21

Physics, Astronomy & Cosmology / Re: What does the term 'time' relate to in time dilation?

« on: 10/12/2018 21:12:40 »

The bit between birth and death is called life. You can count  the clock ticks if you like, but generally the earth goes round the sun between 50 and 100 times. History suggests that this sequential and repetitive event is not affected by the death of anyone.

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Physics, Astronomy & Cosmology / Re: Can UVA and UVB rays pass through a wood ply roof with a bitumen felt covering?

« on: 07/12/2018 10:42:09 »

No.

23

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

« on: 02/12/2018 23:52:23 »

"God's morality" dictates whatever you want it to dictate. That is the reason for inventing gods.

There has been a lot of confusion here between socialism and a command economy - by no means synonymous. The essence of socialism is "from each according to his means, to each according to his needs". Provided that your tax system is fair and not a disincentive, and your benefit system meets all and only basic needs, you can rely on most people's desire for acquisition and comfort to propel a very pleasant and sustainable  society, as in Scandinavia.

Things go wrong when the state dictates rather than provides. Committees are fairly good at responding to demand, but not at predicting it, nor at innovating solutions or changing direction when things go wrong. The best disasters occur when the product must be presented to the dictator on a fixed date - prototype planes fall out of the sky and unripe corn is harvested to meet the command target, where in a less centralised society a test pilot or farmer can say "let's give it another week" and the shareholders realise that a duff product won't sell in a free market. Common sense (usually) prevails in our constitutional monarchy, where the "official opening date" is rarely set before the actual service commences.

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Physics, Astronomy & Cosmology / Re: Will refinement of measurement standards alter how we think of uncertainty?

« on: 21/11/2018 16:37:44 »

To expand a bit (now that my internet connection is a bit faster!)

If we measure something, we know that there are some uncertainties associated with the procedure itself, which will produce a different number if we repeat the experiment: these are random uncertainties. If we can separate out the random uncertainties to a set of independent uncertainties a, b, c in different parts of the measurement, we add them as though they are random noise, so the amplitude of the overall random or experimental uncertainty is √(a² + b² +c²...)

We also know that there are small discrepancies between the national standards or laboratory sub-standards of length, time, voltage, or whatever against which you and I make our individual measurements These contribute to the systematic uncertainty √x²+ y²+z²....) of our measurements.

Defining the units of measurement in terms of fundamental constants means that the systematic uncertainty of anyone's measurement is now reduced to the random uncertainty with which we have determined those constants, and the universal value is now fixed by definition and consensus rather than by reference to a lump of metal or whatever.

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Question of the Week / Re: QotW - 18.11.18 - Does a Great Dane recognise a Chihuahua as another dog?

« on: 21/11/2018 12:04:47 »

I haven't observed such extremes but it is clear that dogs recognise a wide range of their own species at a considerable distance - way beyond the point at which I can distinguish a small dog from a cat or a fox.

It isn't a matter of smell: my dogs used to attack a stone statue of a cat! And there are distinct class and trade preferences: my gundogs would only socialise with other gundogs, though  they tolerated (nearly) all canines. Why "nearly"? Whilst my flatcoat retriever recognised standard poodles as fellow gundogs, her golden retriever companion embarrassingly snarled at any dog or human with curly hair.

Fascinating animals.

26

Technology / Re: Could a microwave be used to heat a home?

« on: 16/11/2018 16:18:21 »

The rate of heating is directly proportional to volts x amps, so to get the same power from a 12 volt unit, you need 10 times the current that you would use in a 120 volt unit. Higher voltages are preferred because you lose less power in the transmission lines.

If you have solar power, consider using a heat pump and a big hot water storage tank, and also underfloor low temperature (30 - 35 degree) heating rather than traditional 50 degree "radiators" (actually convectors) - "low and slow" is best if your property is adequately insulated.

27

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

« on: 14/11/2018 08:05:39 »

Without delving too deeply into the definition of morality or ethics, I think we can usefully approach the subject through "universal". The test is whether any person considered normal by his peers, would make the same choice or judgement as any other in a case requiring subjective evaluation.

This immediately  leads to a sampling question. "Turn the other cheek" would be considered normal and desirable in some peer groups, whilst "an eye for an eye" might be de rigeur for others. Both strategies have evolutionary validity: think rabbits, which outbreed their predators, and lions, where only the strongest male gets to breed.

Homo sapiens is an odd creature We breed too slowly to survive as prey, and are too weak to be predators, but a very complex collaboration allows us to farm and hunt all we need. That said, although we can see the value of large scale collaboration (like bees and ants) it takes a long time to acquire the knowledge and skills needed to participate, so the small "family" unit (including communes and kibbutzim) is a prerequisite of survival.

Thus we grow up with at least two loyalties, to the immediate family that supports us, and to the wider community that supports the family. No problem if we have infinite resources and unlimited choice, but the decisions we make in restricted circumstances are what defines our morality, and it is fairly clear from daily accounts of religious wars and magistrates' court proceedings that either  there is no universal concept of right and wrong, or that it can be set aside for personal gain.

28

Physics, Astronomy & Cosmology / Re: Does true randomness exist?

« on: 02/11/2018 10:34:45 »

You can go on adding untestable hypotheses ad lib, until you end up with a religion. This will give you an excuse to kill people, so I won't encourage it by continuing an otherwise pointless discussion.

29

New Theories / Re: Re: Have gravity modification experiments been conducted?

« on: 18/10/2018 22:57:36 »

The rate of a clock will be altered by the proximity of any mass. The question is whether it changes if the mass spins. There is no obvious reason why it should.

But it does at least explain a phenomenon that has puzzled me for years. The instrument panel of a light aircraft contains all sorts of delicate machinery which seems to work for years and years despite being boiled, frozen, vibrated, bounced around, flown to silly altitudes, subject to loads of g in all directions, kicked as you get in the plane, sneezed and vomited over, and parked on the grass in all weathers. Except for the clock. Probably the simplest, most robust, most mature piece of kit on the panel, and they never work. Electric or mechanical, all certified airworthy, and I've never known one to actually tell the time. Now I understand why - there are at least three gyroscopes on the same panel!

30

Technology / Re: Are heavier-than-air hybrid airships a good idea?

« on: 18/10/2018 16:54:18 »

True, a GE 90 could lift 6 times its own weight compared with the contraprop 4 times, giving a payload of 40 tons per engine if we ignore the weight of the rest of the aircraft. You can eliminate the need for a tank altogether by just flying your machine over the enemy and literally blowing them away.

31

Just Chat! / Re: Would it be wrong to try to get somebody kill themselves?

« on: 15/10/2018 18:03:46 »

Generally considered a Bad Thing to try to persuade anyone to kill himself, but I think it is equally indefensible to bully anyone into staying alive if he genuinely wants to die. This is where the law needs a serious review. Suicide and attempted suicide are not illegal in the UK if you are sufficiently fit and compos mentis to make all the arrangements yourself. The problem arises always and only if you are so sick, disabled, distressed  or mentally incompetent that you need help: it is a crime to assist a suicide.

So suicide is statutorily denied to those who need it most. In a marginal case, where someone seeks legal assistance (as with Dignitas) it has been argued that a third party purchasing your plane tickets or driving you to the airport, is guilty of assisting a suicide. Fortunately the Crown Prosecution Service has usually ruled that such prosecution would not be in the public interest thus establishing a sensible precedent that should be embodied in statute.

32

Physics, Astronomy & Cosmology / Re: How did scientists measure the mass of the Earth and other planets?

« on: 14/10/2018 09:37:33 »

from: alancalverd on 12/10/2018 09:49:23Using F = GmM/r2 you can calculate the force on a falling object of mass m in terms of M, the mass of the earth, and G, which we assume to be a universal constant.

That doesn’t work.  We’re trying to compute at least a rough G and M here.  We don’t know either of them yet.  We do know force F is 9.8 newtons for a 1KG mass.  We can assume we know r.  We therefore know the product of G and M, but not either separately.

We don't "know" F = 9.8 N/kg. We define the newton as1 kg.m.s^-2, and have a measured value of g = F/m

We don't "assume" r: it is a measured value - see below.

As I said, Cavendish (1797) measured G directly. 

Maskeleyne estimated the density of the planet from an experiment in 1774 at Schiehallion, measuring the deflection of a plumb bob on two sides of the mountain. In fact, that was a measure of G rather than ρ_earth since the assumption that the mountain was homogeneous was more defensible. Newton had considered the possibility but rejected the experiment as too difficult.

As F = ma, we can measure the acceleration of a falling object or the period of a pendulum to get a value for F/m

F=ma works (F 9.8 = 1 (mass) * 9.8 m/sec acceleration), but that doesn’t yield either mass of Earth M nor G, which are the two things we’re trying to determine here.
The pendulum thing is a function of acceleration (9., not of the mass of Earth.  Put a pendulum in a rocket accelerating at that rate and it will have the same period as here on Earth.  It tells you nothing about the mass of the Earth under you.

You have missed the point. If we know F/m we know GM/r², and we have known r since 200 BC (Eratosthenes), so all that was required for an accurate estimate of M was Cavendish's determination of G. The reason for using a pendulum  is that we know the period T = 2π√(Lm/F) for small oscillations, which is a simpler experiment than measuring the acceleration of an object in free fall.

The most amazing fact in all this, hundreds of years and thousands of experiments later,  is that the gravitational masses of all bodies are identical to their inertial masses. Why?

Interestingly, Hipparchus estimated the radius of the moon's orbit around 130 BC, though his assumptions regarding the earth's orbit of the sun were somewhat wild.

33

Technology / Re: Are heavier-than-air hybrid airships a good idea?

« on: 14/10/2018 09:25:39 »

A disk-shaped craft is not as subject to side-winds as a slab-side , like an airship or a tube .

All aircraft are equally susceptible to crosswind in flight. You fly through a "box of air", and if the box is moving sideways or backwards in relation to your intended track, you have to go faster and change course to compensate. It's not the same as crosswind on the ground, which has the same effect as on a car or boat - you weathercock into wind depending on the shape of the vehicle and the position of its wheels or keel.

Obviously the effect is less significant at higher airspeeds, but the increase of windspeed with altitude means that even at 500 kt you need to correct for wind drift and going backwards at 150 kt and 30,000 ft is one of the joys of propeller aircraft.

34

General Science / Re: How does proton beam therapy work?

« on: 05/10/2018 18:00:46 »

Some energy is deposited all along the track, but the interaction crossection increases as the proton slows down, so a lot of energy is deposited at the end of the track. Wikipedia has a good entry under "proton bragg peak".

For a crude analogy, imagine a hot cannon ball flying through a mountain of butter. Initially, it is travelling quickly so there is very little time to transfer heat energy to each meter of butter, but when it stops moving, it can deposit all its residual heat at one point.  So the butter will re-congeal along the track but form a molten pool around the stationary missile.

35

Chemistry / Re: How to calculate the valency of 'Al' in 'Al2O3'?

« on: 05/10/2018 09:34:10 »

Even if you don't know the valency of oxygen, suppose the valency of Al is x and O is y, then you need the simplest integer solution to 2x = 3y, which is obviously x = 3, y = 2.

36

Physics, Astronomy & Cosmology / Re: Is the universe expanding faster than the speed of light?

« on: 24/09/2018 22:35:58 »

Einstein was all about relativity, not absolutism. As others have pointed out here, the energy required to accelerate a mass increases towards infinity as its speed approaches c, but speed relative to what?

If objects A and C are converging towards their midpoint B, there is no reason why they cannot both be travelling at 0.6c relative to B , but neither can see the other approaching at greater than c (with a blue shift). From which we can derive time dilatation and all the rest of that wonderful stuff that happens in colliders.

Moving swiftly from subatomic particles to the observable universe (isn't physics wonderful?) we can observe the redshift of distant galaxies to be consistent with supraluminal Doppler shift. The standard explanation is that they aren't moving away from us, but the space between us is expanding. That's where I get off this particular bus!

37

Question of the Week / Re: QotW - 18.09.24 - Why do I feel the pressure of wellies in water?

« on: 24/09/2018 17:11:07 »

The sensory system adapts quite quickly to even, constant stimuli. That is how we manage to work in a noisy environment or acclimatise to warm or cold weather. The dominant sensation of standing in a pool or bath is the temperature difference between air and water, but this sensation disappears after a few seconds. The even pressure distribution of water produces a weak sensation that is noticeable at the air/water junction but is quickly accommodated as it carries no threat or useful information.

Wellies tend to collapse unevenly producing isolated, moving,  small areas of pressure on the skin, which could be threatening (rapid current, or a snake) or otherwise important, so the system recognises the changes in pressure distribution

The same considerations apply to wearing a stocking which suddenly develops a hole.

38

Physics, Astronomy & Cosmology / Re: Is the universe expanding faster than the speed of light?

« on: 23/09/2018 23:40:49 »

Is there any reason why it shouldn't?

Here's a thought experiment. Assume that the universe U is an infinite array of roughly isotropically distributed bits of stuff. Assume that for whatever reason (and Schwarzchild gives us a perfectly good one) we can only observe a finite sphere X within that array. And assume that "gravity sucks". Now since U >> X, there is more stuff outside X than inside it, so X will expand due to the "suction" of (U - X) - i.e.the stuff outside - so the density of X will decrease, and the rate of expansion will increase.

Suppose we have reached the stage  at which points on the boundary of X are receding from us at 0.55c. Nothing in that statement to contradict relativity. But diametrically opposite points, say to the north and south of us, are moving at 1.1c relative to each other. All this means, relativisitically, is that an observer at N will not have any information about S, which is fine, but the diameter of the observable universe is increasing faster than c.

39

General Science / Re: If I wear ANY battery watch, in by a weeks time, the battery is dead. Why?

« on: 23/09/2018 23:15:58 »

The only common chemicals that attack gold at body temperature are "aqua regia", a mixture of concentrated hydrochloric and nitric acids, and mercury, which forms an amalgam used in mining and metal recycling.

If your sweat rots a gold watch, you would be wise not to travel by air - it is illegal to import aqua regia or mercury into the passenger compartment.

If anything other than aqua regia or mercury attacks your gold watch, ask the Fraud Squad or the local Trading Standards Office to visit your jeweller.

40

General Science / Re: If I wear ANY battery watch, in by a weeks time, the battery is dead. Why?

« on: 21/09/2018 23:04:34 »

If  the  cellphone charge duration is normal, then the problem lies with your wrist environment. Sweaty wrists, or failure to remove a cheap watch before washing the dishes, bathing the dog, working on a trawler, or scrubbing up, can corrode the battery contacts. Judging from your extensive  career and life experience, I find it unlikely that your grandfather had many  battery-operated watches as they did not exist before 1960.

In a spirit of scientific enquiry, you could invest in a solar-recharged watch. Accurist  and Citizen make reasonably-priced "eco-drive" watches that are well sealed against the usual insults (mine regularly cycles up to 15,000 ft and I will admit to some sweaty moments) and never need to be opened - the one I am wearing now has run continuously for over 10 years. As a control, you could wear a self-winding mechanical watch on the other wrist, and see which one fails first.