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Physics, Astronomy & Cosmology / Re: Why do the Earth's satellites orbit longer than the Moon's?

« on: Today at 19:36:52 »

Why do artificial satellites of the Earth rotate by inertia longer than artificial satellites of the Moon?

I agree that satellites orbit because they have inertia and a velocity relative to the parent body, which gives them angular momentum.

From a different viewpoint...
The Earth is surrounded by an atmosphere which extends beyond 100km (getting less dense the farther you go)
- This means that satellites in Low Earth Orbit (LEO) continually lose angular momentum, and their orbit decays
- The International Space Station at around 400km altitude would eventually burn up in the atmosphere if it did not get regular rocket boosts
- In contrast, the Moon has no atmosphere - any transient events like meteor impacts just result in a slightly less dense vacuum in the vicinity of the impact.
- So I expect that an artificial satellite at 150km altitude above the Moon would remain in orbit much longer than an artificial satellite at 150km above the Earth.

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Question of the Week / Re: QotW - 23.12.08 What are black holes made of?

« on: 03/12/2023 20:14:13 »

Mass and/or energy (any kind): but concentrated into such a small radius that the escape velocity exceeds the speed of light.

The most common way that black holes form today is when a massive star (>10 times the mass of the Sun) explodes as a supernova. The remnant left behind will be a black hole.

Less massive stars can form a dense neutron star; if two neutron stars later collide, they could also turn into a black hole.

The James Webb Space Telescope has revealed large active galaxies, soon after the Big Bang. These are believed to have supermassive black holes at the centre. Theoreticians are inventing new mechanisms that would allow these supermassive black holes to form in the hot, dense early universe, without first forming stars.

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COVID-19 / Re: What is "long covid" and what different types are there?

« on: 28/11/2023 09:39:29 »

Chronic Fatigue Syndrome is a well-known condition that can last for months after an infection of mononucleosis or some viruses.
- The causes are not well understood
- Some COVID patients seem to suffer something similar.
https://en.wikipedia.org/wiki/Chronic_fatigue_syndrome

COVID attacks cells via the ACE2 receptor on their surface. This receptor is involved in regulating blood pressure.
- This receptor appears on many cell types including lungs (the normal route of infection), lining of blood vessels, kidneys, and nerve-supporting cells.
- Loss of nerve-supporting cells can kill nerve cells, resulting in COVID's most distinctive symptom: Loss of taste and smell.
- If this spreads into the brain, it could be responsible for the death of brain cells - a study of patient MRIs before and after COVID showed a reduction in brain volume (faster than normal shrinkage). This could be responsible for the "brain fog" symptom which is often reported by long COVID sufferers.
- Since every organ depends on a flow of oxygen and glucose, inflammation of the blood vessels (and resulting blood clots) could damage every organ in the body (depending on where the blood clots occur), resulting in a wide array of medical conditions.

Probably, Long COVID symptoms will eventually be grouped into 5 or more clusters of symptoms - where one person may be suffering from 1 or more of these clusters.
https://en.wikipedia.org/wiki/Long_COVID

(Overlap with Paul)

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Plant Sciences, Zoology & Evolution / Re: Can Animals Appreciate Beauty ?

« on: 15/11/2023 20:34:20 »

Humans are very visually-oriented.

Other animals rely more on other senses.

I can imagine a dog appreciating a beautiful scent that would be lost on a human.

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Physics, Astronomy & Cosmology / Re: Are All The Planets Moving Away From The Sun Due To Sun Losing Mass ?

« on: 15/11/2023 09:13:18 »

In about 500 million-1 billion years (please correct  if inaccurate)

The estimate from Wikipedia is that the Sun will turn into a Red Giant in about 5 billion years.
https://en.wikipedia.org/wiki/Red_giant#The_Sun_as_a_red_giant

Either way, I will be gone by then...

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Physiology & Medicine / Re: How Does The Placebo Effect Work ?

« on: 14/11/2023 20:32:05 »

The mind does have considerable control over your body, and how you feel about your body (eg pain sensitivity).
- So telling your mind that someone you trust has prescribed this medication will probably make you feel better, even if it does nothing for an underlying biological problem.
- We saw this during the COVID pandemic, where a significant fraction of the US population believed that Ivermectin had protected them from COVID and/or cured them - because Donald Trump told them (with no clinical evidence at all, initially).

That's why the gold standard for clinical trials is to test a proposed medication/treatment against an identical-looking placebo.
- That tells you (and anyone funding your health) that paying for this medication is better than doing nothing (for free).
- Often, the placebo effect is bigger than the effect of the proposed medication! But the small effect of the medication is visible because you can "subtract out" the placebo effect.
- However, large studies have shown that Ivermectin is no better than placebo at treating COVID (with the possible exception of trials in some 2/3 world countries, where the proven de-worming effect of Ivermectin may have reduced the known immune-suppressing effects of parasitic worms, allowing a more robust immune response to COVID).

I have heard of a 2010 study that demonstrated that the placebo effect really makes people feel better, even if they know they are taking an inert substance!.
See: https://en.wikipedia.org/wiki/Placebo#Mechanisms

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Technology / Re: Why Don't Smartphones Have Antennae?

« on: 10/11/2023 23:50:14 »

If you have an old 4G iPhone like mine, there is a metallic pattern in the case, visible from the outside, with gaps in-between. Mine has:
- One metal stripe along the top (above where you hold it)
- One metal stripe along the bottom (below where you hold it)
- A large metal area across the back and sides (where you hold it)
- These metal stripes (antennas) are linked to internal circuitry, which can pick up signals from 2 antennas (or, in more recent models, more than 2 antennas), for transmission and reception.

When the original iPhone was introduced, reception was horrible. It turns out that your hand covered the antennas, attenuating the signals.
- People complained that it was a design flaw, and wanted their money back
- Apple retorted that people were holding it all wrong
- But when you look at the Apple iPhone ads, all the advertising models were holding it wrong, too.

Lesson: Don't just test it in a lab, test it in the real world, too!

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Technology / Re: Why is my sattellite receiver behaving oddly?

« on: 30/10/2023 05:54:54 »

Perhaps your satellite receiver had been preconfigured to use certain transponders?
- If a channel was shuffled to a different transponder, it would no longer be found.
- The blind scan checks all the transponders it can find.

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Physics, Astronomy & Cosmology / Re: Why isn't my salt water evaporating?

« on: 15/09/2023 09:09:02 »

May be hygroscopic salts in there - they absorb moisture from the atmosphere (which BC was alluding to).
https://en.wikipedia.org/wiki/Hygroscopy

In extreme cases, this becomes "deliquescence" - these compounds absorb water from the atmosphere, and dissolve in the extracted water.
https://en.wikipedia.org/wiki/Hygroscopy#Deliquescence

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Physics, Astronomy & Cosmology / Re: Could the James Webb telescope had saved NASA hundreds of millions?

« on: 29/08/2023 22:16:29 »

The James Webb Space Telescope cost $US10 billion. It was a gross budget blow-out!
- They considered canceling it several times, but then it became "too big to fail"

The JWST cannot take pictures of Pluto/Charon in anywhere near the same detail as an up-close fly-by.
- And it cannot take pictures of farther objects, either, like Arrakoth in the Kuiper Belt
- JWST is an infra-red telescope, and you need a much larger dish to get the same resolution as a visible-light telescope like Hubble. JWST has similar resolution to Hubble.
- Hubble was used in selecting Arrakoth as a target to visit beyond Pluto. Hubble can only see it as a faint dot of light, which moves against the background stars as Earth goes around its orbit.
- But New Horizons took high-resolution images of Arrakoth from up-close. Not possible with Hubble.

https://en.wikipedia.org/wiki/486958_Arrokoth

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Physics, Astronomy & Cosmology / Re: Why Is The Oooooooort Cloud Spherical ? ..and Not an Accretion Disc

« on: 08/08/2023 23:11:19 »

I expect it is because:
- The temperature is so low this far from the Sun, that most of the volatiles have condensed onto compact icy comets and asteroids. So there is effectively no wind resistance out there.
- The density of the icy bodies this far from the Sun is so low that they hardly ever collide with each other.
- The average size of the bodies is pretty low (eg Pluto-sized and smaller), so they don't interact strongly via gravity (we do see the occasional long-period comet)
- That means they don't exchange angular momentum, and so they stay on their original, random trajectories, forming a diffuse ball.

In contrast, the inner accretion disk (nearer the Sun) was full of hot gas, with asteroids frequently running into each other and exchanging angular momentum through collisions and (for the bigger objects) through gravitational interactions.
- As a result of gravitational interactions, many objects would have plunged into the Sun, or been thrown out of the Solar system altogether
- Eventually, after many interactions, the whole inner solar system would have settled down to the average angular momentum of the original cloud from which the inner solar system formed.
- This resulted in a planar arrangement of the planets with a common axis. This then minimised subsequent interactions, and is a fairly stable arrangement (if isolated from outside influences).

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Plant Sciences, Zoology & Evolution / Re: Has natural selection been nullified in humans ?

« on: 03/07/2023 10:46:26 »

Some Australian crows carry burning twigs from forest fires

In Australia, I have heard them called "fire eagles", rather than crows/corvids.
- The Wikipedia link describes it as a "black kite"
https://en.wikipedia.org/wiki/Fire_adaptations#Animal_use_of_fire

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COVID-19 / Re: Is it all over in Brazil?

« on: 27/06/2023 10:33:46 »

The proportion of the population that must be vaccinated against COVID-19 to begin inducing herd immunity is not known.

You can make some guesstimates based on:
- R₀: The average number of other people who will be infected by one infected individual
- Vaccine Efficacy: The percentage of people who will be protected if they are given the vaccine. Call it E

The original Wuhan virus had R₀ ≈ 2.5, and RNA vaccine efficacy E ≈ 90%.
- The minimum number to be vaccinated = 1-E/R₀ ≈ 1-0.9/2.5 = 64% of the population
- That is why early predictions said that if most people had the vaccine, it would protect the population as a whole against the spread of the virus

Two years later: It is thought that some of the more recent COVID variants have R₀ ≈ 12.
- Against these variants, the original vaccine against Wuhan strain had efficacy E ≈ 40%.
- The minimum number to be vaccinated = 1-E/R₀ ≈ 1-0.4/12 = 97% of the population
- Even well-vaccinated countries have trouble meeting this high target for vaccination
- Throw in the fact that vaccine protection declines with time, so E reduces significantly 5 months after previous vaccination or exposure

Which is why there are now bivalent vaccines available, which provide a higher efficacy against the recent variants
- And people are encouraged to get a booster every 6 months
- And still wear a mask if there is an outbreak in your area.

The COVID emergency may be over, but COVID is not over.
- A significant difference between COVID and smallpox is that smallpox only infects humans
- COVID infects many mammal species, so there is an animal reservoir of the virus that will continue mutating, and potentially jumping back into the human population. That makes elimination practically impossible with our current technologies.

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General Science / Re: Horizontal Lightning Conductors

« on: 22/06/2023 23:05:48 »

Lightning consists of one or a few closely-spaced DC impulses. A Fourier analysis shows that many frequencies make up this DC impulse.

I have done lightning testing on telecommunications equipment with a test generator that could generate several waveforms that corresponded to various standards, with peak voltages around 10,000 Volts or so. As I recall, two of them were DC impulses like the graph shown by BC (with rapid rise and slower fall, but different time constants). A third had an AC component, which you might get if the surge were inductively/capacitively coupled from an adjacent wire; in this case you don't get such a strong DC component, the higher frequencies are enhanced compared to the lower frequencies, and you get some "ringing" from the inductive/capacitive resonance.

The most spectacular example of lightning testing was at a visit to a national lightning test facility, which could generate impulses over a million volts. Understandably, the test engineer checked everything over very carefully before he took his hands out of his pockets!

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General Science / Re: Horizontal Lightning Conductors

« on: 22/06/2023 09:47:52 »

old-fashioned heavy iron down-pipes from the gutters

"Skin effect" means that high-frequency components (like lightning impulses) only travel through a thin skin on the outside of the conductor.
- That is why lightning down-conductors are made of a braid of many fine wires (with a large surface area) instead of one big conductor.
- Iron has poor conductivity
- I expect the magnetic interaction with iron would also impair the ability to handle high-frequency components...
https://en.wikipedia.org/wiki/Skin_effect

Horizontal Lightning Conductors

There is another instance where you get horizontal lightning conduction, with Fulgurites on a beach.
- The lightning turns the sand into a glassy tube
- When the lightning reaches the more conductive water, it sometimes turns into a branching pattern at the level of the water table...
https://en.wikipedia.org/wiki/Fulgurite

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General Science / Re: How does ChatGPT work?

« on: 19/06/2023 09:36:51 »

I volunteer at a group providing computer skills for retirees, and today's topic was "AI".

The guy presenting on AI today asked ChatGPT and Bard to provide a description of AI. One of them even produced a lesson plan, with a time allocation for each topic!

One thing we discovered is that when a large number of people try to contact ChatGPT simultaneously from the same IP@, it gets suspicious, and starts asking for birthdates, email addresses and phone numbers (for 2-Factor Authentication).
- It obviously suspects an AI robot attack!

PS: Bard was a lot more forgiving - we ended up with half the group on ChatGPT, and half on Bard.

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Geology, Palaeontology & Archaeology / Re: What are Rocks & Sand?

« on: 06/06/2023 10:16:46 »

sand is Valuable, besides We got soo much of it,

Sand is quite valuable, forming a large component of our roads and reinforced concrete buildings.

But not any sand will do:
- There are huge deserts in the world, but apparently wind-blown sand is too "smooth", and does not form a strong bond
- There are huge oceans in the world, but the salt in marine sand causes "concrete cancer"
- So that basically leaves river sand, of which there is a very limited supply. Dredging river sand disrupts ecosystems and is unsightly for the people who live near the rivers.

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Geology, Palaeontology & Archaeology / Re: Is there enough lithium?

« on: 01/06/2023 10:16:30 »

Apparently, Lithium has no significant new sources in the whole universe...
- Lithium was produced in the early universe, making up 10^-9 of matter from the Big Bang
- When Lithium finds its way into the core of stars, it is rapidly fused into heavier elements (ie the amount is reducing)
- When stars fuse lighter elements, they "skip over" Lithium, so no new Lithium is being produced in stars

So, it is important to wisely use what Lithium we have (and recycle what we no longer use)...
- Lithium is great for mobile applications, due to its low density
- But for stationary power storage, other chemistries are becoming competitive, like flow batteries

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COVID-19 / Re: What's included in protein-based vaccines?

« on: 29/05/2023 11:11:36 »

Every virus has a different set of proteins, so vaccines for different viruses need to emulate different proteins in order to create an effective immune response.
- But a useful target is the protein by which a virus enters the human (or animal) cell. Gum that up with antibodies, and the virus can't infect a cell - these are called neutralising antibodies.

Ultimately, the mRNA vaccines are also protein vaccines - they expose a protein to the immune system, which then targets that protein with antibodies.
- The difference is that the mRNA sequence provides the template from which the target protein is manufactured by ribosomes.

If you are talking about COVID specifically:

Like other coronaviruses, SARS-CoV-2 has four structural proteins, known as the S (spike), E (envelope), M (membrane), and N (nucleocapsid) proteins; the N protein holds the RNA genome, and the S, E, and M proteins together create the viral envelope.

The only proteins exposed to the immune system are S, E & M.
- Most existing vaccines target the Spike (S) protein, to create neutralising antibodies.
- Someone who has been infected will also develop antibodies to E & M proteins. These won't neutralise the virus, but they will draw the attention of the immune system to an infection.

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Chemistry / Re: Silicon based Life?

« on: 18/05/2023 10:41:48 »

Why haven't We seen any Fossilised Evidence of of (Silicon based Life) as Yet?

How do you know? We walk on silicon-based rocks all the time! We don't have any criteria for recognising any that may once have been alive.

As BC mentions above, at room temperature, silicates are solid, but 100km or so beneath our feet, they become plastic, and might show characteristics like movement, metabolism, reproduction, etc.
- Volcanic action can bring up rock samples from these depths, but we would just see it as a solid volcanic rock.
       - What would limit life is the amount of available energy:
       - At the surface of the Earth, we have a solar energy input of about 700W/m²
       - Under the oceans, heat flow is around 0.1 W/m² (and only 0.6 W/m² through the crust), although there are big variations, such as at volcanoes and mid-oceanic ridges.
       - This 4 orders of magnitude difference in available energy suggests that any life processes down there might be extremely slow

Perhaps we could keep our eyes out for some form of information storage: a silicate-based genetic code in rocks, which would provide the instructions for reproducing a silicate-based life form?