Astroblog

I’ve been thinking about comets a lot lately, trying to image C/2009 G1, reading “The Hunt for Planet X”, wondering why Galileo was so wrong about them and recently reading a creationist blog post on them. The latter referred to a very interesting pre-publication article. And I’d like to discuss this article, as this illuminates not only the origins of comets but also how science is done.

Where do comets come from? They scream in from the outer dark, briefly flash across the skies to our wonder then return to the dark. Actually, there are multiple answers to this question. We can divide comets into two basic types, long period comets and short period comets. Long period comets have their origin deep within the outer reaches of the solar system, they will return to the warm embrace of the Sun on geological time scales, if at all. Short period comets come from within the solar system, and can be further divided into Jupiter Family comets, with an orbital period of 20 years or less whose orbits extend not much further than that of Jupiter, and Halley Family comets, with periods of between 20 to 200 years.

Source Hubble Space Telescope:

But this is only part of the answer to “where do comets come from?” Comets are basically dirty snowballs, and as the Jupiter Family comets swing close to the Sun, a little bit of them evaporates. The spectacular (or not so spectacular) tails that awe us are composed of comet material boiling away into space [1]. It’s obvious that comets can’t last forever, and eventually the ice will evaporate away, leaving an inert core of rubble and dust which can no longer be called a comet (or in extreme cases disintegrate into a stream of rubble).

We have examples of comet death in the spectacular decay of comets such as 73P/Schwassmann-Wachmann 3 and the asteroid/comets with feeble tails. The time for a Jupiter family comet to melt away is much shorter than the age of the solar system (around 300,000 years compared to the age of the solar system, around 5 billion years, Earth is around 4.5 billion years old). Any comets present in short period orbits at the formation of the solar system will be long gone by now, but we obviously have active short period comets. Where do they come from?

Source: Wikipedia One of the first proposals was that the short period comets, like the long period comets, come from the icy dark that surrounds the solar system, and represent long period comets captured into short period orbits by interactions with the planets. However, while some short period comets may come from this source, it’s unlikely that most of them do.

Firstly the short period comets are more or less confined to the orbital plane of the solar system (but not as much as the planets), where as long period comets come from all over. Secondly, simulations showed that too few comets could be captured this way to explain the number of comets we see.

On the basis of dynamic simulations, it was predicted that there was a band of icy objects lying beyond the orbit of Neptune, and comets were the results of small icy bodies being bounced into the inner solar system by gravitational interactions with the planets and other icy bodies. A hunt for these objects soon found them [2], and the discovery of the Kuiper Belt a spectacular vindication of cometary theory. Now, cometary objects at that size are too small to be seen, even by the Hubble telescope, but counts of larger objects found that the size distribution followed a power law (with of course, more objects at smaller sizes than at larger sizes), and extrapolation of the best surveys indicated there was more than enough objects in the Kuiper Belt to account for the number of comets we see today.

Which brings us to this paper by Volk and Malhotra. They have done one of the most exhaustive simulations of cometary dynamics to date, to estimate the necessary population of Kuiper Belt objects needed to produce the number of comets we see today, with some of the best estimates of the actually Kuiper Belt population.

And they come up with a shortfall, there appears to be too few small icy objects in the Kupier Belt to account for the number of comets we see. Now, I’ve been simplifying things a bit. The cometary group they were studying is a subset of the short period comets, the Jupiter family comets. These are the most numerous short period comets. Also, the Kuiper belt is not homogeneous, being made up of the classic Kuiper Belt, the Scattered Disk and various bodies in orbital resonance, such as the Plutinos. The work of Volk and Malhotra refers to the origin of Jupiter Family Comets from the Scattered Disk. Anyway, they concluded that there is over two order of magnitude fewer objects in the scattered disk than is needed to provide the number of comets we see today. The creationists have seized upon this, if we have too many comets they say, and comets cannot survive a long time, therefore the soar system must be young. Lets leave aside the numerous problems with this argument and look at the actual paper for a moment.

Source: NASA and A. Feild
The authors calculate that around 1 x10⁸ objects are required to provide the number of comets we see today, but they estimate that there are only 3x10⁵ comet-sized objects in the Scattered Disk of the Kuiper Belt. This is a fairly large discrepancy, but then we come to the confidence interval. The 95% confidence interval, that is the region where we are statistically confident there is a 95% chance the true number of comet sized objects lies in that region, runs from 1x10⁵ to 2x10⁸.

Oh.

In other words, our degree of uncertainly about the true number of Kuiper belt objects is so large, that it may very well be sufficient to explain the number of comets we see. Now, I am not a professional astronomer, nor do I play one on TV. But I do do a heck of a lot of curve fitting and statistics, so I am qualified to comment on this.

Why it's hard to count faint Kuiper belt objects Source: Fig1 Bernsetin et al., 2004.

Why is there such a large error range? As I mentioned before, we can’t actually see comet sized objects in the Kuiper Belt, even with Hubble. We have to estimate their number by extrapolating from the size distribution of objects we can see. Early estimates gave lots of objects, later surveys, looking at dimmer objects, gave a smaller estimate. The Volk and Malhotra article uses a Hubble survey estimate, which looked at the faintest objects yet. However, it is very easy to miss objects, as the history of astronomy shows when we have missed very large icy objects (automated systems may miss faint objects, the survey may be looking where objects aren’t etc.). So the Hubble estimates come with very large error bars. Other deep surveys (although not as deep as Hubble) find more objects.

Estimates of small Kuiper belt objects from one of the most recent surveys ( Source: Fig6, Fraser et al., 2008). Note the large error bars.

The very best we can say is that we do not have enough data to make a definitive estimate of the number of comet-like Kupier objects, and we will have to wait until better, deeper surveys are done.

The creationist says

“The simplest explanation as to why we can still see short-period comets is that the solar system is young.”

No. Making the solar system young does not solve this (non)issue. Apart from requiring physics (the kind that allows our digital watches to run, mobile phones and car GPS’s to work) to be badly broken (we would have notice d by now if physics was that badly wrong), it explains nothing about comets that we would like to know. Why is the median age of Jupiter family comets 300,000 years, (not possible in a 6-10,000 year old solar system) and where do the nearly exhausted comets come from if the solar system is much younger than the median comet lifetime? Why are there 2:3 and other resonances in the Kuiper belt, resonances that will take millions of years to form? If the solar syetm is only 6-10,000 years old, why are comets in the inner solar system at all (and why do comets have the chemical composition of Kuiper Belt objects?)

There are a number of ways we can resolve the comet “problem”. Better estimates of Kuiper belt objects may show there are enough objects, other parts of the Kuiper Belt may supply comets, comet fragmentation (as seen with 73P/Schwassmann-Wachmann 3), as the objects move from the Kuiper Belt to the inner solar system, may provide more objects. There are a number of different ways which we can resolve this, none of which break normal physics, and you can bet that astronomers will investigate them.

Now, let’s have a look at how science operates. The Kuiper belt was a prediction bsed on our knowledge of comets, and that prediction was spectacularly confirmed. Instead of resting on their laurels, astronomers tried to estimate the numbers of Kuiper Belt objects, to see if it fit with their theories. Despite early agreement, they kept on refining their estimates. And when their estimates seemed problematical, did they hide them? No, they are published in open forums so that people can understand and work on the problem.

On the other hand, creationists only prediction was that the Kupier Belt was a figment of “evolutionists” imagination, when found, they did nothing but carp about it. Creationism has yet to give up on any of its theories, does no original work, and basically acts as a reaction to the findings of researchers.

Whatever the resolution to the Kuiper Belt conundrum, you can bet it will be scientists, not creationists, who will work it out Cand that creationists won’t like the answers)

[1] It doesn’t actually boil, but rather sublimes, vigorously.
[2] “soon” involving hundreds of person hours, years of patient watching and poring over plates and digital media, but “soon” in the context of how long it took to discover Pluto, for example.

Labels: Astronomy, comets, creationism

Back in March I wrote a post “Occam's Razor and the Two World Systems”, where I discussed the idea propounded by William Wallace that Occam’s razor failed when applied to the transition from the Ptolemaic geocentric solar system to the Copernican heliocentric solar system. Mr Wallace replied on that post and I promised to respond, but life (and various asteroids and comets) got in the way. Rather than add new comments there. I’ll take this up as a new post.

The starting point of the old post was Mr Wallace’s contention:

I have a counter example, from science, fully described here, where Occam's razor fails. The idea that the "simplest explanation is probably right" was falsified when Tycho Bryhe [sic] rejected heliocentricism for geocentricism, based on his analysis of the best available data.

Once again, I must emphasise that Occam’s razor is not the simplest explanation is probably right, but let’s pick up on Mr. Wallace’s comments. I’m going to do this slightly out of order, so as to address particular issues that arise from the quotes more logically

There were three systems, Copernicus, classical Ptolemaic, and neo-Ptolemaic (or Tycho's). Both Ptolemy's and Tycho's could be accurately described as geocentricism. I just want to clarify that I don't think I ever claimed--and I certainly never intended to claim--that Tycho held a purely Ptolemaic view of the solar system. He held a geocentric view, more consistent with Ptolemy's than Copernicus's (though Kepler's was arguably more akin to Copernicus's than Tycho's was to Ptolemy's.)

This is so very, very wrong. Tycho’s system was not a neo-Ptolemaic system, as I describe below, Tycho explicitly rejected the Ptolemaic system based on observational evidence. Tycho accepted most of Copernicus’s system, but balked at a moveable Earth and produced a Heliocentric-geocentric system (not a geocentric system, for another animation of the respective systems see here). Indeed it is an “inverted Copernican” system, basically a remapping of the Copernican system into a reference frame centred on Earth. Tycho’s system contains nothing of Ptolemy; there are no equants, no great epicycles and so on[1]. I’ll emphasise that again, Tycho’s system was not a re-jigging of Ptolemy, it was an inversion of Copernicus’s system.

The only point of similarity between Ptolemy’s system and Tycho’s system was the Earth was immobile in both systems, but in Tycho’s, everything but the Moon rotated around the Sun. Again, the key features of Ptolemy’s system, the equant and the great epicycles were absent from Tycho’s formulation. This may seem like mere pettifoggery, after all, both Ptolemy and Tycho had the Earth as the centre of the solar system, but Ptolemy’s system was not just “everything goes around the Earth”. Tycho had explicitly rejected the Ptolemaic system, all the key features of the Ptolemaic system, taken all the planets and placed them around the Sun and grounded his system on a reformulation of the Copernican system.

Tycho's system required fewer calculations and was consistent with all known measurements. Copernicus's was not consistent with observations.

Tycho's system required roughly the same number of calculations as Copernicus’s, as it was basically Copernicus’s translated into Earths reference frame [2]. Tycho fully accepted Copernican geometry, used Copernican planetary models, and where he did not explicitly do his own calculations, he used planetary tables based on Copernican models. Where Tycho was more accurate than Copernicus, it was through his greater observational accuracy, not any superiority of his model. Indeed, when he talks of his system, he notes that he has kept all the superior features of the Copernican system. Ironically, it was Tycho’s own measurements of the opposition of Mars in 1582 that showed that the Copernican model was in better agreement with the observed retrograde motion of Mars than the Ptolemaic model.

And yet Copernicus' model defied experimental verification, using the best astronomical apparatus of the time (Tycho's).

Actually, that’s not true. There was a key test of the Ptolemic vs Copernican system and the Copernican system came through with flying colours. It was one of the key results that forced Tycho to reject the Ptolemaic system. However, that test tends to be ignored these days because of modern commentators focusing on the issue of stellar parallax.

We now know that stellar parallax could not be measured with instruments available then. It wasn’t obvious back then, and there were several attempts to measure the stellar parallax. Tycho established that the stellar parallax had to be less than 0.1 degree. However, this did not falsify the Copernican model as negligible parallax was an a priori feature of the Copernican system.

Tycho was well aware of this, and his argument was not that the lack of parallax showed that the Earth did not move, but that if the parallax was so small the distance between Saturn and the stellar sphere would have to be 700 times the distance between the Sun and Saturn. A provident creator would not be so wasteful of space and produce such asymmetry in his creation.
So we can see that the stellar parallax argument as used by Tycho is in the end not a scientific argument.

So what was the measurement that convinced Tycho to reject the Ptolemic system? Ironically, it was a parallax measurement. One of the predictions of the Copernican system was that Mars would be closer to Earth than the Sun at opposition; while in the Ptolemaic system Mars would always be more distant from the Sun. Tycho’s careful measurements of the parallax of Mars showed that Mars was closer to Earth than the Sun. Similar measurement of Venus’s parallax by Tycho also confirmed Copernican predictions. Copernicus was right, Ptolemy was wrong, and Tycho unhesitatingly rejects Ptolemy. But in the end Tycho rejects both Ptolemy and Copernicus.

My ultimate point about Tycho's rejection of Copernican theory was that applying principles of modern science sometimes leads one astray.

Erm. Mr. Wallace. This is what you said:

I have a counter example, from science, fully described here, where Occam's razor fails...

In this case, it was Tycho’s system failed the Occam’s razor test. It requires more assumptions than the Copernican system, introduces two centres of motion, introduces more problems with motions and whilst reproduces the retrograde motions of the planets, it no longer provides these motions as a natural consequence of the orbits of the planets. Altogether it is far less economical than the Copernican system and no longer explains the precession of the equinoxes and is affected by solar eccentricity (and this was pointed out at the time). Occam’s razor doesn’t fail, because Occam’s razor was not used to make the choice between the Copernican and Tychonian systems. What was the basis of Tycho’s rejection of the Copernican system? Let Tycho tell us in his own (translated) words.

Since all these results [parallax measurements of Mars and Venus] did not all agree with the Ptolemaic hypotheses I was urged afterward to put more and more confidence in the Copernican invention. The exceedingly absurd opinion that the Earth revolves uniformly and perpetually nevertheless made up a very great obstacle, and in addition the irrefutable authority of the Holy Scripture maintained the opposite view. [emphasis added]

It wasn’t the principles of modern science that lead Tycho astray.

[1] The equant was seen as a particular drawback to Ptolemy’s system, if one wanted to explain planetary motion in terms of uniform circular motion, and Copernicus reformulation of planetary motion so that the equant was dispensed with. This was seen by contemporary astronomers and mathermaticians and a great benefit of the system. Tycho too saw the equant as something undesirable, and the great epicycles of Ptolemy as unparsimonius.
[2] Tycho got rid of Librations, but generally his scheme was Copernicus’s recast from Earth’s frame of reference.
[3] Ironically, Tycho’s system made the acceptance of the Copernican system easier. It made people familiar with the Copernican system, and smashed key sections of Aristotelian physics which have proved a barrier to acceptance of he Copernican system.

Sources:
The Reception of Copernicus’s Heliocentric Theory 1973 ed Jerzy Dobryzcki, Reidle esp Chapter 3.
Copernicus, On the Revolution of the Heavenly Spheres (translation AM Duncan 1976) Barnes & Nobel
The Copernican Revolution 1957, TS Kuhn Harvard University Press
The Book Nobody Read 2004, O Gingerich, Walker & Company

Labels: Copernicus, creationism, Galileo

Over at Todd Woods blog until recently you could have found this interesting (and unintentionally amusing) statement.

"Anyone who knows me at all knows that I break down creationist biology into four main components: design, natural evil, systematics, speciation, and biogeography."

This automatically channels the Monty Python Spanish Inquisition sketch "Amongst our weapons are ..." Anyway, he fixed the numeric discrepancy (without acknowledging the blogosphere who pointed it out), but minor typographical errors are not the reason biologists laugh at creationists [1]. The reason we laugh at them is they are so gormless about biology.

The point Dr Wood is making in that post is that animals (hey what about plants and protists) are "designed" to kill, which implies that God had an *active* role in making all things loathsome and nasty (cue Monty Python again), rather than us sinful humans being at fault and the world just degenerating.

It is mildly amusing to watch the gears mesh, then miss completely obvious points. The problem is he is trying to explain the biological world without a understanding of even basic biology.

"Look at bears, for example. There you have a baramin with species that will
eat just about anything, and if you include the giant panda, then you have an
obligate herbivore. It would be easy to say that predators are merely predators
because of behaviour. At the Fall, God just modified their behaviour from eating
just plants to eating meat."

Cue the Gary Larson cartoon of two bears watching children at play, and one says "Come on Harvey, look at these claws, these teeth, they're not for eating nuts and berries".

Now, Dr. Wood uses the bear as a too simple example, rather than a key exemplar. But the real point is the biology as stated is wrong. Simple examination of the biology of the Panda shows it's a carnivore modified to be a herbivore, rather than bears being herbivores modified to be carnivores (indeed the Pandas gut is only very slightly modified from the carnivore version). Again, this is why biologists laugh at Creationists, they just can't get the biology right.

Natural evil is a real problem for Creationists [2], if all was good and warm and fuzzy with no death before the fall, and only degeneration is allowed, where do carnivores with their splendid adaptations for killing and eating come from? The cannot allow themselves to consider evolution of adaptations (they will allow some evolution, but only trivial amounts). Bear evolution, for example, where bears originate from a long line of carnivores, is shut out from their minds. Carnivory has a long history, with even some bacteria (including the superbly named Vampirococcus) being predators.

In example after example, Dr. Wood shows that biology is an incoherent mess without the unifying thread of evolution. And he doesn't get it. Yeah, Man sins, evil and death enters the world and suddenly Drosera start thinking, "Hey, I've got these sticky tentacles, and digestive enzymes, I'm gonna get me some MEAT".

And what about the saphrophitic fungi eh? Why do they get short shrift, or Vampirococcus?

As another example of the problem Creationist have with biology, I will refer you "Fellowship, Creation and Schistosomes" (Mace, Simms and Wood 2003) where Dr. Wood and others try to explain how the Schistosome parasites could have arisen [3]. Note that this is a technical publication, a supposedly serious research article like a scientific paper. Now this was published back in 2003, and Dr. Woods position has eh, evolved a bit since then, but the paper is still cited as an example of serious creationist "research" and neatly illustrates the problems therein.

Now creationists have a problem with Schistosomes. These are parasites that have a complicated life cycle. The eggs of the parasite are released in the faeces and if they come in contact with water they hatch into a miracidia, a free-swimming larva. The larva then has to infect a snail of the correct genus within one or two days. Inside the snail, the larva undergo asexual reproduction. After this another free-swimming larva, the cercaria, is shed into the environment and must infect a suitable vertebrate host. Once the cercaria penetrates the skin of the host it loses its tail and becomes a schistosomule. The worms then migrate through the blood vessels ending in the mesenteric veins where they mate and start laying eggs in the vessels of the intestinal wall. The eggs burrow through through the tissues and are passed in the faeces.

Now the creationist problem, as stated by Dr. Wood, is that since everything was perfect before the fall, disease only entered the world after the fall. Also, the only post-fall developments could be degenerative, we can't have any of the nasty evolution stuff (at least on anything other than trivial scales). But the complex life cycle of the Schistosomes clearly is a complex adaptation not a degeneration. How can they explain this?

They decide that the Schistosmes were originally symbiotic (or mutualistic in some way), and then degenerated into a parasitic condition, possibly by infecting the wrong host. This is in itself not a silly idea, organisms that normally live elsewhere and accidentally get into the wrong tissues of the host can be an important source of disease, like gangrene. But it's how they go about trying to establish this reveals their ignorance of biology.

First they try and establish that Schistosomes are a "Created Kind" and compare them to the liver flukes (Fasciola). Now liver flukes also have a complex life cycle similar to the Schistosomes, with larva in molluscan hosts, and the formation of cecaria. On the basis that Schistosomes are sexual, and Fasciola are hermaphrodite and have a slightly different gut architecture, and ones cercaria must be eaten (but still actively burrows in through the gut lining), but the other directly invades via the skin, that they must be two separate "Created Kinds".

Well, we couldn't have any of that nasty adaptive evolution happening could we? Except of course that Schistosomes and Fasciola are but two representatives of trematoda, a large class of parasitic worms that include a two host cycle, with one stage usually occurring in molluscs, the second usually in vertebrates. The cercaria is common to all the trematodes. As well as the liver flukes, there are the lung Flukes (Paragonimidae) and the intestinal Flukes (Fascicoliposis). These are just the ones of disease importance to humans. Then there is Bivesiculidae, the Transversotremidae, the Lepocreadidae and on and on. Indeed there are over 180 families of trematodes which have the life cycle and larval stages (including cercaria) associated with Schistosomes. Indeed, the Schistosomes considered by Wood et al are just one genus in the family Schistosomatidae, venous system specialists that infect reptiles, birds and mammals (including humans). Then there is the family of Spirochidae, blood vessel generalists that infect reptiles and Sanguinicolidae, blood system generalists that infect fish (See diagram above right, taken from Brant and Loker, 2005). None of this is new, and was well established by 2003, when Wood's paper was published.

Now, Mace et al compare just two genera, and pronounce them separate "created kinds", but when we have hundreds of genera, all with the Mollusc - Vertebrate infection cycle involving cercaria, then the idea that all these genera represent "created kinds" is just a trace unparsimonious (and can invoke Occam's Razor). Or we could say that the "created kinds" just extend to the organ system specialization, but then there is massive diversity which means a whole lot of adaptive evolution is going on, with over a hundred of genera in any given family forming adaptive diversity in a variety of hosts and environments. And creationists just don't like admitting to adaptive evolution.

Then there is the one diagram in the article, which show hybridization data (which amusingly replicates the data shown by DNA studies, but those more modern studies imply that the Schistosomes and other trematodes are related by common descent[4]) and infection data. Since the Schistosomes that infect humans have a broad species infectivity, so they take this as evidence that Schistosomes came to cause disease by invading the wrong host. However, the fact that even the Schistosomes that have a very narrow specificity (about a third of them) also are parasites too is a strong strike against this idea. Note that all Schistosomes are parasites. Indeed the over 1000 trematode species we know are all parasites.

Note the pattern here? Mace et al, want the Schistosomes to be "fallen" versions of a mutalistic organism. What kind of mutualism requires a worm to burrow into the circulation and live in blood vessels is not explained, and there are no good examples of such extreme invasive mutualism. Indeed, given the extreme diversity of trematode worms, and the blood inhabiting trematodes (Schistosomatidae, Spirochidae and Sanguinicolidae) the utter lack of a mutatistic version of the trematodes is very telling.

What's more, it is hard to fit the actual biology of Schistosome infection with a "fallen" status. Schistosomes are venous specialists (except for one species that specializes in arteries). They must lay their eggs against the flow of blood, and engage in precision placement of eggs, the females back into the smallest venules and release eggs where the veins clamp down on them and hold the egg in place. The eggs them migrate through the blood vessels and into the gut. Such precision adaptations are incompatible with a "fallen" state.

The recurring observations that supposedly "fallen" species have significant adaptations to perform their parasitism and carnivory has resulted in Dr. Woods musings. Creationists cannot accept that evolution is responsible for adaptations, so they must tie themselves in knots trying to explain how God designed in these adaptations (before of after the fall), and thus produced natural evil.

This is why biologists laugh at creationists, they make their arguments with almost complete ignorance of biology, and even then the evidence of their cartoon version forces them into a corner that only evolution can get them out of.

Oh yeah, the reference list. Almost all the real biology articles are very elderly, and ignores the comprehensive reviews of the evolution of of the Schistosomes that were easily available when the article was written. Note the authors did no original research themselves, just reviewed data obtained by real biologists.

References:
Brant SV and Loker ES (2005) PLOS Patogens, 1, e38
Platt TR and Brooks DR (1997) The Journal of Parasitology 83, 1035-1044
Cribb TH, Bray RA and Littlewood DTJ (2001) International Journal of Parasitology 31, 997-1011.
Mace SR, Simms BA and Wood TC (2003) Impact 357, i-iii

[1] With my ability to create typographic errors, who am I to complain?
[2] As an Australian, growing up with our major religious traditions being Anglican, Catholic and Presbyterian, I find the literalists bemusing. We all get the idea that Genesis is filled with analogy and metaphor, heck I got that at age 10, when I was getting gold stars for my Bible verses, so why can't the Creationists get it?
[3] While not a professional parasitologist, I spent 3 years a a laboratory Scientist, and one of my duties was parasite analysis. So I know my way around parasites. It only took me a few minutes to find several contemporaneous references to the evolution of schistosomes that Mace et al didn't cite.
[4] The creationists accept that species which hybridize are related by common descent, just like real biologists do. And the DNA data show that hybridizing species are most closely related to one another, and other similar species are related, and slightly less similar species are related , and so on with no sign of the "discontinuity" the creationists insist on to separate "created kinds".

Labels: creationism

Over at my blogging buddy ERV's place, there's a lot of arguing going on [1], which at one stage segued into a silly argument about Occam's Razor and the Heliocentric vs Geocentric theories (See William Wallace's post and subsequent posts, and William Wallace's own webpost on this topic).

First off, Occam's razor is not "Choose the simplest explanation". Occam said entia non sunt multiplicanda praeter necessitatem, which can be translated as "entities must not be multiplied beyond necessity"[2]. These days, in science we understand Occam's razor as (quoting Wikipedia)

When multiple competing hypotheses are equal in other respects, the principle recommends selecting the hypothesis that introduces the fewest assumptions and postulates the fewest entities.

As well, you need to understand that in describing cosmological systems, the preference from antiquity to Galileo was to explain heavenly motions in terms of uniform, circular motions. We may see this as weird or foolish now, but it wasn't back then. Now, on to William Wallace.

I have a counter example, from science, fully described here, where Occam's razor fails. The idea that the "simplest explanation is probably right" was falsified when Tycho Bryhe [sic] rejected heliocentricism for geocentricism, based on his analysis of the best available data.

Okay, there are several things wrong with this. One: Occam's razor isn't "simplest explanation is probably right" (see above). Secondly, Tycho Brahe didn't reject heliocentrism for geocentricism.

A little explanation. If the Earth is the centre of the universe, with the planets and fixed stars revolving around it with uniform circular motions, you should see the all the planets move smoothly across the sky at the same pace and the same brightness. But they don't. The inner planets, Mercury and Venus, never move far from the Sun and change radically in brightness. The outer planets move across the sky, then stop, reverse their motion, stop again, then move back the way they were going in the first place. Any geocentric explanation has to explain this retrograde motion.

Ptolemy solved these problems. First, he had the planets not orbit the earth directly, but orbit a point that itself orbited the Earth. This is the epicycle (for an animation see here). Mercury and Venus required somewhat more complicated solutions (see here for an animation of Mercruy in a geocentric system) than for the outer planets. Even so, this system still didn't quite work. Ptolemy also introduced the equant, the planets did not in fact orbit the Earth, but orbited a point some distance away from the Earth. It worked, and the Ptolemaic geocentric system was the standard description of the Solar system until the time of Copernicus. Copernicus, of course, moved the centre of the universe to the Sun, and had all the planets, including Earth, revolve around that.

Now, in terms of Occam's razor, Copernicus's heliocentric system should have been an outright winner. Both the Ptolemaic system and the Copernican system explained retrograde orbits and the reason Venus and Mercury never travelled far from the Sun, but the Copernican system got rid of the ad hoc large epicycles, the retrograde motions of the planets were natural results of the planets orbiting the Sun, as was the explanation of Venus and Mercury not moving far from the Sun, as they orbited inward of the Earth. Most importantly, the ad hoc equant was removed. In the time of Copernicus, getting rid of the equant was more important than many other innovations, as the equant represented a departure from the ideal of explaining the solar system using uniform, circular motion.

The Copernican system also explained things the the Ptolemaic system didn't. It gave a natural explanation for the ordering of the orbits of the planets, it also explained the precession of the equinoxes and the seasons. So, not only did the Copernican system explain the same facts as the Ptolemaic system with fewer ad hoc assumptions, it had greater explanatory power. It did require the assumption that the Fixed stars were very far away, but that was no more ad hoc than the Ptolemaic assumption that they were relatively near. So, in the Occam's razor stakes, it should have won hands down, and indeed the Copernican system was popular with northern European astronomers. Southern European, not so much.

So, along comes Tycho Brahe, he makes the most accurate observations of the stars and planets ever and he ....

Rejects the Ptolemaic system. He makes a system where all the planets bar Earth orbit the Sun (animation here) and the Sun with its panalopy of planets orbits the Earth. It's a kludge, an obvious kludge. The orbits of Mars and the Sun intersected for criminies sake! In terms of Occam's razor, Tycho's helio-geocentric system fails, introducing unwarranted assumptions. The parallax issues was a problem, but again, if the fixed stars were far away (as they were), then parallax would not be observable.

The observations of the phases of Venus by Galileo, and of the Moons of Jupiter meant that the Ptolemaic system (which Tycho rejected) was wrong. Interestingly, Galileo observed changes in the timing of the eclipses of the Moons of Jupiter that could be accounted for only by the parallax changes due to an orbiting Earth, but didn't realise it. In 1613 the annually periodic seasonal variations of sunspot trajectories across the sun's disc was discovered. This fitted with the hypothesis of a rotating earth, and not one of an orbiting Sun. Then in 1729 James Bradley discovered stellar aberration, which can be only explained by a rotating, orbiting Earth. Finally in 1838 , Friedrich Bessel made the first successful measurements of stellar parallax.

Tycho did not accept the Copernican system, but neither did he accept the geocentric Ptolemaic system. His rejection of the Copernican system in favour of a more complicated system of his own devising was a failure of Tycho, not of Occam's razor. Indeed, the Earth does rotate around the Sun, and choosing Occam's razor would have given us the right result (As Tycho's system made more unwarranted assumptions than the Copernican system - Stars close to earth, two centres of rotation and crossing orbits).


[1] Well, it's basically over now, that's what I get by being distracted by the sky. But I have a point I want to make.
[2] He also said Pluralitas non est ponenda sine necessitate which translates as "plurality should not be posited without necessity."

Labels: Copernicus, creationism, Galileo

In my other blog life I blog about evolutionary biology over at the Pandas Thumb. There I have helped point out some of the many errors in Michael Behe's anti-evolutionaty arguments. Behe has a new boook out, "Edge of Evolution", which is mostly the same old tired stuff over again. But in one remakable image (figure 7.4) he claims that HIV has evolved no new protein/protein binding sites. Now over at endogenous retrovirus blog, ERV completely demolishes this claim. As his claim is central to his argument that complex things can't evolve by mutation and natural selection, ERV's post is pretty devestating for Behe. Go over and read it, it's a very good essay (and I'm not saying that just because I drew a diagram for ERV).

Labels: biology, creationism, Intelligent design

That's a question Discovery Insitute fellow Paul Nelson asks. I give an answer over at the Pandas Thumb.

Labels: creationism, Intelligent design, pandas thumb