Saturday, January 29, 2005

Prenatal nutrition, obesity and lifespan

It has been shown that babies with low birth weight suffer more health problems in adulthood. For example they are more susceptible to obesity and Type 2 diabetes in adulthood. The idea is that if the mother is pregnant during a famine then the child will compensate by eating more than normal whenever food is available. However, trying to fatten up a low birth weight baby immediately after they are born may actually be detrimental. A group in Cambridge (Ozanne and Hales, Nature 427 , 411-412 (2004)) showed that mice that were born to underfed mothers experienced a fast catch up in weight if they were nursed by normally fed mice. However, their lifespans were significantly shortened. On the otherhand, if a normal birth weight baby was nursed by an underfed mother, their lifespan was actually lengthened. This experiment can never be repeated in humans so we'll never know if carries over. However, it appears that to maximize the health of a baby, all mothers should make sure they eat enough when pregnant and maybe eat a little less when they are nursing.

Wednesday, January 26, 2005

Nova Science Now

I watched Nova Science Now yesterday on PBS and found it to be very well done. The episode covered a variety of interesting topics including mirror neurons, which is pretty cutting edge. Nova was one of my favourite shows when I was a child but I thought it started to lose focus in recent years. I think they tried to gather a broader audience by diluting the science. I'm glad they've revitalized this venerable franchise.

Tuesday, January 25, 2005

World Year of Physics 2005

This is the world year of physics to celebrate the 100th anniversay of the Annus Mirabilis when Einstein published five papers that changed the world. The first paper that year was on the photoelectric effect in which he argues that light must be quantized. He won the Nobel Prize in 1921 for this work. The second paper is perhaps the least well known (although supposedly the most cited) and is his PhD dissertation on how to estimate the size of sugar molecules and Avogrado's number using the viscosity and diffusion of sugar in solution. The third was on the theory of Brownian motion based on the kinetic theory of heat. The fourth was the first paper on special relativity where he gives all the consequences of the relativity of inertial motion imposed by electrodynamics. The fifth is on the relationship between inertia and energy where he gives the famous equation although as m=E/c^2. The five papers were all published in German in the journal Annalen der Physik and have been newly translated into English. These discoveries formed the basis of 20th century physics. There will be festivities celebrating these achievements and physics in general all over the world. Find out what's happening in your neck of the woods at the official World Year of Physics 2005 website.

Monday, January 24, 2005

Low Carb Diets

Anyone interested in losing weight these days has probably either tried or has thought about trying a low carb diet like the Atkins Diet or the South Beach Diet. The popularity of these diets amazes me. I suppose I can see the appeal - Hey, just cut back on pasta, bagels and potatoes and eat all the hamburgers, bacon bits and steaks you want! Krispy Kreme Donuts, which was one of the few success stories of the late nineties, is hitting hard times in part because Americans are now shunning carbohydrates.

The big question is whether the weight that was lost will remain off for two years. It has been documented that a low carb diet does result in greater weight loss after a few months compared to a conventional diet (Foster et al. NEJM 348:2082-2090, 2003), but after one year there isn't much difference. My prediction is that in a year or two from now there will be a major backlash against these diets. My bet is that the next big trend will be high fiber diets (which makes more sense to me). In future posts, I'll talk more about the science and mythology of diets.

Thursday, January 20, 2005


February's issue of Technology Review (MIT's alumni magazine) has a provocative article on Aubrey de Grey and his pursuit of immortality. De Grey, who's day job is computer support for a genetics lab in Cambridge, proposes an ambitious research agenda to extend life. His ideas are actually not that outlandish. He identifies seven weak points in human physiology such as cell degeneration, mutations, and accumulation of "junk" that lead to our eventual demise. His proposal is that bio-engineering methods could be developed to repair these problems.

It may or may not be possible to live forever but I think there is an entirely different issue that he doesn't address and that is the capacity of our brains is finite. Thus, no matter how long our bodies may live, our memories of it are limited. A Hopfield neural network of associative memory for example has a maximum capacity of about 10% of the number of neurons in the network. After that, memories will begin to overwrite each other. We do have 10^12 neurons or so but that is still a finite number. So an immortal person (without infinite memory capacity) at some point will either overwrite earlier memories or not remember anything new and be frozen in time (like the character in the film Memento).

Any form of memory augmentation could extend the problem but never completely cure it. Thus an immortal person could never have a sense of self that extended over their entire lifetime. They would only remember bits and pieces of it. A second problem is that they would need to adapt and evolve to changing environments. In any case, they would be forced to morph into different beings throughout their lives.

Nature already has a form of immortality and it is called reproduction. It was designed specifically to deal with these issues. De Bray claims that if people had to choose between living forever or having children they would all opt for the former. I personally would not. To me it makes much more sense to try to maximize the chances of survival for our species (or perhaps all forms of life) rather than myself. I would much rather spend our resources trying to make the short lives we have better than spend it on Ponce de Leon's pipe dream. By just eliminating malaria, tuberculosis and AIDS alone we could significantly improve and extend the lives of many.

Tuesday, January 18, 2005

The theory of everything

There seems to be two types of high energy physicists, those that work in string theory and those that deride it. The main criticism of string theory is that it cannot generate predictions that are testable with current or even near future experiments. Thus it is really a field of mathematics or philosophy. However, both sides seem to agree that a search for a fundamental theory resides at extremely high energies.

Those of us working at eV energy scales, have a very different picture. Philip Anderson probably was the first to voice this alternative view in his famous article "More is Different" (Science 177:393-396, 1971). In that article he noted that each energy scale has its own organizing principles and fundamental laws. Hence, chemistry is not just applied physics, biology is not applied chemistry and psychology is not simply applied biology. Each of these disciplines must be studied in their own right.

On this point, I think most high energy physicists just don't get it. I saw a talk many years ago by Erich Vogt, the then director of the Canadian cyclotron TRIUMF, who started his seminar by outlining the 6 steps or so it takes to construct a cow. He started with quarks and built up the complexity step by step until he got to molecules. After that he said you just combine them and make a cow. I thought that perfectly typified the attitude of some particle physicists who feel that anything bigger than an atom is just quantum mechanics and thus it is just a matter of working out the details. Anderson pointed out that knowing the so-called fundamental laws wouldn't tell us how a magnet works. Spontaneous symmetry breaking is more important to magnetism than quantum mechanics or any other underlying theory.

The aura of high energy physics really dimished for me when I learned statistical mechanics and the renormalization group. Here I found that microscopic interactions may not matter at all. When you go to a large enough scale only gross properties like how many dimensions and symmetries determine the properties of a system. As you go to larger and larger scales, the parameters of the system "flow" in parameter space and eventually converge to a fixed point. Thus, it doesn't matter what the initial condition was, just the location of the fixed point. I think a fixed point theorem is as fundamental a law as any other. Perhaps someday, string theorists will find that there isn't a unique theory. A whole family of theories would be consistent with our universe and that would be perfectly fine by me.

Sunday, January 16, 2005

Mathematics, Science and Wigner

The Hungarian physicist Eugene Wigner once wrote a paper on "The unreasonable effectiveness of mathematics in the natural sciences" in Communications in Pure and Applied Mathematics, vol. 13, No. I (February 1960). In it he wrote that ``the enormous usefulness of mathematics in the natural sciences is something bordering on the mysterious and that there is no rational explanation for it''. I know there have been many times when I have been utterly incredulous that any of my theories or calculations could actually have any connection to reality. However, after learning more about biology and particularly neuroscience, I have come to see this as less of a conundrum.

One of the premises of Wigner's thesis is that mathematics is purely invented and hence cannot possibly have any connection to the outside world. To him, it is then uncanny that something as arcane as complex numbers or abstract algebra could have any applications to reality. I believe that mathematics is not a fanciful invention but comes comes directly from a brain that has been shaped by the natural world through evolution. For me, mathematics is a reflection of the mind and science is a mapping of that mind to the external world.

I started to think this way when I found out that mice and other supposedly lower animals know the basics of set theory and logic. For example, Howard Eichenbaum at Boston University showed that rats understand transitivity i.e. if A is bigger than B and B is bigger than C then A is bigger than C. Logic is not only a product of the human mind but is an innate skill that is built in and confers a benefit for survival. With basic logic operations hard wired, all that is needed to generate mathematics is the ability to combine logical statements. Perhaps, the ability to recursively combine arbitrarily complex propositions may be what distinguishes humans from other species. The ability for language may be inextricably linked to the ability for mathematics.

If mathematics is a product of a brain that was shaped by the external world then it is not surprising that we can describe that world with mathematics. To me, the unreasonable effectiveness of mathematics is a tautology. The real question is Why is the world regular enough to allow for mathematics to arise? That I have no answer. However, if a universe has enough consistency that being able to develop logic has a survival advantage, then mathematics will arise and be the natural language to describe it.

Friday, January 14, 2005


Check out the images from Saturn's moon Titan taken by the Huygens probe of the Cassini-Huygens mission. It was launched on October 15, 1997 and has looped around the sun twice getting a boost from Venus and Jupiter on the way. It's pretty amazing to think that we can send a craft to an object over a billion kilometers away moving at 10 km per second, then land on it and send a photo back. You gotta hand it to the rocket scientists!

Thursday, January 13, 2005

How many humans can the world take?

I just came back from a lecture by Jared Diamond who talked about his latest book - Collapse: How Societies Choose to Fail or Succeed. It made me think about what the capacity of the world is if we relied entirely on energy arriving from the sun. The power density of solar radiation is about 1.4 KW/m^2 and the surface area of the earth is about 10^8 km^2. So in 12 hours of daylight we receive about 10^16 MJ. The energy consumption of the US is close to 10^12 MJ per day which amounts to about 10^4 MJ per person. An average person eats less than 10 MJ per day. If the entire world lived as Americans we would use almost 10^14 MJ per day. I may be off by a factor of 10 or so but it looks like we're cutting it pretty close. If the world starts to live as Americans and we convert about 1% of the total sunlight into useable energy, then 6 billion people is at full capacity. I think this is a pretty good argument for us to conserve a little bit more (or we better get those fusion reactors up and running).

The Matrix and Body Heat

``The Matrix'' , while a somewhat entertaining film, was based on a premise that made no sense thermodynamically. In the film, the machines use humans confined to pods as an energy source. In an attempt to salvage a modicum of self-consistency, the confined humans were fed using a source derived from deceased humans. Unfortunately, this can never work. A person weighing say 70Kg requires about 2000 Kcal of food energy per day, a little less if they're inactive. A gram of tissue has about 5 Kcal of combustible energy. If a person is converted entirely into a food source, this would yield about 350,000 KCal, enough to sustain a person for only 175 days. Thus, the human generator could never achieve a chain-reacion necessary for a self-sustaining energy source.

However, we as humans do produce plenty of body heat, about 100 watts when we're still and a lot more when we're exercising. This is enough to power many of our devices. It is reported in this week's Nature Materials, that a group from the University of Toronto has devised a polymer based photovoltaic cell that is sensitive to infrared radiation. This paves the wave for making clothing someday that can use sunlight, ambient light, and body heat to power the toys of the modern road warrior.