量子世界的不可逆性

物理学家首次通过实验确认了热力学不可逆性在量子世界也是成立的。这意味着，在量子层面你也不能自动还原一个打碎了的鸡蛋。

物理学家将液体chloroform(CHCl3)放于核磁共振仪内部的超导磁铁中心（NuclearMagneticResonance,NMR）.利用给样品外加的前向（Forward）和反向（reverse）磁脉冲，驱动碳原子核自旋远离平衡态从而产生不可逆熵（irreversibleentropy）.

Thephysicists,TiagoBatalh?oattheFederalUniversityofABC,Brazil,andcoauthors,havepublishedtheirpaperontheexperimentaldemonstrationofquantumthermodynamicirreversibilityinarecentissueofPhysicalReviewLetters.

Irreversibilityatthequantumlevelmayseemobvioustomostpeoplebecauseitmatchesourobservationsoftheeveryday,macroscopicworld.However,itisnotasstraightforwardtophysicistsbecausethemicroscopiclawsofphysics,suchastheSchr?dingerequation,aretime-symmetric,orreversible.Intheory,forwardandbackwardmicroscopicprocessesareindistinguishable.

Inreality,however,weonlyobserveforwardprocesses,notreversibleoneslikebrokeneggshellsbeingputbacktogether.Itsclearthat,atthemacroscopiclevel,thelawsruncountertowhatweobserve.Nowthenewstudyshowsthatthelawsdontmatchwhathappensatthequantumlevel,either.

Observingthermodynamicprocessesinaquantumsystemisverydifficultandhasnotbeendoneuntilnow.Intheirexperiment,thescientistsmeasuredtheentropychangethatoccurswhenapplyinganoscillatingmagneticfieldtocarbon-13atomsinliquidchloroform.Theyfirstappliedamagneticfieldpulsethatcausestheatomsnuclearspinstoflip,andthenappliedthepulseinreversetomakethespinsundergothereverseddynamics.

Iftheprocedurewerereversible,thespinswouldhavereturnedtotheirstartingpoints—buttheydidnt.Basically,theforwardandreversemagneticpulseswereappliedsorapidlythatthespinsflippingcouldntalwayskeepup,sothespinsweredrivenoutofequilibrium.Themeasurementsofthespinsindicatedthatentropywasincreasingintheisolatedsystem,showingthatthequantumthermodynamicprocesswasirreversible.

Bydemonstratingthatthermodynamicirreversibilityoccursevenatthequantumlevel,theresultsrevealthatthermodynamicirreversibilityemergesatagenuinemicroscopicscale.Thisfindingmakesthequestionofwhythemicroscopiclawsofphysicsdontmatchourobservationsevenmorepressing.Ifthelawsreallyarereversible,thenwhatarethephysicaloriginsofthetime-asymmetricentropyproductionthatweobserve?

Thephysicistsexplainthattheanswertothisquestionliesinthechoiceoftheinitialconditions.Themicroscopiclawsallowreversibleprocessesonlybecausetheybeginwithagenuineequilibriumprocessforwhichtheentropyproductionvanishesatalltimes,thescientistswriteintheirpaper.Preparingsuchanidealinitialstateinaphysicalsystemisextremely







































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