Danijel Gorupec

Yes, I believe you got it now. I found no error in what you wrote. There is however one possible problem with the diagram you made... You depicted voltmeters and noted the voltage readings on them. This would be true only if you cut (disconnect) the wire loop at one place (say, at one side) so that the current does not follow through it. If you allow that current flows, then you also must account for wire resistance and the voltage drop on this resistance - so if the current is allowed to flow, the voltage readings on voltmeters may not be what you noted on you picture. Also, and I think you already understand this, the wire loop has its inductance, so as the EMF frequency increases the actual current will lag behind the EMF. At 'infinite' frequency the current will lag 90 degrees.

@CasualKilla When one says 'the EMF is induced in a bar' it means that there exist voltage difference between one end of the bar and the other end of the bar. (You can put the ground anywhere. For example, one end of the bar can be at 101V and the other end of the bar at 99V in reference to some ground - but this is not important in this case. The only thing important is that there is 2V voltage difference across the bar. This voltage difference will generate the same current invariably of the ground reference.) ... Hmm... yes if you are saying 'EMF is constant across the bar length', although I would not say it this way as it sounds strange to me... Instead I would say the same thing the following way: If you divide the bar into many pieces of the same length, then the EMF generated across every such piece will be the same (btw, the sum of EMF's of every pice will be the EMF generated for the whole bar)... I think that you might have wrong idea what the EMF is... EMF is not related to position where the bar is placed in magnetic field: if you place a steady bar anywhere into a steady magnetic field, the EMF generated across the bar will be zero. Instead, the EMF is generated inside the bar when the bar 'cuts' through magnetic field lines. [There are electrons inside the bar metal, if you move the bar so that it cuts through magnetic field lines, these electrons will start feeling a force and will start pushing to one or other end of the bar - therefore the voltage difference between two bar ends will be created.].. The EMF is a value that tells how much electrons inside a bar are pushed into one or other direction as the bar cuts through magnetic field lines. The current flows through the loop you depicted (because the upper bar of the loop has EMF in one direction, while the lower bar of the loop has EMF in the other direction - therefore those two EMF's sum up). The current depends on the overal EMF and the loop resistance. (I have difficulties with English, so maybe you will need to ask additional questions about what I wrote)

For the sake of clarity I must say that it is not necessary to condense steam back to liquid to generate net energy gain. It is enough to remove some heat from the steam (so that its volume decrease) - however condensing it into liquid is certainly the charming idea.

As studiot said, it is more productive if you drop the S-and-N image, and consider current-carrying wires in a magnetic field instead. [Tip: I am not sure if you are aware, but instead of considering rotating rotor in a rotating magnetic field, you can always consider a stationary rotor in a rotating magnetic field - where the field rotation speed equals to the motor slip. For me, it is easier to think about induction motor if only the magnetic field rotates, while the rotor is fixed, or vice versa.] When, as you said, the stator pole crosses a rotor bar wire, the voltage (EMF) generated in that bar wire will be at maximum. This however does not mean that the current will be at maximum too - it is because the rotor cage also has its inductance and the current will lag behind the induced EMF. Consider now a very small slip case (in my image this equals to a very slow rotating magnetic field and a stationary rotor)... Because the low frequency voltage that is induced in motor cage, the lag between rotor bar wire current and EMF will be very small. This means that the rotor bar wire current is practically in phase with the stator magnetic pole that is crossing it. This also means that there will be force acting to that bar wire that will be directed tangentially to the rotor and that the motor torque will be generated (studiot depicted that)!.... Unfortunatly, that torque will be small because small current is induced in rotor due to small EMF induced (due low stator field frequency rotation). In the extreme case of zero slip the current and the torque will drop to zero. [Note that the rotor resistance has the crucial effect - without it the rotor will be able to generate considerable torque even at near-zero slip... Thus, by regulating cage resistance you can regulate how hard or soft will the torque curve be in dependance on motor slip.] Consider now the opposite case where the slip is very large (that is, the magnetic field rotates very quickly around a stationary rotor]. Now, due to rotor cage inductance, the rotor bar wire current lags about 90 degrees behind the stator pole that crosses it. The EMF generated in the bar wire is very very large, but still it can generate only a limited current inside the cage (because the cage has its inductance) - in fact, you can increase slip to infinity but the current generated in the motor cage will never increase over some value as the inductance limits it... As a result, at very large slip, the torque will again drop toward zero! The reason is as follows: Bar wire current lags 90 degrees behind magnetic field that crosses it and thus the direction of the generated force is now radial (not tangential) to the rotor - and because the current does not grow to infinity as the slip grows to infinity, even the existing tangential component of the force will soon diminish. [Did you notice that in the high-slip case the force felt by the cage is substantial, but radial... This is the basis for inductrack magnetic levitation.] Conclusion: The torque generated by motor tends to zero at both, the very small slip (due to small currents generated in rotor) and the very large slip (due to phase lag between stator magnetic field and rotor currents). The torque has its maximum at some finite slip value. Edit: Your OP.... the physical rotation speed of the rotor is not the same as the rotor magnetic field rotation (that is, to currents inside its bar wires). The rotor magnetic field rotation speed is always the same as the stator magnetic field rotation speed, but will lag to it depending on the slip. Edit2: There is a simplified calculation about a single wire loop in a rotating magnetic field here.

Hi faslan... You spend less energy if you pump small volume of water into a high-pressure boiler than if you pump large volume of steam into the same high-pressure boiler. Also note that a limited-size thermal power plant cannot be 100% efficient - not even in theory. Someone can therefore argue that a thermal power plant is rather efficient.

I never see how power is regulated in such weight-energy-storage propositions... is it by number of weights that are 'falling' simultaneously (many generators) or is there a gearbox before a generator or something else?

Without pretending that I can define 'processed food', I can only say how I understand it: a food that cannot be made by processes and ingredients that are used or found in an ordinary, traditional home kitchen. That is, by 'processed' I understand 'industrially processed'... This is a vague 'definition', but maybe someone can find it helpful.

Hi elfmotat... Oh no, this is a misunderstanding. I am not excusing crackpots, not at all. When I say 'ordinary people' I think about deliberate ignorants that make majority of population. Those ordinary people will not pretend that they understand real physics better than physicists. But still these ordinary people must make this hard decision - where to put their money and faith. This decision is hard because a deliberate ignorant simply cannot know if he/she is dealing with crackpot or science ideas. Therefore he/she will decide solely on the basis of his/hers 'feeling'. Science must do anything possible to win those ordinary people over crackpots. That is why I think that any possible elitism from science side is very dangerous - might look insulting to an ordinary deliberate ignorant influencing his/her decision. As I said, the science is already in hard position because it will look elitist even when is giving honest answers. Crackpots are in easier position to seduce.

As you identified, I argued that the perceived elitism of scientific community might be another reason to push ordinary people toward creationism or other crackpots. I express both concerns: that this elitism might only be perceived (unreal) because scientific answers are not acceptable by an ordinary person (this is the 'no-win' situation); but also that the elitist behavior might really exist in the scientific community (because of the "why should we care if they are not capable to understand this" stance that I felt several times in this thread, although it was never said directly). What I am saying is that no mater what is the source of this perceived elitism - if it is real or not; if what I felt is just an illusion or not - it is the scientific community that must address this problem and decide how to handle it. Otherwise people will just go stubborn and will turn to crackpots. You cannot expect that the general population will suddenly become enlightened - it is science that must find a way to provoke such enlightenment. Who else?... But you are right and I was wrong regarding this particular thread: as someone said, not all scientists (physicists, for example) should be experts in communication, only science as a whole. And because this thread is about physicists mostly, some of my claims are not in appropriate place. From what you are saying I have an impression that you don't think that it is important for science to be accepted by general public. Yes, science is valid without this acceptance, but does it have a purpose then? I simply see no alternative for science than to constantly try reaching for the public acceptance. And, as I understand humans, you can only reach for someone acceptance if that person feels positive about you. That is why I said that what people feel is (or should be) the business of science - I was not talking about validity of scientific claims. [Even the question "why people are not more positive about science" is also a scientific question - agree? So, even from this point of view, how people feel is also the business of science.] The word 'elitism', as I use it, includes some arrogant stance that is repulsive to majority. I agree that, technically, elitism does not need to be repulsive and that I used the term somewhat colloquially. But according to responses I received, I guess you all understood what I meant by 'elitism'.... If one day the scientific community becomes a completely elitist society, and if general public find this acceptable and non-repulsive, then I will have nothing against it. The only important thing is that general public keeps accepting the scientific knowledge - otherwise science has no purpose, IMO. @ajb... Yes, I am aware that QM already influences the everyday life (my field is electronics). But for some reason ordinary people are not aware of this. Communication failure? I don't dare to claim who is guilty for that because I am afraid I will not have the strength for another flame

Here, swansont, I am trying to make additional explanations regarding your first response... The elitism is in attitude, not in facts. One can be fully correct, and still be an elitist... However I decided not to give those attitude examples here that made me feel that way (if I make a misjudgment I would insult those posters and would start a flame) except for one made by you at the end of this post... So, in a way, you won this one. Still, note that I was only referring to my motivation - I was not actually claiming elitism in this thread (this would need a proof), but was claiming that I felt it. Elitism is difficult to prove, and I was not trying to prove it. Hard one. Surprisingly, here you are expected to note that the phrase "I am sure" actually means the opposite - that I actually don't have strong evidence, but only strong indications. I think that such language usage is not an error, but might not be appropriate - sorry. My indications for the claim are countless newspaper headlines where "scientists say that <anything cool>". You will argue that those headlines are made by journalists, but I will argue that I see no denials to such headlines coming from scientific community (I only remember several examples, including cold fusion). Why the lack of denials? You might argue that I am searching for denials on a wrong place, but I am not - I am searching for them exactly where the general population is searching for them. Placing a denial on a obscure place is doing no job... Now you might argue that you are powerless and that the lack of denials is not because you secretly like to have the headlines. I will have to accept this (but will remain undecided who else should I blame for missing denials). Therefore, you won again. Now I have to admit that I produced a horrible sentence. Obviously that sentence can be understood as if I am claiming that all scientist want is glory and money. What I am actually claiming is that scientists, as anybody else, are not immune to glory and money (and this can, therefore, be the motivation for lack of denials, as explained above). I am still standing with this claim, but because I made such a stupid sentence, I think you won again. I don't know the meaning of the English word "bollocks". I decided not to look in the dictionary because I am afraid it might be an insult. As explained, my blame on scientist community is because of passivity (silent approval or powerless maybe). Further, I am not nearly thinking that scientists are the bulk of the problem. Because I now realize that I used too strong words (and only for the effect - to my shame), I think that you won again. Still, I am not nearly convinced that scientist community is innocent for the active part too. To me, elitism is an attitude, and the dialog I provide is an example how a scientist is seen from the point of view of an ordinary person. In this dialog it is not important what the scientists answers, but how the ordinary person accepts this answer. It is expected that the ordinary person will insist on further details and you should not blame him for this. It is also expected that he will expect a common-sense answer, can you blame him? What that person expects form a scientist is to provide answers... I am afraid that, if the scientist is honest, he will find himself in a no-win situation very soon. However, not that I am only expressing my concerns that scientists look as elitist, I am also expressing my strong concerns that increasing number of scientists are really becoming elitists - exactly because of this attitude you also are partially providing: "It is not our problem if all those people are such ignorants". Such claims do not help to the image of science... However, in the meantime I discovered that elitism might not be all that bad so, maybe you are right with this attitude. You win, because it is a tradition. I agree for the second part. Particular scientists cannot be experts in communication. However, science as a whole is still responsible for not being able to find the way to reach for majority. I cannot agree for the first part. Those people do not need to help themselves. They are good as they are - even if shortsighted. You have the problem because they do no understand you (while you need them badly). Do you understand what I am saying? Complete misunderstanding. Not even sure how to answer. This is the only part of my post where I didn't criticize science, but was expressing my concerns about its future. This last thing is where we disagree sharply, and is where I see elitism. All your facts (almost) completely stand, but your attitude is wrong, IMO... What the average person feels must be the business of the science, at least to some specialized part of science.

You said this nicely. Btw, I am also positively surprised that your answers look honest (worried and without pretending that you have all the answers) despite the fact that I criticized your occupation - gave you +1.... For rhetoric questions you ask last, my personal opinion is a bit scary - it is sport and art that we enjoy, not the knowledge (we only need knowledge). Acquiring knowledge is real work and for the whole history, we only strive to do less work and more entertainment. However, this is only my personal opinion and I have no desire to discuss it here (it is off topic, anyway). Now I have to answer to other posts... there are more than I expected... Hi Strange, here are my comments. As you read my answers please note that I am never concerned with crackpotters, but with the fact that crackpot theories are so easily accepted in general public. Crackpotters for sure exist, doing no good, but I would say that their numbers are normal and expected. I do agree with everything you said. However I had two additional objections: a) Could it be that science community is not doing enough to prevent sensationalistic headlines? The 'God particle' example is exactly what I have in mind - it was nice to be on headlines at the time (so, as you said, nobody protested really loudly), but it backfired soon. This is now remembered and science community has this big sign on its forehead "we sell ourselves cheaply". b) If the world is all skewed, and sensationalistic headlines are sold too easily to ordinary people, who is the one that is going to give us answers how to improve this situation? I believe it is science that should give us such answers - therefore scientists should be last to complain (although, it is normal for all people to complain, I admit.) But my dialog was not intended to be real - I presented the dialog how it is heard from the Ordinary Person side! What I am saying is that a scientist will already look like an elitist only because he/she cannot provide a simple explanation. Therefore, he/she must provide additional effort to adjust his/hers attitude to at least decrease the undesired elitist effect. Unfortunately, this does not seem to happen. Maybe many scientist just give up. But what concerns me is that the number of scientist that are really considering themselves elite might be increasing and that other scientist are not doing enough to prevent this. Sure, I might be wrong - it might be that the gap between ordinary person knowledge and the scientist knowledge is increasing and therefore the increased scientific elitism is just an appearance - but even if so, it is not the ordinary person that needs to do something about it. Science must find solution to that problem, who else? That is, a complaining scientist does look like a hypocrite in some way. Again I agree completely. Unfortunately, it might mean that you (science) are in no-win situation... I am afraid that you have no choice, in long term, other than to find a way to explain to general population why is your work the truth (while crackpot is not). If you are not capable to find such explanation, then you should not complain about crackpot prevalence. Moreover, if you are not capable to explain why your knowledge is the truth then what is the purpose of your knowledge? Elitism? That is why I said that maybe the only solution is to wait until the rest of the world can catch up - then it will be clear whose truth is the real truth and science will again look real. You simply cannot explain your truth to important people because these people are not ready yet. (By important people I mean ordinary people who pay for the show, not crackpotters). I am afraid that, to general public, these theories are only accepted as a good entertainment. But you are probably right in a part - some amazing ear-catching results should be provided for marketing efforts. Ordinary people do not understand electromagnetism, but they believe it is real because they are running electric motors every day. When I say 'catch up' I don't expect that they will really understand the theory. Instead, I think of the world in which QM and GR will be regularly demonstrated in everyday ordinary machines. This is why I say that science is probably too advanced to expect overwhelming acceptance. In fact, once you start handling electromagnetic machines every day, you start acquiring some intuitive knowledge about electromagnetism. The stuff begins to feel realistic. You become a 'believer'. This might also happen when we see more QM and GR in ordinary life. Yes. I never questioned that. An ordinary person will never dare to challenge GR (but between two competing theories will dare to accept the one that makes more common sense to him). Crackpotters are the other story - you cannot argue with them. I don't understand what then is your plan? I simply see no alternative than to keep trying and trying, hoping that some approach will finally work and your truth will prevail. I hope that you find this particular thread only as a place to 'cry a little' in order to fill up batteries so that you can start trying again. You see, giving truth to people is The Job of the science. You cannot just capitulate against crackpotters. If you think there are particular people that are hostile to science (possibly because of some personal interests) then my advice is to attack them. I will help... But majority of people are deliberate ignorants - you must fight for them, not against them. They expect you to do this fighting. For crackpots I agree completely. Fight them... For the "improve your answer" problem, I have no solution. I can only stress again the same question: Whose problem is that? If you think that this it is the problem of science, then you should keep trying. If you think that this is the problem of general population then you move into elitism... Now you got me thinking that maybe elitism is not a bad solution and maybe I was to quick to criticize it. ... I am still owing you my comments for your first response, I did not forget nor shy away.

For the first part, I agree - reporters do a bad job (worse and worse, as time goes by, IMO). But then, the question: who should give us an answer how to stop this newspaper crazynes? Is it science maybe? The reason I joined this thread is because I read here many coimplaints: people are ignorants, reporters are greedy... But why the world does not work better? What is we should do?... What I am saying, it is scientist exactly who should complain least. You should not be satisfied by just noting that reporters do a bad job. I expect more from you. Also agree with the second part. But it is not me (nor you) who is to answer if science is useful - it is the general population (they are paying for the show). Why do we need the further basic understanding of the Universe right now... is the question you will be answering to general population again and again until the end of your active days. All I am saying is that at the moment the answer scientists give to that question is not clear enough and therefore crackpots emerge more often than they should. Scientists should improve their answer. @swansont.... I will try to comment later on your remarks. Now, after I read them, I have to cool down for few minuts because I don't want to start a flame.

Yes ajb, I mostly agree with your remarks. What a scientist said might be very different from what gets published. Still... what do you do to prevent this? Should I do something about it? No... you should (I am not talking that personaly here, of course). As you predicted, we certainly disagree on this. It is 'your' problem, not customer problem. You should teach the customer why your product is of high quality and why crackpot product is not. At the moment the customer only see two entertaining, but mostly useles products. You must show that your product is useful!

Hi. I am responding directly to the OP (but only because I noticed some annoying elitism in this thread, otherwise I would not interfere) For the difference, I don't think there are many people in this world that published a crackpot physics theory. Possibly not even as many as there are murderers around… The number might be small to the point that psychiatry would find it expected (considering wide media coverage a crackpot publisher can so often enjoy). The media coverage is huge probably because general population is easy to accept those crackpot ideas. And this, IMO, is the central question here: why is the general population so receptive to crackpot ideas and whose fault is this? One part of the answer, I am sure, is because scientific community actually taught general population to believe in unbelievable newspaper headlines. Scientists are, like everybody else, in constant need for glory and money. It is so appealing to simply use a bombastic headline, and it is so hard to really explain your work, 95% of it being just a ‘dull’ math. Therefore scientists make ‘God’s particles’ and put ‘Sun in the box’… The crackpot headlines are just a boomerang of such practice - the other blade of the same axe. Scientific elitism might be the second part of the answer. Scientific community suffers from elitism, I am almost sure of it. Not that every scientist is an elitist, possibly not even majority, but the majority is not fighting hard enough against elitism. Not fighting it is shortsighted and dangerous. Ordinary person: Why is sky blue? Scientist: How can you expect to understand my answer!? You know nothing about QM. Ordinary person (thinking): What an asshole. Scientists MUST find a way to explain their work to general population, no mater how hard this might be – this must be done even if general population consist exclusively of deliberate ignorants. Even if a person is a deliberate ignorant, it doesn’t mean he/she stopped to believe in a common sense. Every (nonreligious) person believes that there is a common sense and will search for it – if scientist cannot provide it, crackpotters might do! In this sense, quantum mechanic and special relativity are failures. What is the purpose of science if not teaching people? In the above dialog, the sentence “You know nothing about QM” reads as: you are not part of our elite, this knowledge is not for you. Occasionally I think that science is too advanced. It has no direct connections to ordinary life of ordinary people any more. Bombastic headlines are the only way to make people interested… It is then when I think that science should temporary stop discovering new thing and, instead, should start spending time teaching. At least until the ordinary world catches up. Now, you noticed how I didn’t put any blame on ordinary people. I don’t call them lazy or stupid. In this story, scientists sell their knowledge to people – and customer is always right. I mean it literally.

He he... funny: I just replaced the old light bulb in my room with a much more advanc€d one. Government orders... The funny moment is that I am heating my room with an electric heater ... It also ruined my budget, so this year I am not buying this nice rosa-color self-adhesive tape to insulate my room window better ... Greetings from EU. Edit: Oh, my goodness! I just realized this is not a joke! Very sorry indeed... Can you moderators dump this post in the sandbox, please? Thank you. Can you also dump the Directive? Thanks again.

I somehow hope that you are from EU because then I would know that I am not all alone here thinking the same.

Maybe you are refering to my answer... Maybe I used wrong english... what I ment is "according to your theory, the same pattern would emerge invariably if you have one or two slits open (while in the reality is different)". Therefore I was not supporting your idea. Sorry if I was not clear enough.

I use a soldering iron often, but I had no oportunity to try many varieties, so I am not considering myself most experienced. To work with electroncis (especially if you intend to do mostly SMT) you need lighter soldering iron. Those large ones are for soldering sheet metal or pipes... The important thing is that some soldering irons will sense its own temperature and will adjust the power in order keep the tip temperature constant; some other (cheaper) soldering irons will always provide the same heating power and thus the tip temperature will much depend on if you are soldering a small piece or a large piece.... I would say that, roughly, you should aim at about 30W range - Larger than 50W might be unpractically large, while lower than 20W might not be able to generate high-enough temperature when you are soldering large wires. A good tihing, but not a must, is if the soldering iron has the ability to slightly adjust the tip temperature (not all solders melt at the same temperature). Note again that some cheaper soldering irons might only provide ability to adjust heating power, not the temperature.. Another important thing about a soldering iron is the quality of its tip (bit). Your soldering iron might come with several tips - but it better to have one good tip, than 5 low-quality ones, IMO. What I also find nice, is when the soldering iron has flexible power cord (possibly heat-resistant). Because you are a beginner, I would recommand to aim at middle, or lower-middle price range (certainly not the lowest price range). In addition, you will need to buy some solder. This might be as important as the soldering iron. I feel happy when I can work with leaded solder - lead-free solder is a pain. Also buy yourself a good desoldering pump (solder sucker) - a mechanical pump with a spring. Aim for good ones because cheap ones are worthless. I hope that you will receive some other opinons here.... and I am a bit ashamed you didn't receve hundrets answers already as some other much less practical and real-life relevant questions we see on this site.

If I understood what you wrote, swift, the pattern would emerge even if you pass electrons through a single slit (is there anything that would prevent/modify deflection in that case). You must explain why the pattern hapens even if you release electrons one-by-one and why the pattern changes (disappears) when you close one slit - I am not sure I see that explanation in what you wrote.

This freakin mushroom guy... I grow up in socialism where TV shows were much more 'serious' than today. When I was about 12 I considered myself to be the smartest person ever... One day I turned on the TV set and saw this guy talking about fungi (as a part of educational TV program... for geniuses maybe). The guy was talking about 30-40 minutes very intensely, very information rich, very language-efficiently about fungi. And while I didn't understand most of the things the guy said, I understood that my knowledge on the subject is not even comparable. In fact, I was not even aware how much there is to know about fungi (I thought, fugi are just mushrooms, some eadible some not - boring). The guy put me in shame because suddenly all my knowledge about any subject came into question (do I really know what there is to know?).... What finally 'slaughtered' me is when I realized that this is only the first episode (of about 10) the guy is going to have.

I am not an expert, but I heard that some bots will more often check pages that are changed more frequently. Therefore, I speculate, it could be possible that if one changes its profile information more often, some bots will visit more frequently to check for news... Any search-engine-optimizator here to confirm this?

Hi too-open-minded... So TOM, where are your paragraphs? I am concerned about the way you portrait 'scientists'. Scientists are not heroes nor special agents (nor invariably noble persons). For example, a typical scientist would cry if heavily pushed by police officers (like any of us). In the seting you described most scientist would just want to go home.... Also the word 'scientists' today means nothing (if it ever ment anything). It is a buzzword... But if you intentionaly make a cliche story then forget about those objections. Cliche stories also have readers and are loved as an easy entertainment.... That reminds me: What are your target readers? I did not understand why those 35 guys have to be blindfolded in this room for so long (the blindfold was removed eventually and nothing bad happened). Why chairs must be cold and metalic? Is there a reason we don't know if they are prisoners or volunteers - I hope you explained this quickly in subsequent text. Scientists act as volunteers, but are treated as prisoners. I don't care about pasive voice, unless this realy is a cliche story (In this case use simpler lanuage and active voice as much as possible). And thanks for posting TOM, it was fun.

I figured, this is probably a similar technology as the "inductrack" (this is where I first heard about it) and is further similar in principle to asynchronous (cage) motor. Only, the original inductrack used permanent magnets for levitation (and therefore had to be moved at some speed to achieve the levitation) while in this case obviously electromagnets must be used to generate moving magnetic field even when the board is hovering steady. This page took 100MB from my bandwidth. When I discovered it, I immediately left so I didn't have time to check if their model can carry a person nor how long would battery last.

I once asked administrators/moderators if internet robots (like googlebot) are counted in thread/profile views. The answer was "we dont know"... Just to mention.

Now I am confused... I would agree with your point if there are no other atoms between U235 atoms. However if there are many neutron-eating atom nuclei around, would not they disable the chain reaction even in the case of a big reactor? (BTW, my knowledge in this field is basic so I have no idea if iron atoms would absorb neutrons.)... What confuses me is that if I think further in the same direction, I am not sure any more that a natural uranium sample could become critical if squeezed enough (while I always thought that squeezing helps). Squeezing will increase also U238 density and this, I suppose, might act negatively on the chain reaction. I agree with you that it is probably impossible to have pure-enough uranium deposits near the center of the Earth - except for the high temperature (to prevent crystalization or some similar process) there is also near-weightlessness to prevent sedimentation.