Tag Archives: cosmology

Book Review: We Have No Idea

I have no idea how I’m going to review this book.

Ok fine, I have some idea.

Jorge Cham writes Piled Higher and Deeper, a webcomic with possibly the most accurate depiction of grad school available. Daniel Whiteson is a professor at the University of California, Irvine, and a member of the ATLAS collaboration (one of the two big groups that make measurements at the Large Hadron Collider). Together, they’ve written a popular science book covering everything we don’t know about fundamental physics.

Writing a book about what we don’t know is an unusual choice, and there was a real risk it would end up as just a superficial gimmick. The pie chart on the cover presents the most famous “things physicists don’t know”, dark matter and dark energy. If they had just stuck to those this would have been a pretty ordinary popular physics book.

Refreshingly, they don’t do that. After blazing through dark matter and dark energy in the first three chapters, the rest of the book focuses on a variety of other scientific mysteries.

The book contains a mix of problems that get serious research attention (matter-antimatter asymmetry, high-energy cosmic rays) and more blue-sky “what if” questions (does matter have to be made out of particles?). As a theorist, I’m not sure that all of these questions are actually mysterious (we do have some explanation of the weird “1/3” charges of quarks, and I’d like to think we understand why mass includes binding energy), but even in these cases what we really know is that they follow from “sensible assumptions”, and one could just as easily ask “what if” about those assumptions instead. Overall, these “what if” questions make the book unique, and it would be a much weaker book without them.

“We Have No Idea” is strongest when the authors actually have some idea, i.e. when Whiteson is discussing experimental particle physics. It gets weaker on other topics, where the authors seem to rely more on others’ popular treatments (their discussion of “pixels of space-time” motivated me to write this post). Still, they at least seem to have asked the right people, and their accounts are on the more accurate end of typical pop science. (Closer to Quanta than IFLScience.)

The book’s humor really ties it together, often in surprisingly subtle ways. Each chapter has its own running joke, initially a throwaway line that grows into metaphors for everything the chapter discusses. It’s a great way to help the audience visualize without introducing too many new concepts at once. If there’s one thing cartoonists can teach science communicators, it’s the value of repetition.

I liked “We Have No Idea”. It could have been more daring, or more thorough, but it was still charming and honest and fun. If you’re looking for a Christmas present to explain physics to your relatives, you won’t go wrong with this book.

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Cosmology, or Cosmic Horror?

Around Halloween, I have a tradition of posting about the “spooky” side of physics. This year, I’ll be comparing two no doubt often confused topics, Cosmic Horror and Cosmology.

cthulhu_and_r27lyeh

Pro tip: if this guy shows up, it’s probably Cosmic Horror

Cosmic Horror

Cosmology

Started in the 1920’s with the work of Howard Phillips Lovecraft Started in the 1920’s with the work of Alexander Friedmann
Unimaginably ancient universe Precisely imagined ancient universe
In strange ages even death may die Strange ages, what redshift is that?
An expedition to Antarctica uncovers ruins of a terrifying alien civilization An expedition to Antarctica uncovers…actually, never mind, just dust
Alien beings may propagate in hidden dimensions Gravitons may propagate in hidden dimensions
Cultists compete to be last to be eaten by the Elder Gods Grad students compete to be last to realize there are no jobs
Oceanic “deep ones” breed with humans Have you seen daycare costs in a university town? No way.
Variety of inventive and bizarre creatures, inspiring libraries worth of copycat works Fritz Zwicky
Hollywood adaptations are increasingly popular, not very faithful to source material Actually this is exactly the same
Can waste hours on an ultimately fruitless game of Arkham Horror Can waste hours on an ultimately fruitless argument with Paul Steinhardt
No matter what we do, eventually Azathoth will kill us all No matter what we do, eventually vacuum decay will kill us all

Current Themes 2018

I’m at Current Themes in High Energy Physics and Cosmology this week, the yearly conference of the Niels Bohr International Academy. (I talked about their trademark eclectic mix of topics last year.)

This year, the “current theme” was broadly gravitational (though with plenty of exceptions!).

IMG_20180815_180435532

For example, almost getting kicked out of the Botanical Garden

There were talks on phenomena we observe gravitationally, like dark matter. There were talks on calculating amplitudes in gravity theories, both classical and quantum. There were talks about black holes, and the overall shape of the universe. Subir Sarkar talked about his suspicion that the expansion of the universe isn’t actually accelerating, and while I still think the news coverage of it was overblown I sympathize a bit more with his point. He’s got a fairly specific worry, that we’re in a region that’s moving unusually with respect to the surrounding universe, that hasn’t really been investigated in much detail before. I don’t think he’s found anything definitive yet, but it will be interesting as more data accumulates to see what happens.

Of course, current themes can’t stick to just one theme, so there were non-gravitational talks as well. Nima Arkani-Hamed’s talk covered some results he’s talked about in the past, a geometric picture for constraining various theories, but with an interesting new development: while most of the constraints he found restrict things to be positive, one type of constraint he investigated allowed for a very small negative region, around thirty orders of magnitude smaller than the positive part. The extremely small size of the negative region was the most surprising part of the story, as it’s quite hard to get that kind of extremely small scale out of the math we typically invoke in physics (a similar sense of surprise motivates the idea of “naturalness” in particle physics).

There were other interesting talks, which I might talk about later. They should have slides up online soon in case any of you want to have a look.

Adversarial Collaborations for Physics

Sometimes physics debates get ugly. For the scientists reading this, imagine your worst opponents. Think of the people who always misinterpret your work while using shoddy arguments to prop up their own, where every question at a talk becomes a screaming match until you just stop going to the same conferences at all.

Now, imagine writing a paper with those people.

Adversarial collaborations, subject of a recent a contest on the blog Slate Star Codex, are a proposed method for resolving scientific debates. Two scientists on opposite sides of an argument commit to writing a paper together, describing the overall state of knowledge on the topic. For the paper to get published, both sides have to sign off on it: they both have to agree that everything in the paper is true. This prevents either side from cheating, or from coming back later with made-up objections: if a point in the paper is wrong, one side or the other is bound to catch it.

This won’t work for the most vicious debates, when one (or both) sides isn’t interested in common ground. But for some ongoing debates in physics, I think this approach could actually help.

One advantage of adversarial collaborations is in preventing accusations of bias. The debate between dark matter and MOND-like proposals is filled with these kinds of accusations: claims that one group or another is ignoring important data, being dishonest about the parameters they need to fit, or applying standards of proof they would never require of their own pet theory. Adversarial collaboration prevents these kinds of accusations: whatever comes out of an adversarial collaboration, both sides would make sure the other side didn’t bias it.

Another advantage of adversarial collaborations is that they make it much harder for one side to move the goalposts, or to accuse the other side of moving the goalposts. From the sidelines, one thing that frustrates me watching string theorists debate whether the theory can describe de Sitter space is that they rarely articulate what it would take to decisively show that a particular model gives rise to de Sitter. Any conclusion of an adversarial collaboration between de Sitter skeptics and optimists would at least guarantee that both parties agreed on the criteria. Similarly, I get the impression that many debates about interpretations of quantum mechanics are bogged down by one side claiming they’ve closed off a loophole with a new experiment, only for the other to claim it wasn’t the loophole they were actually using, something that could be avoided if both sides were involved in the experiment from the beginning.

It’s possible, even likely, that no-one will try adversarial collaboration for these debates. Even if they did, it’s quite possible the collaborations wouldn’t be able to agree on anything! Still, I have to hope that someone takes the plunge and tries writing a paper with their enemies. At minimum, it’ll be an interesting read!

Bubbles of Nothing

I recently learned about a very cool concept, called a bubble of nothing.

Read about physics long enough, and you’ll hear all sorts of cosmic disaster scenarios. If the Higgs vacuum decays, and the Higgs field switches to a different value, then the masses of most fundamental particles would change. It would be the end of physics, and life, as we know it.

A bubble of nothing is even more extreme. In a bubble of nothing, space itself ceases to exist.

The idea was first explored by Witten in 1982. Witten started with a simple model, a world with our four familiar dimensions of space and time, plus one curled-up extra dimension. What he found was that this simple world is unstable: quantum mechanics (and, as was later found, thermodynamics) lets it “tunnel” to another world, one that contains a small “bubble”, a sphere in which nothing at all exists.

giphy

Except perhaps the Nowhere Man

A bubble of nothing might sound like a black hole, but it’s quite different. Throw a particle into a black hole and it will fall in, never to return. Throw it into a bubble of nothing, though, and something more interesting happens. As you get closer, the extra dimension of space gets smaller and smaller. Eventually, it stops, smoothly closing off. The particle you threw in will just bounce back, smoothly, off the outside of the bubble. Essentially, it reached the edge of the universe.

The bubble starts out small, comparable to the size of the curled-up dimension. But it doesn’t stay that way. In Witten’s setup, the bubble grows, faster and faster, until it’s moving at the speed of light, erasing the rest of the universe from existence.

You probably shouldn’t worry about this happening to us. As far as I’m aware, nobody has written down a realistic model that can transform into a bubble of nothing.

Still, it’s an evocative concept, and one I’m surprised isn’t used more often in science fiction. I could see writers using a bubble of nothing as a risk from an experimental FTL drive, or using a stable (or slowly growing) bubble as the relic of some catastrophic alien war. The idea of a bubble of literal nothing is haunting enough that it ought to be put to good use.

Why Your Idea Is Bad

By A. Physicist

 

Your idea is bad…

 

…because it disagrees with precision electroweak measurements

…………………………………..with bounds from ATLAS and CMS

…………………………………..with the power spectrum of the CMB

…………………………………..with Eötvös experiments

…because it isn’t gauge invariant

………………………….Lorentz invariant

………………………….diffeomorphism invariant

………………………….background-independent, whatever that means

…because it violates unitarity

…………………………………locality

…………………………………causality

…………………………………observer-independence

…………………………………technical naturalness

…………………………………international treaties

…………………………………cosmic censorship

…because you screwed up the calculation

…because you didn’t actually do the calculation

…because I don’t understand the calculation

…because you predict too many magnetic monopoles

……………………………………too many proton decays

……………………………………too many primordial black holes

…………………………………..remnants, at all

…because it’s fine-tuned

…because it’s suspiciously finely-tuned

…because it’s finely tuned to be always outside of experimental bounds

…because you’re misunderstanding quantum mechanics

…………………………………………………………..black holes

………………………………………………………….effective field theory

…………………………………………………………..thermodynamics

…………………………………………………………..the scientific method

…because Condensed Matter would contribute more to Chinese GDP

…because the approximation you’re making is unjustified

…………………………………………………………………………is not valid

…………………………………………………………………………is wildly overoptimistic

………………………………………………………………………….is just kind of lazy

…because there isn’t a plausible UV completion

…because you care too much about the UV

…because it only works in polynomial time

…………………………………………..exponential time

…………………………………………..factorial time

…because even if it’s fast it requires more memory than any computer on Earth

…because it requires more bits of memory than atoms in the visible universe

…because it has no meaningful advantages over current methods

…because it has meaningful advantages over my own methods

…because it can’t just be that easy

…because it’s not the kind of idea that usually works

…because it’s not the kind of idea that usually works in my field

…because it isn’t canonical

…because it’s ugly

…because it’s baroque

…because it ain’t baroque, and thus shouldn’t be fixed

…because only a few people work on it

…because far too many people work on it

…because clearly it will only work for the first case

……………………………………………………………….the first two cases

……………………………………………………………….the first seven cases

……………………………………………………………….the cases you’ve published and no more

…because I know you’re wrong

…because I strongly suspect you’re wrong

…because I strongly suspect you’re wrong, but saying I know you’re wrong looks better on a grant application

…….in a blog post

…because I’m just really pessimistic about something like that ever actually working

…because I’d rather work on my own thing, that I’m much more optimistic about

…because if I’m clear about my reasons

……and what I know

…….and what I don’t

……….then I’ll convince you you’re wrong.

 

……….or maybe you’ll convince me?

 

The Quantum Kids

I gave a pair of public talks at the Niels Bohr International Academy this week on “The Quest for Quantum Gravity” as part of their “News from the NBIA” lecture series. The content should be familiar to long-time readers of this blog: I talked about renormalization, and gravitons, and the whole story leading up to them.

(I wanted to title the talk “How I Learned to Stop Worrying and Love Quantum Gravity”, like my blog post, but was told Danes might not get the Doctor Strangelove reference.)

I also managed to work in some history, which made its way into the talk after Poul Damgaard, the director of the NBIA, told me I should ask the Niels Bohr Archive about Gamow’s Thought Experiment Device.

“What’s a Thought Experiment Device?”

einsteinbox

This, apparently

If you’ve heard of George Gamow, you’ve probably heard of the Alpher-Bethe-Gamow paper, his work with grad student Ralph Alpher on the origin of atomic elements in the Big Bang, where he added Hans Bethe to the paper purely for an alpha-beta-gamma pun.

As I would learn, Gamow’s sense of humor was prominent quite early on. As a research fellow at the Niels Bohr Institute (essentially a postdoc) he played with Bohr’s kids, drew physics cartoons…and made Thought Experiment Devices. These devices were essentially toy experiments, apparatuses that couldn’t actually work but that symbolized some physical argument. The one I used in my talk, pictured above, commemorated Bohr’s triumph over one of Einstein’s objections to quantum theory.

Learning more about the history of the institute, I kept noticing the young researchers, the postdocs and grad students.

h155

Lev Landau, George Gamow, Edward Teller. The kids are Aage and Ernest Bohr. Picture from the Niels Bohr Archive.

We don’t usually think about historical physicists as grad students. The only exception I can think of is Feynman, with his stories about picking locks at the Manhattan project. But in some sense, Feynman was always a grad student.

This was different. This was Lev Landau, patriarch of Russian physics, crowning name in a dozen fields and author of a series of textbooks of legendary rigor…goofing off with Gamow. This was Edward Teller, father of the Hydrogen Bomb, skiing on the institute lawn.

These were the children of the quantum era. They came of age when the laws of physics were being rewritten, when everything was new. Starting there, they could do anything, from Gamow’s cosmology to Landau’s superconductivity, spinning off whole fields in the new reality.

On one level, I envy them. It’s possible they were the last generation to be on the ground floor of a change quite that vast, a shift that touched all of physics, the opportunity to each become gods of their own academic realms.

I’m glad to know about them too, though, to see them as rambunctious grad students. It’s all too easy to feel like there’s an unbridgeable gap between postdocs and professors, to worry that the only people who make it through seem to have always been professors at heart. Seeing Gamow and Landau and Teller as “quantum kids” dispels that: these are all-too-familiar grad students and postdocs, joking around in all-too-familiar ways, who somehow matured into some of the greatest physicists of their era.