1

Just Chat! / Re: Is METAPHYSICS a dirty word?

« on: 19/09/2021 11:53:40 »

According to Wikipedia, nearly all (97%) of articles in Wikipedia end up in the article about Philosophy...
...if you go down the rabbit hole, and click on the first non-pronounciation hyperlink in every article that you come to
See: https://en.wikipedia.org/wiki/Wikipedia:Getting_to_Philosophy

So (at least in Wikipedia hyperlinks), Philosophy itself is the subject that deals with the first principles of things, including abstract concepts such as being, knowing, substance, cause, identity, time, and space.
...The Wikipedia article on Metaphysics points back to the article on Philosophy...

2

Physics, Astronomy & Cosmology / Re: Could we fathom/plumb the depth of the Earth?

« on: 17/09/2021 22:26:54 »

The 2012 version of Total Recall imagined the working class living on the far side of the Earth, and commuting to work via an elevator drilled through the center of the Earth. 42 minute commute time isn't too bad (but not as good as working from home during lockdown).

They had the sensible idea that when you came out the far side, you would be upside-down, so the elevator turns upside down as it is passing through the center of the Earth - that way, gravity is pointing the right way when the elevator stops on the far side.

Someone obviously told them that there is a zone of weightlessness at the center of the Earth, and in this clip, you can see the star playing in zero gravity as they pass through the core of the Earth (they make use of it later in the film, too).

Of course, while the elevator is in free-fall, it would be weightless all the way through the Earth (until they stopped on the far side). And they had a funny idea about where the antipodes are, relative to London...

3

Physics, Astronomy & Cosmology / Re: Could we fathom/plumb the depth of the Earth?

« on: 17/09/2021 10:56:31 »

if we lower/ed that mass/weight

Since we are in Physics Land, we assume that the rope has:
- zero mass (so it doesn't keep falling towards the center of the Earth, even after the central mass has come to rest),
- and is inextensible and inelastic (so that releasing and stopping the mass doesn't set up long-period oscillations)

4

Physics, Astronomy & Cosmology / Re: Could we fathom/plumb the depth of the Earth?

« on: 17/09/2021 10:12:10 »

if we lower/ed that mass/weight slowly, why wouldn't it just stop at the point of max g?

If we lowered the mass 100m, stopped it, then lowered it another 100m....
- When the mass gets to the center of the Earth and we stop it - when we release it, it will just stay there.

5

Physics, Astronomy & Cosmology / Re: Could we fathom the depth of the Earth?

« on: 16/09/2021 10:20:04 »

Why China?

It might be before your time: "The China Syndrome" movie was a box-office hit in 1979.

The title comes from the myth that if a nuclear reactor had a meltdown, it would melt its way all the way through the Earth, and come out on the other side (China being an exotic and distant location on "the other side of the Earth").
See trailer: https://www.imdb.com/title/tt0078966/

PS: This came up on another thread, talking about nuclear meltdowns.

6

General Science / Re: I can see infrared and hear ultrasound. Am I an abnormal human?

« on: 10/09/2021 08:41:41 »

Naked eye! ... i just want to know why?

The human eye has 3 color-sensing cones: roughly red, green and blue (some color-blind people can see only 2 colors directly).

But within these, there is genetic variation - there are two common variants of the red gene that have different spectral responses, allowing some women to see 4 colors directly.

It is possible that you have a genetic variant that has slightly better response into the longer wavelengths, or near-infra-red.
- These infra-red photons carry low energy, so it is hard for infra-red photons to activate chemical pathways
- Without these sensitive pathways being activated all the time by normal body temperature...

You could test this by getting a prism to split light into a rainbow.
- Get another person to mark on a piece of paper the limits of visible light they can see, from red to violet.
- Then you mark the paper with the colors you can see, from red to violet
- See how much they overlap. The Sun produces quite a lot of light at infra-red, and glass is somewhat transparent to near-infra-red.
- For a better test, use a diffraction grating, but they tend to be harder to obtain than a prism.

See: https://en.wikipedia.org/wiki/Tetrachromacy#Humans

7

New Theories / Re: Does Free-Will Exist ? (Lightweight topic......... lol)

« on: 09/09/2021 22:05:23 »

This discussion is often debated in the context of a "Block Universe" hypothesis: If you could imagine someone outside our spacetime, they would be able to see the:
- The past and future trajectory of the Earth around the Sun
- Your past and future trajectory on the Earth (do I go to Edinburgh or Paris for my vacation?)

See: https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time)

At the other extreme, the "Many Worlds" hypothesis suggests that maybe one version of you goes to Edinburgh while another version of you goes to Paris.

What makes these pure philosophy is that they assume things that are untesteable by us:
- We are locked inside our space time, so we can't see a block universe
- We are locked inside one world, and we cannot see many worlds.

8

General Science / Re: compressing hot air

« on: 05/09/2021 11:37:51 »

transfer hot air from a fir room to the liquid

Should I assume that "fir room" is intended to be a "fire room", like the boiler room of a power station?
- Some ideas for collection of CO₂ from a power station involve passing the exhaust gases through a solution which absorbs CO₂ (eg a sodium hydroxide solution).
- There are certainly concerns about the loss of efficiency that this imposes on the power station (so it produces even more CO₂!).

The first point is to say that the exhaust gases from a fire are extremely hot.
- Ceramics are one of the few materials that can withstand such temperatures and corrosive gases.
- Conventional air compressors use components like rubber gaskets and oil to lubricate bearings. These will rapidly fail if you use them to pump the products of combustion
- You would be more successful to use the air compressor to compress the cool air being fed into the fire, so the hot exhaust gas is also under pressure, and can be fed into the liquid.
- You should also note that the exhaust gas from a fire is well above the boiling temperature of water, so the water will quickly evaporate away unless it is kept under pressure and/or actively cooled.

See: https://en.wikipedia.org/wiki/Carbon_capture_and_storage#Capture

...but I may have misinterpreted "fir" - maybe a sauna room was intended?

9

Physics, Astronomy & Cosmology / Re: Lagrangian Mechanics - Why is Nature lazy?

« on: 03/09/2021 23:13:12 »

Chemists and Biologists don't seem to assume that every system they want to study and model can be represented with a Lagrangian

When a protein molecule is being assembled by a ribososome, it is a linear string, continually being jostled by water molecules, salt ions and general thermal motion.
- Rather than forming random strings, balls and knots, it usually ends up in a very particular shape.
- This is due to the fact that some parts of the protein are more positive or more negative, or more hydrophobic or more hydrophilic, so some shapes will be more stable than others (ie the protein end up with a slightly lower potential energy, and the water molecules end up with a slightly higher kinetic energy: that sounds a bit like a Lagrangian?)
- This process is slightly aided by the protein being formed in several strands, and being spliced together or cut apart by enzymes, or self-assembling due to mutual attraction.
- There are several genetic spellings for each amino acid (61 sequences which specify 20 amino acids), not all of which have equal concentration in the cell. I have seen suggestions that some of these "slower" DNA sequences might allow more time for the protein to settle into the right conformation before adding more protein units.
 
Biologists have the opposite problem: they have a string of DNA letters, from which they can deduce a list of transcribed mRNAs (or read the mRNAs directly). They can tell what linear string of proteins will be produced by a ribosome, but it's incredibly hard to tell what shape the protein will fold into.
- The traditional method was to extract and purify the protein, crystallize it and obtain X-Ray diffraction images. But some proteins are resistant to crystallization, or break down too easily, or just fold into a different shape when the water is removed (all these problems plagued Rosalind Franklin when studying the structure of DNA).
- Computer modeling has been used to find shape of a protein with the lowest potential energy, but it requires big computers
- More recently, it has been turned into a competitive computer game: FoldIt
 

- It looks like Google has got into the act with AlphaFold, which is trying to emulate humans playing FoldIt, but taking into account the success rating from tests in the lab to produce a better prediction.

See: https://fold.it/portal/info/about
https://deepmind.com/blog/article/AlphaFold-Using-AI-for-scientific-discovery

10

General Science / Re: Science Photo of the Week

« on: 02/09/2021 23:57:03 »

This little diversion was a reference to the movie "The Dish", which is about the role that the Parkes radio-telescope played in televising Armstrong's first steps on the Moon, back in 1969.

There is a scene in the movie where the operators of the telescope point the dish straight up, and then play cricket in the bowl. No-one will confirm or deny that such an event actually took place.

See trailer with Sam Neil at: https://www.imdb.com/title/tt0205873/

11

Physics, Astronomy & Cosmology / Re: Lagrangian Mechanics - Why is Nature lazy?

« on: 02/09/2021 23:13:00 »

Do the biologists ... use similar equations?

The Least Action Principle refers to "the action is stationary (no change) to first order".
- This implies that there is some differentiable function describing all the alternative ways the system could evolve
- And the path taken is the path which has a stationary point (minimum, maximum or saddleback in some multiple-dimensional space).

However, at its heart, biology is based  on genetics, which is digital rather than analog.
- Differentiating digital functions rarely produces stationary points (unless the digital function is a constant)
- So genetic changes in the offspring can produce no visible change, or a total change (death), or somewhere in between.
- And it can be the same change (eg changing a DNA letter from A to C) that can produce any of these wide variety of outcomes
- One change (A to C) might change the color of your hair, while the same change in a different part of your DNA may provoke early-onset diabetes.
- Both changes have the same action. But because it is digital rather than analog, the effort to make the change is non-zero, so the code is (in some sense) preserved; the system does not slide smoothly from one state to another state, as implied by the Least Action principle, guided by a Lagrangian.

12

Physics, Astronomy & Cosmology / Re: What is the gravity, GTD, temperature and pressure at the centre of the Earth?

« on: 31/08/2021 23:24:54 »

So, if a nuclear reactor could actually melt its way into/thru the Earth, where would it end up?

The premise in the title of the 1979 movie "The China Syndrome" was that a nuclear reactor would melt its way all the way through the Earth and come out at China (supposedly a distant and exotic place on the other side of the Earth).
https://www.imdb.com/title/tt0078966/

At the start of the outer core, where the max/imum g is, or the centre?

Silicate materials like concrete are not dense enough to sink into the core - they would stay in the mantle.
- The mantle is pretty viscous stuff - how far the reactor components would fall into the mantle depends on convection currents - if there are upwards convection currents in the mantle, then it won't sink at all, and will stay near the crust. But if there are descending convection currents in the mantle, it will descend faster.
- How far into the core it travels depends on solubility - iron and nickel components will dissolve into the nickel/iron core, but silicates will not.
- How far the radioactive iron would travel into the core depends on whether the core is solid or liquid. Movement through liquid outer core is assisted by convection, but it will be blocked at the inner solid solid core.
- Uranium forms stable oxides, which means it will tend to stay in Earth's crust (a "lithophile"), rather than descend to the core and dissolve in the iron core.
See the orange lithophile group of elements here: https://en.wikipedia.org/wiki/Goldschmidt_classification

In reality, at a real reactor meltdown like Fukushima, it is believed that a majority of the radioactive omelet of fuel/cladding/reactor shell is still in the basement of the reactor building.
- This is because it takes a careful mix of enriched uranium and moderator to sustain an ongoing nuclear chain reaction.
- Once you mix the fuel with concrete and rock, and dilute the moderator with normal water, the chain reaction slows down enormously, and the heat output is significantly reduced.
- It is still an environmental disaster with a large exclusion zone, but it won't get through enough of the Earth's crust  to join the subducting basaltic ocean plate descending into the mantle below Fukushima (which is what caused the massive earthquake and tsunami in the first place).

13

Physics, Astronomy & Cosmology / Re: How does a/the Mach-Zehnder interferometer really work?

« on: 31/08/2021 22:46:38 »

One of the fastest growing applications of the Mach-Zehnder interferometer is in the internet, used to modulate laser beams in optical fibers.

This uses a crystal whose optical phase delay is affected by the applied electrical field (eg GaAs). By splitting the optical path, and applying an electrically-controlled phase delay, the laser can be recombined to produce constructive or destructive interference.

This can be done in simple ways, just turning the laser on and off (typically used up to 25Gbps), or in more sophisticated ways by superimposing a modulation which varies in both amplitude and phase (typically used at 100Gbps and above).   

The first example on the following page gives the general idea, but it also describes some more effective (and complex) implementations further down.
See: https://www.synopsys.com/glossary/what-is-a-mach-zehnder-modulator.html

14

Physics, Astronomy & Cosmology / Re: What is the gravity, GTD, temperature and pressure at the centre of the Earth?

« on: 30/08/2021 23:14:12 »

What is the ... temperature and pressure at the centre of the Earth?

This part of the question is somewhat independent of the gravitational aspects
- Although temperature and pressure do affect density a bit

It is thought that the center of the Earth is a nickel/iron alloy similar to that seen in metallic meteorites.
- It is still cooling down from the temperature of Earth's formation
- A process which is slowed by the decay of radioactive elements

It is thought that the center of the Earth has a temperature of around 5,400C, which is similar to the surface of the Sun.
- The pressure is about 3 million times atmospheric pressure at the surface.

See: https://en.wikipedia.org/wiki/Earth%27s_inner_core

An upcoming space probe will visit a metal-rich asteroid Psyche, which may give some ideas about the center of the Earth. Planned launch date is July 2022.
https://en.wikipedia.org/wiki/Psyche_(spacecraft)

15

Just Chat! / Re: In defence of U

« on: 30/08/2021 23:00:14 »

If I deserve a cap/ital, then surely so do U, too.

The Dutch/Flemish language does exactly that.
- The translation of "you" is "U"
- This language had a big impact on Old English, with the invasion of the Angles et al
- But Dutch (unlike English) has an ongoing project to simplify the spelling and align it with pronunciation.
- The last time English tried that, we ended up with colour/color and many other inconsistencies, and I think they are too scared to try again!

See: https://en.wikipedia.org/wiki/History_of_Dutch_orthography

16

Physics, Astronomy & Cosmology / Re: How does a diffraction grating really work?

« on: 28/08/2021 11:34:19 »

I'd start with a CD unless you have a "proper" grating handy.

I'd start by explaining that before we had USB sticks or the cloud, we had CDs...

17

Physics, Astronomy & Cosmology / Re: How does a diffraction grating really work?

« on: 27/08/2021 22:44:07 »

If you have a beam of light falling on a Diffraction Grating (DG), you can treat every ridge on the diffraction grating as a source of light, with aligned phases.

So you could model a 1-dimensional diffraction grating, with 1,000 ridges (light emitters) every 1 um.
- Then you calculate the distance to the screen (in micrometers)
- And sum the phase and intensity of the light striking the screen from each ridge
- Repeat for every position on your screen
- Repeat for about 7 colors in the rainbow.

This computer model uses Huygens wave model of light. There are no photons to "go through" any ridge of the diffraction grating; the wave is emitted from all of the ridges.

With the terminology: There are reflective diffraction gratings, and transparent ones...
https://en.wikipedia.org/wiki/Diffraction_grating

18

Physiology & Medicine / Re: Why does holding a mouthful of water stop you from crying when chopping onions?

« on: 27/08/2021 22:32:49 »

I can't see how holding water in your mouth should help.
But one I can understand is cutting onions in a pot of water. The water dissolves the irritating chemicals so they aren't aerosolized*, and get in your eyes.

*aerosolized: A COVIDism.

19

Chemistry / Re: Did they really use Cordite in Little Boy, AB?

« on: 23/08/2021 22:33:13 »

It was hot burning, and they wanted to maximise the heat?

Once the explosive nuclear chain reaction starts, mere chemicals can't hope to compete with the heat.

20

Physics, Astronomy & Cosmology / Re: Do quantum computers imply a many-worlds interpretation?

« on: 23/08/2021 22:29:21 »

Shor's algorithm sounds fantastical, to me.

Publicly described quantum computers have around 50 qubits, and the quantum state decays very quickly (very short "coherence time"). Getting these qubits to work together on a single computation is a real challenge.
- It is thought that to get practical quantum computers, you would need quantum error-correction. This is in its infancy.
- So probably the only practical error correction is to run the algorithm many times, on many quantum computers in parallel, and see if any of them manage to produce a valid answer.
- Fortunately, for applications like this, it is much easier to test if the answer is correct than it is to generate the answer in the first place.

Shor's algorithm is about factoring numbers -  and the the most lucrative application of this would be breaking the Diffie Helman key exchange, which uses typically 1,024 or 2,048 bit keys. So you would need a quantum computer significantly more advanced than the ones publicised by the likes of Google and IBM.

Of course, if such quantum computers existed, in some dark, cold, dungeon, they wouldn't tell us about them!
There are projects going on around the world to produce quantum-resistant encryption codes, and some candidates have been identified.
See: https://en.wikipedia.org/wiki/Post-quantum_cryptography

...is just putting it in/between 2 basis states, which we intend to measure, later on? Nothing more.

There is nothing easy about quantum computers.
- Getting a single qubit into an initial state is fairly easy, but doing it for hundreds or thousands is very difficult
- Measuring a single qubit is fairly easy, but doing it for hundreds or thousands is very difficult
- Getting 2 qubits to interact in the intended way is difficult, but doing it for hundreds or thousands is extremely difficult

The following research found a potential way to synchronise many qubits...

https://newsroom.unsw.edu.au/news/science-tech/missing-jigsaw-piece-engineers-make-critical-advance-quantum-computer-design