Monday, June 25, 2012


A cross section of a stromatolite showing
its characteristic dome layering.

The Pilbara region of western Australia is home to some of the Earth’s oldest rock formations, and among these formations are remnants of an ancient reef. This is no ordinary reef made of coral; it’s made of stromatolites—structures left behind by microorganisms that lived 3.5 billion years ago.
   Stromatolites are formed by algae that live on the floors of shallow seas. They collect fine layers of sediment that are built up into cones, domes and other shapes over many millions of years. A stromatolite is not alive, but is a structure made by living things. When we find one in the fossil record, it's like finding an ancient footprint. 
   The microbes that make these structures are almost never preserved as fossils inside the stromatolites. And without that it's difficult to be sure an ancient stromatolite is proof of life. But the Pilbara stromatolites are not only biologically produced, but possibly the oldest evidence of life on our planet. 
   These amazing structures have a pronounced conical shape unlike any kind of ripple pattern you would expect to see on a lifeless sea floor. Instead, stromatolites are sedimentary structures formed usually by cyanobacteria (also known as blue-green algae) that live in the interface between sediment and ocean. By examining these structures, we can tell that some lived almost 3.5 billion years ago, forming near rocky coastlines with shallow water environments. They have not been found in deep water areas that were also in existence at that time. 
Stromatolites at Shark’s Bay are fossil ancestors
to the oldest known life form on Earth.
   These fossilized ecosystems provide a glimpse of the Earth’s early history and possibly the beginning of life on our planet, as they are thought to be largely responsible for oxygenating Earth’s early atmosphere. Could life have existed even earlier? It’s possible. We don’t have any geologic record of the first 500 million years of Earth’s existence, so there is a pretty big gap in our knowledge. What's not preserved in the fossil record is how life formed in the first place. One thing we do know: that stromatolites are very distinctive and these structures are nearly identical with those of living stromatolites found at Shark's Bay, in western Australia. That means these living fossils have a family tree that stretches back farther than any other living thing on the planet. Remarkable, when you think about it.

Monday, June 18, 2012

Dragons of the Cretaceous

A giant azhdarchid compared to a giraffe.
Illustration by Mark Witton/University of Portsmouth.
Pterosaurs are flying reptiles that lived between about 230 and 65 million years ago. Although often called dinosaurs, they are actually a distinct branch of reptiles that independently evolved flight. There have been about 100 species of pterosaurs discovered so far, and one group of about a dozen species—the azhdarchids—is of particular interest. 
   In 1971, a student from the University of Texas working at Big Bend National Park discovered a long, hollow fossil bone that was from an enormous azhdarchid wing. Excavations recovered more wing bones, but no body bones could be found. It was named Quetzalcoatlus northropi, after the feathered snake god worshipped by the Aztecs. Eventually, other specimens of Quetzalcoatlus were found at the park. These specimens were smaller but more complete than the original, and by comparing them with the massive bones of the original, they were able to calculate the body size of the original specimen. This creature had an estimated wingspan of 10 meters or more and a height of over 5 meters. With a massive, elongate head, long, stiff neck and long hind limbs, this was a real-life dragon (minus the fire-breathing of course!) and the largest flying creature to have ever been found.
   Despite their huge size, azhdarchids were able to quickly launch themselves into flight from level ground by leaping from all four limbs from a standstill, without the need of cliffs. Azhdarchids were most-likely flap-gliders—capable of short bursts of powered flight while covering long distances by soaring on thermal currents.
A group of Quetzalcoatlus northropi, foraging
on a Cretaceous fern prairie. Illustration by
Mark Witton/University of Portsmouth.
   Azhdarchids have often been portrayed as feeding by grabbing fish as they skimmed the water, but it is more likely that they stalked their prey on land. Their fossils are usually found with terrestrial dinosaurs more typically found in semi-arid inland plains. They were well suited for walking and had feet that were small and padded, better for walking on solid ground rather than wading. They preyed on small dinosaurs and other animals up to the size of a large dog. Like a modern-day stork, an azhdarchid would pick up its prey, toss it to the back of its mouth, and swallow it whole.

Monday, June 4, 2012

Photons Are Weird

Single-slit pattern (above) and
double-slit pattern (below).

The classic double-slit experiment is a really good example of an amazing quantum phenomenon: interference of photons, and gets to the heart of the wave-particle duality that photons exhibit. Imagine a small light source in an otherwise dark room. On the other side of the room is a photographic plate. If we shoot a stream of photons at it, the plate will turn black. If we place a sheet of metal in between the two, the photographic plate will not turn black. 
   Now, let’s cut two thin parallel slits in the sheet metal and run a few experiments. If we cover the right slit and turn on the light, after a while we will get a black band on the film where the light has passed though the left slit. Next, open the right slit and  cover the left slit. That will give us another black band that partially overlaps the first one.
   Next, with a fresh photographic plate, open both slits. Surprisingly, you do not get the same pattern on the plate. Instead, you will get a zebra striped pattern—a series of dark and light bands. the light bands are areas of destructive interference, a well-know property of waves. This is where the light going through both slits cancel out.
   Let’s try doing the whole experiment again, but this time we will turn down the intensity of the light source so that only one photon comes through at a time, like a dripping faucet. When we cover one slit and expose the plate for a short while, we will see a few dark dots appear where the  individual photons landed. If we let it run some more, we get more dots, and if we let it run long enough we will reproduce the results of the first experiment. The photons are arriving randomly, and only after enough of them accumulate do we see the original pattern.
   Finally, let’s run this new experiment with both slits open. Remember, only one photon is passing through either slit at a time. Yet in this experiment, the photon do not land where the destructive interference took place [The figure at right is the result of a double-slit experiment showing the build-up of an interference pattern of single electrons. The numbers of electrons are 11 (a), 200 (b), 6,000 (c), 40,000 (d), and 140,000 (e).] This is despite the fact that photons can and do arrive at the very same spot if only one slit is open. In other words, if the left slit is open and the rights slit is closed, or if the right slit is open and the left slit is closed, the photon can reach the point where destructive interference takes place. But if both slits are open, not a single photon will get to that same spot. How can a single photon destructively interfere with itself? How would a photon, about to pass through the right slit, know whether or not the left slit was closed? The truth is hard to explain, but in reality the photon is not passing through either slit. It is instead going through both paths at the same time, destructively interfering with itself. 
   The double-slit experiment is remarkable in that it clearly  expresses the puzzling nature of the quantum world. The great physicist Richard Feynman said that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment: "One would normally think that opening a second hole would always increase the amount of light reaching the detector, but that's not what actually happens. And so saying that light goes either one way or the other is false. I still catch myself saying, Well, it goes either this way or that way, but when I say that, I have to keep in mind that I mean in the sense of adding amplitudes: the photon has an amplitude to go one way, and an amplitude to go the other way." I told you photons are weird!