Wednesday, September 1, 2010

A Short Lesson in Thermodynamics as it Relates to Life

I was recently presented with the statement that “life is the only force that counters entropy”. This can be taken to mean one of two things. The first is that life results in a net decrease in entropy.  The second is that while life may result in a net increase in entropy, entropy of the life form itself is reduced and life is the only such system that can do this. The first argument is nonsense, as it violates the 2nd Law of Thermodynamics.  The second statement is just plain false; there’s nothing thermodynamically unique about life. Let’s look at both situations to understand why.

The first order of business is to define entropy. There are lots of valid definitions and all valid definitions can be shown to be equivalent. One definition of entropy is the amount of energy that cannot be used to do work. Doing work requires energy. For example, if you want to move something, you need to expend energy.  Except for theoretical systems, whenever energy is expended by a system, some of the energy is wasted—not all the energy can be used to do work. For example, when you move an object, you expend energy to flex your muscles, and this is translated into kinetic energy (energy of motion). As your muscles flex, heat is created. This heat is sent into the environment and is not used to move the object. Thus, entropy is increased in an amount related to the wasted energy.

The inefficiency of systems that do work is at the crux of the 2nd Law of Thermodynamics. In a theoretically perfect system, all the energy would go to do work and no heat would be generated. a perfect system can at best use all energy to do work (resulting in no net decrease in entropy). An imperfect system (e.g., a real world system) will have an inefficiency so that some energy is wasted. The wasted energy produces a net increase in entropy. No system can do work to produce more energy than is needed to do the work. This would be a perpetual motion machine and it would generate a net decrease in entropy.

We can now investigate whether life counters entropy. The answer is no, at least not in the global, net sense. Life can do work (e.g., grow, reproduce, move, etc.), but all these processes involve an inefficient expenditure of energy. Life increases entropy. Period.

Now let’s look at the second possible meaning of “life is the only force that counters entropy.” To do this, let’s consider a different, but equivalent measure of entropy. Entropy is also a measure of organization (or disorganization). A system that moves toward more order has a decrease in entropy. The existence of such a system may seem impossible given the discussion above, but it is not. A system can have a decrease in entropy at the expense of its surroundings. The surroundings will suffer a larger increase in entropy than the decrease in the system, resulting in an overall increase in total entropy. There are a multitude of examples of systems in which entropy is decreased.

Life is indeed one such example. Cells and organisms are assembled from a more disorderly system of atoms and molecules. Life takes in nutrients and assembles these into orderly functioning life systems. So, in this sense, one could say that life counters entropy. This of course ignores that life, on the whole, actually increases entropy when life plus its environment is considered. The outstanding question is whether life is the ONLY system to do so. The answer is clearly and resoundingly, “no”.

Well known examples of systems that have a reduction in entropy are air conditioners and refrigerators, crystal formation, and planet formation. The true list of such systems is possibly infinite. Air conditioners produce cold air from hot air. The cold air is slightly more ordered than the hot air. Entropy decreases. But this is only true if the waste heat is neglected. Anyone that has spent even a small amount of time around an air conditioner knows that in order to produce cold air, hot air—much hotter than originally ingested—must be
ejected. So, in net, entropy increases. Entropy is lower in the cold air, but it is higher in the hot air, and the net effect is an overall increase in entropy. Crystals are very orderly. This order represents a reduction of entropy compared to the original unorganized, uncrystalized molecules. But, as in the previous example, the environment suffers a larger increase in entropy in order to allow for crystallization. Cyrstalization generates waste heat and, therefore, an increase in entropy. The net change in entropy is positive. Solar Systems form from a collection of gases and dust. These spiral in and accrete into larger bodies, eventually becoming planets—a more orderly arrangement. But, this process generates heat, and the entropy generated from solar system formation exceeds the reduction in entropy associated with greater organization.

So, there you have it. The first interpretation is wrong and the second interpretation is wrong. Life does not counter entropy, at least in the global sense. Life increases entropy. The second interpretation is wrong even if life is viewed in isolation from its surroundings. In such a situation, life produces a reduction in entropy in the isolated life system, but there is nothing unique about this process. Lots of systems, in absence of their environment, constitute a reduction in entropy. Life is not the “only” system to do so.

Making a statement such as “life is the only force to counter entropy” is at best misleading, because it implies that life has the overall effect of decreasing entropy. It can’t do this, because of the 2nd Law of Thermodynamics. If the proponent of such a statement tries to hide behind the canard of only talking about the system in isolation, they will find themselves once again in the wrong corner, because life is not in any way unique in this regard. In short, there’s nothing thermodynamically special or unique about life.

Sunday, January 3, 2010

The Death of Planets

The Death of Planets.  No, not the planets themselves.  The idea or concept of what we have recently called planets is doomed, with the demotion of Pluto to a dwarf planet being the most recent step in a process that began with Copernicus.  Then end game will be the realization that the concept of a planet as a meaningful astronomical class of objects is antiquated and archaic.  The concept of planet is no longer compatible with what we know about the Universe.  The bickering by a minority of astronomers about the definition of a planet, and which has achieved tremendous exposure in the public arena, is as pointless as the flat Earthers arguing about whether the flat Earth is rectangular or circular. 

Since humans first looked up at the night sky, they noted that there were some objects, often brighter than the rest that did not follow the other objects in their regular movement across the sky.  The ancient Greeks labeled these objects as “The Wanderers”, or as we know them, planets.  Thousands of years ago, the segregation of these special objects from the rest of the celestial chaff made sense; they were clearly different than all the lights in the night sky, and there was no mistaking what was a planet and what was a star.

       The path of Mars against the fixed stars as viewed from Earth. Click on Image to see animated version.  (Image Source: Unknown) 

Then came Nicolaus Copernicus (19 February 1473 – 24 May 1543) and Galilei Galileo (15 February 1564 – 8 January 1642).  In less than a century, the crystalline spheres upon which the stars rode and the Ptolemic epicycles that described the motion of the planets, came crashing down.  The wanderers were in orbit about the Sun, not the Earth, and they were not just points of light, but other worlds.  Some like Jupiter, had moons, just like Earth.  Luna, the Earth’s moon, had mountains and craters.  And there was more, much more.  The skies were crowded with stars that could not be seen with the naked eye, but which revealed themselves through magnification (this includes nebula and galaxies which could not be easily distinguished from individual stars back then).  The origin of the “milkiness” of  the Milky Way became obvious—it was composed of stars, packed so closely together in such great density that they appeared as a hazy cloud. 

Until 1781, the number of known planets remained at six, all of which could be observed by the naked eye without the aid of a telescope (Mercury, Venus, Earth, Mars, Jupiter, and Saturn).  Sir William Herschel discovered Uranus in 1781.  Neptune was discovered by a Ph.D. student, Johann Galle in 1846, based on predictions provided by Urbain Le Verrier, which explained the small orbital perturbations of Uranus.  The search for a ninth planet (at the time needed to explain additional perturbations in the orbits of Neptune and Uranus, but no longer needed now that the masses of the previous eight planets are better constrained) was ended in 1930 with the discovery of Pluto by Claude Tombaugh.  Pluto was a bit of an odd planet.  It was very small, but more importantly, it orbited in a plane about the Sun that was significantly different than all the other planets.  Neither Uranus, Neptune nor Pluto can be seen without a telescope.

The orbits of the classical planets about the Sun.  All but pluto orbit in nearly the same ecliptic plane. (Image Source: Unknown)

Oddly enough, what we now call an asteroid named Ceres, was discovered in 1801 by Giuseppi Piazzi.  Like the other known planets at the time, it was a wanderer, in orbit about the Sun between Mars and Jupiter.  By all accounts Ceres was a planet, and although not known at the time, it was spheroidal like a planet.    Indeed, it was considered a planet at the time of its discovery.  Within a year or two, the inner Solar System began to get a lot more crowded.  Pallas, Juno, and Vesta were discovered, also in orbit about the Sun between Mars and Jupiter.  It was not long before dozens and dozens of these objects were found.  By the turn of the century, there were hundreds.

The known objects in the inner Solar System, including asteroids (white) and trojans (green).  Hundreds of additional objects are discovered every year.  (Image credit: Unknown; data source: Minor Planet Data Center).

For no other reason than their location between Mars and Jupiter, these objects which had all the same orbital properties of the classical planets, were put into a new class of objects, called asteroids, as suggested by Sir Herschel (the discoverer of Uranus).  However, for many years, the terms planets and asteroids were used interchangeably in the scientific literature.  By the time thousands of the objects had been discovered, the need to distinguish within the literature the classical planets from these minor planets led to the acceptance of the term asteroid (as well as minor planet), which has remained to this day.  Importantly, at the time, the distinction between asteroids and planets was merely one of convenience. 

Asteroid and former planet Ceres, as viewed through the Hubble Space Telescope by my friend and colleague J. Parker at Southwest Research Institute.  Ceres is massive enough that its shape is spheroidal.  (Image credit:  NASA)

For more than a hundred years after the discovery of the first asteroid, Ceres, the known objects in the Solar System consisted of the Sun, the classical planets and their moons, the asteroids, and the occasional comet.  Then, suddenly, the population of the Solar System once again exploded.

Dave Jewitt and his former graduate student Jan Luu found the first Kuiper Belt Object (KBO) in 1992.  KBOs are found outward from the orbit of Neptune to ~55 AU.  Pluto is within this region and is one of the  largest KBOs.  Thousands of these objects are now known to exist, including Eris, which is thought to be slightly larger than Pluto, and Quaoar, Makemake, Haumea, and Ixion, all of which are at least half as large as Pluto.  Pluto is not alone in the outer Solar System, nor is it unique. 

 Known Outer Solar System Objects consist mainly of KBOs. (Image source: Unknown; Data obtained from Minor Planet Center).

Beyond the Kuiper Belt there has been theorized to exist the Oort cloud, filled with additional icy debris left over from the early formation of the solar system.  It is from the Oort cloud that comets may originate.

We now know (thanks to increasingly powerful telescopes and spacecraft exploration) that objects in the Solar System come in a remarkable variety of shapes, sizes and compositions.  The classical planets are all spheroidal.  The inner planets and asteroids are rocky.  The outer planets are gaseous or liquid metal.  The KBOs are icy (probably mostly methane and water ice).   The largest of the asteroids and KBOs are also spheroidal; this an inescable consequence of physics—at some point an object becomes large enough that its self gravity causes it to collapse upon itself and reaches so-called hydrostatic equilibrium.   At the very small end of the size distribution, we know there exist small grains of dust.  Thus, the Solar System is populated by objects smaller than a dust grain and as large as the Sun with a continuum of objects in between.

Asteroids can occasionally present a hazard to the Earth (just ask the dinosaurs).  Recent efforts to survey the asteroid belt for those objects that may pose a future hazard has resulted in a very well characterized size distribution.  As the size of asteroids decreases, the number of asteroids increases.  In other words, there are just a few very large asteroids (Ceres, Vesta), and thousands upon thousands of the very small (under 100 m).  

The size distribution of asteroids surveyed by the Sloan Telescope.  The population number is normalized the population of 10 km objects.  The asteroid population is a continuum, as is the population of objects in the Solar System.  (Image credit:  Unknown).

If the origin of the classical planets and their moons, the asteroids, the KBOs, and the uncountable dust grains were different, these different origins might serve as a means by which to establish a scientific categorization.   However, the origin of all these objects is the same.  All the objects are the result of countless collisions and gravitational collapse of a protoplanetary disk.   It is the collision and accretion that results in the nearly log-normal size continuum of objects in the Solar System.  Asteroids are just the bits of rubble left over that were not accreted by larger objects.   The asteroids themselves may very well be conglomerations of smaller bits of dust and rock rather than the more solid monolithic structures of the science fiction genera.  KBOs are the bits of rubble in the Outer Solar System that were not accreted.

So here we are today.  We inherited the term planet, originally used to describe the wandering nature of objects against the more predictable background stars.  Millennia ago, we were aware of only a handful of these objects.  In time we found that the wanderers were not only distinct from stars in their journey across the sky, but that they were not stars at all; they really were in a class by themselves.  The idea of planet made sense.  Then, we found that there were more than just a handful of planets.  First there were dozens, then hundreds, then thousands, then tens of thousands!  It still makes sense to distinguish these objects from stars, as they are quite clearly different (for example, there is no nuclear fusion and they are not at the center of the Solar System). 

What does not make scientific sense is to further scientifically categorize the range of solar system objects.  Doing so inherently requires defining arbitrary defining lines in a size continuum.  While such dividing lines can certainly be legislated by such bodies as the International Astronomical Union (IAU), they have no meaningful basis, and such an exercise merely brings to light the hubris of man trying to sort nature into boxes and bins.

In time, the concept of a planet will find its way to the dust bins of astronomical rubbish, atop celestial spheres and the geocentric model of the Solar System.  Astronomers of the future will recognize the continuum of objects in our Solar System and in extra solar systems, too.  Any distinction between objects will be one of convenience rather than of scientific purpose, just as asteroids were once distinguished from planets solely for convenience.  Historians of science will study the transition of the Classical Planetary Model to the Modern Solar System Continuum Model, and children of the future will chuckle at the silly argument made over whether Pluto is a planet, just as we all chuckle over the argument of a circular or rectangular flat Earth.

The concept of a planet is dead.