A further clarification. Apart from Huffingtons, the worst penalty condition is to be in Nidd. This is a river in Yorkshire which features in every version of the game, and in the beginner's version is assumed to be part of the London Undergound, despite being a long way away and not a railway station or anything resembling one. If you're a novice player who happens to be French and are therefore playing Mornington Croissant, the River Nidd is also part of the Paris Metro.

Being in Nidd is such a long-established rule that it almost makes sense, since the consequence is that you have to miss a turn until you're out of it, which differs from Huffingtons in that you're forbidden to move into Huffingtons on purpose but may do so by mistake, the consequences of which are baffling but temporary. Nidd, on the other hand, means that you get no moves at all until the referee decides you're out of it, which may take some time.

Prior to 1903, since Nidd is technically a London Underground station, deliberately putting yourself in it was a legal move, so long as it was perpendicular to a line between two points, thus making it the shortest distance. However, once the offside rule was introduced at the championship level, the prohibition on moving south of the river, which was meant to apply only to the River Thames, meant that putting yourself in Nidd, and thus in a river a long way north of London, put every other player in Quadruple Huffingtons. It was this glaring loophole which caused five-time world champion Hezekiah Pipstraw to lose the Clothtoucher Cup to a tortoise.

I trust that's all clear. In any case, it has always been an illegal move to put yourself in Nidd on purpose in January, because you'll probably get pneumonia.

Oh, by the way, I don't know whether it's true genius or just beginner's luck, but by choosing Vauxhall, a classic though frankly predictable first move, as her extremely unorthodox second move, elizabeth has blocked my progress up the first quadrant I was hoping to dominate by taking King's Cross. In fact, as a result of the Gruntfuttock Amendment (remember - this is the Mousebender Variation with no Double Huffingtons), I have no legal moves other than to redouble myself and hope she makes a mistake.

Therefore, Finsbury Park again.

(See how this works...?)

For the benefit of elizabeth plus any lurkers, here's the explanation I was only going to give if you needed it, because apparently you do. Mornington Crescent is a game invented almost 40 years ago by people writing for BBC Radio for the sole purpose of annoying a producer who was an intellectual snob. The idea was to create a game so elaborate that nobody could possibly figure out the rules unaided, then pretend that it was so well-known that no explanation was needed, thus putting this guy in a position where he couldn't bring himself to ask what the heck was going on for fear of looking ignorant and stupid, and therefore had to pretend to understand the rules, not realizing that everybody saw through his bluff because they knew they'd made it all up and explained it to nobody.

The game works on 3 levels. Firstly, the basic idea is absurdly simple. The first player can name any station on the London Underground except Mornington Crescent. Everyone then takes turns to name a station which links up with the previous one, and whoever gets to Mornington Crescent wins.

The second level is that the moves permitted between stations (or, in more advanced versions of the game, between thousands of other locations as well) have absolutely nothing to do with how the actual stations physically connect up. Instead, they're governed by insanely complex rules allegedly going back centuries which you'd need a lifetime's dedication and an IQ of 800 to fully understand. Any player may at any time challenge anyone else's move if they think it contravenes an obscure rule they happened to remember. It's also considered polite to compliment any moves that go unchallenged, citing the subtle rules that player has cleverly taken advantage of. There is usually a referee whose word is unquestionable, who tends to take a very active part in the game, frequently disallowing moves that are illegal because of some rule so obscure that nobody remembered it (most players find it particularly difficult to remember which moves will put them in Huffingtons).

The third level is that there aren't really any rules whatsoever. The only rule that actually applies is that, whenever any player for any reason decides the game has gone on long enough, they can instantly win by announcing they've reached Mornington Crescent. This is the only move that's never challenged, because everybody else automatically assumes it was both valid and clever.

The skill of the game lies in justifying every move you make by quoting incredibly complex rules that you made up on the spot, sometimes combining them with rules which, over the years, have been mentioned often enough to be almost comprehensible as actual rules (though not quite) and are therefore an established part of the game and can be found online. For example, being in Huffingtons is a severe penalty condition which can occur either as the result of a bad move, or because another player forced you there by making a very good move. However, since being in Huffingtons costs you points, and winning the game doesn't actually involve the number of points you have, it's not clear why this should matter. Though of course any player in Huffingtons is obliged to be upset about it.

All moves can be challenged, though any challenge can be counter-challenged. The basic rule here is that any challenge will work if it sounds convincing enough, unless it's met fairly quickly by an equally eloquent counter-challenge. Since none of the rules are ever really explained, if a player wishes to justify a move by citing a rule which almost makes sense, it is customary to explain this rule only in terms of how it affects an even more obscure rule, which is of course not explained at all.

In general, all players who are challenged must accept that challenge if they can't come up with a more eloquent rebuttal, but if they can, the challenger should nearly always admit they were wrong, unless they can come up with something even better. Compliments for good moves can be made at any time, and are assumed to be entirely valid, as are any rules they quote, so a wily player may congratulate somebody else on a clever move using a very obscure rule, then use that rule themselves on their next move. Which would matter if this was an actual game you could actually win.

Does everybody get it now?

Well elizabeth, you've been both astute and audacious there, since you've left yourself dangerously open to Huffingtons. However, you're slyly taking advantage of the fact that there are no Double Huffingtons, so you might get away with it.

Oh, by the way, did I mention that we are of course using the Mousebender Variation, therefore if any player puts you in Huffingtons before your next move, in the absence of Double Huffingtons, they become Gundy for the day and can give you your Palfreys? Which, unless you perform a legally permissible Craffit Sidestep, would put you in Triple Huffingtons, and I don't think you'd like that at all. However, you're safe for now. But not for long...

My move: Finsbury Park.

Superstring theory is at present unfalsifiable for the reason I've already given: it would need a ridiculously huge machine to test it (the diameter of the machine has to be enormous for technical reasons you can't get around unless general relativity is completely wrong, which is very unlikely indeed). The basic idea is that fundamental particles which appear pointlike are in fact scrunched-up tangles of "string" with multiple extra dimensions, but this only applies over incredibly tiny distances. The number of extra dimensions has changed over the years, but it's never been infinite - superstring theory used to have a lot more dimensions, but the most viable versions of it have now settled down to either 10 or 11 (that's the usual 3 space and 1 time, plus another 6 or 7 spacial dimensions). In some versions of the theory, the 7th extra dimension is a lot bigger than the others, and only applies to gravity. However, since the thought-experiment based on this idea suggests that if the 11th dimension was bigger than a tiny fraction of a millimeter we could detect it fairly easily, the fact that we haven't implies that either this is wrong, or the 11th dimension, despite being bigger than the others, is still very small indeed.

Superstring theory is a thought experiment because, in addition to the extreme practical difficulty of testing it, most of the people working on it admit that it's an abstract mathematical model that tries to make sense of things we can neither observe nor imagine. But if the theory works, it doesn't really matter whether or not these extra dimensions literally exist, so long as the mathematical model which assumes that they do describes reality better than the previous version.

I can't offhand think of any entry-level books on this particular topic, since it's one of the most complex ideas ever conceived by the human brain, it's still under development, and it quite possibly deals with things nobody has anything like the brainpower to genuinely understand. But generally, you can find quite a lot of texts that aren't overly technical that get into the less insanely complicated aspects of this kind of thing, including the thought experiments you're particularly interested in.

The classic "Two Slit Experiment" is both a thought experiment, in that you can do it in your head and logically figure out what has to happen, and a real experiment in that it's very easy to actually do, but unfortunately logic and reality give completely different results. Since it's mechanically simple but at the same time incredibly baffling, studying it until you more or less sort of understand it (if you really, truly do understand it, congratulations - you're the smartest person on the planet and then some!) is probably a necessary first step before moving on to anything more complicated. Most popular books on quantum physics will give you a detailed account of this experiment in easy-to-understand terms (if they don't, dump that book 'cos it's rubbish). The trick is not to grasp the mechanics of it, which shouldn't tax anyone's brain, but to see what's going on there and why it's so weird. I'm describing a real experiment here, but you don't have to actually do it (though I personally have, and I assure you that it gives results just as odd as the books say it does). You just need to think about the thought-experiment version until something clicks in your head - you'll definitely know when it has. Then you can move on to stuff like quantum entanglement.

If we're talking about specific books, offhand I would say that anything not too technical by Richard Feynman is a good bet. By the way, he won the Nobel prize for physics, so he probably knew what he was talking about. Avoid anything which in any way mentions eastern philosophy or religion, new age beliefs, or telepathy. I promise you that none of this has anything whatsoever to do with real physics.

This thread is misnamed. If it's supposed to be about psychology experiments, which it seems to be so far, it should say so. A "thought experiment" is by definition something you don't actually do, either because it's wildly impractical - for example, it would involve somehow harnessing all the energy in the Universe - or because it's completely pointless. Shrodinger's Cat is an example of this, because although the experimental setup is quite simple, the bizarre result you're trying to observe - a cat which is simultaneously alive and dead - vanishes whenever you try to look directly at it, so you can never actually see it (there are also at least two other reasons why this experiment can never succeed).

Thought experiments are useful because they allow you to mentally explore aspects of science which can't be directly tested without worrying about your inability to do so, which may lead you to discover things which can be tested after all, thus indirectly confirming what you thought in the first place, or may reveal huge logical problems suggesting that your theory is broken. Some of general relativity's predictions about gravity have for the past century been a borderline thought experiment, since gravity is very hard to study because it's such a weak force, but have been assumed to be true because all the testable predictions work out just fine. However, in the next few years, we'll be launching satellites which can do the experiment for real. If they get the expected result, Einstein posthumously becomes officially even smarter. If they don't, Einstein is still very smart indeed, but there's a peculiar gap in his theory which the next generation of theoretical physicists will need to fill somehow, but that's OK because they like a challenge.

Superstring theory, in which the Universe has many extra dimensions but only over ridiculously short distances, is taken very seriously because if you take it to be true it solves a lot of problems, but it's definitely in the realm of thought experiments for the foreseeable future and probably forever, because unless science is completely wrong about a great many things, there's absolutely no way to test these ideas directly without building a machine similar to the Large Hadron Collider, only with a circumference the size of the the orbit of Neptune. But by assuming the Universe is 11-dimensional in a really subtle way that doesn't show, problems can be solved that otherwise couldn't be.

The interesting thing about this theory which we'll almost certainly never actually be able to test is that it doesn't need to be literally true to work. Quantum physics is so weird that "quantum weirdness" is a perfectly serious scientific term, and we're talking about the next level down, so the truth may be completely unimaginable by humans. But a mathematical model which approximates that unimaginable truth does the job pretty well. And since all those extra dimensions, assuming they exist in the first place, only matter over distances smaller in relation to the diameter of a proton than a proton is in relation to the solar system, whether or not we're 11-dimensional doesn't have any practical consequences whatsoever, so don't expect to turn into Doctor Manhattan any time soon.

So, if the thread title isn't a misprint, superstring theory is my favorite thought experiment, mainly because I'm impressed by the sheer audacity of those versions of it which suggest that everything is 10-dimensional apart from gravity, which radiates across the 11th dimension, losing almost all of its strength in the process, therefore there's a parallel universe where everything is insanely heavy that you could get to (a bad idea, since you'd instantly be squashed flatter than all previous definitions of flat) by moving less than a tenth of a millimeter, if you could do that in the 11th dimension. Which is 6 more dimensions than Mister Mxyzptlk knows about, so it's a good trick if you can do it!