Skip to main content
Loading Explore guides…
Oppenheimer and the Atomic Bomb · Acedra
Home
Explore
Physics
Oppenheimer and the Atomic Bomb
Sign in to save
Chat
If YouTube asks you to sign in, open the video on YouTube.
Watch on YouTube
Reload player
Transcript
89 segments
Copy
Hide video
0:00
J. Robert Oppenheimer might be the most important physicist to have ever lived. He never won a Nobel Prize, but he changed the world more than most Nobel Prize winners. Under his leadership, the best physicists of the 20th century built the atomic bomb forever changing the course of history. - If there is another world war, this civilization may go under. - [Derek] He has affected every war waged and every peace settled since the end of World War ii.
0:32
He also created a way for humanity to destroy itself. - Now I am become death,
0:39
the destroyer of worlds. - [Derek] This video is about how to build an atomic bomb, the life of Oppenheimer, and why serious scientists were worried about the explosion setting fire to the atmosphere, ending all life on earth.
0:56
Part of this video is brought to you by Wren.
1:01
When J. Robert Oppenheimer was 21, he placed an apple laced with toxic chemicals on the desk of his physics tutor. The tutor, Patrick Blackett, was an experimentalist and he had hounded Robert to do more of what he thought Robert wasn't very good at, experimental work. Oppenheimer had already been spending his days in a corner of JJ Thompson's basement laboratory, attempting to make thin films of beryllium, which were used to study electrons. But Oppenheimer was clumsy and not good at this work.
1:33
He was soon avoiding his duties in the lab, spending his time listening to lectures and reading physics journals. It was 1925 and the 21 year old Oppenheimer was becoming fascinated by the new field of quantum mechanics. Despite being surrounded by brilliant physicists like Rutherford and Chadwick, Oppenheimer was deeply unhappy. He wrote, "I'm having a pretty bad time. The lab work is a terrible bore, and I'm so bad at it that it's impossible to feel that I'm learning anything."
2:04
A friend walked in on him, lying on the floor of his room, which he called "A miserable hole", groaning and rolling from side to side in emotional anguish. It was in this state that Robert attempted to poison Blackett. The specifics are lost to history. There are conflicting reports if Oppenheimer used cyanide or something he found in the lab, which would've just made Blackett sick. This story sounds unbelievable, but Oppenheimer himself confirmed it. Luckily, Blackett did not eat the apple,
2:36
but the attempted poisoning became known to the Cambridge University authorities. Robert's parents were visiting their son from the US at the time, and Julius Oppenheimer successfully lobbied Cambridge not to press criminal charges. Due to his family's wealth, Robert wasn't even expelled from Cambridge on the condition that he had periodic counseling sessions with a psychiatrist in London. (train horn blaring) In the summer of 1926, Robert traveled to the University of Göttingen.
3:05
The chairman of the department was Max Born, who just two years earlier had coined the term, "Quantum Mechanics". Born was reportedly a thoughtful and gentle teacher and had nurtured the work of Werner Heisenberg, Wolfgang Pauli, Enrico Fermi, and Eugene Wigner, basically the who's who of quantum mechanics. The class that Oppenheimer was in was also extraordinary, including luminaries like Paul Dirac and John Von Neumann, where the academic culture at Cambridge focused on experimental physics.
3:36
Göttingen was all about theoretical physics, and under Max Born's mentorship, Oppenheimer thrived. His mental health improved, and he found a community of people who were as obsessed with physics as he was. On November 14th, 1926, Robert wrote to Frank, his younger brother, "You would like Göttingen. I find the work hard, thank God, and almost pleasant." Robert was thriving and his talent was being recognized. Born later wrote, "He was a man of great talent and he was conscious
4:08
of his superiority in a way which was embarrassing and led to trouble." When Oppenheimer was 23, he graduated with his PhD in physics. He wrote his thesis in German on the quantum theory of continuous spectra. All in all, he published more than a dozen papers in the two years he was at Göttingen. Many of them expanded upon the work of Werner Heisenberg, who was just three years older than Oppenheimer. The two eventually met in 1927, the same year Heisenberg published his groundbreaking paper
4:39
on the Quantum uncertainty principle. By all accounts, the pair got along well. There was no way to know that just 15 years later, they would be deadly rivals attempting to build the first nuclear bomb. Oppenheimer for the USA and Heisenberg for Nazi Germany.
4:57
At the time, it was thought that getting significant amounts of energy out of radioactive atoms was impossible. Ever since the discovery of radioactivity by Henri Becquerel, Marie Curie, and Pierre Curie in the late 1890s, it was known that radioactivity was a passive process. Unstable atoms would just decay at random, unpredictable times, and surely there was no way to control that. In 1933, Ernest Rutherford, Oppenheimer's old boss from Cambridge wrote that,
5:28
"Anyone who expects a source of power from the transformations of these atoms is talking moonshine." That same year, Albert Einstein said, "That there is not the slightest indication that nuclear energy will ever be obtainable. It would mean that the atom would have to be shattered at will."
5:46
So how would you break an atomic nucleus? Well, you could take a proton and accelerate it through a large electric field and then smash it into a nucleus. This is exactly what John Cockcroft and Ernest Walton did in 1932. They accelerated protons into lithium nuclei, breaking them apart. The pair would later win a Nobel Prize for this work, but a proton is positively charged, so it's repelled by all nuclei, which are also positively charged. So to give them a hope of overcoming this barrier,
6:19
Cockcroft and Walton had to use 250,000 volts to accelerate the protons. Even then, only about one in a billion protons actually hit and split a lithium nucleus, so this would not be an effective way to get energy. But there is another way.
6:39
In 1932, the neutron was discovered this subatomic particle that's about 0.1% heavier than a proton, and it has no electric charge, so a neutron would not be repelled from a nucleus. And in 1933, Leo Szilard was thinking about how you could use neutrons to split nuclei. "It suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbed one neutron, such an element if assembled sufficiently large mass,
7:12
could sustain a nuclear chain reaction." But the thing is, nobody knew if there was an element that had a kind of nucleus that would do that.
7:24
On the 29th of January, 1939, Louise Alvarez, a promising young physicist, was getting a haircut while reading "The San Francisco Chronicle", and suddenly he got outta the chair halfway through the haircut and ran to Oppenheimer's office. Alvarez read an article about how two German chemists, Otto Hahn and Fritz Strassmann had successfully split an atom of uranium by bombarding it with neutrons. Oppenheimer was not impressed. "That's impossible." He reportedly told the Young Alvarez
7:54
proceeding to mathematically prove on his blackboard why fission could never be achieved.
8:00
But the next day, Alvarez had repeated the experiment and invited Oppenheimer to see it. Alvarez later recalled that, "In less than 15 minutes, he not only agreed that the reaction was authentic, but also speculated that in the process, extra neutrons would boil off. That could be used to split more uranium atoms and thereby generate power or make bombs." When a single atom of uranium-235 splits apart, it loses a little bit of mass, which is released as energy.
8:31
Following Einstein's mass energy equivalence. That is a tiny amount of energy, about 20 times less than the amount required to raise a grain of sand, the thickness of a piece of paper. But atoms are also tiny. In a one kilogram lump of uranium, there are about a trillion, trillion atoms, so the energy quickly adds up. Soon, almost everyone was convinced in August of 1939, Einstein, who just six years earlier believed that nuclear bombs were impossible, signed his name to a letter
9:03
addressed to President Franklin Roosevelt. The letter actually written by Szilard warned Roosevelt of the possibility of nuclear weapons. It also pointed out that Germany had access to uranium from the mines in Czechoslovakia, which was recently taken over by the Nazis. Roosevelt began an informal uranium committee to discuss this topic, but then for two years, nothing happened.
9:28
In 1941, Roosevelt upgraded the informal uranium committee to the S-1 committee, which would report directly to the White House. The explicit goal was to develop an atomic bomb, and in May, 1942, Oppenheimer was hired onto the committee to be the coordinator of rapid rupture. So why was he selected? (dramatic music) Well, after completing his PhD, Oppenheimer became a physics professor, first at UC Berkeley, and then at Caltech, the brilliance he had shown under Max Born's tutelage didn't fade.
10:05
Indeed, it blossomed into a remarkable but strange physics career. In the 15 years after finishing his PhD, Oppenheimer made important contributions to everything from nuclear physics to quantum field theory, and even astrophysics. He had a number of Nobel Prize winning ideas. One of his students, Willis Lamb, became a Nobel laureate, but Oppenheimer himself was nominated three times, but never actually won the Nobel Prize. When asked why he thought that Oppenheimer never won the Nobel Prize,
10:36
Murray Gell-Mann said that, "He didn't have Sitzfleisch" A German word that translates to sitting flesh. The ability to sit down in a chair for a long time and do the hard work. "He never wrote a long paper or did a long calculation. He didn't have the patience for that." Wolfgang Pauli also said, "His ideas are very good, but his calculations are always wrong."
11:03
But Oppenheimer was amazing with people. He was a natural and charismatic leader, and this combination, his charisma and his ability to generate great ideas would serve him well in the next phase of his life. (upbeat music)
11:21
On the 18th of September, 1942, general Leslie Groves was put in charge of the Manhattan Project. - I was responsible for the development of the atomic bomb. - [Derek] On day one, he ordered 1200 tons of uranium ore. The next day, he ordered to buy the Oak Ridge site where the ore would be refined. The next month in a surprising move, he chose Oppenheimer to be the science director of the soon to be established Los Alamos laboratory. Oppenheimer had just been selected to be the chief architect
11:52
of the atomic bomb.
11:55
The military establishment had concerns. Oppenheimer did not have a Nobel Prize, so would the scientists hired for the project, respect his opinion, and follow his leadership? Oppenheimer also had no prior administrative experience over a large project like this. Furthermore, he was a theoretical physicist, who according to Isidor Rabi, "Was a very impractical fellow. He didn't know anything about equipment." And then there was the problem of Oppenheimer's political stance.
12:24
He had links to the Communist Party, including his wife, Catherine, who was a member of that party. But Groves was impressed by Oppenheimer. He valued his overwhelming ambition. He also knew that Oppenheimer's ability to understand problems not just in physics, but chemistry, engineering, and metallurgy would be invaluable. Groves thought that Oppenheimer was a "Real genius." Saying that why Oppenheimer knows about everything. "He can talk to you about anything you bring up. Well, not exactly.
12:55
He doesn't know anything about sports." The two men couldn't have been more different. Oppenheimer weighed half as much as Groves despite both of them being nearly six feet tall. Ideologically, Oppenheimer was a communist. Groves, a staunch conservative, but Groves was convinced that Oppenheimer would be the person that would build the atomic bomb before the Nazis, and that was all that mattered. Isador Rabi later commented that, hiring Oppenheimer for this role was
13:24
"A real stroke of genius on the part of General Groves, who was not generally considered to be a genius." The Manhattan Project needed a location. Somewhere isolated to keep the project secret safe from enemy attack. And while no one wanted to admit it, somewhere that was sparsely populated, just in case there was an accident,
13:44
Oppenheimer proposed Los Alamos, New Mexico. He had fallen in love with the harsh desert in the mountains of New Mexico when he was in his twenties. In 1929, Oppenheimer wrote to a friend. "My two great loves are physics and New Mexico. It's a pity they can't be combined." But Oppenheimer had severely underestimated the logistical challenge ahead. (gentle music) In 1943, Oppenheimer estimated that he'd need about six scientists supported by a handful of engineers and technicians to make a bomb.
14:20
He was off by two orders of magnitude. 764 scientists would end up working for the Manhattan Project. 302 of which would work at the Los Alamos site.
14:33
Over 600,000 people in total were involved with the making of the atomic bomb.
14:40
By this point, making the atom bomb didn't seem impossible. It seemed likely.
14:47
On the 2nd of December, 1942, a team of physicists at the University of Chicago led by Enrico Fermi created the world's first artificial nuclear reactor Pile-1. It consisted of 45 tons of uranium and uranium oxide and 330 tons of graphite blocks. Horrifyingly enough. It was located under the stands of the football field. It generated about half a watt of power, and if you can make a nuclear power plant, you can make a bomb. The only real difference between the two
15:20
is how many neutrons hit the next atom, causing it to split and release more neutrons. If on average that number is one, there will be a stable self-sustaining chain reaction, but it won't grow. If it's less than one, the reaction will die down, and if it's more than one, the reaction will grow. This is known as the multiplication factor, K. Nuclear reactions are similar to pandemics in this way.
15:48
The simplest way to make a nuclear bomb is to get enough fissile material close together that it creates a runaway chain reaction. That amount is known as the critical mass. With uranium-235, you need about 52 kilograms. Forming a sphere with a diameter of 17 centimeters. If you use plutonium-239, the critical mass is much smaller, only around 10 kilograms, which would create a sphere only 10 centimeters wide. For the first few years, the scientists worked on a bomb with a gun type design.
16:20
Inside a gun type bomb, you have two slabs of uranium-235, both of which are below the critical mass. Then using a conventional explosive like cordite, you rapidly fire one towards the other. So the combined mass is higher than the critical mass. When the uranium bullet is about 25 centimeters away, the nuclear chain reaction begins resulting in an atomic explosion. Despite the simple design, it is not very efficient. Only a small percentage of the uranium undergoes fission,
16:53
so the total yield of the bomb is much smaller. You also run into some unexpected problems, like how do you make sure the uranium slides smoothly through the barrel? Well, you use oil to lubricate the barrel, but all the synthetic oils the scientists tried would dry up. In the end, the only oil they could find that would work was the oil from sperm whales.
17:20
Only about 0.7% of naturally occurring uranium is U235, the fissile fuel for nuclear bombs. When U235 absorbs a neutron, it briefly becomes U236, and then it rips itself roughly in half and releases on average 2.4 neutrons per fission. But when you get uranium outta the ground, most of it is U238, which doesn't undergo fission. So to make a nuclear bomb, the scientists use gigantic mass spectrometers to separate out and concentrate the U235, and the resulting substance was uranium
17:57
with a much higher concentration of U235. In other words, it was enriched. There was another option though. In early 1941, a new element was discovered or rather synthesized. When a neutron is absorbed by a nucleus of uranium-238, it turns into uranium-239. U239 is unstable, so it decays into Neptunium, which then becomes plutonium.
18:25
There are three things that matter for this story. First is that plutonium-239 is a great fuel for a nuclear bomb. It has a critical mass of only about 10 kilograms. Second, it is cheaper to make than to separate uranium-235 and third, it reacts too quickly to be used in a gun type device. It would fizzle, meaning only a tiny fraction of the fuel would undergo fission. But there is a way to make a bomb using plutonium. Critical mass changes depending on the density of the material.
18:56
Under normal pressure conditions, six kilograms of plutonium-239 won't explode. But if you compress it, the atoms get closer together and the chance of a stay neutron hitting the nucleus increases. So the higher the density, the lower the critical mass. So if you set off conventional explosives around a ball of plutonium, you can get it compressed enough to start a nuclear chain reaction. And this was the whole idea behind the implosion bomb design.
19:29
There are a couple ways to cheat lowering the critical mass. For one thing, you surround the sphere with a material that reflects neutrons decreasing the amount of nuclear fuel you need to start a chain reaction. You can also have a neutron source, something that kick starts the chain reaction. For the first implosion bomb, scientists created a device called the urchin, which was a tiny pellet weighing just seven grams, and it would sit at the heart of the bomb.
19:57
It was made of beryllium and polonium separated by a layer of nickel and gold. The idea was that when the explosives detonated, the shockwave would mix the beryllium and polonium together, and then the alpha particles from polonium would cause the beryllium to release a flood of neutrons, which would set off the nuclear chain reaction.
20:19
At least that was the hope. An atomic bomb had never been made before. Oppenheimer and the rest of the scientists at Los Alamos needed to act quickly. It was already 1945, and Truman wanted to test the weapon before the start of the Potsdam conference. That's where Truman, Churchill, and Stalin would come together to plan the post-war piece. The conference began on the 17th of July. The earliest date that everything could be ready for the bomb was just one day earlier.
20:49
So that is when the test was scheduled. It was Codenamed Trinity.
20:57
The night before Oppenheimer was nervous. There were so many things that could go wrong. The last test firing of the explosives without the actual plutonium core was a failure. To calm himself, he recited a stanza from the Bhagavad Gita, the sacred Hindu poem. He had actually translated the Gita from the original Sanskrit in himself. In battle, in forest, at the precipice in the mountains, on the dark great sea, in the midst of javelins and arrows in sleep, in confusion, in the depths of shame,
21:34
the good deeds a man has done before defend him.
21:41
Perhaps more terrifying than the idea of the bomb not working was that it would work too well.
21:50
Around 1942, Oppenheimer discussed with Arthur Compton a terrible possibility that a nuclear test could end the world. The worry was that the nuclear bomb would create temperatures so hot that fusion would occur.
22:08
A tiny fraction of the atmosphere, just one part in 2 million is hydrogen gas. But the worry was that at high enough temperatures and pressures that hydrogen could fuse to together releasing energy, this energy would fuse more hydrogen. It could also break apart the hydrogen from water vapor causing that to fuse as well. That would release even more energy causing yet more fusion until the entirety of the Earth's atmosphere would become a giant fusion bomb.
22:40
Recalling his conversations with Oppenheimer in 1959, Compton said, "Nor was this all that Oppenheimer feared. The nitrogen in the air is also unstable, though in less degree. Might not it, too, be set off by an atomic explosion in the atmosphere." Most of the scientists quickly realized how unlikely this scenario was, and they continued on with the project, so no one took the idea too seriously. But the thought of starting a fusion reaction with a fission
23:09
weapon would become very important after the war. (dramatic music) The Trinity test was scheduled for 4:00 AM but it was delayed due to a storm. So at 5:29 and 21 seconds, the gadget, the world's first nuclear bomb detonated, the high explosive squeezed the core of plutonium inwards. The shockwave mixed the beryllium and polonium releasing a flood of neutrons. The urchin worked. It jump started the nuclear reaction, and now there was no way to stop it.
23:47
Just six kilograms of plutonium created an explosion that was equivalent to nearly 25,000 tons of TNT. The New Mexico mountains were illuminated brighter than in daytime. The shockwave was felt from over 160 kilometers away. The mushroom cloud rose to 12 kilometers into the sky. It was so hot that the desert sand melted into a glassy mineral, now known as trinitite. Fortunately, the blast did not set fire to the atmosphere.
24:24
On August 6th, 1945, the Boeing B29 Flying Fortress dropped little boy, a gun type nuclear bomb with 64 kilograms of enriched uranium. The nitro cellulose ignited pushing the slugs of uranium-235 together, tipping it over its critical mass. The blast from the explosion equivalent to 15,000 tons of TNT, killed nearly 70,000 people. Another 70,000 would die from burns and radiation poisoning in the following months. Three days later, an implosion type bomb, like the gadget, was dropped on Nagasaki
25:02
killing an estimated 80,000 more people.
25:07
More than 95% of the 225,000 people killed in the bombings of Hiroshima and Nagasaki were civilians. Most were women and children.
25:21
In 1965, recalling the moments after the Trinity test, Oppenheimer said that he thought of another verse from the Ghita. - He knew the world would not be the same.
25:34
Few people laughed,
25:38
few people cried. Most people were silent.
25:45
I remembered the line from the Hindu scripture, the (speaking foreign language).
25:54
The Vishnu is trying to persuade the prince
26:00
that he should do his duty and to impress him takes on his multi-armed form and says,
26:12
'Now I am become death the destroyer of worlds.'
26:19
I suppose we all thought that one way or another.
26:23
- [Derek] After the war, Oppenheimer was a national hero. His portrait was on the cover of Time Magazine, and he became a household name.
26:32
In 1947, he became the director of the Institute of Advanced Study at Princeton. He also became the chairman of the General Advisory Committee, where he became an advisor on nuclear weapons related issues. He used his position to argue for arms control.
26:50
(dramatic music) In August, 1949, the Soviet Union tested their first atomic weapon,
27:00
and the US military quickly decided that the best course of action was to develop a more powerful bomb. The hydrogen bomb, known as "The Super", Oppenheimer was against the development of The Super on ethical grounds and the worry that it would start an arms race. But Truman's administration pushed through, and three years later, Ivy Mike, the first hydrogen bomb was tested in the Marshall Islands. It had a yield of 10.4 megatons of TNT. That's 400 times more powerful than the Trinity test.
27:40
A hydrogen bomb is actually three bombs in one, a conventional bomb, a fission bomb, and a fusion bomb. The conventional explosives trigger a fission reaction, which increases the temperature and pressure enough to fuse deuterium and tritium together, releasing a huge amount of energy. In 1961, the Soviet Union tested the Tsar Bomba, the most powerful explosion ever detonated. It was another five times more powerful than Ivy Mike around 2000 times more powerful than Trinity.
28:17
This kind of arms race was exactly what Oppenheimer had feared, in part, due to his opposition to the hydrogen bomb. And due to his calls to avert a nuclear arms race, Oppenheimer was essentially put on trial to revoke his security clearance. He had been surveilled while he was working for the Manhattan Project, but that surveillance didn't stop after he left. Many of the wiretaps were illegal and warrantless. Oppenheimer was questioned about his ties to the Communist Party,
28:43
including his affair with Gene Tatlock, a Communist party member while he was leading the Los Alamos lab. He was essentially accused of treason and espionage. In December, 1953, Oppenheimer had his security clearance suspended. His face now grim and in black and white was once again on the cover of time. His security hearings were international news.
29:09
In 1964, German playwright Heinar Kipphardt wrote a play about Oppenheimer's life. Oppenheimer was sent a copy of this script, and he hated it so much that he threatened to sue. He especially despised the final scene where the character of Oppenheimer realizes the evil of his work. And I quote, "We have been doing the work of the devil." To Oppenheimer, it was always more complicated than that. - I think that it probably was assumed, it certainly was always assumed at Los Alamos
29:42
that if the war were not over and not clearly
29:52
to be brought to a conclusion by the diplomatic means
29:58
this weapon would play a part. At the time, the alternative,
30:06
the campaign of invasion was certainly much more terrible for everyone concerned.
30:13
I think that Hiroshima was far more costly in life
30:21
and suffering inhumane than it needed to have been,
30:27
to have been an effective argument for ending the war.
30:33
This is easy to say after the fact. - In 1965, he was asked about the recent proposal of talks with the Soviet Union to halt the proliferation of nuclear weapons, and his response was, "It should have been done the day after Trinity. Later that same year, he was diagnosed with throat cancer. He was a lifelong smoker, and he died on the 18th of February, 1967 aged 62.
31:05
Advances in science and technology have given humans tremendous power to make the world better or worse, and nuclear war isn't the only thing we have to worry about. Human caused climate change poses a serious threat to ecosystems and the most vulnerable people around the world. As I record this, we are in the midst of a heat wave, and my folks up in Canada are suffering in the smoke from wildfires. This is where this video sponsor Wren comes in.
31:31
Their mission is to take action on climate change. On their website, you can calculate your carbon footprint, which can help you identify ways to reduce your impact. And if you like, you can offset your carbon footprint with a simple monthly subscription. While individual action is important, Wren understands that this alone is not enough, which is why your subscription funds not only carbon removal and offsets, but also lobbying to enact climate policy and land conservation projects.
31:59
I am particularly fond of their new refrigerant destruction project. We actually made a video about Thomas Midgley who invented Freon, which is a refrigerant that is both destructive to the ozone layer and a very potent greenhouse gas. Well this project permanently destroys harmful refrigerants like CFCs, HCFCs, and HFCs to stop them from getting into the atmosphere. And this will reduce global warming and prevent the further destruction of the ozone layer. It's a win-win.
32:28
I personally offset my emissions with Wren, and for the first a hundred people to sign up using the link in the description, I will personally pay for the first month of your subscription. This is an idea I came up with myself because I think it's so important that for those of us who can help, that we pool our resources together to have a greater impact. So I hope you take the time to check out Wren. I wanna thank them for sponsoring this part of the video, and I wanna thank you for watching.
Here to help you learn
Sign in to keep a private conversation with your course tutor.
Ask me anything about the material…
Sign in to chat