1

The Environment / Re: Is there any hope of effective action on climate change?

« on: Yesterday at 22:59:48 »

Really large scale deployment of biofuels is, with currently available technology, a non starter; their production takes up far too much land. Small scale deployment might work, maybe 10% of current fuel use (enough for airlines). The rest will probably have to electrify.

The technology- and more importantly- costs of the technology are making major strides and are likely to see widespread deployment by about 2025. All the technologies, like batteries, biofuels, solar, wind, electric cars etc. actually look like they are going to interwork really, really well.

There are some worrying areas, like shipping, it may be that a lot of shipping will have to switch to going overland instead. Electrifying ships seems to be beyond the current state of art, unless there's nuclear power involved.

2

Physics, Astronomy & Cosmology / Re: What is the maximum force of the smallest magnet?

« on: 15/09/2018 22:29:28 »

Magnetic monopoles don't exist, and cannot be constructed.

3

Geology, Palaeontology & Archaeology / Re: To what extent can Biblical accounts of Noah's flood be believed?

« on: 15/09/2018 22:14:05 »

You can't really believe it. Noah's ark story is just a retread of one chapter of the Epic of Gilgamesh.

4

Physics, Astronomy & Cosmology / Re: What is the effect of MRI on body?

« on: 15/09/2018 22:11:46 »

Which bit of  "the iron in your body is not attracted to magnets because it is not metal" causes  people difficulty?

It's not exactly true that the 'iron in your body is not attracted to magnets', or it's deceptive, ferrohemoglobin is paramagnetic.

5

Physics, Astronomy & Cosmology / Re: What is the maximum force of the smallest magnet?

« on: 15/09/2018 18:50:52 »

Two 5x5x5mm cube neodymium magnets will attract each other with a force of around 10N (1 kgf), depending on the grade of magnet. But that's not the maximum possible, you can assemble smaller magnets into Halbach arrays and get greater force over a smaller region of attraction.

6

Geek Speak / Re: Where can I get a totally free VPN?

« on: 15/09/2018 18:05:45 »

I don't think people charge for onion routing. Dog slow of course, but very secure and cheap.

7

Physics, Astronomy & Cosmology / Re: What is the effect of MRI on body?

« on: 15/09/2018 18:01:47 »

A lot of materials aren't affected by MRIs, and many pacemakers are MRI compatible these days:

https://www.bhf.org.uk/informationsupport/heart-matters-magazine/medical/ask-the-experts/pacemaker-and-mri-scan

With iron evenly distributed within tissues, the amount of iron necessary to cause the MRI to tear you apart, would have been fatal well before you ever reached the MRI. Haemochromatosis causes measurable differences on MRI scans, but there's no obvious mechanical issues that affect the patient due to the magnetic field.

The real problem is highly ferromagnetic, sharp pieces of metal within the body. A normal neodymium magnet saturates at about 0.5 Tesla, and most people that have played with them know they can pinch your fingers quite painfully. In a strong magnetic field like an MRI iron saturates at 1.5-2 Teslas, it's 3 or 4 times more magnetic than neodymium.

The pull on individual pieces goes as the product of the two strengths; the MRI would have several Tesla of field strength, and the iron would be 1.5-2, so you can imagine how strongly the field would be tugging on pieces of iron in the patient.

But if you were to divide the same amount of iron throughout the body, the pressure would divided over the whole volume and you wouldn't even notice it.

8

Technology / Re: Is it safe to sleep near a wifi router?

« on: 13/09/2018 23:25:09 »

WiFi is a ridiculously low power version of radio, and has no known or conceivable ill-health effects at all.

9

Physics, Astronomy & Cosmology / Re: Are there metals that shift from soft to hard?

« on: 13/09/2018 23:22:34 »

If you're a man, yes.

There's also magneto-rheological and electro-rheological fluids that can do stuff a bit like that, but they're not metals.

And there's also non-Newtonian materials like Oobleck.

10

Technology / Re: Can an Impulse Engine be made?

« on: 10/09/2018 00:56:12 »

It's not an opportunity unless you have a plan that can actually achieve it.

11

Technology / Re: Can an Impulse Engine be made?

« on: 09/09/2018 23:10:05 »

FYI the problem with most engines isn't the low exhaust velocity-  velocities can trivially be made arbitrarily large, even lightspeed (photon rocket). The problem is getting very high exhaust velocity at the same time you get very high thrust. That implies extremely, ridiculously high power. Normal rockets give you that because the nozzle doesn't directly touch most of the flow. But if you're trying to heat a flow electrically to raise the temperature then you've got to build electrical stuff with very high power/weight ratio. This is beyond the state of the art.

12

Technology / Re: Can an Impulse Engine be made?

« on: 09/09/2018 21:33:51 »

An engine that only reaches a peak of 300F is inevitably horribly inefficient. The laws of thermodynamics cannot be circumvented; you're trying to turn a hot fluid into a fast fluid, that's a heat engine. You seem to be unaware that high pressures do not raise the speed of sound in fluids- and fluids leave an orifice at only sonic speeds (supersonic speeds, if the orifice has an expansion nozzle as well).

Very high pressure engines enjoy plenty of thrust, but the exhaust velocity and efficiency is not in any way raised due to that. For that you need high temperatures.

13

Technology / Re: Can an Impulse Engine be made?

« on: 09/09/2018 19:57:29 »

You are thinking of combustion-powered projectiles , I am using a continuing shockwave .  Their velocity is definitely pressure-driven .  Combustion is so weak !

Carnot is not to be so easily circumvented.

What's the peak temperature at the shockwave, and what's the exhaust temperature?

14

Physics, Astronomy & Cosmology / Re: How do you steer a rocket following take-off?

« on: 09/09/2018 18:11:41 »

To be optimal, the trajectory has to be smooth (otherwise you could have cut the corner and made it more optimal), and you can show that nearby optimal trajectories (going to slightly different orbits) can't branch from it in mid flight. That means that all the trajectories are slightly different going right back to the take-off point, so a rocket optimally takes off at slightly different angles to end up at different orbits.

15

Technology / Re: Can an Impulse Engine be made?

« on: 09/09/2018 04:27:27 »

What is needed is a nuclear-mechanical drive .  This would be a nuclear-powered  high pressure system , fielding 1 million psi.+ , & yielding exhaust velocities approaching 1M mph ! The pressure-chamber would be held at 250°F .

No.

To get high exhaust velocities, you need high temperature differences (Carnot efficiency rules these kinds of situations). Pressure is helpful to allow you to make use of very high temperatures, but makes no difference on its own.

16

Plant Sciences, Zoology & Evolution / Re: I have fish in my free-standing water tank - how is this possible?

« on: 09/09/2018 01:04:11 »

The presence of identical freshwater fish species in different rivers has always intrigued me.The simplest model of an island(such as Great Britain) is a pyramid with a dry apex and water runing down the sides. The rivers are not connected at the watershed, and the salt sea at the base prevents freshwater fish from migrating between rivers at their mouths. But somewhere in the middle of the stream you will find the same native species (pike, perch, roach)  in each river, supported and  predated by pretty much the same ecosystem The probability of identical evolution of the top species in isolated biomes is negligible,and fish can neither fly nor walk across the land.

The least unlikely solution I have been offered is accidental transfer of pregnant fish or eggs, by birds, and I guess that might account for fish in  a tropical water tank.

Not all that long ago, a few tens of thousands of years, Great Britain used to be connected to the mainland of Europe, because the sea level was a lot lower due to massive glaciation. Lots of rivers that now individually lead to the sea used to join up before proceeding to the sea in what are now the English and Bristol channels, and also, the glaciers themselves did wacky things, like damming up stuff and changing the topography by grinding grooves in the land, and when they melted, huge freshwater flooding happened that could easily push fish around.

17

Physics, Astronomy & Cosmology / Re: How do you steer a rocket following take-off?

« on: 08/09/2018 18:24:01 »

I'm not a rocket scientist, but I sometimes play one on Wikipedia (this is achieved by referring to actual rocketry textbooks- not making stuff up!)

The optimum trajectory isn't actually vertical initially, but it is quite steep- any time a rocket isn't firing at least slightly sideways it's wasting fuel because it needs to go sideways very fast to reach orbital speed. By Pythagorus, a rocket that is thrusting just off vertical gets little loss of vertical thrust, but gains a large amount sideways thrust.

In practice, a rocket will normally take off vertically and then quickly begin pitching once it clears the pad equipment. A lot of rockets then head east, because that gains the rotation of the Earth, but depending on the mission and launch site, most of them head not due east.

One interesting detail, rockets often don't follow the optimal trajectory initially, they do a gravity turn:

https://en.wikipedia.org/wiki/Gravity_turn

the difference is that with the optimum trajectory the rocket is angled to point upwards relative to it's trajectory somewhat to fight gravity, but that means it would get a lot of air drag and tends to buckle- a gravity turn means the rocket is always turned directly into the air until it gets past the point of 'maxQ' - the point of maximum air drag; and then it resumes the optimum trajectory. That means the rocket can be built more lightly, and this overall improves the amount of payload it can carry. Furthermore, the designers can put fins on the rear of the vehicle, these keep it mostly pointed in the right direction for a gravity turn, but don't work at low speeds, but they seem to be less common these days for some reason, but are very obvious on the Saturn V.

Evan's numbers are indeed a bit off, a low altitude (180km) orbit requires the rocket to achieve a horizontal velocity of about 7.8 km/s. However, it takes about 9.3-10 km/s of 'delta-v' to achieve that including gaining altitude and air drag (delta-v is the speed change the rocket would achieve if everything else was the same, but there was no air, and there was no gravity and it fired in a straight line) An escape orbit requires a velocity of 11.2 km/s, but because of having to fight gravity it will need more delta-v than that (about 13 km/s). These are pretty much minimal numbers from the equator, a lot of rockets have to work harder than this, it depends on the mission.

18

Plant Sciences, Zoology & Evolution / Re: I have fish in my free-standing water tank - how is this possible?

« on: 08/09/2018 16:53:21 »

Maybe a king fisher caught a live fish, and was carrying it to it's chicks, and then accidentally dropped it.

19

Physiology & Medicine / Re: Is household dust bad for my lungs?

« on: 05/09/2018 22:28:18 »

The second biggest risk after smoking is general air pollution from traffic and so forth. If you live in a heavily polluted location, you should move. Something like one person in 14 dies from air pollution.

Things like chalk are negligible risk, chalk is non toxic.

20

Technology / Re: Do wind-powered cars exist?

« on: 05/09/2018 15:59:11 »

It's difficult to see how it could be integrated.