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Physics, Astronomy & Cosmology / Re: What is the difference in APPLICATION of characteristic and continuous spectrum?

« on: 29/06/2020 23:04:46 »

We don't use the entire continuous spectrum for radiography.  Low energy photons are absorbed in superficial tissue so contribute to skin burning but not to image formation. High energy photons penetrate further through tissue but discriminate less between low atomic number (low Z) and high Z absorbers, so you get less contrast between bone (containing calcium and phosphorus) and soft tissue (mostly carbon and water). So we limit the x-ray tube voltage to less than 150 kV and remove the low energy part by filtration through aluminium or copper, leaving a spectrum with a peak intensity around 40 - 70 keV which forms the most useful images for general radiology. A tungsten anode yields a fairly clean initial continuous spectrum (the characteristic lines at 60 keV are a small fraction of the useful energy) that can be tailored to the  thickness of the part to be x-rayed by simply altering the tube potential.

In mammography we are trying to image subtle differences in soft tissue density and pick out tiny calcifications. This is best done with a very narrow spectrum to minimise surface dose (from a low energy component) and low-contrast fog (from high energy photons).  It turns out that the molybdenum Kα characteristic line at 17.5 keV does the job very well. If we restrict the anode voltage to less than 30 kV and absorb the higher energy part of the continuum with a Mo filter, we get a beam that is close to ideal as long as we can compress the breast to minimise the overall absorption and scatter of the useful photons.

It would be really great to get rid of the low energy component altogether, but whilst a radionuclide source would give us a genuine line spectrum, the effective source diameter would be too large -  we need a point source with a high dose rate to get a sharp image.

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Technology / Re: Are heat pumps viable in areas of high density housing?

« on: 23/06/2020 00:51:04 »

Ground source heating is fraught with problems, not only of infrastructure but also of diminishing efficiency in times of high demand as the ground freezes around the pipes. Airsource heating is a lot easier to install and maintain: I use a single fan unit of less than one meter square to heat and provide hot water for a family in a 1600 sq ft (160 sq m) building. It works with air temperatures down to about -5 degC and a reversible unit would also provide summer cooling (very important in high density cities). No problem with increasing source impedance - you just blow more air through on a cold day. Wind speed is irrelevant as the fan is rated for zero wind, and any air movement just helps a bit.

The real key however is efficient building design. Mine is massively insulated with a huge hot water tank. I have architect friends with a completely passive house in Maine, where even in mid winter a south-facing glass wall provides all the heat they need to keep their huge concrete slab floor warm. A builder friend in England has made three terraced town houses with passive, heat-exchange and air-source heating that demonstrate how to achieve high density and thermal efficiency. 

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Physics, Astronomy & Cosmology / Re: Is EM radiation power affected by mass of the source?

« on: 16/06/2020 23:21:32 »

As it should be. A volt is a potential difference of one joule per coulomb, and an amp is one coulomb per second. Power is the rate of doing work, i.e. joules per second, whatever the source. Same thing with an engine burning fuel (joules per kilogram) at a given rate (kilograms per second). Hence hybrid vehicles!

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COVID-19 / Re: What's the current opinion on masks to reduce Covid-19 spread?

« on: 07/06/2020 12:59:26 »

Aha! The Cummings-Johnson design: glittery, with a hole in the middle.   

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General Science / Re: Is consciousness fundamental?

« on: 07/06/2020 10:45:03 »

Welcome! I have no positive recommendations but a word of caution.

Be very critical of texts that mention "observer effects". Many physics textbooks use the term "observer" to mean "anything that is changed by an event". A lot of amateurs take the word literally, and assume it means that consciousness, however you define it, affects the result of an experiment - you will come across this particularly in the "double slit experiment".

In many classic experiments the "observer" was a photographic film, so either the film was sufficiently psychic to know that a human was going to look at it some time later, or consciousness is irrelevant.

And full marks for a clear exposition of the question!

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Physics, Astronomy & Cosmology / Re: Is this gyroscopic antigravity?

« on: 01/06/2020 18:31:52 »

A rotating object defines a direction along the axis of rotation. Then any other vector defines "sideways".

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Physics, Astronomy & Cosmology / Re: Is this gyroscopic antigravity?

« on: 01/06/2020 08:24:44 »

Eric Laithwaite was an interesting character. His work on electromagnetic suspension and motor systems was inspirational and timely, and he contributed some great ideas in the aerodynamics of buildings, but his lack of understanding of the conservation of angular momentum was baffling.

Interestingly, Eric Braithwaite also served in the RAF and studied physics, but became a schoolteacher, author and later a lecturer in English, not engineering.

The odd thing is that Laithwaite's RAF career was as a test engineer on autopilots, so he should have understood the theory of gyroscopes pretty well.

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COVID-19 / Re: Does the UK record cold viruses?

« on: 30/05/2020 23:28:56 »

It's a pity that the Common Cold Research Unit was closed just as the techniques of analytical virology were accelerating.

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COVID-19 / Re: Why doesn't the UK just repurpose South Korea's COVID-19 app?

« on: 29/05/2020 21:49:08 »

Politics.

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Physiology & Medicine / Re: Will there be a drug that will enable us to live to 120 in 20-30 years time?

« on: 27/05/2020 16:03:34 »

It's been done without drugs. Jeanne Calment made it to 122 by, she said,  giving up smoking when she was 95.

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Physics, Astronomy & Cosmology / Re: Do electromagnetic waves have a fixed speed?

« on: 23/05/2020 11:42:24 »

Deriving the speed of light from Maxwell's equations is similar to Bell's inequality in that both are universally accepted but I have never seen a simply understood explanation for either.

try www.maxwells-equations.com  The video is excellent and links the weird-looking 3-dimensional vector calculus to actual laboratory experiments with electromagnets and capacitors.

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COVID-19 / Re: Infections: Which is the best metric?

« on: 11/05/2020 22:39:19 »

I think "staying indoors" is just a simplification of "restrict your interactions". You are unlikely to catch a respiratory infection in the middle of a field with nobody around, in your own car, or even in a large garden, but modern society crowds people together at work, in shops, etc., so "stay indoors" actually means "don't mix with strangers".

I'm in contact with people in a small mining town in Australia. The "native" population and the few resident managers would have no problem as they are hundreds of miles from any source of infection, but the shift workers who fly in for "2 weeks on/1 week off" may be infective, so the permanent residents have to isolate themselves from the migratory group.

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General Science / Re: Cheese shelf life, why?

« on: 11/05/2020 22:25:44 »

Surface to volume ratio, and infection.

Cheese is produced in large blocks, either sterile or infected with specific bacteria that produce a local flavour. The blocks are covered with a protective wax rind or salt, so everything that happens inside is controlled and predictable.

When the block is cut for retail, this highly nutritious material is exposed to oxygen and all the bugs and yeasts floating about in your kitchen, so what was once a slow maturing process is now a playground for anything and everything that eats cheese. Prepacked wedges are prepared in fairly sterile conditions so may last almost as long in your refrigerator as the original bulk cheese but as soon as you open the pack you are turning it into a potential can of worms.

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Cells, Microbes & Viruses / Re: Are 85 Covid-19 deaths ON AVERAGE a day per 60 milllion a reason to PANIC?

« on: 04/05/2020 22:01:12 »

There's a significant difference between rapid decisive action and panic. Indeed they are polar opposites. An immediate lockdown of all nonessential interaction would have prevented COVID spreading much beyond the initial dozen or so cases in the UK. And it wouldn't have been all that damaging: the economy picks up pretty well after Christmas and New Year when primary and secondary industries effectively close down for 5 - 10 days.

So the trick for future epidemics (and there will be more - nature is indifferent to the fate of humanity)  is to determine in advance what activities and industries are essential and how those workers will be segregated into nonoverlapping shifts and squads (the lights don't go out at Christmas), then how a short, sharp lockdown will be policed and essential supplies rationed and distributed. We have had blackout (once) and petrol rationing (a few times) imposed  in my lifetime, immediately and at the stroke of a pen. Taxes change overnight and everyone adapts. I've participated in plans, preparations and exercises  for a 7 day transition to nuclear war, with a reasonable chance that they would have succeeded in keeping the immediate survivors alive. You get a lot more warning of an epidemic, and if you impose early lockdown and quarantine, it can be contained and eliminated. But it takes long-term commitment and planning, which is anathema to modern government.   

This thread began 6 weeks ago with 85 deaths per day. Had we had sensible plans and decisive action, that could have been the peak, but faffing and fumbling about has led to daily totals of hundreds of deaths and thousands of severe infections, massive economic impact, disruption of health and social services, and a probable death toll exceeding 40,000 this year plus maybe a million rendered unemployable. All completely unnecessary, as demonstrated in Korea.

6p - proper planning prevents piss-poor performance!

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Just Chat! / Re: Why Do Scientists Refuse To Experiment

« on: 03/05/2020 15:43:31 »

You may have stumbled upon the obvious. Half the people in any courtroom are lying. It is the job of the judge and jury to determine which half is the more plausible.

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Just Chat! / Re: Why Do Scientists Refuse To Experiment

« on: 03/05/2020 15:38:55 »

I think the key is in the word "rational".

Or you might look at the smug and inhumane behaviour of believers over the last 2000 years and decide it would be unethical to associate with them.

Or you might ask which Him? The protestant or the catholic, since belief in either is apparently a reason to despise the other? Or the prophet in whose name you must slaughter infidels, oppress women and ban anything that looks or sounds enjoyable?

You don't need faith to behave decently but it provides an excuse for evil.

Yes, I've looked into it, and quickly put the lid back on that seething mass of superstition and corruption.

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Just Chat! / Re: why would a scientist accept the bible

« on: 01/05/2020 21:36:00 »

What white lie?

You choose. All I require is that it is harmless and that you know it is untrue. Would $15M be acceptable?

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Just Chat! / Re: why would a scientist accept the bible

« on: 01/05/2020 21:34:18 »

Though I have no respect for the analytical intellect of Christians, they do seem to be a pretty canny bunch when it comes to money. Now I can't imagine Constantine, King James or the National Council of Churches paying their editors for simply saying they wrote something. Surely they had to actually deliver a product? And given the differences between, say, the KJV, RSV and NEB, it's pretty obvious that the dates bear some resemblance to the truth.

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Just Chat! / Re: why would a scientist accept the bible

« on: 01/05/2020 00:13:22 »

 

women who went to any kind of religious service more than once a week had a 33% lower chance than their secular peers of dying during the 16-year study-follow-up period. .......................  worshippers were 55% less likely to die during the up to 18-year follow-up period than people who didn’t frequent the temple, church or mosque.

So it's nothing to do with Jesus, then. Or even monotheism. Indeed if you include Buddhism, any deities at all. Apologies for applying the scientific method.

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COVID-19 / Re: Why can't we re-use PPE?

« on: 30/04/2020 19:35:16 »

By the time you have resterilised a latex glove, both inside and outside, it's pretty useless as a glove, even if it isn't torn or punctured. Likewise single-use aprons: very difficult to sterilise with ethylene oxide, not washable, and distorted to uselessness by steam. You might collect them, remove the blood and gunk (by hand!)  and sterilise in bulk with a cobalt irradiator, but the machines are expensive and pretty well full of urgent items like syringes, and if you knowingly begin with infected material, you can't rely on batch sampling to assure product sterility - you have to test every one!

The point of a HEPA filter respirator is to trap infectious droplets. So if you have a washable (plastic) matrix, you end up with a large volume of infected water and a wet filter with a few viruses. So you dry the filter with moving air and disperse virus (a) into the public water system and (b) into the factory air. Bag, bin and burn, please.

Cloth gowns can be laundered but they are surprisingly fragile and have a hard life in the ward or theatre, so they need to be inspected before repacking to make sure there are no gaping holes or missing tapes - all very labor-intensive in an expensive sterile area, so may not be first choice, and in the case of high infectivity, at some point you will have a bin full of evil to store in the ward then move along a public corridor and unpack in the laundry. Bag, bin and burn paper if you want to save lives.

Hospitals do clean and re-use crutches but most are stolen or at least forgotten because the discharged patients don't know how to return them! Crutch hunting and wheelchair tracking is a major activity of the Red Cross!