21

Physics, Astronomy & Cosmology / Re: Why must c be an absolute "speed limit"?

« on: 20/02/2018 15:42:17 »

Objects are held together by electrostatic forces, which are mediated by photons that travel at the speed of light. That means if you wobble an electron it takes a tiny moment for the forces on the proton to respond; and also when an object moves the electric field is no longer spherically symmetric, it becomes ellipsoid.

It takes an infinite amount of energy to make an object go faster than light, but if somehow an object found itself going faster than light, the electric fields wouldn't keep up with the electrons and protons, so they would no longer be bound together as atoms and molecules and the object would fall to pieces.

22

Physiology & Medicine / Re: Could a placebo effectively replace an antipsychotic?

« on: 18/02/2018 19:05:06 »

Congratulations.

However if a placebo was as good as an antipsychotic then something went really, really wrong in the licensing and testing process.

23

General Science / Re: Why are some stains more stubborn / harder to remove than others?

« on: 18/02/2018 18:46:20 »

so tends to soak in and stay in fabrics

So is it binding to non-polar moieties in the chemicals that make up the fabric? Like the cotton fibres, for instance?

I don't think so, or not significantly, it's just more difficult to mobilise. You can mobilise it with solvents or even oils. Lycopene is actually a solid, and in tomato sauce it's presumably dissolved in the small amounts of fats and oils naturally in the tomato, and they would soak into the fabric, so detergents help as well. Lycopene is also broken down by UV, so leaving it out in the sun helps.

24

General Science / Re: Why are some stains more stubborn / harder to remove than others?

« on: 18/02/2018 16:23:22 »

Stains are typically pigments.

Tomato colour is mostly lycopene. Lycopene is very, very red, and is non polar and therefore insoluble in the 'universal solvent' aka water, and so tends to soak in and stay in fabrics, but is soluble in solvents:

https://en.wikipedia.org/wiki/Lycopene#Staining_and_removal

It also has a nasty habit of diffusing into plastics where it can't be removed.

Red wine, coffee etc are different, it's the tannins that are widely present in plants that are the biggest problem. I'm not entirely sure what the chemical basis is, but googling around seems to imply that tannins actively bond to fabrics as they dry. Best to remove them while they're still wet, but the stain can often still be removed later by using acids or alkalines to break them down.

25

Plant Sciences, Zoology & Evolution / Re: Are cats domesticated?

« on: 15/02/2018 17:03:38 »

So far as I know, all dogs have it. When some Russians domesticated foxes they ended up with floppy ears, which suggests that they too have Williams syndrome.

My suspicion is that a lot of mammals can get it since humans and dogs aren't all that closely related, so I'm wondering what a cat with it would be like. I don't expect all dogs are equally affected, it may well vary a bit depending on details of their genetics. But a dog that doesn't have it, is probably a wolf not a dog and very difficult to handle. Wolves are tameable, but tend to only be friends with one human.

26

Plant Sciences, Zoology & Evolution / Re: Are cats domesticated?

« on: 14/02/2018 23:11:00 »

There's two things, there's tame, which means it's trained to not bite you or run away. And then there's domesticated, which means it's genetically different from the wild form to better interact with humans.

Dogs are domesticated and tame, they have different features from the wild form, they love humans and they have floppy ears which wolves don't have.

Cats... not so much. They have perky ears, and they like humans, particularly when you feed them, but they're mostly tamed, not domesticated, genetically, they're very little different to wild cats.

What have the ears got to do with it? Well, there's a human disease called Williams disease, which makes people that have it, very, very friendly, and good at communicating, but otherwise somewhat mentally impaired. It also damages collagen product which can cause heart disease and other problems. Dogs have this disease, and that a lot to do with why they like humans so much, and it makes their ears floppy.

Cats.. don't. And their ears are like wild cats.

Analysis shows that cats have very few genetic changes to fit them to humans, they haven't all been explained, but they're mostly genes for coat patterns.

27

Physics, Astronomy & Cosmology / Re: Is this calculation of simultaneity right?

« on: 09/02/2018 00:41:15 »

One interesting thing I worked out a while back, if you move a clock at a very slow speed along a moving object, then you get the same result as the Einstein synchronisation method. The overall sum of the time dilation, does not tend to zero. In fact it depends on the distance and the speed of the object. Indeed, that's where the lack of simultaneity comes from, it's just another effect of time dilation really.

28

Physiology & Medicine / Re: Is coffee more enjoyable after a meal?

« on: 30/01/2018 22:09:13 »

So far as I know coffee triggers the release of sugars from your liver, and that's one reason you feel more lively when you've had a cup.

However, if you're hungry then the glycogen stores in your live (i.e. glucose stores) may be running low, and in that case your body will release hormones to create glucose from other sources, such as breaking down muscle; which will make you feel bad and make you feel even more hungry.

Anyway that's my rough understanding.

29

General Science / Re: How Dense Can Water Get? Can water under pressure become a solid?

« on: 27/01/2018 15:06:09 »

Well, they usually say it's almost incompressible. The compressibility of water is about 1% at 200 atmospheres (compressibility varies, but is about 4.4 to 5.1 x 10^-10/Pa). So for example, the pressure at 11km depth at the bottom of the Mariana Trench is about 1000 atmosphere, so the water will be compressed about 5%.

Actually if you look at the diagram, if you just increase the pressure from normal liquid water, along most of the curve you find you form Ice VI, which, while not metastable, is also interesting:

http://www1.lsbu.ac.uk/water/ice_vi.html

If you read the page, it too has a slightly higher density than water, even the high pressure water around it which has been compressed, so it too would sink.

Unfortunately that would take several times the pressure to form than you find at the bottom of the Mariana Trench, so humans will never see a snowy bottom in our Bathyspheres.

30

General Science / Re: How Dense Can Water Get? Can water under pressure become a solid?

« on: 26/01/2018 19:13:56 »

And still nobody had really answered it!

31

Physics, Astronomy & Cosmology / Re: How long would a mile-long capacitor take to discharge?

« on: 23/01/2018 18:33:06 »

Your mental model of this is wrong.

Think of a water tap connected to the mains.

When you open the tap, the water starts flowing, instantly.

Why is that? If you think about it, there's a huge column of water going all the way to the nearest water tower, and there's only a few tonnes of pressure behind it as head, but there's a mass- thousands of tonnes of water per square metre per kilometer. It should barely dribble out to start with and then gradually get faster and faster. But it doesn't! Why not?

The reason is- there's a bit of give in the pipe, and as you open the tap, the water gets squeezed out like toothpaste until finally the whole column is flowing.

And the electricity is fairly similar; the electrons repel each other really, really, REALLY strongly, so the electric 'fluid' is largely incompressible. The slight 'give' in a water pipe is exactly like capacitance, it can store a little extra electricity, which is plenty enough to get the electricity flowing; but as with the give in the pipe it doesn't take many electrons to flow to completely discharge the entire capacitor in your example. So even though the electrons flow very slowly, because the electric forces are so very strong, electricity can carry a lot of power.

Feynman gave an example where he imagined if there was suddenly an electrical imbalance of 1% in the electrons in your finger, you would explode at pretty much the speed of light. That's how incredibly strong the electric forces are.

32

Geek Speak / Re: How to make a cryptomining website?

« on: 21/01/2018 02:34:34 »

Provided you have a splash screen giving them the choice to not read your site and not mine that's probably legal.

Otherwise it's theft of electricity; I'm not sure whether in the UK that's considered hacking.

33

Geek Speak / Re: Would decrypting of German communications have gone well with modern computers?

« on: 13/01/2018 15:07:28 »

The thing is, even with brute force you still have to have a model of the plaintext data.

In addition you don't have enough letters in your plaintext. Because it's shorter than the keyspace, it's probable there's multiple valid plaintexts that could produce that encrypted output.

I know.
I also know I'm not the one who said " Most of them will fail in six memory lookups at the first hurdle."
So, show me how most of the zillion possibilities are excluded in 6 lookups.

The standard way that the Enigma was used was that the beginning of the plaintext was 3 characters, repeated.

So if you run a trial decrypt on the first and 4th character, and they aren't the same, then you've ruled out those settings and you can go onto the next.

Except when the rightmost wheel has rolled over and bumped the next wheel, which for the sake of this not being a cryptographic forum, only the rightmost wheel has moved.

So if: P'(W1'(W2'(W3'(R(W3(W2(W1(P(e1))))))))))  != P'(W1+4'(W2'(W3'(R(W3(W2(W1+4(P(e4))))))))))

Then you can stop immediately, and go onto the next wheel settings.

And that's the typical case: that they DON'T match. If they do match, you check the next letter of the station code, and only if they match do you check the third. That reduces the amount of work done per setting by thousands. If the result is a plausible station code then you can perform a much longer check on the rest of the alleged plaintext looking for plausible bigrams.

It would also be instructive to see how many clock cycles it takes to actually "run" this
P'(W1'(W2'(W3'(R(W3(W2(W1(P(x))))))))))
I suspect it's over 1000

Nah. It's 6 additions and 9 memory lookups, there's pipelining so the additions and lookups will happen in parallel, and the table lookups should fit in the on-chip caches. If they don't fit, then yes, it would be very slow, and you've picked the wrong processor for this workload. Also, processors these days are superscalar, so you can write the code to run several decode attempts running in parallel on different cores, this kind of thing parallelises really, really well.

34

Geek Speak / Re: Would decrypting of German communications have gone well with modern computers?

« on: 12/01/2018 06:10:53 »

The lookup is just, in C:

rotorArray[rotorPosition + inputCharacter]

You can avoid having to worry about index overflow by making the array big enough (36*2) and repeating the rotor wiring settings on the end in the array, so you don't have to test for it or do modulo arithmetic on it.

Only the first rotor clicks on in most cases. You do have to worry about that but it doesn't change the order of magnitude of the answer; in most cases the second rotor doesn't click on.

edit: it's actually likely to be less than 9 lookups; you can cache a single lookup table for all the centre W2'(W3'(R(W3(W2())))))) permutations and reuse that over and over; that should bring it down to 5 lookups.

35

Plant Sciences, Zoology & Evolution / Re: Why do insects waste energy buzzing about in swarms?

« on: 05/01/2018 21:18:45 »

I think it's to do with predation and geometry. If an insect is sitting, and a predator flies directly towards it, it won't see it coming until it's far too late, the size of the predator on the eye will be very very small, then small, then suddenly huge, but by then it's lunch- there's no lateral movement on the retina, it's really hard to spot the predator coming. (This happens with aircraft- aircraft pilots find it really hard to spot other aircraft that are on a collision course.)

Whereas if you're flying in a straight line, things are better, the insect will see the predator coming unless the predator is tracking the insect (dragonflies tend to sit on the ground, and then fly up to an insect keeping a constant angle to its eye to avoid being spotted too early.)

So house flies take this to another level, instead of flying in straight lines from A-B, they make lots of zig-zags. Unless the predator can make the SAME zigzags then the fly can spot the relative movement and take avoiding actions.

From the point of view of the dragonfly though, even if it can track the fly, there's a big problem- it can only track the movements of one fly at a time! So if there's a cloud of flies all doing their own things, if it flies towards the cloud, the cloud will all simultaneously take avoiding action except for one fly that it tracks which won't be able to spot it.

Except even then the fly WILL notice all the rest of the cloud scarpering for no obvious reason, and immediately take evasive maneauvers, and is virtually certain to survive!

Anyway, that's my theory.

36

Just Chat! / Re: Will inflation of currency ever end?

« on: 31/12/2017 04:52:50 »

Inflation is, to some extent, deliberate.

While it can be caused by many things, keeping inflation at a couple of percent is desirable because it keeps the economy away from a deflationary spiral. Deflationary spirals tend to be self-reinforcing and economists don't know how to kick an economy out of one very well. Inflation can be self reinforcing too, but economists think they have a better handle on them.

Governments deliberately create inflation if there is not enough; to oversimplify- they "print extra money" and that creates inflation. That doesn't always work though, if the economy is at the negative bound then it doesn't work. Still, in most normal times it works great.

37

Technology / Re: Can you power a jet engine by combining hydrogen and oxygen?

« on: 25/11/2017 23:47:22 »

The Reaction Engines A2 is the aircraft version, and doesn't pull 4g, it's relatively normal takeoff and acceleration, it's just that it keeps accelerating to Mach 5, and it should have tremendous range because liquid hydrogen is so very light.

https://en.wikipedia.org/wiki/Reaction_Engines_A2

Skylon is the orbital version, that's much more sporty, it pulls 3g on the way up:

https://en.wikipedia.org/wiki/Skylon_(spacecraft)

38

Technology / Re: Can you power a jet engine by combining hydrogen and oxygen?

« on: 23/11/2017 16:17:16 »

If you mean can you carry oxygen with you, yes, but it tends to burn too hot for gas turbine rotor blades and they tend to burn out. Normal jet engines use air because the nitrogen dilutes the combustion reaction and keeps it cooler. It also avoids having to carry (liquid) oxygen because it's expensive, weighs down the plane and reduces range.

So if you restrict yourself to the oxygen in the air, the answer is a resounding yes, you can use hydrogen for powering jet engines, and it burns and works extremely well and doesn't get too hot or anything. Indeed, Hans von Ohain built the very first working jet engine and that used hydrogen for fuel!

Hydrogen fueled airliners are on the drawing board, and actually work better than normal aircraft, they could fly much further on a single load of fuel, non stop half way around the world even supersonically, but the liquid hydrogen they would need to burn is relatively expensive, and basically because the hydrogen has very low density, the aircraft would be very big and few existing airports would be able to handle them.

39

General Science / Re: How close are we to building a human memory-erasing machine?

« on: 16/05/2016 03:44:55 »

Not sure about a machine, but some drugs block memory formation and have been used to attempt to remove existing memories.

http://www.wired.com/2012/02/ff_forgettingpill/

The theory behind it is that memory is active; when you recall something you're actually relearning it each time, so if you take the drug and then recall it, you're attenuating the memory.

40

General Science / Re: Is it true that for healthy people PROBIOTICS don't make a difference?

« on: 15/05/2016 02:16:35 »

Stomach acid is a pretty excellent way to sterilise bacteria, and probiotic bacteria don't really make it past that; and if they do, they have to compete with all the other bacteria that are already there.

IRC nobody has ever shown that probiotics make any long term changes to stomach flora.

Fecal transplants are a proven way to do that though, but that's a whole other thing.