21

Physics, Astronomy & Cosmology / Re: How are physical units defined?

« on: 02/08/2022 10:23:14 »

we just assume that distances are somehow axiomatic

A lot of people (most people?) are confused when they first run into concepts from Special Relativity whereby observers in different frames of reference will will come up with a different distance and time between the same same two events.

22

Just Chat! / Re: What is on your bucket list?

« on: 01/08/2022 23:00:46 »

I ticked one item off my bucket list earlier this year - I went to see the Aurora (the Australis version).
An overnight plane flight to around latitude 60° South, zig-zagging above the cloud-tops to give everyone good views of the aurora.

23

General Science / Re: Can you use capillary action to clean water or reduce salinity

« on: 01/08/2022 22:55:29 »

say for instance the salinity was reduced by 20% in the second cup.
Then the 2nd cup could be used as the starting cup up for a third cup to reduce salinity by another 20%

You can interpret this the right way or the wrong way.

Wrong way: If each paper towel reduces the concentration of salt by 20%, then after 5 paper towels, the concentration of salt would be 0%

Right Way: If each paper towel reduces the concentration of salt by 20%, then there is 80% of the salt left.
After 5 paper towels, there is 0.8⁵ salt left, or 33% of the original concentration.

24

General Science / Re: Can you use capillary action to clean water or reduce salinity

« on: 01/08/2022 11:49:47 »

If capillary action made water run uphill...

That's what trees do - a fine capilliary in the trunk raises the water quite high, and transpiration does the rest...

But it uses solar power to drive the water cycle.

25

General Science / Re: Can you use capillary action to clean water or reduce salinity

« on: 31/07/2022 22:28:51 »

Having just failed a COVID test

So you are saying that you failed, but the test method was a success...
- Get well soon!

PS: In Australia, demand for PCR capacity has dropped, so some laboratories are now routinely checking for COVID, Influenza (A & B) & RSV; several respiratory viruses circulating in our winter.

26

Technology / Re: What can I learn from electro-optical characterisation testing of Si detectors?

« on: 31/07/2022 22:24:28 »

what information can be learned from performing these tests.

It sounds like you have already learned a great deal from the testing you are doing.
- And as you try to turn the individual test results into a coherent overview, I am sure you will learn even more!
- If it is a university exercise, then the purpose will be learning about photodetectors, and learning about laboratory work, learning about experimental error, and learning how to write reports
- If it is an exercise in industry, then someone will have some ulterior motive, like imagining some application. If you knew the application, it may allow you to make more focused conclusions.
          - It is quite possible that the intent is to select a set of photodetectors that cover the widest available range of wavelengths. In that case, what you are doing is developing the calibration curve that will be installed as the factory default in the software. Presumably, this initial default will be refined by some factory or field calibration method?

If you have more time, you might also learn more by seeing how:
- The performance varies with temperature (dark current increases significantly with temperature)
- Select devices from different manufacturing batches, to see how device performance will vary between individual devices.

27

Physics, Astronomy & Cosmology / Re: How many levels of orbit can a planetary system sustain?

« on: 31/07/2022 22:08:05 »

Could we have something orbit the moon that is 1/(1.2x10^2) the mass of the moon?

The Moon/Earth system is very "similar" in size: The Moon is 1/80 the mass of the Earth. Some have called it a "twin planet"
- Probably due to the method of formation: The current popular theory is a Mars-sized body smashing into the Earth.
- But other ratios in our Solar system are more extreme:
- Mars has the tiny Phobos & Deimos
- Jupiter & Saturn have significant sized moons, but the planet is huge.

When we come to smaller objects:
- Pluto & Charon are fairly similar in size
- There are a number of double asteroids
- Some comets look like they were originally two objects which stuck together, with descriptions like "duck" and "snowman"

28

The Environment / Re: Is global warming man-made?

« on: 31/07/2022 07:55:53 »

I even heard that when CO₂ ppm reach some high value it will have the opposite effect of an greenhouse gaz.

The atmosphere of Venus is 95% CO₂, and the surface temperature is a whopping 467 °C, or 872 °F.

That doesn't sound like the opposite of a greenhouse effect to me (even allowing for the fact that Venus is closer to the Sun than the Earth).
https://en.wikipedia.org/wiki/Atmosphere_of_Venus

29

General Science / Re: Can you use capillary action to clean water or reduce salinity

« on: 30/07/2022 23:47:18 »

can it be used make dirty water into clean drinking water?

Complicated? 2 cups and a tissue. Does not answer the question I ASKED

The average human adult needs about 2 liters of drinking water per day (if they are willing to give up sanitation).

"2 cups and a tissue" will not produce 2 liters of fresh water per day, so "No" is still the answer.

Reason: It takes real energy to separate water from impurities.
- Capilliary action is driven by surface tension (which operates once) and/or evaporation (which doesn't deliver drinkable water).
- Extracting fresh water from sea water takes energy. In lifeboats, additional fresh water can be produced by a solar still or by a semi-permeable membrane and muscle power.

https://www.usgs.gov/special-topics/water-science-school/science/desalination

30

Cells, Microbes & Viruses / Re: 'When' would you use nucleotides to construct phylogenetic trees?

« on: 30/07/2022 02:01:06 »

You might use protein amino-acid sequences when:
- Temperatures are so high that fossil DNA has been corrupted beyond our ability to read, but there is some intact protein available.
- When you are looking for functional changes in a protein, rather than non-functional genetic changes in the protein.
- But untangling and reading the sequence of a protein is not easy, and different techniques need to be developed for every different protein

In most cases, if you can recover intact DNA, you would use it, because:
- You can read the sequence of every protein in the same way, without having to develop a new technique for every protein
- You can read changes in "non-protein-coding" DNA, which often changes more rapidly than protein-coding segments, allowing the researcher to chart even very close family relationships
- Techniques for reading DNA have accelerated exponentially over the past few decades, making it the cheapest and fastest method available
- COVID-19 has resulted in the deployment of far more PCR machines and DNA sequencers than ever before...
- If you can't read nuclear DNA, mitochondrial DNA will also give some clues (there are more copies of mitochondrial DNA in each cell than nuclear DNA)

31

Physiology & Medicine / Re: Can hot tubs give us some benifits of exercise?

« on: 28/07/2022 23:52:36 »

The optimum temperature for sperm production is a bit less than the body's core temperature of 37C.

A hot tub above 37C will impair sperm function.

Note that long immersion in temperatures above 37C can result in heat stroke.

32

The Environment / Re: Is global warming man-made?

« on: 28/07/2022 12:16:47 »

Only 43 % of the emmission finish in atmosphere (the rest land elsewhere)

Much of the rests lands on the 70% of not-land = 70% ocean & seas.
- It is making the oceans more acidic, which means that marine animals have to expend more energy maintaining bones and shells

A small amount actually lands on the 30% of land, and is slowly absorbed by rocks (eg basalt, like much of Mauna Loa).
- A larger amount is actively absorbed by trees on land, plants in peat bogs and algae in the oceans. How long it stays out of atmospheric circulation depends on whether the forest burns, the peat bog dries out, and whether the algae sinks to the bottom of the ocean.
- It is thought that the average residence time of CO₂ in the atmosphere is about a century.

Is global warming man-made?

I would say that women played a large part in it, too...

33

The Environment / Re: Is global warming man-made?

« on: 25/07/2022 22:43:49 »

man only changed "the mean" temperature of the earth

With the Earth's current spin, we can expect a certain day/night temperature excursion; with its current axial tilt and orbital eccentricity we can expect a certain seasonal temperature variation around the mean.

So if you increase the mean temperature, you also increase the peak temperatures, so you break record high temperatures more often (and record low temperatures less often). With higher mean temperatures comes hotter summers, dryer forests, and more severe wildfires (under the right conditions).

Some effects are rather non-linear; for example, the chance of hurricanes increases significantly when the sea surface temperature exceeds 26C. So if you increase the mean temperature, you can expect far more hurricanes. But even without hurricanes, higher sea temperatures means more evaporation, and (under the right conditions), heavier rainfall and more flooding.

With habitat fragmentation, species can't migrate poleward, and we can expect to see accelerating species extinctions.

the sun is the major source of heat for at least the inner planets, its output must be the primary determinant of their temperatures

Yes, but not in the way you mean.
- Over a timescale of billions of years, the Sun will get hotter, as it burns more of its fuel.
- But that's not the timescale here. We are looking at changes over a period of hundreds of years, since humans used the power of coal to mine more coal (and a similar effect with petroleum production), in a positive feedback cycle.

without a proper experimental test

We do have a proper test - it's called a Climate Attribution test.
- It uses supercomputers to model the Earth's atmosphere, with and without the CO₂ humans have added since the start of the Industrial Revolution.
- What this shows is things like "Extreme climate event X will occur Y% more often due to human-added greenhouse gases."
- So this is evidence that humans have increased both the mean and extreme temperatures (and their impact on society)

Some people may claim that this is just statistics - but saying that "smoking increases your chance of lung cancer" is just statistical - but that doesn't invalidate smoking as a major cause of lung cancer.

https://en.wikipedia.org/wiki/Attribution_of_recent_climate_change

34

Plant Sciences, Zoology & Evolution / Re: How do birds know when & where to find water in Australia?

« on: 25/07/2022 09:38:38 »

Probably the most spectacular example of this is Lake Eyre/Kati Thanda in South Australia.
- Scientists don't know how the birds know to migrate there
- The Lake does fill in La Nina years, and La Nina affects the whole of Eastern Australia, so this may be a good hint to the birds that the lake may be filling...

https://en.wikipedia.org/wiki/Lake_Eyre#Birds

35

Just Chat! / Re: What is worse...?

« on: 24/07/2022 22:30:34 »

The worst would be getting all of them at once.
Of course, Mad Cow Disease may not manifest itself for 10 or 20 years...

36

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

« on: 23/07/2022 23:53:27 »

the thing about Mathematics:   It often looks easy when you know how to do it

I have heard that the mathematician Gauss had a reputation for working on a mathematical problem until he solved it. And then, knowing it was true, he looked for the most elegant way of showing that it was true - but in a way that made it difficult for other mathematicians to use the learnings he had gained in discovering the result.
https://mathworld.wolfram.com/NonconstructiveProof.html

37

Plant Sciences, Zoology & Evolution / Re: Great maggot mystery: how did maggots get into my frozen chicken?

« on: 23/07/2022 12:20:53 »

Could a fly detect odor molecules from meat diffusing through cling-wrap?
Could a fly's ovipositor penetrate cling-wrap?

38

Physics, Astronomy & Cosmology / Re: Can we utilise antimatter to store energy from solar power?

« on: 23/07/2022 12:17:16 »

matter and antimatter to be mutually repulsive: Just the standard theory

An electron (charge -1) has as its antiparticle: the positron (charge +1). Since opposite charges attract, these will attract each other strongly. That is the standard theory.

However, when you come to uncharged particles (eg anti-neutrons and anti-Hydrogen), the electric field is minimal, and gravitation has a chance to assert itself.

At one time, some cosmologists tried to explain the dominance of matter in our part of the universe by imagining that matter and anti-matter would gravitationally repel each other to opposite ends of the universe.

Most physicists today expect that matter and antimatter will have equal gravitational attraction
- This comes from Einstein's Mass-Energy equivalence.
- A Hydrogen atom has a mass/energy of 939 Mev/c²
- An anti-Hydrogen atom has a mass/energy of 939 Mev/c²
- In Relativity, the gravitational force is between the units of Mass-Energy, and is always attractive
- This is why CERN is creating, storing & experimenting on anti-Hydrogen, to confirm these theories

39

Physics, Astronomy & Cosmology / Re: Can we utilise antimatter to store energy from solar power?

« on: 23/07/2022 01:37:59 »

Storing positrons is fairly well understood - before the LHC was built at CERN, the 27km tunnel was used for LEP: the Large Electron-Positron collider. This stored electrons and positrons in a very good vacuum, guided by electric and magnetic fields. It cost about a billion swiss francs to build (around  $US 1 billion, in 1990 dollars).
https://en.wikipedia.org/wiki/Large_Electron%E2%80%93Positron_Collider

A more difficult item to store is is anti-hydrogen, since it doesn't have such a strong electric or magnetic field as positrons. Recent tests showed that scientists could store anti-hydrogen for 16 minutes (for use in studies of the gravitational attraction of anti-matter, among other things).
https://home.cern/science/physics/antimatter/storing-antihydrogen

40

General Science / Re: What be the pressure (at the sea level) in a column of water - diameter 1 meter

« on: 21/07/2022 10:53:24 »

I am targeting escaping gravity

How does a big column of water help you do that?