21

New Theories / Re: Can a photon be visualized ?

« on: 07/04/2019 14:43:51 »

The particle model doesn't predict the speed of light in vacuo because there is nothing there to absorb and re-emit particles.

Maxwell's equations show that you don't need any medium to transmit light, and imply that the speed of light in vacuo is independent of direction, with values of ε₀ and μ₀ chosen arbitrarily to rationalise the system of units in use - they don't refer to the properties of a medium but are the "fudge factors" that make electrostatics and electromagnetics mutually consistent.

22

Physics, Astronomy & Cosmology / Re: Why don't atmospheric gases separate according to specific gravities?

« on: 30/03/2019 00:34:25 »

They do separate, despite thermal and turbulent mixing. The upper atmosphere does contain slighty less oxygen and a lot less carbon dioxide than the lower atmosphere.

23

Physiology & Medicine / Re: what do adheliolysis and laminectomy have in common?

« on: 23/03/2019 08:09:39 »

1. Worth checking the website, at least - they seem to offer very short waiting times: www.welcomebackcentre.com

2. Yes, sadly, there are all sorts of jokes about orthopedic surgeons, but having had a hip joint replaced by an old friend who really does do woodwork in his spare time, I've nothing but respect!

24

Physiology & Medicine / Re: what do adheliolysis and laminectomy have in common?

« on: 22/03/2019 07:38:58 »

Google "upright MRI Canada". There is apparently only one,  in BC, but several (around 80) within reach in the USA (try Chicago and New York - I haven't found one in New England, despite trying to set up a clinic in Boston myself!) and at least half a dozen in the UK. Worth a visit - a transatlantic flight costs less than an MRI scan!https://www.uprightmri.co.uk/london.php has the best images - not a commercial as I no longer own the company!

Be careful in your choice. Most "open" MRIs can't actually scan you standing or sitting. The FONAR upright system is a big beast: it can accommodate almost any size and posture and is the most technically advanced. Paramed is a close second, but considerably easier to install so may offer a more local service. I owned a FONAR and two Parameds and have referred family and friends to either. Esaote make a smaller unit with some limitations, mostly used for limbs but if you are reasonably slim it will produce loadbearing images of the spine. Other systems may be available (including a venerable GE "double donut" that never went into volume production) but I haven't worked with them.

The trick is to use a steel-frame magnet rather than the air-cored superconducting solenoid of "conventional" MRI. This allows us to orient the field perpendicular to the patient (which improves signal capture) and put the magnetising coils out of the way, so you walk in between the poles. Imaging is a bit slower than a tunnel unit, but that isn't a problem for skeletal anatomy as the target doesn't pulsate or squirm. The downside of steel magnets is an upper limit on field strength of about 1tesla, and a lot of mechanical and electrical engineering: the big machine weighs over 150 tons and eats 200 kW of electricity, which is why there aren't many around.     

25

Physiology & Medicine / Re: what do adheliolysis and laminectomy have in common?

« on: 21/03/2019 08:16:10 »

It's a general term for relieving adhesion between bits that shouldn't be stuck together by scar tissue.

https://paindoctor.com/treatments/adhesiolysis describes its specific application in spinal stenosis. Clever stuff and less invasive than fusion or laminectomy, but if you can get an upright MRI beforehand, that will give a more reliable indication of the correct target area than a recumbent scan.

26

Physiology & Medicine / Re: where does excess bone go in a laminectomy?

« on: 18/03/2019 08:49:38 »

Laminectomy diagrams show complete removal of the spinous process and its supporting arch laminae to decompress the nerve sheath, so any cutting debris will have been removed by suction during the cutting process. 

27

Technology / Re: What is the net environmental impact of biofuels?

« on: 15/03/2019 09:53:05 »

The net impact has not been impressive to date. Fossil hydrocarbons have an inherently high energy density because they have been dehydrated and compressed over zillions of years, so in principle you  can just pick up a lump of coal or a bucket of crude oil and set fire to it, whereas anything derived from recently living plants has to be harvested, dried, compressed, and possibly boiled or fermented before it becomes useable. Even domestic firewood needs more energy input from chainsaws and trucks than coal, per unit heat output, because you are handling around 50% water in the raw product.

The more subtle problem is in fact water. Fermentation produces low molecular weight alcohols which are usefully combustible (though less energy dense than diesel) but hygroscopic, and also tend to corrode many rubber and plastic parts, so there's a limit to the amount of biofuel you can use in an existing internal combustion or jet engine. It takes as much energy to build a new car as it will use in its life, so rapid fleet replacement is not energy-efficient, even if you can design an engine that tolerates low-mw fuels. Except for methane, which has a very low energy density but can be used in a petrol engine with very little conversion.

28

New Theories / Re: Split from "How fundamental is time?"

« on: 13/03/2019 23:48:09 »

No. An EM wave is information

29

New Theories / Re: Split from "How fundamental is time?"

« on: 11/03/2019 11:25:17 »

Planck length and planck time are often considered to be the shortest measurable quantities but there is no lower limit to the shortest calculable values. For instance unit Planck time is about 5 x 10^-44  seconds, but the is no reason why I can't discuss a period of 10^-45 s elapsing between events: I just won't perceive them as sequential.

30

Physics, Astronomy & Cosmology / Re: Do we go round in circles?

« on: 10/03/2019 12:43:34 »

Before even attempting to go any further, I should check, and possibly improve, my understanding of the mathematical concept of infinity.  Hopefully I will post my thoughts soon.

That path leads to insanity, as many mathematicians have shown!

However most physicists and engineers seem to die with their brains relatively intact, having decided that infinity is a tool determined by context and either means  "Limit_x→0 {A/x}" where A is pretty well anything (physics) or "too big or far away to make any practical difference" (engineering).

Your departure would diminish the challenge and value of these boards! Trying to explain something I think I understand to someone who thinks he doesn't, really gives the grey matter a good workout, and reading the opinions of experts in fields I haven't even thought of is just as much fun. 

Just as Eddington said "the student of physics must become accustomed to having his common sense violated five tims before breakfast", so anyone paddling in the shoals of science must be prepared for some fairly robust exchanges with the whales that explore the deep waters.   

31

Physics, Astronomy & Cosmology / Re: Is all motion relative?

« on: 20/02/2019 14:50:35 »

As an electron has mass, it cannot be infinitesimal since that would imply infinte density - infinities are evenless palatable than infintesimals!

32

Physics, Astronomy & Cosmology / Re: How much wind to blow over a truck?

« on: 13/02/2019 23:59:53 »

The wind force F on a flat surface is ˝ρv²A where
ρ = air density
v = wind speed
A = surface area.

To tip a truck over, you need to move its center of gravity outside the wheelbase.

Consider the wind force F to act through the midpoint of the side of the truck. This gives a turning moment of hF/2 where h is the height of the truck. The restoring moment is Ww/2 where W is the weight of the truck and w is the wheelbase width. If hF> Ww, the windward wheel will lift.

Counterintuitively, it may be easier to tip a loaded truck than an empty one, because adding load raises the CG. If W is constant, you need the same force F to lift the windward wheel off the ground, but with a higher CG you don't have to lift it so far in order to tip the vehicle over.

In practice, things get more complicated. A gust can  cause an articulated truck to snake, and the resultant centrifugal force can tip it even though the initial gust couldn't.

33

New Theories / Re: How do we measure the energy of a photon?

« on: 13/02/2019 23:49:41 »

Having spent many happy hours measuring the energy of photons by various means over many years, I find it surprising to be told that it can't be done.

But then anyone who thinks "waves of intensity" belongs in a scientific discussion has clearly been reading too many Mills & Boone bodice-rippers and not enough elementary physics texts.

34

Physiology & Medicine / Re: Can a nuked thyroid gland regenerate?

« on: 09/02/2019 00:03:30 »

Total ablation of the thyroid is not always required or achieved. It is entirely normal to review all "lifetime" medications from time to time to better approximate normality.

35

New Theories / Re: How do we measure the energy of a photon?

« on: 28/01/2019 18:23:15 »

The answers I have been getting refer to measurig the energy of particular types of photons in particular sets of circumstances. Nothing like a generalised  PhotonMeter that I would have thought would have been dead easy to construct.
ps I would prefer to be referred to as mxplxxx rather than the very unfriendly "the questioner"

A bolometer will in principle directly measure the energy of any and all photons. As with rockets, the science is indeed dead easy but the engineering is extremely difficult. It took my team 15 man-years to measure the absolute energy of a radiotherapy photon beam to a reproducibility and traceability of 0.1%.

And it all depends on what you mean by "directly". Time is what comes out of the NIST primary standard atomic clock, but you wouldn't use it to decide when to stop boiling an egg. Length is defined in terms of time and the speed of light, but you wouldn't use radar to buy curtains. So we use different gadgets to measure the energy of different photons, and most easily, in a tiny weeny bit of the electromagnetic spectrum, we just use our eyes.

Interesting to note that an unpronounceable assembly of consonants can be considered a friendly form of address. But  Llanfair­pwllgwyngyll­gogery­chwyrn­drobwll­llan­tysilio­gogo­goch is probably the only word in any human language that  has three consecutive l's, so I'll assume our friendly questioner is Welsh. Iechyd da.

36

Technology / Re: Is there a material that can receive 0.5kW of heat in one hour ?

« on: 27/01/2019 17:24:45 »

You need to understand the difference between heat (the total kinetic energy of all the molecules in an object) and temperature (the average kinetic energy of a molecule in an object). Some materials retain their integrity at high temperatures, some conduct heat very slowly.

The amount of heat required to raise the temperature of a substance is its specific heat capacity, expressed in joules per kilogram.degree.

The amount of heat required to change the state of a substance is the latent heat of  fusion or evaporation expressed in joules per kilogram.

The rate of heat transfer across a substance depends on its thermal conductivity, expressed in watts per meter.degree

So if you can state exactly what property you are looking for, someone can direct you to the appropriate table of material properties.

A device like an oven glove needs to have a low thermal conductivity, and a melting point well above the highest temperature you are likely to encounter. A dry cotton towel is the standard tool in restaurant kitchens because it contains plenty of air, which has a very low conductivity A wet towel is useless because although its heat capacity is about 3000 times that of a dry one, water is a very good conductor of heat.

You also need to specify your starting conditions. The latent heat of melting ice is very useful as a heat shield at low ambient temperatures, but the latent heat of vaporisation of water is 7 times as large.

37

General Science / Re: How does an aircraft altimeter work?

« on: 25/01/2019 10:18:17 »

Indeed.  The altimeter has a "subscale" which you can reset to pressure values given by Air Traffic Control or an automatic weather station. The numbers are preceded by a 3-letter code.

QFE  is the atmospheric pressure at the threshold of the runway in use.  Its use should be obvious! An altimeter set to QFE reads "height".

QNH  is the current pressure at sea level. This is important once you have taken off, because the terrain below you is mapped as height above sea level, and at low level (generally below 6000 ft in  the UK) everyone else will be flying with the same QNH, so  you use it for terrain and traffic avoidance. As atmospheric pressure  varies with time and place, ATC will transmit "regional QNH" from time to time. This is the lowest forecast pressure for the next 2 hours in the region, and it can vary quite sharply as you transit the regional boundary. An altimeter set to QNH reads "altitude".

QNE is the standard pressure (1013.25 hPa) setting that everyone uses above the "transition altitude" (6000 ft or whatever the national regulations state). The altimeter reading with QNE set on the subscale, is reported as "flight level" in hundreds of feet,so 12,000 ft is reported as "FL120". This regulates traffic separation in airways: you are well clear of the deck so you can rise and fall with the actual local air pressure, knowing that everyone else at your flight level will be going in roughly the same direction.

There is some debate about the safest way of organising things. As Colin says, some countries dislike QFE and leave it up to the pilot to aim for the published altitude of his destination, using QNH. This avoids possible confusion and reduces workload on departure, but you can't assume that the forecast regional QNH, which is an estimate for the whole region in 2 hours' time, actually applies here and now, so you call for local QNH when you are 10 miles out and end up twiddling a knob and doing mental arithmetic at the same time.

Just to add to the fun, some aircraft have terrain clearance radar that looks ahead, and military and survey aircraft (particularly helicopters) may also have look-down radar that reads actual height above the immediate terrain

38

New Theories / Re: How do we measure the energy of a photon?

« on: 24/01/2019 13:03:11 »

E = hf is Planck's definition of h. How can that be controversial?

Regarding integration: Way back in the dawn of time I built a calorimeter to measure the energy deposited by photons emitted from a cobalt-60  source.  The integration time was around 100 seconds, and knowing the activity of the source and the solid angle covered by the calorimeter (plus a whole lot of corrections you can find in the textbooks), we could calculate the number of photons involved. It became the first UK national primary standard of absorbed dose for radiotherapy.

I don't think there is any doubt about the "nature" of a photon. It is what is emitted when a charged particle accelerates or changes state. The only confusion arises in the minds of people who confuse a mathematical model of a photon with an actual photon, because there are two useful models.

Back to the philosopher's favorite animal - the cow. There is no doubt as to the nature of a cow. Just like photons, you can see some of them. You can model a cow in terms of various ratios: grass input to milk output (a useful model of a live cow) or gross weight to edible pies (a useful model of a dead cow), but neither model tells you "what a cow is".

39

Physics, Astronomy & Cosmology / Re: Can two protons differ in mass?

« on: 18/01/2019 00:13:03 »

Indeterminacy would apply equally to the mass of both particles, because the concomitant of indeterminacy is indistinguishability.

40

General Science / Re: General question on the efficiency of a car in different gears at the same speed

« on: 16/01/2019 23:52:50 »

In the bad old days we used to have a manifold vacuum gauge that effectively indicated efficiency: a wide-open throttle or low RPM reduces the pressure differential across the throttle butterfly, so max vac at any given speed indicates minimum fuel consumption. Whilst this was usually achieved in the highest gear with a 3 or 4 speed box, it often turns out to be in 4th gear at 40 mph with a 6-speed box: in any higher gear the engine will be turning on the low-speed side of the power and torque curves.

Modern cars with 6 speed boxes tend to have multifunction display panels, one function of which is "instantaneous MPG". Worth investigating the max MPG at various constant speeds on a runway or a flat motorway section (preferably at 3 a.m. if you don't want trucks hooting at you). Police instructors generally recommend 3rd, 4th or 5th in any restricted area, saying that the economy of a modern fuel injected car in the 30 - 50 mph range is not improved by using a higher gear and speed control is better in the lower gears.