NS 2759: Seven wonders of the quantum world (series)http://www.newscientist.com/article/mg20627596.000-seven-wonders-of-the-quantum-world.htmlet seq.* 05 May 2010 by Michael Brooks[Comments added.]

From undead cats to particles popping up out of nowhere, fromwatched pots not boiling--sometimes--to ghostly influences at adistance, quantum physics delights in demolishing our intuitionsabout how the world works. Michael Brooks tours the quantum effectsthat are guaranteed to boggle our minds.

1. Corpuscles and buckyballs2. The Hamlet effect3. Something for nothing4. The Elitzur-Vaidman bomb tester5. Spooky action at a distance6. The field that isn't there7. Superfluids and supersolidsAnd finally: Nobody understands

Michael Brooks was the Science party candidate for the constituencyof Bosworth in the UK general election this week

1. Corpuscles and buckyballs

IT DOES not require any knowledge of quantum physics to recognisequantum weirdness. The oldest and grandest of the quantum mysteriesrelates to a question that has exercised great minds at least sincethe time of the ancient Greek philosopher Euclid: what is light madeof?

History has flip-flopped on the issue. Isaac Newton thought lightwas tiny particles--"corpuscles" in the argot of the day. Not allhis contemporaries were impressed, and in classic experiments in theearly 1800s the polymath Thomas Young showed how a beam of lightdiffracted, or spread out, as it passed through two narrow slitsplaced close together, producing an interference pattern on a screenbehind just as if it were a wave.

So which is it, particle or wave? Keen to establish its reputationfor iconoclasm, quantum theory provided an answer soon after itbowled onto the scene in the early 20th century. Light is both aparticle and a wave--and so, for that matter, is everything else. Asingle moving particle such as an electron can diffract andinterfere with itself as if it were a wave, and believe it or not,an object as large as a car has a secondary wave character as ittrundles along the road.

That revelation came in a barnstorming doctoral thesis submitted bythe pioneering quantum physicist Louis de Broglie in 1924. He showedthat by describing moving particles as waves, you could explain whythey had discrete, quantised energy levels rather than the continuumpredicted by classical physics.

De Broglie first assumed that this was just a mathematicalabstraction, but wave-particle duality seems to be all too real.Young's classic wave interference experiment has been reproducedwith electrons and all manner of other particles (see diagram).

We haven't yet done it with a macroscopic object such as a movingcar, admittedly. Its de Broglie wavelength is something like 10^-38metres, and making it do wave-like things such as diffract wouldmean creating something with slits on a similar scale, a task waybeyond our engineering capabilities. The experiment has beenperformed, though, with a buckyball--a soccer-ball-shaped latticeof 60 carbon atoms that, at about a nanometre in diameter, is largeenough to be seen under a microscope (Nature, vol 401, p 680).

All that leaves a fundamental question: how can stuff be waves andparticles at the same time? Perhaps because it is neither, saysMarkus Arndt of the University of Vienna, Austria, who did thebuckyball experiments in 1999. What we call an electron or abuckyball might in the end have no more reality than a click in adetector, or our brain's reconstruction of photons hitting ourretina. "Wave and particle are then just constructs of our mind tofacilitate everyday talking," he says.

2. The Hamlet effect

A WATCHED pot never boils." Armed with common sense and classicalphysics, you might dispute that statement. Quantum physics wouldslap you down. Quantum watched pots do refuse to boil--sometimes.At other times, they boil faster. At yet other times, observationpitches them into an existential dilemma whether to boil or not.

This madness is a logical consequence of the Schrödinger equation,the formula concocted by Austrian physicist Erwin Schrödinger in1926 to describe how quantum objects evolve probabilistically overtime.

Imagine, for example, conducting an experiment with an initiallyundecayed radioactive atom in a box. According to the Schrödingerequation, at any point after you start the experiment the atomexists in a mixture, or "superposition", of decayed and undecayedstates.

Each state has a probability attached that is encapsulated in amathematical description known as a wave function. Over time, aslong as you don't look, the wave function evolves as the probabilityof the decayed state slowly increases. As soon as you do look, theatom chooses--in a manner in line with the wave functionprobabilities--which state it will reveal itself in, and the wavefunction "collapses" to a single determined state.

This is the picture that gave birth to Schrödinger's infamous cat.Suppose the radioactive decay of an atom triggers a vial of poisongas to break, and a cat is in the box with the atom and the vial. Isthe cat both dead and alive as long as we don't know whether thedecay has occurred?

We don't know. All we know is that tests with larger and largerobjects--including, recently, a resonating metal strip big enoughto be seen under a microscope--seem to show that they really can beinduced to adopt two states at once (Nature, vol 464, p 697).

The weirdest thing about all this is the implication that justlooking at stuff changes how it behaves. Take the decaying atom:observing it and finding it undecayed resets the system to adefinitive state, and the Schrödinger-equation evolution towards"decayed" must start again from scratch.

The corollary is that if you keep measuring often enough, the systemwill never be able to decay. This possibility is dubbed the quantumZeno effect, after the Greek philosopher Zeno of Elea, who devised afamous paradox that "proved" that if you divided time up into eversmaller instants you could make change or motion impossible.

And the quantum Zeno effect does happen. In 1990, researchers at theNational Institute of Standards and Technology in Boulder, Colorado,showed they could hold a beryllium ion in an unstable energyconfiguration rather akin to balancing a pencil on its sharpenedpoint, provided they kept re-measuring its energy (Physical ReviewA, vol 41, p 2295).

The converse "anti-Zeno" effect--making a quantum pot boil fasterby just measuring it--also occurs. Where a quantum object has acomplex arrangement of states to move into, a decay into alower-energy state can be accelerated by measuring the system in theright way. In 2001, this too was observed in the lab (PhysicalReview Letters, vol 87, p 040402).

The third trick is the "quantum Hamlet effect", proposed last yearby Vladan Pankovic of the University of Novi Sad, Serbia. Aparticularly intricate sequence of measurements, he found, canaffect a system in such a way as to make the Schrödinger equationfor its subsequent evolution intractable. As Pankovic puts it: to bedecayed or not-decayed, "that is the analytically unsolvablequestion".

3. Something for nothing

"NOTHING will come of nothing," King Lear admonishes Cordelia in theeponymous Shakespeare play. In the quantum world, it's different:there, something comes of nothing and moves the furniture around.

Specifically, if you place two uncharged metal plates side by sidein a vacuum, they will move towards each other, seemingly withoutreason. They won't move a lot, mind. Two plates with an area of asquare metre placed one-thousandth of a millimetre apart will feel aforce equivalent to just over a tenth of a gram.

The Dutch physicist Hendrik Casimir first noted this minusculemovement in 1948. "The Casimir effect is a manifestation of thequantum weirdness of the microscopic world," says physicist SteveLamoreaux of Yale University.

It has to do with the quantum quirk known as Heisenberg'suncertainty principle, which essentially says the more we know aboutsome things in the quantum world, the less we know about others. Youcan't, for instance, deduce the exact position and momentum of aparticle simultaneously. The more certain we are of where a particleis, the less certain we are of where it is heading.

A similar uncertainty relation exists between energy and time, witha dramatic consequence. If space were ever truly empty, it wouldcontain exactly zero energy at a precisely defined moment in time -something the uncertainty principle forbids us from knowing.

It follows that there is no such thing as a vacuum. According toquantum field theory, empty space is actually fizzing withshort-lived stuff that appears, looks around a bit, decides it

doesn't like it and disappears again, all in the name of preventingthe universe from violating the uncertainty principle. For the mostpart, this stuff is pairs of photons and their antiparticles thatquickly annihilate in a puff of energy. The tiny electric fieldscaused by these pop-up particles, and their effect on free electronsin metal plates, might explain the Casimir effect.

Or they might not. Thanks to the uncertainty principle, the electricfields associated with the atoms in the metal plates also fluctuate.These variations create tiny attractions called van der Waals forcesbetween the atoms. "You can't ascribe the Casimir force solelyeither to the zero point of the vacuum or to the zero point motionof the atoms that make up the plates," says Lamoreaux. "Either viewis correct and arrives at the same physical result."

Whichever picture you adopt, the Casimir effect is big enough to bea problem. In nanoscale machines, for example, it could causecomponents in close proximity to stick together.

The way to avoid that might be simply to reverse the effect. In1961, Russian physicists showed theoretically that combinations ofmaterials with differing Casimir attractions can create scenarioswhere the overall effect is repulsion. Evidence for this strange"quantum buoyancy" was announced in January 2009 by physicists fromHarvard University who had set up gold and silica plates separatedby the liquid bromobenzene (Nature, vol 457, p 170).

4. The Elitzur-Vaidman bomb-tester

A BOMB triggered by a single photon of light is a scary thought. Ifsuch a thing existed in the classical world, you would never even beaware of it. Any photon entering your eye to tell you about it wouldalready have set off the bomb, blowing you to kingdom come.

With quantum physics, you stand a better chance. According to ascheme proposed by the Israeli physicists Avshalom Elitzur and LevVaidman in 1993, you can use quantum trickery to detect alight-triggered bomb with light--and stay safe a guaranteed 25 percent of the time (Foundations of Physics, vol 23, p 987).

The secret is a device called an interferometer. It exploits thequantumly weird fact that, given two paths to go down, a photon willtake both at once. We know this because, at the far end of thedevice, where the two paths cross once again, a wave-likeinterference pattern is produced (see "Quantum wonders: Corpusclesand buckyballs").

To visualise what is going on, think of a photon entering theinterferometer and taking one path while a ghostly copy of itselfgoes down the other. In Elitzur and Vaidman's thought experiment,half the time there is a photon-triggered bomb blocking one path(see diagram). Only the real photon can trigger the bomb, so if itis the ghostly copy that gets blocked by the bomb, there is noexplosion--and nor is there an interference pattern at the otherend. In other words, we have "seen" the bomb without triggering it.

Barely a year after Elitzur and Vaidman proposed their bomb-testingparadox, physicists at the University of Vienna, Austria, hadbrought it to life--not by setting off bombs, but by bouncingphotons off mirrors (Physical Review Letters, vol 74, p 4763).

In 2000, Shuichiro Inoue and Gunnar Bjork of the Royal Institute ofTechnology in Stockholm, Sweden, used a similar technique to showthat you could get an image of a piece of an object without shininglight on it--something that could revolutionise medical imaging."It would be very useful for something like X-ray scanning, if therewere no radiation damage to the tissue because no X-rays actuallyhit it," says physicist Richard Jozsa of the University ofCambridge.

Josza is the brains behind perhaps the most eye-rubbing of suchtricks: using a quantum computer to deliver the output of a programeven when you don't run the program. As the team that implementedhis idea in 2005 showed, quantum physics does at least retain somesemblance of classical decency: to deliver a sensible answer, thecomputer does need to be switched on (Nature, vol 439, p 949).

5. Spooky action at a distance

ERWIN SCHRÖDINGER called it the "defining trait" of quantum theory.Einstein could not bring himself to believe in it at all, thinkingit proof that quantum theory was seriously buggy. It isentanglement: the idea that particles can beed in such a waythat changing the quantum state of one instantaneously affects theother, even if they are light years apart.

This "spooky action at a distance", in Einstein's words, is aserious blow to our conception of how the world works. In 1964,physicist John Bell of the European Organization for NuclearResearch (CERN) in Geneva, Switzerland, showed just how serious. Hecalculated a mathematical inequality that encapsulated the maximumcorrelation between the states of remote particles in experiments inwhich three "reasonable" conditions hold: that experimenters havefree will in setting things up as they want; that the particleproperties being measured are real and pre-existing, not justpopping up at the time of measurement; and that no influence travelsfaster than the speed of light, the cosmic speed limit.

As many experiments since have shown, quantum mechanics regularlyviolates Bell's inequality, yielding levels of correlation way abovethose possible if his conditions hold. That pitches us into aphilosophical dilemma. Do we not have free will, meaning something,somehow predetermines what measurements we take? That is notanyone's first choice. Are the properties of quantum particles notreal--implying that nothing is real at all, but exists merely as aresult of our perception? That's a more popular position, but ithardly leaves us any the wiser.

Or is there really an influence that travels faster than light?Cementing the Swiss reputation for precision timing, in 2008physicist Nicolas Gisin and his colleagues at the University ofGeneva showed that, if reality and free will hold, the speed oftransfer of quantum states between entangled photons held in twovillages 18 kilometres apart was somewhere above 10 million timesthe speed of light (Nature, vol 454, p 861).

Whatever the true answer is, it will be weird. Welcome to quantumreality.

6. The field that isn't there

HERE'S a nice piece of quantum nonsense. Take a doughnut-shaped

magnet and wrap a metal shield round its inside edge so that nomagnetic field can leak into the hole. Then fire an electron throughthe hole.

There is no field in the hole, so the electron will act as if thereis no field, right? Wrong. The wave associated with the electron'smovement suffers a jolt as if there were something there.

Werner Ehrenberg and Raymond Siday were the first to note that thisbehaviour lurks in the Schrödinger equation (see "Quantum wonders:The Hamlet effect "). That was in 1949, but their result remainedunnoticed. Ten years later Yakir Aharonov and David Bohm, working atthe University of Bristol in the UK, rediscovered the effect and forsome reason their names stuck.

So what is going on? The Aharonov-Bohm effect is proof that there ismore to electric and magnetic fields than is generally supposed. Youcan't calculate the size of the effect on a particle by consideringjust the properties of the electric and magnetic fields where theparticle is. You also have to take into account the properties whereit isn't.

Casting about for an explanation, physicists decided to take a lookat a property of the magnetic field known as the vector potential.For a long time, vector potentials were considered just handymathematical tools--a shorthand for electrical and magneticproperties that didn't have any real-world significance. As it turnsout, they describe something that is very real indeed.

The Aharonov-Bohm effect showed that the vector potential makes anelectromagnetic field more than the sum of its parts. Even when afield isn't there, the vector potential still exerts an influence.That influence was seen unambiguously for the first time in 1986when Akira Tonomura and colleagues in Hitachi's laboratories inTokyo, Japan, measured a ghostly electron jolt (Physical ReviewLetters, vol 48, p 1443).

Although it is far from an everyday phenomenon, the Aharonov-Bohmeffect might prove to have uses in the real world--in magneticsensors, for example, or field-sensitive capacitors and data storagebuffers for computers that crunch light.

7. Superfluids and supersolids

FORGET radioactive spider bites, exposure to gamma rays, or anyother accident favoured in Marvel comics: in the real world, it'squantum theory that gives you superpowers.

Take helium, for example. At room temperature, it is normal fun: youcan fill floaty balloons with it, or inhale it and talk in a squeakyvoice. At temperatures below around 2 kelvin, though, it is a liquidand its atoms become ruled by their quantum properties. There, itbecomes super-fun: a superfluid.At room temperature, helium is normal fun. Close to absolute zero,though, it becomes super-fun

Superfluid helium climbs up walls and flows uphill in defiance ofgravity. It squeezes itself through impossibly small holes. It flipsthe bird at friction: put superfluid helium in a bowl, set the bowlspinning, and the helium sits unmoved as the bowl revolves beneathit. Set the liquid itself moving, though, and it will continue

gyrating forever.

That's fun, but not particularly useful. The opposite might be saidof superconductors. These solids conduct electricity with noresistance, making them valuable for transporting electrical energy,for creating enormously powerful magnetic fields--to steer protonsaround CERN's Large Hadron Collider, for instance--and forlevitating superfast trains.

We don't yet know how all superconductors work, but it seems theuncertainty principle plays a part (see "Quantum wonders: Somethingfor nothing"). At very low temperatures, the momentum of individualatoms or electrons in these materials is tiny and very preciselyknown, so the position of each atom is highly uncertain. In fact,they begin to overlap with each other to the point where you can'tdescribe them individually. They start acting as one superatom orsuperelectron that moves without friction or resistance.

All this is nothing in the weirdness stakes, however, compared witha supersolid. The only known example is solid helium cooled towithin a degree of absolute zero and at around 25 times normalatmospheric pressure.

Under these conditions, the bonds between helium atoms are weak, andsome break off to leave a network of "vacancies" that behave almostexactly like real atoms. Under the right conditions, these vacanciesform their own fluid-like Bose-Einstein condensate. This will, undercertain circumstances, pass right through the normal helium lattice--meaning the solid flows, ghost-like, through itself.

So extraordinary is this superpower that Moses Chan and Eun-SeongKim of Pennsylvania State University in University Park checked andre-checked their data on solid helium for four years beforeeventually publishing in 2004 (Nature, vol 427, p 225). "I hadlittle confidence we would see the effect," says Chan. Nevertheless,researchers have seen hints that any crystalline material might bepersuaded to perform such a feat at temperatures just a fractionabove absolute zero. Not even Superman can do that.

8. Nobody understands

It is tempting, faced with the full-frontal assault of quantumweirdness, to trot out the notorious quote from Nobel prize-winningphysicist Richard Feynman: "Nobody understands quantum mechanics."

It does have a ring of truth to it, though. The explanationsattempted here use the most widely accepted framework for thinkingabout quantum weirdness, called the Copenhagen interpretation afterthe city in which Niels Bohr and Werner Heisenberg thrashed out itsground rules in the early 20th century.

With its uncertainty principles and measurement paradoxes, theCopenhagen interpretation amounts to an admission that, as classicalbeasts, we are ill-equipped to see underlying quantum reality. Anyattempt we make to engage with it reduces it to a shallow classicalprojection of its full quantum richness.

Lev Vaidman of Tel Aviv University, Israel, like many otherphysicists, touts an alternative explanation. "I don't feel that Idon't understand quantum mechanics," he says. But there is a highprice to be paid for that understanding--admitting the existence of

parallel universes.

In this picture, wave functions do not "collapse" to classicalcertainty every time you measure them; reality merely splits into asmany parallel worlds as there are measurement possibilities. One ofthese carries you and the reality you live in away with it. "If youdon't admit many-worlds, there is no way to have a coherentpicture," says Vaidman.

Or, in the words of Feynman again, whether it is the Copenhageninterpretation or many-worlds you accept, "the 'paradox' is only aconflict between reality and your feeling of what reality ought tobe".

COMMENTS

Take The Next StepThe Copenhagen interpretation was an attempt by a generation ofstunned scientist to bridge the difference between our classicalsense of the universe and its quantum reality. It distorts thereality of quantum behavior and results in confusion, apparentparadoxes and general misunderstanding."If you don't admit many-worlds, there is no way to have a coherentpicture," says Vaidman.The "coherent picture" that Vaidman refers to is reconcilingEinstein's sense of causality with entanglement. Although theMulti-World interpretation (MWI) is an improvement over theCopenhagen interpretation, it still creates confusion andmisunderstanding by failing to accept the truth of the non-localityof wave-functions. It generates fantasies of alternate co-existingrealities where every choice is explored. "What if Hitler won?"scenarios. In its detail, the MWI doesn't actually support thisidea, but as a bad explanation, it spawns them.Take the next step and move past Einstein's causality. Embracewave-function non-locality and what it means. Space and time are notfundamental properties of the universe, they are emergent.

Take The Next StepMon May 10 09:23:45 BST 2010 by LizaI've read your previous comment on the according to you falseparticle-wave duality, and you mentioned locality as well. Still,most books on physics for non-physicists still mention the dualityas an established theory. Do you have any real reason to assume theduality is false, and non-locality is normal, except for yourpersonal opinion? What you say may make sense or could just as wellbe nothing but wild speculation, but since I'm not a physicist, Idon't have the knowledge to make up my mind.

Take The Next StepMon May 10 17:08:56 BST 2010 by David Allen@Liza "...most books on physics for non-physicists still mention theduality as an established theory."Yes, most books are based on the dominant. and very entrenched,interpretation of quantum mechanics (QM), the Copenhageninterpretation.Duality is an explanation, not a theory, QM is the theory. Dualityattempts to explain why under some conditions we see "classical"particle-like behavior and under others we see QM wave-likebehavior.@Liza "Do you have any real reason to assume the duality is false,and non-locality is normal, except for your personal opinion?"

Yes I have reasons to reject duality as an explanation:The word "duality" implies equivalence, which isn't true. Particlelike models of QM systems can't describe everything that is seen inexperiments, however the wave-function models can. In other wordsthe wave-function models are complete, they can describe both thewave-like behavior and the particle-like behavior. Particleinterpretations are incomplete, and unnecessary.Wheeler's delayed choice experiment can't be explained by dualityand wave-function collapse. It can only be explained by thewave-function models.Both duality and wave-function collapse are bad generalexplanations. In the right circumstances however, the ideas behindthem can be used to simplify the math, making real-world problemsmore tractable.As for non-locality:The issues surrounding non-locality are harder to tease apart, so myconclusions are certainly just my opinion. This will give you someidea of the different approaches:http://en.wikipedia.org/wiki/Principle_of_localityThe Copenhagen interpretation doesn't reject non-locality, in factit needs it to support wave-function collapse. The Many-Worldsinterpretation (MWI) does reject non-locality, but other than thatis basically in line with my wave-only perspective.MWI's rejection of non-locality appears to be based on a desire toavoid an explanation that contradicts special relativity. I claimhowever that special relativity only applies to energy in space, andnot to quantum information. The non-local behavior of entanglementdoes not contradict it. I also claim that non-local entanglement maynot necessarily allow simultaneity, or specific ordering to beestablished between frames of reference. This would also avoid acontradiction with special relativity. However I think that theremay be a way to falsify this second claim.Wild Speculation:The wild speculation I engage in is that the Universe has no truespacial dimensions, time and space are emergent from quantumdecoherence. The flow of time and expansion of space are tied to therate of decoherence, but the rate of decoherence slows down as spaceexpands due to fewer interactions. The relative rates of growth inspace vs. flow of time makes the universe appear to grow at anexponential rate to observers within the universe. When there is nomore matter and all energy stretches to the ultimate quantum levels,the dominant quantum interactions become entanglement. The universebegins to collapse, and as it does the rate of entanglementincreases. To observers inside the universe, this collapse wouldappear to be inversely exponential. The distortions in observer timemean that at the point of maximum expansion, the universe appears tohave just been created, exploding almost instantly to its currentsize. At the point of maximum density, the universe appears to havegone on almost forever in this high density state of slowingcollapse.

Take The Next StepTue May 11 15:53:59 BST 2010 by LizaHey thanks! I had to read your comment thrice in order to understandwhat you are explaining (no fault of yours), and it's surelyinteresting. True, if the wave-function models can explain allwhat's observed, and the particle-based models can't, there's noreal reason to hang on to the particle idea except that it's easierto visualise and understand for most people. I guess non-localitygets rejected because it seems beyond what we can comprehend,magical even. The whole decoherence/dimensions speculation is

fascinating, but have you ever tried to make some calculations tosee if it holds up?PS: it's pretty nice to read comments on this topic from people whomake sense, rather than the usual Zephir-style nonsense.view thread

Quantum Wonders: Nobody UnderstandsSun May 09 15:28:00 BST 2010 by andworin order to understand the quantum world it is important to take thenext quntum leap. Specifically it is important to look at thequantum world at a very much smaller scale.Please see a recently published article (currently online) onexactly this topic. Entitled "The formulation of harmonicquintessence and a fundamental energy equivalence equation" PhysicsEssays 23: 311-319

Quantum Wonders Nobody UnderstandsMon May 10 00:00:32 BST 2010 by Julian MannI do not agree with Vaidman that we are forced into acceptingparallel universes. See my comments on the quantum Hamlet Effect andthe existence of Classical Time, Anti-time(Quantum World) and Nultime.These concepts suffice to explain all the anomolies in QuantumMechanics, reconcile it to relativity etc. I have noticed that whenscientists do not understand something in Physics, they invoke suchconcepts for which there is no experimental evidence for existence

Quantum Wonders Nobody UnderstandsMon May 10 15:20:16 BST 2010 by andworThank you for your insightful comments. I am also exploring theconcept of what you term "anti-time".Specifically I was considering an adaptation to the Wheeler andFeynman theory, where time is symmetric. This effectively means thatelectromagnetic processes go backwards in time as well as forwards.But in the presence of the "perfect reflector " in the past, i.e.The Big Bang, then the half that goes backwards in time getsrefelected forwards in time to arrive when it left, and continues onits forward journey. This means that the whole signal effectivelygoes forward in time.This requires also that the future is the "perfect absorber" andsince the recent discovery of the accelerating Universe then we dohave our "perfect" absorberOf course entanglement is the archetypal example of where this mightbe happening. Equally well the electomanetic signal does not need togo all the way back to the BIG Bang it just needs to go back to thepoint in time when the electromanetic effect/signal was created,exactly as in entangled particles/photons.Any thoughts