Month: September 2025

Seeing Shapes and Shadows: How Portrait Drawing Changes Our View of Faces | September 12 2025, 15:44

It’s interesting that even a little experience in drawing portraits makes one see patterns in other people’s faces that you wouldn’t think about otherwise. For example, you look at someone’s face, and some points on the face converge into an equilateral triangle. Or the shadow from the sun forms a notable pattern. Or some lines are strictly parallel or perpendicular. And at that moment, you feel like grabbing a pencil and trying to sketch it. At this point, it seems that achieving a likeness is a piece of cake.

Or you notice that a silvery dress is the darkest thing in the picture and probably needs to be depicted almost in black. With highlights, of course. This contradicts the notion that “a silvery dress is just a shiny white.”

Sometimes you look at someone’s face, reassured that the typical proportions are maintained, or, conversely, that they are not. There are also optical illusions. They are the most interesting. It’s when it seems that some point exactly divides a segment in half, but as soon as you measure, it turns out not to be the case.

It’s also interesting that our eyes deceive us about what lines are and what are not lines. Here, it would be more correct not to use the word line” but edge.”

Solar Simplicity: How Turkey’s Homes Heat Water with the Sun | September 11 2025, 06:44

I constantly see such panels on almost every house in Turkey. Of course, my first thought was that these were solar photovoltaic panels for generating electricity. But the second thought — they are expensive, there shouldn’t be so many of them, plus typically just two panels on a roof seems too few. I started googling.

It turned out, these are solar water heaters, more precisely, flat-plate solar collectors. The system is simple, reliable, and inexpensive — that’s why they are installed on every other house.

The principle of operation: the panels consist of an absorber (usually copper or aluminum plates with a black coating), a transparent cover (low-iron glass for greenhouse effect), and thermal insulation (glass wool or stone wool). A heat-carrying fluid circulates in the tubes — either water or antifreeze (glycol).

Solar rays heat the absorber up to 60-90°C, the heat transfers to the fluid, which by the principle of thermosiphon (natural convection, without a pump) rises to the tank, which is usually nearby. The tank is a thermos of 100-300 liters, with insulation, so the water stays hot for 2-3 days.

This too was a surprise. I actually thought the tanks were just metal and heated up in the sun by themselves. That’s how it was in Baku. It turns out, no, and so they are white here, not black.

In Turkey, with over 2000+ hours of sunshine a year, such a system covers 70-90% of the hot water needs for a home. The efficiency of the collector is 40-60% (depending on the model and angle of installation, optimally 30-45° to the horizon for the latitude of Antalya). For a family, this costs from 500-1500 euros, with a payback period of 3-5 years due to savings on gas/electricity. Electricity is expensive in Turkey. Plus, government subsidies and tax incentives encourage installation.

Probably, there are also electric panels, but I haven’t seen them yet.

Debunking Kelvin: The Dynamics of Wake Angles at Different Speeds | September 10 2025, 12:03

I look from the boat at the water and wonder whether the divergence angle of the waves depends on speed or not? Started Googling. Turns out, according to Kelvin, it’s constant at any speed, and amounts to 39 degrees (or 19.47 =arcsin(1/3) from the axis). But then I found a paper where the authors studied satellite images and disproved Kelvin, stating that with increasing speed, the wedge indeed narrows slightly (“Ship wakes: Kelvin or Mach angle?”, authors: Marc Rabaud and Frédéric Moisy)

Exploring Airport Security: How Baggage Scanners Work | September 02 2025, 20:29

The day after tomorrow, I am flying to Amsterdam (and then to Turkey), and I remembered that I had an unanswered question to myself about how baggage scanners work at the airport. Of course, I knew that it was essentially computer tomography, X-rays and all that, but I wanted more details. And below is the response as to why they ask you to take out water, and why sometimes they do not.

It turns out that modern scanners can not only see the shape of objects but also determine what material they are made of. How does a regular scanner work? Dense materials (such as metal) absorb a lot of radiation and appear bright or opaque in images. Less dense materials absorb little radiation and appear dark. Hence laptops, for example, had to be taken out — not because the scanner couldn’t recognize them, but because their dense components (battery, boards) could be used to hide other prohibited items behind them. So, it has long been not just scanners, but computer tomography — in essence, the bag or suitcase is scanned from all sides, then a 3D image is created. It seems like everyone knows this.

But I mentioned that they understand the materials items are made from. How?

It turns out that the scanner uses dual-energy X-ray technology. It scans the object with two beams of rays of different energy levels (high and low). Since materials absorb radiation differently depending on the energy of the ray and their atomic composition, the system analyzes this difference. Based on the absorption ratio of the two beams, the effective atomic number Z — a key characteristic, a kind of “elemental fingerprint” of the substance, is calculated.

The problem is that this “fingerprint” of water (~7.4) and many explosives are almost identical. This is precisely why water was banned. Relying only on this parameter would mean receiving a huge number of false alarms.

Here is where computer tomography (CT) comes into play. The scanner creates an accurate three-dimensional (3D) model of the contents of the bag. From the 3D model, the system obtains the exact volume (V) of each object. Based on data on the absorption of X-rays, its mass (m) is calculated. Then it’s simple: ρ=m/V.

That is, the system does not make a decision based on one parameter. It plots each detected substance on a two-dimensional graph with axes “Z — density.” On this graph, water and explosives, having almost the same atomic number, occupy completely different positions due to different densities.

And that’s precisely why water can sometimes be carried through. Smart machines simply do not mark it as something significant, but still identify it as water. Then procedures follow. If the airport has updated the machines, but not the procedures, they will ask to dispose of the water. But also, not all machines are updated everywhere, and at the same airport, it depends on which line is open at the moment.

The cost of such a scanner is $300-400 thousand.

The scanners for people work differently. They use millimeter waves. They pass through clothing and reflect back from the skin. Water absorbs them significantly, so they penetrate only a couple of millimeters. The system registers the reflected signal and constructs a three-dimensional map of the body surface and objects under the clothing. But it does not show this — instead, it displays a simplified contour of a person and shows on it what ML found unusual. Therefore, by the way, many try to carry various items inside themselves, knowing that such a scanner absolutely cannot see it.

Echoes of Anthrax: The Amerithrax Investigation Unveiled | September 02 2025, 13:33

From the museum of the day before yesterday. Probably, some of you remember the notorious case in 2001: shortly after the 9/11 attacks, the USA experienced a series of bioterror attacks: someone mailed letters containing powder with anthrax spores (Bacillus anthracis). This led to the deaths of 5 people and infected 17, but it could have ended much worse for the entire planet. The investigation, known as “Amerithrax,” was conducted by the FBI in collaboration with other agencies and became one of the most complex in history.

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For those who might not know — the inhalational form of anthrax has a mortality rate of 85–90% without treatment. Symptoms appear after 6 days, by which time dozens will be infected. It can’t be destroyed — spores remain viable for decades in the soil. For example, on the Scottish island of Gruinard, they lingered for nearly 50 years after wartime testing. Only after 50 years had passed and after 280 tons of formaldehyde solution had been sprayed across all 196 hectares of the island, and the most contaminated topsoil around the dispersal site had been removed, did the island become relatively safe. Thus, anthrax could easily be more terrifying than a global nuclear war.

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So, returning to the subject. Initially, suspicions fell on various individuals, including Iraq or Al-Qaeda, but no evidence was found.

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The key breakthrough was scientific examination. Scientists analyzed the anthrax strain from the letters — it was the Ames strain used in American laboratories. Using microbial forensics (genetic analysis), they identified unique mutations in the spores that narrowed the source down to flask RMR-1029 in the USAMRIID (United States Army Medical Research Institute of Infectious Diseases) laboratory at Fort Detrick, Maryland.

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In other words, every living being has names and genealogy from birth, it’s just a matter of willingness to dig into the genealogy. Apparently, controlled substances have their own registry office, so to speak.

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Bruce Ivins, a microbiologist who worked there, was the custodian of this flask and had direct access (although more than 100 others did as well).

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Later, investigators gathered circumstantial evidence. Ivins had been working late at the lab just before the mailings in September and October 2001, which was inconsistent with his usual schedule. He could not convincingly explain these hours. Moreover, in early September 2001, he was vaccinated against anthrax, which seemed suspicious. The FBI also accused him of attempting to mislead the investigation: he allegedly provided false anthrax samples to divert suspicion and attempted to frame colleagues. In 2001, Ivins sent an email to colleagues offering the Ames strain for analysis, which might have been an attempt to cover his tracks.

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Behavioral signs also played a role. Ivins suffered from depression and suicidal thoughts, especially after another suspect (Steven Hatfill) was cleared in 2008. In June 2008, he was hospitalized in a psychiatric clinic, where during therapy, he made statements that the FBI interpreted as “denials without denial” — for example, that he “had no heart for killing” and did not remember participating in the attacks.

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By 2008, the investigation had narrowed down to Ivins. When he learned that charges were being prepared against him, on July 29, 2008, he took a lethal dose of Tylenol (acetaminophen). Formal charges were never brought. In 2010, the FBI officially closed the case, declaring Ivins the sole perpetrator.

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However, the conclusions remain controversial: the US National Academy of Sciences noted in 2011 that the genetic examination was not convincing enough for a definitive conclusion, and some microbiologists, victims’ families, and politicians demanded further investigation. As of now, no new discoveries have been made, and the case is considered closed.