Archive for the ‘Biology’ Category

Q: How/when will the world end?

Monday, April 26th, 2010

Physicist: To answer this question definitively would require the destruction of at least a couple dozen other worlds.  But failing that, guesswork:

The little things (people): In the short term (less than several million years) the biggest threat the Earth faces is people.  We’ve already got the Holocene mass extinction going for us, now we’ve just got to step up our game and go for broke.  Hey, Coalition of the Willing!  I think I saw North Korea stealing your cup cakes.  Also, it’s too cold in the winter.  Couldn’t we burn a teraton of coal or something?

Boring, Regular Extinction: It we (Homo sapiens) follow the same fate as all of our predecessors and cousins (homo habilis, rudolfensis, georgicus, ergaster, erectus, cepranensis, antecessor, heidelbergensis, rhodesiensis, and neanderthalensis, for example), then it’s very likely that we’ll be extinct within the next 100,000 to 1,000,000 years.  Statistically speaking anyway.

Total carbon re uptake: Over very large time scales the sun is getting brighter (along the lines of about 10% per billion years).  Astrophysicist Brownlee and paleontologist Peter Ward have written a book espousing the idea that this gradual brightening will cause the Earth to heat up and the natural chemical processes that absorb CO2 from the air (and lock it away in sediment) will speed up.

"Main sequence stars", which include the sun, are surprisingly stable for a very long time. They do change a little, increasing their brightness by about 10% every billion years.

They figure that inside of 500 million years there won’t be enough CO2 in the atmosphere to support plant life, and that would be the end of complex life.  Although life has been around for at least 3.5 billion years, the interesting stuff (animals) have only been around for about 500 million years.  So if Brownlee and Ward are right, we’re only about halfway done (not nearing the end).

It may seem strange to talk about the loss of CO2 being the end of the world when we so often talk about the dangers of too much CO2.  The difference is in the time scales.  The spike of CO2 we worry about today is on the scale of centuries, while the long term absorption of CO2 is on a time scale a million times larger (unnoticeable in the short term).

Dynamo shutdown: The Earth’s magnetic field is the result of iron rich (electrically conductive) stuff flowing around in the Earth’s core.  The currents are driven by radioactive heating which causes convection, specifically the decay of radioactive potassium, uranium, and thorium.  The half-lives of these materials are 1.25 billion, 4.5 billion, and 14 billion years respectively, so most of the original fuel has already been used up.

The exact nature of magnetic dynamos is not terribly well known, and is still an active area or research.  We don’t know for certain what the minimum energy input is needed to keep the damn thing running.  We do know that it’s certainly possible for a planetary magnetic dynamo to shut down (Mars’ shut down at least a couple billion years ago).  If our dynamo shuts down, then our magnetic field will vanish and (in fairly short order) the atmosphere will be stripped away by solar wind, as happened on Mars.

Never-ending Summer: The increase in the Sun’s output will make it too hot for liquid water on Earth in about 1 billion years.  With the oceans boiled away the pressure everywhere on Earth will be about the same as the pressure on the ocean floor.  The difference between Venus and Earth will be academic.  No matter what else happens before then, this will be the end of life on Earth.

SPF 5,000,000,000,000,000: Somewhere around 5 to 7 billion years from now the Sun will start to run out of fuel.  Ironically this will actually make the core hotter as it collapses in on itself.  The top layers will fluff up and (probably) envelope all the inner planets, including Earth.  For obvious reasons this is called the “red giant” phase of the Sun’s life.  The solar system will eventually settle down with the gas giants still in place, the inner solar system missing, a white dwarf star where the Sun used to be, and the trans-neptunian stuff completely unaffected.

The Sun as we see it today (yellow), and the Sun in its fluffier red giant phase (red).

Lights out: If by “end of the world” you mean “end of the universe”, then a good end of everything is the end of the age of stars.  The universe started out made up of about 75% hydrogen, but today is only about 70% hydrogen.  Stars are almost completely powered by hydrogen fusion, so assuming that the consumption of the universe’s hydrogen is stays constant (which isn’t a particularly good assumption), then there will be almost no stars left in 250 to 300 billion years.

The big rip: Not only is the universe expanding, but the speed of that expansion is increasing.  The expansion is a little hard to picture because the expansion isn’t about things moving away from each other in space, it about the space in between things actually expanding.  Right now the effect is small enough that it can only be seen on huge, inter-galactic, scales.  But eventually the expansion with be so rapid that the space between the planets and their stars will increase so fast that the planets will be pulled into open space, and not long after than (as in a couple of months or so) the space between atoms will increase so fast that everything will be completely torn apart and atomized.  This is called the “big rip”.  Some estimates put the big rip about 20 billion years out, and some say it won’t happen at all.

Posted in -- By the Physicist, Astronomy, Biology, Paranoia, Physics | 1 Comment »

Q: Is it of any coincidence that mathematics is able to describe physical reality – given that both are inventions of the human mind?

Friday, March 5th, 2010

Physicist: There’s a lot of math that doesn’t describe physical reality at all, and even some (few) mathematicians who feel that
“applicability” is just another word for “impurity”.  The ability of math to describe reality is just a consequence of the fact that reality is nice and consistent.

The fact that the math we use (addition, subtraction, geometry, calculus, whathaveyou) works is no coincidence at all.  Mathematics literally evolves in the sense that, if something doesn’t work, then people will ignore it.  So if you have a theory that \pi = 7, great, but no one will use it because it’s patently, provably false.  It doesn’t describe reality (in this case the reality that the ratio of the circumference to the diameter of a circle is \pi), so it goes the way of the Woolly Mammoth.

π=7

I assume that this question is about perceived reality (colors only exist in the brain, whereas in reality there is no “blueness” or “redness”), and not physical reality.  The fact that we can only describe (mathematically and otherwise) the reality we perceive does guide the direction of mathematical research, and as we perceive more we find that the field of math expands accordingly.  For example; number theory wasn’t much more than a hobby before digital communication and RSA encryption, and differential geometry was mostly a nuisance (and anal-retentive over-generalization) until general relativity cropped up.  Now these are both thriving fields of research (in computer science and physics, respectively).

However, just because something works in your head has absolutely no bearing on whether or not it will work in reality (which you would expect if the physical world were created by our minds).  Very good, very reasonable ideas get shot down by experiment every day, and we are constantly surprised.

Philosopher: If we assume the external world exists (independent of our minds), Math’s correspondence to reality is no more coincidental than the correspondence to reality of theories stated in any other language.  This isn’t dependent on the existence of mathematical objects, and it’s not dependent on Mathematical truths existing independently of humans (though I think they do).  If we assume the external world is merely an “invention of the human mind”, then the correspondence of Math to the world is even less coincidental, since the same thing is the author of both.

Posted in -- By the Physicist, Evolution, Math, Paranoia, Philosophical | 1 Comment »

Q: If you were to break down an average human body into its individual atoms, and then laid the atoms out in a single straight line, how far would it stretch?

Wednesday, March 3rd, 2010

Physicist: Atoms are a little “fuzzy”, so their exact size is a little tricky to define.  So taking their size in terms of bond length, and looking at the most common elements in the human body (by mass: 65% oxygen, 18% carbon, and 10% hydrogen), you’ll find that 1kg of person will stretch about 7 trillion km.  So an average (80kg) human would extend about 550 trillion km, or about 14 billion loops around the equator, or 1.4 billion trips to the moon, or about 58 light years.

So you can fit a rich man through the eye of a needle, but be sure to coil him up after you string him out.  Otherwise the process will take at least 58 years.

Posted in -- By the Physicist, Biology, Brain Teaser | No Comments »

Q: Do aliens exist?

Saturday, January 30th, 2010

Physicist: Yuppers.  In as much as the probability that they don’t is effectively zero.

The statistics on this are a little weak, since we only have one real data point.  If you define intelligent life as tool-using, then (based on the age of the oldest tools and the oldest fossils, and the progress of the Earth to date):  Intelligent life has existed 0.06% of Earth’s history, and animal life has existed for about 16% of Earth’s history.  Moreover, the vast majority of life on Earth (and the toughest) is microbial.  So by “yuppers”, I mean that space bacteria almost certainly exists.

As far as the fancy aliens (with their lasers and tentacles) that I assume the question is really about: probably.  The universe is crazy big.  However, stars are far apart (especially around here), and the likelihood of finding intelligent life is really low.

In the last decade there have been some surprising results from the panspermia people.  It seems to be entirely possible, even likely, for life to get kicked from planet to planet and even from star to star.  The three difficulties are getting off a planet, surviving in space, and landing somewhere else.  During a major impact the material immediately around the impact is vaporized.  A little farther out and things are pulverized.  Just beyond the “automatically dead zone” is a thin ring where material from the planet’s surface can be thrown into space smoothly (no more than a couple hundred G’s) and without excessive heating.  Although no animals could survive the shock, massive G forces have very little impact on single celled life (too small to slosh).

There’s a wide variety of life from Earth that does fine in space.  Things like Water Bears, and some bacteria can put up with the cold and radiation, and are more than happy to drop into a state of suspended animation for the trip (forever if they have to).  The classic example is a few cells of Streptococcus that survived on the moon (on Surveyor 3′s camera) between 1967 to 1969.

Something you may notice, if you collect large meteorites, is that although the surface tends to be pretty messed up, the interior is frequently quite intact.  Although the fall looks pretty impressive, the heat and fireball don’t have time to cook the meteors all the way.  In fact the hottest parts of the meteor vaporize during the fall, which serves to keep it cool (like sweat, but like… a rock version).  Although it’s unlikely for living things on any one rock to make it through all three stages intact, keep in mind that there are actually many rocks flying around that have been knocked off of planets in the past.  There are so many, that one of the cheapest ways to collect samples from Mars or Venus is to go to Antarctica.  (If you find a rock sitting on top of a 3 miles of ice, where do you think it came from?)  One of the biggest “life is out there” stories came from exactly this source.

Here’s the point: If there’s life anywhere it’s likely to spread everywhere, like… well, like life.  Panspermists think that life may have started on some other planet around some other star, and that this life then infected the Earth.  This would help explain why the Earth was covered with sophisticated (microbial) life almost immediately after it was capable of supporting life at all.  Or to spin it around, if there’s life here (check) it’s had over 3 billion years to get blasted out into the nearby universe.

 

Mathematician: There are compelling reasons to think that life exists on other planets (perhaps even on a huge number of other planets). If life spontaneously arose on earth from a soup of molecules through an evolutionary process, then all you need for life to be created is the right planetary conditions, the proper raw materials, and a sufficient amount of time. The right conditions may include things like being close enough to a sun that the planet is reasonably warm, but far enough from that sun so that it isn’t  burnt to a crisp. The right materials probably include carbon and water among other things. In any event, once you get these things right, you just add time (a billion years probably would suffice) and viola, life is born. That means that for earth to house the only living organisms in the universe, these requirements would have to have been met one time and one time only in all the billions of galaxies that have formed during the 14 billion year history of our universe. That sure sounds pretty unlikely.

Here’s another way to think about it: there is some probability p that a randomly selected planet will form life on it within a billion years. If p is sufficiently small, then there would be almost no chance of any life forming, including our own, and hence we should not exist. If p is sufficiently large, then life would exist almost everywhere in the universe. The only way that we should expect to be the one and only planet with life is if p is just right to produce about one planet with life over all the years and on all the planets that have ever existed. But we have no evidence whatsoever indicating that p should be perfectly balanced in this way, indicating that the chance of alien life is a good one.

But does technologically advanced alien life exist? Well, if life occurs on many other planets, then we should expect technologically advanced life to occur on at least some of them. Whatever caused natural or sexual selection to select for high levels of mammalian intelligence on earth could lead to intelligent aliens as well. On the other hand though, if technologically advanced civilizations tend to wipe themselves out fairly quickly (say, within a hundred thousand years) or if the process that creates highly intelligent life requires sufficiently rare conditions, then advanced aliens could certainly be the exception rather than the rule.

Posted in -- By the Mathematician, -- By the Physicist, Astronomy, Biology, Evolution, Physics | 1 Comment »

Q: Is the total complexity of the universe growing, shrinking or staying the same?

Friday, January 22nd, 2010

The complete question was:

If you were to look at the universe as an organism, was the early universe a simpler organism than the present-day organism?  Is the total complexity of the universe growing, shrinking or staying the same?  And how do you measure that?

Physicist: Absolutely.  The total complexity of the universe is increasing, due to the inevitable march of entropy (or information), which is exactly the measure of complexity.  A more intuitive way to talk about complexity and entropy is: can you predict what you’ll see next?  If you look at part of a checker board, you can probably guess what the whole thing looks like, so the board is predictable and has low entropy.  In the early universe matter was distributed pretty uniformly, almost all of it was hydrogen, almost everything was the same temperature, and there were no complex chemicals of any kind (going back far enough everything was ionized).  So if you’d seen one part of the universe, you’ve pretty much seen all of it.

This is actually a chess board.

No surprises.

Nowadays the universe is full of a wide variety of different elements with very complicated ways to combine together, matter shows up hot, cold, as plasma, as proteins, in stars, and clouds, and not at all.  The amount of data it would take to accurately describe the universe as it is now utterly dwarfs the amount that it would take to describe the early universe.  On an atom-by-atom basis, in the early universe you could grab an atom at random and feel fairly confident that: it’s hydrogen, it’s ionized, it’s about “yay” far away from the other nearby hydrogen, etc.  Today you’d probably be right if you guessed “hydrogen” (about 3/4 of the universe’s mass is still hydrogen), but you’d have a really hard time predicting anything beyond that.

Oddly enough, life is surprisingly uncomplex compared to say, dirt or sea water.  If you look at a single cell in your body, you’ve already got a pretty good idea of what you’ll see everywhere else in your body.  Admittedly, we are more complex than single celled life, but most of that is a symptom of being physically bigger.

Posted in -- By the Physicist, Biology, Entropy/Information | 2 Comments »

Q: Why does oxygen necessarily indicate the presence of life?

Wednesday, January 20th, 2010

Physicist: Short answer: Life is the only thing that makes lots of oxygen.

This question comes in the context of a conversation about the Kepler mission.  So far (as of January 11, 2010) 424 “exoplanets” have been discovered and confirmed in orbit around other stars.  It’s worth pausing to take a minute and say, “holy shit!”.  Most civilizations throughout the ages have been aware of Mercury, Venus, Mars, Jupiter, and Saturn, and so was it for tens of thousands of years.  Between 1781 and 1930 we found 4 more: Uranus, Ceres, Neptune, and Pluto.  (It’s been slow.)

Since 1992 we’ve found over 400 new planets around other stars and, depending on where you draw the line, between 7 and several new dozen dwarf planets around our star.  It may have nothing to do with the question, but I think it’s worth knowing.

Pluto has friends.

New Dwarf Planets

Unpause.  Due to the difficulties in measurement, the vast majority of the exoplanets discovered so far are bigger than Jupiter, and orbit their parent star closer than Earth orbits the Sun.  Kepler holds the promise of detecting Earth sized planets, and brings us a step closer to detecting life around other planets (even if there were life on gas giants, it would be so alien we wouldn’t know what to look for).  Kepler works by waiting for eclipses.  When a planet passes in front of its star, the star appears to dim a little (if someone around another star were to see Earth do this, the Sun would appear to dim by about one part in 10,000).  Even better, by staring really (really) hard you can actually see light that has passed through the atmosphere of those planets, and now you’re talking chemical analysis!

If you look around at the other planets in our solar system you’ll notice that they all have something in common: their atmospheres are all chemically stable.  The other atmospheres, CO2, Hydrogen, Helium, Methane, etc., don’t do much more than just blow around.  You can’t start a fire anywhere other than Earth.

This is why we can't have nice things

Oxygen: The jerk of the elements. It's corrosive, burns like crazy, and is generally reactive and unstable.

Oxygen, on the other hand, is about as stable as a drunk unicyclist.  When you find oxygen in nature (and by “nature” I mean “other than Earth”) it’s always already tied up in the molecules of something else (such as in water or granite).  As soon as oxygen is released it tends to immediately combine with things around it.  It has been estimated that, left on it’s own, atmospheric oxygen will be completely absorbed by chemical processes within a few hundred years, and that’s not including big fires and whatnot.

The only known process that actually releases O2 into the air in any real quantity is photosynthesis.  So, observing oxygen in the atmosphere of other planets implies photosynthetic life.

Posted in -- By the Physicist, Astronomy, Biology, Physics | No Comments »

Q: Would it be possible to kill ALL of Earth’s life with nuclear bombs?

Friday, January 15th, 2010

Physicist: Probably not.  We could kill all of the large (insects and up) life no problem.  Hell, we’re doing all right by mistake so far.  There are about 30,000 nuclear weapons in the world today, so in what follows I’ll assume the worst case scenario; that all of them are evenly spaced across the Earth’s land masses and set off.  That should put them about 70km apart (in a grid).

Certainly everything on the surface within several dozen km of a nuke will be dead (like, really dead) but surprisingly, several feet of dirt or stone offer remarkable protection from the light and fire of the initial blast.  Not directly under the explosion, but pretty close.  It takes an amazing amount of energy to heat up and/or move dirt, so while the surface may be heated to red hot, the ground underneath can stay surprisingly cool.

So sure, you’ve kicked the legs out from under the ecosystem, but how do you ensure that you get everything?  Fall out and nuclear winter are a good place to start.  Nuclear winter  is caused by dust thrown up in the air blocking out sunlight.  The “sunlight blocking” shouldn’t last for more than a few weeks, but it takes very little time to starve all the plants and plankton that rely on sunlight.  Or really just plankton, since you’re not going to find plants left standing within 35km of a nuke.  Now, whatever survives (burrowing critters, seeds) will have to contend with ash instead of food, and radioactive fallout.

Modern weapons are fairly efficient, in that they use up almost all of their fissionable material when detonating.  The initial flash involves a lot (as in “holy shit”) of radiation that mostly takes the form of gamma rays.  Gamma rays are just high energy photons, so they’re gone immediately.  Unfortunately, when fissionable stuff splits it breaks up into smaller isotopes which also tend to be highly radioactive.  Most of these by products have short half lives.  There’s a strong correlation between an isotope having a short half-life and the isotope radiating especially high energy crap when it decays.  So most of the nasty stuff goes away pretty quick.  The glaring exceptions to this are Caesium-137 and Strontium-90, which both have half-lives of about 30 years (and are delicious).  Today the background radiation of Hiroshima is due primarily to Caesium, and that accounts for very little radiation total.

Basically, in order to survive the worst case scenario you have to: 1) live under ground or underwater, 2) be highly resistant to buckets of radiation, 3) not be particularly bothered by losing the sun for a while, and 4) not be particularly sad about the surface of the Earth burning and then freezing (or continuing to burn, just not as much.  Some of the jury is still out).

A creepy blind fish from an Australian cave, a Pompeii Worm from a black smoker vent, and a Tardigrade (Water Bear) from freaking everywhere. The last two are harder to kill than werewolves.

We live in the largest ecosystem on the planet, but we definitely don’t live in the only one.  There are fungus driven ecosystems deep in caves scattered around the world for example that may be safe.  If however those caves can exchange air with the outside (or are forced to by a bomb for example), then the radiation would probably wipe out everything in there too.  At the bottom of the ocean you can find black smokers, usually at the edge of tectonic plates.  Black smokers are vents that spew out super-heated acid water laced with poison.  I can only assume that the creatures that live down there must have been kicked out of every other clubhouse on the planet.  These ecosystems depend only on heat and material from beneath the Earth’s crust, and as such are completely independent of the Sun.  Although, poetically, since they depend on the nuclear decay of heavy metals in the Earth that were produced in at least one supernova more than 5 billion years ago, they still rely on a Sun, just not our Sun.  The creatures in the black smoker ecosystems have to deal with radioactive crap flying out of the vents all the time, so they may be able to put up with fallout that manages to drift all the way down to them.  Also, back in the 1950′s a bacterium called “Deinococcus radiodurans” was discovered that flourishes in radiation upto 3 million rads.  By comparison, 1000 rads is usually fatal to people.  3,000,000 rads means that the glass of the test tube you’re keeping this bacteria in is going to turn purple and fall apart long before the bacteria dies.

I mean, how does that evolve?  Where in the hell is this bacteria finding an environment that horrible?

Finally, Water Bears.  God damn.  Those guys don’t die.  Ever.  You can freeze them (-272°C), boil them (151°C), dry them out, irradiate them (500,000 rads), and even chuck them into space (seriously… space!), and they couldn’t care less.

So as long as there’s liquid water somewhere on Earth (even ultra-high pressure acid water) there will almost certainly be life.  We would probably be more successful (at killing everything) with toxins and run-away global warming.  So, if we could turn Earth into another Venus.

It worth noting that if this post seems a little “guessy”, it is.  A lot of research has been done on the subject.  The United States alone has detonated at least 1,054 weapons in tests, injected at least 18 people with plutonium, and exposed many more to radiation.  The exact results of all these tests are largely classified (as in fact were the tests themselves).  And of course, the world has never been destroyed by an all encompassing nuclear disaster.  Hence the guess work.

However, we have fossil evidence of microbial life dating back about 3.8 billion years, and the moon’s marias were still being created (by really, really big impacts) until about 3 billion years ago.  So we can expect that the Earth was subject to several ocean-boiling impact events since life started, and we’re still here (suck on that, space!).

Posted in -- By the Physicist, Biology, Physics | 2 Comments »

Q: Why?

Wednesday, December 2nd, 2009

Mathematician: From time to time, people like asking us questions such as “Why?”, while steadfastly refusing to explain what the heck they are talking about. The best example of this was a naked guy who approached our “Ask a Mathematician / Ask a Physicist” booth at Burning Man. In an attempt to respect everyone’s right to not explain themselves, we’ll make a series of guesses about what those folks might be trying to get at, and briefly respond to each of these possible questions.

1. “Why do we exist?”

Mathematician: We exist because our ancestors were at least slightly better at passing down their genetic material than other people. If the environment of earth happened to be just a tad bit different, then other genes besides our own would have been favored, and we would not be here today. If the environment had been a little more different still, then not only would we not be here, but the human species would not even be here. Some other creatures (possibly of great intelligence) would now be romping around this planet. In conclusion, we exist because the process of evolution works, because our planet happened to have the right conditions for evolution to begin, and because conditions changed over time such that human genes (and more specifically, our ancestor’s genes) happened to be favored for survival. We all got very, very lucky.

Physicist: If the many-worlds hypothesis holds (it totally does), then everything that’s possible happens in some version of the universe.  If you can ask the question “Why do we exist?”, then you’ve already restricted your attention to the (possibly very small) set of universes where intelligent life exists.  This argument is called the “anthropic principle“.  So the reason we exist is because there is at least some vanishingly small chance that we could.

2. “Why does existence exist?”

Mathematician: Nobody knows. A related question that nobody knows the answer to is “why does consciousness exist?”  For example, why aren’t we like computers, going about our business without ever “feeling” anything or having any “internal, personal experience”? There are a few possible answers to this question. Perhaps we don’t really have consciousness and we just think we do (though this answer strikes me as bullshit). Another possibility is that consciousness is something that emerges automatically (call this a property of the universe) when you have a sufficiently complicated system with the right components, and that we have consciousness merely because evolution happened to produce those special components in our brain (so in that case consciousness is a side effect of some useful brain parts). A third option is that consciousness was created by the process of evolution because it has a specific survival use (perhaps it is a handy way to get creatures to spread their genes).

Physicist: What else is existence going to do?

3. “Why is the universe the way it is?”

Mathematician: I consider this to be a deep mystery. According to an old Finnish creation story, the sky is a piece of an egg and the sun is its yolk, but this strikes me as unlikely. Perhaps physicists will one day be able to shed light on this question.

Physicist: If there’s some kind of rhyme or reason behind the beginning, then go figure out who was doing the reasoning and ask them.  If, however, the universe was randomly generated, then it may follow the same rules for random generation that everything else in the universe does.  The probability that a particle is generated is proportional to the number of states it can be in.  So if an electron can be in two states and a pion can be in three, then the pion is 3/2 times more likely to be created than an electron.  More states means more entropy.  The universe may be the way it is because our particular set of physical constants (speed of light, gravitational constant, planck’s constant, etc.) cause the universe to have tremendous complexity (many many states). If the constants were different, then everything might have already fallen into black holes, or complex molecules may not form, or there may be no stars or solar systems, etc.  Each of these circumstances have a much lower entropy than the universe we see today. So (perhaps) the universe is the way it is so that it can maximize it’s own entropy.

4. “Why are we the way we are?”

Mathematician: We have eyes because there is a Sun near our planet that is bouncing light off of everything, and it is mighty useful (for survival) to be aware of this radiation. It is so useful, in fact, that eyes have been created (from scratch) multiple times on the evolutionary tree. But why do we have two eyes, rather than one? In part, it probably serves as a back up system: since vision is so critical to survival on this planet, and since eyes can easily be pierced or go faulty, it’s much safer to have two. Another reason to have two eyes is because it allows us to have true 3D vision (which again, likely increases a creatures chance of survival). Why do we have arms and hands? Because our ancestors walked on four legs (which is useful for fast running, and our arms developed from these front legs) and manipulating the world around us is damn useful (thank you fingers!). Why do we have legs? Well, because that’s one of the best mechanisms that evolution found for getting around. These arguments can be repeated for most body parts. The point is that we are the way we are because the environment of earth during our evolution made the traits that we have useful for survival. We could have been very different had conditions been a little different, but things like eyes and legs are so incredibly useful that it is very likely we would have at least had those.

Physicist: Evolution favors what works.  Things that work, and have thus evolved over and over in different places here on Earth, include: limbs, blood, eyeballs, nervous systems, sex, socializing, fighting and fleeing, eating and pooing, smelling, breathing, moving, not looking at the sun, … As for why we’re bipedal, hairless, and whatnot: sometimes these things happen.  If you’ll notice, there aren’t a lot of other creatures that share these traits.  If we were something else we’d wonder about that too.

5. “Why am I me and not someone else?”

Mathematician: Just chance, mostly, but you are also changing into a new person every moment, with new cells, new memories, new ideas and new behaviors.

Physicist: You’re no one at all before you’re born.  Who you are now is who you’ve become.

Posted in -- By the Mathematician, -- By the Physicist, Evolution, Philosophical, Quantum Theory | 2 Comments »

Q: What’s the highest population growth rate that the Earth can support?

Tuesday, November 24th, 2009

Physicist: Zero.

Populations tend to grow exponentially, which is why the growth rate is defined as R in P = AeRt, where A is the population at time t=0, and P is the population at any other time t.  If the average growth rate is greater than zero, then the population will grow exponentially forever, which is sadly impossible.  Here’s why:

If the population could grow forever, then eventually the total mass of Humans would be greater than the mass of the Earth, which makes no damn sense.  What were we eating?

The population could grow forever if we found a way to colonize other star systems.  However, even with speed-of-light ships we could only colonize something like \frac{4}{3} \pi (Ct)^3 planets in time t, since we can’t travel faster than light (this is the volume of a sphere that expands at the speed of light, C).  The population density in the colonized area of the universe would then look like \frac{\textrm{Population}}{\textrm{Volume}} = \frac{3A e^{Rt}}{4 \pi (Ct)^3} \propto \frac{e^{Rt}}{t^3}, where “\propto” means “proportional to”.  You’ll notice that if R is greater than 0, then as time increases the population density goes up forever, which makes no damn sense.  If you don’t notice, then just graph it.

What this ultimately means is that the average, over all time, of the number of children that a person has is 2 or less.  No way around it.

We’ve had a lot of visits from reddit.com with a little confusion over the line “populations tend to grow exponentially” (my bad).  What I should have written is “populations tend to grow exponentially under the assumption that they have not yet begun to saturate the available resources”, but I figured that might be pushing the discussion.  The less that resources are available, the more the population will tend to level off.  Both the exponential growth and leveling off can be modeled using various scalings of the logistic function.  Try graphing: \frac{1}{1+e^{-t}}, and e^t.  You’ll find that they line up almost exactly until near t=0, where resources begin to dwindle.

Posted in -- By the Physicist, Biology, Math | 3 Comments »

Q: What is the meaning of life?

Thursday, October 29th, 2009

Mathematician: I’m glad you asked. The theory of evolution with natural selection sheds some light on the question of why humans exist, which in turn relates to the meaning of life. First of all, let me get this out of the way: while evolution is still called a “theory”, it has a tremendous amount of evidence in support of it (including gradual transitions in the fossil record, radiocarbon dating, DNA analysis, laboratory experiments, etc.) and, as biologist Richard Dawkins is known to say, it is only a theory in the sense that the “theory of gravity” is a theory. From a scientists perspective, evolution is a fact. But what does that have to do with the meaning of life? Well, evolution tells us that human beings share a common ancestor with apes, not to mention with pigs, dogs, cats, rats, plants, and bacteria. If historically the conditions on earth had been very slightly different than they were, the best traits for survival would have been different also, and therefore we would expect that some other species besides humans (possibly with intelligence as great as ours) would now dominate this planet. Hence, evolution tells us that humans have the capacities that they do now simply because those capacities helped our ancestors survive long enough to have children, or made them more effective at finding mates.

I believe most people will agree that “What is the meaning of life?” is a question that is meaningless when it is applied to the lives of rodents, insects, or bacteria. As we are simply evolutionary offshoots of these creatures, what makes us think that this question will have any more significance when applied to us? The primary characteristic that differentiates us from these other creatures is our powerful brains that have incredible capacity for abstract though (including the ability to consider questions like “what is the meaning of life?”). Each of us exists today because our ancestors managed to survive. Their survival occurred both because they were well adapted to their environment, and because they got very, very lucky. Our existence then is, in some sense, a happy accident, and lacks the deep cosmic significance that questions like “what is the meaning of life?” presuppose. Fortunately, however, our brains are very adept at discovering meaning in all sorts of places. Life does not require an all encompassing, universal meaning or purpose in order for us to find that our own lives are meaningful, and that is a truly wonderful thing! We can feel totally fulfilled despite being little more than happy accidents of evolution. Of course, “meaning” is a very real and important emotion, but that does not make it an objective property of things. Fortunately, there is no reason to fret over life itself having no ultimate purpose. We ourselves can find something that fills us with a sense of purpose, which for practical purposes is just as good.

Physicist: Every now and again a question comes along that implies more than it asks.  Questions like: “What’s that blue thing?”, or “Who ate this?”.  If there’s nothing blue or eaten around, then these questions don’t make sense.  The same is true of the classic: “What is the meaning of life?”.

Posted in -- By the Mathematician, -- By the Physicist, Evolution, Philosophical | 1 Comment »