1

Physiology & Medicine / Re: which scans are best NMRI or PET

« on: 12/09/2019 23:45:00 »

Calculating "committed dose" is actually a lot easier for short life nuclides than long ones, since we don't have to worry about precise retention and excretion coefficients. It's quite a process of measuring the administered activity, estimating the decay throughout the imaging process, adding up all the detected activities, and assigning organ sensitivities to the bits where the labelled stuff ends up. Fortunately it's all been done many times so for a routine procedure the required total activity is calculated according to body weight and the committed dose is estimated from standard tables.

NHS hourly costs include a lot more than consultant fees. Assuming he is an in-house consultant he will have a secretary, a nurse, a lab technician, a registrar and maybe a couple of students directly in tow, plus all the secretarial, housekeeping and administrative overheads, rent, heating, lighting, amortisation of the capital kit……. £500 per consultant-hour is indeed pretty much what it costs to run an outpatient session in a teaching hospital. For comparison, it's a hell of a lot cheaper than keeping a fighter plane in the sky, and the pilot will be lucky to get more than £50 per hour.

Best wishes for a definitive diagnosis and successful treatment! 

2

Technology / Re: How can renewable energy farms provide 24-hour power?

« on: 02/09/2019 19:03:12 »

Experience, though it was some time ago. The essence of a supercap is a very thin dielectric layer

C = ε_mε₀A/d

where ε_m is the relative dielectric constant of the dielectric material m and ε₀, of the vacuum. A is the area of the plates and d is the thickness of the dielectric. Modern dielectrics can have very high values of ε but the manufacturing process is always a compromise between practical values of A and d. Mechanical fracture or chemical failure at a point in the dielectric produces a local short circuit through which the capacitor discharges, with catastrophic results.

Things may have improved - I hope so!

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Physics, Astronomy & Cosmology / Re: If earth rotates why don't hanging items swing?

« on: 19/08/2019 23:38:33 »

Look at a Foucault pendulum. It uses the conservation of angular momentum to demonstrate the rotation of the earth.

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Physics, Astronomy & Cosmology / Re: Matchstick physics and central european myths

« on: 09/08/2019 23:35:30 »

Two possible explanations

1. Matches are made of dry wood, which floats. Some are waxed, so they never sink.

2. Nobody had cursed you.

If you let the match burn completely, you may end up with a carbon residue that might eventually sink, especially if you add a bit of soap to the water. Or you could use  vodka or petrol, in which case more of the matches will sink. 

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

« on: 07/08/2019 14:56:34 »

differentiating between a sequence, which converges towards a terminal value that it can never reach, one that converges towards no identifiable limit and one that converges towards an endless decimal fraction, does seem to be making little more than a conceptual distinction, however valuable the concepts might be in mathematics.

It's enormously important!

In the first case I can write down a rational number (in this case 1) that is just a bit larger or in this case smaller than  any term in the sequence. So if you reported your position every hour as 1.1, 1.01, 1.001...miles from my house, I would know exactly where to look for you.

In the second case, I would know you are horribly lost and not recoverable because you are moving away from me at about 1 mph.

In the third case, although I couldn't write down a rational number that tells me where you would end up, I know it would be less than 2.7183 miles away.

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Just Chat! / Re: What impact will BREXIT have on Science & Society?

« on: 04/08/2019 11:54:08 »

Was it a majority of pensioners that voted for Brexit, many of whom are now dead a year later?

Not many dead. About 5% per annum.

I voted for Brexit because I'm fed up with paying a membership fee to a club whose primary function is to bankrupt my country. I look forward to a time when I can once again recruit staff on the basis of ability, not nationality.

7

The Environment / Re: Hottest day - Cambridge

« on: 26/07/2019 22:07:20 »

Oddly, the Met Office doesn't seem to record temperature at Cambridge airport, where it actually matters! However at 1400 - 1500 yesterday the Met Office station out of town at Histon recorded 41C whilst the temperature in town at Chesterton was "only" 38C.

Whilst we generally regard cities as heat islands, it is noticeable that Mediterranean cities have tallish buildings with narrow streets, so the bit where people live and do business is mostly in shadow, with sunlight reflected from roofs and the considerable thermal inertia of brick and stone evens out the day and night temperatures. So it is entirely possible that a village green  surrounded by low buildings gets a lot hotter than a city street during the day.

The hottest places I can find on yesterday's map were villages at Mereworth (Kent) 44C and Saxilby (Lincolnshire) 43C, again a few degrees hotter than the nearest town.   

If Helsinki really is west of Cambridge, that would explain why I sometimes get lost, but my copilot puts it down to having the map upside down.

You can certainly brew up in a traffic jam but official weather stations tend not to be at roadsides. Slow traffic in town  will heat the air but a good solid motorway tailback probably won't have much effect until the ambient temperature exceeds 25C, at which point everyone will start his engine to run the aircon!

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Physics, Astronomy & Cosmology / Re: why are some heavenly bodies tilted on their axis and not others?

« on: 24/07/2019 08:45:18 »

Assume that heavenly bodies are formed by the gravitational accretion of random dust.

If two particles approach each other, but not in a straight line, there will be some angular momentum about their barycentre (mutual center of gravity). So the clump will spin, as angular momentum is conserved.  The axis of the spin is perpendicular to the plane in which they finally approached, and is independent of any other factor. So as a planet grows, it will develop a spin vector all of its own.

Now consider two large particles: a planet and a much larger star. What looks like planetary capture is actually the same phenomenon: two large particles orbiting around their barycenter, but the barycenter is close to or even inside the star, so we tend to consider the planet(s) to be orbiting around the star. But the plane of orbit has no necessary relation to the spin of either body.   

Our moon does not appear to spin when seen from earth because it seems to be a chunk of Earth thrown off by some explosion or collision, rather than an independently-formed  accretion.   Other moons do spin, with spin axes oriented sort-of-close to their planet, and other planets spin and orbit in pretty random orientations relative to ours.

9

Technology / Re: Why are the temperature readings abnormally high in Southern Portugal?

« on: 21/07/2019 20:54:21 »

Check your data by googling "METAR FARO". Look for the "allmetsat" website. This gives you the most recent (METAR)and "best forecast" (TAF) data for all the world's commercial airports.

Check also for "obvious" errors - US reports will be in Fahrenheit, satellite infrared data may be visible bare ground (standard met report is air at 1.5 m above ground in the shade), and just to make it interesting, local time in Portugal is an hour behind the rest of Europe!

Then check for other anomalies. You'd expect Faro to be cooler than, say, Seville during the day and warmer at night, for instance.

And remember that "the region around" a coastal airport is going to be quite a bit hotter than the airport during the day if there's no cloud. The BBC forecast may well be based on airport data.

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Physics, Astronomy & Cosmology / Re: Can a test of reciprocal time dilation be constructed from this?

« on: 10/07/2019 23:39:01 »

If that's all it does, it seems to only measure my speed component in the direction the gun is pointed, a fraction of my actual speed if I'm not driving directly towards or away from the gun.

Which is why it is very difficult to defend a charge of speeding against a Doppler radar gun - it can't indicate a higher speed than the target.

Alice could count the rate of rotation of her wheel (I've reduced them to unicycles to save engineering) which is of course how surface vehicles measure speed. Now suppose Bob's vehicle emits a light pulse for each rotation of the wheel. As they approach, Bob appears to be going faster than his measured speed, and as they recede, slower.

Suppose we fit an elastic cylinder to Bob's wheel. As it spins, it stretches, by centrifugal force. At constant speed, its diameter is constant. So Alice can estimate Bob's speed by the diameter of the cylinder, either by looking or
 by asking Bob to look and radio her, with no need for a time base. She could have an identical cylinder on her wheel, and using her own clock, can calibrate the speedometer. I can't think of a relativistic reason why Bob's cylinder should appear to contract perpendicularly to the velocity vector, so she now has two different values for their relative velocity.

11

Just Chat! / Re: Can science prove God exists?

« on: 10/07/2019 18:22:23 »

Does the Roman empire still exert an influence today via its religion?

Philistines, Romans, the Inquisition, Communism, Fascism, even the British Empire....all came and went. We're still here. And, if modern-day antisemites are to be believed, we're still in charge of everything. The answer seems to be in having a faith that doesn't seek to convert or conquer others, or even argue very loudly for our precepts. It would be nice to be ignored for a few thousand years, but as Topol pointed out "If you look at a map, Israel is about the size of Yorkshire. If you look at a newspaper, it's bigger than Russia."   

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Physics, Astronomy & Cosmology / Re: Is the shift in the wavelength of light cancelled during free fall?

« on: 08/07/2019 08:47:12 »

True. But the Doppler shift depends on the relative velocity of source and receiver, and the gravitational shift, on their relative gravitational potentials. If both are in free fall towards a point gravitational source but at different altitudes, or even widely separated at the same altitude, all these numbers will be time-variant, so you need to consider the flight time of your photon from source to receiver.

I prefer to avoid "observer" in this forum as some people think physics is bound up with "consciousness" - whatever that is. AFAIK physics preceded anything that might be described as conscious!

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Plant Sciences, Zoology & Evolution / Re: Why do gulls and crows go after eagles?

« on: 28/06/2019 23:39:31 »

Because eagles, kites, buzzards and the like, prey on smaller birds. It's something of a "bomber v fighter" combat: if a bird of prey can catch a small bird unawares on the ground, it's an easy lunch, but once airborne a couple of more manouverable birds can confuse and attack a heavier beast with a large turning circle. Once the larger bird has perched it is no longer a threat as they generally need to dive to gain initial flying speed whilst the shorter-winged corvids can climb rapidly from a standing start.

Crows are particularly good at close combat in groups - highly intelligent and sociable, and they spend a lot of time practicing close formation flying in thermals. Seagulls I think are less aerobatic but very aggressive and fairly heavily armoured compared with crows. 

The unequal fight seems to be between sparrowhawks and pigeons - the hawk appears to single out its prey in flight and strikes with a slashing dive, too quickly for the pigeons to organise a defence. My garden is often littered with pigeon debris.


 

14

Just Chat! / Re: Can science prove God exists?

« on: 26/06/2019 00:05:23 »

First, determine what you mean by God. Then decide what you would consider proof of existence. Now look for that proof. Or, if you want to be scientific, look for disproof.

So far, I haven't come across anyone whose definition and chosen proof have stood up to scrutiny.   

You could postulate "somebody must be in charge of everything" which may not be disprovable, but he's clearly not on the side of homo sapiens, so I'm an antitheist.

15

Technology / Re: Will this 2-probe oscilloscope produce an XY display?

« on: 24/06/2019 11:56:33 »

Can find no email contact for this firm only phone, but the blurb is there.

Recommend using the "chat" tab on their website - they are very helpful when it's active.

16

Technology / Re: Will this 2-probe oscilloscope produce an XY display?

« on: 23/06/2019 22:39:29 »

Indeed, Pico, Hantek and other serious professional DSO plug-ins use a PC for control and display. The nice thing is that if you are serious about an art/entertainment installation, you can use the PC slave display output to drive a damn great projector.

Lissajous figures from raw stereo music are likely to be very messy but Pico software includes all sorts of filters, so you may be able to isolate the bass for a cleaner picture.

Funny how life has got more complicated and simpler at the same time. The old BBC "Tomorrow's World" introductory sequence (1965) had the music displayed on an analog scope and  captured on cine film - it probably took about 20 minutes to rig the whole take, from stuff just lying around in every recording studio!

17

Technology / Re: Will this 2-probe oscilloscope produce an XY display?

« on: 23/06/2019 15:18:03 »

It will do the job if the scope has XY display facility. Picoscope and Hantek definitely do, and as I think I remarked earlier, Pico have been generous in the past with software help and mods, so should be no problem getting rid of the graticule and displaying on as large a screen as you like. 

Other DSO systems are available - these are the only two I have used personally, and I've had less success with Hantek software mods.

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Physics, Astronomy & Cosmology / Re: Is it possible to connect a second analogue display to an oscilloscope?

« on: 21/06/2019 22:46:05 »

It's not the most obvious feature as most people want to make measurements from a digital scope, but if you want to display a static waveform or Lissajous figure you can use a Picoscope plug in to a PC to capture the data, transfer it to Excel, then draw whatever graph you want from the spreadsheet, with or without a grid.

Other plugins (e.g. Hantek) are available but I think the Pico software is particularly transparent and adaptable.

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New Theories / Re: Can I convince you that consciousness (and therefore spacetime) is involved?

« on: 12/06/2019 08:20:05 »

Define consciousness, and we'll talk.

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Technology / Re: Why do aeroplane propellers appear curved?

« on: 09/06/2019 15:45:25 »

"I say, chaps, the prop seems to have stopped but we are still flying"

"Praise the Lord"