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Quantum on the treadmill, when Schrodinger asked, What is life?
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Most people know Erwin Schrdinger because of a cat, a cat in a box, alive and dead at the same time.
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One of the strangest thought experiments ever associated with physics.
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But Schrödinger asked another question that may have had an even bigger impact outside physics.
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What is life?
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Not in the philosophical sense.
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He wanted to know something much more concrete.
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How can a living organism exist at all?
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How can something as complicated as a human being maintain its structure, reproduce itself, repair itself, and pass incredibly detailed instructions from one generation to another when everything underneath it is made from atoms obeying the strange laws of physics?
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In 1944, Schrdinger published a very small book with a very big title, What is Life?
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The Physical Aspect of the Living Cell.
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And that little book helped inspire an entirely new way of thinking about biology.
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It even helped point scientists toward DNA.
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A physicist walks into biology.
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By the 1940s, Schrdinger had already helped create quantum mechanics.
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We have talked about his wave equation and the radical idea that electrons should not simply be imagined as little planets orbiting a nucleus.
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Think about yourself.
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Your body contains trillions of cells.
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Those cells divide.
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They manufacture proteins, they repair damage, they reproduce structures with astonishing precision, and somehow the instructions for building another human being can be packed inside microscopic cells.
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Schrödinger wanted to know how.
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More specifically, he wanted to understand heredity.
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How could biological instructions remain stable for decades or even thousands of generations?
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Remember, this was 1944.
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Scientists knew chromosomes were involved in heredity, but DNA had not yet been established as the famous information molecule we think of today.
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Watson and Crick would not publish the double helix structure of DNA until 1953.
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Schrdinger was looking at the problem almost a decade earlier, and he approached it like a physicist, the problem of biological information.
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Imagine writing a book, then imagine shrinking the entire book until
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it is far too small to see.
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Now imagine that tiny book sitting inside every cell of your body.
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Each time a cell divides, the book must be copied, not approximately.
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It must be copied with extraordinary accuracy.
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Then those instructions must somehow be used to build proteins, tissues, organs, eyes, skin, bones, and eventually an entire organism.
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What kind of physical object could do that?
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Schrdinger reasoned that whatever carried hereditary information had to have two properties that seemed almost contradictory.
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It had to be incredibly stable.
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But it also had to contain an enormous amount of variation.
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A perfectly repetitive structure would be stable, but it could not encode very much information.
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A highly irregular structure might contain lots of information.
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But how could it remain stable enough to survive generation after generation?
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Schrdinger proposed a fascinating answer.
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He called it an a periodic crystal.
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The a periodic crystal.
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Think about an ordinary crystal.
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Salt is a simple example.
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Its atoms arrange themselves in a repeating structure.
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The same pattern occurs again and again.
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That repetition gives the crystal stability.
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But repetition is not very useful for storing complicated information.
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Imagine a book containing a continuous string of A's.
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It is very orderly, but it does not say much.
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Schrdinger imagined a different kind of molecular structure, something that had the stability of a crystal, but did not simply repeat the same pattern.
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And a periodic crystal.
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Its structure could vary from place to place, and those variations could contain information.
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Imagine instead a CG, TAACG, T T A C G.
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That should look familiar.
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That is essentially how DNA works.
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DNA is built from sequences of chemical bases, adenine, cytosine, guanine, thymine, ACG, and T.
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The order matters.
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Change the order and you change the information.
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Schrödinger did not know the structure of DNA.
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He did not discover DNA.
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He did not predict the double helix.
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But he reasoned that the hereditary material would have to be something remarkably similar to what DNA turned out to be.
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A stable molecular structure whose non-repeating arrangement could encode information.
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That alone would have made what is life remarkable.
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But Schrdinger went one step further.
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The code script.
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Schrodinger suggested that chromosomes contained what he called a code script.
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Think about how extraordinary that phrase is.
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This was before the genetic code had been discovered, before molecular biology had matured, before scientists understood how DNA sequences were translated into proteins.
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Yet Schrdinger was already imagining heredity as something resembling information encoded in a physical structure.
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The chromosome was not merely a chemical object.
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It contained instructions.
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In modern language, we might say something like biology is partly an information processing system implemented in chemistry.
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That idea would become foundational to molecular biology.
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Some of the young scientists who read Schrdinger's book would eventually help create that field.
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James Watson read it.
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Francis Crick read it.
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Physicists and chemists began moving into biology because they believed some of life's deepest mysteries might be understandable using physics.
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Suddenly the question changed.
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Instead of asking what mysterious force makes life alive, scientists increasingly began asking, where are the instructions stored?
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And how are those instructions copied?
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Those questions eventually led directly into the age of DNA.
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But Schrdinger had another problem.
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Information was only half of Schrdinger's puzzle, and there was something else about life that seemed strange.
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The universe tends toward disorder.
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This comes from one of the most important ideas in thermodynamics, entropy.
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Very loosely, physical systems tend to move from highly organized states toward more probable disorganized states.
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Ice melts, hot coffee cools, buildings decay, machines wear out, yet living organisms seem to do something completely different.
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A seed becomes a tree, an embryo becomes a human being, a cut heals, cells continually rebuild themselves.
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If your body somehow maintains an astonishingly complicated structure for decades, life seems to create and maintain order.
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So Schrodinger asks, How can life apparently resist entropy?
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His answer became one of the most famous parts of what is life.
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He said living organisms feed on what he called negative entropy.
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Eating negative entropy.
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Schrodinger did not mean that organisms somehow violate the second law of thermodynamics.
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They don't.
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The trick is that living organisms are open systems.
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You constantly exchange matter and energy with your environment.
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You eat, you breathe, you absorb energy, you release heat, you produce waste.
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Your body uses energy from the environment to maintain its internal organization.
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Think about your workout on the treadmill.
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Your body is an incredibly organized machine.
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Muscles contract, your heart pumps, your lungs exchange oxygen and carbon dioxide, and its cells convert chemical energy into motion.
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But while your body maintains that organization, it releases heat into the room.
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Energy is dispersed.
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Entropy increases in the larger environment.
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Life maintains local order by increasing disorder elsewhere.
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Schrdinger's phrase, negative entropy, is not generally how physicists describe the process today, but the insight behind it was powerful.
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Life does not escape thermodynamics.
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Life survives because it is constantly exchanging energy and matter with its surroundings.
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Stop those exchanges and biological order eventually disappears.
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And here quantum mechanics sneaks back into the story.
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This is where the story becomes especially interesting.
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Remember who was asking these questions.
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Schrdinger, one of the architects of quantum mechanics.
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And he knew that at the microscopic level, the world is governed by quantum physics.
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Atoms obey quantum rules.
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Chemical bonds ultimately depend on quantum mechanics.
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Molecules behave the way they do because electrons occupy quantum states.
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So the hereditary molecule Schrdinger was imagining had to exist in that quantum world.
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And yet heredity is extraordinarily reliable.
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An organism can preserve biological information for generations.
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How can the microscopic quantum world produce something as stable as biological heredity?
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Schrdinger realized that molecular structure provided the bridge.
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Quantum mechanics allows certain atomic arrangements to be extremely stable.
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Chemical bonds can lock atoms into structures that persist for long periods, and that stability makes it possible to store information physically.
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And that is one of the deeper ideas hidden inside what is life.
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The weirdness of quantum mechanics does not make stable life impossible.
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In some sense, quantum mechanics makes stable molecules possible, and stable molecules make biological information possible.
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Physics becomes biology and this is where I think the history becomes beautiful.
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Quantum mechanics began by asking strange questions about light in atoms.
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Why did hot objects emit energy in discrete amounts?
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Why did atoms produce specific spectral lines?
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Why couldn't electrons simply orbit nuclei like planets?
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Planck introduced quanta, Bohr introduced quantum jumps, Heisenberg abandoned electron trajectories, Schrdinger introduced waves.
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And then Schrdinger looked at biology and realized that those same physical laws had to explain something completely different.
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How does an organism remember how to build itself?
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That question helped pull physics into biology.
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Less than ten years after what his life was published, Watson and Crick proposed the double helix structure of DNA, and suddenly Schrdinger's mysterious aperiodic crystal had something very close to a physical identity, DNA, a stable molecule, a non-repeating sequence, a chemical structure capable of storing biological information.
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Schrdinger didn't discover DNA.
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It is important not to exaggerate the story.
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Schrodinger did not discover DNA.
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Scientists such as Friedrich Mischer had identified DNA decades earlier.
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Oswald Avery, Colin McLeod, and Macklin McCarty provided crucial evidence in 1944 that DNA carried hereditary information.
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Rosalind Franklin and Maurice Wilkins produced essential structural evidence.
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Watson and Crick proposed the double helix.
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Many scientists built molecular biology.
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Schrödinger's contribution was different.
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He helped frame the question.
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He asked physicists to look at life and say, there must be a physical mechanism capable of storing enormous amounts of information with extraordinary stability.
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Sometimes asking the right question changes science almost as much as answering one.
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And that may be the bigger Schrdinger story.
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We tend to remember Schrdinger for the cat, but perhaps we should also remember him for the cell, because Schrdinger helped uh connect two of the greatest scientific revolutions of the 20th century.
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The first was quantum mechanics, the second was molecular biology, and the bridge between them was information.
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Atoms could form molecules.
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Molecules could store patterns, patterns could encode instructions.
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Instructions could reproduce organisms, physics could become chemistry, chemistry could become biology, and biology could become information, which leaves us with a remarkable idea to think about while finishing today's treadmill session.
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Schrödinger spent much of his life helping us understand a world where particles behave like waves, certainty gives way to probability, and nature becomes strange at the smallest scales.
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Then he looked at something as familiar as a living cell and asked, How does all of that quantum weirdness produce something capable of remembering what it is?
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His answer was not complete.
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But the question helped open the door to DNA, and perhaps that is one of the recurring lessons in the history of quantum mechanics.
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Every time physicists thought they were studying something incredibly small, they eventually discovered that the implications were incredibly large.