Guru Magazine - - Contents - ROSS HARPER• COM­PLEX­ITY GURU

Er­win Schrödinger ev­i­dently didn’t care too much about cats. But who can blame him when his at­tempt at a high-cal­i­bre joke went so long with­out ap­pre­ci­a­tion. Ross Harper puts an end to the con­fu­sion and shows that physi­cists re­ally need to work on their sense of hu­mour.


Let’s get a cat. Now let’s stick it in a box. Now let’s throw in some con­fu­sion over whether or not the cat in­side is liv­ing or dead. Con­grat­u­la­tions! You’re an ex­pert in quan­tum physics. Feel smarter? Didn’t think so.

The prob­lem is, this fre­quently quoted thought ex­per­i­ment by Er­win Schrödinger is rarely fully un­der­stood (or at least not fully ap­pre­ci­ated). The sce­nario was in fact de­signed to high­light a flaw in quan­tum physics, not as a tool to ex­plain it, or to make things in any way clearer. Time and time again, how­ever, this pesky lit­tle hy­po­thet­i­cal is rote-learnt and re­gur­gi­tated as a (per­haps ill-con­sid­ered) dis­play of knowl­edge. But no more, I say! This ar­ti­cle should hope­fully clear up a few things, and arm you with the abil­ity to si­lence that an­noy­ing din­ner party know-it-all. So what’s go­ing on? Well, we take a cat and put it in a box – that much is true. But this box also con­tains a flask of poi­son. The flask is rigged to a de­vice that will re­lease the poi­son when one of the atoms in a small amount of ra­dioac­tive sub­stance (also in­side the box) breaks down or ‘de­cays’. Dur­ing the ex­per­i­ment, there is a chance that one of the atoms will de­cay, re­leas­ing the poi­son, and thus killing the cat. There is also the equally prob­a­ble chance that no atom will de­cay (leav­ing our four-legged hero un­harmed and free to con­tinue won­der­ing why it’s in a box at all). Upon clos­ing the lid, we have no way of telling whether the cat is alive or dead. Ei­ther sce­nario is equally prob­a­ble, and un­til we lift the lid to see, we just don’t know. Now comes the sci­ence. A fun­da­men­tal prin­ci­ple of quan­tum physics is some­thing called ‘quan­tum su­per­po­si­tion’. This is the idea that, un­til mea­sured, a sys­tem ex­ists in all its the­o­ret­i­cally pos­si­ble states si­mul­ta­ne­ously. So some­thing that is nor­mally ei­ther black or white can in­stead be con­sid­ered a kind of grey. In our case, the quan­tum su­per­po­si­tion prin­ci­ple im­plies that, while in the box, the cat’s state is ac­tu­ally a com­bi­na­tion of liv­ing and dead. This con­clu­sion is ob­vi­ously lu­di­crous. It goes against com­mon sense and con­jures up im­ages of de­ranged cat-zom­bies claw­ing their way through cat flaps (when ev­ery­one knows cat-zom­bies don’t have claws). Un­for­tu­nately, most peo­ple don’t go much fur­ther with Schrödinger’s ex­per­i­ment, but walk away with a mes­sage that goes some­thing like, “There’s this cat, and it’s kind of alive, but also kind of dead. Who knows? Sci­ence is crazy!” This in­ter­pre­ta­tion isn’t ex­actly stretch­ing the bound­aries of knowl­edge. Clearly in to­day’s so­ci­ety, the jux­ta­po­si­tion of life and death is a non­sense eas­ily swal­lowed. But Schrödinger was mak­ing an en­tirely dif­fer­ent point al­to­gether. The fact is, quan­tum me­chan­i­cal ef­fects are only re­ally rel­e­vant on a very small scale – for ex­am­ple, at an atomic level. Elec­trons (the small par­ti­cles whizzing around the cen­tre of an atom) can be lo­cated in dif­fer­ent po­si­tions, and have a dif­fer­ent prob­a­bil­ity of be­ing found in par­tic­u­lar places at a given time. So tak­ing into ac­count quan­tum su­per­po­si­tion, un­til it’s ob­served, we con­sider a sin­gle elec­tron to be

ev­ery­where, rather than in just one spe­cific place. This has an im­por­tant im­pli­ca­tion: by mea­sur­ing the po­si­tion of an elec­tron we ac­tu­ally force it into that lo­ca­tion, and in do­ing so, change the sys­tem. It’s a pretty cool idea. By sim­ply ob­serv­ing the world around us, we are chang­ing the way it acts (cue delu­sions of gran­deur). So let’s get back to our cat-in-a-box. The crea­ture’s life de­pends on whether or not a ra­dioac­tive atom de­cays; the two are in­ex­tri­ca­bly linked. At this small scale, the atom is gov­erned by quan­tum ef­fects, and can there­fore be con­sid­ered as both de­cayed and un-de­cayed

at the same time. But hang on a minute: this also means quan­tum ef­fects will in­di­rectly gov­ern the cat, right? So, it too is in a state of… liv­ing death? Schrödinger is mess­ing with us! He has pur­pose­fully paired a quan­tum sys­tem (the ra­dioac­tive atom) with a more nor­mal sys­tem (the cat) to leave us won­der­ing. He raises an im­por­tant ques­tion: when does a quan­tum sys­tem stop ex­ist­ing as a su­per­po­si­tion of states and adopt just one? We’re not sup­posed to ac­cept the con­clu­sion of the ex­per­i­ment: we’re sup­posed to laugh at how ridicu­lous it is. Clearly there is still some fine-tun­ing to be done with quan­tum the­ory. As it hap­pens, there are some prob­lems with Schrödinger’s ex­per­i­ment – in par­tic­u­lar, the ques­tion of what con­sti­tutes a mea­sure­ment. We’re led to be­lieve the only mea­sure­ment taken is the ac­tion of us open­ing the box and look­ing in­side (at which point, the sys­tem would be forced into one par­tic­u­lar state). How­ever, isn’t the poi­son-re­leas­ing de­vice a piece of ap­pa­ra­tus de­signed to mea­sure ra­dioac­tiv­ity? And what about the cat it­self? Is there a min­i­mum level of in­tel­li­gence re­quired from the ob­server? The term ‘ mea­sure­ment’ is am­bigu­ous, and as a re­sult, things start to fall apart a lit­tle bit. (The de­bate con­tin­ues to rage to­day.) But don’t lose faith in old Er­win. He hasn’t failed us. Oh no. His aim was to spark dis­cus­sion, and spark dis­cus­sion he most cer­tainly has. Just don’t ask him to look af­ter your cat while you’re on hol­i­day. So the next time you find your­self in the pres­ence of an in­suf­fer­able know-itall preach­ing the gospel of half-alive, half-dead cats, quot­ing Schrödinger, and ex­plain­ing how com­pli­cated it all is and why you couldn’t pos­si­bly un­der­stand, just take a mo­ment, fur­row your brow and say, “Dude… it’s a joke”. [Dis­claimer: No cats were harmed in the writ­ing of this ar­ti­cle.]

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