaker or Stronger?
August 26, 2008

Martin Rees is an astrophysicist famous for his work on Big Bang theory and galaxy formation. In Just Six Numbers, he explains what the universe would be like if gravity was a bit stronger or weaker compared to other forces.

Despite its importance for us… gravity is actually amazingly feeble compared with the other forces that affect atoms… The gravitational attraction between [two] protons is thirty-six powers of ten feebler than the electrical forces, and quite unmeasurable. Gravity can safely be ignored by chemists when they study how groups of atoms bond together to form molecules.

How, then, can gravity nonetheless be dominant, pinning us to the ground and holding the moon and planets in their courses? It’s because gravity is always an attraction… On the other hand, electric charges can repel each other as well as attract… But any everyday object is made up of huge numbers of atoms (each made up of a positively charged nucleus surrounded by negative electrons), and the positive and negative charges almost exactly cancel out. Even when we are ‘charged up’ so that our hair stands on end, the imbalance is less than one charge in a billion billion. But everything has the same sign of gravitational ‘charge’, and so gravity ‘gains’ relative to electrical forces in larger objects… An apple falls only when the combined gravity of all the atoms in the Earth can defeat the electrical stresses in the stalk holding it to the tree. Gravity is important to us because we live on the heavy Earth.

Sand grains and sugar lumps are, like us, affected by the gravity of the massive Earth. But their self-gravity — the gravitational pull that their constituent atoms exert on each other, rather than on the entire Earth — is negligible. Self-gravity is not important in asteroids, nor in Mars’s two small potato-shaped moons, Phobos and Deimos. But bodies as large as planets (and even our own large Moon) are not rigid enough to maintain an irregular shape: gravity makes them nearly round. And masses above that of Jupiter get crushed by their own gravity to extraordinary densities (unless the centre gets hot enough to supply a balancing pressure, which is what happens in the Sun and other stars like it). It is because gravity is so weak that a typical star like the Sun is so massive. In any lesser aggregate, gravity could not compete with the pressure, nor squeeze the material hot and dense enough to make it shine.

Gravitation is feebler than the forces governing the microworld by the number N, about 1036. What would happen if it weren’t quite so weak? Imagine, for instance, a universe where gravity was ‘only’ 1030 rather than 1036 feebler than electric forces. Atoms and molecules would behave just as in our actual universe, but objects would not need to be so large before gravity became competitive with the other forces. The number of atoms needed to make a star (a gravitationally bound fusion reactor) would be a billion times less in this imagined universe. Planet masses would also be scaled down by a billion. Irrespective of whether these planets could retain steady orbits, the strength of gravity would stunt the evolutionary potential on them. In an imaginary strong-gravity world, even insects would need thick legs to support them, and no animals could get much larger. Gravity would crush anything as large as ourselves.

Galaxies would form much more quickly in such a universe, and would be miniaturized. Instead of the stars being widely dispersed, they would be so densely packed that close encounters would be frequent. This would in itself preclude stable planetary systems, because the orbits would be disturbed by passing stars — something that (fortunately for our Earth) is unlikely to happen in our own Solar System.

But what would preclude a complex ecosystem even more would be the limited time available for development. Heat would leak more quickly from these ‘mini-stars’: in this hypothetical strong-gravity world, stellar lifetimes would be a million times shorter. Instead of living for ten billion years, a typical star would live for about 10,000 years. A mini-Sun would burn faster, and would have exhausted its energy before even the first steps in organic evolution had got under way. Conditions for complex evolution would undoubtedly be less favourable if (leaving everything else unchanged) gravity were stronger. There wouldn’t be such a huge gulf as there is in our actual universe between the immense timespans of astronomical processes and the basic microphysical timescales for physical or chemical reactions. The converse, however, is that an even weaker gravity could allow even more elaborate and longer-lived structures to develop.

Gravity is the organizing force for the cosmos… [It] is crucial in allowing structure to unfold from a Big Bang that was initially almost featureless. But it is only because it is weak compared with other forces that large and long-lived structures can exist. Paradoxically, the weaker gravity is (provided that it isn’t actually zero), the grander and more complex can be its consequences. We have no theory that tells us the value of N. All we know is that nothing as complex as humankind could have emerged if N were much less than 1,000,000,000,000,000,000,000,000,000,000,000,000.

Buy Just Six Numbers.

The Joy of Chemistry
August 24, 2008

Oliver Sacks is a neurologist whose book Awakenings was made into a film starring Robin Williams. In this essay for The New Yorker, he revels in the excitement and heroism of scientific discovery.

My parents and my brothers had introduced me… to some kitchen chemistry: pouring vinegar on a piece of chalk in a tumbler and watching it fizz; then pouring the heavy gas this produced, like an invisible cataract, over a candle flame, putting it out straightaway. Or taking red cabbage, pickled with vinegar, and adding household ammonia to neutralize it. This would lead to an amazing transformation, the juice going through all sorts of colors, from red to various shades of purple, to turquoise and blue, and finally to green. I enjoyed these experiments, I wondered what was going on, but I did not feel a real chemical passion [until I] remet Uncle Dave, and saw his lab and his passion for experiments of all kinds.

It was through reading Mary Elvira Weeks’ Discovery of the Elements… that I got a vivid idea of the lives of many chemists, the great variety, and sometimes vagaries, of character they showed; and the relation (sometimes) between their characters and their work. Here I found quotations from the early chemists’ letters, which portrayed their excitements (and despairs) as they fumbled and groped their way to their discoveries, losing the track now and again, getting caught in blind alleys, though ultimately reaching the goal they sought.

If Humphry Davy was the first chemist I had ever heard of, he was also the one I most warmed to. I loved reading of his experiments with explosives and electric fish; his discovery of incandescent metal filaments and electric arcs; of catalysts… of the physiological effects of… laughing gas… He appealed to me especially because he was boyish and impulsive, the way he danced with joy all around his lab when he first isolated potassium, in 1807, and saw the shining metallic globules burst and take fire…

It was through reading these accounts that I first realized one could have heroes in real life. There seemed to me an integrity, an essential goodness, about a life dedicated to science. I had never given much thought to what I might be when I was “grown up” – growing up was hardly imaginable – but now I knew: I wanted to be a chemist. A sort of eighteenth-century chemist coming fresh to the field, looking at the whole, undiscovered world of natural substances and minerals, analyzing them, plumbing their secrets, finding the wonder of new and unknown metals.

See here for details on how to read the full essay.

Seeing Through Stone
August 24, 2008

Richard Fortey loves fossils. In Trilobite! he waxes poetic about the trilobite’s stone eyes.

Trilobite eyes are made of calcite. This makes them unique in the animal kingdom.

Calcite is one of the most abundant minerals. The white cliffs of Dover are calcite… Limestones (which are calcite) have been used to build… the sublime crescents of Bath, the pyramids of Gizeh, the amphitheatres and Corinthian columns of classical times. Polished slabs composed of calcite deck the doors of Renaissance churches in Italy, still grace the interiors of Hyatt–Regency hotels, or conference halls, or wherever architects wish to suggest the dignity that only real rock seems to confer.

The purest forms of calcite are transparent. In building stones and decorative slabs it is the impurities and fine crystal masses that provide the colour and design… The dark red of the scaglio rosso so typical of Italian church doors is a deep stain of ferric iron. But when a calcite crystal grows more slowly in nature, then it may acquire its perfect crystal form, and be glassy clear…

Look into a crystal of Iceland spar and you can see the secret of the trilobite’s vision. For trilobites used clear calcite crystals to make lenses in their eyes; in this they were unique. Other arthropods have mostly developed ‘soft’ eyes, the lenses made of cuticle similar to that constructing the rest of the body.

The science of the eye demands a little explanation. It all depends on the optical properties of calcite… If you break a large piece of crystalline calcite it will fracture in a fashion related to its fine atomic structure… You are left with a regular, six-sided chunk of the mineral in your hand, termed a rhomb… The clear calcite of this not-quite-a-cube treats light in a peculiar way. If a beam of light is shone at the sides of the rhomb it splits in two; this is known as double refraction. The rays of light so produced are the ‘ordinary’ and the ‘extraordinary’ rays: their course is determined, just like the shape of the rhomb, by the stacking of the individual atoms. There is a huge specimen of Iceland spar on the first floor of London’s Natural History Museum through which you may peer to see two images of a Maltese cross, one generated by the extraordinary, and the other by the ordinary rays. But there is one direction, and one direction only, in which light is not subjected to this optical splitting… from this direction it does not split into two rays at all but passes straight through.

If to travel back to the time of the trilobite is a historical sea-change then there can be nothing stranger than the calcareous eyes of the trilobite. And pearls are chemically the same as the trilobite’s unblinking lenses, being yet another manifestation of calcium carbonate, although pearls are exquisite reflectors of light rather than transmitters of it… The trilobite saw the submarine world with eyes tessellated into a mosaic of calcified lenses… his stony eyes read the world through the medium of the living rock.

Buy Trilobite!

August 19, 2008

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