Did you know that all the nuclear bombs we exploded in the '40s-50s-60s have permanently contaminated all the steel the world has produced since then? All steel in the world is divided into two categories: pre-1945 non-contaminated steel, and post-1945 contaminated steel.

How?
Nuclear bombs (including the tests) create a lot of radioisotopes that are not found in nature. For example Cobalt-60 (a radioactive version of Cobalt-59). And since the 1945 Trinity test, these have all dispersed in the atmosphere.

What does this have to do with steel?
The process of manufacturing steel uses atmospheric air (or atmospheric oxygen). Some of the radioisotopes like Cobalt-60 get pulled in too, and as a result, any steel produced after 1945 has embedded in it some of these radioisotopes, sitting there emitting gamma rays.
Why do we care about traces of radioisotopes in steel? When making Geiger counters, or medical devices to check for radioactive contamination, or astronomical instruments, the emissions from the contaminated steel would interfere with the measurements.

So we need pre-1945 steel
Pre-1945 non-contaminated steel is called "low background steel". And when we need it, where do we get it from?

We go, find pre-World-War-II sunken shipwrecks, and then salvage the steel (and lead) from those to build our instruments which need heavy radiation shielding.
This is why the search for Dark Matter depends on sunken ships: theatlantic.com/science/archiv…
I vaguely remember some TV serial where a forgery is detected because the steel used in the supposed-ancient-artifact was not low background steel. I don't know whether that's a real thing, or just fiction, but it's a great story. (We need more such stories in science teaching.)
Apparently, the use of radioactive isotopes to test for forgery is a real thing (thanks @achillesHeelV2):
Just to be clear: the contamination of the atmosphere (and steel) with radioisotopes is not considered a health hazard (or even a significant environmental problem) because the amounts are really tiny.

In fact, there are a lot of naturally occurring radioisotopes also. For example, Carbon 14 is regularly created naturally by cosmic rays. All of us are constantly breathing in all these radioisotopes. In fact, the entire concept of carbon-dating depends upon this.

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More from @NGKabra

22 Dec 20
1/ I've now taught my "Defense Against the Dark Arts" course (teaching teenagers about misinformation, fake news, misleading ads, etc) to 4 batches, and it has been a very rewarding experience for me

A quick thread on my learnings and suggestions for you

genwise.in/events-3/defen…
2/ My biggest learning has been how quickly children figure out things with the help of the internet as long as they are just pointed in the correct direction.

They don't need teachers. They need people who can motivate them and point them in the right direction.
3/ Consider the exercise in this photo. I gave them no other coaching, but within minutes most of them had figured out why each of these ads was misleading and they were able to articulate and discuss subtleties like "The product is a good product, but the claim is exaggerated." Image
Read 13 tweets
24 Nov 20
If, like me, you're confused about the Oxford/AstraZeneca (Pune-walla!) vaccine, which is 70% effective, but also 90% effective, then here is an explanation. 🧵
The Oxford vaccine requires two shots one month apart. The original design called for two equal shots. However, someone made a mistake and a small fraction of the volunteers got only half a dose in the first shot, and the full dose in the second shot.
🧵
Later, analysis of the data showed—to everybody's surprise—that the ½+1 dose regimen was 90% effective and the standard 1+1 dose regimen was only 66% effective.

The average of these two regimens across all volunteers is 70%, and that is what has to be reported 🧵
Read 13 tweets
29 Sep 20
As a child, I always wondered why Indian words like राम always got written as "Rama" in English. To me "Ram" seemed like a much better fit for the pronunciation. I thought it was because of British arbitrariness.

Only recently I learnt the real reason for this. /1
Consider गम. How would you pronounce it? Like the English word "gum" right? As a Hindi/Marathi speaker, I can't imagine any other pronunciation.

But, for a Sanskrit speaker, things are different. गम् is pronounced "gum". In गम the full म is pronounced, so it becomes "gum-uh" /2
The extra "-uh" sound which is the difference between गम and गम् is called a schwa

And modern Indian languages like Hindi and Marathi all have an (unwritten) rule that the schwa at the end of a word isn't to be pronounced

This is called schwa deletion en.wikipedia.org/wiki/Schwa_del… /3
Read 13 tweets
27 Sep 20
Ok, let's do this. What are good YouTube channels that you watched (or still watch) because they're fun to watch, but you also ended up learning from them?

Only recommendations from people born this century are allowed

@gauravjoshi2000 @arshkabra @KabraRuhi @ShahAvanti02
Include a link to the YouTube channel, what it's about, and why you liked it. And tag others who could add to the list. Younger kids deserve better recommendations than the ones from the previous century.
YouTube for youngsters: Epic Rap Battles of History. This did more for @ArshKabra's history knowledge than ICSE History ever did.
Read 40 tweets
24 Sep 20
This is a non-foodie thread for foodies.

Recently, I've been stanning @krishashok's fabulous threads and videos on the science of cooking. Practical tips, not just theory. For Indian as well as non-Indian food.


Follow him if you like cooking

But /1
In a completely different way @prachi_ has been cooking and instagramming heartwarming stories of dishes-with-memories-attached shared by her friends and readers over at instagram.com/via_dil/

Follow if you like food, feel-good, feel-bad, feel-better all rolled into one

But /2
But all this has been reminding me of an incident shared by my mom about 45 years ago

My dad was a building contractor in Malegaon, and for some reason, my parents were temporarily staying in a shed on the site. Some of the construction workers were also staying there /3
Read 10 tweets
23 Sep 20
We all have serious misconceptions about the income distribution in India. I ran a survey last week and compared the answers with real data

Check out the survey here: forms.gle/noyd9GPMeY2Yya…

The gaps between perception and reality are stunning. A thread with some highlights
/1
What's the average household monthly income of the top 1% of the country? Most people think its ₹5L or more.

Reality: it is 85k.

88% of you are in the top 0.5% of the country, and almost none of you know this fact /2
The average monthly household income in India is 22k. For a family of 4.7 members.

People in metros don't realize this because, in the metros, we don’t really have the “rest of India”. 87% of all the people living in metros are from the top 40%. We never meet the bottom 60%. /3
Read 6 tweets

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