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the left end of the rod is an observation event at a given position at a given time: $x_{1}$ and $t_{1}$ for $S$ or $x_{1}^{'}$ and $t_{1}^{'}$ for $S^{'}$
the right end of the rod is an observation event at a given position at a given time : $x_{2}$ and $t_{2}$ for $S$ or $x_{2}^{'}$ and $t_{2}^{'}$ for $S^{'}$

Note that what you visually observe on a photograph is a different measurement to the more precise/easy to calculate two event measurement. On a photograph, it seems you might not even see the contraction in some cases as mentioned at en.wikipedia.org/wiki/Terrell_rotation

Measuring a length means to measure the

x_{2} - x_{1}

difference for a single point in time in your frame (

t 2 = t 1

So what we want to obtain is

x_{2}^{'} - x_{1}^{'}

for any given time

t^{'} 2 = t^{'} 1

In summary, we have:

L L^{'} = x_{2} = x_{2}^{'} - x_{1} - x_{1}^{'} t_{2}^{'} = t_{1}^{'}

(1)

By plugging those values into the Lorentz transformation, we can eliminate

t_{2} a n d t_{1}

, and conclude that for any

t_{2}^{'} = t_{1}^{'}

, the length contraction relation holds:

L^{'} = \frac{L}{γ}

(2)

The key question that needs intuitive clarification then is: but how can this be symmetric? How can both observers see each other's rulers shrink?

And the key answer is: because to the second observer, the measurements made by the first observer are not simultaneous. Notably, the two measurement events are obviously spacelike-separated events by looking at the light cone, and therefore can be measured even in different orders by different observers.

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Roguelike by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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Survivorship bias by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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Cyrillic script by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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2013 DNS census MX records by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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Let' see if there's anything in records/mx.xz.

mx.csv is 21GB.

They do have " in the files to escape commas so:

mx.py

import csv
import sys
writer = csv.writer(sys.stdout)
with open('mx.csv', 'r') as f:
    reader = csv.reader(f)
    for row in reader:
        writer.writerow([row[0], row[3]])

Would have been better with csvkit: stackoverflow.com/questions/36287982/bash-parse-csv-with-quotes-commas-and-newlines

then:

# uniq not amazing as there are often two or three slightly different records repeated on multiple timestamps, but down to 11 GB
python3 mx.py | uniq > mx-uniq.csv
sqlite3 mx.sqlite 'create table t(d text, m text)'
# 13 GB
time sqlite3 mx.sqlite ".import --csv --skip 1 'mx-uniq.csv' t"

# 41 GB
time sqlite3 mx.sqlite 'create index td on t(d)'
time sqlite3 mx.sqlite 'create index tm on t(m)'
time sqlite3 mx.sqlite 'create index tdm on t(d, m)'

# Remove dupes.
# Rows: 150m
time sqlite3 mx.sqlite <<EOF
delete from t
where rowid not in (
  select min(rowid)
  from t
  group by d, m
)
EOF

# 15 GB
time sqlite3 mx.sqlite vacuum

Let's see what the hits use:

awk -F, 'NR>1{ print $2 }' ../media/cia-2010-covert-communication-websites/hits.csv | xargs -I{} sqlite3 mx.sqlite "select distinct * from t where d = '{}'"

At around 267 total hits, only 84 have MX records, and from those that do, almost all of them have exactly:

smtp.secureserver.net
mailstore1.secureserver.net

with only three exceptions:

dailynewsandsports.com|dailynewsandsports.com
inews-today.com|mail.inews-today.com
just-kidding-news.com|just-kidding-news.com

We need to count out of the totals!

sqlite3 mx.sqlite "select count(*) from t where m = 'mailstore1.secureserver.net'"

which gives, ~18M, so nope, it is too much by itself...

Let's try to use that to reduce av.sqlite from 2013 DNS Census virtual host cleanup a bit further:

time sqlite3 mx.sqlite '.mode csv' "attach 'aiddcu.sqlite' as 'av'" '.load ./ip' "select ipi2s(av.t.i), av.t.d from av.t inner join t as mx on av.t.d = mx.d and mx.m = 'mailstore1.secureserver.net' order by av.t.i asc" > avm.csv

where avm stands for av with mx pruning. This leaves us with only ~500k entries left. With one more figerprint we could do a Wayback Machine CDX scanning scan.

Let's check that we still have most our hits in there:

grep -f <(awk -F, 'NR>1{print $2}' /home/ciro/bak/git/media/cia-2010-covert-communication-websites/hits.csv) avm.csv

At 267 hits we got 81, so all are still present.

secureserver is a hosting provider, we can see their blank page e.g. at: web.archive.org/web/20110128152204/http://emmano.com/. security.stackexchange.com/questions/12610/why-did-secureserver-net-godaddy-access-my-gmail-account/12616#12616 comments:

secureserver.net is the name GoDaddy use as the reverse DNS for IP addresses used for dedicated/virtual server hosting

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Rust library by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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Synopsys by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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NIST Post-Quantum Cryptography Standardization by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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This post-quantum cryptography competition by NIST is a huge milestone of the field.

It was mind blowing when in 2022, after several years of selection, one of the 7 finalists was broken on a classical computer, not even in a quantum computer! news.ycombinator.com/item?id=30466063 | eprint.iacr.org/2022/214 Breaking Rainbow Takes a Weekend on a Laptop by Ward Beullens. Dude announced he had a break a few days before submission: twitter.com/WardBeullens/status/1492780462028300290 On Twitter. He's so young. Epic.

Edit: and then, after the third round, things were a bit unclear, so they made a fourth round with 4 choices out of the 7 from round 3, and in August 2022 one of the four was broken again on a classic CPU!!! OMG: arstechnica.com/information-technology/2022/08/sike-once-a-post-quantum-encryption-contender-is-koed-in-nist-smackdown/

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Drosophila melanogaster by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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Electronics vendor by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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Assertion (software development) by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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United States diplomatic cables leak by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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GlobalFoundries by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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AMD just gave up this risky part of the business amidst the fabless boom. Sound like a wise move. They then fell more and more away from the state of the art, and moved into more niche areas.

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Anaerobic organism by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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JavaScript library by

Ciro Santilli 35  Updated 2025-01-29  +Created 1970-01-01

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 Pinned article: ourbigbook/introduction-to-the-ourbigbook-project

Welcome to the OurBigBook Project! Our goal is to create the perfect publishing platform for STEM subjects, and get university-level students to write the best free STEM tutorials ever.

Everyone is welcome to create an account and play with the site: ourbigbook.com/go/register. We belive that students themselves can write amazing tutorials, but teachers are welcome too. You can write about anything you want, it doesn't have to be STEM or even educational. Silly test content is very welcome and you won't be penalized in any way. Just keep it legal!

Video 1.

Intro to OurBigBook

. Source.

We have two killer features:

topics: topics group articles by different users with the same title, e.g. here is the topic for the "Fundamental Theorem of Calculus" ourbigbook.com/go/topic/fundamental-theorem-of-calculus
Articles of different users are sorted by upvote within each article page. This feature is a bit like:
- a Wikipedia where each user can have their own version of each article
- a Q&A website like Stack Overflow, where multiple people can give their views on a given topic, and the best ones are sorted by upvote. Except you don't need to wait for someone to ask first, and any topic goes, no matter how narrow or broad
This feature makes it possible for readers to find better explanations of any topic created by other writers. And it allows writers to create an explanation in a place that readers might actually find it.
Figure 1.
Screenshot of the "Derivative" topic page
. View it live at: ourbigbook.com/go/topic/derivative
Video 2.
OurBigBook Web topics demo
. Source.
local editing: you can store all your personal knowledge base content locally in a plaintext markup format that can be edited locally and published either:
- to OurBigBook.com to get awesome multi-user features like topics and likes
- as HTML files to a static website, which you can host yourself for free on many external providers like GitHub Pages, and remain in full control
This way you can be sure that even if OurBigBook.com were to go down one day (which we have no plans to do as it is quite cheap to host!), your content will still be perfectly readable as a static site.
Figure 2.
You can publish local OurBigBook lightweight markup files to either OurBigBook.com or as a static website
.
Figure 3.
Visual Studio Code extension installation
.
Figure 4.
Visual Studio Code extension tree navigation
.
Figure 5.
Web editor
. You can also edit articles on the Web editor without installing anything locally.
Video 3.
Edit locally and publish demo
. Source. This shows editing OurBigBook Markup and publishing it using the Visual Studio Code extension.
Video 4.
OurBigBook Visual Studio Code extension editing and navigation demo
. Source.
Internal cross file references done right:
Infinitely deep tables of contents:
Figure 6.
Dynamic article tree with infinitely deep table of contents
.
Live URL: ourbigbook.com/cirosantilli/chordate
Descendant pages can also show up as toplevel e.g.: ourbigbook.com/cirosantilli/chordate-subclade

All our software is open source and hosted at: github.com/ourbigbook/ourbigbook

Further documentation can be found at: docs.ourbigbook.com

Feel free to reach our to us for any help or suggestions: docs.ourbigbook.com/#contact

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