1

Chemistry / Re: What things can block x-ray imaging at airport customs?

« on: Yesterday at 23:43:02 »

get undetected in truck costums

Customs officials regularly X-Ray shipping containers.

Definitely do not ship people inside trucks - that is likely to turn you into a mass murderer.

2

Question of the Week / Re: QotW - 22.07.11 - How dangerous is an explosion on the moon?

« on: Yesterday at 12:10:09 »

In the very-near vacuum of the Moon, shrapnel would travel farther (not slowed by atmosphere like on Earth, and it would not fall to ground as quickly as on Earth).

The shockwave behaviour will also be different:
- One the Moon, an explosion will produce a single outward shockwave of gas from the explosion
- On Earth, the hot gas expands, then atmospheric pressure collapses the hot bubble, which then rebounds, producing a series of in-and-out shockwaves.

Given the current density of people on the Moon, an explosion there is likely to kill fewer people than the same explosion on Earth.

3

Geology, Palaeontology & Archaeology / Re: Could There Be Dinosaurs Floating In Space ?

« on: Yesterday at 02:54:07 »

The amount of energy required to launch a piece of rock into orbit is enough to melt rock.
That same amount of energy would destroy flesh and bone.
https://en.wikipedia.org/wiki/Microspherulite#Aerial_environment

4

Physics, Astronomy & Cosmology / Re: What is a black hole made of?

« on: 05/07/2022 11:30:02 »

As the event horizon tries to form in the first place any matter inside gets expelled.

This statement seems to imply that gravitation gets reversed during the formation of a black hole. That seems to violate everything we know about gravitation (without actually having a validated theory of quantum gravity).

As I understand it, gravitation always attracts and never repels. The material on a 1-way trajectory towards the center of a black hole doesn't suddenly reverse direction (I exclude elliptical orbits that do not intersect the event horizon).

Part of the problem is that different frames of reference will see different things as a test particle approaches an event horizon of radius (say) 10km:
- From the frame of reference of a test particle approaching the singularity: It will speed up to a significant fraction of c, and pass the the 10km radius in a short time, without experiencing anything special.
- From the frame of reference of a distant observer: A test particle approaching the singularity will speed up to a significant fraction of c, but will continue to emit severely red-shifted photons from just outside the 10km event horizon, for (potentially) a long time.
- However, while an event horizon is growing, my guess is that any photons emitted from an infalling particle will find themselves inside the new event horizon, and will not reach a distant observer.

5

New Theories / Re: Is Mathematics Unintuitive?

« on: 04/07/2022 11:30:34 »

There are some people for whom mathematics is very intuitive. Évariste Galois was denied any mathematical education - partly because he immediately jumped to the answer, and didn't show any working (he didn't need any intermediate steps). Not helped by the fact that France was in a class war at the time, and coming from a poor family, he did not get on with the nobility in the schools, or the people running the schools for the nobility.

He invented a field of mathematics now called Galois fields, which has been used in cellphones and optical fiber networks.
https://en.wikipedia.org/wiki/%C3%89variste_Galois

6

General Science / Re: electromagnetic waves detail

« on: 03/07/2022 10:57:10 »

"Lines of force" was translated by my old physics teacher as equipotential contours

In a "bar magnet" magnetic field, the equipotentials would be a a set of nested doughnut-shaped surfaces.
- The Earth's magnetic field can be roughly approximated as a bar magnet
- If electrons orbited the equipotential surface, they would need to circle the entire planet
- This does not align with the observed linear structures in the aurora
- These lines of force seem to follow the path of a small "test compass", from one pole to the other; a path rather than a surface

This is equivalent to the situation with an electric dipole, where the lines of force follow the path of a small test charge. I expect these would cross the equipotential surfaces at right angles.

7

New Theories / Re: What makes Riemann's Hypothesis Hard to Prove?

« on: 03/07/2022 10:45:45 »

some problems are impossible to solve- That's the incompleteness theorem

The Incompleteness Theorem applies within a specific domain of mathematics - it may be impossible to prove some true statements within the axioms of that system.

However, some mathematical breakthroughs occur when applying results from a quite different domain of mathematics.
- Effectively, this extends the original set of axioms with an additional set of axioms over a different domain
- There may be additional true statements within the new, extended set of axioms that are unproveable within that extended set of axioms.

8

Just Chat! / Re: Why the fuss about "neguinho" (nigger) common in English until 1960

« on: 03/07/2022 10:33:19 »

Politically correct terms change over time.
- Even terms that are introduced as being more politically correct are later canceled.

negro (Spanish) ≡ black (English)

I understand that at one time, "negro" was unacceptable in the USA, but "black" was acceptable.
- I'm not sure what is acceptable in the USA today.

The current trend in Australia is to use "First Nations".

9

New Theories / Re: How Many Numbers Exist?

« on: 03/07/2022 07:06:58 »

∞^∞ = ∞

However, 2^∞ > ∞
(At least for the counting numbers), so this one seems suspect.

∞−∞ and ∞/∞) are not generally well-defined.

In practice, this means that you need more information to determine the answer.

For example, L'Hopital's rule allows you to calculate
Limit of a/b as a→∞  and b→∞
in those scenarios where you know the derivative of a and b (and both aren't infinite).

10

General Science / Re: electromagnetic waves detail

« on: 02/07/2022 23:32:16 »

surely some harmonics would be detectable

The dispersive nature of the plasma means that these are pure sine-waves - all the harmonics are removed, and separated in time.
Before it is dispersed, you get a wideband click, which would be audible on LW AM radio.

11

General Science / Re: electromagnetic waves detail

« on: 02/07/2022 13:47:45 »

Here are some more:
https://space-audio.org/sounds/EarthWhistlers/ewhist.html

How a disturbance could travel along an imaginary line, I don't know.

Electrons spiral around magnetic lines of force, producing the visible lines of the aurora.
- Iron filings line up along the field lines from a permanent magnet - the traditional high-school demonstration to visualize invisible lines of magnetic force.

Lightning impulses can generate pulses of electromagnetic energy (and even X-Rays, positrons and gamma rays) in the upper atmosphere.
- These pulses of energy start with energy at all frequencies (this sounds like a click or crackle).
- But the plasma in near-Earth space transmits carries this impulse in such a way that different frequencies travel at different speeds, producing the whistler effect.
- In a sense, this is what happens to light from the Sun when we see a rainbow - different frequencies are separated because they travel at different velocities.

12

Just Chat! / Re: Best oil for frying "fish n chips"?

« on: 30/06/2022 08:54:23 »

french fries (chips to you guys)

Belgians will always remind you that "French Fries" were actually invented by a Belgian...
- The town of Namur is in Belgium today
- But the nation of Belgium did not exist in the 1600s
https://www.tastingtable.com/740270/the-origin-of-french-fries-might-surprise-you/

13

Technology / Re: What Question Could You Ask To Determine Sentience Of An AI ?

« on: 29/06/2022 22:58:01 »

It's highly likely ai has surpassed some animal life forms in consciousness.

AI researchers would dearly love to match the unconscious capability of insects - dragonflies routinely dodge obstacles and catch prey in flight, their brain and eyes using a miniscule amount of energy in a tiny volume. Drone designers would love to match that!

Similarly, early self-driving cars used about a kilowatt of CPU, while humans do it using perhaps 2 Watts out of the 20 Watts consumed by our brains.

14

General Science / Re: How much of me is original?

« on: 29/06/2022 22:53:11 »

Some cells are not replaced during your lifetime. This applies to the lens of your eye, and brain cells.
- Most parts of the cell are generated from instructions in the DNA, using new raw materials from your diet
- But if the cell does not divide after birth (eg brain), it can continue to use the same DNA with which you were born
- Some parts of the brain do generate new cells (eg hippocampus), but this does not apply to most of the brain
- There will be spot repairs to DNA when it is damaged by natural radioactivity or metabolic stress, and these repairs would be made from "new" atoms in your diet.

I understand that some of these DNA ages were determined by measuring the radioactive content of DNA from different tissues. The isotopic mix in the diet changed noticeably in the years that atmospheric nuclear testing was underway.
https://www.sciencefocus.com/the-human-body/what-cells-in-the-human-body-live-the-longest/

15

Technology / Re: Are solar panels worthwhile?

« on: 29/06/2022 10:08:10 »

I should think Australia is not far from being carbon neutral?...just like Canada

Australia is one of the highest CO2 emitters in the Western world (per capita), at 17 tons per capita per year
- Behind Canada at 18.6
- But more than USA at 15.5 tones

This is due to:
- Base load is mostly burning coal
- One of the largest states (Victoria) uses brown coal, which is saturated with water; the water must be driven off before you can get it to burn - very inefficient!
- Political backlash against nuclear power over many years (even though Australia exports a lot of uranium ore)
- A carbon tax implemented in 2011 was quickly reversed by a change of government
- It is a fairly flat and dry country, so not much hydro power
- Far from plate boundaries, so not much geothermal power
- Neglected transmission infrastructure, so it is hard to feed in renewables
- The fossil fuel industry has a lot of money from exports, which funds their vigorous campaign to keep burning fossil fuels locally.
- NIMBY: Even if most people want clean power, everyone says "Not In My Back Yard"!

See: https://www.worldometers.info/co2-emissions/co2-emissions-per-capita/

16

Physics, Astronomy & Cosmology / Re: What is a black hole made of?

« on: 28/06/2022 23:22:22 »

...the 'Firewall model' of black holes where all the mass of a black hole is concentrated at the event horizon

Let's take the scenario where a neutron star draws more and more material from an orbiting star onto its surface - until it collapses into a black hole perhaps 10miles across.

Within that 5 mile radius of the event horizon, there would already have been a considerable amount of matter while it was still a neutron star. So that can't really still be stuck on the event horizon.

If it were possible to observe the formation of the black hole (eg with a neutrino telescope), the core of the neutron star might be creeping inwards, just a micrometer from the event horizon - but the event horizon would be bounding outwards by yards and miles. So that material would no longer be stuck on the event horizon (from the viewpoint of a distant observer).
https://en.wikipedia.org/wiki/Neutron_star#X-ray_binaries

17

Physics, Astronomy & Cosmology / Re: Why is the Barycenter Equation expressed in terms of mass?

« on: 28/06/2022 23:09:39 »

There's no extended transition zone where the weakening gravity between the Earth and moon...

One possible result of the Moon drifting away from Earth into its own orbit is called a "Horseshoe orbit".
- Both Earth and Moon remain at the same average distance from the Sun
- But the Moon drifts ahead of the earth in its orbit, until it catches up on the other side, then reverses course, forming the horseshoe shape.
- When the Moon is "ahead" of the Earth in its orbit,  the Earth will accelerate towards the barycenter, and accelerate slightly in its orbit
- When the Moon is "behind" the Earth in its orbit,  the Earth will accelerate towards the barycenter, and decelerate slightly in its orbit
- When the Moon is on the far side of the Sun, the Earth will still accelerate towards the barycenter, but this acceleration is miniscule compared to its acceleration towards the Earth-Sun barycenter
See https://en.wikipedia.org/wiki/Horseshoe_orbit

18

Physics, Astronomy & Cosmology / Re: What is a black hole made of?

« on: 28/06/2022 03:56:17 »

What is a black hole formed from?

Most of the black holes formed today come from the collapse of the core of a star.
- The temperature of a star is so high that electrons are separated from the nucleus of an atom, forming a plasma
- Since electrons & nuclei are "sub-atomic", you could say that black holes are formed from sub-atomic particles.
- As the core of the star collapses, many of the protons and electrons are converted to neutrons (a sub-atomic particle), releasing a burst of neutrinos (which mostly escape). I vaguely recall that around 90% of the energy released in a supernova is in the form of neutrinos.

There is some speculation from cosmologists that, during the early stages of the Big Bang, some black holes may have formed even before protons, electrons and neutrons were  formed. Atoms can't exist at these extreme energy levels, so you might call these "pre-atomic" particles(?)
- Cosmologists think that some of these relic black holes may still exist today

black holes might be viewed not as objects at all but  as extreme spacetime  curvature

Once a black hole has formed, the extreme curvature of spacetime effectively pinches off the singularity from the view of those outside the event horizon.
- At this stage, the only thing you know about the black hole is its mass, spin, and electrical charge.
- At this point, it really doesn't matter what the black hole formed from, because that is not detectable from outside.

19

Physics, Astronomy & Cosmology / Re: Why is the Barycenter Equation expressed in terms of mass?

« on: 28/06/2022 03:30:58 »

Why is the Barycenter Equation expressed in terms of mass?

Because it literally means "the center of mass"...
- I guess if you wanted to be pedantic, it might be debatable whether the "bary-" part is talking about mass or weight...
- but since the ancient Greeks breathed air and lived within a couple of kilometers of sea level, it doesn't make too much of a difference
- unless you were trying to detect an adulterated gold crown!

20

Geek Speak / Re: Going from android to Iphone

« on: 27/06/2022 10:07:38 »

I had a recent changeover problem between laptops.
- Microsoft teams refused to say I was active on the new laptop
- Because I was still signed in on the old laptop (even though it was powered down)

It seems that Microsoft Teams can cope with someone having both a laptop and a smartphone
- But someone with two laptops is unthinkable!