Life as Physics
Staying Far From Equilibrium
What Schrödinger Asked in 1944
Erwin Schrödinger gave a series of lectures in Dublin during the war, published as a short book asking what life looks like from the point of view of physics. He had no biological training and the book contains mistakes. It is also one of the most consequential things written about biology in the twentieth century, because it asked the right questions in a language that made them answerable.
His central puzzle was inheritance. A gene had to be small – a few thousand atoms – and it had to be stable across generations, essentially perfectly. At that size, thermal jostling should scramble anything. Schrödinger reasoned that the carrier had to be a solid rather than a liquid, held by chemical bonds strong enough to resist ordinary heat, and it had to carry information in a sequence that does not repeat. He called it an aperiodic crystal.
That is a fair description of a strand of DNA, arrived at nine years before its structure was known. Both Crick and Watson said the book influenced them. Whatever else it got wrong, it demonstrated that thinking about life in terms of stability, information, and thermal noise was going to be productive.
Nothing Here Breaks the Second Law
A living thing is spectacularly organized, and organization is exactly what the second law says should decay. This observation gets misused constantly, so it is worth stating the resolution precisely rather than waving at it.
The second law applies to closed systems. Nothing alive is closed. An organism maintains its own order by exporting disorder to its surroundings at a greater rate, and the books balance with room to spare. You are not defying anything. You are running a very efficient drain.
The specific accounting for Earth is elegant. Sunlight arrives as a small number of high-energy photons from a source at about six thousand degrees. Earth radiates the same total energy back out as a much larger number of low-energy infrared photons at around minus eighteen degrees. Energy in equals energy out, so nothing is accumulating. What arrives concentrated leaves spread out, and that difference is a flow of order into the system and disorder out of it.
Schrödinger put it as living on negative entropy, which he later regretted as clumsy phrasing. The cleaner statement is that life is not powered by energy, since energy is conserved and never used up. It is powered by the difference between concentrated energy and diffuse energy, and that difference is the finite resource.
Swimming Where Coasting Is Impossible
A bacterium lives in a world where the physics of motion works nothing like ours. At its scale, the viscosity of water dominates completely over inertia, and the consequences are strange.
A bacterium that stops swimming stops moving within about the width of a single atom. There is no coasting, no glide, no momentum worth speaking of. Purcell, who laid this out in a famous lecture, put the human equivalent as swimming in a pool of molasses while forbidden to move any part of your body faster than a centimeter a minute.
This produces a hard theorem. Any swimming stroke that looks the same played backward achieves exactly nothing. A scallop opening slowly and snapping shut quickly works for a scallop, because inertia carries it. At bacterial scale there is no inertia to carry anything, so the shell simply returns to where it started. Speed makes no difference whatsoever.
So microscopic swimmers need strokes that are not their own mirror image in time. Bacteria solved it with a rotating helix – a corkscrew turning always in one direction, driven by a rotary motor embedded in the cell wall that is one of the few true wheels in biology. Sperm cells solved it with a wave traveling along a tail, which also has a direction. Both are answers to a physics constraint, not a biological preference.
Machines in a Hurricane
Inside every cell are molecular machines that do mechanical work. Kinesin walks along filaments carrying cargo, one foot after the other, in steps of about eight nanometers. The rotary motor that makes the cell’s energy currency turns like a turbine driven by a flow of protons across a membrane. Muscle is billions of tiny motors pulling in unison.
The physics of these is not the physics of engines. At that size, the random hammering of water molecules delivers forces comparable to the forces the motor itself generates. A macroscopic engine works despite thermal noise, which is negligible for it. A molecular motor works inside noise that is larger than its own signal.
The trick is not to fight the noise but to bias it. The motor is jostled in both directions constantly, and it spends energy not on pushing but on preventing the backward steps – closing the door behind it rather than shoving forward. That is a ratchet, and it is a different operating principle from anything in a car. It also has a hard constraint: a ratchet that worked without an energy supply would be a perpetual motion machine, which is why the energy currency is consumed at every step.
Precision Out of Sloppy Parts
Copying DNA has an error rate around one in a billion. The chemistry alone cannot deliver that. The energy difference between pairing correctly and incorrectly is only worth a factor of a hundred or so against a background of thermal noise, which would mean an error every hundred letters or so.
The gap is closed by checking the same decision more than once, and by spending energy to discard the intermediate rather than allowing it to settle back. Because each check is somewhat independent, the error rates multiply rather than add, and a short series of sloppy checks produces one precise outcome. The price is paid in energy, and paying that price is the only way to buy accuracy beyond what equilibrium chemistry allows.
This is a general result rather than a quirk of biology. Reliability beyond the thermal limit always costs energy, and the same reasoning sets the minimum energy to erase a bit of information in a computer. Life is not exempt from the trade. It is unusually good at paying.
Ordinary Physics, Unusual Conditions
Nothing above suggests life needs new physics. Every mechanism described is ordinary electromagnetism and ordinary statistical mechanics, applied where thermal energy is comparable to the energies holding structures together – the same regime as soft matter, which is not a coincidence, since a cell is a crowded colloid full of polymers.
What is incomplete is the theory rather than the ingredients. Statistical mechanics is superb at systems in equilibrium and much weaker away from it, and everything alive is permanently away from it. There is no general framework that predicts what a driven system will organize itself into, which means there is no principled way to say what should happen when energy flows through matter for long enough.
Several proposals exist. Some argue that systems driven hard tend to arrange themselves to dissipate energy more effectively, which would make life’s emergence something closer to expected than to freakish. These are interesting and none is established, and the claim that thermodynamics makes life probable is currently a research direction rather than a result. Treat confident statements in either direction with care.
Separately, this page is deliberately not about quantum effects in biology. Those are a narrower and much-hyped question with a small number of genuine cases. The physics here is classical and is doing nearly all of the work.
A Different Set of Questions
Looking at life this way does not reduce it to anything. It gives a different set of questions. Not what a molecule does, but what it costs; not what a structure is, but what flow maintains it; not why an organism is complicated, but what physical constraints made that particular kind of complication the available one.
The answers keep turning out to be sharp. Bacteria have corkscrews because reciprocal strokes cannot work at that scale. Copying is accurate because energy was spent to make it so. You are warm because staying organized requires continuously dumping disorder into the room. None of that is metaphor. It is the same physics as the rest of this site, running in the one place where it produced something that could eventually ask about itself.
