My name is Ulrich Mohrhoff. I teach physics at the Sri Aurobindo Inter­na­tional Centre of Edu­ca­tion in Pondicherry, India. In 2011 I pub­lished a text­book with the pre­pos­terous title The World According to Quantum Mechanics: Why the Laws of Physics Make Per­fect Sense After All.

Here is how that came about.

While still in high school, I learned that the tides act as a brake on the Earth, grad­u­ally slowing down its rota­tion, and that the angular momentum lost by the Earth is trans­ferred to the Moon, causing the latter to slowly spiral out­wards, away from Earth. I still vividly remember my puz­zle­ment: how — by what mech­a­nism or nat­ural process — did angular momentum get trans­ferred from Earth to the Moon? Just so Newton’s con­tem­po­raries must have mar­veled at his theory of gravity. Newton’s[1] response is well known:

I have not been able to dis­cover the cause of those prop­er­ties of gravity from phænomena, and I frame no hypotheses.… to us it is enough, that gravity does really exist, and act according to the laws which we have explained, and abun­dantly serves to account for all the motions of the celes­tial bodies, and of our sea.

In Newton’s theory, grav­i­ta­tional effects had been simul­ta­neous with their causes. The time-​​delay between causes and effects in clas­sical elec­tro­dy­namics and in Einstein’s theory of gravity made it seem pos­sible for a while to explain “how Nature does it.” One only had to trans­mo­grify algo­rithms that served to cal­cu­late effects (having spec­i­fied their causes) into nat­ural processes by which causes pro­duce effects. This is how the elec­tro­mag­netic field — a cal­cu­la­tional tool — came to be thought of as a phys­ical entity in its own right, which is locally acted upon by charges, which locally acts on charges, and which medi­ates the action of charges on charges by locally acting on itself.

Today this sleight of hand no longer works.

Clas­sical states are algo­rithms that assign trivial prob­a­bil­i­ties to mea­sure­ment out­comes. Because the prob­a­bil­i­ties are trivial (either 0 or 1), these states can be re-​​interpreted as col­lec­tions of pos­sessed prop­er­ties and described without ref­er­ence to “mea­sure­ment.” Quantum states are algo­rithms that assign prob­a­bil­i­ties between 0 and 1, and this is why they cannot be so described. Again, while the clas­sical laws cor­re­late mea­sure­ment out­comes deter­min­is­ti­cally (which is why they can be inter­preted in causal terms), the quantum-​​mechanical laws cor­re­late mea­sure­ment out­comes prob­a­bilis­ti­cally (which is why they can not be so inter­preted). In at least one respect, there­fore, physics is back to where it was in Newton’s time — and this with a vengeance.

According to Dennis Dieks,[2] Pro­fessor of the Foun­da­tions and Phi­los­ophy of the Nat­ural Sci­ences at Utrecht Uni­ver­sity and Editor of Studies in His­tory and Phi­los­ophy of Modern Physics,

the out­come of foun­da­tional work in the last couple of decades has been that inter­pre­ta­tions which try to accom­mo­date clas­sical intu­itions are impos­sible, on the grounds that the­o­ries that incor­po­rate such intu­itions nec­es­sarily lead to empir­ical pre­dic­tions which are at vari­ance with the quantum mechan­ical predictions.

But, seri­ously, how could anyone have hoped to get away for good with passing off cal­cu­la­tional tools — math­e­mat­ical expres­sions or equa­tions — as phys­ical enti­ties or nat­ural processes? Was it the hubristic desire to feel “poten­tially omni­scient” — capable in prin­ciple of knowing the fur­ni­ture of the uni­verse and the laws by which this is gov­erned? Or was it the pres­tige pro­vided by the care­fully cul­ti­vated image of physi­cists as having priv­i­leged access to Truth?

There is another reason why we cannot hope to explain “how Nature does it.” If quantum mechanics is the fun­da­mental the­o­ret­ical frame­work of physics — and while there are a few doubters, nobody has the slightest idea what an alter­na­tive frame­work con­sis­tent with the empir­ical data might look like — then its for­malism not only defies reifi­ca­tion but also cannot be explained in terms of a “more fun­da­mental” frame­work. We some­times speak loosely of a theory as being more fun­da­mental than another but, strictly speaking, “fun­da­mental” has no comparative.

What remains pos­sible is to explain “why Nature does it.” When effi­cient cau­sa­tion fails, tele­o­log­ical expla­na­tion remains viable.

The ques­tion we shall be asking our­selves is: what does it take to have stable objects that “occupy space” while being com­posed of objects that do not “occupy space”? And the answer we shall obtain is: quantum mechanics and spe­cial rel­a­tivity for starters (and thus rel­a­tivistic quantum mechanics), plus the Stan­dard Model and gen­eral rel­a­tivity, at least as effec­tive theories.

An approach that rejects the very notion of quantum state evo­lu­tion, like the one pre­sented here, runs the risk of being dis­missed as an onto­log­i­cally sterile instru­men­talism. Yet it is this notion, more than any other, that blocks our view of the onto­log­ical impli­ca­tions of quantum mechanics. For instance, whereas the sup­po­si­tion that quantum states evolve leads to the con­clu­sion that the spa­tiotem­poral dif­fer­en­ti­a­tion of the phys­ical world is com­plete, one of the onto­log­ical impli­ca­tions of quantum mechanics is that the spa­tiotem­poral dif­fer­en­ti­a­tion of the phys­ical world is incom­plete; it does not go “all the way down.”

This is not simply a case of one word against another, for the world’s incom­plete spa­tiotem­poral dif­fer­en­ti­a­tion fol­lows from the manner in which quantum mechanics assigns prob­a­bil­i­ties, which is testable, whereas the world’s com­plete spa­tiotem­poral dif­fer­en­ti­a­tion fol­lows from an assump­tion about what is the case between mea­sure­ments, and such an assump­tion is “not even wrong” in Wolf­gang Pauli’s felic­i­tous phrase, inas­much as it is nei­ther ver­i­fi­able nor falsifiable.

Again, under­standing the cen­tral role played by mea­sure­ments calls for a clear dis­tinc­tion between that which mea­sures and that which is mea­sured. This in turn calls for a rig­orous def­i­n­i­tion of the fre­quently mis­used and much maligned word “macro­scopic.” But it is pre­cisely the world’s incom­plete spa­tiotem­poral dif­fer­en­ti­a­tion that makes such a def­i­n­i­tion pos­sible. The cen­tral role played by mea­sure­ments there­fore cannot be under­stood without rejecting the notion that quantum states evolve.

For at least twenty-​​five cen­turies, the­o­rists — from meta­physi­cians to nat­ural philoso­phers to physi­cists and philoso­phers of sci­ence — have tried to model reality from the bottom up, starting with an ulti­mate mul­ti­plicity and using con­cepts of com­po­si­tion and inter­ac­tion as their basic explana­tory tools. If the spa­tiotem­poral dif­fer­en­ti­a­tion of the phys­ical world is incom­plete, then the attempt to under­stand the world from the bottom up — whether on the basis of an intrin­si­cally and com­pletely dif­fer­en­ti­ated space or space­time, out of locally instan­ti­ated phys­ical prop­er­ties, or by aggre­ga­tion, out of a mul­ti­tude of par­ti­cles — is doomed to failure. Reality is struc­tured from the top down.

We have seen (or will be seeing) why the well-​​established laws of physics are just so. They have the par­tic­ular form that they do because they are pre­con­di­tions of the pos­si­bility of objects that “occupy space” while being com­posed of objects that do not “occupy space.” The fact that reality is struc­tured from the top down allows us to address a fur­ther ques­tion: why are stable objects that “occupy space” com­posed of objects that do not “occupy space.”

The Mys­tique

“The quantum is that embar­rassing little piece of thread that always hangs from the sweater of space-​​time. Pull it and the whole thing unravels.” — Fred Alan Wolfe

The Tech­nique

“The fact that we find our­selves in a quantum world where mea­sure­ment is pos­sible… will surely involve the same sort of expla­na­tion as the fact that we find our­selves in a world where we are able to exist as carbon-​​based life forms.” — Jef­frey Bub

The Cri­tique

“I feel that the real joke that the eternal inventor of enigmas has pre­sented us with has absolutely not been under­stood as yet.” — Albert Einstein

1. [↑] Newton, I. (1729). The Math­e­mat­ical Prin­ci­ples of Nat­ural Phi­los­ophy: Trans­lated into Eng­lish by Andrew Motte.

2. [↑] Dieks, D.G.B.J. (1996). The quantum mechan­ical world­pic­ture and its pop­u­lar­iza­tion. Com­mu­ni­ca­tion & Cog­ni­tion 29 (2), pp. 153–168.