Abstract: In Sewell’s discussion of entropy flow, he defines “Entropy-X”, which is a very useful concept that should clarify misconceptions of entropy promulgated by Wikipedia and scientists unfamiliar with thermodynamics. Sewell’s important contribution is to argue that one can and should reduce the “atom-entropy” into a subsets of mutually exclusive “entropy-X”. Mathematically, this is like factoring an N x M matrix into block diagonal form, by showing that the cross terms between blocks do not contribute to the total. Each of the blocks in the diagonal then correspond to a separately computed entropy, or “Entropy-X”. This contribution not only clarifies many of the misunderstandings of laypersons, it also provides a way for physicists to overcome their confusion regarding biology.
– 0 –
Introduction: Entropy was initially discussed in terms of thermodynamics, as a quantity that came out of the energy, temperature, work formulas. Ludwig Boltzmann found a way to relate billiard-ball counting statistics to this thermodynamic quantity, with a formula he had inscribed on his tombstone: S = k ln(Ω). The right-hand-side of this equation, contains a logarithm of the possible ways to arrange the atoms. The left-hand-side is the usual thermodynamic quantity. Relating the two different worlds of counting and heat, is this constant “k”, now called the “Boltzmann constant”.
Now for the shocking part. There is no theory that predicts its value. It is a conversion constant that is experimentally determined. It works best when the real physical system approximates billiard balls–such as noble gasses. The constant gets progressively worse, or needs more adjustments, if the right-hand-side becomes N2 (diatomic gas) or CO2 (triatomic). Going from gas to liquid introduces even more corrections, and by the time we get to solids we use a completely different formula.
For example, a 1991 Nobel prize was awarded for studying how a long oily molecule moves around in a liquid, because not every state of rearrangement is accessible for tangled strings. So 100 years after Boltzmann, we are just now tackling liquids and gels and trying to understand their entropy.
Does that mean we don’t know what entropy is?
No, it means that we don’t have a neat little Boltzmann factor for relating thermodynamic-S to computer statistics. We still believe that it is conserved, we just can’t compute the number very easily. This is why Granville Sewell uses “X-entropy” to describe all the various forms of order in the system. We know they must be individually conserved, barring some conversion between the various types of entropy in the sytem, but we can’t compute it very well.
Nor is it simply that the computation gets too large. For example, in a 100-atom molecule, the entropy is NOT computed by looking at all the 100! permutations of atoms, simply because many of those arrangements are energetically impossible. Remember, when Boltzmann described “ln(Ω)” he was calling it the possible states of the system. If the state is too energetic, it isn’t accessible without a huge amount of energy. In particle physics, this limitation becomes known as “spontaneous symmetry breaking”, and is responsible for all the variation we see in our universe today.
So rather than counting “atom states”, we assemble atoms into molecules and form new entities that act as complete units, as “molecules”, and then the entropy consists of counting “states of the molecule”–a much smaller number than “states of the atoms of the molecules”. Molecules form complexes, and then we compute “states of the molecular complexes”. And complexes form structures, such as membranes. Then we compute “states of the structures”. This process continues to build as we approach the size of a cell, and then we have to talk about organs and complete organisms, and then ecologies and Gaia. The point is that our “unit” of calculation is a getting larger and larger as the systems display larger and larger coherence.
Therefore it is completely wrong to talk about single-atom entropy and the entropy of sunlight when we are discussing structural entropy, for as Granville and previous commentators have said, the flow of energy in sunlight with a coherence length of one angstrom cannot explain the decameter coherence of a building.
So from consideration of the physics, it is possible to construct a hierarchical treatment of entropy which enables entropy to address the cell, but in 1991 we had barely made it to the oily liquid stage of development. So on the one hand, unlike many commentators imagine, physicists don’t know how to compute an equivalent “Boltzmann equation” for the entropy of life, but on the other hand Granville is also right, we don’t need to compute the Boltzmann entropy to show that it must be conserved.
– o –
Mathematical Discussion: Sewell’s contribution is to recognize that there must be a hierarchical arrangement of atoms that permit the intractible problem of calculating the entropy to be treated as a sum of mutually exclusive sub-entropies.
This contribution can be seen in the difference between “Entropy-chromium” and “Entropy-heat” that he introduces in his paper, where Entropy-chromium is the displacement of chromium atoms in a matrix of iron holding the velocity of the atoms constant, and Entropy-heat considers a variation in velocities while holding the position constant. These two type of entropy have a large energy barrier separating them on the order of several eV per atom that prevent them from interconverting. At sufficiently high temperature–say, the temperature at which the iron-chrome allow was poured–the chromium atoms have sufficient mobility to overcome the energy barrier and move around. But at the present room temperature, they are immobile. So in the creation event of the chromium bar, the entropy was calculated for both position and velocity, but as it cooled, “spontaneous symmetry breaking” produced two smaller independent entropies from the single larger one.
Now the beginning of this calculation is the full-up, atom entropy where everything is accessible. This “big-bang” entropy gives at least 7 degrees of freedom for each atom. That is, the number of bins available for each atom are at least 7–one for the species, 3 that give the position in x,y,z and 3 that give the velocity in Vx,Vy,Vz. We could subdivide species into all the different quantum numbers that define atoms, but for the moment we’ll ignore that nuance. In addition, the quantization of space and velocity into “Planck” sizes of 10^-34 meters, means that our bins do not always have a real number, but have a quantized length or velocity specified by an integer number of Planck sizes. But again, the real quantization is that atoms don’t overlap, so we can use a much coarser quantization of 10^-10 meters or angstrom atomic lengths. The reason this is important, is that we are reducing ln(Ω) by restricting the number of states of the system that we need to consider.
But if S = ln(Ω), then this means we are mathematically throwing away entropy! How is that fair?
There is a curious result in quantum mechanics, that says if we can’t distinguish two particles, then there is absolutely no difference if they are swapped. This is another way of saying that their position entropy is zero. So if we have two states of a system, separated by a Planck length, but can’t tell the difference, it doesn’t contribute to the entropy. Now this isn’t to say that we can’t invent a system that can tell the difference, but since a Planck length corresponds to light with gamma-ray intensity, we really have to go back to the Big Bang to find a time when this entropy mattered. This reconfirms our assumption that as a system cools, it loses entropy and has fewer and fewer states available.
But even this angstrom coarse graining in position represented by “Entropy-Chromium”, is still too fine for the real world because biology is not made out of noble gasses, bouncing in a chamber. Instead, life exists in a matrix of water, an H2O molecule of nanometer size. Just as we cannot tell the difference if we swap the two hydrogen atoms in the water molecule around, we can’t tell the difference if we swap two water molecules around. So the quantization entropy gets even coarser, and the number of states of the system shrink, simply because the atoms form molecules.
A very similar argument holds for the velocities. A hydrogen atom can’t have every velocity possible because it is attached to an oxygen atom. That chemical bond means that the entire system has to move as a unit. But QM tells us that as the mass of a system goes up, the wavelength goes down, which is to say, the number of velocities we have to consider in our binning is reduced as we have a more massive system. Therefore the velocity entropy drops as the system becomes more chemically bound.
And of course, life is mostly made out of polymers of 100’s or 1000’s of nanometers in extent, which have even more constraints as they get tangled around each other and attach or detach from water molecules. That was what the 1991 Nobel prize was about.
Mathematically, we can write the “Big Bang” entropy as a N x M matrix, where N is the number of particles and M the number of bins. As the system cools and becomes more organized, the entropy is reduced, and the system becomes “block-diagonal”, where blocks can correspond to molecules, polymer chains, cell structures, organelles, cells, etc.
Now here is the key point.
If we only considered the atom positions, and not these molecular and macro-molecular structures, the matrix would not be in block diagonal form. Then when we computed the Boltzmann entropy, ln(Ω), we would have a huge number of states available. But in fact, biology forms so much structure, that ln(Ω) is greatly reduced. All those cross-terms in the matrix are empty, because they are energetically inaccessible, or topologically inaccessible (see the 1991 Nobel prize). Sewell is correct, there is a tremendous amount of order (or reduced entropy) that is improperly calculated if life is considered as a ball of noble gas.
Let me say this one more time. Sewell is not only correct about the proper way to calculate entropy, he has made a huge contribution in arguing for the presence of (nearly) non-convertible forms of sub-entropy.
– o –
Thermodynamic Comment: Some have argued that this “sub-entropy” of Sewell’s can be explained by some sort of spontaneous symmetry breaking due to the influx of energy. We have talked about “cooling” causing spontaneous symmetry breaking, which is consistent with the idea that higher temperatures have higher entropy, but the idea that “heating” can also drive symmetry breaking and lowered entropy is incoherent. This is simply because thermodynamically d(TS) = dE, or dS = d(E/T), which is to say, energy brings entropy and temperature simultaneously.
Let’s look at this a bit closer and apply it to the Earth. The Sun is at 5500K, and therefore its energy comes principally in the form of yellow photons. The Earth global temperature averages out to about 300K, so it emits infrared photons. In thermodynamic equilibrium, the energy entering has to balance the energy leaving (or the Earth would heat up!). Since the entropy of the photons hitting the Earth have almost twenty times less than the entropy of the photons leaving the Earth, the Earth must be a source for entropy. Ergo, sunlight should be randomizing every molecule in the Earth and preventing life from happening.
Does this make sense? I mean everybody and their brother say that entropy can decrease if you have a heat engine in the system. Energy comes into your refrigerator as low entropy energy. Energy billows out of the coils in the back as high entropy heat. But inside the fridge is a low-entropy freezer. Couldn’t this apply to Earth? (E.g., compensation argument.)
Well, if the Earth had a solar-powered refrigerator, and some insulation, and some fancy piping, yes. But of course, all that is an even more low entropy system than the electricity, so we are invoking bigger and bigger systems to get the entropy in the small freezer to go down. The more realistic comparison is without all that special pleading. Imagine you leave your freezer door open accidently and leave for the weekend. What will you find? A melted freezer and a very hot house, because your fridge would run continuously trying to cool something down that warmed even faster. In fact, this is how dehumidifiers function. So your house would be hot and dry, with a large puddle of coolish water in the kitchen. Some fridges dump that water into a pan under the heating coils, which would evaporate the water and we would then be back to our original state but at higher temperature. Would the entropy of the kitchen be greater or smaller than if you had unplugged the fridge for the weekend? Greater, because of all that heat.
And in fact, this is the basic argument for why objects that sit in space for long periods of time have surfaces that are highly randomized. This is why Mars can’t keep methane in its atmosphere. This is why Titan has aerosols in its atmosphere. This is why the Moon has a sterile surface.
If the Earth is unique, there is more than simply energy flow from the Sun that is responsible.