1

Physics, Astronomy & Cosmology / Re: Correlation vs association

« on: Yesterday at 23:35:29 »

a is the independent variable, b is the dependent variable

I know that this is normal terminology, but it implies a causal direction.
- If there is some variable in your experiment that you can easily control, and another variable that you can easily measure, then it is fair to say that "when I changed variable x, variable y changed in a (linear/parabolic/exponential) manner"

However, when it comes to complex things like the impact of obesity in a human population on heart attacks:
- There is no easy way to control obesity in a whole population
- There is no easy way to control heart attacks in a whole population
- There are many factors which can cause heart attacks (eg genetics, congenital problems, education on exercise, stress)
- There are many factors which can cause obesity (eg genetics, income, education on healthy diet, stress)
- So the easiest thing to do is to do some sort of scatterplot of obesity vs age of first heart attack
- Then do a regression line through it, to conclude that  "with increased variable x, variable y changes in a (linear/parabolic/exponential) manner"
- You could hypothesize that obesity contributes to heart attacks (since the obesity was present before the first heart attack), but it's not guaranteed: Someone who has an underlying heart condition may be predisposed to a sedentary lifestyle, which may make them obese.
- You could make comments like "For patients with BMI > 30, a weight reduction of 1 kg is associated with a delay of z years in age of first heart attack."

2

Chemistry / Re: How can I find the optimum ΔH and ΔS for passive T control?

« on: Yesterday at 23:11:59 »

I don't get it.

Another on the sideline of practicality: It could make sense in a desert, where it gets well above 30C almost every day (including winter), and well below 15C almost every night (even in summer).

You would need two tubs of your phase change material which can be selectively exposed to outside air or inside air.
- During the day, you expose one tub to the outside air to heat it, while the other (cold) tub is exposed to inside air to keep the house cool.
- Reverse it overnight: Expose the hot tub to the inside to heat the house, while the other tub is exposed to the outside air to cool it, ready for the next day.

Effectively, you want a system that will produce a 12-hour phase change* in the outside temperature cycle.
     * This is a temporal phase change, achieved by a physical phase change

For it to work well, your house would need to be well-insulated, and you would need fans to circulate air past the tubs of hot/cold materials.

3

New Theories / Re: Is this why Ivermectin appeared to be the answer to Covid?

« on: Yesterday at 22:51:56 »

We recently had a national election in Australia, and party had a policy that included permission to use ivermectin for COVID (among other things). In my state, they received 0.03% of the vote...

The figurehead of another party imported $1M of ivermectin. He had to destroy it, as ivermectin can only be distributed by doctors, for proven conditions like intestinal parasites. They got 0.25% of the votes.

My guess is that almost half of the US population would approve of ivermectin.

4

Radio Show & Podcast Feedback / Re: Feedback on: Question Of The Week in Podcast vs Forum

« on: Yesterday at 22:24:09 »

QOTW is a question, and the Forum is about answering questions, so it's still worthwhile posting QOTW on the forum.

Often the discussion on the forum will take a different approach than the expert (or multiple different approaches).

5

That CAN'T be true! / Re: How do people survive lightning strikes?

« on: Yesterday at 10:15:57 »

I can think of a couple of critical organs between the legs that might be in the path of the lightening

If the Earth Potential Rise is (say) 30,000 Volts, and it dissipates over a distance of (say) 100m, that is 300 Volts per meter in the soil.

A typical pace might be 0.5m, so 150 Volts between the legs will produce an electric shock, but not anything like the voltage of a direct lightning strike.
- The ground resistance will also provide some series impedance to the current in the legs
- If you are standing, the distance between your feet might be closer to 0.2m
- If you are walking, half of the time, there will be only 1 foot on the ground (ie perhaps a small current in one foot, but nothing up the legs - and that one foot may have a plastic-soled shoe on it)
- If you are running, most of the time there will be only 1 foot on the ground

The human condition is much better than for (say) a cow. The distance between front and rear feet is more like a meter (300V in the above scenario), and there are some very electric-sensitive organs between these locations, like the heart and spinal cord.
- Most of the time, cows have at least 2 feet on the ground, and no shoes

6

Radio Show & Podcast Feedback / Re: Feedback on: Question Of The Week in Podcast vs Forum

« on: Yesterday at 00:29:35 »

It would be good if the podcast team could remember to put an entry in the "QOTW" forum around the time the question is asked on the podcast.
- Then we could pontificate until the nominated expert provides the "official" answer.
- At one time, the podcast team sometimes mentioned some of the ideas on the forum, as well as playing the response from the expert...

7

That CAN'T be true! / Re: How do people survive lightning strikes?

« on: 23/05/2022 11:18:14 »

how can a human body tolerate that amount of heat?

The human body is largely composed of water, which has a very high heat capacity.

The water is slightly salty, which makes it slightly conductive
- Probably slightly more conductive than a tree-trunk, so don't lean on a tree trunk in a storm!
- But the voltage drop across 2 meters of ionised air is very low, so some of the energy may bypass the human body (at least, in survivors)

even if only for a nanosecond?

Standard lightning test waveforms have a rise time of 1μs, and a fall time of 50μs.
So you could loosely say that the average duration of a lightning impulse is around 50μs, or 1/20,000 of a second.

8

Physics, Astronomy & Cosmology / Re: How can I pass a large DC current through mercury?

« on: 23/05/2022 11:05:05 »

Ampere longitudinal forces in conductor

This video shows a fountain created by passing electricity through an alloy of Sodium and Potassium (NaK).
It is a demonstrator for the type of coolant planned for some Fast Breeder reactors.

As I recall, they point out that the success of this fountain is largely due to the low density of NaK. If you tried to do it with Mercury, the high density of Mercury would make for a rather disappointing display.

PS: NaK should not be kept in your home. It reacts with moisture or oxygen in the air, and most other things with which it comes into contact, including humans...

9

General Science / Re: How do we make synthetic gasoline?

« on: 22/05/2022 22:49:40 »

Methanol

In Brazil, they make engines that can take up to 100% ethanol, and automatically adjust to the mixture.
My car says that it can use up to 10% Ethanol.

The problem is that plastic fittings can be damaged by organic solvents like ethanol and methanol.
So buy a car from Brazil before you try this at home, and don't use Methanol (there are tax reasons for a small amount of Methanol).
https://en.wikipedia.org/wiki/Ethanol_fuel_in_Brazil

10

Physics, Astronomy & Cosmology / Re: How can I pass a large DC current through mercury?

« on: 22/05/2022 22:42:59 »

If you want to put a lot of current through metal (such as happens during aluminium smelting, for example), you use a low voltage, much lower than 5 Volts - and very thick wires.
https://en.wikipedia.org/wiki/Hall%E2%80%93H%C3%A9roult_process

Note that mercury is volatile, and the invisible fumes are toxic, so you should not have mercury in your home (except, perhaps, sealed in an old-fashioned thermometer); in the lab, it should be kept in a fume cupboard when uncapped.

11

That CAN'T be true! / Re: How do people survive lightning strikes?

« on: 22/05/2022 01:17:17 »

Most people who are impacted by lightning suffer due to Earth Potential Rise, which affects a radius of many meters around the lightning strike.
- This causes a high voltage between the feet, which causes a current up one leg, and down the other - avoiding major critical organs.
- The affected people can be seemingly hit at random - if both feet are equidistant from the strike, there will be no potential difference.

There is something called the "skin effect" in wires, where high-frequency components tend to stay in the outer skin of the wire.
- In the rarer case where a person is directly hit by lightning, "skin effect" takes on a dual meaning - with waveform time constants around 10μs-50μs, there are a lot of high-frequency components, and much of the current tends to flow around the skin rather than the core.

However, even in the survivors, long-term neurological problems often occur, as well as short-term problems like damaged eardrums and burns also occur.

https://en.wikipedia.org/wiki/Lightning_strike#Strikes

12

Just Chat! / Re: a suitable pseudonym

« on: 22/05/2022 01:03:17 »

I've never met a scientist who I would call remotely autistic.

Until around the 1980s (perhaps, until the release of Rainman, in 1988), autism was commonly seen as a development problem of infants, who were often locked up in institutions. Since virtually all scientists are adults, they could not be autistic, by definition.

Now it is recognised as:
- occurring in adults too (usually children who grew up outside an institution, and learned to adapt)
- occurring in a spectrum, from very mild to very severe
- more severe cases often being seen as children of parents with mildly autistic tendencies (ie a strong genetic contribution) - with Silicon Valley being a particularly intense hotspot

We can now look back and see scientists who were almost certainly autistic, eg
- Cavendish, who was brilliant, but could never talk directly to people, but used correspondence.
- Mendel, who spent years breeding peas
- Darwin, with his intense childhood focus on collecting bugs
- Even many of the early researchers into childhood autism! (in one case, because they had an autistic child themselves)
- It is that often-introverted focus on collecting information in some specific area, often with a numeric focus that means someone with autistic tendencies is more likely to become a scientist, engineer, musician or perhaps an artist (in contrast to movie star or a socialite, for example).
- A text-based forum like this one is likely to attract people with some of these characteristics...

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

You spend your whole life trying to find something you enjoy, and then everyone tells you to shut up about it.

13

Physics, Astronomy & Cosmology / Re: What happens when photons leave the sun?

« on: 19/05/2022 00:39:01 »

how much Doppler shift is there?

Each year, around 3-4th January (perihelion, closest to the Sun) and 3-4th July (aphelion, farthest from the Sun), the Earth is neither moving towards or away from the Sun, and there is no Doppler shift. Around October there is a maximum Doppler blue shift (Earth moving towards the Sun) and around May there is a maximum Doppler red shift (Earth moving away from the Sun).

As others have pointed out, all year long there is a gravitational red shift (or "Einstein shift"), as photons "climb out" of the Sun's gravitational well. Depending on your frame of reference, you could say some of the following things:
- Photons don't lose speed as they always travel at c
- Photons don't lose kinetic energy, since the photon is massless
- Photons do lose momentum
- The photon frequency does decrease

One way of looking at this frequency decrease is to consider Einstein's time dilation.
- Deep in a gravitational well (eg the surface of the Sun), time moves more slowly, compared to an observer on the surface of the Earth
- So a Hydrogen atom emitting a spectral line on the surface of the Sun will do so more slowly than a Hydrogen atom emitting the same spectral line on the surface of the Earth
- So when you compare the frequency of the light from the Sun with the frequency of light generated here on Earth, you will find the light from the Sun has a lower frequency.

Einstein shift will be a maximum at aphelion, and minimum at perihelion.

photons are on their 1,000,000 year journey out of our sun

Under the extreme conditions of a supernova, energy from the core of a star can reach the surface much more quickly - just hours.

But neutrinos don't interact much with matter; traveling at (pretty much) the speed of light, they can reach the surface of the star in just seconds.
- To date, such a "race" has only been observed once, when a supernova occurred in the nearby Large Magellanic Cloud (a dwarf galaxy orbiting our Milky Way galaxy).

https://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions

14

Chemistry / Re: What colour is paint before the colour is added?

« on: 19/05/2022 00:10:29 »

'White' paint is a base with a bright white pigment, probably titanium dioxide, added.

Before titanium dioxide, lead oxide was often used as the base. Lead is a neurotoxin, and children sometimes picked up lead from flaking paint, or even lead paint on kid's toys. Thankfully, lead paint (and leaded petrol) is now banned.

Even farther back in time, lead oxide was used in makeup.

There have also been cases where arsenic was used as a green tint...

15

Plant Sciences, Zoology & Evolution / Re: Why doesn't evolution revert when environments change back?

« on: 18/05/2022 23:17:15 »

Ecosystems can revert back when conditions change: In the exclusion zone around the abandoned Chernobyl reactor (in Ukraine), many native animals have returned.
- But this only happens if those species (and their genes) haven't been driven into extinction.

A similar thing happens with individual species: There are different variants of genes, and these gene variants can come back - provided they haven't been driven to extinction.
- An often-quoted (but somewhat controversial) example is the peppered moth, which developed a dark color in polluted city areas during the industrial revolution, but the original light color persisted in less-polluted rural areas. As pollution levels decreased in cities, the light colored variant spread back into previously polluted areas.
See: https://en.wikipedia.org/wiki/Peppered_moth#Evolution

If you take something like the dinosaurs, so many genetic variants went extinct during the meteor impact that these variants can't come back. Another group of animals (the mammals) eventually filled the newly vacated ecological niche, and now those dinosaur variants can't come back, because there is no ecological niche for them (except in Jurassic Park).

16

Physiology & Medicine / Re: CKD-EPI equation for eGFR

« on: 17/05/2022 22:19:52 »

2021 CKD-EPI in european units

That is an oxymoron.

If it uses different units for inputs and/or outputs, then the equation will be different, and it is no longer the 2021 CKD-EPI equation.

It is now the jinjon  equation, and since jinjon  does not have the reputation (or published, peer-reviewed papers) of the National Kidney Foundation, no-one will know what that is, and no-one will use it.
So stick to the 2021 CKD-EPI equation, in its original units.

See, for example: https://academic.oup.com/clinchem/article-abstract/68/4/534/6509513?redirectedFrom=fulltext

17

Physiology & Medicine / Re: What is the CKD-EPI equation for eGFR?

« on: 17/05/2022 12:07:05 »

It looks like this 2021 CKD-EPI equation was developed by the National Kidney Foundation, based in New York. So it would make sense that it uses measures common in the USA.
https://www.kidney.org/

They provide the equation here: https://www.kidney.org/professionals/kdoqi/gfr_calculator/formula
- Some of the units are metric, eg ml,  mg and m²
- Some of the units are semi metric, eg dl
- Some of the metric units are mis-spelled, eg mL & dL
- Some of the units are "convenient" units, like kidney filtration time in minutes and patient age in years
- Some of them are constants which appear to have been derived from some regression analysis, like 142 and 0.9938

So if you want the results in "European" units, you will have to define what those "non-American" units are.
- For example, you could easily calculate patient age in seconds, if you think that is more European/Metric, but I don't think that is a good idea; age in years is a convenient measure
- If you arbitrarily change the units, that may change the constants in non-obvious ways
- So I suggest that if you are quoting a measurement of 2021 CKD-EPI, I suggest that you use the standard units, as that allows you to compare results between countries.

The following website shows the "normal" range, and even lets you compare with earlier versions of the formula:
https://www.mdcalc.com/ckd-epi-equations-glomerular-filtration-rate-gfr#pearls-pitfalls

18

Question of the Week / Re: QotW - 22.05.16 - Can we generate electricity through the spinning of the Earth?

« on: 17/05/2022 11:48:16 »

This has already been discussed, here: https://www.thenakedscientists.com/forum/index.php?topic=84653.0
Please add any additional comments there...

19

Chemistry / Re: How well understood is the Chemistry of the trans-uranic elements?

« on: 17/05/2022 11:46:54 »

I wonder if Schrodinger's cat thought experiment needs to be re-written.

Sean Carrol is a cat-lover, and has rewritten the Schrodinger's cat thought experiment: Instead of a radioactive decay releasing a poison gas, it releases an anesthetic gas.
- So the question becomes: "Is the cat awake or asleep?".

20

Technology / Re: Should games be used to train the military?

« on: 16/05/2022 10:05:51 »

On the weekend, I went to an airshow, and the Army was letting members of the public try out their VR helicopter flight simulators - effectively a video game with realistic hand/foot controls.

Judging by the number of people ahead of me in the line who crashed and burned, it's much cheaper to use a game than a real helicopter. (In one case, the apprentice pilot flew straight through a dozen residential homes...)

I was able to take off, fly around a bit, and land (with a lot of help!).

VR_Helicopter_simulator_cropped.jpg (91.38 kB . 720x493 - viewed 301 times)