omenti

(way) beyond black swans

a mnipost

 

Fundamental change comes from discovery - something that science is wired to do.  The process of science is admittedly messy and a bit inefficient, but it works and has vastly expanded our understanding in so many areas. There's this myth about how it works -  the scientific method kids had to memorize in school is a not very accurate model of how things really operated - at least the areas I know. All of the pieces are there, but they're generally non-linear and can spread out over long periods of time.   What drives people are the rare magical moments.  For most of us they're rather small, but the pleasure in discovery is enormous.  It's very hard work as it's so easy to fool yourself.  Some of the most dramatic discoveries come when you're looking for something else - if you're careful enough to recognize them. 

 

One of my favorite examples is a 24 year old astronomy grad student who fundamentally put a dent in astronomy and physics.  Her discovery involved a huge amount of careful work.  Her advisor was awarded the Nobel Prize in Physics while she got nothing -after all - she was a woman and a student in the 60s.  I've been lucky enough to have had dinner with her - she sees her lack of the Nobel underscoring how sexist science and the Nobel committee are.  

 

The interview is excellent and something to show to kids for inspiration as well as dealing with the imposter syndrome.  She's done so much for women in science.

 

not buying it yet

Extraordinary claims require extraordinary evidence

    Carl Sagan - Cosmos

Several people have been in contact with questions about a new physics paper on a room temperature superconductor. It’s a big thing it’s real and if it can be developed into something practical. I skimmed the paper at first and then gave a more careful read. So here are a few early comments.

 

First the background. Superconductors conduct electricity without resistance, but there are caveats. The most useful ones need to be cooled to liquid helium temperatures. At normal atmosphere pressures that’s about 4.2 K .. really cold! (absolute zero is 0 K). Helium is very expensive, in short supply if you want scale and expensive to liquify and keep in a liquid state. LHe temperature superconductors are used in superconducting magnets that produce very high magnetic fields - physics experiments, maglev vehicles and MRI machines are major users. For the last 25 years or so we’ve had superconductors that work at liquid nitrogen temperatures. LN₂ is comparatively inexpensive and fairly easy to handle. The problem is they don’t support high electric currents or magnetic fields.. There’s been a lot of focused research working on this with the best materials known as ReBCO (rare earth barium copper oxides) - there’s a largish family. They do support relatively high current densities and magnetic fields at liquid hydrogen temperatures. LH₂ is between LHe and LN₂ in temperature. It’s much cheaper than liquid helium. Currently the cost of LH₂ temperature superconductors is about four times that of conventional materials in high field magnets. Getting down to the same cost would be a big thing.

 

There has been work on much higher temperature superconductors - some work suggested it could be done at room temperature, but at very high pressures. That was a couple of years ago, but there are questions with the experimental work and the very high pressure requirement makes it impractical. But if it turns out to be real, it could provide useful insight.

 

So onto the current paper. It shows superconductivity at and somewhat above room temperature (up to 400 K!) and at sea level pressures.

 

Reading the paper I see a number of red flags. It hasn't been peer reviewed yet.  None of the authors is known in the field and they’re from an institute that doesn’t have a track record. Important discoveries are sometimes made outside of the field, but it’s VERY rare. They talk about critical (magnetic) field and critical current. Both numbers are extremely low - far too low to be practical. Also critical current doesn’t mean anything .. the right metic is critical current density. It’s not a typo .. their graph shows current rather than current density. More troubling is they don’t find a Tc (critical temperature).. they only state it’s still superconducting at 400 K. The fact they can’t make it go away raises red flags.

 

You also need to demonstrate the Meisner effect - the exclusion of magnetic fields - it gets technical, so I won’t go into details, but just noticing a drop in resistance isn’t enough. They do claim a Meisner effect, but their graph doesn’t show it. It looks like garden variety diamagnetism.

 

There are a few other technical issues, but I think I've made my point.

 

They offer a theory of what’s going on, but it’s ad hoc-ish. That’s not really a problem - an experimental result doesn’t need a theory. But why do they try? red flag.

 

It should be easy to replicate. Their technique strikes me as sloppy. It’s possible they’ve discovered something. We’ll know quickly as this should be straightforward to try and replicate. If it’s real and the magnetic field is what they show, it isn’t practical.  It would offer a window into a different class of materials to try.

 

For now, color me skeptical. Extraordinary claims require extraordinary evidence. They don’t offer much in the way of evidence. I worry we have another Pons and Fleishmann paper.  A big sin was P-F publicly claimed they had made a fusion breakthrough with cold fusion and the media jumped onto it. At least these guys don’t seem to be banging the publicity drum.

 

LH₂ class superconductors may be very practical for many commercial applications in the next five to ten years. That’s a very important area that could become commercial soon. It would be even better if it can work at LN₂ temperatures, but so far it’s been disappointing.

 

a return to mount hydrogen?

Usually she buys the same number as her age, but this year she rented a small helium tank that ended up filling 56 balloons.  For the past ten years Jheri celebrates her birthday by giving balloons away to kids and adults who look like they could use one. 

 

We think of helium in terms of balloons but, particularly in its liquid form, it's uniquely suited to a number of important applications.  First discovered when astronomers analyzed the spectrum of a total solar eclipse in 1868 and then later found on Earth, it became part of a race between an English and Dutch scientist.  Not because getting there first would lead to uses, but rather to scale a peak. 

 

By the mid 19th century Michael Faraday managed to get down to about -130°C.  It seemed almost impossibly cold at the time, but wasn't enough to liquify oxygen, nitrogen and hydrogen.  He conjectured the trio were permanent gases. About that time Sir William Thompson (later known as Lord Kelvin) deduced from the emerging science of heat, that there was a coldest temperature.  An absolute zero  -273.15°C.  Well below Faraday's mark, it gave hope that much more was possible.  Sir James Dewar, at the Royal Institution in London, had been refining techniques to go to very low temperatures and took up the challenge.  It didn't take long for him to liquify oxygen and nitrogen, but hydrogen proved more difficult.   The Dutch scientist Kamerlingh Onnes was also entered the game.  It was an expensive and even dangerous quest.  One of the great stories of science follows.  Onnes was the good guy, Dewar not so much.  I won't go into it, but there's an excellent half hour audio history with a transcript.  Listen to it - seriously - then the title of this post might make sense) Without spoiling things too much, Dewar got there first, but helium had been discovered.  There was another gas to try and liquify and race got much more serious.  

 

Almost all of the helium on Earth is produced by the radioactive decay of Uranium and Thorium and is trapped in a few pockets around the planet.  That wasn't known at the time and it was basically unobtainium.  Dewar's personality caused him serious delays and his relentless push led to serious accidents that injured himself and lab assistants.  Onnes was banished from his town ..  (listen to the audio)

 

Now that he had liquid helium, Onnes, began to study the bizarre liquid.  Curiously the electrical resistance in mercury lowered to liquid helium temperatures fell to zero.  Onnes coined the term superconductivity.  A few other materials have this property.  It allows you to build extremely powerful magnets which are used in MRI machines, maglev trains, particle accelerators and fusion reactor research and more.  Almost all of the practical superconductors require liquid helium temperatures.  Being able to do this at warmer temperatures would be revolutionary.

 

In 1986 I was a the big annual meeting of the American Physical Society when Bednorz and Müller of IBM announced they had found superconductivity in a ceramic at liquid nitrogen temperatures.  Pandemonium broke out and it was on the front page of the New York Times the next day. It's very unusual for a Nobel Prize to be awarded soon after a discovery, but they were in Stockholm for the ceremony the next year.   The material wasn't terribly useful, but it opened up new area of research.  

 

Superconductivity at liquid nitrogen temperatures (77 K, -196°C)  is appealing as LN₂ is very inexpensive and relatively safe.  (Karrie and I have both made ice cream with it:-)  More progress has been made with superconductivity at liquid hydrogen temperatures  (20 K, -253°C).  LH₂ is flammable, tricky to handle, and somewhat more expensive, but is almost free when compared with helium.  And now practical superconductors for a few uses exist at LH₂ temperatures.

 

If Mount Hydrogen can be economically scaled for a wide range of superconductors, any number of applications where high magnetic fields are important become much more practical. Cheaper MRI, maglev, large motors and generators and beyond. It could bring electric aviation closer.  Moving to LN₂ superconductors levels much larger scale applications emerge.  Perhaps important sections of the electric grid, public transportation and nuclear fusion reactors (the magnets, not the fuel.. plus it's probably a loooong way off)

 

It quickly gets technical, but a class of superconducting materials that work well at liquid hydrogen temperatures have been around for awhile, but manufacturing them (you find yourself looking at single crystals over a kilometer long) is difficult and expensive.  But demand, driven applied science work in fusion reactors,  has soared recently and new manufacturing techniques have appeared.  Production has grown from meters of wire to hundreds of kilometers.  Costs are now within a factor of four or five of a tipping point for widespread use.  The problems ahead are difficult, but probably solvable. In ten or fifteen years we may see the beginning of widespread use of these materials and even serious progress at liquid nitrogen temperatures. 

 

The audio piece I linked to describes the lowest temperature made on Earth in about 2004.  The record has fallen - it seems likely the coldest spots in the Universe are in physics labs around the Universe - hopefully more than just the ones on this planet.

 

 

the aether and ethernet

I just came across a piece by Om celebrating Ethernet's 50th birthday. A must-read if you don't know the history of this foundational technology.  Bob Metcalfe called it Ethernet as a play on the luminiferous aether that was used to explain how light could populate through the universe

 

I won't go into how the concept of the luminiferous aether came about, other than it was something of a hack that was needed given the knowledge of the day even though it was clear much about it was wrong.  Science is full of explanations that become dated with new discoveries.  Sometimes the old models work so well that they still find use for calculations and pedagogy.  Newtonian mechanics is still used in intro physics and is good enough for SpaceX, but falls apart for the map on your smartphone. 

 

By the mid 19th century issues with the luminiferous aether were beginning to fill books.  Issues that made it appear more like magic than science.  It had to be a fluid to completely fill space, it had to be massless,  it had to be billions of times stiffer than steel and have zero viscosity, it had to be perfectly transparent, and so on.  Most people would probably say "we don't understand it", ignore thinking about it and just proceed with what they had the tools to work on. 

 

A few experimentalists tried to prove or disprove its existence. Michelson and Morley showed there was no difference in the speed of light caused by the motion of the solar system.  That and Einstein's Special Theory of Relativity pretty much put a nail in the coffin of the luminiferous aether.  The vacuum of space back then was a void.

 

All well and good except it isn't.

 

Michael Faraday, playing around with electricity and magnetism nearly two hundred years ago, had the remarkable insight to think of electric and magnetic fields to explain how electric charges and magnets worked at a distance.  He didn't have the math to create a robust theory, but his insight is one of those astonishing events that changed history and one of the great moments in science.  Next, around the time of the American Civil War, James Clerk Maxwell published his equations - the first field theory that united the electric and magnetic forces.  

 

The move to towards field theories in fundamental physics continued and Quantum Field Theory is the most accurate theory in all of science.  The Standard Model, how we think about the Universe (except for gravity) is a collection of about 17 fields (depending on how you count) - one is associated with each fundamental particle.  "Particle" as a tiny ball that might be charged or have some other property isn't what's happening.  Rather there's a field that fills space for each type of particle.  Electromagnetism uses two - the photon and electron fields.  Neutrons and protons use up and down quark fields, the Higgs field basically allows the universe as we know it to exist, and so on..

 

What we think of a particle is an excitation of the appropriate field.  A proton is something we learn about in grade school.  It isn't fundamental, but rather made of three quarks.¹ So a proton is a collection of up and down quark excitations.  To a physicist the vacuum isn't a void.. it contains all of these fields.  If unexcited there won't be a particle (although they crackle with virtual creation and extinction all the time .. so fast that you mostly don't note it), but if you whack the vacuum hard enough, there's enough energy to excite one or more of the fields and a particle appears.   The large hadron collider is able to concentrate enormous amounts of energy in tiny spaces to do just that - whack the vacuum.

 

So the universe is filled with these fields - it's fair to think of them as a fluid of sorts.  The view of an aether is a nice description of the vacuum, except the banishment of the luminiferous aether as made the use of the term taboo.  

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¹ A proton is much more complex than just three quarks, but for the purpose of a non-technical description.  And further on there are some deep mysteries about the vacuum indicating there's much to learn.  

 

 

on entropy

I've written a few pieces on the entropy physicists talk about as well as the sort-of-not-really related entropy of computer science.  Popular culture calls it disorder, but that's misleading.  

Derek Muller posted a good discussion that doesn't assume any background but gets much of the core right (that's a difficult balance!)   He spends a lot of time on these videos and it shows.   

An added note.  The twenty photons out for every photon in is an average, but is fairly accurate.  Your body emits photons too.. in fact they're a bit more energetic than what the Earth gives off, but not quite enough to say nineteen photons out for every one in..  

 

Entropy became formalized in statistical mechanics.  The introductory paragraph of the textbook States of Matter by David Goldstein is one of my favorites.

 

INTRODUCTION: THERMODYNAMICS AND STATISTICAL
MECHANICS OF THE PERFECT GAS

Ludwig Boltzmann, who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics.

Perhaps it will be wise to approach the subject cautiously. 

 

LIGHTS ALL ASKEW IN THE HEAVENS: Men of science more or less agog

One of the best New York Times headlines ever (although sexist) referred to the total solar eclipse on May 29, 1919 - the longest solar eclipse in over 500 years.  Two teams looked to see if the apparent position of stars near the eclipsed solar disc would be shifted as Einstein had predicted a few years earlier.  Sir Arthur Eddington's team, on the island of Principe, had clear weather and was able to confirm Einstein's prediction.  Much to his dismay, Einstein became a public superstar. 

 

General Relativity makes predictions that seem strange as we're used to the everyday force of gravity as described by Newton.  Black holes are probably the most famous prediction of something that pops out of the physics.  They're not massive bodies, but rather a region in space associated with enough mass in a compact region that nothing, including light,  that falls in can ever leave.  It's the stuff of good and bad science fiction and good science.

 

A black hole the mass of the Sun would have a six kilometer diameter.   A five solar mass black hole would be thirty kilometers across  You may object as the volume of a sphere goes as the cube of its radius.  It turns out black hole's diameter increases linearly with mass. A massive black hole can be much larger than you might expect.¹

 

Besides the date, the motivation for this post is a video from NASA showing how big some of the supermassive black holes at the centers of galaxies can get.  It turns out there's a limit ( it's a bit technical) and the largest black hole shown here is close.  But just wow!

 

 

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¹ Blame General Relativity - at least that's the usual approach.  You can see it by working out the Schwartzschild radius, but that requires a bit of differential geometry and, trust me, I'm not clever enough to give a quick non-technical explanation...  and not for a lack of trying. But there's a simple way to see it.  The event horizon is a limit in spacetime rather than an object.   You can use a Newtonian analogy and high school physics to think about it.  Consider the escape speed some radius out from a spherically symmetric mass - it's a spherical surface where the sum of its kinetic and potential energy is zero.  Fix the velocity and you get

r = 2GM/v²  

So no surprises. You get a similar scaling in General Relativity and you then realize you're not describing an object.

 

there's no cow on the ice

A close friend and I have been exchanging emails several times a week for nearly thirty years.  I hadn't heard from him in a week, so I asked if everything was alright.   He replied with a single line:

Der er ingen ko på isen.

He's very Danish and, although I'm barely read-only in that tongue, I knew immediately there was nothing to worry about.  The idiom means there's no cow on the ice.   Danes happen to be a practical people. If your cow was on the ice, you'd have something to worry about. 

 

Idioms mean more than the words that make them up are a wonderful window into a culture.  They can come from different periods, subcultures, age groups, common experiences and so on.   And they can hold on well past the time they were coined. 'Hun stikker ikke op for bollemælk' means 'she doesn't stick up for milk dumplings' and is still in use even though the farming references are lost on almost everyone.

 

Courtesy of a couple of Danish friends, I keep a list of Danish idioms and think I understand where they came from a bit more.¹  I'm sure they don't think about where they come from, just didn't think about 'it's raining cats and dogs', until I heard the Danish equivalent which translates to 'it's raining shoemaker's apprentices'.  We're like the fish who doesn't realize it's swimming in water. 

 

A few years ago I started wondering about quantum leap/quantum jump - the idiom that is entirely different from what quantum [anything] means in physics and chemistry. Of course it's so wired in popular culture that it's completely displaced the original meaning. 

 

Pulling out my tiny text two volume OED with its magnifying glass, I find an early use in 1649 referred to a share or allotment: “Poverty is her portion, and her quantum is but food and raiment.”  In about 1870 it was first used in physics to describe the the smallest quantity of electric fluid. At the turn of the century Planck and Einstein started using it in the sense that light consists of small and measurable pieces of energy.  Something very small. In the 20s as quantum mechanics developed the energy change in an atom or molecule was discrete - it was quantized.  In physics and chemistry a quantum jump or leap represents a tiny amount of energy.

 

Quantum mechanics is the study of this tiny world that has properties that are counter to what we're used to experiencing.  It includes particle/wave duality, superposition, entanglement and so on.  By the 50s a deeper understanding had emerged.  It was becoming clear the subatomic world could be more accurately described by fields rather than just particles and forces.   That's what I think about when I hear quantum.  To shift gears to  culture outside of physics I must 'at sluge en kamel' - swallow the camel.

 

According to the OED (I had to resort to the electronic version in the library as my copy is too old) the first use of 'quantum leap' to mean 'very large' came in a 1956 document describing the US-Soviet balance of power:

“The enormous multiplication of power, the ‘quantum leap’ to a new order of magnitude of destruction.”

 It had been used as its opposite and somehow it caught on.  This isn't uncommon with words and phrases..  smart, nice, awful, awesome and many others have been turned around.  There's probably a term for this.

 

I was going to write something the Schrödinger's Cat thought experiment as it has also become an idiom, but that can wait as it's fascinating for other reasons.

 

Så er den ged barberet

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¹ Note that I use English idioms to explain some of them

At stå op før fanden får sko på -- "get up before the devil puts on his shoes"

Spis lige brød til -- "have some bread with that" (telling someone to calm down)

Hold da helt ferie -- "take a whole holiday"  (no way!)

Lokummet brænder -- "the toilet is burning" (there are big problems)

Der er ugler i mosen -- "there are owls in the bog" (something suspicious is going on)

Man kan ikke både blæse og have mel i munden -- "you can't blow and have flour in your mouth at the same time" (you have to choose)

Skægget i postkassen "beard in the mailbox" (caught red handed)

At gå som katten om den varme grød "To walk like a cat around hot porridge" (to beat around the bush)

Så er den ged barberet "The goat is shaved"  (done with a big task .. big enough for a celebration)

 

 

physics isn't about looking for some grand truth

 

It turns out we make it up as we go along and that's a foundational piece of why it's been so successful.

 

Apparently there's been some discussion about the connection of physics and some quest for an ultimate truth on a social media site I gave up.  A few people reached out and asked my opinion.  Starting down the path as a teenager, I thought science was after some deep and ultimate truth.  Some grand and beautiful underpinning of the Universe.  I was wrong.  It took time, but finally as a grad student I had come to know better.

 

What physics does is build mathematical models that describe observations. (why math works so well is another discussion - but it just does)  The hypothesis models that are robust enough to survive different experimental techniques and are predictive become accepted theory.   It's very much an iterative process and the models get progressively better with more predictive models coming into use as appropriate.  The Standard Model is the best we have for particle physics.  It has some warts, but it's far and away the most accurate theory devised.  General Relativity is the benchmark for gravity.  Both are under constant study and that study has led to radically different views of the Universe and how it has and will evolve. 

 

But to the question from the birdsite.  What is real?  Is a proton real and what is it made of?  What about the spin of a particle?  Is dark matter or the dark force real?  For the electron and the quarks and gluons that make up protons and neutrons, the most accurate and way to think about them is as excitations in fields that spread throughout the universe.  You, the computer screen you're looking at, your dog and your lunch are vast assemblages of field excitations.  It's far from intuition and these models are rarely taught until the junior year and that's usually at a high level as the math is a bit intense.  It may seem weird, but it's much more predictive than anything else. And gravity being spacetime that changes it's curvature around mass and mass that responds to the curvature of spacetime isn't exactly Newton's falling apples.  

 

Current accepted theory addresses serious flaws in earlier work. The earlier work was good enough given the sophistication of measurements, but tools improve with time.   It turns out most  (but not all!) of applied physics and engineering doesn't have to worry about these deeper descriptions as these are fields that usually apply to things roughly our size moving a low speeds.  There's an interesting notion called complementarity that tells you it's okay to use less precise, but easier to understand, models when you don't need extreme accuracy.  We live in a world where models with a Newtonian underpinning are usually valid.  

 

In a way it's how we perceive things. We only sense a tiny amount of the electromagnetic spectrum, our hearing misses quite a bit, our sense of time misses large and small intervals and our size is roughly between that of the Sun and a hydrogen atom.  We don't perceive much of the reality around us - we have to use our imaginations to figure out how to ask the right questions and see more deeply.

 

So we build models.  We don't work out the equations of motion, but we walk, throw balls, drive cars and so one. We have a fair amount of information for these simple actions.   Physical models quickly get more difficult.   How do you predict an earthquake and its severity?  When and where tornadoes will do damage?  We build approximations, but that's all they are.  Better models will come using the scientific process. Even if it's a messy undertaking, it's proven to be the best approach we have.  And there's a foundational principle you have to deal with  - one that Feynman famously articulated:

The first principle is that you must not fool yourself and you are the easiest person to fool.

More complex and social sciences come into play. Predicting markets and economic models don't have the twelve digit accuracy seen in the Standard Model .. they generally don't make it past the decimal point.  They're also a stab at reality.    Social models are often poorly constructed with bad data and analysis - a major "AI" issue these days.  And how we understand each other as people.   We all build models.  Hopefully they're good enough.

 

For a long time I believed you could change minds about the causes and threat of global warming through education.Silly me -  it turns out to be a fool's errand. We all have different views of reality.  We can witness an event and see differing realities.  These lenses can be very different and culturally driven.  Quite a bit of work has been done trying to understand how this works.  Recently a podcast chat with  psychologist Jer Clifton was recommended by Corinne.  I give it two thumbs up and recommend it to anyone who has to deal with other people in almost any capacity.  It may even make you think twice about how you interact with others.

 

fusion!

The short version is it’s a remarkable technical and scientific achievement. Controlled human-made power from fission is easy - last week marked the 80th anniversary of the first reactor. Fusion - what powers the stars - is much more difficult on Earth. The necessary temperatures are far greater than those found in most stars. A star like the Sun gets away with a very slow and low power density reaction that would be impractical for generating power on Earth. It turns out the power density of the core of the Sun is about a quarter of your metabolic density and close to that of a lizard. To get around this different fuels are needed along with much higher temperatures. Still, if you can only figure it out, it sounds like the perfect goal - fuel from water and no radioactive waste.

 

The first serious attempt at magnetic confinement look place about 60 years ago .. it, along with every other effect until this last week failed. Those that achieved fusion required more input power than the power they produced. The figure of merit is Q: the ratio of the power of the fusion reaction divided by the power required to ignite and sustain it.

 

The laser confinement experiment announced focused a short pulse of light - about a billionth of a second - from 192 lasers onto a small gold container that vaporized to generate x-rays that collapsed a diamond coated fuel pellet of deuterium and tritium. The power of the laser light was 2.05 MJ (megajoules — a gallon of gasoline is 121 MJ, a hot dog in a bun is about 1.5 MJ) and the fusion explosion yielded 3.15 MJ. A Q of about 1.5. What isn’t mentioned is the efficiency of the lasers or the facility isn’t included - or the efficiency of extraction power from the miniblast. The lasers are less than one percent efficient so you can see there’s a long way to go.

 

This facility has been running for about a dozen years. It has been solving enormous engineering and applied science problems along the way as well as contributing to the pure science of understanding plasmas at this scale and temperature. Even the fuel pellets are very difficult to make and test. They have to be extraordinary symmetrical and most are rejected in testing - usable pellets are very expensive.  What has been achieved is not a prototype, but rather a triumph of instrumentation technology along with the control of some difficult to manage parameters. 

 

Moving this line of fusion forward seems like a long shot. A factor of five improvement may be possible with a redesign, but they need more than a thousand. Not impossible, but there’s a long path ahead.

 

There are other design types that confine microwave heated plasmas. Getting it right and working will take many years - probably decades and then there’s the issue of what does a facility cost compared to other types of power stations.

 

I hope work goes forward, but not at the cost of workable technologies that address global warming *now*. There’s so much that can be done, but very little political will.

 

2 december 1942 - a half watt

4:00 PM 2-Dec-1942

Compton: The Italian navigator has landed in the New World.

Conant: How were the natives?

Compton: Very friendly

 

Almost exactly 80 years ago the first human-made self sustaining nuclear reaction took place under the stands of a vacant football field at the University of Chicago.  It's a remarkable story and something of a race with Nazi Germany.  Fortunately so many competent physicists had fled Germany that the Nazi bomb project was a failure.  It turns out the wikipedia article on the subject is an accurate and readable summary of the dawn of the pre-manhattan project effort to get a handle on nuclear fission.  The piece  gets a bit technical here and there, but you can skip over those parts if you like.  I'll try and give a high level view of what a sustained reaction is without going into the quantum mechanics of the process.

__________

 

Nuclear fission is the process where the nucleus of an atom breaks apart into smaller pieces releasing energy (often quite a bit) in the process.  The nucleus of an atom is a tight cluster made up of positively charged protons and electrically neutral neutrons.

All of those positive changes packed together - you might ask why don't they fly apart? After all, electromagnetism causes like charges repel.  It turns out a very strong force, cleverly called the strong force, acts on neutrons and protons binding them together.  A difference between the two forces becomes important.  Electromagnetism is a long range force while the strong force only acts over short subatomic distances. To first order you can think of the strong force acting only between immediate neighbors while the electromagnetic force acts on all of the protons.

If the nucleus gets big enough the total repulsive electromagnetic force of all of the protons overcomes the closest-neighbor strong force and breaks the nucleus up in to smaller pieces releasing energy in the process (nuclear fission)

Most of the elements we're used to have nuclei that are too small to break up on their own.  A few are on the borderline. In the case of uranium the addition of a single neutron is enough to undergo fission and in the process it releases energy and, on average somewhat more than two extra neutrons along with a couple of nuclei that are smaller than uranium.¹

In nature uranium usually isn't concentrated and the neutrons from fission are absorbed by other materials.² But if you concentrate uranium you get to a point where each fission has a high probability of striking another uranium nucleus and so on - creating a sustained reaction. Unchecked, with enough purity, and you get an atomic bomb. At lower concentrations and with the ability control the amount of neutron absorption you get a controlled nuclear reactor that liberates a useful amount of energy.

 

A number of interesting characters were involved in these early experiments.  Enrico Fermi was the key player in the first sustained reaction.  Physics was getting specialized and he was probably the last physicist who was both a great theorist and experimentalist. 

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¹ I've left out a lot.  I haven't mentioned which isotope of uranium for example.  Also - you may have noticed that adding an extra neutron shouldn't increase the electromagnetic force as the neutron is neutral.  It turns out the uranium nuclei is so close to breaking up that the kinetic energy of the addition neutron adds just enough to break things up.  A nice example is adding a neutron to U-235.  One possible fission chain  n + U-235 → Rb-92 + Cs-140 + 3n + 200 MeV of energy. 

Also details of the strong force weren't known at the time..  A clear hypothesis didn't arrive until about 30 years later and deeper details of the mechanism took almost thirty more..

² A natural formations of uranium t pure enough to initiate sustained nuclear reactions was discovered in Gabon.  It operated for a few hundred thousand years producing less than a megawatt of power.