Science

I’ve been too busy to contribute much to the laws of physics discussion, and now I’m about to hop on a plane to bluegrass country. But I am sincerely seeking the best way to make this point clear, so one more quick try. And I do appreciate the back-and-forth thus far; sometimes frustrating, but certainly very useful to me.

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.

One hundred years ago, you would not have heard that story, because it hadn’t yet been put together.

But — here’s the important part — one thousand years from now, you will still hear precisely that same story.

There might be new layers underneath, but it won’t be necessary to refer to them to give a sufficient answer to the original question. There will certainly be much greater understanding of the collective behavior of these underlying particles and forces, which is where most of the great work in modern science is being done. And hopefully there will be a deeper story about why we have the laws we do, how gravity and quantum mechanics play together, how best to interpret quantum mechanics, and so on.

What there won’t be is some dramatic paradigm shift that says “Oops, sorry about those electrons and protons and neutrons, we found that they don’t really exist. Now it’s zylbots all the way down.” Nor will we have discovered new fundamental particles and forces that are crucial to telling the story of everyday phenomena. If those existed, we would have found them by now. The view of electrons and protons and neutrons interacting through the Standard Model and gravity will stay with us forever — added to and better understood, but never replaced or drastically modified.

I’m not actually trying to say something controversial. I think it is pretty unambiguously correct, once I actually say it clearly. But it’s something I think is not as widely appreciated as it really should be.

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.

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Seriously, The Laws Underlying The Physics of Everyday Life Really Are Completely UnderstoodRead More »

Einstein tells us that the time you experience between two events depends on the path you take through the universe. In particular, it can depend on the curvature of spacetime along your trajectory. At a quick-and-dirty level: clocks in a strong gravitational field tick more slowly than ones far away from any gravity. (At the event horizon of a black hole, they wouldn’t tick at all.)

Or not so far away: James Chin-Wen Chou and colleagues at NIST have measured the difference in clocks that are separated by 33 centimeters in elevation. That’s one foot for you Americans. (See NPR, Science News, press release. And because this is a blog rather than Old Media, I’ll even link to the research paper in Science.) As predicted, the elevated clock ticks faster by a factor of (1 + 4×10^-17). If you stand on a chair, you’ll move into the future that much faster.

Not a surprise, of course; it’s a straightforward application of general relativity. Still, we need to look pretty hard to find GR showing up on human scales. These guys worked very hard!

Not sure why people don’t make a bigger deal out of this fact. Physicists (and scientists more generally) are infamous for making grandiose claims about how close we are to Figuring It All Out, only to be shocked by some sort of revolutionary discoveries soon thereafter. Personally I have no idea how close we are to a comprehensive theory of absolutely everything. But I do know how close we are to having a comprehensive theory of the basic laws underlying the phenomena we encounter in our everyday lives — without benefit of fancy telescopes or particle accelerators or what have you. Namely, we already have it! That seems to be worth celebrating, or at least remarking upon, but you don’t hear it mentioned very much.

Obviously there are plenty of things we don’t understand. We don’t know how to quantize gravity, or what the dark matter is, or what breaks electroweak symmetry. But we don’t need to know any of those things to account for the world that is immediately apparent to us. 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. But these are manifestations of the underlying laws, not signs that our understanding of the laws are incomplete. Nobody thinks we’re going to have to invent new elementary particles or forces in order to understand high-T_c superconductivity, much less predicting the weather.

All we need to account for everything we see in our everyday lives are a handful of particles — electrons, protons, and neutrons — interacting via a few forces — the nuclear forces, gravity, and electromagnetism — subject to the basic rules of quantum mechanics and general relativity. You can substitute up and down quarks for protons and neutrons if you like, but most of us don’t notice the substructure of nucleons on a daily basis. That’s a remarkably short list of ingredients, to account for all the marvelous diversity of things we see in the world.

A hundred years ago it would have been easy to ask a basic question to which physics couldn’t provide a satisfying answer. “What keeps this table from collapsing?” “Why are there different elements?” “What kind of signal travels from the brain to your muscles?” But now we understand all that stuff. (Again, not the detailed way in which everything plays out, but the underlying principles.) Fifty years ago we more or less had it figured out, depending on how picky you want to be about the nuclear forces. But there’s no question that the human goal of figuring out the basic rules by which the easily observable world works was one that was achieved once and for all in the twentieth century.

You might question the “once and for all” part of that formulation, but it’s solid. Of course revolutions can always happen, but there’s every reason to believe that our current understanding is complete within the everyday realm. Using the framework of quantum field theory — which we have no reason to doubt in this regime — we can classify the kinds of new particles and forces that could conceivably exist, and go look for them. It’s absolutely possible that such particles and forces do exist, but they must be hidden from us somehow: either the particles are too massive to be produced, or decay too quickly to be detected, or interact too weakly to influence ordinary matter; and the forces are either too weak or too short-range to be noticed. In any of those cases, if they can’t be found by our current techniques, they are also unable to influence what we see in our everyday lives. We have very little idea how big the region of our understanding is, compared to all that there is to be understood; but we know that it’s bigger than what we need to understand to make sense of the world we see with our unaided senses.

That’s pretty amazing. Science will certainly push forward along the frontier of phenomena that are too big or small or subtle to be detected without delicate instruments, as well as along the much more jagged and unpredictable frontier of how the basic laws play out in complicated ways. But getting the basic laws right is an extremely impressive accomplishment, especially for good old human beings who have only been doing science systematically for a few centuries. Way to go, human beings!

(See follow-up posts here and here.)

Sorry for the radio silence around here of late. I don’t know about anyone else, but I’ve been traveling like a mad person. The good news is that I just got back from UC Davis, where I had the chance to meet John Conway for the first time in person.

The bad news is: no time for blogging. But I recently received an email pointing out that some links have died in an old post, which I proceeded to update. And that gave me the idea of stooping to a classic blogospheric move in times of sparse content: reposting old stuff! So here is the post in question, from several years ago. If people don’t complain too loudly, maybe we’ll dig up some more ancient blogging and bring it back to the surface.

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Quantum mechanics, as we all know, is weird. It’s weird enough in its own right, but when some determined experimenters do tricks that really bring out the weirdness in all its glory, and the results are conveyed to us by well-intentioned but occasionally murky vulgarizations in the popular press, it can seem even weirder than usual.

Last week was a classic example: the computer that could figure out the answer without actually doing a calculation! (See Uncertain Principles, Crooked Timber, 3 Quarks Daily.) The articles refer to an experiment performed by Onur Hosten and collaborators in Paul Kwiat‘s group at Urbana-Champaign, involving an ingenious series of quantum-mechanical miracles. On the surface, these results seem nearly impossible to make sense of. (Indeed, Brad DeLong has nearly given up hope.) How can you get an answer without doing a calculation? Half of the problem is that imprecise language makes the experiment seem even more fantastical than it really is — the other half is that it really is quite astonishing.

Let me make a stab at explaining, perhaps not the entire exercise in quantum computation, but at least the most surprising part of the whole story — how you can detect something without actually looking at it. The substance of everything that I will say is simply a translation of the nice explanation of quantum interrogation at Kwiat’s page, with the exception that I will forgo the typically violent metaphors of blowing up bombs and killing cats in favor of a discussion of cute little puppies.

So here is our problem: a large box lies before us, and we would like to know whether there is a sleeping puppy inside. Except that, sensitive souls that we are, it’s really important that we don’t wake up the puppy. Furthermore, due to circumstances too complicated to get into right now, we only have one technique at our disposal: the ability to pass an item of food into a small flap in the box. If the food is something uninteresting to puppies, like a salad, we will get no reaction — the puppy will just keep slumbering peacefully, oblivious to the food. But if the food is something delicious (from the canine point of view), like a nice juicy steak, the aromas will awaken the puppy, which will begin to bark like mad.

It would seem that we are stuck. If we stick a salad into the box, we don’t learn anything, as from the outside we can’t tell the difference between a sleeping puppy and no puppy at all. If we stick a steak into the box, we will definitely learn whether there is a puppy in there, but only because it will wake up and start barking if it’s there, and that would break our over-sensitive hearts. Puppies need their sleep, after all.

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Repost: Quantum InterrogationRead More »