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Physics, Astronomy & Cosmology / Re: why do a lot of people confuse between interference and diffraction?

« on: Yesterday at 16:16:44 »

When a guitar string is plucked, it vibrates and produces a wave that travels along the string. This wave is a longitudinal wave, which means that the particles of the string move back and forth in the same direction that the wave is traveling.

Wrong, obviously. Not a good starting point for an essay.

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Just Chat! / Re: Is there a universal moral standard?

« on: 01/06/2023 00:41:00 »

Never mind ice cream. Every book for sale at the airport bookshop is a "best seller".

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Just Chat! / Re: A question for our paragon of aeronautical erudition, Alancalverd

« on: 31/05/2023 16:49:40 »

We did try, but the main harbor road in Porec is named after Nikola Tesla and the first bloke we met at dinner was a retired physics teacher, then somebody started dropping parachutists out of a Cessna Caravan and somehow it all seemed a bit like being at home.....

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Just Chat! / Re: Why is Brexit a right-wing cause?

« on: 31/05/2023 16:36:44 »

....before the NHS was established.

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Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 30/05/2023 23:40:34 »

ChatGPT seems like a longwinded philosopher rather than someone who actually understands information theory. Try Wikipedia - the entry seems to have been written by folk who know what they are taking about.

Strictly, of course, a code is a string that identifies a longer string stored in the receiver, whereas a cipher simply substitutes one symbol at a time so that the decrypt contains the same number of bits or whatever as the encrypted string. Thus "BMBO" is a simple cipher for "ALAN", but "ALAN" is a code for "an old geezer in Cambridge with nothing better to do with his time".

The unexpected message will only contain information if the receiver has some preconception of what the sender means. Thus ALAN or even .-  .-..  .-  -. will probably denote at least "a British bloke" to many earthlings but conveys nothing at all to a Martian tree frog.

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Just Chat! / Re: A question for our paragon of aeronautical erudition, Alancalverd

« on: 25/05/2023 20:46:58 »

I paid someone else to fly me (and the boss) in a fairly serviceable 737  to somewhere I didn't have to think about physics for a whole week. Croatia is recommended.

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Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 25/05/2023 10:10:15 »

What is a bottle, first of all?

A bottle is something we make (or imagine) to contain something else. It is a member of the set of containers, which includes boxes, cages, and finite bounded universes.

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Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 25/05/2023 09:53:41 »

Energy is a conserved quantity in newtonian mechanics, but interchangeable with mass (another newtonian conserved quantity)  in relativistic mechanics. Nothing more, nothing less.

School curricula tend to be written by educationalists, not teachers, and therefore serve only to confuse the student and put him off "difficult" subjects like physics. Fact is that physics is really dead easy because it is about what happens (dynamics) or doesn't happen (statics) - stuff you see in everyday life.  Quite unlike history (deciding which account of stuff you never experienced is less unreliable) languages (the grunts made by apes who look like us but live somewhere else) literature (many books of bad English written about a few lines of good English) or religion (you'd be prosecuted for selling any other product that doesn't work).

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Geology, Palaeontology & Archaeology / Re: No Evidence of the Exodus?

« on: 15/05/2023 14:15:51 »

If exodus didn't happen, why am I here and not there?

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Physics, Astronomy & Cosmology / Re: Can Light Experience 'Time'

« on: 14/05/2023 08:49:12 »

Time is what separates sequential events*. A photon cannot experience sequence as it has no memory.


*an original statement, but apparently my predecessor Prof A Einstein said "time is what prevents everything from happening at once", an arguably anthroponormative definition but having the same import.

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Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 14/05/2023 00:09:34 »

New readers:

Note the difference between physicists ("consider a spherical cow in a vacuum.......") and mathematicians ("an instantaneous sample of all the photons in the universe......").

Or as others have put it, "Rocket science is two equations. Rocket engineering is a lot more complicated."

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New Theories / Re: Universal Utopia? What's The Universal Terminal Goal?

« on: 13/05/2023 11:34:55 »

There would be some optimal and balanced compositions for different cases in different conditions and importance.

A well-known problem with databases. The customer always asks for an accurate, up-to-date, database. The supplier asks "which do you really want?"  Big problem with medical research: we know exactly who entered the trial 5 years ago, but we don't know who died yesterday. So how many people do we need to recruit in order to decide whether the procedure actually extends life? The quicker we get the answer, the more lives we can extend (or not harm) by our proposed intervention, but the less confidence we have in that decision. So we recruit a bigger sample, but the trial may end up doing more harm if the intervention turns out to be harmful, so we gradually expand the numbers if it looks promising, but at some point the cost of the trial will exceed any profit we might make by putting the procedure on the market, or if we expand too slowly someone will come up with a better solution.....

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Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 13/05/2023 11:18:48 »

I think you have answered your own question quite elegantly.

Einstein said that repeating an action and hoping for a different result is madness, but two random samples of the same thing is not a repeat - definition of randomness!  So the question is what level of confidence you require to assert that they are samples of the same distribution.

The power of the χ² test is it can tell you not only the extent to which your samples may be said to be representative, but also if the fit is "too good" - evidence of a failure of the mechanism, such as a bit of the primary beam getting through your supposed filter.

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Just Chat! / Re: Can someone assist me with some scientific terms?

« on: 13/05/2023 06:24:33 »

We start with observations and develop hypotheses that model them and predict the next observations. When we have gone through the loop a few times we can identify theories (a guess at the mechanism behind the obs) and laws (robust mathematical projections of what will happen in the wider universe, regardless of the mechanism).

Thus we have a law of gravitation F = GMm/r²  that is good enough to put a satellite into orbit, but no idea of how bodies actually attract each other apart from the theory that mass bends spacetime.

Underlying all this is the common language of mathematics which is based on the mutual acceptance of certain axioms such as those of Euclidean geometry (similarity, identity, parallelism, etc) with useful theorems (statements that can be rigorously proved as long as the axioms are true) such as Pythagoras.

As far as I can see, a conjecture is a mathematician's hypothesis: a  theorem that looks reasonable but hasn't been proved.

The difference is that mathematical proof is singular and positive (if A then B, QED) whereas scientific proof  is continuous and negative (we haven't found a system that doesn't obey Newton's laws).

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Question of the Week / Re: QotW - 23.05.05 - How fast does evolution happen?

« on: 08/05/2023 17:34:13 »

I reckon no other creature comes close to man in terms of sexual activity.

You clearly have not studied bonobos or sparrows - God knows how they find time to eat!

You could calculate the R factor for any STD, but you need two measures: the "known source" factor - how many people will be infected from a single source, and the "source density" factor - what proportion of the sexually active population is actually infectious at any time. The success of the COVID  virus is in its long pre-symptomatic infectious period, meaning that the second factor was several times the preponderance of symptoms, but most STDs express themselves before the carrier has infected many others. Problem with AIDS is, I think, the subtlety of its early symptoms.

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Just Chat! / Re: If Religion Wants To Survive...

« on: 07/05/2023 15:05:54 »

Is this a good deed?

Report: Kim Jong Un fed uncle alive to 120 starved dogs

Starving a dog is never a good deed.

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Just Chat! / Re: A question for our paragon of aeronautical erudition, Alancalverd

« on: 05/05/2023 16:33:21 »

One limiting factor with propellors is tip speed. As it approaches Mach 1 you get lots of noise and stress for very little additional thrust. Anyone who has heard a T6 Harvard plodding round the sky at 200 mph will know the buzz-saw noise of its huge 2-blade prop. You can get more thrust at high speed by having more blades, but each blade is working in the turbulent wake of its predecessor at low airspeeds, where you want lots of  thrust for takeoff and landing. The compromise rarely exceeds an airspeed of Mach 0.6 (~ 400 mph) with the prop tips just beginning to scream.

The compressor fan of a pure jet isn't required to produce thrust but to stuff lots of subsonic air into the combustion chamber. Interceptors like the Lightning have prominent passive nose cones to slow down the airflow, and the intakes of the Concorde Olympus engines are still something of a trade secret. Thrust mainly comes from the exhaust gases.

Subsonic airliners are optimised for a different mission: long efficient cruises at Mach 0.9 or thereabouts, so you can use a multi-blade bypass fan with short blades and a very coarse pitch, and tolerate the extra fuel burn needed to get off the deck as it's a small part of the mission.  The "jet" bit is mostly there to drive the fan rather than the other way around!

Not sure about your sooty RB211s. There are all sorts of resonances happening in the cruise and it wouldn't surprise me if this leads to occasional pulsatile overrichness and a bit of smoke - jet fuel is like diesel but cheaper! I recall as a passenger of Ryanair (my favorite airline) being intercepted by a couple of French jets who left impressive smoke trails like old tractors as they climbed towards us at full thrust: the smoke gradually thinned as they accelerated and the mixture optimised around 20,000 ft and 600 kt.

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Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 05/05/2023 09:34:46 »

And, is the universe an entity? If it is, what kind of bottle does it go in?

A really big one. Or, if you are of a mathematical turn of mind, a Klein bottle, which resolves the dispute between Big Bang and Continuous Creation.

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Just Chat! / Re: A question for our paragon of aeronautical erudition, Alancalverd

« on: 03/05/2023 20:02:51 »

Dunno about paragon, but it's certainly a subject I take seriously - the alternative being a fiery death. The Wisdom of the Aeronauts says:

After 100 hours, you know everything

After 1000 hours, you realise you don't know everything

After 10,000 hours you know you will never know everything.

Currently hovering around the 1000 hour mark, so on the steep part of the learning curve.

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Physics, Astronomy & Cosmology / Re: Talking about Physics

« on: 02/05/2023 09:25:12 »

Interesting that the Wikipedia article, when discussing sequential S-G  systems, talks about "measuring" when a particle passes through a nonhomogeneous magnetic field. When we use magnetic fields to select regions for analysis by spin resonance (e.g. MRI) we talk about polarising or forcing, not measuring*.  Wikipedia partially redeems itself with

Given that the input to the second S-G apparatus consisted only of z+, it can be inferred that a S-G apparatus must be altering the states of the particles that pass through it.

(my italics)

which is much more reasonable, and also makes sense if applied to the 45 degree optical polariser.

I may be a bit pedantic in distinguishing between segregating (black sheep to the left, white  to the right) and measuring (counting the sheep in each pen after segregation) but that's the residual chemist in me: qualitative and quantitative analysis are not the same thing. The "triple S-G" experiment simply selects white sheep then arbitrarily paints them red or blue.


*the measurement phase of MRI comes later: we listen to the radiofrequency emission as the selected spins relax and realign to the primary field.