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The Sciences

Seriously, The Laws Underlying The Physics of Everyday Life Really Are Completely Understood

Cosmic VarianceBy Sean CarrollSeptember 29, 2010 5:51 PM

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While the primary purpose of last week's post on the laws of physics underlying everyday life was to convey information like a good blog post should, there was another agenda as well: to test the waters. This is an issue I've been thinking about a lot lately, but I wanted to get a better idea for how it's perceived in the outside world. I honestly wasn't sure whether there would be more of "you arrogant physicist, we don't have any idea what the laws are" or "you moron, why are you wasting our time with this self-evident crap?" So much for that ambiguity. Responses, for example at Fark and Reddit but even here in our very own comment section, displayed a greater than average internetitude, defined as a tendency to not read the article, set up straw men, and miss the point. But at least the direction of disagreement was fairly uniform. The issue under discussion is important, so it's worth taking the time to counter the three most common arguments, from completely silly to almost-sensible.

1. Are you serious? There's so much we don't understand: turbulence, consciousness, the gravitational N-body problem, photosynthesis...

To which my years of academic training have prepared me to reply: duh. To conclude from my post that I was convinced we had a full understanding of any of those things represents, at a minimum, a rather uncharitable reading, given that no one in their right mind thinks we have such an understanding. Nevertheless, I knew people would raise this point as if it were an objection, which is why I was extra careful to say "We certainly don’t have anything close to a complete understanding of how the basic laws actually play out in the real world — we don’t understand high-temperature superconductivity, or for that matter human consciousness, or a cure for cancer, or predicting the weather, or how best to regulate our financial system." And then, at a risk of being repetitive and boring, I added "Again, not the detailed way in which everything plays out, but the underlying principles." And for emphasis there was something about "the much more jagged and unpredictable frontier of how the basic laws play out in complicated ways." Nevertheless. The distinction I'm drawing is between the laws underlying various phenomena, and how the phenomena actually behave, especially on macroscopic scales. Newtonian gravity provides an excellent example of the difference: we certainly know the laws underlying the behavior of gravitating particles in the Newtonian regime, but that obviously does not mean we have a complete solution to the N-body problem, or even a qualitative understanding of how large collections of particles behave. It's the difference between knowing the rules by which chess is played, and being a grandmaster. Those are not the same thing. In particular, taunting "you're no grandmaster!" is not actually a refutation of the claim that I know the rules of chess. My claim was that we know the basic equations governing the behavior of matter and energy in the everyday regime -- not that we have a complete understanding of every observable phenomenon. It is of course completely legitimate not to care that we know the basic underlying laws. You may not think that's interesting, or very important. That's fine, I certainly wasn't making any claims at all about priority or importance or interestingness. But it should still be possible to understand the claim I was making, and judge it on its own merits, such as they are. Let me just emphasize how non-trivial the claim is. First, that there is such a thing as an "underlying" set of laws. That is, that we can think of everyday objects as being composed of individual pieces, such that those pieces obey laws that are the same independently of the larger context. (Electrons obey the same equations of motion whether they are in a rock or in a human heart.) That's the reductionist step. Again, for people who enjoy taking offense: this is not to say that the reductionist description is the only interesting one, or to imply that the right way to attack macroscopic problems is to reduce them to microscopic ones; only that the microscopic laws exist, and work, and are complete within their realms of validity. And second, that we know what those laws are. There's nothing in the everyday world that is inconsistent with Standard Model particles obeying the rules of quantum field theory, plus general relativity to describe gravity. Amazing.

2. We don't even understand gravity! And the Second Law of Thermodynamics! And quantum mechanics! (Magnets! How do they work???)

Unlike the previous objection, this one is not correct-but-misplaced, it's just wrong. But it's wrong in an interesting way. We actually do understand gravity: it is described by Einstein's general relativity. Not deep down at the quantum level, of course, but that's very far from the world of the "everyday." You might try to make some profound epistemological claim that we don't really understand gravity, we just have a set of rules that it unambiguously obeys. Fine; I would argue that this isn't an especially helpful distinction in this case, but in any event it's beside the point. What I meant was that we have a clear set of rules that are unambiguously obeyed. That's also true for the Second Law -- it was explained by Boltzmann. Sure, we have to invoke a low-entropy boundary condition at the Big Bang, but guess what? The Big Bang is not within the realm of our everyday experience. Even the collapse of the wave function, which comes closest to a true mystery, doesn't qualify. For one thing, wave function collapse isn't something you see happening in your kitchen on an everyday basis. But more importantly, we do have a theory that describes what happens, handed down to us by Bohr and Heisenberg. You might think that this theory is unsatisfying and incomplete, and I would be extremely sympathetic. But it fits all the data we have. I'm not trying to make a deep philosophical point about the meaning of "understanding"; just noting that things obey laws, and in the everyday regime we know what those laws are.

3. You're too presumptuous. New physics might be required to understand consciousness, or wave function collapse, or...

This comes closest to an actual argument, and I wish that the entire conversation could have focused on relatively sensible points of this form. But ultimately, I don't buy it, not even close. Take consciousness as an example. Obviously there are a lot of things about the workings of the human mind that we don't understand. So how can we be so sure that new physics isn't involved? Of course we can't be sure, but that's not the point. We can't be sure that the motion of the planets isn't governed by hard-working angels keeping them on their orbits, in the metaphysical-certitude sense of being "sure." That's not a criterion that is useful in science. Rather, in the face of admittedly incomplete understanding, we evaluate the relative merits of competing hypotheses. In this case, one hypothesis says that the operation of the brain is affected in a rather ill-defined way by influences that are not described by the known laws of physics, and that these effects will ultimately help us make sense of human consciousness; the other says that brains are complicated, so it's no surprise that we don't understand everything, but that an ultimate explanation will fit comfortably within the framework of known fundamental physics. This is not really a close call; by conventional scientific measures, the idea that known physics will be able to account for the brain is enormously far in the lead. To persuade anyone otherwise, you would have to point to something the brain does that is in apparent conflict with the Standard Model or general relativity. (Bending spoons across large distances would qualify.) Until then, the fact that something is complicated isn't evidence that the particular collection of atoms we call the brain obeys different rules than other collections of atoms. What would be a refutation of my claim that we understand the laws underlying everyday phenomena? Easy: point to just one example of an everyday phenomenon that provides evidence of "new physics" beyond the laws we know. Something directly visible that requires a violation of general relativity or the Standard Model. That's all it would take, but there aren't any such phenomena. A century ago, that would have been incredibly easy to do; the world of Newtonian mechanics plus Maxwell's equations wasn't able to account for why the Sun shines, or why tables are solid. Now we do understand how to account for those things in terms of known laws of physics. I am not, as a hopelessly optimistic scientist from the year 1900 might have been tempted to do, predicting that soon we will understand everything. That's an invitation to ridicule. Indeed, we know lots of cases where the known laws of physics are manifestly insufficient: dark matter, dark energy, electroweak symmetry breaking, the Big Bang, quantum gravity, the matter/antimatter asymmetry, and so on. We might answer all these questions soon, or it might take a really long time. But these are all rather dramatically outside our everyday experience. When it comes to everyday phenomena that are incompletely understood, from consciousness to photosynthesis, there is every reason to believe that an ultimate explanation will be obtained within the framework of the underlying laws we know, not from stepping outside that framework. An impressive accomplishment.

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