Why Sean Carroll is wrong
Sean Carroll, who I do respect, has blogged no less than four times about the idea that the physics underlying the “world of everyday experience” is completely understood, bar none. His most recent post on this subject claims to have put it all into a single equation. In his response to critics he has made a number of interesting claims including arguing – correctly – that there is a misperception about the nature of scientific theories. They aren’t simply right or wrong. They have ranges of applicability. This is one of my greatest pet peeves, in fact. It drives me bonkers when people, for instance, claim that Einstein proved Newton was wrong. No, that in fact is not true. Einstein proved Newton was only correct within a certain range of validity. We rely on Newton having been correct every single day when we open a door or drive a car. That’s not the issue here. So what is the issue?
Let’s look at Sean’s claim one more time: that the physics underlying the world of everyday experience is perfectly well understood. To quote from one of Sean’s earlier posts on the subject,
If you were to ask a contemporary scientist why a table is solid, they would give you an explanation that comes down to the properties of the molecules of which it is made, which in turn reflect a combination of the size of the atoms as determined by quantum mechanics, and the electrostatic interaction between those atoms. If you were to ask why the Sun shines, you would get a story in terms of protons and neutrons fusing and releasing energy. If you were to ask what happens when a person flexes a muscle, you would hear about signals sent through nerves by the transmission of ions across electromagnetic potentials and various chemical interactions.
And so on with innumerable other questions about how everyday phenomena work. In every single case, the basic underlying story (if that happens to be what you’re interested in, and again there are plenty of other interesting things out there) would involve the particles of the Standard Model, interacting through electromagnetism, gravity, and the nuclear forces, according to the principles of quantum mechanics and general relativity.
As simple as that sounds, what is he really trying to say? It certainly appears as if he is implying that one can draw a direct line from the Standard Model (via the equation that he is now rolling out on his tour of England) to doors closing and muscles flexing. Yet, when anyone challenges him on the fact that emergence and complexity, not to mention the quantum-classical contrast, are not sufficiently well-understood he (and his supporters) dismiss the argument as “tiresome.” But he has fallen into his own trap by overextending the validity of a theory.
So let’s review what we know, without question.
- We know the classical physics (and the “classical” part is crucial here) that describes how things like doors open and close, buildings stand up, and so on. On an everyday level, this involves Newtonian mechanics. Thus we know how macroscopic objects work at non-relativistic energies and speeds to great precision.
- We know the classical physics of electromagnetism and we know that it helps govern how macroscopic objects interact (when you press on a door it is really electromagnetic repulsion between molecules that mediates the interaction between your hand and the door).
- We know the classical physics of macroscopic objects at relativistic energies to a great precision.
- We know the quantum physics that tells us how molecules are held together, i.e. we know chemistry.
- We know the quantum physics that tells us about the sub-atomic particles that make up the atoms (and thus molecules) that constitute these things, i.e. we know QED and QCD.
What don’t we know? Well, for starters, we do not know where the quantum world ends and the classical world begins. In other words, we’re not 100% certain how Numbers 4 and 5 above connect to Numbers 1, 2, and 3. For example, Sean’s equation seems to imply that spacetime and gravity, at least as it regards everyday objects, is entirely explained by the two terms Sean has identified in that equation. But Sean’s equation is fundamentally quantum in nature. How can one include a gravitational term in a fundamentally quantum equation and claim it explains all of the physics underlying everyday life when we as yet have no well-developed theory of quantum gravity? Do we know for certain that the gravitational interactions that affect everyday life can be traced to that one term? Sean blithely claims
We don’t understand the full theory of quantum gravity, but we understand it perfectly well at the everyday level.
Really? That’s a rather tall claim. Likewise, by including a group of terms broadly labeled “quantum mechanics” he is implying that we fully understand quantum mechanics, at least as it regards the physics of everyday life. Presumably all of chemistry comes out of this particular set of terms, but there are an awful lot of things about quantum mechanics that we just don’t know. Sean has conveniently brushed over some of the more complex aspects of biology in his description of muscle flexing (and then dismisses this criticism as “tiresome”). But there are legitimate questions that could be asked about just how some of these neuro-chemical processes can legitimately come out of those terms (or others as we are apparently supposed to “not take them too seriously”) in Sean’s equation. Thus even if I reluctantly granted him the gravity claim, he’s dodging certain problems with biochemistry by claiming criticism on this point is “tiresome” (if you do not see the problem in this, perhaps you should review a list of basic logical fallacies, notably this one).
Aside from the rather nebulously labeled term “other forces,” Sean also fails to account for certain interpretational problems inherent in the Standard Model, some of which have a direct bearing on everyday life. In my very first FQXi essay I argued the point that there is an interpretational problem with the exclusion principle (and hence the spin-statistics theorem). As we understand it at present, the Standard Model only fully explains three of the four fundamental forces of nature (already a problem for Sean’s claim as I have stated above). Nevertheless, if we assume an extension of the Standard Model will someday include gravity, then
the four fundamental interactions would each be accompanied by a mediator particle – the photon for the electromagnetic, vector bosons (W+, W−, and Z0) for the weak nuclear, gluons for the color, and gravitons for the gravitational … Higher order micro- scopic interactions, such as the strong nuclear, possess their own mediator particle (e.g. the meson). One can theoretically use these as the building blocks for ordinary macroscopic matter with one glaring exception: the extended structure of the atom. In addition to two of the fundamental interactions, ‘building’ an atom requires invocation of the Pauli Exclusion Principle (PEP). PEP may be understood in the context of the Standard Model via the spin-statistics theorem – fields ultimately possess certain commutation properties that manifest themselves, after the action of a field operator, as bosons or fermions, the latter obeying PEP. In other words, we would find that a certain field has to be commuting (or perhaps anti-commuting) or else we get, in the words of Tony Zee, ‘a nonvanishing piece of junk’ in our mathematics …
So, as Tony Zee also put it,
[i]t is sometimes said that because of electromagnetism you do not sink through the floor and because of gravity you do not float to the ceiling, and you would be sinking or floating in total darkness were it not for the weak interaction, which regulates stellar burning. Without the spin statistics connection, electrons would not obey Pauli exclusion. Matter would just collapse.
Now here’s the rub. We often connect the physics of everyday life with the physics underlying everyday life, but sometimes we have trouble. In the former we like to do things such as draw free-body (force) diagrams to describe how the forces acting on a macroscopic object balance out. While the following example is not part of everyday life, it is illustrative of the problem (since we know that the exclusion principle plays a major role in keeping all of matter from collapsing in on itself). Consider a stable, macroscopic chunk of a white dwarf star. Now draw a free-body diagram of that chunk. In the radial direction there is, of course, a force due to gravity acting in the negative r direction and so I can draw an arrow and label it. Now there ought to be an equal magnitude arrow pointing in the opposite direction since the chunk is stable. But there isn’t because there is no force preventing collapse. It is PEP that prevents collapse and acts against gravity here and PEP, in the Standard Model, is not a force.
That example was simply illustrative. A similar argument can be made about all matter. Indeed, as Tom Moore said to me once, PEP does present a problem for the interaction picture that is painted by the Standard Model. Within the realm of the Standard Model itself, there is no problem. But this is precisely where Sean falls into his own trap by over-extrapolating the realm of applicability of a theory: the interaction picture of the Standard Model matches up well with standard Newtonian physics except for this one case. And we do not yet know why. That alone should be enough to refute Sean’s claim. The claim is dubious at best and at worst is misleading enough to beguile even the best science journalists especially when it comes from someone as well-known as Sean Carroll.
A cynic might say that the purpose of such a claim is merely to sell books. But I think Sean really believes his claim. Either way it’s another case of the particle physics and cosmology community making grandiose claims that are eaten up by the public, giving the impression that this sub-field of physics has a monopoly on truth, particularly when it comes to fundamental questions. And not only is that wrong but it is potentially harmful to science.
Oh, and by the way, just because we have an equation that works doesn’t mean we understand it. If you don’t believe me then google “interpreting the quantum mechanical wave function.”