TRANSKRIP 🔗
WEBVTT
00:00:00.080 --> 00:00:04.000
Quantum on the treadmill, when Schrodinger asked, What is life?
00:00:04.400 --> 00:00:10.000
Most people know Erwin Schrdinger because of a cat, a cat in a box, alive and dead at the same time.
00:00:10.160 --> 00:00:13.679
One of the strangest thought experiments ever associated with physics.
00:00:13.839 --> 00:00:19.120
But Schrödinger asked another question that may have had an even bigger impact outside physics.
00:00:19.359 --> 00:00:19.920
What is life?
00:00:20.079 --> 00:00:21.519
Not in the philosophical sense.
00:00:21.760 --> 00:00:23.839
He wanted to know something much more concrete.
00:00:24.000 --> 00:00:26.079
How can a living organism exist at all?
00:00:26.239 --> 00:00:40.399
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?
00:00:40.640 --> 00:00:45.679
In 1944, Schrdinger published a very small book with a very big title, What is Life?
00:00:45.840 --> 00:00:47.920
The Physical Aspect of the Living Cell.
00:00:48.079 --> 00:00:53.039
And that little book helped inspire an entirely new way of thinking about biology.
00:00:53.200 --> 00:00:56.000
It even helped point scientists toward DNA.
00:00:56.320 --> 00:00:58.960
A physicist walks into biology.
00:00:59.759 --> 00:01:03.840
By the 1940s, Schrdinger had already helped create quantum mechanics.
00:01:04.000 --> 00:01:11.599
We have talked about his wave equation and the radical idea that electrons should not simply be imagined as little planets orbiting a nucleus.
00:01:23.359 --> 00:01:24.159
Think about yourself.
00:01:24.400 --> 00:01:26.159
Your body contains trillions of cells.
00:01:26.480 --> 00:01:27.439
Those cells divide.
00:01:27.680 --> 00:01:38.400
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.
00:01:38.640 --> 00:01:40.480
Schrödinger wanted to know how.
00:01:40.640 --> 00:01:43.519
More specifically, he wanted to understand heredity.
00:01:43.680 --> 00:01:49.120
How could biological instructions remain stable for decades or even thousands of generations?
00:01:49.359 --> 00:01:51.439
Remember, this was 1944.
00:01:51.599 --> 00:01:59.599
Scientists knew chromosomes were involved in heredity, but DNA had not yet been established as the famous information molecule we think of today.
00:01:59.840 --> 00:02:05.040
Watson and Crick would not publish the double helix structure of DNA until 1953.
00:02:05.359 --> 00:02:14.000
Schrdinger was looking at the problem almost a decade earlier, and he approached it like a physicist, the problem of biological information.
00:02:15.120 --> 00:02:20.000
Imagine writing a book, then imagine shrinking the entire book until
Load full transcript
it is far too small to see.
00:02:20.159 --> 00:02:23.680
Now imagine that tiny book sitting inside every cell of your body.
00:02:23.840 --> 00:02:27.599
Each time a cell divides, the book must be copied, not approximately.
00:02:27.759 --> 00:02:30.479
It must be copied with extraordinary accuracy.
00:02:30.719 --> 00:02:38.080
Then those instructions must somehow be used to build proteins, tissues, organs, eyes, skin, bones, and eventually an entire organism.
00:02:38.240 --> 00:02:40.159
What kind of physical object could do that?
00:02:40.319 --> 00:02:46.400
Schrdinger reasoned that whatever carried hereditary information had to have two properties that seemed almost contradictory.
00:02:46.639 --> 00:02:48.240
It had to be incredibly stable.
00:02:48.400 --> 00:02:51.840
But it also had to contain an enormous amount of variation.
00:02:52.000 --> 00:02:57.199
A perfectly repetitive structure would be stable, but it could not encode very much information.
00:02:57.439 --> 00:03:00.960
A highly irregular structure might contain lots of information.
00:03:01.120 --> 00:03:04.960
But how could it remain stable enough to survive generation after generation?
00:03:05.199 --> 00:03:07.439
Schrdinger proposed a fascinating answer.
00:03:07.599 --> 00:03:10.639
He called it an a periodic crystal.
00:03:11.199 --> 00:03:12.719
The a periodic crystal.
00:03:13.840 --> 00:03:15.520
Think about an ordinary crystal.
00:03:15.680 --> 00:03:17.199
Salt is a simple example.
00:03:17.439 --> 00:03:20.159
Its atoms arrange themselves in a repeating structure.
00:03:20.400 --> 00:03:22.719
The same pattern occurs again and again.
00:03:22.879 --> 00:03:25.280
That repetition gives the crystal stability.
00:03:25.439 --> 00:03:28.960
But repetition is not very useful for storing complicated information.
00:03:29.199 --> 00:03:31.759
Imagine a book containing a continuous string of A's.
00:03:31.919 --> 00:03:34.159
It is very orderly, but it does not say much.
00:03:34.400 --> 00:03:42.879
Schrdinger imagined a different kind of molecular structure, something that had the stability of a crystal, but did not simply repeat the same pattern.
00:03:42.960 --> 00:03:44.319
And a periodic crystal.
00:03:44.479 --> 00:03:48.879
Its structure could vary from place to place, and those variations could contain information.
00:03:49.039 --> 00:03:54.000
Imagine instead a CG, TAACG, T T A C G.
00:03:54.319 --> 00:03:55.360
That should look familiar.
00:03:55.520 --> 00:03:57.520
That is essentially how DNA works.
00:03:57.680 --> 00:04:04.560
DNA is built from sequences of chemical bases, adenine, cytosine, guanine, thymine, ACG, and T.
00:04:04.719 --> 00:04:05.439
The order matters.
00:04:05.599 --> 00:04:07.840
Change the order and you change the information.
00:04:08.080 --> 00:04:10.560
Schrödinger did not know the structure of DNA.
00:04:10.719 --> 00:04:12.400
He did not discover DNA.
00:04:12.560 --> 00:04:14.479
He did not predict the double helix.
00:04:14.639 --> 00:04:21.519
But he reasoned that the hereditary material would have to be something remarkably similar to what DNA turned out to be.
00:04:21.680 --> 00:04:25.680
A stable molecular structure whose non-repeating arrangement could encode information.
00:04:25.839 --> 00:04:28.079
That alone would have made what is life remarkable.
00:04:28.240 --> 00:04:30.480
But Schrdinger went one step further.
00:04:30.800 --> 00:04:32.319
The code script.
00:04:33.040 --> 00:04:37.279
Schrodinger suggested that chromosomes contained what he called a code script.
00:04:37.439 --> 00:04:39.600
Think about how extraordinary that phrase is.
00:04:39.839 --> 00:04:50.160
This was before the genetic code had been discovered, before molecular biology had matured, before scientists understood how DNA sequences were translated into proteins.
00:04:50.319 --> 00:04:58.079
Yet Schrdinger was already imagining heredity as something resembling information encoded in a physical structure.
00:04:58.240 --> 00:05:01.120
The chromosome was not merely a chemical object.
00:05:01.279 --> 00:05:02.399
It contained instructions.
00:05:02.560 --> 00:05:09.439
In modern language, we might say something like biology is partly an information processing system implemented in chemistry.
00:05:09.600 --> 00:05:13.360
That idea would become foundational to molecular biology.
00:05:13.600 --> 00:05:17.920
Some of the young scientists who read Schrdinger's book would eventually help create that field.
00:05:18.160 --> 00:05:19.279
James Watson read it.
00:05:19.439 --> 00:05:20.480
Francis Crick read it.
00:05:20.639 --> 00:05:28.160
Physicists and chemists began moving into biology because they believed some of life's deepest mysteries might be understandable using physics.
00:05:28.319 --> 00:05:29.680
Suddenly the question changed.
00:05:29.920 --> 00:05:36.879
Instead of asking what mysterious force makes life alive, scientists increasingly began asking, where are the instructions stored?
00:05:37.040 --> 00:05:38.959
And how are those instructions copied?
00:05:39.199 --> 00:05:42.720
Those questions eventually led directly into the age of DNA.
00:05:43.120 --> 00:05:45.279
But Schrdinger had another problem.
00:05:46.240 --> 00:05:51.199
Information was only half of Schrdinger's puzzle, and there was something else about life that seemed strange.
00:05:51.519 --> 00:05:53.199
The universe tends toward disorder.
00:05:53.360 --> 00:05:57.279
This comes from one of the most important ideas in thermodynamics, entropy.
00:05:57.519 --> 00:06:03.759
Very loosely, physical systems tend to move from highly organized states toward more probable disorganized states.
00:06:04.000 --> 00:06:11.120
Ice melts, hot coffee cools, buildings decay, machines wear out, yet living organisms seem to do something completely different.
00:06:11.279 --> 00:06:17.040
A seed becomes a tree, an embryo becomes a human being, a cut heals, cells continually rebuild themselves.
00:06:17.120 --> 00:06:23.600
If your body somehow maintains an astonishingly complicated structure for decades, life seems to create and maintain order.
00:06:23.759 --> 00:06:27.360
So Schrodinger asks, How can life apparently resist entropy?
00:06:27.519 --> 00:06:30.560
His answer became one of the most famous parts of what is life.
00:06:30.720 --> 00:06:34.879
He said living organisms feed on what he called negative entropy.
00:06:35.120 --> 00:06:37.040
Eating negative entropy.
00:06:37.759 --> 00:06:42.399
Schrodinger did not mean that organisms somehow violate the second law of thermodynamics.
00:06:42.560 --> 00:06:43.040
They don't.
00:06:43.199 --> 00:06:45.839
The trick is that living organisms are open systems.
00:06:46.000 --> 00:06:48.800
You constantly exchange matter and energy with your environment.
00:06:48.959 --> 00:06:53.120
You eat, you breathe, you absorb energy, you release heat, you produce waste.
00:06:53.279 --> 00:06:57.519
Your body uses energy from the environment to maintain its internal organization.
00:06:57.680 --> 00:06:59.680
Think about your workout on the treadmill.
00:06:59.839 --> 00:07:02.079
Your body is an incredibly organized machine.
00:07:02.399 --> 00:07:09.120
Muscles contract, your heart pumps, your lungs exchange oxygen and carbon dioxide, and its cells convert chemical energy into motion.
00:07:09.279 --> 00:07:13.439
But while your body maintains that organization, it releases heat into the room.
00:07:13.680 --> 00:07:14.720
Energy is dispersed.
00:07:14.879 --> 00:07:17.360
Entropy increases in the larger environment.
00:07:17.519 --> 00:07:20.800
Life maintains local order by increasing disorder elsewhere.
00:07:20.959 --> 00:07:29.519
Schrdinger's phrase, negative entropy, is not generally how physicists describe the process today, but the insight behind it was powerful.
00:07:29.759 --> 00:07:31.759
Life does not escape thermodynamics.
00:07:31.920 --> 00:07:36.720
Life survives because it is constantly exchanging energy and matter with its surroundings.
00:07:36.879 --> 00:07:40.800
Stop those exchanges and biological order eventually disappears.
00:07:41.360 --> 00:07:44.639
And here quantum mechanics sneaks back into the story.
00:07:45.519 --> 00:07:47.759
This is where the story becomes especially interesting.
00:07:48.079 --> 00:07:50.160
Remember who was asking these questions.
00:07:50.480 --> 00:07:53.199
Schrdinger, one of the architects of quantum mechanics.
00:07:53.279 --> 00:07:57.279
And he knew that at the microscopic level, the world is governed by quantum physics.
00:07:57.439 --> 00:07:59.040
Atoms obey quantum rules.
00:07:59.199 --> 00:08:01.839
Chemical bonds ultimately depend on quantum mechanics.
00:08:02.000 --> 00:08:05.920
Molecules behave the way they do because electrons occupy quantum states.
00:08:06.079 --> 00:08:11.439
So the hereditary molecule Schrdinger was imagining had to exist in that quantum world.
00:08:11.600 --> 00:08:13.839
And yet heredity is extraordinarily reliable.
00:08:14.000 --> 00:08:17.279
An organism can preserve biological information for generations.
00:08:17.600 --> 00:08:23.279
How can the microscopic quantum world produce something as stable as biological heredity?
00:08:23.519 --> 00:08:26.720
Schrdinger realized that molecular structure provided the bridge.
00:08:26.879 --> 00:08:31.279
Quantum mechanics allows certain atomic arrangements to be extremely stable.
00:08:31.439 --> 00:08:38.960
Chemical bonds can lock atoms into structures that persist for long periods, and that stability makes it possible to store information physically.
00:08:39.120 --> 00:08:42.159
And that is one of the deeper ideas hidden inside what is life.
00:08:42.240 --> 00:08:45.759
The weirdness of quantum mechanics does not make stable life impossible.
00:08:46.000 --> 00:08:53.200
In some sense, quantum mechanics makes stable molecules possible, and stable molecules make biological information possible.
00:08:53.600 --> 00:08:58.879
Physics becomes biology and this is where I think the history becomes beautiful.
00:08:59.039 --> 00:09:02.879
Quantum mechanics began by asking strange questions about light in atoms.
00:09:03.039 --> 00:09:05.759
Why did hot objects emit energy in discrete amounts?
00:09:05.919 --> 00:09:08.399
Why did atoms produce specific spectral lines?
00:09:08.559 --> 00:09:11.759
Why couldn't electrons simply orbit nuclei like planets?
00:09:11.919 --> 00:09:18.799
Planck introduced quanta, Bohr introduced quantum jumps, Heisenberg abandoned electron trajectories, Schrdinger introduced waves.
00:09:18.879 --> 00:09:25.039
And then Schrdinger looked at biology and realized that those same physical laws had to explain something completely different.
00:09:25.279 --> 00:09:27.840
How does an organism remember how to build itself?
00:09:28.000 --> 00:09:30.639
That question helped pull physics into biology.
00:09:30.879 --> 00:09:50.399
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.
00:09:51.120 --> 00:09:53.600
Schrdinger didn't discover DNA.
00:09:54.399 --> 00:09:56.799
It is important not to exaggerate the story.
00:09:57.039 --> 00:09:59.120
Schrodinger did not discover DNA.
00:09:59.279 --> 00:10:03.039
Scientists such as Friedrich Mischer had identified DNA decades earlier.
00:10:03.200 --> 00:10:11.120
Oswald Avery, Colin McLeod, and Macklin McCarty provided crucial evidence in 1944 that DNA carried hereditary information.
00:10:11.360 --> 00:10:15.200
Rosalind Franklin and Maurice Wilkins produced essential structural evidence.
00:10:15.440 --> 00:10:17.759
Watson and Crick proposed the double helix.
00:10:17.840 --> 00:10:20.000
Many scientists built molecular biology.
00:10:20.159 --> 00:10:22.080
Schrödinger's contribution was different.
00:10:22.240 --> 00:10:23.679
He helped frame the question.
00:10:23.919 --> 00:10:32.720
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.
00:10:32.960 --> 00:10:37.519
Sometimes asking the right question changes science almost as much as answering one.
00:10:38.080 --> 00:10:40.480
And that may be the bigger Schrdinger story.
00:10:41.759 --> 00:10:53.120
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.
00:10:53.279 --> 00:10:59.039
The first was quantum mechanics, the second was molecular biology, and the bridge between them was information.
00:10:59.200 --> 00:11:01.120
Atoms could form molecules.
00:11:01.279 --> 00:11:04.960
Molecules could store patterns, patterns could encode instructions.
00:11:05.200 --> 00:11:18.159
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.
00:11:18.320 --> 00:11:28.639
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.
00:11:28.960 --> 00:11:37.120
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?
00:11:37.440 --> 00:11:38.720
His answer was not complete.
00:11:38.960 --> 00:11:45.519
But the question helped open the door to DNA, and perhaps that is one of the recurring lessons in the history of quantum mechanics.
00:11:45.759 --> 00:11:52.960
Every time physicists thought they were studying something incredibly small, they eventually discovered that the implications were incredibly large.