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Question of the Week / Re: QotW - 24.12.12 - Is there any way to harness nuclear energy other than steam?

« on: Today at 20:31:45 »

Many nuclear reactors use water for cooling, but other designs have been used:
- Gas-cooled reactors https://en.wikipedia.org/wiki/Gas-cooled_reactor
- Liquid-metal cooled reactors (eg using liquid sodium or liquid lead as a coolant) https://en.wikipedia.org/wiki/Liquid_metal_cooled_reactor

One proposed design for a helium gas-cooled reactor would use the Helium gas to directly turn a gas turbine, potentially achieving high efficiency. https://en.wikipedia.org/wiki/Gas_turbine_modular_helium_reactor
- But as far as I know, most reactors separate the nuclear cooling circuit from the power generation circuit with a heat exchanger; this assists maintenance of the turbines since they should not become radioactive. Normally, water/steam is the working fluid feeding the power-generating turbines.

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Question of the Week / Re: QotW - 24.12.12 - Is there any way to harness nuclear energy other than steam?

« on: Today at 20:21:43 »

Researchers investigated nuclear-powered propulsion, where:
- In the atmosphere, the nuclear reactor heats the air, which expands and propels the airplane
- In space, the nuclear reactor heats a propellant, which expands and propels the spaceship
       - Water would not make a good propellant, as it's latent heat is too high

Nuclear-powered airplanes were not put into production, as an airline crash would spread nuclear waste across a wide area.
- A nuclear-powered spacecraft would be less of a problem, provided the reactor was not fired up before reaching orbit (ie only low-radioactive materials present), and they were very confident it would never return to Earth.

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

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Question of the Week / Re: QotW - 24.12.12 - Is there any way to harness nuclear energy other than steam?

« on: 07/12/2024 08:48:10 »

beta emitters ... to power ultra low power equipment

For maximum power output, the ideal operating voltage for a typical beta emitter is around 500,000 Volts,
- which is a bit high for low-power electronics (which typically operate at < 1V)

See a typical beta particle energy spectrum here: https://en.wikipedia.org/wiki/Beta_decay#Description

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Physics, Astronomy & Cosmology / Re: Will AI really replace supercomputers for weather-forecasting?

« on: 07/12/2024 07:33:32 »

AI at the minute is a targeted learning programme (narrow ai) that would only develop the same methods as existing programmes

Current supercomputer models are based on solving the Navier-Stokes equations, and they forecast the future weather from the current weather.
- There is a process of comparing the predictions with the actuals (when they happen, 2 weeks later), which feeds into manually optimizing future versions of the software
- For example, the effects of clouds are not obvious, and are not explicitly covered in the Navier Stokes equations, so various cloud corrections are put into the model "by hand"
- Landform features are included as "boundary conditions" for the flow of air
- https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations

Typical AI models use comparison of predicted vs actual weather as the primary input, with minimal manual tweaking
- They do not explicitly insert the Navier-Stokes equations (which are impossible to solve analytically, and extremely hard to solve numerically), but the TPU calculates output signals from its artificial neurons, based on weighted inputs (a very different calculation).
- The designers may not even explicitly insert the landform
- They let the model work this out for itself, after observing its predictions vs the actual weather (from historical records)

One thing is clear - they will definitely need parallel supercomputers for the training phase!

Another limitation of AI systems is that a lot of data is implicit, and it's not clear how to change the AI model for new situations
- For example, if a large volcanic eruption dumps a lot of sulphate particles in the atmosphere (as Mount Pinatubo did in 1991), it will change the atmospheric behavior. If sulphate reflectivity is not an explicit input variable to the AI model, with a range of examples in the training data, the AI model may not cope with a new volcanic explosion. This can be handled more directly in a numeric model.
- If there is a sudden change in ocean currents in the Atlantic, this will be outside the range of AI training data, and the results may be misleading (unless sea surface temperatures are part of the input data).

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Physics, Astronomy & Cosmology / Will AI really replace supercomputers for weather-forecasting?

« on: 06/12/2024 21:43:30 »

On today's podcast, an interview with researchers from Google DeepMind reported results of an AI doing Medium-Range weather forecasting (up to 2 weeks in advance).
- They reported huge reductions in energy consumption, number of CPUs and computation time when using a Google TPU (AI chip) compared to thousands of CPUs in a conventional supercomputer.
- Suggesting that weather forecasting would no longer need supercomputers
- The Google chip started with the current state of the world's weather as input.

5 years ago, I listened to this interview with the European Medium-Range Weather Forecasting team.
- My vague recollection was that something like 60% of their Supercomputer usage was to produce a definition of the current state of the weather, and only 40% was predicting the future of the weather, out to 2 weeks
- There are many different sources of data on the current weather, and they don't always agree
- The data collection points are closer together over land than over sea or deserts
- And so it takes a huge amount of supercomputer analysis to integrate them all to define the current global weather

So while AI might will be able to reduce supercomputer usage by 40% for weather prediction, there is still significant research if forecasters are to reduce the other 60% of supercomputer usage.

https://omegataupodcast.net/326-weather-forecasting-at-the-ecmwf/

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Geek Speak / Re: What is 'maths'?

« on: 04/12/2024 01:53:43 »

Clocks "linearise" time.

Most clocks also quantise time.
- "Grandfather" clocks used a pendulum which swung at 1Hz, to activate the hands on the clockface.
- Mechanical wristwatches used a spring and escapement mechanism to drive the hands on the wristwatch.
- Early wristwatches & today's cheap wall clocks use a crystal vibrating at 32,768Hz, which they divide using 15 flip-flops to 1Hz, driving the display mechanism
- Atomic clocks use a Cesium 133 resonance at 9,192,631,770Hz, which they divide down to 1Hz.

Water clocks are pretty analog, but in the details, they are also quantised at the level of water molecules...

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Geek Speak / Re: What is 'maths'?

« on: 03/12/2024 06:46:01 »

time is linear.

There is a debate about whether time is continuous or quantized; unfortunately, at this point in time, we don't have much evidence either way.
- At the scale of General Relativity, time is treated as continuous
- When you get down to the detailed behavior of quantum fields in the vicinity of a black hole event horizon, you end up with some stubborn infinities, which have so far resisted attempts to unify General Relativity and Quantum Field Theory.
- One possible interpretation is that time is actually quantized (at some level).
- In support of this, some researchers found that when they used a computer to simulate cases where these singularities appear in the theory, they don't see them in the computer. One aspect of computer simulation is that you have to quantize the simulation in both time and space
- and/or maybe space is quantized; as Einstein showed, Time and Space are both aspects of a unified Spacetime.

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Physics, Astronomy & Cosmology / Re: Could the universe be smaller than the visible universe?

« on: 03/12/2024 01:06:13 »

Einstein's equations gave two solutions

A brief history of modern cosmology: Around 1915, Einstein produced a field equation describing the cosmos. The calculation generated an (unknown) constant of integration (as integration always does).
- Einstein's original paper omitted mention of this constant. It was mentioned in a later version.
- As usual in mathematics, you try to find some known condition that allows you to identify the value of this integration constant
- Einstein followed the cosmological assumption of his day that the universe was eternal and unchanging, and this "known condition" allowed him to define a value for this constant.
- Around 1925, Belgian priest Georges Lemaitre (perhaps with a Bible-inspired cosmology) saw in these equations the possibility of what we now call a "Big Bang"  creation of the universe. Same equation, different range of values for the cosmological constant, and different initial conditions.
- In 1929, astronomer Edwin Hubble saw that the universe was expanding. This confirmed Lemaitre's suggestion. At this point, the cosmological constant was assumed to be zero. Same equation, different value for the cosmological constant.
- In the 1970s, Vera Rubin saw that galaxies rotated in a strange way, and this led to the idea of Dark Matter. Same equation, same value for the cosmological constant, but introducing the concept that the mass of the universe is a combination of "normal" matter, and the dominant "Dark" matter.
- In the 1990s, the accelerating expansion of the universe was discovered, attributed to Dark Energy. Same equation, different value for the cosmological constant.

Einstein's field equation has proved remarkably resilient for over a century.
- New theoretical, computational and experimental techniques for investigating it have repeatedly confirmed it to an incredible accuracy
- With applications including black holes, GPS, gravitational waves, gravitational lensing and frame dragging
- Discoveries in this time have changed the interpretation of various parameters in the equation, but the equation itself has stood the test of time.
- Some very recent satellite measurements of the expansion of the universe have called into question just how constant this "cosmological constant" truly is. We will need more data to confirm if it has changed, and if so, in what manner
- An engineering "law": "All constants are variable" (under different conditions)

https://en.wikipedia.org/wiki/Einstein_field_equations#The_cosmological_constant

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Just Chat! / Re: Should smartphones be banned from the dinner table?

« on: 03/12/2024 00:29:07 »

Australia has recently taken the radical step of banning under-16s from holding a social media account.
- So this might slightly reduce the number of people on cellphones at the dinner table

Of course, there are still a few steps to be resolved:
- The legislation doesn't come into force for another year
- It doesn't spell out exactly which social media platforms will be included and excluded (it may be left to the discretion of some government official?)
- It is unclear how the age of a person will be determined (although there is a field trial of several candidate methods that is supposed to report in mid-2025)
- I am sure the whole process will be appealed by various interested parties...

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Chemistry / Re: Shrink ray or growth ray, how could they work?

« on: 02/12/2024 08:20:20 »

I have heard that it is possible to increase the size of a hydrogen atom considerably by hitting it with just the right wavelength to promote the electron from the ground state to n=very large.
- I am sure that in this state, it would not act anything like a conventional Hydrogen atom in its chemical reactions, etc
- I am sure that this state would be very unstable, so that collisions with other atoms would disrupt this delicate state.
- I am sure that these orbitals would rapidly decay by emitting a photon (unless it was a "forbidden" transition)

You can't subtract energy from the ground state to shrink the atom.

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Geek Speak / Re: What is 'maths'?

« on: 02/12/2024 07:40:17 »

why is there so much maths?

Because different maths is required for different situations.
- A child may say that the number after 12 is 13 (when they are talking about the natural numbers)
- Except when they are talking about a clock, in which case the  number after 12 is 1 (a modified form of modulo arithmetic)
- Both are valid answers, in their own domain

There is a debate about whether maths is invented or discovered.
- Some ancient Greek mathematicians believed that geometry held an objective reality in nature, and humans just discovered it

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General Science / Re: Can an electron travel a bit in a copper wire for inertia?

« on: 02/12/2024 01:36:45 »

I'm afraid that electrons won't travel a few millimeters after you cut the wire, because electrons are always bumping into the atoms of the copper wire, which effectively halts their forward motion.

The references below suggest that:
- An electron will collide with an atom about every 10^-14 seconds
- An electron will travel about 10^-8 meters between collisions with an atom

If you remove the electrical field, the electrons will not regain their drift speed after this collision.

https://www.wyzant.com/resources/answers/770983/what-is-mean-free-path-for-the-conduction-electrons-in-copper-assuming-that
https://www.wyzant.com/resources/answers/770982/what-is-the-mean-free-time-between-collisions-for-the-conduction-electrons-

Electrons do have momentum: Electrons colliding with atoms in a conductor causes the atoms to shift position, a process called electromigration.
- This is a major problem in the most advanced semiconductor chips, where current must flow through finer and finer wires (down to as little as 5nm wide, in the latest chips).
https://en.wikipedia.org/wiki/Electromigration

All of this changes in superconductors, where electrons (or electron pairs) can propagate through the material without interacting with the superconducting material, so they have a long mean free path, and mean free time.
- But the chemical reactions driving AA batteries stop at the extremely low temperatures required by current superconductors!

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Physics, Astronomy & Cosmology / Re: Could the universe be smaller than the visible universe?

« on: 02/12/2024 01:13:29 »

PS: We still don't know if the universe is finite or infinite.

If the universe is infinite, the concept of a "smaller size" is meaningless (or at least, ill-defined).

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Physics, Astronomy & Cosmology / Re: Could the universe be smaller than the visible universe?

« on: 02/12/2024 01:11:26 »

If the universe were finite and shrinking (one of the possible solutions of Einstein's equations), the light from distant galaxies would show galaxies at more distant places than they are "today" (bearing in mind that "simultaneous" does not really exist in Einstein's equations). This would allow an answer of "YES" to: "Could the universe be smaller than the visible universe?"

However, Hubble's observations (both Hubble the man, and the space telescope named after him) show that distant galaxies are red shifted, and moving away from us, which is different solution to Einstein's equations.
That means that the light we see from distant galaxies would show galaxies closer than they are "today"

The Nobel-winning discovery of Dark Energy in the 1990s show that the expansion is accelerating (another solution to Einstein's equations).

So according to this latest information, the answer is "NO" to: "Could the universe be smaller than the visible universe?"

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Question of the Week / Re: QotW - 24.12.06 - How long does it take a galaxy to form?

« on: 02/12/2024 01:01:41 »

Following results from the James Webb Space Telescope (JWST): Much quicker than cosmologists previously thought: Less than 250 million years after the Big Bang.
- I am sure that with further observations from JWST, even quicker examples will be found

Galaxy distance and age are estimated from their red shift.
- Extreme red shifts move the emission lines into the infra-red region of the spectrum; JWST specializes in infra-red observations and spectroscopy, so it has detected galaxies much older than previously known (eg from the Hubble Space Telescope, which uses visible light).
https://en.wikipedia.org/wiki/James_Webb_Space_Telescope#Bright_early_galaxies

Cosmologists have debated various ways that galaxies could form - did (1) the central black holes form from the stars of the galaxy, or (2) did the stars of the galaxy form around pre-existing black holes?
- It is felt that quite a long time would be required for process (1) to reach the observed size of central black holes; much more than the observed 250 million years.
- Process (1) would require the universe to expand and cool down significantly before gas could collapse to form a star; these stars would need to burn into stellar-mass black holes, and then combine into supermassive black holes by slowly radiating away gravitational energy. This would have formed a core around which galaxies could have formed in the early universe.
- Some cosmologists are wondering about process (2): if there may have been some large black holes formed in the early universe, either in the Big Bang itself, or by gravitational collapse of gas when the density of the universe was greater than it is today.
https://en.wikipedia.org/wiki/Black_hole#Primordial_black_holes_and_the_Big_Bang

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Physics, Astronomy & Cosmology / Re: How does H20 wear away stone?

« on: 02/12/2024 00:34:36 »

Water can dissolve some components of stone (eg limestone), releasing the less soluble parts to be carried downstream.
- Moving water can carry sediment, which abrades the rock
- Water can get into cracks and freeze; expanding ice breaks off bit of rock.
- Salty water can get into rocks and evaporate; the salt crystals break the rock grains
- Frozen water (ie glaciers) can break off rocks, which then abrade other rocks.
- Ocean waves carry a lot of momentum, which breaks and erodes rocks

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Physiology & Medicine / Re: What Are The Advantages To Taking Vinegar Everyday?

« on: 26/10/2024 01:09:10 »

As a small molecule, vinegar might be absorbed directly in the gut, to be further processed by the liver.

But is it possible that vinegar is an indirect nutrient?
- It is produced by a narrow set of microbes
- Among the multitude of microbes in your microbiome, are there varieties which can metabolise vinegar, producing other useful products?
- And are there pathogenic bacteria which are kept under control by vinegar?

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Question of the Week / Re: QotW - 24.11.01 - If dinosaurs hadn't gone extinct, would humanity have emerged?

« on: 26/10/2024 01:01:26 »

It is possible that evolution got locked into a dead-end, with large, fast, meat-eaters
- These preyed on heavily-armoured vegetarians at ground level
- The very large vegetarians that could eat tree leaves were probably protected by sheer bulk

There were a lot of smaller dinosaurs and mammals scurried around in the undergrowth.

If your computer locks up, reset it... 

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Question of the Week / Re: QotW - 24.10.25 - Are isotopes distributed similarly on other planets?

« on: 20/10/2024 03:51:25 »

Radioactive C14 present at the formation of the Earth would all be gone (half-life 5,700 years), but it is continually produced by cosmic rays smashing into Nitrogen in the atmosphere.
- If a planet had little Nitrogen in the atmosphere (like Mars), then there would be less C14.

Stable C13 and C12 isotopes are produced in supernova explosions, but the ratio of them would depend on the type of star exploding.
- Red giants shed a lot of their atmosphere, and the carbon present here would be primarily C12, formed by fusing helium.
- So the isotopic composition of carbon in a planet or star would be determined by the mix of events seeding the molecular clouds from which they formed.

Even on Earth, the ratio of C12 and C13 varies slightly in different places, due to plant photosynthesis (plants prefer C12 to C13).

More dramatically:

at the time of the Permian extinction 252 Mya when the [C12/C13] ratio changed abruptly by 1%

Presumably, a large source with a different C12/C13 ratio was dumped into the atmosphere.
For more detail on this event:
https://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event#Dating

With elements having very different origins, the concentration would vary dramatically with the events forming their molecular gas clouds.
- The heavier elements (Silver and above) are mostly formed in neutron star collisions/disruptions.
- A difference in the proximity of these events would make a big difference in the concentration of heavy elements in a star or planet
See periodic table at: https://en.wikipedia.org/wiki/Nucleosynthesis#Timeline

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Chemistry / Re: Dexpan alternative question

« on: 13/10/2024 07:43:58 »

I wouldn't have found it, because I wouldn't have known what to search for...

But Google is really effective.

AI systems are even more "effective", in the sense that if they don't know the answer, they will just invent an answer.
- But they are less effective at providing a list of references
- And they consume a lot more electricity than Google queries!