This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from October 2018, with only one of the images that originally appeared with it, because that’s essential to the explanation.

Big news in the study of tiny things dropped last week while I was traveling to Washington DC for a meeting: the ACME Collaboration has a new paper in Nature reporting the latest results of their search for a permanent electric dipole moment of the electron. This is huge because it suggest that the new particles predicted by theories of physics beyond the Standard Model should have masses greater than could be directly detected at the Large Hadron Collider.

A lot of people wrote stories about this last week– the collection of links I retweeted from airports includes an NSF press release, a Science News article, and a a post from fellow Forbes blogger Brian Koberlein. Those will give you the basic outline of what’s being reported; in this post, I want to dig a little deeper into the physics at work.

Most of the coverage of this adopts the framing of talking about this as measuring the “shape” of the electron, something that’s been standard for news releases since at least 2011 when I wrote a ScienceBlogs post about an experiment from Ed Hinds’s lab. This provides a nifty visual analogy– most of the press stories quote Dave DeMille of Yale comparing this to shifting a two-nanometer-thick slice from the south pole to the north pole of a perfect sphere the size of the Earth. (Note to the planetary science crowd: Yes, we know that the Earth is not a perfect sphere, but bulges out at the equator by far more than that.) What’s less clear from a lot of these stories, though, is what this has to do with particle physics. For that matter, what does it even mean to talk about the electron having a shape?

As physicists constantly have to emphasize when talking about electron spin, the electron is not literally a tiny ball of charge. In fact, to the best of our knowledge, the “bare” electron is a featureless point. If you could turn off all of its interactions with the rest of the universe, an electron would be infinitesimally tiny and basically uninteresting.

Happily, electrons do interact with the rest of the universe, which is what allows us to measure their properties. Because the universe is quantum, though, those interactions mean that we never get to see a “bare” electron: instead, we see some combination of the “bare” electron and its interaction with the rest of the universe. These interactions change the energy of the electron, and we can use the light absorbed and emitted by an electron to determine its energy to very high precision. We can then look at how that energy changes when we apply other kinds of fields.

The most significant type of interaction is simply between the charge of the electron and an applied electric field (say, from another charged particle nearby). This creates a very large energy shift that makes the electron “want” to get closer to positive charges and farther away from negative charges. This “electric monopole” interaction absolutely dwarfs any other interaction you might be interested in.

The next most significant interaction is a “magnetic dipole” interaction between an applied magnetic field and the intrinsic spin of the electron. This is a tiny shift, but you can see its effects because it’s not symmetric: a magnetic field in one direction will increase the electron’s energy by a tiny amount, and a field in the opposite direction will decrease it by the same amount. If you do something to trap an electron in place, most of its energy comes from the electric monopole interaction with whatever’s trapping it, but if you switch a magnetic field back and forth between two directions, you’ll see a tiny difference between the two states that you can measure using spectroscopy. This shift is something like a millionth of the energy of a typical electronic state in an atom or molecule, but measuring energy differences at that level is pretty routine for atomic physicists.

What ACME is looking for (along with a bunch of other experiments) is an “electric dipole” interaction, which has a mix of the characteristics of the other two. Like the electric monopole interaction, it’s an energy shift caused by an applied electric field, and like the magnetic dipole interaction, it depends on the direction of the applied field, shifting up for one direction and down for the other. Again, this is absolutely minuscule compared to the energy from the monopole interaction, but if you use the monopole interaction to stick an electron to an atom or molecule so it can’t simply move in the direction of the electric field, you can hope to pick this up as a shift in the energy of the electron that changes direction when you change the direction of the field.

The strength of this electric dipole interaction is measured by a thing called an “electric dipole moment,” and in classical electromagnetism, you calculate these all the time for macroscopic distributions of charge. A perfect sphere of charge would have zero dipole moment– no matter what direction you apply the field in, you get the same total energy. Any “lopsided” distribution will have a non-zero dipole moment, which is what leads to the “shape of the electron” characterization of this experiment: a sphere of charge with a tiny “bump” on one pole and a corresponding dent on the other will give you an electric dipole moment, and then you can use the charge and the radius to calculate how big a “bump” you would need to generate a particular value of the electric dipole moment.

But what does any of this have to do with particle physics? Well, again, we never truly see a “bare” electron, only the combination of the electron and its interactions with the rest of the universe. Those interactions include not only the fields applied in the course of an experiment, but also the inescapable vacuum electromagnetic field. Quantum physics tells us that you can never have nothing at all– there’s always zero-point energy around, and the electron interacts with these zero-point fields. In the Feynman picture of these things, that interaction takes the form of a cloud of “virtual particles” surrounding the electrons, and mediating its interactions with those applied fields.

These “virtual particles” are what make precision spectroscopy one of the well-established methods for searching for exotic physics. The interaction between the electron and the virtual particles leads to a shift in the electron’s energy, and in keeping with the “everything not forbidden is mandatory” nature of quantum physics, those virtual particles include absolutely everything, up to (in principle) bunnies made of cheese.

That might seem like a recipe for madness, but happily, the size of the shift caused by the appearance of a particular type of virtual particle decreases as the mass of that particle increases, and the number of virtual particles involved. When theorists try to predict the energy of an electron in some experiment, they don’t have to calculate the effect of every conceivable particle in huge numbers, just those particles whose mass is small enough for small numbers of them to cause a shift in energy large enough to be detected by the experiment. Or, to turn things around, when experimentalists measure some shift in the energy of an electron in some experiment, they can work backwards and determine the mass of the virtual particles that caused it.

In the case of the new ACME results, what they have is actually a lack of a shift: they apply a variety of electric and magnetic fields to a sample of cold thorium monoxide molecules, and look for an energy shift that changes with the direction of the electric field in the way they would expect if the electron has an electric dipole moment. They don’t see a shift that’s bigger than the uncertainty in their experimental measurements, which lets them put a hard upper limit on the size of any possible electric dipole moment of the electron: it has to be smaller than 0.000000000000000000000000000011 e-cm (one e-cm being the dipole moment you would get from an electron and a positron separated by one centimeter).

That hard upper limit for the electron’s electric dipole moment leads to a hard lower limit for the mass of any hypothetical particle with the right characteristics to cause an electric dipole moment. If you assume that the dipole moment would be created by “one-loop” Feynman diagrams, the simplest type shown in the figure above, the minimum mass for these particles would be about 30 TeV, or a bit more than double the maximum energy available at the Large Hadron Collider. If you allow for the possibility that the “one-loop” contribution is zero (something that’s not too difficult to arrange theoretically, but kind of inelegant), and it’s “two-loop” contributions that matter, the lower mass limit drops down to around 3 TeV, which is still pretty huge.

This is, obviously, a pretty stringent constraint on possible theories. Some years back, I wrote an article for Physics World that included a graph of predictions for various exotic theories, and this result takes a big bite out of that chart– they’re at the 10^-29e-cm level now. There’s still some wiggle room for theorists, but this is yet another strong piece of evidence that whatever beyond-the-Standard-Model physics is out there is something very different than the simplest models that particle theorists find aesthetically appealing.

This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from October2016, without the images that appeared with it, which were from the publicity materials sent out by the Nobel Foundation.

Most years, when I write about the Nobel Prize in Physics, I can feel free to pitch it a little on the high side, because there are large numbers of media reports giving a more basic version of the story. This year, as I grumbled on Twitter a while ago, that hasn’t been a great assumption. Between the U.S. Presidential election sucking up media attention and the kind of esoteric nature of the prize, the coverage has been… sparse. Even outlets that are generally really good, like the New York Times, offer only brief and sketchy descriptions. (Philip Ball’s piece for Prospect in the UK is a rare exception.) And some of the less-good outlets offer, well, lazy and dismissive pieces that devote more words to jokes about baked goods than explaining the physics. As a result, my piece from last Tuesday is standing more alone than I expected it to, and as a result is less illuminating than it ought to be.

This is genuinely important physics, though, and deserves better than the duck-and-run treatment it’s mostly gotten, so I’ll circle back and try to explain the general nature of the Nobel-winning work in a broader context, without so much specific and detailed physics content. The work that Haldane, Kosterlitz and Thouless won for has genuinely transformed the way people think about condensed matter physics, and as such is richly deserving of the wider recognition that’s supposed to come with the Nobel. Explaining how and why goes all the way back to the question of mindsets and approaches that I’ve blogged about before.

The fundamental reason why Haldane, Kosterlitz and Thouless needed to do what they did is that they’re working in a subfield where the simple and straightforwardly reductionist approach that characterizes physics seems to run into trouble. As I’ve said before, physics largely works by abstracting away messy details to get down to simple universal behavior. In the introductory mechanics course I’m teaching this term, for example, we start by treating complicated macroscopic objects — baseballs, people, cars — as featureless points, and ignore things like air resistance and friction when considering simple motion. Later on, we’ll add those complicating factors back in, but taking them away lets us explore the basic underlying laws.

This works great for a wide range of physics subfields — from huge astrophysical objects down to subatomic particles — but there are areas that are unavoidably complex. One of the biggest of these is the subfield of “condensed matter,” which tries to study the properties of vast assemblages of atoms making up solid or liquid systems. In condensed-matter systems you’re worried about the collective behavior of many more particles than you have any hope of counting. As Phillip Anderson pointed out in a famous paper from 1972, these collective behaviors aren’t necessarily obvious, even when the underlying rules governing the interactions between particles are simple and well-understood. More is different, in Anderson’s phrase, but more importantly, more is difficult.

There are still reductionist things you can do to try to understand these systems, and strip out some of the complexity– you imagine perfect crystals free of dirt and defects, extending infinitely far in all directions, and weak interactions between individual particles. And you can talk about condensed-matter systems in terms of collective properties that are a little more abstract than the individual motions of the particles making the system up. These methods get you a handful of problems you can solve with pencil and paper, and give a conceptual framework for thinking about this stuff, that you can use to draw some very general conclusions about what sort of collective states are possible.

As with most of physics, though, you exhaust the pencil-and-paper problems pretty quickly. Once you start adding complicating factors back in — defects in the lattice, strong inter-particle interactions, boundaries to the system — things get very messy, very quickly. And unlike basic classical mechanics where you can think about the motion of just a few objects, it’s extraordinarily difficult to simulate these systems in a direct way, because the collective behavior you’re after demands many more particles than you can readily keep track of.

This leads to a situation where the very simplified models physicists can usefully work with in a fairly direct way — in the case of Kosterlitz and Thouless a two-dimensional fluid, or in the case of Haldane, a one-dimensional string of quantum magnets — don’t obviously capture the complete reality. In particular, the Berezinskii-Kosterlitz-Thouless system I wrote about the other day is one where the simplest theoretical models say that superfluid behavior — where particles flow without resistance — shouldn’t be possible in a two-dimensional system, while some experiments seemed to show a low-temperature transition to exactly that sort of behavior. So there must be more going on than can be captured with the tools that were readily available in the late 1960’s and early 1970’s.

What Kosterlitz and Thouless (and Berezinskii independently) did was to bring in another high-level way of looking at collective behavior that simplifies the problems, namely looking at the topology of the system. You can sort of see how this works in the two-dimensional superfluid problem: if you imagine looking down on a sheet of fluid, a superfluid flow just looks like all the particles moving in the same direction at the same speed, say, from left to right across the screen of whatever you’re using to read this. Breaking that up requires particles to move in other directions — toward the top or bottom of the screen, and even back from right to left — otherwise, they’d still be part of the superfluid flow. This diversion necessarily introduces some circular motion, little eddies in the flow where it looks like the fluid is spinning clockwise or counter-clockwise. And the amount and rate of flow possible in these “vortices” is governed by straightforward quantum-mechanical rules, meaning that each has an energy and an entropy associated with it.

The Nobel-worthy realization here is that these “vortices” in the flow change the topology of the system — each individual vortex looks a bit like a whirlpool with a hole punched through the center. And topology is all about classifying sheets by the numbers of holes punched through them. When you bring the math of topology to bear on the problem of vortices in a two-dimensional fluid, you find that there are conditions where these vortices tend to pair up — loosely, one spinning clockwise and the other counter-clockwise — in a way that cancels out their disruption of the flow so you still basically have a superfluid. Put a bit more energy into the system, though — by raising the temperature — and these vortices split up, destroying the superfluid flow. There’s a sharp transition between the basically-superfluid phase and the not-superfluid phase, and that behavior emerges very naturally from the topological picture (you’re basically going from a sheet with no holes in it to a sheet with holes in, which is necessarily a sudden jump), while it’s extremely difficult to understand from trying to look at the microscopic motion of individual particles.

That’s the transformative aspect of this work: describing systems containing enormous numbers of atoms in terms of their topology offers a whole new way of looking at problems that can’t readily be solved using a lower-level description. In a sense, it’s re-enabling reductionism: you can’t easily understand what’s going on in condensed matter by breaking things down to the level of individual particles, but if you think about a higher-level description, you can classify systems very simply in terms of topology, and this lets you solve problems that you couldn’t hope to work out with other methods.

So, this is one of those Nobel Prizes where the achievement is not so much the specific technical problem that was solved — like the prize for blue LED’s in 2014 — but the introduction of a new tool that broadens the scope of problems that physicists can attack. To attempt an analogy for non-scientists, bringing topology into condensed matter is a bit like introducing the “Intentional Fallacy” to the study of literature — it allows you to ask and answer questions that you couldn’t even consider asking under previous methods. It’s not just an excuse to bring baked goods to a press conference, it’s a game-changing innovation in physics, and for that richly deserves the acclaim that comes with a Nobel Prize.

This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is the text of the original post, from August 2017, with only one of the images that appeared with it, which is necessary for the explanation.

A bit more than eight months ago, I blogged about an experiment from the ALPHA collaboration at CERN, about an experiment where they did some rudimentary spectroscopy of antihydrogen. Not even a year later, they’re back, with another paper on antimatter spectroscopy, which is cool enough to need another blog post. This time around, they’re measuring the “hyperfine splitting” in antihydrogen (here’s a news story from Physics World), and explaining why this is a particularly valuable thing to study requires a bit of background about how atoms are put together.

These days, everybody learns in grade school that an atom looks a bit like a little solar system, with a positively charged nucleus orbited by negatively charged electrons. Hydrogen is the simplest atom of all, with a nucleus containing a single proton, orbited by a single electron. This picture isn’t quite right, of course, but it’s a good lie-to-children — that is, a simplified conceptual picture you can use at the start to get a handle on things that are a more complicated than the simple version.

One of the biggest ways this “solar system” picture is wrong is that the orbits of the electron are nothing like those of a planet in the actual Solar System. A planet is very much a classical sort of object, so you could speed it up or slow it down by a tiny amount, and it would happily continue orbiting, just a tiny bit closer or farther away from the Sun. An electron in an atom, on the other hand, is very much a quantum-mechanical object, and can only exist in orbits that have very particular energies. You can’t make an arbitrary change in the energy of an electron, you can only change it by certain very specific amounts, and when you do, the energy added or subtracted has to come from or be carried away by a photon of light.

The simplest such model was worked out by Niels Bohr in 1913, and does a nice job of getting the electron energies in hydrogen from just thinking about the electrostatic attraction between the electron and the proton. With the development of quantum mechanics in the 1920s, we got a better handle on what, exactly, the energies of the electron in hydrogen ought to be, and more importantly what effects were being left out that change those energies.

There are a collection of effects that shift the energy of the electron in an atom that sort of get lumped together as “fine structure,” because they take states that have exactly the same energy in the very simplest analysis and split them apart by a small amount. This means that instead of a single frequency of light that can be absorbed to change the electron energy, you have two very similar (different by less than a percent) frequencies. The most interesting of these effects has to do with the “spin” of the electron, which is an inherent property making the electron behave like a tiny magnet. For certain types of orbits, the electron energy shifts up or down a tiny amount due to an interaction between the spin and the orbit. You can think of it fairly loosely in terms of a current loop: from the electron’s perspective, it’s a magnet at rest being orbited by a proton. That orbiting proton creates a magnetic field, and the electron’s energy goes up or down a tiny bit depending on whether the magnet associated with the electron spin is aligned with the field the proton makes or not.

This isn’t the full story, though, because the proton also has a spin, and behaves like a tiny magnet. Which leads to the “hyperfine” effect (so called because the energy splitting it generates is much smaller than the fine structure, and physicists aren’t good with names). If the electron and proton spins are lined up with each other (both up or both down), the energy goes up a bit as the intrinsic magnetic field of the proton raises the electron’s energy; if they’re pointing in opposite directions, the energy goes down a bit. Among other things, this takes the “ground state” of hydrogen and splits it into two states. This ground-state splitting is incredibly important for astronomy, because the universe is full of clouds of cold hydrogen atoms in the ground state that can move back and forth between these two levels by emitting radio waves with a wavelength of around 21 centimeters. Many radio telescopes are specifically designed to look for this “21-centimeter line” in hydrogen, and we’ve learned an enormous amount about the universe by studying this light.

The paper from the ALPHA team that I discussed back in December was measuring the first kind of these energy effects: they looked at the light needed to drive anti-hydrogen atoms from the ground state to the lowest excited state, and that energy is mostly determined by the simple electrostatic interaction between the electron and proton (or positron and the anti-proton). The energy involved is several million times greater than the hyperfine splitting, and the wavelength of the light in question is several million times shorter than the 21-centimeter line, at 121 nanometers.

The current experiment is measuring that several-million-times-smaller hyperfine splitting, using basically the same principle as the earlier measurement: they collect a bunch of antihydrogen in a magnetic trap, hit them with light of the appropriate frequency, and see how many atoms are left. If the frequency of the light they’re hitting the anti-atoms with matches the transition frequency, the resulting state change causes atoms to fall out of their trap and annihilate with ordinary matter in the walls, which they can detect.

The process is complicated by the fact that their trapped atoms are held in a whopping huge magnetic field, which shifts the energy of the electron orbits. They’re rescued by a quirk of atomic structure, though, which is that in the high-field limit, the states split into two groups of two, giving two transition frequencies that differ by exactly the hyperfine splitting. This lets them do something that looks a lot more like ordinary spectroscopy. In the December paper, they fixed the laser at the frequency for ordinary hydrogen and confirmed that it caused losses of antihydrogen. In this paper, they vary the microwave frequency to find the maximum loss for one of the transitions, then increase the frequency by approximately the hyperfine splitting for hydrogen, and repeat the process to find the maximum for the other. The difference between the two frequencies of maximum loss gives the hyperfine splitting in antihydrogen.

Since hydrogen is the simplest atom, and the only one whose properties can be calculated exactly, this hyperfine splitting has been extensively studied, and measured to impressive precision. The exact frequency of light absorbed or emitted when a hydrogen atom switches ground states is 1,420,405,751.773 Hz, plus or minus about 0.001 Hz. The ALPHA team can’t quite match that phenomenal precision, but it’s a very respectable first effort: 1,420,400,000 Hz plus or minus 500,000 Hz.

So, why is this an interesting measurement? Well, for one thing, the precision to which the hyperfine splitting is known makes it an attractive target. It’s also a whole lot easier to work with microwaves than the vacuum ultraviolet lasers needed for the earlier work, which is part of why the December paper involved a fixed laser frequency. (This is not to say that it’s easy in any objective sense, though — one of the issues they face is that one of the two frequencies they use is much harder to get into their very complicated magnetic trap apparatus than the other, so they have a huge disparity in the intensity of the radiation hitting the trap.)

There are also some physics reasons to think the hyperfine splitting might be a good place to look for differences between hydrogen and anti-hydrogen. The hyperfine interaction is between the spin of the proton and the spin of the electron, which means it’s both very weak (since the magnetic field generated by either is tiny) and very short-range (because it’s a dipole interaction, rather than the direct charge-charge interaction). Much of the shift happens thanks to interactions when the electron is inside the nucleus, and that’s exactly the sort of scenario where you’d expect exotic physics to show up.

They have a long way to go before they get to the sort of precision where anybody might expect differences between matter and antimatter to show up. (In the very simplest models of high-energy physics, there’s absolutely no difference, but those models can’t fully explain why the Big Bang created enough extra matter to make, well, us. So there’s got to be some difference somewhere.) This experiment, maybe even more than the December one, is a promising step in the development of high-precision spectroscopy of anti-matter.

This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is a test of copying the entire original post, from October 2018, including the images that appeared with it.

The 2018 Nobel Prize in Physics was announced this morning “for groundbreaking inventions in the field of laser physics.” This is really two half-prizes, though: one to Arthur Ashkin for the development of “optical tweezers” that use laser light to move small objects around, and the other to Gérard Mourou and Donna Strickland for the development of techniques to make ultra-short, ultra-intense laser pulses. These are both eminently Nobel-worthy, but really aren’t all that closely related, so I’m going to split talking about them into two separate posts; this first one will deal with the Mourou and Strickland half, because I suspect it’s the less immediately comprehensible of the two, and thus probably needs more unpacking.

What Mourou and Strickland did was to develop a method for boosting the intensity and reducing the duration of pulses from a pulsed laser. This plays a key role in all manner of techniques that need really high intensity light, from eye surgery to laser-based acceleration of charged particles (sometimes touted as a tool for next-generation particle accelerators), or really fast pulses of light such as recent experiments looking at how long it takes to knock an electron out of a molecule. This kind of enabling of other science is exactly the kind of thing that the Nobel Prize ought to recognize and support, so I think this is a great choice for a prize.

The technique Mourou and Strickland invented, as part of Strickland’s thesis research, is called “chirped pulse amplification,” and relies heavily on one of the central facts of wave physics, namely that making a pulse with a short duration in time requires a wide spread in frequency (and vice versa). They exploit this frequency spread in a clever way to circumvent the limits imposed by the fact that too-high intensity can damage the crystals used to amplify laser pulses.

But why does a short pulse need a wide range of frequencies? You can see this by looking at what happens when you start adding pulses of light with slightly different frequencies. The figure below shows a single frequency wave at the bottom, with combinations of 2, 3, and 5 slightly different frequencies above .

When we add a second frequency, the single smooth wave breaks up into a series of “beats,” regions where there’s some wave behavior separated by regions where the two different frequencies cancel each other out. This is a familiar phenomenon to anybody who’s ever tried to tune two similar musical instruments: when they’re trying to play the same note but not quite in tune, you hear an irritating pulsing tone, that gets slower as you bring the two into tune.

As you add more frequencies, the general beat structure remains, but the width of the region with obvious wave behavior gets smaller. This is a very general phenomenon relating to waves, and applies to anything with wave nature: sound waves, light waves, even the matter waves associated with quantum-mechanical particles. If you add together lots of waves with slightly different frequencies, you end up with a series of narrow pulses where you see intense wave activity, separated by wide regions where not much is happening.

You can use these physics to make a pulsed laser pretty easily, simply by finding a gain medium that will amplify light over a broad range of frequencies. One common such medium is titanium atoms embedded in a sapphire crystal, which will let you amplify light over a range extending from the visible spectrum across a huge swathe of the near infrared. Bang one of these Ti:sapph crystals in between two mirrors, pumps some energy into the system, and you can get a “mode-locked” laser in which the presence of a bunch of different frequencies of light leads to short pulses of light with each pulse containing a wide range of frequencies.

You can look at these in two complementary ways: one measurement you can make is to look at the overall intensity as a function of time, in which case you see a very short pulse. The other is to look at the intensity as a function of frequency, in which case you see a wide spread of different frequencies, each with a little bit of intensity.

If you want to make a really intense ultra-short laser pulse, you quickly run into the problem that there’s only so much any one amplifier crystal can take. When the intensity of light gets to great, the material can be damaged, and that limits what you can do with one of these lasers.

This problem seems insurmountable if you think of the pulse only in the intensity-versus-time sense, when the amplifier is getting All The Frequencies at once. If you look in intensity-versus-frequency, though, you can see that none of the individual frequencies contributing to the pulse have enough intensity on their own to pose a problem. So, the trick Mourou and Strickland figured out is to separate those out, so the amplifier has to deal with only a few frequencies at a time.

There are several waves of doing this– the cartoon provided to the media by the Royal Swedish Academy of Sciences (all the way up at the top of this post) shows it being done with a pair of diffraction gratings to spread out the different frequencies so that some follow a longer path than others. You can also send the pulse through a length of optical fiber, in which some frequencies of light travel faster than others (this is why you can use glass prisms to study the spectrum of light or make iconic album covers). Either way, you end up with a longer laser pulse in which the high-frequency light arrives first while the lower-frequency light straggles in some time later. This is referred to as a “chirped pulse,” because the chirp of a bird has the same sort of frequency structure: high frequency at the start, low at the end (or vice versa).

The chirped pulse gets you a longer duration, but more importantly, it spreads out the intensity so that it’s always below the damage threshold for the amplifier. Then you can safely boost the intensity of each of the individual frequencies in the pulse, which leaves you with a more intense but longer pulse. Then you just reverse the chirping process, using a pair of diffraction gratings to make the high-frequency light on the leading edge travel a slightly longer path than the low-frequency light on the trailing edge, in such a way that all the frequencies arrive at the same time, but now with many times the intensity.

Careful use of this technique can get you pulses of femtosecond duration with crazy intensities– 10^25 watts per square centimeter or thereabouts. These enormously intense fields can do all sorts of violent and interesting things to matter, opening a huge range of possibilities.

Chirped-pulse amplification is one of those extremely clever tricks that’s easier to say in words than to do in the lab, so it’s impressive that Mourou and Strickland were able to make it work. And having demonstrated it, lots of other people started imitating and refining the technique, which has found applications all over physics, and even in medicine. For that, they richly deserve the Nobel Prize.

—

There is, of course, another important feature of this year’s prize, namely that Donna Strickland is the first woman in 55 years to share a Nobel Prize in Physics, and only the second woman not named “Marie Skłodowska Curie.” So, in addition to being excellent and prize-worthy science, this award is also a long-overdue step toward correcting that shameful history. I focused on the physics above, because there’s no shortage of coverage hitting the “first woman in far too long” angle, but it’s an important development, so I can’t not mention it.

This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from April 2020, without the images that appeared with it, some of which were lost during a Forbes platform change a couple of years ago; I may try to recover them and replace them in this post later.

There’s a story over at Popular Mechanics with the eye-catching headline “Why Do We Keep Trying To Prove Einstein Wrong?” about a new experiment using ultra-stable atomic clocks at the top and bottom of the Tokyo Skytree to test the prediction of General Relativity that clocks run slower near a large mass than farther away. My quick and flippant response to this was “Karl Popper.” My second, only slightly less flippant response was “Because nobody has heard of Jean Richer.”

To unpack these a bit, Karl Popper was the philosopher of science who I last mentioned back in January in the context of his disparaging take on “normal science.” He’s best known for his definition of the process of science as a process of falsification of hypotheses: Popper argued that while science can never definitively prove something to be true, a particular theoretical model can be proven false if contradicted by experiments, and that the progress of science is best understood as a sequence of falsifying models.

This has fallen out of favor with philosophers for a bunch of reasons, but remains popular with scientists, particular physicists, I think because it provides a clear methodological target. If you’re working on a new theory, “What predictions can I make that could be tested against experiment?” is a good guide, and similarly a scientist planning a new experiment can fairly reliably find a path forward by asking “What do I need to do to falsify this model?” More modern philosophical models of science, which tend to give a larger emphasis to social phenomena, don’t provide the same kind of conceptual guidance to the uncertain working scientist.

One result of Popper’s enduring popularity is that descriptions of experiments tend to be framed in very Popperian terms, particularly when seeking media attention. Whatever you’re doing sounds more exciting when cast as an attempt to falsify some theoretical prediction, and as that Popular Mechanics piece notes, the more famous the better. Saying you’re testing a prediction of Einstein’s is about as good as it gets.

And, really, this experiment, from the group of Hidetoshi Katori (who I’ve met several times) at RIKEN is a great example of that. What they really have here is a proof-of-principle demonstration that their ultra-precise strontium optical lattice clocks are useful outside of the highly controlled environment of a physics laboratory. To do that, they need to take them somewhere else and demonstrate that they can measure some tiny but well-understood frequency shift. And what they’ve accomplished with these clocks really is remarkable: if they run two clocks for about a day, they can detect a frequency difference between the two of a few parts in 10¹⁸. That is, if you could use these to time the same one-second interval, you would expect them to differ by about one attosecond, or the time it takes for light to travel the length of a small molecule.

This is an extremely impressive achievement, and being able to move this out of the lab to some other location is amazing, but it’s only really impressive to great big nerds like me, who are fascinated by precision measurements generally. If you want to get attention from reporters and not just atomic physicists, you need a way to make this seem less technical and more exciting. So, you set your clocks up at a local landmark, and you find a way to frame it as the latest entry in the fine tradition of experiments testing the effect of gravity on time. As they note in the paper, this goes back to the early 1960’s when Pound and Rebka shot gamma rays up and down a stairwell and showed that there was a frequency shift matching the prediction of General Relativity.

Of course, this also fits in another venerable tradition, which leads to my slightly-less-flippant answer to the “Why do we keep testing Einstein?” headline, “Because nobody has heard of Jean Richer.” To be fair, I wouldn’t’ve known to connect this with Jean Richer, either, except I happen to be writing a book about the history of timekeeping, and literally just this week wrote the section about Richer.

Richer (who was French, so it’s most likely pronounced “Ree-SHAY”) was an astronomer in the second half of the 1600’s, who in 1672 was dispatched to Cayenne in French Guiana to make some observations of the planet Mars during the period of its closest approach to the Earth. They hoped to detect a small difference between the observations of Richer in South America and simultaneous measurements by Jean-Dominique Cassini back in Paris. This parallax measurement would establish the distance between Earth and Mars, and help nail down the true size of the Solar System.

Richer was equipped with the finest equipment that the 1670’s had to offer, including some state-of-the-art pendulum clocks to help with the timing of his measurements. He was also asked to make measurements of the length of a “seconds pendulum,” a pendulum that takes exactly two seconds to swing back and forth (and thus, in a clock, would “tick” once per second). This length is very close to one meter, and was in fact being considered as a possible basis for defining the meter, since the period of an ideal pendulum depends only on its length and the strength of gravity. A meter defined in terms of a seconds pendulum would tie the standard of length to a fundamental constant, providing a simple and universal definition that could be used anywhere.

In the course of his measurements, though, Richer discovered that his clocks, which worked perfectly in France, ran a bit slow in Cayenne. And when he measured the length of a seconds pendulum, he found it was almost 3mm shorter than in Paris. When this was reported, some people thought it was simply an error on Richer’s part, most notably Christiaan Huygens, who was nursing a grudge because he felt Richer had mishandled his pendulum clocks during a trial at sea. Subsequent measurements in equatorial latitudes found the same thing, though: a seconds pendulum near the equator is shorter than one at European latitudes.

One prominent scientist who believed Richer from the beginning was Isaac Newton (who was not yet ISAAC NEWTON in 1673, but getting there), who recognized that Richer’s measurement was telling him something about gravity: the strength of gravity near the equator was very slightly less than that in Europe. This, in turn, told Newton something about the shape of the Earth: that it bulges outward slightly near the equator, making the surface slightly farther from the center, and making the strength of gravity that determines the period of a swinging pendulum very slightly weaker. Newton included this in his Principia Mathematica, ensuring that Richer’s secondary task would be remembered far better than the parallax measurement that was the main reason he was sent across the Atlantic. (Which, just for the record, he did very well.)

So, while you can frame the recent experiment with atomic clocks in Tokyo as a test of Einstein’s theory of General Relativity, it also fits squarely in a 350-year-old tradition of using clocks to measure gravity. The variation they find between the top and bottom of the Tokyo Skytree is a lot smaller than what Richer measured— about five trillionths of a percent compared to three-tenths of a percent for Richer— which just shows how far our clocks have come since 1672. And, in fact, in the modern paper, they talk about using these clocks for exactly this purpose: a transportable clock that’s good to a few parts in 10¹⁸ can be used to measure variations in gravity with amazing precision, and could track geophysical processes like continental uplift or magma movement underground.

However you like to frame it, as a Popperian attempt to falsify a hundred-year-old theory, or a Richer-esque milestone in the 350-year history of geodesy, it’s a very cool experiment. And one that’s exceptionally well-timed (#SorryNotSorry) for my current project of writing about the history of timekeeping.

This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from January 2016, without the images that appeared with it, some of which were lost during a Forbes platform change a couple of years ago; I may try to recover them and replace them in this post later.

It’s been quiet here for a couple of weeks, as I was off visiting family for the holidays and then at a meeting in South Carolina. It’s a New Year though, so let’s ease back into things with a simple and non-controversial topic. Say, the Many-Worlds interpretation of quantum physics.

OK, maybe that doesn’t seem simple or non-controversial, but it’s what’s on my mind. Whenever I give general-audience science talks, as I did in South Carolina, I get questions about Many-Worlds, and frequently hear the same basic objection, namely that it’s “too complicated.” This is primarily a response to a popular metaphor regarding the theory, though, not the reality– in fact, when looked at in the right way, Many-Worlds is actually the simplest way to resolve the issues in quantum mechanics that demand interpretations in the first place.

Since this keeps coming up, and I’ve recently put thought into it, let me take another crack at explaining what’s really going on with “Many-Worlds,” and why it’s actually the simplest game in town.

What’s The Problem?

The fundamental issue is the question of superposition and measurement that drove people like Einstein and Erwin Schrödinger away from quantum physics even though they played a pivotal role in creating the theory that the rest of us know and love. Quantum physics deals in probabilities, not certainty, and the mathematical wavefunctions we calculate within the theory will contain pieces describing multiple possible outcomes right up until the moment of measurement. And yet, we experience only a single reality, with any given measurement having one and only one outcome.

This is the point of Schrödinger’s infamous thought experiment with a cat (one of them, anyway). In a 1935 paper that served as a kind of parting shot on his way off to study other topics, he suggested an “absurd” scenario involving a cat sealed in a box with a device that has a 50% chance of killing the cat in the next hour. The question he posed is “What is the state of the cat at the end of that hour, just before you open the box?” Common sense would seem to suggest that the cat is either alive or dead, but the quantum-mechanical wavefunction describing the scenario would include an equal mixture of both “alive” and “dead” states at the same time.

To Schrödinger, and many other classically inclined physicists, for a cat to exist in a superposition like this is clearly ridiculous. And yet, he noted, this is what we’re expected to accept when applied to photons and electrons and atoms. There has to be some way to get from the quantum situation to a single everyday reality, and the lack of a satisfactory means to handle this was, for them, a fatal flaw in quantum physics.

The traditional way of handling this is via “wavefunction collapse,” which gives measurement an active role in the process. In collapse interpretations, the usual equations of quantum physics apply during intervals between measurements of the state of a quantum system, but during the measurement process itself, something else happens that takes the system from a spread-out quantum superposition into a single classical reality. This brings with it a whole host of problems associated with defining what counts as a measurement, and how this “collapse” is brought about, and so forth, but for many physicists, that was regarded as a necessary concession to reconcile empirical reality with the enormously successful predictions of quantum physics.

“Many-Worlds” came along in the late 1950’s, when a Princeton grad student named Hugh Everett III pointed out that you don’t really need to have the wavefunction collapse. Instead, you can simply have the wavefunction continue along as it was before, retaining the multiple branches corresponding to the different possible measurement outcomes. In this view, we “see” a single outcome because we’re part of the wavefunction, with everything entangled together. So, Schrödinger’s infamous cat is both alive and dead before the box is opened, and after it’s opened, the cat is alive-with-a-happy-Schrödinger, and dead-with-a-sad-Schrödinger. If Schrödinger goes on to tell his fellow cat-physics enthusiast Eugene Wigner the results, then the cat is alive-with-a-happy-Schrödinger-and-a-happy-Wigner and dead-with-a-sad-Schrödinger-and-a-sad-Wigner. And so on.

Many Metaphorical Worlds

This fixes the problem of needing a mysterious collapse mechanism, but at the cost of carrying around all these extra versions of ourselves and our experimental subjects. Which seems like a problem, because we don’t see those things. when I go to the playground with my kids, they’re not bumping into a dozen other versions of themselves going down the slide.

The explanation here is really that we’re constrained to only perceive a single reality at a time, and that perception is entangled with the outcome we’re seeing. Which is kind of a weird idea, so Everett and others (notably Bryce DeWitt) came up with a metaphor in hopes of making things clearer, as is commonly done in physics. And, as too often happens (see Hossenfelder on Hawking), the metaphor introduced a whole new line of confusion.

The metaphor is basically the common name of the interpretation: Many Worlds. That is, the claim is that the different branches of the universal wavefunction in which the different outcomes happen and are perceived to happen are effectively separate universes. There’s a “world” in which Professor Schrödinger opens his box to find a live cat, and in that universe he goes on to share the good news with his buddy Wigner, and there’s an entirely separate “world” in which Professor Schrödinger has his day ruined, and goes on to bum Wigner out, and so on. There’s a “world” for every possible sequence of events, and these are completely separate and inaccessible to one another. The world bifurcates whenever a measurement is made, and subsequently there are two universes with different histories evolving in parallel.

As long as you don’t take this too seriously, it gets the right basic idea across. The problem is, people take it too seriously, and thus you end up with an array of misconceptions and objections to things that aren’t really Many-Worlds. For example, you’ll sometimes run into the argument that it’s all garbage because it requires the creation of an entire new universe worth of material for every trivial measurement. In fact all it really says is that the single universe worth of stuff we already has exists in an expanding superposition state– nothing new is brought into existence by measurement, existence just gets more complex.

The “too complicated” argument that I mentioned at the start of this post is another example, arguing on essentially aesthetic grounds that having extra universes running around is overly baroque. But I would argue that this really ought to run the other way: the “extra universes” business is an oversimplification brought in for calculational convenience.

Detecting Other “Universes”

To explain what I mean, we need a concrete example of how one might go about trying to detect the presence of the additional branches of the wavefunction that metaphor turns into “other universes.” Fortunately, this is something we do all the time in physics, through the process of interferometry, which is at the heart of the best clocks and motion sensors modern technology has to offer.

The simplest illustration of this is what’s called a “Mach-Zehnder interferometer,” illustrated above. A beam of light (or an electron, or an atom, or any other quantum object) falls on a beamsplitter that has a 50% chance of sending the light on either of two perpendicular paths. Each of these leads to a mirror, which redirects it to a second beamsplitter with a 50% chance of sending light to either Detector A or Detector B. Then you look at how much light reaches each of your detectors.

Now, if you imagine the incoming photons of light behaving like classical particles, you can easily convince yourself that each detector gets 50% of the light. On the other hand, if the light behaves like a wave and takes both paths at once, it turns out that if both path 1 and path 2 are of exactly equal length, 100% of the light you sent in will end up at Detector B, with nothing at all at Detector A. If you make one path a little longer than the other, you can flip these (100% at A, 0% at B), then flip them back, and so on. The amazing thing about quantum physics is that you can get both of these at once, sort of: a single photon of light sent in can only be detected at one of these, like a particle, but if you repeat the experiment many times with the same conditions, you’ll see the wave behavior. If you set up your interferometer with exactly equal path lengths, and shoot in 1000 photons one after the other, you’ll get 1000 of them at B, and zero at A.

Mathematically, this happens because of the way we calculate probability distributions from wavefunctions. The wavefunction of a photon passing through the interferometer contains two pieces, one corresponding to passing along path 1, the other along path 2. To get the probability distribution, we have to add these together, then square the wavefunction, and that process gives you the interference effect. So this is really, in some sense, a device for detecting exactly the sort of extra wavefunction branches that become the “other universes” of Many-Worlds.

So, all we need to do is to throw a cat into one of these devices, and see what happens, right? The problem here is that there’s a stringent condition buried in the above: “if you repeat the experiment many times with the same conditions, you’ll see the wave behavior.” And as your system becomes more complicated, it becomes exceedingly difficult to maintain the same conditions.

By way of illustration, let’s imagine a specific form of complication, in the form of an imaginary demon (like cats, a common feature of physics thought experiments) squatting atop Mirror 1 in the apparatus. This demon is equipped with a device– a small glass plate, say– that it can stick in the path of the light to flip the probabilities at the output of the interferometer. If the glass plate is out, the path lengths are exactly equal, and none of the light makes it to A, but if it’s in, it delays one path just enough that 100% of the light goes to A, and none to B.

Such a demon can easily obscure the existence of the second branch of the wavefunction by randomly sticking the plate in for half of the experimental trials. If you do this 1000 times, you’ll get roughly 500 photons at A and 500 photons at B, and conclude that they behave like classical billiard balls. But this isn’t eliminating a branch of the wavefunction, just obscuring it. If the canny physicist running the experiment employed a recording angel to watch over the demon and note which trials included the plate, the data could be sorted into two subsets, one with 500 photons at A and the other with 500 photons at B. Each of these would individually show the existence of two components of the wavefunction, thwarting the demon’s nefarious plan.

What’s this got to do with the “other universes” of Many-Worlds? Well, if you wanted to see the presence of other branches of the wavefunction, you would need to do something conceptually very much like this– make a measurement that “splits” the system into two components, then recombine them and look for an interference effect. This only works, though, if you can consistently guarantee the same conditions through enough repetitions of the experiment to reconstruct the probability distribution with reasonable accuracy. And that becomes exponentially more difficult as you add to the complexity of the system. For something like a cat, you’re fighting against, I dunno, 10³⁰ “demons” in the form of environmental interactions and so on that change the conditions for a given run and thus change the probability. That’s pretty much hopeless, and the end result is that your experiment looks classical, like you’ve picked out a single definite outcome and. Which is what you expect from either a “collapse” interpretation that pruned away all the other branches of the wavefunction, or the metaphorical version of Many-Worlds where you’ve split them off into separate universes.

But neither of those is exactly true. In fact, just as with that pesky demon, the real and correct calculation of the probability distribution for a particular run of the experiment includes both branches of the wavefunction, along with information about the exact conditions of that specific run. If you could keep track of all the necessary details, you could subdivide your experiments to show that both the “live” and “dead” parts of the cat contribute to the final result. It’s not that those pieces don’t exist, or aren’t affecting the outcome of your experiment, it’s just that you can’t see the effect, because you can’t keep track of enough information to repeat the experiment consistently enough to see the effect.

The “multiple universes” of Many-Worlds, then, aren’t a baroque add-on to an otherwise simple universe, they’re just a calculational convenience. The proper way to determine the outcome of a particular set of experiments would be to include all of the zillions of terms in the wavefunction that correspond to different possible chains of events. Since you can’t keep track of enough of those to detect them in the final result, though, you end up getting the same basic result as if you had a single isolated branch of the wavefunction, which is much easier to deal with. So we cut those out just to make the math easier. It’s essentially the same thing I do when I neglect to include the gravitational influence of Jupiter when I calculate how long it takes a satellite to orbit the Earth. Strictly speaking, that effect ought to be included, but it would complicate the calculation tremendously without significantly affecting the result, so I might as well ignore it. But the physical universe isn’t constrained to do calculations the same way I do, and effortlessly includes all those tiny effects.

Now, this is not to say that there aren’t problems to be solved with Many-Worlds– there are, chief among them an ongoing effort to understand how probability works out from within the wavefunction. On a conceptual level, though, contrary to a common misconception, it’s actually a simple and elegant solution to a problem that otherwise demands some ugly kludges to sort out.

This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from May 2016, without the images that appeared with it, some of which were lost during a Forbes platform change a couple of years ago; I may try to recover them and replace them in this post later.

Last week, I wrote a big long post about why entanglement doesn’t allow faster-than-light communication, and included a passing mention that was related somehow to the “Many-Worlds” Interpretation of quantum physics. That post was long already, so I didn’t have room to say more, but that remark probably deserves unpacking.

Now, entanglement as a general phenomenon is kind of mysterious, so saying that it can help you understand another totally mysterious quantum idea might seem less than perfectly helpful. the nice thing about entanglement, though, is that even if you don’t think about the underlying mechanism at all, it provides a very concrete set of results, verified in countless experiments, that we can use to ground our thinking about deeper issues.

A generic entanglement experiment looks like this:

Two experimental physicists, traditionally named “Alice” and “Bob” share a pair of particles emitted from a source of some sort (represented as a canonical black box, because it doesn’t matter how it works), whose states may or may not be entangled. These could be any of a huge range of systems, but for the sake of concreteness, let’s imagine that they’re electron spins. Two electrons fly out of the box, one to Alice and one to Bob, and the scientists measure what direction the magnetic moment is pointing, recording a 1 for up and a 0 for down. Their measurements are tables of numbers that look kind of like this:

Alice and Bob each record a random string of 1’s and 0’s, split 50/50; nothing about the individual lists tells you anything about entanglement. The only way to distinguish between entangled and not-entangled particles is to compare Alice’s list of measurements to Bob’s. If the particles are entangled, they’ll correlate nearly perfectly (maybe the occasional experimental error creeping in, shown in red), while if they’re not entangled, they’ll only agree about half of the time.

Of course, the universe is much bigger than just Alice and Bob and their two electrons. So let’s imagine a third person, usually called “Eve” in a communications contest, who’s mad at Alice and Bob for some reason, and wants to mess up their experiment. Eve inserts an extra element in the path between the black-box source and Bob’s detector that rotates the spin of Bob’s electron. This is easy enough to do by, say, putting an extra magnet in the path of the beam.

When this happens, Bob’s detector will give very different results than Alice’s. When Alice detects a spin-up electron and records a “1,” Bob ought to also get a spin-up electron. Instead, thanks to Eve’s meddling, he’s getting an electron whose spin has been rotated 45 degrees from the “up” direction, and thus has a 50/50 chance of being detected as spin-up or spin-down. In this scenario, they’ll find lots of cases where their spin measurements disagree, and conclude that their particles are not, in fact, entangled.

This does not mean, however, that Eve has broken the entanglement between Alice’s particle and Bob’s. On the contrary, that entanglement is still there, it’s just hidden from view. But if Alice and Bob know about Eve, Bob can adjust his detector to account for her meddling:

When Bob turns his detector to compensate for Eve’s extra rotation, Alice and Bob will recover their nearly-perfect correlation between measurements. Once entangled, the particles remain entangled even though Eve’s messing with them, provided Alice and Bob make the right set of measurements. As long as they know that Eve’s meddling, and approximately how much she’s changing things, they can correct for that outside influence.

This toy-model scenario explains why physicists studying entanglement need to work very hard to isolate their experiments from the outside world. Very few physicists have honest-to-God malicious interlopers changing their experimental parameters, but there are lots of random environmental factors that can play the role of “Eve.” A stray magnetic field in one part of the apparatus can rotate the electron spin just as effectively as an evil electromagnet, so experiments are carefully shielded, and numerous sanity-check tests are done to ensure that Bob’s making the right sort of measurements to correlate with the measurements Alice is making.

The killer for these kinds of experiments are unmeasured interactions with the external environment. If you know there’s something perturbing the experiment, you can correct for it, and see the entanglement that’s really there. You can’t correct for interactions you don’t know about– the “unknown unknowns” in Donald Rumsfeld’s immortal taxonomy of problems— and that’s what ultimately limits our ability to do entanglement experiments. It’s not that the entanglement itself is fragile– it’s actually pretty robust, nearly as hard to truly destroy as to create in the first place– but the ability to measure it depends on knowing exactly what measurement you ought to be making.

What’s this got to do with the Many-Worlds Interpretation? Well, the whole reason we have Many-Worlds and all the other interpretations of quantum physics is that when we look at the everyday world around us, we don’t see all the weird stuff quantum physics says ought to be there. We don’t see superposition states, with cats that are alive and dead at the same time, and we don’t generally notice entanglement-type correlations between distant measurements unless we work really hard to find them.

One traditional way of dealing with this is through a “collapse” type interpretation, saying that there’s some as-yet-unknown mechanism that pushes superposition states toward a single definite state, and destroys correlations between entangled particles. There are a bunch of variants of this, differing in how, exactly, they talk about the collapse process.

Many-Worlds takes a different approach, saying that superposition states and entanglement correlations continue to exist, but are hidden from us by unmeasured and random interactions with the environment. We don’t see quantum superpositions of actual cats not because the cats are really in a single state, but because of the undetected meddling of trillions of Eves. If we knew in detail exactly what the environment was doing to perturb our measurements, we could correct for it and see the quantum effects that are really there, but not only do we not know what the environment is doing, what it’s doing is changing all the time, making it impossible to compensate. The universe is really in a massive superposition of massively entangled states, but we can’t see it because of all the environmental perturbations screwing up our experiments.

It should be noted that this process of unmeasured perturbations hiding quantum effects is not unique to Many-Worlds– it gets the catch-all name “decoherence,” and shows up in basically every modern approach to quantum physics. Its role is (arguably, at least) more important in Many-Worlds type approaches than others, but it’s a real process regardless of your choice of interpretation. The process is often explained very badly– one of the things I worked hardest at when writing How to Teach Quantum Physics to Your Dog was trying to find a better way to talk about decoherence than most of the popular treatments out there– but really, it’s just a bunch of undetected Eves messing with your experimental measurements.

This post is part of a series of posts originally written for my blog at Forbes.com that I’m copying to my personal site, so I have a (more) stable (-ish) archive of them. This is just the text of the original post, from February 2018, without the images that appeared with it (which were mostly fairly generic photos ).

A couple of weeks ago, fellow Forbes blogger Ethan Siegel took to his keyboard with the goal of making me sigh heavily, writing a post about interpretations of quantum physics calling the idea that you need an interpretation “the biggest myth in quantum physics.” Ethan’s argument boils down to noting that all of the viable interpretations known at present make identical predictions about the probability of getting particular outcomes for any experiment we might do. Therefore, according to Ethan, there’s no need for any interpretation, because it doesn’t really matter which of them you choose.

As an experimentalist by training and inclination, I am not without sympathy for this point of view. In fact, when I give talks about quantum mechanics and get the inevitable questions about interpretations, I tend to say something in that general vein– that at present, nobody knows how to do an experiment that would distinguish between any of the viable interpretations. Given that, I say, the choice between interpretations is essentially an aesthetic one.

When, then, the heavy sigh on reading Ethan’s post? There are two reasons why I had that reaction, and am writing this post in (belated) response.

The first reason is best explained via a historical analogy. For this, I would point to the infamous arguments between Albert Einstein and Niels Bohr that culminated in the famous paper by Einstein and his colleagues Boris Podolsky and Nathan Rosen that introduced the world to the physics of quantum entanglement. Einstein, Podolsky, and Rosen proposed a thought experiment involving a pair of particles prepared in such a way that their individual states were indeterminate but correlated– most modern treatments make it a two-state system, so that the measurement of an individual particle has a 50/50 chance of coming up with either outcome, but when you make the same measurement on both particles, you’re guaranteed to get the same answer.

This phenomenon turns out to be a rich source of interesting physics to explore, as you can tell from the fact that I’ve written at least ten posts about it (this one, and the nine links in the second paragraph). There’s a thriving subfield of physics that has grown out of the ideas expressed in that one paper, generating both fascinating theoretical approaches and useful experimental technology.

That said, though, for the first three decades after it was published in 1935, the “EPR paper” was pretty much a footnote to physics. As late as the early 1980’s, Abraham Pais’s magisterial scientific biography of Einstein dispenses of it very quickly, as a sort of brief mis-step during the declining years of Einstein’s scientific career.

Why was such an important paper so lightly regarded? Because it was, initially, an argument about interpretations of quantum physics. That is, Einstein, Podolsky, and Rosen used the correlation between separated measurements to argue that quantum mechanics must be incomplete– that the measurement outcomes that orthodox quantum theory says are indeterminate are in fact determined in advance, and just hidden from us. They didn’t disagree with the prediction of the quantum theory, only its interpretation.

The EPR paper has the title “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?”, and in keeping with Betteridge’s Law of Headlines, they answered “No,” saying that a deeper theory was needed. Niels Bohr wrote a hasty response, also with the title “Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?” arguing the opposite. The specifics of the counter-argument aren’t all that important (it’s also not a very well-written paper…); the important thing is that most practicing physicists viewed the whole thing as pretty much an “Is not!” “Is too!” exchange, a purely philosophical matter with no practical importance.

How did the EPR paper get lifted out of obscurity, then? Because an Irish physicist named John Stewart Bell looked into the problem, and thought seriously about what the claims being made really meant, and realized that the sort of model Einstein, Podolsky, and Rosen favored would place hard limits on certain types of measurements. Those limits would not be the same in quantum theory, so a clever experiment can distinguish between them. Bell pointed this out, and not long afterward a small number of physicists began working on those experimental tests. From that seed, much of the modern edifice of quantum information physics has sprung.

So, my first response to Ethan’s line of argument is a reminder of Bell’s work. Just because we don’t currently know of an experiment that can distinguish between interpretations of quantum physics does not mean that we will never know how to do such an experiment. It may be that some future physicist, successor to Bell, will think long enough and deeply enough about quantum foundations to come up with an experiment for which different interpretations genuinely make different predictions. That would be pretty amazing, and for that reason I think it’s worth having some people put some effort into thinking about this stuff.

I would say, though, that even if nobody ever comes up with an experimental test of quantum interpretations– or, to take an extreme case, somebody manages to prove that no such experiment can ever be done– it’s still worth thinking about them, and picking a favorite. The choice between them is currently an aesthetic choice, and may remain so forever, but as anyone who has ever decorated a house can tell you, aesthetics are not nothing.

That is, the way you choose to think about “what’s really going on” in quantum physics may not make any difference in the outcomes you predict for a given experiment, but it will shape the way you think about what experiments to do. If you favor an ontological theory involving a physical collapse of the wavefunction, that may lead you to explore certain lines of inquiry– looking at the speed of that collapse, say– that are most easily conceptualized in that sort of picture. If, on the other hand, you’re more of an epistemic interpretation type, you might instead choose to pursue different lines, ones more easily interpretable in terms of information. If you’re a Bohmian, you’ll think about particle trajectories, and if you like one of the retrocausal approaches you’ll think about future measurements affect past conditions, and so on.

The argument that you should care about quantum interpretations, even in a world with no measurable distinction between them, is the same as any other argument in favor of diversity. Different aesthetic choices influence the way you think about the world, and that change in thinking can lead to new and different approaches to… everything, really. It may not be possible to distinguish old-school Copenhagenism from the latest Many-Worlds hotness in any given experiment, but someone who favors Many-Worlds may be led to do experiments that would never happen in a world where only Copenhagenists exist, depriving us of a different angle on the quantum world.

So, while I don’t think anything Ethan said is wrong in the sense of being incorrect, I sighed heavily on reading it because it’s the wrong take in the sense of being far too narrow and constrained a way to look at quantum phenomenon. Quantum physics operates in a way that runs very counter to everyday experience, and that makes it a theory of unparalled richness. So rich and weird a set of phenomena deserves as diverse a set of approaches as we can bring to bear on it, and that’s ultimately why quantum interpretations matter.

——

(If you’re not happy with that “Let a thousand interpretations bloom” take, you should read Scott Aaronson’s post on quantum interpretations, which predates Ethan’s post, and makes an affirmative argument for Many-Worlds. Actually, you should read that even if you like the above…)