Let's Eliminate All the Interpretations of Quantum Mechanics!

… with John Gribbin

i) Introduction

ii) What is Interpretation?

iii) Shut up and calculate!

iv) Limits to John Gribbin's Pluralism

v) Only Maths?

vi) Waves and Particles

vii) Conclusion

This piece doesn't claim to offer a conclusive case for the elimination of all the interpretations of quantum mechanics (QM). It simply raises the possibility of elimination and then offers a few arguments in that direction.

The prime motive here is that, at least at present, there's no way of establishing which interpretation is the true/correct/etc. one. Secondly, the multiplicity of interpretations both confuses the issue and leads to scepticism towards many of them. Thirdly, some interpretations are so convoluted and wacky that laypersons and even physicists themselves have only aesthetic reasons to believe in them.

This is particularly true of the Many Worlds Interpretation. I can see no reason whatsoever for the layperson to accept it other than it can be taken to “explain the phenomena” (a phrase which is being used in contrast to Bas van Fraassen's "save the phenomena"). That is, in the way that panpsychism, idealism, Marxism, theism, etc. can all be taken to explain the phenomena. However, explaining phenomena is often very cheap and easy, even if it is neat and tidy. (Note Albert Einstein's rejection of David Bohm's theory.)

The first section introduces John Gribbin's position as well as a potted history of positions (which features Paul Dirac, Richard Feynman, etc.) which can be taken to lead towards a possible eliminitivism This section also deals with both the shut-up-and-calculate mantra and instrumentalism and how they can be taken to lead to eliminitivism. Then the simple question “What is Interpretation?” is asked. After that, Gribbin's pluralism is tackled and seen to be not very convincing (at least not from a philosophical point of view). The section “Only Maths?” is self-explanatory. Finally, the issue of wave-or-particles is discussed in the context of the elimination of interpretations of quantum mechanics.

What is Interpretation?

To state the obvious: the interpretations of quantum mechanics are, well, interpretations. Thus it's hard to grasp what kind of standing (philosophical or scientific) a particular interpretation of quantum mechanics could (or should) actually have. Indeed the very word “interpretation” seem to deflate what it is that's being done – at least from a strictly scientific perspective. 

Does an interpretation become something else when it is proven or simply established to be true or correct? Do any interpreters of quantum mechanics believe that their own interpretations can be proven or shown to be true? What would that even mean?

When it comes to all interpretations of QM, the mathematical theory is taken as a given (or at least parts thereof). Thus the interpretation is something that's over and above the mathematical theory. (This is complicated by many laypersons' views on what exactly scientific theories are.)

In effect, the interpretation attempts to tie the mathematical theory to the world (or to reality). This is even the case when that (mind-independent) reality is rejected or simply seen as being philosophically problematic.

In the end, all interpretations ask and answer similar questions. Nonetheless, not all of these questions are to do with "what is real”. They can also be questions of measurement, determinism/what is random, and so on. Thus such questions aren't always about things – such as particles, waves, parallel worlds and so on.

Shut up and calculate!

John Gribbin (an astrophysicist who's been writing on quantum mechanics for over 40 years) has an interesting and radical position on the interpretations of quantum mechanics (QM). Thus, in this piece, I'll often bounce off Gribbin's words. However, although Gribbin doesn’t cite a case for the elimination of the interpretations of QM, he does strongly (for want of a better word) deflate them.

Following on from that, there's no reason why an eliminativist position on QM interpretations can't be advanced. By this I don't simply mean the instrumentalist, technological or “philistine” position which is best summed up by N. David Mermin's well-known quote, “Shut up and calculate.” Indeed it may even be possible to recruit both Paul Dirac and Richard Feynman to the eliminitivist cause.

Dirac, for example, once compared particles to the pieces of chess. He wrote:

“I can describe the situation by comparing it to the game of chess. In chess, we have various chessmen, kings, knights, pawns and so on. If you ask what chessman is, the answer would be that it is a piece of wood, or a piece of ivory, or perhaps just a sign written on paper, or anything whatever. It does not matter. Each chessman has a characteristic way of moving and this is all that matters about it. The whole game of chess follows from this way of moving the various chessmen.”

To put it bluntly, Dirac's position seems to be eliminativist when it comes to particles. Yet if it is eliminativist, then why did he speak of “particles” at all? That's unless, of course, the word “particle” is simply shorthand for specific types of behaviour or movement. Yet this simply raises the question:

What is it that behaves or moves?

Richard Feynman also had a problem with seeing particles as particles. He hinted at the possibility that we have a wave-particle duality simply because there are no particles or waves in the first place. He wrote:

“Things on a very small scale behave like nothing that you have any direct experience about. They do not behave like waves, they do not behave like particles, they do not behave like clouds, or billiard balls, or weights on springs, or like anything that you have ever seen.”

Like Dirac, Feynman was emphasising behaviour, not particles or things.

From a purely scientific point of view, it's easy to agree with Mermin's well-known phrase which was quoted earlier. (Although he wasn't putting his own position.) So it's not a surprise that the science writer Philip Ball says that “[s]ome scientists feel the same way today”. Ball himself writes:

“Quantum theory works. It allows us to calculate the shapes of molecules, the behaviour of semiconductor devices, the trajectories of light, with stunning accuracy.”

Thus the “theory works”; though “without our knowing what it's about”. And that, surely, wouldn't be such a bad thing if physicists also believed that there's no real (or available) answer to the what-is-it-about question. Perhaps some (or even many) physicists do believe that.

So what do the words, “What the maths mean”, mean? What does the maths describe? What is there beyond the maths (if anything)? Does the mathematics alone give us a full understanding?

Interestingly enough, Mermin also once quipped that

"new interpretations [of QM] appear every year. None ever disappear."

Now that's another reason to be sceptical of – or deflationary towards – the many interpretations of quantum mechanics.

One can also take a positive pragmatic (or instrumentalist) position on these many interpretations of quantum mechanics. Alternatively, one can take a pessimistic position on them. The theoretical physicist Steven Weinberg takes the latter option. He writes:

“My own conclusion is that today there is no interpretation of quantum mechanics that does not have serious flaws. This view is not universally shared. Indeed, many physicists are satisfied with their own interpretation of quantum mechanics. But different physicists are satisfied with different interpretations. In my view, we ought to take seriously the possibility of finding some more satisfactory other theory, to which quantum mechanics is only a good approximation.”

Now is Weinberg's position philosophical/ontological in nature? Is he saying that it's not all about prediction, experiment, observation, etc. – it's also about what is? In other words, is it a realist position on the interpretations of quantum mechanics?

The physicist David Finkelstein also notes the problems with these different interpretations; though he doesn't really hint at any metaphysical concern. He tells us that “[q]uantum theory was split up into dialects” and that this was the case because “[d]ifferent people describe the same experiences in remarkably different languages”. Consequently, this pluralism may seem fine except for the fact that all “[t]his is confusing even to physicists”.

As hinted at above, the philosophical position called instrumentalism comes the closest to the position that rejecting all interpretations (or the need for interpretations) of quantum mechanics is a good idea. Yet it's still a philosophical position, if not an actual interpretation of quantum mechanics.

Having said that, interpretation is not even an issue for the many physicists and technologists in white coats. In quantum field theories, for example, the structures described are abstract (or purely mathematical). That is, they exist without any clear interpretations of the quantities which are part of the theory.

Yet, on the other hand, just as we can deflate QM interpretations, so we can also deflate the Technophile/Philistine Interpretation (as it were) of QM. For instance, Gribbin says that

"[j]ust how an electron, or a beam of electrons, get from A to B does not matter to an engineer designing, say, a TV set".

Put that way, it's hard to imagine any scientist not being interested in how electrons "get from A to B". Of course we may not be able to speak of the in between of A and B - and that's a recurrent theme in QM. (This is a position best highlighted by the relationalist - or interactionist - interpretation of QM; such as that offered by Lee Smolin and Carlo Rovelli.) Gribbin himself highlights the problem in this way:

"If you have a set of parameters describing the system in state A, you can calculate the probability that it will be in state B after a certain time. But there is nothing which tells you what is going on in between."

Yet to many quantum theorists there is (effectively/simply) no "in between"!

Finally and more relevantly, there may be a philosophical case that can be given for eliminativism when it comes to the interpretations of QM. And this is something the shut-up-and-calculate brigade never offered.

Limits to John Gribbin's Pluralism

Gribbin's actual position is one of the “complementarity” of all the different interpretations of QM. That is, no interpretation (or no set of classical physical concepts) can refer (at the same time) to a singular quantum system. This of course can be traced back to Niels Bohr's theoretical stance on both the wave and particle descriptions (see later section).

Prima facie, Gribbin's position seems to be one of relativism. Or, at the very least, one of conceptual pluralism. For example, Gribbin believes that

“none of them [quantum mechanical interpretations] is anything other than a conceptual model designed to help our understanding of quantum phenomena”.

His primary reason for stating the above is that all the interpretations he considers “make the same predictions”. Sure, this is hardly an original point. People have been saying this about all the interpretations of quantum mechanics since at least the 1950s. Yet surely even if we see the essential limitations of these interpretations, and the fact that they don't offer us a complete picture, that doesn't also mean that all of them must be treated equally. That simply doesn't follow.

Take a murder that hasn't be solved and which has engendered numerous interpretations (or explanations). Does that mean that all interpretations should all be treated equally? Having said that, if they're all in the same ballpark (as, arguably, all/most QM interpretations are), then perhaps they are all equal.

Gribbin himself also believes that all interpretations of quantum mechanics are effectively in the same ballpark. Thus can we also argue that all the interpretations are in the same ballpark as purely metaphysical/ontological theories? Or, more simply, that they're all actually metaphysical/ontological theories? The fact that QM interpretations are about physical (or scientific) realities doesn't really seem to make any difference here. The fact is that they go beyond the maths, predictions, experiments, observations, etc. And that means that they're (almost by definition) philosophical in nature. (That's not necessarily a criticism.) However, can we say (as already stated) that the interpretations of QM are also in the same boat as, say, panpsychism, monism, idealism, theism, Marxism and whatnot? If they are, then the interpretations may never be able to smoothly bridge the gap between the mathematical theory and the interpretations.

As stated, Gribbin is a pluralist (as are many scientists and philosophers when it comes to scientific theories, concepts and models). Gribbin, in the following, again talks about the interpretations of quantum mechanics:

“I stress, again, that all such interpretations are myths, crutches to help us imagine what is going on at the quantum level and to make testable predictions. They are not, any of them, uniquely 'the truth'; rather, they are all 'real', even where they disagree with one another.”

This is heavy stuff. Of course what's true of quantum interpretations (or “myths”) isn't also true when it comes to mathematical theories. (The above is certainly problematic for string theory, for example, for the simple reason that Gribbin talks about such theories/interpretations making - unique - “testable predictions”.)

Despite all the above, Gribbin happily acknowledges that the quantum interpreters believe that their interpretations are true. He writes:

"[T]he interpreters and their followers will each tell you that their own favoured interpretation is the one true faith, and all those who follow other faiths are heretics."

And that passage comes straight after Gribbin tells us that

"[a]t the level of equations, none of these interpretations is better than any other".

Thus, logically, "none of the interpretations is worse than any of the others, mathematically speaking". Though all this hinges on precisely how we're to take the phrases "at the level of equations" and "mathematically speaking". That, of course, is the main issue of this piece.

Gribbin also gets all psychological or aesthetic when he concludes (as the very end of a book) that we

“are free to choose whichever one gives you most comfort, and ignore the rest”.

Again, there may well be an argument that all interpretations are superfluous when it comes to predictions, technology, etc. However, that certainly doesn't mean that all interpretations are “equally good”. They may be equally good in the sense that they don't make the slightest bit of difference when it comes to to mathematical theory, predictions and (quantum) technology; though are they equally good in literally every respect?

Gribbin also talks in terms of what he calls a “physical model” of “mathematical concepts”. He writes (in his Schrodinger's Kittens and the Search for Reality) that “a strong operational axiom” tells us that

“literally every version of mathematical concepts has a physical model somewhere, and the clever physicist should be advised to deliberately and routinely seek out, as part of his activity, physical models of already discovered mathematical structures”.

Yet even in Gribbin's case, it's still clear that a “mathematical concept” comes first and only then is a “physical model” found to square with it.

Only Maths?

Do mathematical physicists require (or need) Gribbin's “conceptual models” or any interpretations of QM? It depends. It depends on what they’re doing and what they want.

One way of interpreting Gribbin's remarks is that only those who know the mathematics can bypass QM interpretations altogether. Thus are all these interpretations the result of commentators (or laypersons) having a mathematical deficit? That can't be true for the simple reason that many well-known QM interpreters have also been adept mathematicians and mathematical physicists. (Not all have taken the eliminitivist position of Paul Dirac – see later section.) Thus if the maths literally gives us everything, then why did all these people feel the need to interpret QM in the first place? For a start, they aren't simply telling stories to assuage the layperson. Yes, they also believe that their own interpretations are true/accurate/etc.

In addition, it may not be the case that knowing all (or much of) the maths of QM (as, say, Richard Feynman did) automatically brings you closer to the truth when it comes to interpretations. For example, say that I acquire literally all the mathematical knowledge which is used and known by all the believers in the Many Worlds Interpretation of QM. Would I then automatically see that this interpretation is the only true one? There will still be a leap from the mathematics to the interpretation. That is, the links between the mathematics and the interpretation will neither be (entirely) mathematical nor (entirely) logical. Indeed it can be argued that they can't be (entirely) scientific either.

On the other hand, Stephen Hawking’s question as to “what breathes fire into the equations” will always hover over the heads of the physicists who don't endorse an interpretation. The science writer Kitty Ferguson (in her The Fire in the Equations) offers a possible Platonist answer to Hawking's question by saying that “it might be that the equations are the fire”. Alternatively, could Hawking himself have been “suggesting that the laws have a life or creative force of their own?”. Again, is it that the “equations are the fire”?

If the maths is “only” a tool for predictions, experiments and suchlike, then the maths alone can't possibly answer all the questions raised by QM interpreters.

The physicist John Archibald Wheeler also provided a powerful riposte to such Platonism (as it were) in physics. We're told that Wheeler used to write arcane equations on the blackboard and stand back and say to his students:

“Now I'll clap my hands and a universe will spring into existence.”

So is the maths simply a tool which doesn't tell us what is? Sure, the “what is the case?” question may end up being bogus (i.e., from an anti-realist or instrumentalist point of view). Nonetheless, surely there must always be something beyond the maths and therefore beyond the predictions, the technological applications, etc. of QM.

A sharp and to-the-point anti-Platonist (as it were) position is also put by the science writer, Philip Ball. He writes:

“... equations purportedly about physical reality are, without interpretation, just marks on paper”.

In other words, what exactly (as Hawking put it above) “breathes fire into the equations [to] make a world”?

The Philip Ball quote above also highlights two other problems:

i) The fact that we can make mistakes about physical reality.

ii) That even if the equations are about physical reality, then they're not one and the same as the physical reality.

Ontic structural realists (in the philosophy of physics), on the other hand, would say that this distinction (i.e., between maths and the world) hardly makes sense when it comes to physics generally - and it doesn't make any sense at all when it comes to quantum physics. Nonetheless, surely there's still a distinction to be made here.

Waves and Particles

Niels Bohr made it clear that only mathematics gives us a true picture of the quantum realm. He wrote:

"We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.”

This, again, is a roundabout of saying that only the mathematics is adequate when it comes to describing sub-atomic phenomena. Thus, by this definition, the words “particle” and “wave” simply can't do the full job. Yes, they can do part of the job; though not the full job. Indeed it's not just the word “particle” that's problematic, thinking of particles as things is too. Bohr wote:

"Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems."

In other words, it can't possibly be about the world “as it is in itself”. Every statement we make about the world comes with a lot of contingent baggage. There is of course a causal set of relations to any x – though those very same relations can be described in an indefinite number of ways.

At the most basic level, “seeing” a subatomic x as a particle or as a wave is an act of interpretation. So is seeing them as both (i.e., within different experimental contexts), as Louis de Broglie and many other did. Finally, seeing x as neither a wave nor a particle may also involve interpretation.

In more general terms, Bohr argued that the reality behind our measurements and experiments is that there is neither a particle nor a wave. But we must still talk about something (his "poetry"). However, that something isn't necessarily a thing as such (this is grammar speaking here). And we give that x (which doesn't need to be a thing, only a something) the name “wave” or “particle”.

So when de Broglie argued (in 1924) that that every moving particle (yes, particle) can be equally described as either a wave or a particle, that may well be because this x is neither a wave nor a particle. Nonetheless, describing x as a wave or a particle still helps both physicists and laypersons.

In more detail and to go back to the 1920s. Erwin Schrodinger was committed to waves. Werner Heisenberg and Paul Dirac to particles. (Heisenberg dealt with pure numbers in the form of arrays – i.e., matrices). In the end and in a strong sense, all this didn't matter because Dirac went on to show/prove that the wave and particle interpretations are equivalent. That is, equivalent when it came to the maths, experiments, predictions and, later, technology.

In actual fact, we can go back to J.J. Thomson, when he showed that electrons are particles. Then, 30 or so years later, his son, George Thomson, showed that electrons are waves.

The American philosopher Ernest Nagel (in his 'The Cognitive Status of Theories') had a different (though related) take on particles and waves. Firstly he discussed their “puzzling characteristics”. These puzzling characteristics seem to be “incompatible”. (Though the word “incompatible” isn't a synonym of “contradictory”.) More precisely, electrons are “construed to have features which make it appropriate to think of them as a system of waves”. Yet, “on the other hand”, electrons “also have traits which lead us to think of them as particles”. They're deemed to be particles because each one has “spatial location and a velocity”. However, “no determinate position and velocity can in principle be assigned simultaneously to any of them”. It's here that Nagel appears to deflate quantum mechanics. He does so by saying that

“many physicists have therefore concluded that quantum theory cannot be viewed as a statement about an 'objectively existing' domain of things and processes... On the contrary, the theory must be regarded simply as a conceptual schema or a policy for guiding and coordinating experiments”.

It's also the case that the words “particle” and “wave” carry far too much baggage. After all, the original wave-particle experiments had water waves in mind. Is that a good thing when it comes to talk about happenings at the subatomic level? Yes, it is if it helps us get of grip of things. The same goes for the word “particle”. So there are a host of good reasons as to why x should be seen as a particle. Yet there are also a host of equally good reasons as to why we shouldn't see it as a particle.

Gribbin himself takes an equally radical position on particles and waves. He writes:

“In the world of the very small, where particle and wave aspects of reality are equally significant, things do not behave in any way that we can understand from our experience of the everyday world...all pictures are false, and there is no physical analogy we can make to understand what goes on inside atoms. Atoms behave like atoms, nothing else.”

It can of course be said that even if it's correct that

“all pictures are false, and there is no physical analogy we can make to understand what goes on inside atoms”

then it may still be the case that (at least for laypersons) that's all we've got. Indeed without the mathematics, all we have are pictures, images and analogies. So we'd better make do with all that. And surely Gribbin isn't arguing that pictures, images and analogies serve no purpose. Indeed he can't be arguing that because his books make extensive use of them. Having said that, all Gribbin's pictures, images and analogies do come along with words of warning (as can be seen in the quotes above).

In fact we can even say that the very use of the words “particle” and “wave” may mean that Gribbin himself is also using pictures or images and/or being analogical. That is, if “in the world of the very small” it's the case that

“things do not behave in any way that we can understand from our experience of the everyday world”

then why is Gribbin using the words “particle” and “wave” in the first place?

Conclusion

One thing is certainly the case: without the maths, we'd have almost (or literally) nothing to "say" about the quantum world – real or otherwise. When it comes to QM, the usual means of ownership aren't available to us. That is, we can't observe, feel, smell or (often) even imagine the quantum world. Thus the maths is all we've got. This is excellently expressed in the following passage from John Horgan:

“[M]athematics helps physicists definite what is otherwise undefinable. A quark is a purely mathematical construct. It has no meaning apart from its mathematical definition. The properties of quarks – charm, colour, strangeness – are mathematical properties that have no analogue in the macroscopic world we inhabit.”

If maths is all we've got, then it's not really a surprise that many physicists (i.e., the more philosophical ones) have said that quantum mechanics doesn't really “say anything about the real world”. Or, at the very least, everything that's said about the quantum world is said by the maths. Thus, all the imagery, picture painting, analogies, etc. aren't really about the quantum world – they're simply the crutches we use in order to get a grip of that world.

So when it's said that Richard Feynman, for example, could only do “quantum theory” (i.e., the maths), then that's not a surprise. That's because the maths may be all we've got. And when we stray beyond the maths into “interpretation”, then we (perhaps by definition) can't help but get things wrong... Or at least this is one sceptical scenario we must consider.

Again, it's not a surprise that Feynman didn't “know what the maths means”. That may be because the phrase “what the maths means” is (almost) meaningless. At the very least, there's a hint that we can't go beyond the maths. Yet it's still the case that so many philosophers, and somewhat less physicists, believe that the maths is only second-best to something far deeper – the interpretation (or ontology) of quantum mechanics.

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Afterthought

In many respects it can be argued that certain interpretations of quantum mechanics aren't really interpretations at all. Or, at the very least, they're less interpretational than their rivals. The Copenhagen Interpretation and QBism (quantum Bayesianism), for example, are good examples. The very fact that there are strong anti-realist or verificationist factors to these positions surely limits the need for interpretation.

Take the electron's journey from A to B mentioned in the piece above. To quote John Gribbin again:

"Just how an electron, or a beam of electrons, get from A to B does not matter to an engineer designing, say, a TV set… If you have a set of parameters describing the system in state A, you can calculate the probability that it will be in state B after a certain time. But there is nothing which tells you what is going on in between."

If Niels Bohr, John Gribbin, QBists, etc. all deny that in between (or simply play it down), then surely there's little room left for interpretation.

Stephen Hawking's Model-dependent Realism

“Traditionally these are questions for philosophy, but philosophy is dead. Philosophy has not kept up with modern developments in science, particularly physics. Scientists have become the bearers of the torch of discovery in our quest for knowledge.” - Stephen Hawking, The Grand Design (2010)

i) Introduction

ii) Why Model-dependent Realism?

iii) The Brain's Models

iv) True Reality and Models

v) Pluralism

vi) Hawking's Constructive Empiricism?

vii) The Aesthetics of Models

viii) Conclusion

Introduction

The words "model-dependent realism" (MDR) were first used in Stephen Hawking and Leonard Mlodinow's book The Grand Design, which was published in 2010. Before that, Hawking had of course already talked about the importance of models in physics. (As a physicist, it would be hard not to stress their importance.)

For example, here's Hawking from 1994 – some 16 years before the The Grand Design:

“If what we regards as real depends on our theory, how can we make reality the basis of our philosophy? But we cannot distinguish what is real about the universe without a theory. I therefore take the view, which has been described as simple-minded or naïve, that a theory of physics is just a mathematical model that we use to describe the results of observations… Beyond that it makes no sense to ask if it corresponds to reality, because we do not know what reality is independent of theory.”

Still, at that time Hawking hadn't turned that emphasis on models into a philosophical position with its very own title: namely, model-dependent realism.

In the following, Hawking expresses what model-dependent realism is in a strong and clear manner:

"There is no picture- or theory-independent concept of reality. Instead we will adopt a view that we will call model-dependent realism: the idea that a physical theory or world picture is a model (generally of a mathematical nature) and a set of rules that connect the elements of the model to observations. This provides a framework with which to interpret modern science."

Thus without theories and models we would be bombarded by a possible “infinite variety of facts” (Karl Popper). Reality, therefore, is “seen” through the lenses supplied by our (sometimes-conflicting) theories or models.

One may wonder why Hawking stresses scientific models rather than scientific theories. (In fact, he sometimes appears to use the two words as virtual synonyms – perhaps for convenience’s sake.) So let the scientist and professor John Holland describe what he takes models to be:

“Although model building is not usually considered critical in the construction of scientific theory, I would claim that it is. Every time a scientist constructs a set of equations to describe the world, such as Newton’s or Maxwell’s equations, he or she is constructing a model.”

Now many would regard that as a fairly loose description of a model. Or, more accurately, a description that’s certainly at odds with our everyday use of the word “model”. Yet in Hawking’s case too, theories can take the form of a “mathematical models”. More specifically, mathematical models are used “to describe the results of observations”.

Why Model-dependent Realism?

One question which needs to be asked here is the following:

Why did Stephen Hawking use the word “realism” to characterise his position?

At an intuitive level, MDR appears to be an anti-realist position, rather than a realist one. After all, if one is stressing models, theories and “mental concepts” (see later section), then isn't that also to stress some kind of anti-realist position?

Perhaps the wording doesn't matter.

Hawking wouldn't have read that much contemporary philosophy of science; so this kind of conceptual clarification might have annoyed him. (Many physicists get annoyed by philosophical “conceptual analysis” because they believe it implies “conceptual conservatism”.) In any case, perhaps because MDR claims that all we have is models, theories and mental concepts, then why can't we be realist about these things instead? That is, not realist about noumena, “objective reality” or “the Real” - but realist about our models, theories and mental concepts instead.

As already hinted at, Hawking didn't have much time for philosophy and on more than one occasion he said that it was “dead”. Yet it's very odd that he didn't realise he was doing philosophy when he wrote these parts of The Grand Design. What's more, Hawking made the following incredible claim:

“Model-dependent realism short-circuits all this argument and discussion between the realist and anti-realist schools of thought.”

So perhaps it wasn't philosophy simpliciter that Hawking was against; but only philosophy “which has not kept up with modern developments in science”. (Many philosophers themselves have said the same about their fellow philosophers.) Thus one must now assume that model-dependent realists have kept up with modern science. However, even philosophy which has kept up with science still remains philosophy. And that's also true of Hawking's own model-dependent realism.

The Brain's Models

Hawking wrote about how the human brain models the world and how scientific models do the very same thing. Indeed he seemed to have taken the brain's models as the basis of (or inspiration for) scientific models . Hawking writes:

"According to the idea of model-dependent realism ... our brains interpret the input from our sensory organs by making a model of the outside world. We form mental concepts of our home, trees, other people, the electricity that flows from wall sockets, atoms, molecules, and other universes. These mental concepts are the only reality we can know. There is no model-independent test of reality. It follows that a well-constructed model creates a reality of its own."

The problem here is that the brain's models are hardwired and (as it were) given – unlike scientific models. Or, should I say, there is a hardwired element on top of which we apply contingent concepts. Thus those hardwired “interpretations” take a similar role to Kant's categories. That is, they are transcendental (in Kant's sense).

This means that we can agree with Hawking on the necessity and importance of scientific models; though also reject his analogy with hardwired brain “pictures”.

The basic gist of the brain's models is that they aren't the same as what it is they model. Indeed it may even follow that they don’t even resemble what it is they model. And if this is the case as far as the brain’s models are concerned, then the situation is even trickier when it comes to scientific models.

As some philosophers have put it, we don’t have (true) representations. (Though philosophers like Jerry Fodor use the word “representation” in a very technical way.) What’s more, it's counterproductive to believe that we do. (This was a line put forward by Richard Rorty and other pragmatists.)

In any case, Hawking goes into a little detail to demonstrate his case:

“In vision, one's brain receives a series of signals down the optic nerve. Those signals do not constitute the sort of image you would accept on your television...”

And then there’s Hawking’s philosophical conclusion. Namely:

“The brain, in other words, builds a mental picture or model.”

One problem that can be broached here is that just because the journey from the world's causal forces to the final “mental picture or model” is both circuitous and complex, that doesn’t in itself mean that the picture (or model) can't be a faithful (or even truthful) representation of those external causal forces. Then again, we can’t deny the contrary either.

Hawking then states something which can be taken as showing that the brain’s models are faithful or even truthful. He cites the following case:

“The brain is so good at model building that if people are fitted with glasses that turn the images in their eyes upside down, their brains, after a time, change the model so that they again see the world the right way up…”

True Reality and Models

Model-dependent realism asserts that all we can know about reality is that it

"consists of networks of world pictures that explain observations by connecting them by rules to concepts defined in models”.

Does that therefore mean (to jump forward a little) that an ultimate “theory of everything” will never be found? Hawking/Mlodinow suggest that this is unclear:

“In the history of science we have discovered a sequence of better and better theories or models, from Plato to the classical theory of Newton to modern quantum theories.”

Hawking then concludes in the following manner:

“It is natural to ask: Will this sequence eventually reach an end point, an ultimate theory of the universe, that will include all forces and predict every observation we can make, or will we continue forever finding better theories, but never one that cannot be improved upon? We do not yet have a definitive answer to this question.”

Hawking puts the well-known pessimistic meta-induction position in the above passage.

The pessimistic meta-induction is applied to previous theories and models and it's argued that this provides us with a very good reason to see a model as being precisely that – a model. Thus if it's (only) a model, then it can't be a piece of reality. (Though models must also belong to reality – what else can they belong to?)

Ontologically, “true reality” is true reality, not a model of true reality.

Can a model simply replicate a piece of reality?

Yes; though not in literally every respect. And even if it could model everything about any given x, then what would be the point of such modelling? That's not what scientific models are meant to do. (“A true model of the universe would be the universe itself.”) However, in everyday parlance things are complicated by the fact that that's exactly what models are – replicas of given bits of reality (i.e., as in model aircrafts, model cars, model soldiers, etc.).

Thus if we must model “true reality”, then it can't actually be that true reality. And if it's not that pure reality, then we have something less than that pure reality. What's more, that's a good thing because (as stated) there's little to be gained from any kind of exact replication – at least not in theoretical physics.

Yet it is also said that those who accept the model-dependent realist position also accept that there is indeed a "reality-as-it-is-in-itself".

Yes, “reality-as-it-is-in-itself” clearly exists. But that doesn't mean that we can access (or describe) it as it is in itself. After all, contingent brains (with their contingent “pictures”) and persons are doing the assessing and describing. And precisely because of that, a perfect model or theory will always be out of the question.

Now what about Hawking's “mental concepts”?

Hawking seemed to reduce scientific models to what he called “mental concepts”. And, as he put it, these “mental concepts are the only reality we can know”. However, does Hawking's own conclusion actually follow? He boldly states:

“It follows that a well-constructed model creates a reality of its own.”

[Even an] anti-realist wouldn’t be too keen on this type of language. As it stands, it's partly true and partly false. It's partly true in that we gain access to “reality” through models, theories and “mental concepts”. Thus, in that basic sense, if we use different models, theories and mental concepts about the same given x, then we’ll get different “realities”. 

The problem is that causal forces are completely written out of this picture. We can accept that, as the philosopher Donald Davidson argued, causal forces don’t "come under a [given] description". Nonetheless, they are still the same causal forces which are described by different models. So such causal forces must constitute some kind of reality which can’t be dispensed with.

In addition, models are part of reality in the sense that they can hardly be anything else. And even if models are seen as abstract objects, models are still part of reality. But does a model also “create a reality of its own”? Again, both yes and no.

Pluralism

Hawking's model-dependent realism is similar to the “poetic naturalism” of theoretical physicist Sean Carroll.

Firstly, here's Hawking writing about how “different theories” can exist side by side:

"It might be that to describe the universe, we have to employ different theories in different situations. Each theory may have its own version of reality, but according to model-dependent realism, that is acceptable so long as the theories agree in their predictions whenever they overlap, that is, whenever they can both be applied."

Sean Carroll, on the other hand, states:

“[T]here is only one, unified, physical world, but many different ways of talking about it, each of which captures an element of reality.”

The obvious question to ask both Carroll and model-dependent realists is:

How do we know if it's the same “element” described by different models or a different element entirely?

In other words, is Carroll correct to argue that “different ontologies” can liv[e] happily alongside “the same underlying reality”?

In addition, it isn't true that “different vocabularies” imply (or entail) “different ontologies”. At a very crude level, if someone uses the name the 'Morning Star' and another person uses the name the 'Evening Star', and both persons know that both names refer to the planet Venus, then we don't have different ontologies on our hands here. Instead, different words, senses, technical terms, etc. can be seen to have the same ontology and therefore they posit the “same underlying reality”.

Both Carroll and Hawking hold that reality should be interpreted and that interpretation takes the form of scientific modelling. Thus if the pluralism of “poetic” elements has it that various models don't coincide in describing the same particular x, then does that mean that many realities exist? (Hawking came close to saying this.) Again, if the models are very different, then how do we know that models/theories have the same x (or phenomenon) in their focus?

It's also argued that

“[w]here several models are found for the same phenomena, no single model is preferable to the others within that domain of overlap”.

Yet just because different models can model any given x, that doesn't also mean that “no single model is preferable”. It's true that no single model need offer us what Hawking calls the “absolute truth”. However, that doesn't also mean that a theory/model can't be superior to its rivals. Of course spelling out a model/theory's superiority may be a tricky business (see the last section); though scientists are already well aware of that.

To clarify this way of thinking about pluralism we can see what Hawking has to say about what physicists call “dualities”. These are “situations in which two very different theories describe the same phenomenon”. So Model¹ and Model² may deal with the same causal forces in different ways. That is, the descriptions are different even when the casual forces are the same. (Though how could we know that?)

Hawking complicates this by hinting that M¹ and M² deal with different “properties”, rather than saying that M¹ and M² say different things about the same properties. To be faithful to Hawking, he writes:

“Each theory can describe and explain certain properties, and neither theory can be said to be better or more real than the other.”

As stated, it’s hard to work out if Hawking meant different models of the same properties; or different models about different properties. It can be argued that properties can only come care of models or theories. Thus different models - by definition - will generate different properties. And if that’s the case, then M¹ and M² can’t be dealing with the same properties in their different ways.

Constructive Empiricism?

Immediately after making a comment about model-dependent realism short-circuiting philosophy, Hawking indulges in some philosophy of his own. Not only that, the philosophy he does indulge in is very similar to Bas van Fraassen's constructive empiricism. Take Hawking's following words:

"According to model-dependent realism, it is pointless to ask whether a model is real, only whether it agrees with observation. If there are two models that both agree with observation ... then one cannot say that one is more real than another. One can use whichever model is more convenient in the situation under consideration."

The words above advance the idea of empirical adequacy, which is an element of van Fraassen's constructive empiricism.

This isn't to say that there isn't something real which underpins observations and models. It's just that “the Real” (to use Slavoj Žižek's favourite term) is basically a Kantian noumenon.

Yet the claim that different models/theories “predict the same phenomena” (i.e., they're all “empirically adequate”) is a little loose as it stands.

Take the philosophical position of panpsychism.

Nothing in panpsychist theory is contradicted by any phenomena or observations. The same applies for many other metaphysical theories. This point is shown when A.J. Ayer commented on monism and pluralism. He said that if monism were true, then the world would be exactly the same as that posited by a pluralist.

Hawking then cites the quark as an example of a model. He wrote:

“[A]ccording to model-dependent realism, quarks exist in a model that agrees with our observations of how subnuclear particles behave.”

This means that all we have is the quark model itself and our observations. We don't need anything “real” beneath, between or behind the model and our observations.

In addition, the empirical adequacy position and MDR are basically instrumentalist in nature. That is, instrumentalists believe that models, concepts and theories should be evaluated solely in terms of their agreement with observations; as well as in terms of their explanatory and predictive power.

Hawking himself appeared to move one step beyond merely emphasising the importance - and indeed necessity - of models: he also stressed what he called “mental concepts”.

Hawking certainly didn't believe that mental concepts and nature/reality are as one. Yet he once wrote that “mental concepts are the only reality we can know”. Furthermore, he stated: “There is no model-independent test of reality.”

This seems to mean that Hawking went further than simply saying that mathematics describes (or perfectly models) nature. After all, he stressed the importance of mental concepts. However, it can still be said that the models of physics are mathematically accurate. Thus even if we require mental concepts to get at these mathematical models, the models can still perfectly capture “reality”.

The Aesthetics of Models

What constitutes a good model?

It's here when certain aesthetic criteria are brought into play. Hawking (as well as other physicists) happily admits this. So here's Hawking's own list of what constitutes a good model:

i) It must be elegant.

ii) It must contains few arbitrary or adjustable elements.

iii) It must agrees with - and explain - all existing observations.

iv) It must make detailed predictions about future observations that can disprove or falsify the model if they are not borne out.

As for the first criterion: the recognition of elegance is partly subjective affair. Nonetheless, saying that the notion of elegance is subjective isn't necessarily a criticism. It depends. Elegance can be cashed out in very scientific (or third-person) ways.

However, there'll always be a aesthetic remainder. That is in the sense that the scientific (or third-person) criteria of an aesthetic criterion will always leave out any mention of the aesthetic term itself.

Even the criterion that models should contain “few arbitrary or adjustable elements” is not itself scientific. The word “arbitrary” will need a definition and explanation because – as it stands - it's quite loaded. Nonetheless, that's not to say that it can't be defined or that arbitrary elements can't be specified.

The final two criteria aren't aesthetic in nature; though they can still be problematic.

Thus Hawking was correct to state that no model meets all these aesthetic criteria. (This quandary has been well-discussed in the philosophy of science.) For example, a theory which scores well when it comes to “elegance” may score poorly when it comes to “[a]gree[ing] with and explain[ing] all existing observations”.

Conclusion

As already stated, it's very odd that Stephen Hawking should have said that “philosophy is dead” on more than one occasion. What did he think his model-dependent realism is? Above and beyond that, it's difficult to decipher why Hawking chose the term “realism” in the first place.

As for the philosophical position of model-dependent realism itself, it has a strong pragmatic - rather than a strictly empiricist - appeal. And, precisely because of that, metaphysical realists and scientific realists will have serious problems with Hawking's philosophical position.