xpected roominess鈥攖hat had rutherford scratching his head in 1910。
it is still a fairly astounding notion to consider that atoms are mostly empty space； and thatthe solidity we experience all around us is an illusion。 when two objects e together in the2geiger would also later bee a loyal nazi； unhesitatingly betraying jewish colleagues； including many whohad helped him。
real world鈥攂illiard balls are most often used for illustration鈥攖hey don鈥檛 actually strike eachother。 鈥渞ather；鈥潯s timothy ferris explains； 鈥渢he negatively charged fields of the two ballsrepel each other 。 。 。 were it not for their electrical charges they could； like galaxies； pass rightthrough each other unscathed。鈥潯hen you sit in a chair； you are not actually sitting there； butlevitating above it at a height of one angstrom （a hundred millionth of a centimeter）； yourelectrons and its electrons implacably opposed to any closer intimacy。
the picture that nearly everybody has in mind of an atom is of an electron or two flyingaround a nucleus； like planets orbiting a sun。 this image was created in 1904； based on littlemore than clever guesswork； by a japanese physicist named hantaro nagaoka。 it ispletely wrong； but durable just the same。 as isaac asimov liked to note； it inspiredgenerations of science fiction writers to create stories of worlds within worlds； in which atomsbee tiny inhabited solar systems or our solar system turns out to be merely a mote in somemuch larger scheme。 even now cern； the european organization for nuclear research； usesnagaoka鈥檚 image as a logo on its website。 in fact； as physicists were soon to realize； electronsare not like orbiting planets at all； but more like the blades of a spinning fan； managing to fillevery bit of space in their orbits simultaneously （but with the crucial difference that the bladesof a fan only seem to be everywhere at once； electrons are ）。
needless to say； very little of this was understood in 1910 or for many years afterward。
rutherford鈥檚 finding presented some large and immediate problems； not least that no electronshould be able to orbit a nucleus without crashing。 conventional electrodynamic theorydemanded that a flying electron should very quickly run out of energy鈥攊n only an instant orso鈥攁nd spiral into the nucleus； with disastrous consequences for both。 there was also theproblem of how protons with their positive charges could bundle together inside the nucleuswithout blowing themselves and the rest of the atom apart。 clearly whatever was going ondown there in the world of the very small was not governed by the laws that applied in themacro world where our expectations reside。
as physicists began to delve into this subatomic realm； they realized that it wasn鈥檛 merelydifferent from anything we knew； but different from anything ever imagined。 鈥渂ecauseatomic behavior is so unlike ordinary experience；鈥潯ichard feynman once observed； 鈥渋t isvery difficult to get used to and it appears peculiar and mysterious to everyone； both to thenovice and to the experienced physicist。鈥潯hen feynman made that ment； physicists hadhad half a century to adjust to the strangeness of atomic behavior。 so think how it must havefelt to rutherford and his colleagues in the early 1910s when it was all brand new。
one of the people working with rutherford was a mild and affable young dane namedniels bohr。 in 1913； while puzzling over the structure of the atom； bohr had an idea soexciting that he postponed his honeymoon to write what became a landmark paper。 becausephysicists couldn鈥檛 see anything so small as an atom； they had to try to work out its structurefrom how it behaved when they did things to it； as rutherford had done by firing alphaparticles at foil。 sometimes； not surprisingly； the results of these experiments were puzzling。
one puzzle that had been around for a long time had to do with spectrum readings of thewavelengths of hydrogen。 these produced patterns showing that hydrogen atoms emittedenergy at certain wavelengths but not others。 it was rather as if someone under surveillancekept turning up at particular locations but was never observed traveling between them。 no onecould understand why this should be。
it was while puzzling over this problem that bohr was struck by a solution and dashed offhis famous paper。 called 鈥渙n the constitutions of atoms and molecules；鈥潯he paper explainedhow electrons could keep from falling into the nucleus by suggesting that they could occupyonly certain well…defined orbits。 according to the new theory； an electron moving betweenorbits would disappear from one and reappear instantaneously in another without visiting thespace between。 this idea鈥攖he famous 鈥渜uantum leap鈥濃�攊s of course utterly strange； but itwas too good not to be true。 it not only kept electrons from spiraling catastrophically into thenucleus； it also explained hydrogen鈥檚 bewildering wavelengths。 the electrons only appearedin certain orbits because they only existed in certain orbits。 it was a dazzling insight； and itwon bohr the 1922 nobel prize in physics； the year after einstein received his。
meanwhile the tireless rutherford； now back at cambridge as j。 j。 thomson鈥檚 successor ashead of the cavendish laboratory； came up with a model that explained why the nuclei didn鈥檛blow up。 he saw that they must be offset by some type of neutralizing particles； which hecalled neutrons。 the idea was simple and appealing； but not easy to prove。 rutherford鈥檚associate； james chadwick； devoted eleven intensive years to hunting for neutrons beforefinally succeeding in 1932。 he； too； was awarded with a nobel prize in physics； in 1935。 asboorse and his colleagues point out in their history of the subject； the delay in discovery wasprobably a very good thing as mastery of the neutron was essential to the development of theatomic bomb。 （because neutrons have no charge； they aren鈥檛 repelled by the electrical fields atthe heart of an atom and thus could be fired like tiny torpedoes into an atomic nucleus； settingoff the destructive process known as fission。） had the neutron been isolated in the 1920s； theynote； it is 鈥渧ery likely the atomic bomb would have been developed first in europe；undoubtedly by the germans。鈥�
as it was； the europeans had their hands full trying to understand the strange behavior ofthe electron。 the principal problem they faced was that the electron sometimes behaved like aparticle and sometimes like a wave。 this impossible duality drove physicists nearly mad。 forthe next decade all across europe they furiously thought and scribbled and offered petinghypotheses。 in france； prince louis…victor de broglie； the scion of a ducal family； found thatcertain anomalies in the behavior of electrons disappeared when one regarded them as waves。
the observation excited the attention of the austrian erwin schr？dinger； who made some deftrefinements and devised a handy system called wave mechanics。 at almost the same time thegerman physicist werner heisenberg came up with a peting theory called matrixmechanics。 this was so mathematically plex that hardly anyone really understood it；including heisenberg himself （鈥渋 do not even know what a matrix is ；鈥潯eisenberg despairedto a friend at one point）； but it did seem to solve certain problems that schr？dinger鈥檚 wavesfailed to explain。 the upshot is that physics had two theories； based on conflicting premises；that produced the same results。 it was an impossible situation。
finally； in 1926； heisenberg came up with a celebrated promise； producing a newdiscipline that came to be known as quantum mechanics。 at the heart of it was heisenberg鈥檚uncertainty principle； which states that the electron is a particle but a particle that can bedescribed in terms of waves。 the uncertainty around which the theory is built is that we canknow the path an electron takes as it moves through a space or we can know where it is at agiven instant； but we cannot know both。
3any attempt to measure one will unavoidably3there is a little uncertainty about the use of the word uncertainty in regard to heisenbergs principle。 michaelfrayn； in an afterword to his play copenhagen； notes that several words in german…unsicherheit； unscharfe；unbestimmtheit…have been used by various translators； but that none quite equates to the english uncertainty。
frayn suggests that indeterminacy would be a better word for the principle and indeterminability would be betterstill。
disturb the other。 this isn鈥檛 a matter of simply needing more precise instruments； it is animmutable property of the universe。
what this means in practice is that you can never predict where an electron will be at anygiven moment。 you can only list its probability of being there。 in a sense； as dennis overbyehas put it； an electron doesn鈥檛 exist until it is observed。 or； put slightly differently； until it isobserved an electron must be regarded as being 鈥渁t once everywhere and nowhere。鈥�
if this seems confusing； you may take some fort in knowing that it was confusing tophysicists； too。 overbye notes： 鈥渂ohr once mented that a person who wasn鈥檛 outraged onfirst hearing about quantum theory didn鈥檛 understand what had been said。鈥潯eisenberg； whenasked how one could envision an atom； replied： 鈥渄on鈥檛 try。鈥�
so the atom turned out to be quite unlike the image that most people had created。 theelectron doesn鈥檛 fly around the nucleus like a planet around its sun； but instead takes on themore amorphous aspect of a cloud。 the 鈥渟hell鈥潯f an atom isn鈥檛 some hard shiny casing； asillustrations sometimes encourage us to suppose； but simply the outermost of these fuzzyelectron clouds。 the cloud itself is essentially just a zone of statistical probability marking thearea beyond which the electron only very seldom strays。 thus an atom； if you could see it；would look more like a very fuzzy tennis ball than a hard…edged metallic sphere （but not muchlike either or； indeed； like anything you鈥檝e ever seen； we are； after all； dealing here with aworld very different from the one we see around us）。
it seemed as if there was no end of strangeness。 for the first time； as james trefil has put it；scientists had encountered 鈥渁n area of the universe that our bra