My observations during the explosion at Trinity (1945) | Hacker News Reader

My observations during the explosion at Trinity (1945)

fermatslibrary.com

153 pointsluisb46 comments

In one of my interstellar medium classes I remember learning about the physicist Geoffrey Taylor's estimate of the yield of the first atomic bomb. At the time, the energy of the atomic bomb was classified information, but photographs of the explosion like this one [1] had been published in newspapers and magazines. Crucially, this photograph was labelled with both the time after the explosion, and a scale bar. With just this information Taylor found it was fairly simple to calculate the energy of the explosion. He ended up publishing two papers about it [2] [3]. In astrophysics, his calculations are now used when modeling the effect of a supernova on the surrounding interstellar medium.

[1]: https://upload.wikimedia.org/wikipedia/commons/thumb/7/77/Tr...

[2]: http://rspa.royalsocietypublishing.org/content/201/1065/159

[3]: http://rspa.royalsocietypublishing.org/content/201/1065/175

Thank you. I had known about Taylor's work from a course on scaling methods in continuum mechanics (it is also one of the introductory examples in Barenblatt's very nice book Scaling), but did not know that it has other applications.

Barenblatt looks like something I need right now -- just put it on my library list for tomorrow. Thanks!

Those WW2 era photos are interesting, and I like the story about Taylor's analysis, but what really spooks me from early atomic bomb tests are the Rapatronic photos by Edgerton.

He took high-speed photography to the ultimate limits, at exposure times of 10^-8 seconds, and the pictures are absolutely eerie looking. Some are like gaping skulls of pure energy, like this one:

https://upload.wikimedia.org/wikipedia/commons/e/e9/Tumbler_...

http://petapixel.com/2014/03/05/rapatronic-camera-atomic-bla...

I didn't know Edgerton name, nor how many things he shot at the time. https://www.youtube.com/watch?v=gspK_Bi0aoQ

From the article: "This is 1/100,000,000th of a second after the first photo."

A 100 millionth of a light-second is about 10 feet, the tower was 100 feet. So that couldn't have been the time between exposures, and they used multiple cameras anyway.

The Wikipedia article https://en.wikipedia.org/wiki/Rapatronic_camera says the exposure times were several microseconds.

I don't think that's the first image in the sequence, just the most famous one. They took a lot of images in sequence. Here they are edited together into a sequence (several series, presumably from multiple tests, unfortunately looks a bit incomplete): https://m.youtube.com/watch?v=KQp1ox-SdRI

I gather that some of the lumpiness is caused by imperfections in the strength and time synchronization of the implosion. Ideally, you'd see spheres. There are also bumps caused by flanges, ports, etc in the casing.

I can't figure out if that is awesome or absolutely terrifying... Imagine what is the equivalent today.

I don't know why (perhaps the other-worldly look of the explosion at such a small amount of time after detonation), but this picture and the picture of the Howitzer delivered mushroom cloud, has always filled me with absolute dread and horror. I think for different reasons.

The former just looks so ... weird, and scary, and like anyone witnessing it would know that's the last thing they'll see. The latter because of the implications of what we have the power to do, and probably won't be able to resist actually doing.

So far, so good.

Obligatory reference to 'The Making of the Atomic Bomb', Rhodes. Terrific book on the people, physics, and politics behind 'The Bomb' and its use.

Any recommendations on biographies of Fermi?

The one by his wife Laura is very interesting: "Atoms in the Family: My Life with Enrico Fermi"

I just love how practically minded these men were. Whatever they had available to them, they made it work somehow. If you want to read another account of the Manhattan Project / first nukes, I highly recommend the book "Surely you're joking, Mr. Feynman!" which contains a wealth of interesting and funny (and sometimes touching) trivia about the life of Feynman and how things went at Los Alamos.

Fermi was famous for this kind of thing, he could calculate the answer to a problem where it seemed like there wasn't enough information as if by magic.

Not calculate, estimate. You come up with a simple mathematical model- often just adding or multipling a few numbers together. It's not magic, he just had a good sense for including all the 0th and 1st order terms.

Imagine to witness a Tsar Bomba https://en.wikipedia.org/wiki/Tsar_Bomba 64km tall mushroom head. About 8 kilometres (5.0 mi) in diameter, was prevented from touching the ground by the shock wave, but nearly reached the 10.5-kilometre (6.5 mi) altitude of the deploying Tu-95 bomber

Wow, and the biggest threat we have today is immigration of (mostly) peaceful people. How quickly we forget.

Due to the blast in the desert, quartz and other minerals in the sand were melted to form Trinitite, a green glass. Would be interesting to hold this piece of history in a museum (according to Wikipedia, it's safe to handle).

https://en.wikipedia.org/wiki/Trinitite

It was all bulldozed into a small "building" on the site as it was too hot to leave around.

There were lots of signs warning about it on the bunker when I visited in 2003/4.

Great resource on Fermi problems and related topics.[1]

[1]https://mitpress.mit.edu/sites/default/files/titles/free_dow...

Trinity site is an interesting place to visit - it's open to visitors twice a year. http://www.wsmr.army.mil/PAO/Trinity/Pages/Home.aspx

What I found most interesting about it, was the lack of interesting there. It could very easily be missed.

Once had a job working in the basement of a building in Los Alamos NM operating a digital scanner to digitize various documents from the lab archives. I encountered a few boxes of photos of various atomic tests - beautiful and spooky.

Seeing a black and white description of an explosion estimated at 10,000 tons of TNT is still mind-boggling. Even harder to believe that within a decade or so there were explosions of 100,000,000 tons of TNT. E=MC2 gets big in a hurry.

No single nuclear weapon detonation ever hit 100,000,000 tons of TNT. There are designs for weapons in that range, and single example at 1/2 that power. However, such designs are impractical as using the same material to carpet bomb an area is more effective.

PS: Before the H-Bomb nuclear weapons where not actually cost effective per ton. Their 'advantage' over high explosives was portability and fear.

The Tsar Bomba was supposed to be in this range, but the allegedly the designer of the weapon started to fear his own creation and decided to tamper with it and tone down the yield.

https://en.wikipedia.org/wiki/Tsar_Bomba

No idea why you have been downvoted

I kind of remember reading that somebody involved in the creation / testing of that bomb made a rough estimate of the area covered by the fallout and decided to ditch the U-238 damper on the second (third?) stage.

Also, getting the plane dropping the bomb to a safe distance before the explosion was kind of an act as it was, if the bomb had gone off at twice the yield, it probably would have killed the crew.

(All this is from rather fuzzy memory, though! If somebody has less fuzzy information, I'd be happy to stand corrected!)

From my own fuzzy memories that's correct - a full 100MT device would have been a pretty awful weapon, I've seen it described as something that could have utterly destroyed Belgium but that would have killed people in Russia with fallout.

I seem to recall that the Tsar Bomb had multiple tertiary stages?

Nitpick: it's tamper not damper.

Also without a fissioning tamper round it's secondary and tertiary stages the Tsar Bomb actually was a "fusion bomb" in that a very high proportion of its yield came from fusion. In most H-bomb designs most of the yield comes from the fissioning of the tamper round the secondary.

With bombs of this type you can chain things on several times and really amp up the yield. The theoretical limit, if there is one, is absolutely nuts, like 500MT or more.

The problem is that beyond a certain yield most of the explosive force punches up through the atmosphere and is wasted. Apart from shock and awe, not sure what the utility of something that big would ever be.

Additionally since a spherical explosion of double the diameter takes eight times as much energy to create, you quickly run into diminishing returns. Lots of smaller bombs is more effective, hence MIRVs: https://en.wikipedia.org/wiki/Multiple_independently_targeta...

Nuclear weapons are so awesome and so utterly insane at the same time.

One pedantic correction is that the term is 'tamper' not 'damper'.

The Tsar Bomba is a 100MT design, there's zero doubt it could achieve that yield. However, like most thermonuclear weapons roughly half of that yield would have come from fission in the natural uranium casing. Essentially every neutron from a fusion reaction has the ability to cause fissioning of U-238, and when you do all the math it works out to release as much energy as the fusion reactions.

But that would have meant that the test would have released 50MT worth of fission byproduct fallout, about an order of magnitude more than the roughly 7-8MT worth of fallout accidentally retired released in the castle bravo test. Since the USSR's test sites were on land, such a release was daunting, so instead they changed the device for the test and used an inert material for the casing. The end result was that the 50MT test was comparatively very "clean" with most of the yield coming from fusion.

Surely the pre-thermonuclear bombs also had a big advantage in ease of delivery? One bomber could do what previously took a thousand, which means that a bomber fleet could destroy a bunch of cities simultaneously, instead of hitting a couple of cities per week as in the Second World War.

The super-large bombs weren't entirely impractical. Hitting an area with multiple smaller bombs is indeed far more effective, if you can accurately deliver a bunch of smaller bombs at once. There was a period where that wasn't a given, and they tried to make up for inaccuracy with explosive power. The 100Mt bomb was never deployed (or, indeed, built other than the one de-rated test article) but both the US and USSR deployed 25Mt bombs.

Carpet bombing a larger area means you need even less precision. Picture throwing a 10 darts at a dart board with a 2 foot radius, vs 1 dart with a 4 foot radius.

Still, even at half design yield the Tsar Bomba had an instantaneous power output equal to 1.4% of the sun, albeit for a few nanoseconds at most.

We did this calculation in a course I took in college. I've completely forgotten how it worked.

Assuming the paper shifted perfectly laterally, I assume you'd just be measuring the difference in the volume of half a sphere ten miles in radius, and a sphere ten miles and 2.5 meters in radius. I think you could use the ideal gas law at that point to figure out the energy.

I'm just guessing though.

I think you're right. Ideal gas law wouldn't apply to the plasma sphere but knowing shockwave strength at some distance you can figure out how much energy was transferred to the atmosphere.

I agree the calculation probably worked by measuring the kinectic energy of that half-shell by measuring its velocity*thickness directly using pieces of paper.

But I wouldn't count the ideal gas law out (see http://physics.stackexchange.com/questions/43696/will-ideal-...). My guess is that the huge temperatures involved will work in favour of an ideal-gas approximation.

Perhaps the more important thing is that things are happening on very short time-scales, far enough from equilibrium and temperature might not even be well defined.

That probably would work better. I tried doing the math and ended up with some really weird answers. Definitely not 10 kt.

Is there any way to determine exactly when this document was written? I've read accounts that indicate that it took some time (weeks, months, years?) before the metaphor of mushroom shaped cloud entered into the vocabulary used by witnesses of atomic weapons. Fermi's account would disprove that assertion if it were written immediately after the test at Trinity.

While reading this, I started wondering if there are any other 3rd party (civilian?) accounts of the event or if the geography / remoteness made it 'invisible' to everyone else? I have a funny visual in my head of a random farmer casually mentioning to his wife a 'weird glow in the horizon' on an otherwise regular Monday morning.

Is there only one page? I'm on an iPad, there doesn't seem to be arrows for next/previous page.