https://www.sciencedirect.com/science/article/abs/pii/S01685...

I can't find a good source but it seems that the radiation shielding properties of polyethylene were discovered some time after WWII. But I think this was an inevitable discovery, not some sort of accident. HDPE has a lot of hydrogen atoms which are perfect for blocking high energy particles without creating secondary radiation. A block of solid hydrogen would be ideal but that's not practical. Water works well but it's heavy and needs a container which adds more weight.

Dense metals and concrete do a fine job but are heavy relative to amount you need to stop the same amount of radiation as HDPE. Plus you get the secondary radiation effects from metals that can harm humans or sensitive electronics. It's like being behind armor that's hit with a round. The armor may stop the round from getting through but very hot fragments of the backside of the armor (spalling) can fly off and injure someone. Now you need additional protection from the spalling too.

Here's a photo from 2017 of a NASA facility using white HDPE sheets to attenuate high energy particles:

https://www.flickr.com/photos/brookhavenlab/33642244296/in/a...

You can see the HDPE sheets in this photo, they can be lowered or raised to adjust attenuation:

https://www.flickr.com/photos/brookhavenlab/52979144013/

A service will allow you upload a file and attach a date and signature. Additionally, you add the email addresses of anyone else that needs to also date and sign. The service then verifies all signatures and then dates and signs. Then it makes a SHA-512 hash, publishes it next to the signatures on its site and then publishes a url including the hash on the blockchain. When someone visits that link, they can see the hash of the file, dates, signatures and a final hash of everything. Anyone wishing to view the actual file must be able to prove their identity using their private key (or alternatively, host an encrypted version for each signer). Thus anyone can prove a few things:

1. You agreed that the document was correct on the date you signed. You can't date your signature any earlier than when the file was uploaded. This prevents you from uploading the wrong version of a document or predating your signature well in the past. It's like signing in front of a notary.

2. The additional person can verify the document is correct (no contract clauses have been edited, a photo is legit, etc). This gives credence to what you've posted, especially important for a contract.

3. The service verifies that the file you uploaded matches what everyone signed and the date everyone supplied is in the past. A requirement could be that everyone involved needs to have their public keys published on a well-known key server that at least verifies email addresses. A better option would be one that verifies through physical identity documents. Even better would be if your national ID card had PKI keys. Regardless, the service ensures that each signature has been tied to some sort of identity and that the signatures are all valid.

4. The hash published on the blockchain again can confirm that at the time of publishing, everyone agreed to the contents of the file and that everyone signed when they did. If someone claims the file was changed, we can simply verify the signatures and as long as no one was impersonated, we can prove that signers A, B, and C all signed a copy of the same document on dates X, Y, and Z.

5. The original document is still available for signer that still has their private key, even if it's expired. Additionally, the service has their own copy of the file that can be subpoenaed if necessary.

The entire system ensures a file has not been changed in any way since it was first published (via the signatures), real people have arrested to its truth (via public key verification), and that the attestation has not changed (via publishing the hash of all signatures on the blockchain). Additionally, nothing can be deleted without signed deletion requests from all signers.

You could potentially do this without the neutral 3rd party but then there's no verification when you actually attested to its truth, only that you published that attestation on the blockchain publication date.

I wish I still had that stuff. It's not I'm any geocities archive I've seen online.

I did really well my freshman year and managed to get deans list. Then I transfered to a much more rigorous school and didn't do well. I didn't enjoy my major, I was living in a house far from campus with three other guys I didn't know and didn't really have any shared interests with. By spring, I was on academic probation. I lived like a hermit and wasn't going to class much or studying hard. I was pretty depressed and my undiagnosed ADHD didn't help. I did have some highschool friends at that school but I hadn't seen them in a while so it was hard making the effort to see them. I got on academic suspension at the end of my second year and had to take my next fall off. I waited the whole summer until I had come back to school to tell my parents because I was so ashamed. I remember my dad was trying to figure out why he hadn't gotten a tuition bill yet and called the bursar office while we were at the beach in August. They couldn't say why he hadn't gotten a bill yet so he handed the phone to me and the lady on the other end is saying "you need to tell them you're suspended." That was a gut punch. I was actually suicidal at this point but I kept it together until I wrote them a long email saying how bad things were. My dad visited me and convinced me to see a therapist. This ended up getting me an ADHD diagnosis and a prescription for Adderall. I stayed in my room a lot of that suspended fall semester but I did join a club sport (while lying about my eligibility as a full time student) which lead to making some friends. When I came back the next spring semester under a new major, I felt ready to crush it and I did. I stayed for the summer and crushed some more classes. I moved in with a friend and got along great with those guys. I was also right next to campus so there was no excuse to missing classes. I met my wife and made a huge turn around. But, those grades from my sophmore year were an albatross around my neck. They ruined my GPA. My in-major GPA was decent but I was still graduating with a 2.6 (freshman year grades didn't apply since they were transfer credits). Luckily, my parents had helped me get internships and my wife's father got me in for an interview at my current employer so GPA wasn't a huge burden to employment.

I really wish I could have dropped any of those sophmore grades. Obviously this policy wouldn't have helped me but I can certainly understand how it could help.