Lecture 14: Radar and the Manhattan Project

foreign and we can start today's material so on Monday's class we talked a bit about some of the kinds of work that was going on uh largely within Germany or at least other parts of of Europe but a lot of it right within Germany itself uh around topics like um the discovery of nuclear fission and early efforts to to put nuclear fission to to more practical use including both for um early ideas about about nuclear weapons as well as for power generation nuclear reactors and we saw that that was d

eeply deeply bound up or kind of caught up in this head-spinning swirl of of the onrush toward the second world war the rise of the Nazis soon overt fighting across Europe and of course much much beyond that before long so that so so Monday's class was largely about developments within Europe and today we'll talk about a range of developments mostly in the United States and so we have as usual three main parts for the class here at the bottom is is my note reminding us again as I just mentioned

about the film The Day After Trinity and I wanted to add that note because the the course material today the lecture for today is mostly going to be looking at kind of conceptual ideas physics and Engineering ideas as well as new institutions or institutional Arrangements in which many physicists began to find themselves and we won't be talking in today's class session about broader uh uh questions ethical moral contextual about the actual use of these new weapons and that's partly I want to mak

e sure that we do have time to talk about that uh at least at least to start those discussions with our optional um discussion section on Monday so today will be not exactly a technical history of the wartime project so we'll get into a bit more of what did physicists mostly in the United States kind of find themselves being being drawn into or being wrapped up with uh during the late 30s and throughout the much of the 1940s so we'll we'll start actually about talking a bit about radar which oft

en gets kind of overlooked these days the drama of the nuclear weapons uh tends to obscure many many other kind of Full Tilt defense projects or weapons projects that physicists and Engineers uh were really immersed in during the war and so we'll talk about radar um for a good chunk of of today and then we'll shift and talk about some aspects of the Manhattan Project and and the film will cover uh other kinds of aspects in addition so let's talk first about radar so radar was was around since be

fore the second world war the the first working units have been developed actually in many countries kind of independently simultaneous Discovery is a phrase that historians will often use there were groups working independent of each other uh in different countries in Europe and the UK uh some in the United States in Japan in the Soviet Union and other places it's been found that all came across similar ideas Came Upon similar ideas in the mid-1930s the idea for radar is to emit electromagnetic

waves Maxwell waves classical radiation let those waves reflect off of some Target some object and then collect the echo collect the rebound uh the the reflected waves that come back to your device and then you can do things like use the fact that these are Maxwell waves they're electromagnetic waves they should be traveling at a constant speed of light and so you can if you have a very good Electronics a good timing then you should be able to tell the different measure the difference between w

hen you sent out your own pulse when you generated your waves and when you received the echo so that gives you the time and if you know the constant speed at which those waves are traveling you can then determine the distance toward that object or your target that was the original idea there were working units as I say in many parts of the world already by the early and mid 1930s more sophisticated units that were developed actually during the second world war so by the early 1940s built in a ve

ry clever Edition which was to measure not just the time of arrival of that Echo but also measure the frequency shift they had very quick Doppler analyzers to measure the shift in the frequency of the risk of the return signal compared to the signal that the unit had sent out so then you could actually measure um the speed at least speed along the line of sight of the target object as well so now these devices could measure both distance and speed of the targets uh already before largely much of

that before the start of the war so both British and us-based researchers had developed these long wavelength radar systems and by long that meant the wavelength was measured in in meters sometimes tens or even hundreds of meters more like kind of radio waves um and those were what what had been operational uh before the start of the war and then once the war actually broke out once UK declared war against Germany and U.S began to mobilize even before it declared war officially that was really

one of the most one of the earliest uh experiences that many many physicists in these parts of the world had with a direct involvement with with military matters so it was a radar project that was for many many physicists and other Engineers physical science-based Engineers this was for many of their first experience working closely on on Direct military projects so one of the first challenges a real a genuine hard research challenge was to design new kinds of Radars that used much shorter wavel

ength waves so make the outgoing signal not meters or tens of meters but more like centimeters or at most say tens of centimeters they want to shrink down the wavelength of the of the outgoing signal and that was because the the nature of the challenge had shifted quite dramatically remember with shorter wavelengths you can you can resolve you can make sharper images of smaller scaled things if you only have a very long wavelength wave you'll never be able to make out short scale short distance

phenomena why would they need suddenly to worry about centimeter scale phenomena instead of meters after all airplanes you know are many meters long or large boats on on the water the problem was beginning with right around the outbreak of the war itself the famous or inFAMOUS German submarines the U-boats had become an enormous threat a very deadly threat to both U.S and UK shipping interests both for commercial shipping but also for for Naval uh you know ships as well now the boats were ordina

rily underwater they were basically impervious to this kind of radar but they but if a part of the boat would would reach the surface often as little as just a periscope just a little you know kind of siding device to allow the the members on the on the German Subs just to sort of see their their targets if that breached the water that would be a couple centimeter scale Target for these Radars so in order to to try to have any hope against this now very deadly force of the German U-boats the rad

ar challenge became to find centimeter scale radar systems not just meters or tens of meters okay so by 1940 uh just a few months into the real heart of the U-Boat campaign uh physicists and Engineers within the UK had had really made a huge Advance they had developed what became called the cavity magnetron and here's one shown uh in the image here you don't get a clear sense of scale this is just a couple centimeters across you could hold it in your hand this is a handheld scale device that cou

ld emit very high power high intensity electromagnetic electromagnetic waves of this shorter wavelength of roughly one or early ones are about three centimeter wavelength instead of meters so you could create very powerful outgoing beams of short wavelength electromagnetic waves by that time however uh the UK was under both bombardment from the air the German uh luftwaffe and the German air force was now doing a very very successful bombing runs uh penetrating London airspace routinely and and w

ith devastating effects so the the um the Blitz the bombing in London and other parts of Britain was was nearly constant uh and so it seemed impossible to kind of scale up this kind of bench top level Research into really full-scale research development and production so they knew how to make one or two of these devices they didn't know how to make factories worth humming around the clock to make hundreds and thousands of these devices because that kind of industrial capacity was under constant

threat from bombardment so in the Autumn of 1940 uh and remember that's more than one whole year before the US even officially entered the war so long before the surprise attack on uh Pearl Harbor which finally uh was sort of the reason why the United States officially entered the Second World War long before that a British delegation came over quite a dangerous journey and they came over by by boat despite the German submarines it was a delegation led by Sir Henry tuzzard it became known as the

tazard delegation they came to the United States and they set up a meeting in Washington DC to try to get U.S based colleagues to partner and ultimately kind of take over the lead on these next steps for radar development they wanted to cooperate but especially have a kind of safer home base uh or headquarters at which this within which this work could then be expanded they came up with lots of blueprints lots of paperwork and also literally one one cavity magnetron they were so rare and they w

ere in such high demand back in Britain they could spare only one to hand over to their U.S colleagues and the idea was between the paperwork the specs and the blueprints and the technical reports and literally the one working device The Hope was that these groups in the US could kind of reverse engineer this thing make many more of them and improve the design so at this meeting which was held in a fancy hotel in Washington DC uh there were it was a small group of U.S colleagues who attended who

met with tazard in his British team but it included a heavy dose of MIT folks so in particular it included van ever Bush who had only just recently left MIT by that point to work full-time in science policy and Science advising in Washington DC but until that time he'd been the dean of engineering at MIT came from an electrical engineering himself uh likewise attending this meeting was Carl Compton the physicist who at this point was president of MIT there was a kind of a dominant MIT presence

in this top secret meeting in the in this Washington DC Hotel uh and at one point Compton actually literally stepped out of this meeting to to place a phone call to one of his assistants who was back here on campus at MIT to see if MIT could spare space to be the the place to build up the headquarters for this new Allied effort and radar the biggest stumbling block was that there was a big faculty parking lot in the middle of campus and uh you might know that then as now faculty parking is like

super precious and rare and so the concern that the president of MIT had was whether the campus could basically take over that parking lot uh to build this temporary laboratory to do work on radar and the grudging response was yes that's where the mit's famous building 20 then wound up being built none of you probably have seen building 20 he was torn down in the mid 90s it was a temporary building literally built out of like plywood it was not meant to Outlast the duration of the war it was in

constant use in fact for 55 years much beloved on campus it was only torn down to make space for what's now the status and here's a picture you can see the 1940s Vehicles it was literally a temporary large scale but temporary facility right in the middle of Campus famous building 20. that became the original headquarters then for the mit-based Allied efforts in radar it became known as a radiation laboratory or just the rad lab okay so this project became one of the largest earliest and largest

uh projects sponsored by this new institution within the United States called the National Defense research committee or the ndrc this was also a kind of brainchild of mit's Van ever Bush so bush had by because of his time in DC he knew a U.S President Franklin Roosevelt he convinced Roosevelt and Roosevelt's immediate circle of advisors that the U.S although it was still not officially at War should begin kind of mobilizing or getting ready it looked like War could spill out and involve the U.S

at any time so Bush's idea was to make a kind of a meeting place an institution that could help connect researchers in science and engineering some at universities some at industrial Laboratories in the private sector connect them with them with U.S military officials the idea was the the military could come to to Bush's organization the ndrc say we really need a better this or a better that could you please get people to work on it so the ndrc would be the kind of meeting ground to help arrang

e these contacts so a little while later roughly one year later still well before the US even entered the war veneaver Bush convinced Roosevelt to actually that that wasn't enough that just being able to arrange for um for uh research contracts was was actually insufficient and that in fact there should be an even an expanded institutional base that became known as the osrd the office of scientific research and development and the idea and that bush would lead that so not only with the osrd kind

of let out contracts like the older model but who actually have a much more active and ongoing role in production it wouldn't just say here's your contract tell us when you're done it would have a a kind of steady oversight role to make sure these things are getting done on time and on budget this next part is really just for uh Professor gensler's benefit but I find it fascinating one of the things that stands out from veneiver Bush's strategy was to use contracts rather than grants let alone

gifts to make it look like these were kind of equal Partners entering into a business arrangement I find this actually very fascinating so the last thing veniver Bush wanted would be anything like a federal takeover or federal bailout of higher education that universities should be independent from the federal government and so instead if it looked like two equal partners coming to do business together like in the private sector we'll have contracts with overhead and all these kinds of affordanc

es of a business to business style contract as opposed to a grant or a gift or anything like that so in actual fact the universities were completely desperate for funds this was now nearly a decade into the Great Depression all of these universities were facing enormously difficult financial times but Bush wanted to maintain the kind of appearance of equal Partners arranging contracts as opposed to anything else that's I find that really interesting okay so what's going on then at MIT here's a p

hotograph from the top there was a rooftop facility temporarily built uh not just in building 20 but even in part of the infinite Corridor this was now on this on the roof of building four part of the you know the infinite card in there so in many many uh kind of sites on campus so the rad lab grew very very quickly it began once the green light was given and they could throw that plywood Palace together building 20 they began attracting staff to it at first they hired 30 physicists most of whom

were not previously at MIT they came from other universities across the country they had three security guards which is a top secret effort right in the middle of Campus two stockroom clerks and one kind of administrative secretary not so big it was led by a nuclear physicist not an engineer or even an electromagnetic expert the physicist Lee dubridge who at the time was at Rochester University in Upstate New York he was recruited to lead to temporarily leave his job and move to MIT to be the k

ind of scientific director for this but it quickly uh grew well beyond physics or even electrical engineering it included meteorology experts in geology what we now call Material Science even Linguistics how do you how do you identify uh signal from noise and so on it grew very rapidly in fact after less than two years of operation the staff numbered two thousands not just 30 and it doubled again before the end of the war so by the end of the war it had 500 academic physicists a very large fract

ion of all the pad PhD physicists in American universities all together a huge fraction were recruited to the rad lab uh it was by by this point spending a million dollars per month if you adjust for inflation to our contemporary currency that's about a 15 million dollar per month budget they're burning through cash very rapidly and this became the largest part of actually very large suite of uh defense projects that were being done at MIT throughout the war totaling again in today's dollars abo

ut one and a half billion dollars MIT became by a very very wide margin the single largest university contractor for wartime projects in the United States and in fact it was even a bigger contractor for these research and development projects than some of the largest industrial companies in the country ATT General Electric RCA Dupont Westinghouse the the research and development contracts from the osrd that came to MIT were three times more than even those huge industrial behemoths now those tho

se companies got huge contracts for production like building airplanes and engines and all the rest but the actual r d MIT became an enormous enormous node of this osrd so the rad lab staff now that you had 4 000 people by late stages in the war they were very busy they designed dozens of different radar systems not just one type of system from that one cavity magnetron but really dozens of variations so you wanted to have say ground to air to get early warning about aircraft uh air to Sea um gr

ound to Sea and so on now these were all in the kind of centimeter range a couple centimeters they were all were of that new type but now adapted to different kinds of um tactical needs they would conduct tests literally from MIT rooftops to see if they're if they're kind of beta Vision beta versions could detect actual aircraft from nearby airports but are now uh the Hanscom Air Force Base uh in the western suburbs in what's now called Logan Airport both Air Force and uh commercial aircraft and

they also trained nearly 10 000 active duty service members from across the United States they would come to campus for very brief very intense training in how to use these new systems and then be shipped out and use them operationally now at first many of the theoretical physicists who were recruited to the rad lab they were even by their own lights pretty arrogant that's not just me saying so many of them came to that conclusion themselves after all they came in saying oh radar that's that's

just classical physics that's merely Maxwell's equations how hard could that be they had many of them had been immersed in these kind of very fancy esoteric ideas about Quantum Theory or Nuclear Physics of the sort we've talked about in recent sessions here they were quickly schooled they learned very quickly that calculating the actual electromagnetic field configurations for real devices not just the kind of they assigned to their students on problem sets was as we physicists say non-trivial m

eans it was really really hard this was not at all an easy task so as you all know and are still learning as we still use in our own more mature research oftentimes it's very very important and very helpful to exploit symmetries to simplify calculations imagine a spherical symmetry or if you really must a cylindrical symmetry so two dimensions of space can be treated the same way and one-third one's different that was going to get you absolutely nowhere when it came to these real world devices l

ike radar here's an example of just some of the so-called components these waveguide components they were already in standard use by this time by the mid-1940s few of these could be uh could be treated as a cylinder none of them could be treated as a sphere and these are just components the actual parts of these radar devices like here's one schematic would be you know putting all these together in these complicated forms there is no symmetry argument that's going to help you there so what the p

hysicists began to learn and again many of them were called this years afterwards was really a whole new way to think about their own calculations and here they credited many of them credited the engineers with whom they were suddenly and often for the first time working very closely credit of the engineers for helping them to learn a whole new way to think about their own calculations to think about effective circuits don't start from kind of individual uh basic parts and even more more basical

ly focus on kind of input output relationships so you might have a particular circuit for part of that radar system that would have a bunch of resistors some in parallel some in series you have capacities you have all these kind of messy electronic components and although it is the case that one can simplify these mathematically the engine and find an effective circuit uh using things like Ohm's law the engineers would say don't even waste your chalkboard time on that stick a lead on over here s

tick a lead on over here what's the current flowing in what's the current flowing out infer an effective overall resistance have an effective circuit based on the input and the output and stop it you don't have time to do this kind of thing plus when you're faced with those crazy crazy shape wave guides even these simplifying mathematics would be no help so several physicists including this gentleman here uh Julian schwinger who spent the war working at the rad lab they later recall that this ne

w approach this kind of engineering input output approach to problem solving really shaped how they thought about research questions even after the war was over and that's a little bit of a a foreshadowing we'll look at some of the lessons that schwinger took from his radar experience when he returned to challenges in quantum theory we'll look at that in a few class sessions okay so by 1943 so in roughly two and a half three years into kind of full-scale operations for the rad lab these kinds of

units were actually developed and deployed all over the the so-called theaters of battle they were not only used in ground-based scanning stations they were also put on board aircraft onboard Naval vessels they began finally to turn the tide against the devastating German submarines the U-Boat campaigns as well as the luftwaffe uh bombing rates over uh over Britain uh it turns out these systems were not only in some sense defensive it wasn't really trying to get early warning of an incoming att

ack though they turned out to be very effective at that these also became very important for for for offensive weapons for for uh for weapons that would that would go and attack the enemy and so one of the most substantial was actually at a related osrd project uh the so-called Applied Physics laboratory associated with Johns Hopkins University uh near Baltimore which was set up in a similar fashion to mit's rad lab and one of the most important things there was developing the proximity fuse so

this would actually embed is really quite amazing embed miniature radar units in the WarHeads in the tips of these artillery shells so now the the each uh each kind of artillery shell that would be fired from these very large cannons would carry its own kind of ranging device so it could tell in real time how close it was getting to it to a given Target so you could then wire these things up to explode to actually uh detonate not any old time but only when they were within some preset distance f

rom the Target and that had an enormous impact on the offense previously these anti-aircraft efforts like shooting these big guns from a naval vessel against incoming aircraft they typically had to fire hundreds of rounds these very expensive rounds to hit a single fast-moving airplane they were had a very bad uh return on investment so to speak they were not very effective once these same shells now were were equipped with these proximity fuses they need it on average two not hundreds to to suc

cessfully strike an incoming aircraft so after the war it became common especially for veterans of the rad lab of the Applied Physics lab to say that nuclear weapons are the sort that we'll talk about for the rest of today and on the film that these weapons might have ended the war but it was radar they said that it won the war so let me pause there and take some questions I see the chat is filling up so fish is right so building 20 is indeed uh on the campus location where where the uh state is

centered now is uh and whether that's an ugly building or a beautiful building we can all decide but indeed that was um it's sort of a it's you know a building of real legend it was really supposed to last like five years and it was in constant use for 55 years much beloved there's a time capsule within status so on the same physical locations when we could all get back on campus I encourage you to go take a look at it for those of you who might us like me still be remote there's a time capsule

in stata of rad lab materials and memorabilia they wanted to have it on the same physical site and it will be sealed until sometime later in the 21st century um and so anyway so that's uh a a large part of mit's role uh and kind of Legacy during the second world war so Jesus asks how is the original radar built if it was so non-trivial yeah good so the the non-trivial part was mostly shrinking it down to short wavelengths and having a means of generating high power high intensity waves of that

short wavelength and also getting very careful Electronics to detect the echo getting even fancier Electronics to detect any kind of frequency shift the Doppler shift the original idea of getting a big basically big radio tower send out large multimeter wavelength waves people have been sort of generating radio frequency waves um since around 1900s the 1890s think about Marconi and other other people so just generating long wavelength radio waves and detecting them that's kind of like what radio

does you have to have a little more sensitivity to get the right radar Echo and get the timing right but that part was indeed closer a kind to trial and error or let's say building on established engineering principles but getting it really um to work with short wavelengths much finer resolution both for time and frequency that was much more tricky uh Gary tells us that uh his his own father was uh experienced in the war he was I say mere Corporal uh yeah so he found himself because you're gett

ing ahead of ourselves here so Gary we'll see photographs of the sorts of experiences your father indeed might have had and also more in the film as well by the way much like Gary's reminding us of um there was a a large number of people who were stationed at places like Los Alamos and other other Manhattan Project sites who were part of what was called the SED the special engineering Detachment these were people who were who were drafted they were not recruited especially for the side of your p

roject many of them had a high school level education had some aptitude for physics and math but we're not kind of full-time science students or engineering students but if they had any aptitude with like using their hands you know like radio kits and so on anything were sent to this special engineering Detachment to these high-powered labs and that's where many of them got their first kind of real exposure to to higher level kind of uh laboratory work and several of them then decided to change

their their life path and many of them then decided to go and study uh these topics more formally in graduate school to get a PhD to become professors when before that they had never dreamed of such things so that SED Detachment played a large kind of unsung role at a lot of these wartime Laboratories uh so Alex is right so not only by the way whether were the proximity fuse is so secret he's absolutely right they could only be fired over the open water in the beginning that they relax that by t

he end of War but the fear was if if one of these uh failed to detonate and it landed on the ground could the enemy troops capture it and then reverse engineering so exactly is Alex very rightly reminds us these proximity fuses especially in the early rounds literally rounds were were limited only to Naval um to Naval operations so if it didn't explode the enemy couldn't capture it I think it's just extraordinary Fisher asked what was the status of radar in Germany uh they had some prototypes ye

ah that's right so again uh that's one of the places where there had been early working units even before the start of the second world war in the 30s um there were again efforts to improve them but much like we saw it in Monday's class everyone especially the Germans thought that Germany would win the war immediately the Blitzkrieg was just blindingly successful uh in the early early months and years so I figured why bother sinking a lot of research you know the one exception to that by the way

was paintamunda the effort to develop uh rockets that really was invested in very heavily and of course it did have a real impact even during the war itself but for a lot of these other kind of high technology wartime projects the Germans tended not to invest so heavily because they thought they'd win and then when the tide turned they said oh we're not winning but now we have to divert resources to to proven Technologies um and then yes and we also have Alex algebraz we can also thank microwav

es for burnt popcorn and so much more like most of our lunch is probably during the pandemic any other questions about about radar or bird Legends okay let's switch gears now let me go back to sharing screen these are great questions let's go back now and and we will shift now to the other sort of most famous project of the osrd the Manhattan Project so I do want to emphasize over the course of the war this office of scientific research and development with venever bush at its head oversaw liter

ally thousands of Defense projects for the military it wasn't like there were only two or three it wasn't just radar proximity fuse and Manhattan Project there were literally thousands of funded projects the the biggest though the most um the largest in terms of personnel and budget and arguably in terms of impact on the course of the war were indeed radar uh and then and then the one we'll turn to now with the project for nuclear weapons so this other main project was officially called The Manh

attan engineer District or the med uh of course it was rapidly shortened to just the nickname The Manhattan Project why was it called the Manhattan Project it actually started the first kind of headquarters for the first very small planning office was in Manhattan it was it was a joint project between this osrd and uh the war department in particular the U.S Army Corps of Engineers long-standing organization within the army and the core then as now has these kind of Regional Offices there's a co

re uh you know to oversee things around say the Mississippi River there's a core office for the Northeast and so on there's a regional structure for the court for the Army Corps of Engineers then as now and it was the Manhattan office that was made in charge of this early on quite modest scale effort to begin investigating uh fission for weapons purposes so they were literally headquartered in New York City and that was an entirely random some of the earliest scientists Consultants on this proje

ct were at Columbia University in New York City including Enrico Fermi who had just as we saw last time had just left Italy upon receiving his Nobel Prize in December of 1938 by January 1939 he was then basically set up at Columbia and many of his colleagues there so that's why it was originally called named for Manhattan very very quickly this project grew well beyond that small little planning office in fact it included more than 30 sites across both the US and Canada over the course of the wa

r it wound up applying employing more than 125 000 people it was a massive massive project far far larger than radar in terms of personnel and of course the overwhelming majority of those people of those 125 000 people who were paid by the Manhattan Project during the war most of them had almost no idea of what the project was actually about there was a very very tight control of information flow and you'll hear more about that even in the film day after Trinity so although these people were off

icially working on the Manhattan Project uh very few of them had any idea about what the project was even aiming to do let alone the relevant details so among these 30 sites there are four that really were most uh critical to the project and that get talked about most often and again you'll hear more about these um in the film now those four are highlighted here on the map uh Chicago Illinois Oak Ridge Tennessee Hanford Washington uh and Los Alamos New Mexico the film is largely about developmen

ts at Los Alamos but it does give us insights into these other four Mains the other three main sites as well we're going to take a tour today at some of the a brief tour of some of the kinds of work that was being done at each of those four uh main sites during the war we're going to start with Chicago that that installation that part of this very large sprawling project was called the metallurgical laboratory they weren't doing Metallurgy these names are always meant to kind of throw off suspic

ion it wasn't actually you know basically Material Science or chemistry but they gave it a name to sound kind of innocuous by this point Fermi had moved from Colombia to the University of Chicago they moved pretty rapidly upon emigrating to the U.S and he was one of the main leaders then of the Chicago met lab as it was known uh for sure the the metallurgical laboratory took the lead on trying to further understand these still quite new fission reactions and that was what Fermi had been working

on even before he even sort of knew it went back from his Rome days and certainly one of his main interests once he arrived at Columbia so the Matlab was focusing on sort of the physics and chemistry of fission one of the first things I did was build this monster here it was known by the code name Chicago pile one because he's literally a pile this was the first working nuclear reactor in the world it was built as some of you may know under the the stadium seating of uh the of the big stag Field

Stadium on campuses they were underground uh squash cords like racquetball courts and they over during the war in secret took over some of those underground facilities to start developing and building the first working nuclear reactor no one above ground was told that they have a bunch of uranium underneath so what was their goal each time a uranium nucleus underwent fission additional neutrons were released this was not so obvious at first with the experiments from Han and Strassman in Berlin

but this is exactly the sort of thing that a lot of U.S based labs and elsewhere began to uh to identify and then to confirm as soon as Niels Bohr arrived in New York and let his U.S based colleagues hear about fission once people began replicating the basic fission reaction in their labs they could then hone in on the other reaction products and so it was clear by fermi's time at the Met lab that more neutrons came out each time a single nucleus underwent fission and that meant there was a pros

pect for getting a chain reaction the first if you inject one Neutron into the system if this nucleus splits and gives out more than one new Neutron each of those could split neighboring nuclei then they would split more and more and more I mean it could become an exponential runaway process known as a chain reaction so what was this pile how did this reactor work it consisted of 57 layers you can count them up I'm not sure all 57 were there yet but it was literally stack upon stock on stack a v

ery closely packed ingredients so most of this most of the weight actually came from graphite bricks carbon you know dense carbon bricks those were not going to undergo fission themselves they're not a nuclear Source they were actually to slow down the neutrons so each time a neutron flies out of one of these recently split nuclei you wanted to interact with some moderating material and it turns out carbon was quite effective the carbon inside these graphite not to absorb the neutron but to slow

it down remember we saw Lisa meitner and Robert Frisch had had recognized that react fission reaction rates in general should rise if the neutron is slowed down because then its Quantum properties will be stretched out more to be comparable in size to the entire Target nucleus so the idea was to use some moderating materials a material that would slow down those neutrons by a few kind of um collisions not to absorb them so that by the time the neutron then found a neighboring uranium nucleus it

would be more likely to induce fission so most of this of these piles are the graphite moderators in between were little chunks of uranium metal some of which would hopefully undergo these these reactions and they're lasted quite critical uh part of this were huge control rods 14 foot long rods of purified cadmium metal y cadmium the idea here was not to slow down the neutrons but to absorb them so cadmium will very readily absorb neutrons so if you want to Halt a chain reaction from say blowin

g up the um entire University of Chicago let alone the lovely Sports Arena here if you want to slow down or halt a fission reaction you start taking neutrons out of the equation you absorb these neutrons before they can find a Target nucleus so the graphite bricks moderate the energy of the neutrons the cadmium control rods take them out of the of the system altogether and then these were movable they could literally be manually the earliest days manually pushed in or pulled out to control the t

he average number of neutrons sort of in play and so with this Arrangement the very first self-sustaining chain reaction that got this reaction to undergo uh not a runaway Chain Reaction a controlled chain of reaction thanks to those cadmium rods that went critical literally underneath the stands here on December 2nd 1942 under fermi's direction that was not quite a full year after the surprise attack on Pearl Harbor so you can see the pace begins to pick up very rapidly once the U.S did actuall

y enter the war the bombing of Pearl Harbor was December 7th 1941 the so-called day that will live in infamy and almost exactly a year later families group in Chicago had produced the first self-sustaining Chain Reaction okay now independent of that kind of in parallel there were developments going on at other uh sites which would also eventually become absorbed into Manhattan Project so another very important one was at Berkeley California where a very young nuclear chemist he was an assistant

professor just a few years past his own PhD and then Glenn seaborg worked with a small team he was a nuclear chemist and they actually finally finally succeeded in doing what Fermi had first thought he'd done back in the mid 1930s so Fermi remember we saw one the Nobel Prize for essentially a mistake everyone including the Nobel committee thought the Fermi had produced nuclei heavier than uranium by Neutron capture although it turns out families group was inadvertently inducing fission well seab

org and his team actually finally so to speak was able to successfully produce transunionic nuclei with the same mechanism that everyone thought had been already been going on they could control it better and measure their products better and so uh early on he and his team produced um neptunium by Neutron capture followed by Beta Decay and then a few months later by very early 1941 seaborg's team had produced the next uh largest element on this new extended periodic table plutonium this was actu

ally a single Neutron capture followed by double beta Decay so the heavy uranium nucleus one Neutron undergoes beta Decay so the proton count has increased by one then a second Neutron within that same nucleus will undergo beta Decay and now you increase the proton number by another step so now you've made element 94. the next month after first making a very small Trace Amounts of this new element seaborg and his colleague Emilio sugray to gray had himself left Italy uh because of Mussolini he'd

been a member of fermi's group in Rome he relocated to Berkeley so the nuclear chemist seaborg and the nuclear physicist in the Legos agree then began studying the properties of this brand new chemical element plutonium in particular we found that plutonium really is subject to nuclear fission much like certain isotopes of uranium within months after that seaborg then went to what was by now this kind of flourishing Manhattan Project site uh the Met lab in Chicago to work more directly with Fer

mi and to continue measuring kind of the properties of plutonium and its uh fission rings so that's that's what largely what's going on in Chicago during this time need more overseeing this entire project the entire Manhattan Project was a member of the Army Corps of Engineers the U.S uh Brigadier General Leslie Groves until that time groves's largest project had been overseeing construction of the Pentagon building literally the building itself was quite new by the late 30s early 1940s and Grov

es who was an engineer a rising member of the U.S Army Corps of Engineers had been like the head contractor he'd overseeing the construction of this enormous enormous strategically important uh headquarters for uh for the war depart so he was seen as someone who could get things done on budget he was then tapped to take over his newest new project of the Manhattan Project and in fact as you again you'll hear more about this in the film Groves was very reluctant to do it uh he actually was very e

ager to to see combat once the U.S actually actively entered the second world war he wanted to be you know leading troops in battle and he thought this very abstract sounding weapons project something about nuclear fission if it were to work at all would have some long-term benefit or impact much down the road Groves was eager to to see you know the the scenes of battle directly nonetheless he was more or less ordered to take us over and he was in the Army he followed his orders so he was he was

then put in charge of this entire project one of the first Maneuvers he did which really just stunned stunned people around him was he asked uh the very young theoretical physicist at Berkeley Robert Oppenheimer to join Groves as the scientific director for this new Los Alamos laboratory and you'll hear a lot a lot about Oppenheimer in the film it actually functions as almost a kind of biography of young Robert Oppenheimer combined with the story of these projects during the war so I won't say

too much now about Oppenheimer but just to say this was a stunning move uh on groves's part standing at the time so Los Alamos began operations and in the spring of 1943. if you may remember the project was established in June of 42 so just getting things like the Chicago lab up and running uh was really the the earliest priority and by the spring then by Spring of 1943 not quite a year into this new project uh this additional site what would eventually become a kind of central coordinating site

at Los Alamos New Mexico was set up as you'll see in the film taking over would have been a very tiny kind of boys school Beyond like K-12 uh Academy in rural New Mexico with like mud caked small little facility Oppenheimer used to enjoy vacationing in that region going on Long camping and horseback riding trips it was actually Oppenheimer who who recommended the site to Grove saying maybe we can requisition this out of the way place that could be well hidden and kept secret so again just a lit

tle bit about oppenheim and here's the poster for the film that you can learn much much more there uh is a there are many many very very good books about Oppenheimer including uh this one that actually received the Pulitzer Prize it's a it's a a spellbinding book as well as one based on enormously impressive research lots more to say about whatever I'll just be brief he was a near contemporary of people like Vander Heisenberg and Wolfgang Powell he was roughly two years younger than Heisenberg s

o pretty similar generation Oppenheimer was a bit of a prodigy he actually went to Harvard very young for his undergraduate studies he had skipped grades you know as a younger student and then he studied for his PhD in a quick postdoctoral jaunt in Europe both in Cambridge England and especially in gerting it he actually did his PhD under the direction of Max Bourne so he was there just as the kind of brand new quantum theory quantum mechanics was emerging he got to meet many many of those folks

when he was a grad student there he came back he was hired to teach both at Berkeley and at Caltech he was hired to be a full-time professor at two universities hundreds of miles apart and both schools were so desperate to get him they agreed to let him spend one semester at Berkeley and the next semester at Caltech is just extraordinary and part of the part of his role was a hoped uh he could do was build up a kind of us-based strength in theoretical physics which was seen I think quite approp

riately quite accurately as really lagging behind the European schools by that point before the war he had basically no experience with either experiments or with any sort of large-scale organization which is part of what made it so shocking when General Groves asked him to play this very large administrative role for the wartime projects okay so as soon as this new facility at Los Alamos began uh to get built up oppenheimer's own former student is postdoc Robert server gave a series of initiati

on lectures for the new recruits who had been asked to come to this place but not told why it was so top secret people were basically not told why they should drop everything and move to nowheresville rural New Mexico so cerber gave what was actually called a quote indoctrination course as if they were joining like a cult it was another physicist Edward Condon who took notes and typed them up and this 24-page document became literally the first technical report of the laboratory it was classifie

d immediate it was it was a top secret report this was Los Alamos report one of which there would be thousands in fact probably tens of thousands then to follow informally it became known as the Los Alamos primer decades later it was Declassified and published and in fact you can uh actually just download the PDF of the original type typescript on the web it's not hard to find so this is from the actual Doc in the actual primer on page one the first thing he tells these people when they uh meet

together in Los Alamos is the following the object of the project is to produce a practical military weapon in the form of a bomb in which the energy is released by a fast Neutron chain reaction in one or more of the materials known to show nuclear fission lest there be any doubt basically we are here to build bombs that's literally the first thing he says in these notes the next thing he does is very very quickly go over the kind of back of the envelope order of magnitude estimates for the ener

gy released every time a single nucleus undergoes fission exactly the sort that we saw last time that Lisa Meiner and Robert Frisch have been doing not so long before and that I work out in more detail in those lecture notes for from Monday's class there's one really interesting shift though I find this pretty fascinating server gets the same answer but he chooses to write the the energy uh associated with these nuclear reactions not in units associated with chemical or nuclear reactions like el

ectron volts or maybe millions of electron volts he writes down the energy in ergs remember that's the unit of energy in the kind of human scaled units centimeters grams and seconds he's now not thinking about individual you know reactions among one or two nuclei he's already thinking about a human scale because the next maneuver he does in these notes is say this is not very much on a human scale right a fly buzzing around your head expands more energy than this however that's for one nucleus u

h that's undergone fission there are 10 to the 25 nuclei in a single kilogram of this stuff so suddenly if you could get this runaway chain reaction that refers to here if you can get lots and lots of these nuclei each to undergo fission they'll each release that amount of energy now you're talking about some enormous uh kind of reaction not just a one isolated nucleus that happened to fission again right on page one of these notes the next thing it does is compare that kind of energy release wi

th the energy associated with conventional chemical explosives like TNT or Dynamite basically those were well known to release something like 10 to the 16 ergs per ton not per kilogram of uh of chemical explosive but per ton so then again right on page one he then takes this ratio to say this is why we're people have gathered here in Los Alamos one kilogram of this stuff sorry of a fissionable isotope would give off the energy equivalent of 20 000 tons of conventional explosives let me just paus

e here to say he's used a code here they were so worried about about secrecy even though they were you know in the middle of nowhere that in the notes he never refers to um uranium-235 here for he refers to substance 25 and the code was take the sorry take the last digit of the atomic number so it's 92 the last digit of the atomic mass 5 that's your code so u235 is 25 u-238 is 28 plutonium is 49 because it's element 94 with atomic mass and so on so that's what server writes on page one as the re

ason to have brought all these people in secret to the Mesa now which material to use by this point there were several known fissionable materials u-238 is actually mostly stable as it had been clarified by this point u-235 is the isotope of uranium that is most readily fissionable however it only exists in Trace Amounts in nature so if you dig up uranium ore out of the ground whether in the African mines or in uh mines out in western United States or elsewhere most of the Iranian that you'll di

g up is the overwhelming majority will be this very stable isotope u-238 less than one percent of naturally occurring uranium is of this fissionable kind and in the meantime this newest element that seaborg and his colleagues had made actually synthesized in the laboratory plutonium that is can be even more fissionable under certain conditions and yet it existed only in micrograms not kilograms so these are the these are the challenges that that server begins telling the recruits this is again h

is hand actually it's Edward condon's hands-drawn chart uh the chart that cerber showed this is the level of detail and accuracy in the Los Alamos primer it's literally taken from the primer this is showing the reaction rates for fission uh in in convenient units for these different types of materials and again you can see here is uranium-235 here's the common kind u-238 here's plutonium the four and the nine so for for slow neutrons are the sort that might earn fresh we're thinking about ones t

hat have been slowed by some moderator like in the Chicago pile the highest reaction rate that had been measured at least was of uranium-235 the highest likelihood to undergo fusion was way up here for two for 235. the problem was when the next uh nucleus fissions the neutrons that come out of that are actually very fast they're at these kind of nuclear energies or at least fractions of the nuclear energy they're more up in this scale in a million times say electron volts or 10 million not a tin

y fraction for very fast neutrons it turns out plutonium is even more susceptible to fission than u235 the challenge is how do you isolate this Trace stuff from this because you want to get a lot of this stuff in one place kilograms Worth or how do you scale up this stuff by a factor of a billion from micrograms to kilograms neither of these seemed pretty seemed at all straightforward that's where some of these other facilities then come in so we'll now look at the Oak Ridge facility very briefl

y Oak Ridge in Tennessee here's an example of this kind of enormous industrial scale operation under the auspices of the Manhattan Project scaled up by the U.S Army Corps of Engineers and now with more and more industrial Partners as well this was literally a Classified City it wouldn't even show up on maps until many years after the end of the war and yet behind the fence in under classified conditions they built the single largest Factory on the at the time on the planet the largest Factory at

least we Under One Roof ever uh with about a mile uh if you walk from here to here the single building was was um like more than a mile in distance this was uh to try to separate uh the the kind of uranium isotope that was now really needed the fissionable one from the common one now these are both atoms of uranium so in terms of any chemical analysis they'll be indistinguishable the chemical properties are the same they're isotopes of a single chemical element so people realize right away in s

erver lecture and this of course as well to separate them you have to turn to physical methods not chemical ones you have to exploit the very tiny percent level Mass difference between uh this slightly lighter version of that more common isotope so here's an example go through very quickly there's a marvelous treatment of this uh by my my friend and colleague Alex wellerstein was a real real expert on on the wartime nuclear projects you can check out his very brief uh essay sorry essay there so

here's what they wound up doing uh in this plant here's what's going on in this huge enormous Factory plant something called gaseous diffusion so you mix the uranium with um with fluorine to make a gas called uranium hexafluoride u46 this is incredibly poisonous incredibly noxious it will burn through many kinds of gaskets and rubbers and metals it was really nasty stuff to work with not just to humans but even to the kinds of uh engineering parts that one would build a factory out of this was h

ard to work with nonetheless it had the property that they could heat it up in equilibrium the molecules of uranium hexafluoride that happens to include the rare lighter isotope of uranium those molecules would be would enter equilibrium with the more common molecules that have to include uh the standard isotope u238 if they're in equilibrium their energies should be about balance the kinetic energy should be about equal but that means that the smaller Mass here of the lighter isotope must be mu

ltiplying a slightly larger velocity that the the molecules with the stuff you want will on average have slightly larger speeds in equilibrium than the more common stuff so put the whole gas Under Pressure force it through these these these uh these Chambers with a permeable membrane and then because you have a larger average velocity for the ones you want for the lighter ones they will diffuse through this chamber uh slightly more quickly so after a short amount of time the ones you want these

small black dots the ones with the fissionable isotope will diffuse throughout the chamber a little bit faster than the ones that are stable that you don't want to focus on not by a lot the enrichment of doing this one cycle is less than 0.4 percent enrichment and so the idea was let's just do that a thousand times literally scale it up like never before with the help of these uh experienced industrial partners and so what this building has is literally thousands of these kind of um cubic meter

scale uh gaseous diffusion units strong one for the other you take the slightly enriched output from here and put in another chamber and I'll put an output output so you do this a thousand times meanwhile the other main installation uh for the Manhattan Project was in Washington state at the Hanford site and by the way one of our a member of our group here Tiffany Nichols is a real expert on Hanford so we should get her to talk about Hanford sometime as well so Hanford during the war was also a

secret facility enormous sprawling industrial site now here the main partner was Dupont although many other industrial Partners there as well and the job at Hanford was not first and foremost to separate the Isotopes of uranium but actually to make more plutonium so their job was to take kind of fermi's Chicago pile take the insights that people like nuclear chemists like Glenn seaborg had learned in the interim and do that at an industrial scale it build enormous reactors that could induce Neut

ron capture within otherwise relatively stable uranium and then induce the production of plutonium and so again you can see that this the scale here uh just to put it in complex there are multiple reactor complexes there was the b b reactor the F complex and many others all told during the war panford required one billion cubic meters of concrete I just think about the the scale of that going on okay let me pause there so that's a kind of lightning tour of some of the technical things happening

at the VAR at the main uh uh Manhattan Project sites we'll talk more about what comes next uh let's see so uh Fisher asks by the time the trans transits were actually discovered do people really know about antimatter or neutrinos oh good yes good so people had ideas nothing like a real evidence yet uh I'm in a bracket I won't spend too much time on that but family had actually done a lot of work creating a theory that included neutrinos and of the week week Decay more generally but it was still

entirely hypothetical and family himself believed these particles would never be detected it was very skeptical at the time we'll come to actually it turns out that the first evidence that neutrinos exist came from these kind of huge nuclear uh projects not during the war but soon afterwards so people had to build enormous industrial scale reactors or weapons to create lots and lots of these things flying out uh before there was any hope to try to detect it and so we could talk more about that b

ut the short answer is people had ideas about the what we would Now call neutrinos and and the kind of beta reactions but but it was still quite fledgling Johann asks was there widespread protests against these drivers very good because part of what we should talk about uh especially on Monday and thereafter uh the short answer is no if you say widespread protest an unequivocal no no way no how partly because these projects were all deeply deeply classified many of these sites literally weren't

even on a map you could not drive to the town of Oak Ridge you wouldn't know it was there during the war and same with Hanford and many other so-called Atomic cities they were nicknamed afterwards so that's part one part two uh what was going to happen with these things wasn't so clear we'll talk more about that uh soon uh part three uh let me leave part three to our discussion it's a great Johan that's a fantastic question and that's exactly what I want to be able to talk about at least at leas

t brooch uh both with the film and with our discussion on on Monday uh good Muriel shared a screenshot uh excellent [Music] um yes Alex is right so part of why Oak Ridge was cited where it was is because it had already access to enormous enormous sources of electricity uh it was really just I mean you saw the the photos there was a huge industrial output um Sarah says we will talk about radioactive contamination oh we will yes very good uh so they did did they Sarah asked did they did people wor

ry about radiation from these materials again sorry it's a great great question did they they were aware of it it becomes controversial I have to say who knew what when so even with with hindsight and declassification and lots of more documentary evidence it's unambiguously the case that many many of these scientists and Engineers knew there was something to think about radiation at the time it's also unambiguously the case that they were quite Cavalier not just with themselves but also and I th

ink even more uh shamefully with all these workers at these huge industrial sites who were handling extremely dangerous materials with minimal uh safety precautions or even basic information it was actually very there's a dissertation on that very topic by grad student in in our own Department uh in in uh in the SDS program some decades ago again based largely on Declassified documents and so on so so there was knowledge that this radiation existed that it was harmful to humans it wasn't clear e

xactly harmful at which doses but it was unambiguously harmful in general there were some precautions taken but nothing like what would come later so so that's a huge huge question that much more of which is learned actually after the weapons are used there's a long-term longitudinal study of victims of the bombings in Japan for example that goes on for years there are then other kinds of experiments uh and and more controlled studies after after second world war and we will so we will talk abou

t that there's a second film we'll watch together a second documentary we'll see in a few class sessions it looks much more directly at the broader environmental impacts including radioactivity uh associated with these very very massive projects and by the way just one more plug I showed the book cover uh my my friend and colleague Kate Brown who's now also professor at MIT wrote this really very compelling very moving book called plutopia on some of the longer term impacts um not just during th

e 1940s but even afterwards uh about these things so that's a very important very good question Sarah we'll have a chance to talk a bit more about that um were these commitments binding ah good no so Johan asks uh were people kind of threatened if they chose to leave no and and yet there is literally one person one person who left Los Alamos Joseph rutblatt his name uh before the project was completed because of what he cited as kind of moral concerns goes back to to Johan's question so it wasn'

t that it was impossible to imagine the consequence of these things some people did some people thought about it and kept working the project some people said that's not my problem some people said someone else would worry about it and one person at Los Alamos said this is my problem and I don't like it and I'm leaving he went on to found the Pug watch movement among other things so again great question we'll talk more about those things uh uh we'll have an opportunity to talk more about that so

on let me personally I want to talk the last part of class is a bit more brief but I I don't want to run too long so let me jump into the next part these are great great questions so the last part is is now how do people actually construct a device that would explode how do you make an actual weapon out of these esoteric sounding things so this last part is actually making bombs very briefly so each time we saw each time a single nucleus undergoes fission a couple extra neutrons were released th

e problem that also is right in cerber's uh initial primer they knew this right from the spring of 1943 was that if you have two small uh a mass of this fissional material then on average more too many neutrons will be close to the edge and so they will they were they're more likely to fuse right outside of the active region than just stick around and cause more fission so you have something like a critical size you if you have a larger volume the same density same properties as more of it then

on average most of the time a new Neutron is released it'll be more likely to encounter another Target another nucleus rather than be close to the edge and kind of fly out and fizzle so this introduces the notion of a critical size from there you can then calculate a critical mass this is there's a huge story here uh you can learn some more about in Peter gallison's really fascinating book called image and logic also this really amazing resource uh written by a team of historians and scientists

several of whom actually applied for and received um top secret clearance so they could actually invest read classified materials even though their own what they wrote about it would then be subject to to uh you know it could only be safe to release so you have some real Insider experts who worked on this other book called critical assembly and a portion of that team is Lillian hoddison who wrote the main piece we read for today so what was happening at Los Alamos was a series of kind of hybrid

computation human almost entirely women volunteer computers they were usually the kind of wives of staff so the people who were first hired were almost exclusively men to work at Los Alamos we'll talk more about the kind of gender Dynamics in the field around this time that we'll see that more squarely in a lecture or two so most of the people who were trained in science or Engineering in the US in this time were men many of them were invited to relocate to Los Alamos with their families so ther

e were uh many uh spouses mostly women spouses who came along and many of them were then able to pick up work at the lab as well as computers as I say the people were named computers they were usually using these handheld mechanical calculators not programmable electronic machines those were just at the moment under development so you have these kind of hybrid human machine Computing teams they would break down complicated iterative calculations to try to do things like calculate the likelihood

for a neutron to um to to leave a region of active material or induce fission will it will it drift uh and diffuse outward or not so with this kind of series of uh early what we could call a numerical simulation I'm just painstakingly slow the scientists were able to estimate the critical size above which you're more likely to be in this regime than that and that was about a radius of nine centimeters if you had purified an all fissionable u-235 and you had a sphere of radius nine centimeters yo

u'd be more likely to have that thing undergo runaway Chain Reaction rather than this loss of neutrons from diffusion that then the size translates to a mass right because you have a constant density of of the metal there so the the critical size was related to a critical mass about 50 kilograms over a hundred pounds of pure u-235 at a time when this existed in tiny Trace Amounts uh so what what uh what had already been figured out and uh server lectures on this in the primer is you can actually

get by with a much smaller size you could get by a size closer to this if you surround the active material the fissional material with something called a tamper a very heavy metal that is that is very uh inert to nuclear reactions so it will neither absorb neutrons nor undergo fission it'll basically just act like a mirror and bounce those neutrons back in a very heavy very inert stable nucleus of which they had ideas of what there might be that was called the tamper and just put that heavy met

al around the the active region then you're going to reflect these neutrons back in you can shrink down by a factor of about three this critical size and then the critical mass becomes about kilogram scale not not um tens or hundreds of kilograms so now the question is how do you how do you get this thing to actually undergo a runaway Chain Reaction so now you know roughly how much stuff you need for that critical mass how do you get it to undergo this very rapid uh energy releasing response sho

ws how to get it to blow up so here again all the who's identified already from the primer they knew about this very early the idea was to get two sub-critical pieces so you're not in danger of either of these pieces those shaded regions here undergoing a runaway Chain Reaction they're each too small neutrons on average will diffuse out before they cause too many fissions so again two sub-critical pieces of this enriched fissionable material and literally shoot them together like from uh from a

musket from a gun so they knew their their existing army guns actually in use that could get muzzle speeds of projectiles that would correspond to uh say a tiny fraction of the second this the velocities were uh were thousands of kind of meters per second uh or centimeters per second I guess so I could get these two sub-critical pieces to be jammed together to make one critical mass in a time within a tiny fraction of a second you'd also need to actually induce then one thing together you have t

o inject at least one Neutron that can start this runaway Chain Reaction they were already thinking about what are called initiators at the time of the primer the idea was to actually have a natural Alpha emitter something that is naturally radioactive like radium or polonium glue that onto one of these pieces attach it to one of the projectiles and have beryllium or some other Target on the other piece it redoing Chadwick's experiment from which he he identified neutrons if you if you have alph

a particles smacking into some materials like beryllium they will produce neutrons so just do that really fast by gluing the two ingredients of Chadwick's experiment into these pieces in the middle of a bomb I've done that fascinating they were so confident about this method I just find this mind-boggling they were so confident they literally never even tested it the first time any device ever went underwent a runaway chain reaction from this u-235 gun method uh assembly was that it was used aga

inst a population in the Japanese city of Hiroshima and you'll see of course much more about the actual use of the weapon and consequences in the film so the very first time a device like this was even exploded at all was actually uh in a military usage on October 6 1945 many of you might know we just passed the 75th anniversary of of these bombings uh this past summer so here's what what is now called the atomic dome for some reason it's still not so clear this one building near Ground Zero was

mostly destroyed to get this kind of Dome structure uh the skeletal girders of the Kiln uh survive that's now called the atomic Dome it was actually kind of industrial management Hall in the middle of hiroshan at the time they're not even testing it the other uh method was actually much much more complicated and again you'll hear more about this in the film and we can talk more about it soon too this became a major Challenge and this is the subject of Lillian hoddison's piece that we read for t

oday the other kind of fissionable material the material that was even more likely to fission and uranium was this plutonium but it had a spontaneous fission rate this was an unnaturally unstable element that's why it doesn't exist on its own on Earth so the it is more likely to kind of blow itself apart in something other than a runaway chain reaction faster than you could get two of those subcritical pieces to join no matter what the muzzle velocity was they were they came to recognize only by

uh summer of 1944 well past the start of the laboratory that any of these assembly methods for this highly unstable plutonium would be too slow so again it's Hardison uh tells us in the reading what they wound up doing was pursuing something called implosion this became a really very very significant technical challenge it leads to all kinds of moral challenges as well I don't want to downplay those I just want to say what what occupied many of these folks uh during these very hectic days of th

e of the war the idea was to then get a tiny little plutonium core actually have separated into tiny little subcritical pieces but near each other surround it with a tamper again to reflect the neutrons back in but then surround that with multiple kinds of conventional explosives that were shaped into what became known as shaped charges so so you want to set up here's the plutonium fissionable stuff here surrounded with different blocks that are shaped very intentionally with different burn rate

s so this kind of basically TNT one kind of chemical explosion would have a certain burn rate you have a different burn right here and different different burn right here so you could actually shape the in-going wave into a spherically symmetric shock wave that goes in instead of going out so you want it with with very high Precision induce an in-going wave that will then crush the plutonium core so that all these subcritical pieces are condensed into a single critical mass even more quickly muc

h much more quickly than any of those kind of muzzle velocity methods of gun assembly can we talk more about that but that was what was was the idea now that created huge challenges both theoretical and experimental how do you calculate the appropriate shapes how do you actually mix these materials to appropriate Purity I mean lots and lots and lots of challenges there so this uh really the leaders were not so confident this would work on its own this they did a test of this became known as the

Trinity test the film that you'll watch before Monday was called the day after Trinity it's referring to this uh now famous test called the Trinity test which happened on July 16th 1945. you can see here's the tests uh uh Bob about to be exploded after Norris Bradbury leaves the assembly so this was arranged uh not too far from Los Alamos plutonium was so rare remember there was just barely eeking out kilograms worth after that entire industrial effort at Hanford that at first the idea was to su

rround this test bomb in an enormous uh steel very thick steel container called literally called jumbo they had to build a special railroad carrier just because this thing wouldn't fit on standard tracks and get it from where it was made I think forged in like Pennsylvania to get it to uh New Mexico ahead of time so that if the bomb didn't work as expected they could scrape off this very rare plutonium and try again in the end they wound up not using it would that just gives you an idea of how e

xperimental this was here's uh one of the rare color photographs of the Trinity test it was so powerful it fused the desert sand into glass there's a special material that was dubbed trinitite glass from the Trinity test that covered the desert floor from these uh from the unleashing of these extraordinary forces so three weeks after that test and just three days after the surprise bombing of Hiroshima a bomb of that kind nicknamed fat man a plutonium implosion bomb was then dropped on the Japan

ese city of Nagasaki again we just passed the anniversary and then you can see uh just just as a quick version here as much more we could talk about the kind of of impacts of of this nuclear weapon on the city there so many many more questions think about of the sort we already uh were beginning in the chat now and I want to uh these are important and very difficult questions we're going to take our time with him on Monday just some things to think about when you do watch the film what did what

got people to work on this did their own motivations change over time why were these things used how was the decision made to use these new weapons what really was the impact militarily strategically on the course of the War uh as imagine van or now how do people react Beyond these projects once the secret was revealed and so on many many hard questions to ask about there so I'll stop there uh a good Alex shares some good uh resources here Scott Manley series is indeed excellent and also I encou

rage you to go check out Alex wellerstein's blog as well tons of stuff so I'll pause there any final questions uh before we turn to our discussion together on Monday okay I'll pause there please remember paper two due this Friday good luck with the paper uh enjoy the film it's a hard film but I hope you'll appreciate the film I should say watch that on your own and and then uh for those who who are interested and able to spare the time we'll meet together at our usual Zoom link uh Monday at 1 pm

Eastern take care everyone see you soon