Tag Archives: physics

The (but I’m Not a) Crackpot Style Guide

Ok, ok, I believe you. You’re not a crackpot. You’re just an outsider, one with a brilliant new idea that would overturn the accepted paradigms of physics, if only someone would just listen.

Here’s the problem: you’re not alone. There are plenty of actual crackpots. We get contacted by them fairly regularly. And most of the time, they’re frustrating and unpleasant to deal with.

If you want physicists to listen to you, you need to show us you’re not one of those people. Otherwise, most of us won’t bother.

I can’t give you a foolproof way to do that. But I can give some suggestions that will hopefully make the process a little less frustrating for everyone involved.

Don’t spam:

Nobody likes spam. Nobody reads spam. If you send a mass email to every physicist whose email address you can find, none of them will read it. If you repeatedly post the same thing in a comment thread, nobody will read it. If you want people to listen to you, you have to show that you care about what they have to say, and in order to do that you have to tailor your message. This leads in to the next point,

Ask the right people:

Before you start reaching out, you should try to get an idea of who to talk to. Physics is quite specialized, so if you’re taking your ideas seriously you should try to contact people with a relevant specialization.

Now, I know what you’re thinking: your ideas are unique, no-one in physics is working on anything similar.

Here, it’s important to distinguish the problem you’re trying to solve with how you’re trying to solve it. Chances are, no-one else is working on your specific idea…but plenty of people are interested in the same problems.

Think quantum mechanics is built on shoddy assumptions? There are people who spend their lives trying to modify quantum mechanics. Have a beef against general relativity? There’s a whole sub-field of people who modify gravity.

These people are a valuable resource for you, because they know what doesn’t work. They’ve been trying to change the system, and they know just how hard it is to change, and just what evidence you need to be consistent with.

Contacting someone whose work just uses quantum mechanics or relativity won’t work. If you’re making elementary mistakes, we can put you on the right track…but if you think you’re making elementary mistakes, you should start out by asking help from a forum or the like, not contacting a professional. If you think you’ve really got a viable replacement to an established idea, you need to contact people who work on overturning established ideas, since they’re most aware of the complicated webs of implications involved. Relatedly,

Take ownership of your work:

I don’t know how many times someone has “corrected” something in the comments, and several posts later admitted that the “correction” comes from their own theory. If you’re arguing from your own work, own it! If you don’t, people will assume you’re trying to argue from an established theory, and are just confused about how that theory works. This is a special case of a broader principle,

Epistemic humility:

I’m not saying you need to be humble in general, but if you want to talk productively you need to be epistemically humble. That means being clear about why you know what you know. Did you get it from a mathematical proof? A philosophical argument? Reading pop science pieces? Something you remember from high school? Being clear about your sources makes it easier for people to figure out where you’re coming from, and avoids putting your foot in your mouth if it turns out your source is incomplete.

Context is crucial:

If you’re commenting on a blog like this one, pay attention to context. Your comment needs to be relevant enough that people won’t parse it as spam.

If all a post does is mention something like string theory, crowing about how your theory is a better explanation for quantum gravity isn’t relevant. Ditto for if all it does is mention a scientific concept that you think is mistaken.

What if the post is promoting something that you’ve found to be incorrect, though? What if someone is wrong on the internet?

In that case, it’s important to keep in mind the above principles. A popularization piece will usually try to present the establishment view, and merits a different response than a scientific piece arguing something new. In both cases, own your own ideas and be specific about how you know what you know. Be clear on whether you’re talking about something that’s controversial, or something that’s broadly agreed on.

You can get an idea of what works and what doesn’t by looking at comments on this blog. When I post about dark matter, or cosmic inflation, there are people who object, and the best ones are straightforward about why. Rather than opening with “you’re wrong”, they point out which ideas are controversial. They’re specific about whose ideas they’re referencing, and are clear about what is pedagogy and what is science.

Those comments tend to get much better responses than the ones that begin with cryptic condemnations, follow with links, and make absolute statements without backing them up.

On the internet, it’s easy for misunderstandings to devolve into arguments. Want to avoid that? Be direct, be clear, be relevant.

In Defense of Lord Kelvin, Michelson, and the Physics of Decimals

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William Thompson, Lord Kelvin, was a towering genius of 19th century physics. He is often quoted as saying,

There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.

Certainly sounds like something I would say!

As it happens, he never actually said this. It’s a paraphrase of a quote from Albert Michelson, of the Michelson-Morley Experiment:

While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. It is here that the science of measurement shows its importance — where quantitative work is more to be desired than qualitative work. An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals.

Now that’s more like it!

In hindsight, this quote looks pretty silly. When Michelson said that “it seems probable that most of the grand underlying principles have been firmly established” he was leaving out special relativity, general relativity, and quantum mechanics. From our perspective, the grandest underlying principles had yet to be discovered!

And yet, I think we should give Michelson some slack.

Someone asked me on twitter recently what I would choose if given the opportunity to unravel one of the secrets of the universe. At the time, I went for the wishing-for-more-wishes answer: I’d ask for a procedure to discover all of the other secrets.

I was cheating, to some extent. But I do think that the biggest and most important mystery isn’t black holes or the big bang, isn’t asking what will replace space-time or what determines the constants in the Standard Model. The most critical, most important question in physics, rather, is to find the consequences of the principles we actually know!

We know our world is described fairly well by quantum field theory. We’ve tested it, not just to the sixth decimal place, but to the tenth. And while we suspect it’s not the full story, it should still describe the vast majority of our everyday world.

If we knew not just the underlying principles, but the full consequences of quantum field theory, we’d understand almost everything we care about. But we don’t. Instead, we’re forced to calculate with approximations. When those approximations break down, we fall back on experiment, trying to propose models that describe the data without precisely explaining it. This is true even for something as “simple” as the distribution of quarks inside a proton. Once you start trying to describe materials, or chemistry or biology, all bets are off.

This is what the vast majority of physics is about. Even more, it’s what the vast majority of science is about. And that’s true even back to Michelson’s day. Quantum mechanics and relativity were revelations…but there are still large corners of physics in which neither matters very much, and even larger parts of the more nebulous “physical science”.

New fundamental principles get a lot of press, but you shouldn’t discount the physics of “the sixth place of decimals”. Most of the big mysteries don’t ask us to challenge our fundamental paradigm: rather, they’re challenges to calculate or measure better, to get more precision out of rules we already know. If a genie gave me the solution to any of physics’ mysteries I’d choose to understand the full consequences of quantum field theory, or even of the physics of Michelson’s day, long before I’d look for the answer to a trendy question like quantum gravity.

Entropy is Ignorance

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(My last post had a poll in it! If you haven’t responded yet, please do.)

Earlier this month, philosopher Richard Dawid ran a workshop entitled “Why Trust a Theory? Reconsidering Scientific Methodology in Light of Modern Physics” to discuss his idea of “non-empirical theory confirmation” for string theory, inflation, and the multiverse. They haven’t published the talks online yet, so I’m stuck reading coverage, mostly these posts by skeptical philosopher Massimo Pigliucci. I find the overall concept annoying, and may rant about it later. For now though, I’d like to talk about a talks on the second day by philosopher Chris Wüthrich about black hole entropy.

Black holes, of course, are the entire-stars-collapsed-to-a-point-that-no-light-can-escape that everyone knows and loves. Entropy is often thought of as the scientific term for chaos and disorder, the universe’s long slide towards dissolution. In reality, it’s a bit more complicated than that.

For one, you need to take Elric into account…

Can black holes be disordered? Naively, that doesn’t seem possible. How can a single point be disorderly?

Thought about in a bit more detail, the conclusion seems even stronger. Via something called the “No Hair Theorem”, it’s possible to prove that black holes can be described completely with just three numbers: their mass, their charge, and how fast they are spinning. With just three numbers, how can there be room for chaos?

On the other hand, you may have heard of the Second Law of Thermodynamics. The Second Law states that entropy always increases. Absent external support, things will always slide towards disorder eventually.

If you combine this with black holes, then this seems to have weird implications. In particular, what happens when something disordered falls into a black hole? Does the disorder just “go away”? Doesn’t that violate the Second Law?

This line of reasoning has led to the idea that black holes have entropy after all. It led Bekenstein to calculate the entropy of a black hole based on how much information is “hidden” inside, and Hawking to find that black holes in a quantum world should radiate as if they had a temperature consistent with that entropy. One of the biggest successes of string theory is an explanation for this entropy. In string theory, black holes aren’t perfect points: they have structure, arrangements of strings and higher dimensional membranes, and this structure can be disordered in a way that seems to give the right entropy.

Note that none of this has been tested experimentally. Hawking radiation, if it exists, is very faint: not the sort of thing we could detect with a telescope. Wüthrich is worried that Bekenstein’s original calculation of black hole entropy might have been on the wrong track, which would undermine one of string theory’s most well-known accomplishments.

I don’t know Wüthrich’s full argument, since the talks haven’t been posted online yet. All I know is Pigliucci’s summary. From that summary, it looks like Wüthrich’s primary worry is about two different definitions of entropy.

See, when I described entropy as “disorder”, I was being a bit vague. There are actually two different definitions of entropy. The older one, Gibbs entropy, grows with the number of states of a system. What does that have to do with disorder?

Think about two different substances: a gas, and a crystal. Both are made out of atoms, but the patterns involved are different. In the gas, atoms are free to move, while in the crystal they’re (comparatively) fixed in place.

Blurrily so in this case

There are many different ways the atoms of a gas can be arranged and still be a gas, but fewer in which they can be a crystal, so a gas has more entropy than a crystal. Intuitively, the gas is more disordered.

When Bekenstein calculated the entropy of a black hole he didn’t use Gibbs entropy, though. Instead, he used Shannon entropy, a concept from information theory. Shannon entropy measures the amount of information in a message, with a formula very similar to that of Gibbs entropy: the more different ways you can arrange something, the more information you can use it to send. Bekenstein used this formula to calculate the amount of information that gets hidden from us when something falls into a black hole.

Wüthrich’s worry here (again, as far as Pigliucci describes) is that Shannon entropy is a very different concept from Gibbs entropy. Shannon entropy measures information, while Gibbs entropy is something “physical”. So by using one to predict the other, are predictions about black hole entropy just confused?

It may well be he has a deeper argument for this, one that wasn’t covered in the summary. But if this is accurate, Wüthrich is missing something fundamental. Shannon entropy and Gibbs entropy aren’t two different concepts. Rather, they’re both ways of describing a core idea: entropy is a measure of ignorance.

A gas has more entropy than a crystal, it can be arranged in a larger number of different ways. But let’s not talk about a gas. Let’s talk about a specific arrangement of atoms: one is flying up, one to the left, one to the right, and so on. Space them apart, but be very specific about how they are arranged. This arrangement could well be a gas, but now it’s a specific gas. And because we’re this specific, there are now many fewer states the gas can be in, so this (specific) gas has less entropy!

Now of course, this is a very silly way to describe a gas. In general, we don’t know what every single atom of a gas is doing, that’s why we call it a gas in the first place. But it’s that lack of knowledge that we call entropy. Entropy isn’t just something out there in the world, it’s a feature of our descriptions…but one that, nonetheless, has important physical consequences. The Second Law still holds: the world goes from lower entropy to higher entropy. And while that may seem strange, it’s actually quite logical: the things that we describe in more vague terms should become more common than the things we describe in specific terms, after all there are many more of them!

Entropy isn’t the only thing like this. In the past, I’ve bemoaned the difficulty of describing the concept of gauge symmetry. Gauge symmetry is in some ways just part of our descriptions: we prefer to describe fundamental forces in a particular way, and that description has redundant parameters. We have to make those redundant parameters “go away” somehow, and that leads to non-existent particles called “ghosts”. However, gauge symmetry also has physical consequences: it was how people first knew that there had to be a Higgs boson, long before it was discovered. And while it might seem weird to think that a redundancy could imply something as physical as the Higgs, the success of the concept of entropy should make this much less surprising. Much of what we do in physics is reasoning about different descriptions, different ways of dividing up the world, and then figuring out the consequences of those descriptions. Entropy is ignorance…and if our ignorance obeys laws, if it’s describable mathematically, then it’s as physical as anything else.

Bras and Kets, Trading off Instincts

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Some physics notation is a joke, but that doesn’t mean it shouldn’t be taken seriously.

Take bras and kets. On the surface, as silly a physics name as any. If you want to find the probability that a state in quantum mechanics turns into another state, you write down a “bracket” between the two states:

$\langle a | b\rangle$

This leads, with typical physics logic, to the notation for the individual states: separate out the two parts, into a “bra” and a “ket”:

$\langle a|$ , $|b\rangle$

It’s kind of a dumb joke, and it annoys the heck out of mathematicians. Not for the joke, of course, mathematicians probably have worse.

Mathematicians are annoyed when we use complicated, weird notation for something that looks like a simple, universal concept. Here, we’re essentially just taking inner products of vectors, something mathematicians have been doing in one form or another for centuries. Yet rather than use their time-tested notation we use our own silly setup.

There’s a method to the madness, though. Bras and kets are handy for our purposes because they allow us to leverage one of the most powerful instincts of programmers: the need to close parentheses.

In programming, various forms of parentheses and brackets allow you to isolate parts of code for different purposes. One set of lines might only activate under certain circumstances, another set of brackets might make text bold. But in essentially every language, you never want to leave an open parenthesis. Doing so is almost always a mistake, one that leaves the rest of your code open to whatever isolated region you were trying to create.

Open parentheses make programmers nervous, and that’s exactly what “bras” and “kets” are for. As it turns out, the states represented by “bras” and “kets” are in a certain sense un-measurable: the only things we can measure are the brackets between them. When people say that in quantum mechanics we can only predict probabilities, that’s a big part of what they mean: the states themselves mean nothing without being assembled into probability-calculating brackets.

This ends up making “bras” and “kets” very useful. If you’re calculating something in the real world and your formula ends up with a free “bra” or a “ket”, you know you’ve done something wrong. Only when all of your bras and kets are assembled into brackets will you have something physically meaningful. Since most physicists have done some programming, the programmer’s instinct to always close parentheses comes to the rescue, nagging until you turn your formula into something that can be measured.

So while our notation may be weird, it does serve a purpose: it makes our instincts fit the counter-intuitive world of quantum mechanics.

Physical Truths, Lost to the Ages

The Hardest Audience Knows Just Enough to Be Dangerous

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You’d think that it would be hard to explain physics to people who know absolutely nothing about physics.

And you might be right, if there was anyone these days who knew absolutely nothing about physics. If someone didn’t know what atoms were, or didn’t know what a physicist was, then yes it would take quite a while to explain anything more than the basics. But most people know what atoms are, and know what physicists are, and at least have a basic idea that there are things called protons and neutrons and electrons.

And that’s often enough. Starting with a basis like that, I can talk people through the Large Hadron Collider, I can get them to picture Feynman Diagrams, I can explain, roughly, what it is I do.

On the other end, it’s not all that hard to explain what I do to people in my sub-field. Working on the same type of physics is like sharing a language, we have all sorts of terms to make explaining easier. While it’s still possible to trip up and explain too much or too little (a recent talk I gave left out the one part that one member of the audience needed…because everyone else would have gotten nothing out of it), you’re protected by a buffer of mutual understanding.

The hardest talks aren’t for the public, and they aren’t for fellow amplitudes-researchers. They’re for a general physics audience.

If you’re talking to physicists, you can’t start with protons and neutrons. Do that, and your audience is going to get annoyed with you rather quickly. You can’t rely on the common understanding everyone has of physics. In addition to making your audience feel like they’re being talked down to, you won’t manage to say anything substantial. You need to start at a higher level so that when you do describe what you do, it’s in enough detail that your audience feels like they really understand it.

At the same time, you can’t start with the jargon of your sub-field. If you want to really explain something (and not just have fifteen minutes of background before everyone tunes out) you need to build off of a common understanding.

The tricky part is, that “common understanding” is more elusive than you might think. For example, pretty much every physicist has some familiarity with Quantum Field Theory…but that can mean anything from “uses it every day” to “saw it a couple times back in grad school”. Too much background, and half your audience is bored. Too little, and half your audience is lost. You have to strike the proper balance, trying to show everyone enough to feel satisfied.

There are tricks to make this easier. I’ve noticed that some of the best speakers begin with a clever and unique take on something everyone understands. That way, people in very different fields will still have something they recognize, while people in the same field will still be seeing something new. Of course, the tricky part is coming up with a new example in the first place!

In general, I need to get better at estimating where my audience is. Talking to you guys is fun, but I ought to also practice a “physics voice” for discussions with physicists (as well as grants and applications), and an “amplitudes voice” for fellow specialists. The key to communication, as always, is knowing your audience.

A Nobel for Blue LEDs, or, How Does That Count as Physics?

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When I first heard about this year’s Nobel Prize in Physics, I didn’t feel the need to post on it. The prize went to Isamu Akasaki, Hiroshi Amano and Shuji Nakamura, whose discoveries enabled blue LEDs. It’s a more impressive accomplishment than it might seem: while red LEDs have been around since the 60’s and 70’s, blue LEDs were only developed in the 90’s, and only with both can highly efficient, LED-based white light sources be made. Still, I didn’t consider posting on it because it’s pretty much entirely outside my field.

Shiny, though

It took a conversation with another PI postdoc to point out one way I can comment on the Nobel, and it started when we tried to figure out what type of physicists Akasaki, Amano, and Nakamura are. After tossing around terms like “device physicist” and “condensed matter”, someone wondered whether the development of blue LEDs wasn’t really a matter of engineering.

At that point I realized, I’ve talked about something like this before.

Physicists work on lots of different things, and many of them don’t seem to have much to do with physics. They study geometry and topology, biological molecules and the nature of evolution, income inequality and, yes, engineering.

On the surface, these don’t have much to do with physics. A friend of mine used to quip that condensed matter physicists seem to just “pick whatever they want to research”.

There is something that ties all of these topics together, though. They’re all things that physicists are good at.

Physics grad school gives you a wide variety of tools with which to understand the world. Thermodynamics gives you a way to understand large, complicated systems with statistics, while quantum field theory lets you understand everything with quantum properties, not just fundamental particles but materials as well. This batch of tools can be applied to “traditional” topics, but they’re equally applicable if you’re researching something else entirely, as long as it obeys the right kinds of rules.

In the end, the best definition of physics is the most useful one. Physicists should be people who can benefit from being part of physics organizations, from reading physics journals, and especially from training (and having been) physics grad students. The whole reason we have scientific disciplines in the first place is to make it easier for people with common interests to work together. That’s why Akasaki, Amano, and Nakamura aren’t “just” engineers, and why I and my fellow string theorists aren’t “just” mathematicians. We use our knowledge of physics to do our jobs, and that, more than anything else, makes us physicists.

Edit: It has been pointed out to me that there’s a bit more to this story than the main accounts have let on. Apparently another researcher named Herbert Paul Maruska was quite close to getting a blue LED up and running back in the early 1970’s, getting far enough to have a working prototype. There’s a whole fascinating story about the quest for a blue LED, related here. Maruska seems to be on friendly terms with Akasaki, Amano, and Nakamura, and doesn’t begrudge them their recognition.

Feeling Perturbed?

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You might think of physics as the science of certainties and exact statements: action and reaction, F=ma, and all that. However, most calculations in physics aren’t exact, they’re approximations. This is especially true today, but it’s been true almost since the dawn of physics. In particular, approximations are performed via a method known as perturbation theory.

Perturbation theory is a trick used to solve problems that, for one reason or another, are too difficult to solve all in one go. It works by solving a simpler problem, then perturbing that solution, adjusting it closer to the target.

To give an analogy: let’s say you want to find the area of a circle, but you only know how to draw straight lines. You could start by drawing a square: it’s easy to find the area, and you get close to the area of the circle. But you’re still a long ways away from the total you’re aiming for. So you add more straight lines, getting an octagon. Now it’s harder to find the area, but you’re closer to the full circle. You can keep adding lines, each step getting closer and closer.

And so on.

This, broadly speaking, is what’s going on when particle physicists talk about loops. The calculation with no loops (or “tree-level” result) is the easier problem to solve, omitting quantum effects. Each loop then is the next stage, more complicated but closer to the real total.

There are, as usual, holes in this analogy. One is that it leaves out an important aspect of perturbation theory, namely that it involves perturbing with a parameter. When that parameter is small, perturbation theory works, but as it gets larger the approximation gets worse and worse. In the case of particle physics, the parameter is the strength of the forces involves, with weaker forces (like the weak nuclear force, or electromagnetism) having better approximations than stronger forces (like the strong nuclear force). If you squint, this can still fit the analogy: different shapes might be harder to approximate than the circle, taking more sets of lines to get acceptably close.

Where the analogy fails completely, though, is when you start approaching infinity. Keep adding more lines, and you should be getting closer and closer to the circle each time. In quantum field theory, though, this frequently is not the case. As I’ve mentioned before, while lower loops keep getting closer to the true (and experimentally verified) results, going all the way out to infinite loops results not in the full circle, but in an infinite result instead. There’s an understanding of why this happens, but it does mean that perturbation theory can’t be thought of in the most intuitive way.

Almost every calculation in particle physics uses perturbation theory, which means almost always we are just approximating the real result, trying to draw a circle using straight lines. There are only a few theories where we can bypass this process and look at the full circle. These are known as integrable theories. N=4 super Yang-Mills may be among them, one of many reasons why studying it offers hope for a deeper understanding of particle physics.

The Many (Body) Problems of the Academic Lifestyle

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I’m visiting Perimeter this week, searching for apartments in the area. This got me thinking about how often one has to move in academia. You move for college, you move for grad school, you move for each postdoc job, and again when you start as a professor. Even then, you may not get to stay where you are if you don’t manage to get tenure, and it may be healthier to resign yourself to moving every seven years rather than assuming you’re going to settle down.

Most of life isn’t built around the idea that people move across the country (or the world!) every 2-7 years, so naturally this causes a few problems for those on the academic path. Below are some well-known, and not-so-well-known, problems facing academics due to their frequent relocations:

The two-body problem:

Suppose you’re married, or in a committed relationship. Better hope your spouse has a flexible job, because in a few years you’re going to be moving to another city. This is even harder if your spouse is also an academic, as that requires two rare academic jobs to pop up in the same place. And woe betide you if you’re out of synch, and have to move at different times. Many couples end up having to resort to some sort of long-distance arrangement, which further complicates matters.

The N-body problem:

Like the two-body problem, but for polyamorous academics. Leads to poly-chains up and down the East Coast.

The 2+N-body problem:

Alternatively, add a time dimension to your two-body problem via the addition of children. Now your kids are busily being shuffled between incommensurate school systems. But you’re an academic, you can teach them anything they’re missing, right?

The warm body problem:

Of course, all this assumes you’re in a relationship. If you’re single, you instead have the problem of never really having a social circle beyond your department, having to tenuously rebuild your social life every few years. What sorts of clubs will the more socially awkward of you enter, just to have some form of human companionship?

The large body of water problem:

We live in an age where everything is connected, but that doesn’t make distance cheap. An ocean between you and your collaborators means you’ll rarely be awake at the same time. And good luck crossing that ocean again, not every job will be eager to pay relocation expenses.

The obnoxious governing body problem:

Of course, the various nations involved won’t make all this travel easy. Many countries have prestigious fellowships only granted on the condition that the winner returns to their home country for a set length of time. Since there’s no guarantee that anyone in your home country does anything similar to what you do, this sort of requirement can have people doing whatever research they can find, however tangentially related, or trying to avoid the incipient bureaucratic nightmare any way they can.

4 gravitons

Stories about physics from someone who's been there