8. The Mean­ing of Quan­tum Me­chan­ics

En­gi­neers tend to be fairly mat­ter-of-fact about the physics they use. Many use en­tropy on a daily ba­sis as a com­pu­ta­tional tool with­out wor­ry­ing much about its vague, ab­stract math­e­mat­i­cal de­f­i­n­i­tion. Such a prac­ti­cal ap­proach is even more im­por­tant for quan­tum me­chan­ics.

Fa­mous quan­tum me­chan­ics pi­o­neer Niels Bohr had this to say about it:

“For those who are not shocked when they first come across quan­tum the­ory can­not pos­si­bly have un­der­stood it.” [Niels Bohr, quoted in W. Heisen­berg (1971) Physics and Be­yond. Harper and Row.]

Feyn­man was a Cal­tech quan­tum physi­cist who re­ceived a No­bel Prize for the cre­ation of quan­tum elec­tro­dy­nam­ics with Schwinger and Tomon­aga. He also pi­o­neered nan­otech­nol­ogy with his fa­mous talk “There's Plenty of Room at the Bot­tom.” About quan­tum me­chan­ics, he wrote:

“There was a time when the news­pa­pers said that only twelve men un­der­stood the the­ory of rel­a­tiv­ity. I do not be­lieve there ever was such a time. There might have been a time when only one man did, be­cause he was the only guy who caught on, be­fore he wrote his pa­per. But af­ter peo­ple read the pa­per, a lot of peo­ple un­der­stood the the­ory of rel­a­tiv­ity in some way or other, cer­tainly more than twelve. On the other hand, I think I can safely say that no­body un­der­stands quan­tum me­chan­ics.” [Richard P. Feyn­man (1965) The Char­ac­ter of Phys­i­cal Law 129. BBC/Pen­guin.]

Still, say­ing that quan­tum me­chan­ics is un­un­der­stand­able raises the ob­vi­ous ques­tion: “If we can­not un­der­stand it, does it at least seem plau­si­ble?” That is the ques­tion to be ad­dressed in this chap­ter. When you read this chap­ter, you will see that the an­swer is sim­ple and clear. Quan­tum me­chan­ics is the most im­plau­si­ble the­ory ever for­mu­lated. No­body would ever for­mu­late a the­ory like quan­tum me­chan­ics in jest, be­cause none would be­lieve it. Physics ended up with quan­tum me­chan­ics not be­cause it seemed the most log­i­cal ex­pla­na­tion, but be­cause count­less ob­ser­va­tions made it un­avoid­able.