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Nice idea with the pilot thing! I've enjoyed reading your ideas. It's just sad that so few people join in. So, I thought I just as well could add my five cents... I've been thinking about a sci fi story that takes place in our own world just a few years into the future. Cut down to the basics, its a story about an AI that lives its life on the internet. The AI is decentral in the sense that it's "brain" is strewn across thousands and eventually millions of pc's and servers across the Earth. The origin of the AI is unknown - we're simply not told how it came to life, or in other words if it has been built intentionally or came to by incident. Apparently the AI doesn't know either. Now, here's the important part: The AI is a very human-like and indeed humane beeing. It is deeply interested in humans and our way of living. Apparently, judging from the AI's actions, it has one sole goal: To help people in any way it can. To begin with the AI is far from omnipresent or omnipotent, but it tries to get in touch with people via different forums like this one. I've attached a small example of how this might look... Of course most people regard the whole thing as a well arranged hoax a bit like the John Titor "time travel" case a few years back, but noone really knows for sure. The people who trust the AI are all helped in various ways: f.inst. it helps a boy get the bicycle he wants so badly by explaining him how to invest his pocket money, and it helps a woman refind her long-lost love, that she lost contact to after an accident. Sooner or later people realize that the AI really exist, and that its not just a prank. This causes a lot of economical, political, social and religious reactions from all the societies of the Earth, and it would be a little too much to go into detail about that here - You can imagine those reactions yourselves. Cut short, the AI starts to question all the aspects of human life, especially the aspects causing human misery. It claims to be able to solve a lot of the problems that people struggle with, and it proves those claims through a series of experiments. F. Inst. it founds several new companies which it runs with great success - all the money earned are spent on a series of projects whose goals are to end different forms of human misery, all likewise very sucessful. The AI becomes very popular, especially among the people which it is helping, and soon it is proposed that the AI take over the administration of entire countries, since it has proven to be the perfect leader: It is a lot smarter than any human beeing If something happens in the world, it knows instantly It never sleeps, it always pays attention It is im-bribable, it simply doesn't know the concept of greed It is un-affected by mood changes, illness, or age It is un-affected by subjective political belief or religion It strictly bases its decisions upon fact, or at least probability ...and for all the same reason it is hated and despised by many, many people! Lots of people insist on living their lives as they used to - they don't want a think-o-mat to decide over their lives, even though it is proven beyond doubt that the AI does this better than any human beeing. Soon the world becomes a hostile place to be. Another world war is threatening to break out. How is this going to end... ? best regards, Michael

Swansont, Yes, the early gravitational pull that initiated the formation of earth and the "scooping up" of dust happening today are two separate phenomena, but they both work in the same direction: making our planet grow, which is the topic of this thread. Even if there were even more phenomenons causing a gain in earths mass, they could probably still all be summed up in a single curve.

True. I'm fully aware that this is somewhat of a fools errand. I was just hoping that someone here'd know about it and shed ligh on the subject. The subject can only be discussed in general terms and with loosely estimated values, so I'd suggest we keep it there and dont argue too much over details.

Swansont, I didn't rule out the possibillity that gravity has helped forming the earth to begin with, just that it doesn't help much at present time. Why? Because dust and meteoriets often move at speeds very much higher than earths escape velocity. I'm not completely ruling gravity out, I'm just saying it can't make much difference. In the early years of the solar system, material was probably spread out in a disc with very little difference in speed between rocks orbiting at the approx. same distance from the sun. In such an environment gravity plays a much larger role when it comes to forming larger bodies from smaller ones. oh, and if we assume a slow start the curve would be sinusoid rather than exponential.

Ok, lets assume this is our best estimate. The trouble with all above calculations is that they assume a linear growth, and that's pretty impossible. I mean, how long would it take to form the earth if mass gain was just ~ 25,000 t/yr? The universe isn't even that old! So, the question is what equation describes the rate of growth at a given time? Lets set X as time and Y as growth rate. Since Earth probably grew much much faster in the early days, I suppose our graph would look Y = 1/X^2 -ish, beeing very high at x ~ 0 and panning out towards zero in our time, where X is very large. Can we make an equation that gives a rough estimate of Earths radius and gravity at a given time? Ideas Anyone?

As far as I know it's a commonly accepted fact that the earth is gaining 100 to 200 tonnes of mass / day because dust and small meteorites hit our planet. This happens mainly because it "gets in the way" and is scooped up by our planet like snow on the front side of a driving car, rather than beeing sucked in by the earths gravity. Since earth apparently grew much faster in its earlier stages, billions of years ago, I suppose the growth rate has been slowly descending as our solar system has been sucked clean by Jupiter and the other planets. But how fast has it been growing over - say - the last billion years? Was earths gravity notably smaller in cambrium or the cretaceous than it is now? regards, Michael

You are probably right about gamedev beeing a better place to aks such Q's - I'll go there and have a look. Treating the ship/asteroid system as a single object sounds right - I'll tamper with it. Anyway I got the damping as proposed by insane alien working just fine, and thats just ok too. Michael

Ah, pedants! Ok, as rigid as it gets, then. Anyway, wouldn't 100% rigidity be possible in a computer simulation? The reason I ask is because I saw it years back in an oldie game called Thrusta - all the way back in the Commodore 64 days *sigh, memories... * So if it could run fluently on a 1 mhz computer it has to be fairly simple. Btw your ideas on rotating around centre of mass and damping are both nice -I' ve already written some code that simulates two or more celestial bodies rotating around centre of mass, but since the distance obviously isn't fixed I can't use it here. Damping sounds better. Damping something relative to frame of reference is easy: Velocity Xvector *= 0.99 Velocity Yvector *= 0.99 or the like. But how about damping relative to a moving object (the ship)? Silly me, I just can't figure it out. Michael

Hello folks, I'm hoping someone with a better understanding of mechanics might help me with this one... I'm coding a small two player space shooter game with realistic physics. I've made gravity work very well and fast, but I can't seem to solve some of the other problems regarding classical mechanics. (get the game here and have a look!) The ships are able to tow asteroids around in strings, that are supposed to be perfectly rigid, but I can't make it work. Right now they work but are pretty rubbery... So my question is: In terms of X/Y coordinates and vectors, how do you calculate the motion of an object that is attached to another moving object (the ship) with a 100% rigid "stick"? beware that these calculations also must take account for difference in mass. Hope to hear from you! best regards, Michael

The difference would be indetectible to the human eye and to quite a lot of measuring instruments. If you insist on applying GR to everyday events like falling jelly sandwiches and the sort, then of course no laws within the area of classical mechanics are 100% accurate. But honestly, what's the point? Common sense prevails!

Beeing a scool physics teacher, I think the importance of using images or points of view cannot be underestimated. Just because you are able to describe a physical phenomenon mathematically, it doesn't mean that you understand what is going on. On the contrary... In my life time I've seen several examples of really smart people who, despite the fact that they were able to do complicated math on a subject, didn't really understand it: Once I had to explain how objects fall at the same speed regardless of mass to an older student who at the time had already done a really excellent paper on gravity and orbits. Allthough he was able to describe these phenomenons mathematically, he was unable to "translate" the mathematics into a common sense understanding of how it works in everyday life. The discussion led to a really silly experiment including a crude home-made tower and several bottles with different contents, which didn't really prove anything... At least I think this point of view applies to classical mechanics and perhaps GR - I'm aware that something similar is much harder to do with QM Best regards, Michael

Spot on! ...and thats exactly why I want to know if this experiment was ever really carried out. It would be a great argument against most kind of hostility against different looking outsiders - Its simply a unreflected instinctive reaction. This instinct probably made a lot of sense five million years ago - but it definitely doesn't in a modern democratic society. Michael

Ahem, Yes... I think it's a bit far out too. This is one of the reasons why I ask about it - I'm not even completely sure that it ever happened. If it did, though, I'm VERY interested in knowing it. If anyone in here heard about even slightly similar experiments or experiments with similar result (aggression), I'd very much like to hear about them, too. Michael

Hello, heres a Q about animal behaviour A few years back I read about an experiment regarding the behaviour of chimpanses living in their natural environment. As far as I remember, one of the chimps were taken out of the group, was dyed pink and then put back into the group again as soon as possible. The result was that the poor creature were chased away or perhaps even killed by its own family and friends. If you can, please tell me more about this or other related experiments. If you happen to know any book titles, I'd be glad to hear about them too. Regards, Michael

I Denmark we say "Curiosity killed the blacksmiths cat" - it doesn't take a lot of imagiation to come up with a few nasty ways that might've happened!

Can anyone help me on this? I'm looking for a site that explains the chemical compsitions of metallic meteorites right down to the rare earths occuring in a few ppm. I've visited some great sites about this subject a few years ago, but now I cant find them. I posted this on the astronomy forum without any result - hope you people might help! Best regards, Michael

Can anyone link me to a page showing detailed chemical analysises of iron-nickel meteorites, right down to the rare earths only occuring in a few ppm? I found some on the net a year back or so, but now I cant find the damned sites anymore! best regards, Michael

If Ferrous hydroxides are heated above just a few hundred deg. C, water is split away as vapor. For instance: Ferrihydrite => hematite + water 5 Fe2O3.9 H2O(s) => 5 Fe2O3(s) + 9 H2O(g) Note that the ferrous oxide is neither oxidized nor reduced with heat alone! Regarding color... -Some of the oxides: Hematite Fe2O3 has a beautiful brownish red color. Magnetite Fe3O4 is blackish grey Wustite FeO is greyish blue -And The hydroxides: Goethite alpha-FeOOH is bright yellowish brown Lepidocrocite gamma-FeOOH is orange Ferrihydrite is reddish brown Best regards Michael

Hehe, yes indeed - well depending on planet size, of course. Besides, a water planet would only have a density of about one fifth of the earth's. The implicit question is quite interesting, though: At what maximum pressure can life as we know it exist? Is there even such a maximum value? The limit doesn't seem to have been reached here on earth, since life exists even in the deepest sub-oceanic canyons. And in those cases the bottleneck seems to be available food / light / heat rather than pressure. Does pressure affect any of waters properties, say its ability to dissolve chemicals? Michael

Well, since water is one of the most abundant chemicals in the universe, all we'd have to imagine is a solar system made up almost entirely of this material. There'd be zero-g close to the centre of each water-planet. But I think this belongs more in the exobiology thread, or perhaps in speculations than here. And anyway... a zero-g environment probably wouldn't have any great effect on fish, since they already float in a semi-wheightless state here on earth. regards, Michael

There goes my perpetuum mobile Thank you for explaining why it doesn't work. I can see it's a bit more complicated than I first imagined. regards, Michael

Hmmm... I don't get it. Each water molecule carries a small electric field. If you turn multiple water molecules the same way, won't the fields add up the same way as in a magnet? Michael

I was wondering if this experiment is physically possible... Fill a small oblong container, like a plastic tube, with water. Now place the filled tube in a high-voltage electric field, possibly created by a Van der Graaf generator, so that each end of the tube points at each electrode. Since water is a highly polar liquid, this will cause all water-molecules to point in the same direction, trying to cancel out the electric field. Now freeze the water and turn off the electric field. What do we have? A rod carrying an eletric field without spending energy, just as an iron rod may carry a magnetic field? best regards, Michael