A Lazy Layman's Guide to Quantum Physics.pdf

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13/04/13 22:00 A Lazy Layman's Guide to Quantum PhysicsPgina 1 de 6 http://www.higgo.com/quantum/laymans.htmA Lazy Layman's Guide toQuantum Physics James Higgo 1999 What is Quantum Physics?That's an easy one: it's the science of things so small that the quantum nature of reality has aneffect. Quantum means 'discrete amount' or 'portion'. Max Planck discovered in 1900 that youcouldn't get smaller than a certain minimum amount of anything. This minimum amount is nowcalled the Planck unit.Why is it weird?Niels Bohr, the father of the orthodox 'Copenhagen Interpretation' of quantum physics oncesaid, "Anyone who is not shocked by quantum theory has not understood it".To understand the weirdness completely, you just need to know about three experiments: LightBulb, Two Slits, Schroedinger's Cat.Two SlitsThe simplest experiment to demonstrate quantum weirdness involves shining a light throughtwo parallel slits and looking at the screen. It can be shown that a single photon (particle oflight) can interfere with itself, as if it travelled through both slits at once.Light BulbImagine a light bulb filament gives out a photon, seemingly in a random direction. ErwinSchroedinger came up with a nine-letter-long equation that correctly predicts the chances offinding that photon at any given point. He envisaged a kind of wave, like a ripple from a pebbledropped into a pond, spreading out from the filament. Once you look at the photon, this'wavefunction' collapses into the single point at which the photon really is.13/04/13 22:00 A Lazy Layman's Guide to Quantum PhysicsPgina 2 de 6 http://www.higgo.com/quantum/laymans.htmSchroedinger's CatIn this experiment, we take your pet cat and put it in a box with a bottle of cyanide. We rig it upso that a detector looks at an isolated electron and determines whether it is 'spin up' or 'spindown' (it can have either characteristic, seemingly at random). If it is 'spin up', then the bottle isopened and the cat gets it. Ten minutes later we open the box and see if the cat is alive or dead.The question is: what state is the cat in between the detector being activated and you openingthe box. Nobody has actually done this experiment (to my knowledge) but it does show up aparadox that arises in certain interpretations. If you dare to think about it (you're not really supposed to), you have to believe one of thefollowing things: m5VAq-- ---:-.-:-.:: +,,.-:: :/. s./+-.-- -, :s+:-.- e+-:.-:.:- -- -+-:.-:.: --. s+-e+-: +: .:: +: ,--+-: .- :.. +- +ee.+- .- +:: e-::.s:. e:+-.:+: ---.- -- -7/. -.-.-:. .: :e:.::.-,. .-.-, :+--e-:.. !^ 5-4 :.---:i .-:- s.::.--: -, e+-+::.:-.-.-:.:- -- -7/. -.-.-:. .: .-:.-----.-:. .:/ ,+::.--:/+--:.,/: :-+-:,.-: -, .-,--+:.------@:: 5-,:.:/ 5-.+e,+::_-,,.. -- 7.+These are the results of the different interpretations of quantum physics. The interpretations allcompete with each other. Otherwise respectable physicists can get quite heated about howsensible their pet interpretation is and how crazy all the others are. At the moment, there's aboutone new interpretation every three months, but most of them fit into these categories. 13/04/13 22:00 A Lazy Layman's Guide to Quantum PhysicsPgina 3 de 6 http://www.higgo.com/quantum/laymans.htmWhat does it mean?The meaning of quantum physics is a bit of a taboo subject, but everyone thinks about it. Tomake it all a bit more respectable, it is better to say 'ontology' than 'meaning' -- it's the samething. There are several competing interpretations and the one thing they all have in common isthat each of them explains all the facts and predicts every experiment's outcome correctly.Copenhagen Interpretation (CI)This is the granddaddy of interpretations, championed by the formidable Niels Bohr ofCopenhagen university. He browbeat all dissenters into submission (with the notable exceptionof Einstein) at a Brussels conference sponsored by a man called Solvay in 1927. Bohr therebystifled the debate for a generation or two.The CI has a bit of a cheek calling itself an interpretation, because it essentially says "thou shaltnot ask what happens before ye look". He pointed out that the Schroedinger equation worked asa tool for calculating where the particle would be, except that it 'collapsed' as soon as you took apeek. If anyone asked why this was, he would say, "shut up and calculate" (or he might as wellhave done).When you do try to take Copenhagen seriously you come to the conclusion that consciousnessand particle physics are inter-related, and you rush off to write a book called The Dancing Wu-LiMasters.More recently, Henry Stapp at the University of California has written papers such as OnQuantum Theories of the Mind (1997). Stapp's central thesis is that the synapses in your brain areso small that quantum effects are significant. This means that there is quantum uncertaintyabout whether a neuron will fire or not - and this degree of freedom that nature has allows forthe interaction of mind and matter.What happens to the cat? You're not allowed to ask.Many Worlds Interpretation (MWI)The various paradoxes that the Copenhagen Interpretation gave rise to (famouslySchroedinger's cat, and Einstein's dislike of "spooky action at a distance") led others to keep ontrying to find a better interpretation.The simplest was put forward by a student, Hugh Everett, in 1957. He simply said that theSchroedinger equation does not collapse. Of course, everyone laughed at him, because theycould see that the photon, for example, was in just one place when they looked, not in allpossible places. But after a couple of decades, this issue was resolved with the concept ofdecoherence - the idea that different universes can very quickly branch apart, so that there isvery little relationship between them after a tiny fraction of a second.This has led to what should strictly be called the 'post-Everett' Interpretation, but is still usuallycalled MWI. It is now one of the most popular interpretations and has won some impromptubeauty contests at physics conferences. Unfortunately it means that billions of you are splittingoff every fraction of a second into discrete universes and it implies that everything possibleexists in one universe or another. This comes up with its own set of hard-to-digest concepts,such as the fact that a 500-year-old you exists in some universes, whereas in others you died atbirth.In 1997, Max Tegmark at Princeton University proposed an experiment to prove that MWI was13/04/13 22:00 A Lazy Layman's Guide to Quantum PhysicsPgina 4 de 6 http://www.higgo.com/quantum/laymans.htmcorrect. It involved pointing a loaded gun at your head and pulling the trigger. Of course, youwill only survive in those universes where the gun, for whatever reason, fails to go off. If youget a misfire every time, you can satisfy yourself -- with an arbitrarily high level of confidence --that MWI is true. Of course, in most universes your family will be weeping at your funeral (orpossibly just shaking their heads and muttering).What happens to the cat? It's dead in half the subsequent universes and alive in the other half.Pilot Waves, Hidden Variables and the Implicate OrderDavid Bohm (1917-1992) was a very brilliant physicist and that's why people went along withhim when he came up with an elegant but more complicated theory to explain the same set ofphenomena (normally, more complicated theories are disqualified by the principle known asOckham's Razor).Bohm's theory follows on some original insights by Prince Louis de Broglie (1892-1987), whofirst studied the wave-like properties of the behaviour of particles in 1924. De Broglie suggestedthat, in addition to the normal wavefunction of the Copenhagen Interpretation, there is a secondwave that determines a precise position for the particle at any particular time. In this theory,there is some 'hidden variable' that determines the precise position of the photon.Sadly, John von Neumann (1903-1957) wrote a paper in 1932 proving that this theory wasimpossible. Von Neumann was such a great mathematician that nobody bothered to check hismaths until 1966, when John Bell (1928-1990) proved he'd bodged it and there could be hiddenvariables after all -- but only if particles could communicate faster than light (this is called'nonlocality'). In 1982 Alain Aspect demonstrated that this superluminal signaling did appear toexist, although David Mermin then showed that you could not actually signal anything. There isstill some argument about whether this means very much.Bohm's theory was that the second wave was indeed faster than light, and moreover it did notget weaker with distance but instantly permeated the entire universe, acting as a guide for themovement of the photon. This is why it is called a 'pilot wave'.This theory explains the paradoxes of quantum physics perfectly. But it introduces a new faster-than-light wave and some hidden mechanism for deciding where it goes -- to create an'implicate order'. That's quite a lot of extra baggage, and scientists like to travel light. Worse still,Bohm went on to become a mystic, identifying his 'implicate order' with Eastern spirituality andspawning books like Fritjof Capra's The Tao of Physics . That's heretical behaviour in the eyes ofany decent physicist.What happens to the cat? It's either dead or alive, of course!Consistent HistoriesThe Consistent Histories interpretation, put forward by Robert Griffiths in 1984, worksbackwards from the result of an experiment, arguing that only a few possible histories areconsistent with the rules of quantum mechanics. It's an interesting idea but not very popularbecause it still doesn't explain how a particle can go through two slits and interfere with itself.Roland Omns, in The Interpretation of Quantum Mechanics (1994) wrote down 80 equations in asingle chapter and came to the conclusion that the 'consistent histories' interpretation was prettymuch the same as Copenhagen, with a few knobs on.What happens to the Cat? Again, you're not supposed to ask.13/04/13 22:00 A Lazy Layman's Guide to Quantum PhysicsPgina 5 de 6 http://www.higgo.com/quantum/laymans.htmAlternate HistoriesThe Alternate Histories Interpretation is quite different, being similar to the Many-WorldsInterpretation, but with the insistence that only the actual outcome is the real world and theones we're not in don't actually exist. Unfortunately this gets us right back to their being somekind of 'collapse'.What happens to the cat? Again, you're not supposed to ask.Time ReversibilityRichard Feynman (1918-1988) was a genius who developed a new approach to quantummechanics. He formalised its crowning achievement, Quantum Electrodynamics, which is themost accurate scientific theory ever devised. He also developed the Feynman Diagram, whichrepresents the interaction of two particles as the exchange of a third particle. This diagram hastime on one axis and space on the other and the interaction can be viewed as happening both inforward and in reverse time.An electron, on its way from point A to point B, can bump into a photon. In the diagram this canbe drawn as sending it backwards not just in space, but also in time. Then it bumps into anotherphoton, which sends it forward in time again, but in a different direction in space. In this way, itcan be in two places at once.There is little doubt that a Feynman diagram offers the easiest way to predict the results of asubatomic experiment. Many physicists have seen the power of this tool and taken the next step,arguing that reverse time travel is what actually happens in reality. Victor Stenger of theUniversity of Hawaii argues strongly for this ontology in his forthcoming book. Of course, for alayman, it is hard to understand why a photon bounces around in such a way that it appears intwo slits at once.What happens to the Cat? It is both dead and alive simultaneously. We don't see this because ofthe macroscopic 'measurement problem'.Transactional InterpretationLike Stenger's, John Cramer's Transactional Interpretation relies on the fundamental time-symmetry of the universe. He argues that particles perform a kind of 'handshake' in the courseof interacting. One sends out a wave forward in time, and another sends one out backwards intime.What happens to the Cat? Ermm...GremlinsA new interpretation, presented for the first time here, is that there are little green gremlinshovering around, going backwards and forwards in time, shaking hands and collapsing withmirth as they poke and prod subatomic particles in a way they calculate most likely to confuseus. This explains all of the observed experimental results, but it does introduce gremlins, andthe need for a further theory about why they should want to confuse us. Using the principle ofOckham's razor, this interpretation will probably not find much popularity among the scientificcommunity although it may be the basis for a new religion. Watch this space.What happens to the Cat? Depends on what the gremlins think will confuse us most.13/04/13 22:00 A Lazy Layman's Guide to Quantum PhysicsPgina 6 de 6 http://www.higgo.com/quantum/laymans.htm