Where Grey Matter meets Dark Matter

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Episode 9 - 9 April 2009

This week we interviewed Andrew Prentice, Department of Mathematical Sciences, Monash University about the origin of the solar system.

The dominant hypothesis of solar system origin, the one that most people would have learned in high school science, is that the solar system was once a big, rotating ball of gas, probably thrown off by the dying remains of a massive stellar ancestor of our sun. (Why was it rotating, you ask? Everything in space is rotating.) As the gas molecules feel the mutual gravitational attraction of their neighouring molecules, the ball contracts, and as it does so it starts spinning faster. Think of the hackneyed, yet handy, example of the ice-skater: as she pulls her arms in to her body she spins faster. For those so inclined, the technical term for this is 'conservation of angular momentum', one of the fundamental assumptions in physics: as the ball contracts, the moment of inertia is decreasing and it needs to spin faster to maintain the same angular momentum.

As the ball contracts further and spins faster, a huge disk - called the 'accretion disk' - forms around the equator. It would look something like a solar system-wide version of Saturn's rings. Now this disk has irregularities in it: some parts are more dense and some parts are less dense. The planets formed in the densest regions, sucking up the material from the less dense areas.

Andrew Prentice, a mathematician at Monash University in Melbourne, has a very different idea, called the Modern Laplacian Theory (MLT). Everybody agrees with the giant rotating gas ball that spins faster as it contracts, but in Andrew's model there is no accretion disk. Rather, as the ball is contracting a 'density inversion' happens at the surface of the ball. In other words, a thin 'crust' of denser gas forms around the outside. (It's not really a hard crust, just more dense than most of the rest of the ball.) The density inversion is represented in the following graph. The green line represents the density as the crust builds up. The centre of the ball is at height=0.0 and the surface is at height=1.0.


As the ball spins, this crust slides down the surface and collects around the equator in a sort of belt, like a hula hoop on a morbidly obese person, shown in the second of the following two pictures:

This belt is then cast off by the ball and it continues to contract, until the ball is dense enough to form the sun. Each of these belts or 'gas rings' (as Andrew calls them) becomes a planet. Here's an 'artist's impression' of the formation (start in the top left and go down the column and then move to the top right and down the second column):

This type of process was proposed by the Marquis de Laplace nearly 200 years ago, but he couldn't figure out how it would work. Indeed, Andrew faced a significant hurdle as well: the model requires something called 'supersonic turbulence' to build up this crust. Each material (liquid, solid or gas, or even superfluid) will vibrate if it is disturbed. The speed of these waves of disturbance define the speed of sound in that material (since sound is just waves of disturbance moving through a medium). So in general, if you make a disturbance in a gas, the waves from the disturbance will travel outward from the source at the speed of sound for that material. However, 'supersonic turbulence' seems to be saying that a disturbance will move through the medium faster than a disturbance will move through the medium. Adds up to trouble. (There are more complicated arguments revolving around shock waves and energy dissipation, but whole textbooks can be written about that stuff.)

But Laplace didn't have access to calculating machines that can grind through billions of calculations per second. Andrew and his student managed to show that with appropriate conditions, supersonic turbulence might have occurred in the early solar system. So the theory is not theoretically untenable.

As for its predictive power, the theory makes several claims. For example, Andrew has been able to predict the composition of several of the planets and their moons to a high degree of accuracy. The reasoning comes from the temperatures at which different materials should have condensed out of the theory. In the following graph, the solid white line plots the density versus distance as predicted by the theory, and some actual observations are plotted plotted with error bars:

Here is a similar graph where the theory was applied to the formation of the Saturnian system (ie. Saturn and its moons):

This paper (from the 39th Lunar and Planetary Science Conference in 2008) contains predictions about what the Messenger probe should have found at Mercury when it flew by in January 2008. One notable prediction is that no potassium-40 (which is radioactive) should be found - and none was. And here's a list of other predictions that you can check out:

The article that Andrew referred to about his predictions for the moons of Neptune before Voyager arrived, which he managed to get published in Japanese in Newton, is shown here:

This is one legitimate way to do science: assume something is true, and then figure out what you can predict with this assumption. If these predictions turn out to be right, then you can be more confident that your assumption is true. But he hasn't convinced everyone; the standard theory is not to be thrown off so lightly. But as we send out more and more probes to the other planets, hopefully the weight of data will decide the issue for us, in true science fashion - like sensible jumpers and socks worn with sandals.

Andrew used a few equations in his interview, and so we'd better explain what they mean. Honestly, they're not that bad. THE EQUATIONS Oh God the equations!!


Pictures courtesy Andrew Prentice.

BETA: In the not too distant past, bloodlines were used for all manner of discrimination. For centuries there was no known mechanism for this theory, but it was apparent to everybody that the people related to them were better than those that weren't - especially repugnent were those 'people' (if you could call them that) that looked different. Wars were fought over whose haemoglobin sat on the throne. As long as you stayed where your lineage put you, you had done your service.

Of course, society (largely) moved into a more enlightened age; where a person's worth was not determined by that of their ancestors, but was made by them alone in a world full of possibility, or so we tell children. A definite improvement.

However, perhaps the old-school had it right all along, perhaps there is something in our blood that determines what sort of person we are and how worthwhile our contribution may be. This is the subject of Gattaca.

The argument of nature vs nurture is probably more hotly contested in this age than at any point in the past. Social, political, economic and scientific viewpoints on the debate fall on both sides. Rather than just a crass guess that the blood of two people determines the blood of the offspring, now we are equipped with sophisticated language like 'DNA', 'genes', 'alleles', 'discrimination' and 'internalised oppression'.

The study of genetics, which began in the 19th Century (with Gregor Mendel and his pea plants), could turn out to be the most transformative thing in the entire tragi-comedy of humans. Crudely speaking, genes are the instructions that tell your bodily machinery how to make you. The genetic code contains information that has a 'large' effect on your bodily hardware. And it is the size of this 'large' that is the topic of debate. Certainly there are genetic traits that make heart disease more common, but smoking and junk food are also risk factors. What is the most important?

At the molecular level, DNA starts with 2 parallel chains of alternating sugar and phosphate molecules. Along each backbone is attached a series of bases (the opposite of acids), in any order you please, called guanine (G), adenine (A), cytosine (C) or thymine (T). This series of GATC is the genetic code. Each of these bases is matched to another base on the other backbone, with G attached to C and A attached to T. So DNA is a double string of GATC letters matched by their pairs on the other backbone. The shape this molecule makes is called a double-helix, which looks something like 2 curly phone cords wrapped around each other.

If that was the end of the story, then we would each be just a smear of gooey slime. Rather, in a complicated chemical process involing another acronym (RNA), these bases tell other molecules what to do, including the production of stuff to build your body. A segment of the DNA that contains the code for something is called a genes. It it the collection of genes that make one embryo different from another.

The world of Gattaca is one where this string of genetic code has come to be regarded as a person's defining characteristic. Everything is determined based on the information in the DNA: entry into school, employment, insurance availability, social status, etc. The aristocrats of old have been replaced by the 'valids', meaning the genetically superior.

In order to be a 'valid' your parents go through an in-vitro fertilisation (IVF) procedure, where all the resulting embryos are examined for genetic defects and positive traits. Only the most superior embryos are then implanted for birth.

And this is not very far from the world of 2009. In the case of scanning for abnormalities, this process is called Pre-implantation Genetic Diagnosis (PGD), and is already taking place in Australia and the US. The latter article also points out that this is illegal in some European countries. And for more cosmetic choices: recently an IVF clinic has announced it can offer parents the choice of hair and eye colour (in addition to gender which has been available for some time) for their children (although it has since withdrawn the offer, because of public outcry, not for technical reasons). It is only a technical hurdle (albeit a rather high hurdle) to a situation where more complex traits can be selected for.

Of course, in our world discrimination based upon race, gender, height, hair colour etc... (read: genes) is illegal, as it is in Gattaca, but in the film it is not taken seriously. And one wonders if the same won't happen to us. In fact, this sort of discrimination is already happening: insurance companies are very interested in the genetic makeup of their valued clients. Predispositions to any ailments make a handy tool for jacking up premiums*, or refusing cover altogether. It's almost as though insurers are not the caring philanthropists that their advertising would have us believe. In the US there is a brouhaha being raised over the privacy of genetic information, and the restrictions placed on employers and insurers to avoid an exact Gattaca-esque situation (article from the Sunday New York Times).

This debate is not likely to go away any time soon, with ethical quandries on all sides: is it ethical for those that can afford genetic selection to have it, while poorer people don't? Is it ethical to allow children to be born with genetic defects when we have the technology to avoid it? Should parents have the final say, or indeed, any say, in the process?

Beats us, but if you have an opinion, I'm sure there are several places on world-wide internet for you to vent them. And you will probably convince no-one.


Quiz: Again, the info for the quiz this week was mostly taken from: Asimov, Isaac (ed.) (1993), The giant book of facts and trivia, Magpie Books, London. But this source is looking shakier all the time.

* Anthony insists that this should be 'premia'. Still, Anita gets the final edit, so too bad for him.

Oops. Mistakes we shouldn't have made but did:

Things got a little confusing when talking about the Nobel men in the quiz. To clarify: Alfred Nobel invented dynamite and instituted the Nobel Prize. Immanuel Nobel was his father. Different sources attribute the invention of plywood to either Alfred or Immanuel.


And of course, Anthony promised the David Lee Roth soundboard - here it is, with rock: http://www.thetyser.com/

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