Category: Top Posts


By Sean Carroll | November 22, 2012 12:59 pm

This year we give thanks for an idea that is central to our modern understanding of the forces of nature: gauge symmetry. (We’ve previously given thanks for the Standard Model Lagrangian, Hubble’s Law, the Spin-Statistics Theorem, conservation of momentum, effective field theory, and the error bar.)

When you write a popular book, some of the biggest decisions you are faced with involve choosing which interesting but difficult concepts to tackle, and which to simply put aside. In The Particle at the End of the Universe, I faced this question when it came to the concept of gauge symmetries, and in particular their relationship to the forces of nature. It’s a simple relationship to summarize: the standard four “forces of nature” all arise directly from gauge symmetries. And the Higgs field is interesting because it serves to hide some of those symmetries from us. So in the end, recognizing that it’s a subtle topic and the discussion might prove unsatisfying, I bit the bullet and tried my best to explain why this kind of symmetry leads directly to what we think of as a force. Part of that involved explaining what a “connection” is in this context, which I’m not sure anyone has ever tried before in a popular book. And likely nobody ever will try again! (Corrections welcome in comments.)

Physicists and mathematicians define a “symmetry” as “a transformation we can do to a system that leaves its essential features unchanged.” A circle has a lot of symmetry, as we can rotate it around the middle by any angle, and after the rotation it remains the same circle. We can also reflect it around an axis down the middle. A square, by contrast, has some symmetry, but less — we can reflect it around the middle, or rotate by some number of 90-degree angles, but if we rotated it by an angle that wasn’t a multiple of 90 degrees we wouldn’t get the same square back. A random scribble doesn’t have any symmetry at all; anything we do to it will change its appearance.

That’s not too hard to swallow. One layer of abstraction is to leap from symmetries of a tangible physical object like a circle to something a bit more conceptual, like “the laws of physics.” But it’s a leap well worth making! Read More


Time-Reversal Violation Is Not the "Arrow of Time"

By Sean Carroll | November 20, 2012 10:01 am

Looks like the good folks at the BaBar experiment at SLAC, feeling that my attention has been distracted by the Higgs boson, decided that they might be able to slip a pet peeve of mine past an unsuspecting public without drawing my ire. Not so fast, good folks at BaBar!

They are good folks, actually, and they’ve carried out an extremely impressive bit of experimental virtuosity: obtaining a direct measurement of the asymmetry between a particle-physics process and its time-reverse, thereby establishing very direct evidence that the time-reversal operation “T” is not a good symmetry of nature. Here’s the technical paper, the SLAC press release, and a semi-popular explanation by the APS. (I could link you to the Physical Review Letters journal server rather than the arxiv, but the former is behind a paywall while the latter is free, and they’re the same content, so why would I do that? [Update: the PRL version is available free here, but not from the PRL page directly.])

The reason why it’s an impressive experiment is that it’s very difficult to directly compare the rate of one process to its precise time-reverse. You can measure the lifetime of a muon, for example, as it decays into an electron, a neutrino, and an anti-neutrino. But it’s very difficult (utterly impractical, actually) to shoot a neutrino and an anti-neutrino directly at an electron and measure the probability that it all turns into a muon. So what you want to look at are oscillations: one particle turning into another, which can also convert back. That usually doesn’t happen — electrons can’t convert into positrons because charge is conserved, and they can’t convert into negatively-charged pions because energy and lepton number are conserved, etc. But you can get the trick to work with certain quark-antiquark pairs, like neutral kaons or neutral B mesons, where the particle and its antiparticle can oscillate back and forth into each other. If you can somehow distinguish between the particle and antiparticle, for example if they decay into different things, you can in principle measure the oscillation rates in each direction. If the rates are different, we say that we have measured a violation of T reversal symmetry, or T-violation for short.

As I discuss in From Eternity to Here, this kind of phenomenon has been measured before, for example by the CPLEAR experiment at CERN in 1998. They used kaons and anti-kaons, and watched them decay into different offspring particles. If the BaBar press release is to be believed there is some controversy over whether that was “really” was measuring T-violation. I didn’t know about that, but in any event it’s always good to do a completely independent measurement.

So BaBar looked at B mesons. I won’t go into the details (see the explainer here), but they were able to precisely time the oscillations between one kind of neutral B meson, and the exact reverse of that operation. (Okay, tiny detail: one kind was an eigenstate of CP, the other was an eigenstate of flavor. Happy now?)

They found that T is indeed violated. This is a great result, although it surprises absolutely nobody. There is a famous result called the CPT theorem, which says that whenever you have an ordinary quantum field theory (“ordinary” means “local and Lorentz-invariant”), the combined operations of time-reversal T, parity P, and particle/antiparticle switching C will always be a good symmetry of the theory. And we know that CP is violated in nature; that won the Nobel Prize for Cronin and Fitch in 1980. So T has to be violated, to cancel out the fact that CP is violated and make the combination CPT a good symmetry. Either that, or the universe does not run according to an ordinary quantum field theory, and that would be big news indeed.

All perfectly fine and glorious. The pet peeve only comes up in the sub-headline of the SLAC press release: “Time’s quantum arrow has a preferred direction, new analysis shows.” Colorful language rather than precise statement, to be sure, but colorful language that is extremely misleading. Read More

CATEGORIZED UNDER: arxiv, Science, Time, Top Posts

Top Ten Amazing Higgs Boson Facts!

By Sean Carroll | November 13, 2012 10:53 am

To celebrate the publication of The Particle at the End of the Universe, here’s a cheat sheet for you: mind-bending facts about the Higgs boson you can use to impress friends and prospective romantic entanglements.

1. It’s not the “God particle.” Sure, people call it the God particle, because that’s the name Leon Lederman attached to it in a book of the same name. Marketing genius, but wildly inaccurate. (Aren’t they all God’s little particles?) As Lederman and his co-author Dick Teresi explain in the first chapter of their book, “the publisher wouldn’t let us call it the Goddamn Particle, though that might be a more appropriate title, given its villainous nature and the expense it is causing.”

2. Nobel prizes are coming. But we don’t know to whom. The idea behind the Higgs boson arose in a number of papers in 1963 and 1964. One by Philip Anderson, one by Francois Englert and Robert Brout (now deceased), two by Peter Higgs, and one by Gerald Guralnik, Richard Hagen, and Tom Kibble. By tradition, the Nobel in Physics is given to three people or fewer in any one year, so there are hard choices to be made. (Read Chapter 11!) The experimental discovery is certainly Nobel-worthy as well, but that involves something like 7,000 people spread over two experimental collaborations, so it’s even more difficult. It’s possible someone associated with the actual construction of the Large Hadron Collider could win the prize. Or someone could convince the Nobel committee to ditch the antiquated three-person rule, and that person could be awarded the Peace Prize.

3. We’ve probably discovered the Higgs, but we’re not completely sure. We’ve discovered something — there’s a new particle, no doubt about that. But like any new discovery, it takes time (and in this case, more data) to be absolutely sure you understand what you’ve found. A major task over the next few years will be to pin down the properties of the new particle, and test whether it really is the Higgs that was predicted almost five decades ago. It’s better if it’s not, of course; that means there’s new and exciting physics to be learned. So far it looks like it is the Higgs boson, so it’s okay to talk as if that’s what we’ve discovered, at least until contrary evidence comes in.

4. The Large Hadron Collider is outrageously impressive. The LHC, the machine in Geneva, Switzerland, that discovered the Higgs, is the most complicated machine ever built. (Chapter 5.) It’s a ring of magnets and experimental detectors, buried 100 meters underground, 27 kilometers in circumference. It takes protons, 100 trillion at a time, and accelerates them to 99.999999% the speed of light, then smashes them together over 100 million times per second. The beam pipe through which the protons travel is evacuated so that its density is lower than you would experience standing on the Moon, and the surrounding superconducting magnets are cooled to a temperature lower than that of intergalactic space. The total kinetic energy of the protons moving around the ring is comparable to that of a speeding freight train. To pick one of countless astonishing numbers out of a hat, if you laid all the electrical cable in the LHC end-to-end it would stretch for about 275,000 kilometers, enough to wrap the Earth almost seven times.

5. The LHC was never going to destroy the world. Remember that bit of scaremongering? People were worried that the LHC would create a black hole that would swallow the Earth, and we would all die. (It was never quite explained why the physicists who built the machine would be willing to sacrifice their own lives so readily.) This was silly, mostly because there’s nothing going on inside the LHC that doesn’t happen out there in space all the time. There was a real setback on September 19, 2008, when a magnet kind of exploded, but nobody was hurt. The current casualty list from the LHC mostly consists of people’s favorite theories of new physics, which are continually being constrained as new data comes in.

6. The Higgs boson isn’t really all that important. The boson is just some particle. What’s important is something called the Higgs mechanism. What really gets people excited is the Higgs field, from which the particle arises. Modern physics — in particular, quantum field theory — tells us that all particles are just vibrations in one field or another. The photon is a vibration in the electromagnetic field, the electron is a vibration in the electron field, and so on. (That’s why all electrons have the same mass and charge — they’re just different vibrations in the same underlying field that fills the universe.) It’s the Higgs field, lurking out there in empty space, that makes the universe interesting. Finding the boson is exciting because it means the field is really there. This is why it’s hard to explain the importance of the Higgs in just a few words — you first have to explain field theory!

7. The Higgs mechanism makes the universe interesting. If it weren’t for the Higgs field (or something else that would do the same trick), the elementary particles of nature like electrons and quarks would all be massless. The laws of physics tell us that the size of an atom depends on the mass of the electrons that are attached to it — the lighter the electrons are, the bigger the atom would be. Massless electrons imply atoms as big as the universe — in other words, not atoms at all, really. So without the Higgs, there wouldn’t be atoms, there wouldn’t be chemistry, there wouldn’t be life as we know it. It’s a pretty big deal.

8. Your own mass doesn’t come from the Higgs. We were careful in the previous point to attribute the mass of “elementary” particles to the Higgs mechanism. But most of the mass in your body comes from protons and neutrons, which are not elementary particles at all. They are collections of quarks held together by gluons. Most of their mass comes from the interaction energies of those quarks and gluons, and would be essentially unchanged if the Higgs weren’t there at all. So without the Higgs, we could still have massive protons and neutrons, although their properties would be very different.

9. There will be no jet packs. People sometimes think that since the Higgs has something to do with “mass,” it’s somehow connected to gravity, and that by learning to control it we might be able to turn gravity on and off. Sadly not true. As above, most of your mass doesn’t come from the Higgs field at all. But even putting that aside, there’s no realistic prospect of “controlling the Higgs field.” Think of it this way: it costs energy to change the value of the Higgs field in any region of space, and energy implies mass (through Einstein’s famous E = mc2). If you were to take a region of space the size of a golf ball and turn the Higgs field off inside of it, you would end up with an amount of mass larger than that of the Earth, and create a black hole in the process. Not a feasible plan. We haven’t been looking for the Higgs because of the promise of future technological applications — it’s because we want to understand how the world works.

10. The easy part is over. The discovery of the Higgs completes the Standard Model; the laws of physics underlying everyday life are completely understood. That’s pretty impressive; it’s a project that we, as a species, have been working on for at least 2,500 years, since Democritus first suggested atoms back in ancient Greece. This leaves plenty of physics that we don’t yet understand, from dark matter to the origin of the universe, not to mention complicated problems like turbulence and neuroscience and politics. Indeed, we’re hoping that studying the Higgs might provide new clues about dark matter and other puzzles. But we do now understand the basic building blocks of the world we immediately see around us. It’s a triumph for human beings; the future history of physics will be divided into the pre-Higgs era and the post-Higgs era. Here’s to the new era!


A Book Full of Particles

By Sean Carroll | November 13, 2012 7:14 am

Publication day! In case it’s slipped your mind, today is the day when The Particle at the End of the Universe officially goes on sale. Books get a bit of a boost if they climb up the Amazon rankings on the first day, so if you are so inclined, today would be the day to click that button. Also: great holiday present for the whole family!

A very nice review by Michael Brooks appeared in New Scientist. (It’s always good to read a review when you can tell the author actually read the book.) Another good one by John Butterworth appeared in Nature, but behind a paywall.

Brief reminder of fun upcoming events:

  • Today (Tuesday the 13th), you can Ask Me Anything over at Reddit, starting at 2pm Eastern, 11am Pacific. Of course you lovely blog readers already know everything worth knowing, but I’m looking forward to dodging personal queries from people around the world. (And hopefully explaining a little physics.)
  • Thursday I’ll be doing an online chat in a platform called Shindig. That’s 6pm Eastern, 3pm Pacific. Fire up your webcam and you can be part of the virtual audience.

FAQ: Yes, you should have no trouble reading and understanding it, no matter what your physics background may be. Yes, there are electronic editions of various forms. Yes, there will also be an audio book, but it’s still being recorded. No, nobody has yet purchased the movie rights; call me. Yes, I know that the Higgs boson is not literally sitting there at the end of the universe. It’s a metaphor; for more explanation, read the book!

Writing this book has been quite an experience. Unlike From Eternity to Here, in this case I wasn’t writing about my own research interests. So for much of the time I was acting like a journalist, talking to the people who really built the Large Hadron Collider and do the experiments there. It’s no exaggeration that I went into the project with an enormous amount of respect for what they accomplished, and came out with enormously more than that. It’s a truly amazing achievement on the part of thousands of dedicated people who are largely anonymous to the outside world. (But for the rest of their lives they get to say “I helped discover the Higgs boson,” which is pretty cool.)

Of course, being who I am, I couldn’t help but take the opportunity to try to explain some physics that doesn’t often get explained. So once you hit the halfway point in the book or so, we start digging into what quantum field theory really is, why symmetry breaking is important, and the fascinating history of how the Higgs mechanism was developed. (I had to restrain myself from going even deeper, especially into issues of spin and chirality, but this is supposed to be a bodice-ripper, not a brain-flattener.) At the end of the book, as a reward, you get to contemplate the role of the internet and bloggers in the changing landscape of scientific communication, as well as all the fun technological breakthroughs that we will get as a result of the Higgs discovery. (I.e., none whatsoever.)

Hope you like reading it as much as I liked writing it.

CATEGORIZED UNDER: Higgs, Personal, Science, Top Posts

South Pole Telescope and CMB Constraints

By Sean Carroll | November 5, 2012 1:12 pm

The South Pole Telescope is a wonderful instrument, a ten-meter radio telescope that has been operating at the South Pole since 2007. Its primary target is the cosmic microwave background (CMB), but a lot of the science comes from observations of the Sunyaev-Zeldovich effect due clusters of galaxies — a distortion of the frequency of CMB photons as they travel through the hot gas of the cluster. We learn a lot about galaxy clusters this way, and as a bonus we have a great way of looking for small-scale structure in the CMB itself.

Now the collaboration has released new results on using SPT observations to constrain cosmological parameters.

A Measurement of the Cosmic Microwave Background Damping Tail from the 2500-square-degree SPT-SZ survey
K. T. Story, C. L. Reichardt, Z. Hou, R. Keisler, et al.

We present a measurement of the cosmic microwave background (CMB) temperature power spectrum using data from the recently completed South Pole Telescope Sunyaev-Zel’dovich (SPT-SZ) survey. This measurement is made from observations of 2540 deg^2 of sky with arcminute resolution at 150 GHz, and improves upon previous measurements using the SPT by tripling the sky area. We report CMB temperature anisotropy power over the multipole range 650<ell<3000. We fit the SPT bandpowers, combined with the results from the seven-year Wilkinson Microwave Anisotropy Probe (WMAP7) data release, with a six-parameter LCDM cosmological model and find that the two datasets are consistent and well fit by the model. Adding SPT measurements significantly improves LCDM parameter constraints, and in particular tightens the constraint on the angular sound horizon theta_s by a factor of 2.7…[abridged]

Here is the first plot anyone should look for in a paper like this: Read More

CATEGORIZED UNDER: arxiv, Science, Top Posts

Wal Sargent

By Sean Carroll | October 30, 2012 3:48 pm

I’m very sad to report that Wallace Sargent, a distinguished astronomer at Caltech, died yesterday. Wal, as he was known, was a world leader in spectroscopy and extragalactic astronomy, with a specialty in studies of quasar absorption lines. He played a crucial role in numerous major projects in astronomy, including serving as the director of the Palomar Observatory. He was awarded numerous major awards, including the Bruce Medal, the Helen B. Warner Prize, the Henry Norris Russell Lectureship, and the Dannie Heineman Prize for Astrophysics.

A glance at Wal’s home page will quickly reveal that he led an active an extraordinarily productive life. Those who knew him, however, will remember a warm and enthusiastic personality who was always happy to talk. He mentored numerous students, and contributed greatly to the spirit of Caltech’s fantastically successful astronomy program. Our thoughts to out to his wife Anneila (also a distinguished Caltech astronomer) and all his friends and family.

CATEGORIZED UNDER: Personal, Science, Top Posts

Nudging Naturalism Just a Bit Forward

By Sean Carroll | October 30, 2012 7:17 am

Greetings from our fifteenth floor hotel room in Boston, where yesterday’s Hurricane Sandy maelstrom has relaxed to a dreary gray calm. The storm was a fierce illustration of the power of Nature — completely different from the power of Naturalism, which is what I spent the last few days discussing with some of the smartest people I know, at the Moving Naturalism Forward workshop (as mentioned).

For me personally, the workshop was a terrific experience, digging into important and fascinating ideas with a collection of extremely smart people. Some minor disappointments right at the beginning, as Patricia Churchland, Lisa Randall, and Hilary Bok all had to cancel at the last minute due to (happily temporary) medical issues. But we plowed bravely forward, and we had about the right number of people to both represent a variety of specialties and yet keep the gathering intimate enough so that everyone was talking to everyone else. This was not a meeting devoting to cheerleading or rallying the troops; it was a careful, serious, academic discussion about the issues we struggle with among people who share the same basic worldview.

Photo by Massimo Pigliucci. L to R: Jennifer Ouellette, Jerry Coyne, David Poeppel, Alex Rosenberg. At the left you can see bits of Dan Dennett, and Owen Flanagan's gesturing hands.

There have already been some write-ups of the proceedings by Massimo Pigliucci (one, two, three) and Jerry Coyne (one, two, three), so I thought I’d offer mine. But in writing it up I saw the brief impressionistic remarks I originally intended to offer grow into something more sprawling and hard to digest. So I’m splitting it up into a few posts: this one, plus I think three more.

Read More

CATEGORIZED UNDER: Philosophy, Science, Top Posts

From Particles to People: The Laws of Nature and the Meaning of Life

By Sean Carroll | October 22, 2012 6:19 am

That’s the charmingly grandiose title of a talk I gave at The Amazing Meeting this past July, now available online. I hope that the basic message comes through, although the YouTube comments indicate that the nitpicking has already begun in earnest. There’s a rather lot of material to squeeze into half an hour, so some parts are going to be sketchy.

There are actually three points I try to hit here. The first is that the laws of physics underlying everyday life are completely understood. There is an enormous amount that we don’t know about how the world works, but we actually do know the basic rules underlying atoms and their interactions — enough to rule out telekinesis, life after death, and so on. The second point is that those laws are dysteleological — they describe a universe without intrinsic meaning or purpose, just one that moves from moment to moment.

The third point — the important one, and the most subtle — is that the absence of meaning “out there in the universe” does not mean that people can’t live meaningful lives. Far from it. It simply means that whatever meaning our lives might have must be created by us, not given to us by the natural or supernatural world. There is one world that exists, but many ways to talk about; many stories we can imagine telling about that world and our place within it, without succumbing to the temptation to ignore the laws of nature. That’s the hard part of living life in a natural world, and we need to summon the courage to face up to the challenge.

Or at least, so you will hear me opine if you click on the link. Curious as to what people think.

CATEGORIZED UNDER: Humanity, Philosophy, Science, Top Posts

Feynman on Initial Conditions, Evolving Laws, and What We Consider Physics

By Sean Carroll | October 18, 2012 8:02 am

We’ve mentioned before that Richard Feynman was way ahead of his time when it came to the need to understand cosmological initial conditions and the low entropy of the early universe. (Among other things, of course.) Feynman actually wrote three different books in the early 1960′s — in his way of “writing books,” which consisted of giving lectures and having others transcribe them — all of which made a point of discussing this problem. The Character of Physical Law was aimed at a popular audience, the Feynman Lectures on Physics were aimed at undergraduate physics majors, and the Feynman Lectures on Gravitation were aimed at advanced graduate students — and in every case he emphasized that we can only account for the Second Law of Thermodynamics by assuming a low-entropy boundary condition in the past, for which we currently have no reliable explanation. (These days we have a larger number of speculations, but still nothing reliable.)

Here’s a video clip from about ten years afterward, in 1973, where Feynman raises a similar point in a conversation with Fred Hoyle, the accomplished astronomer and a pioneer of the Steady State cosmology. (Thanks to Ronan Mehigan.) They don’t go into details, but Feynman introduces the idea as a kind of meta-issue in physics:

“What, today, do we not consider part of physics, which we may ultimately be part of physics?”

His answer (which should be cued up here at the 7:10 mark) is the initial conditions of the universe, as well as the possibility that the physical laws themselves evolve with time. (Conversation continues for a tiny bit in the followup video. Listen on to hear Feynman explain how he doesn’t like to speculate about things.)

What’s interesting is that now, four decades later, it’s commonplace to address the issue of initial conditions in a scientific context, and even to consider the evolution of local physical laws, as we do with the multiverse and the string theory landscape. I’m not sure what is the precise history of this endeavor, but in the very same year this interview was aired, Collins and Hawking wrote an early paper asking why the universe is isotropic. In 1979, Dicke and Peebles published “The Big Bang Cosmology — Enigmas and Nostrums,” which set out many of the puzzles that Alan Guth would attempt to address with the inflationary universe scenario. When we marry inflation with the idea of a landscape of vacua (whether from string theory or elsewhere), we naturally are led to the idea of an evolving set of local physical laws, raising the possibility that we might be able to actually explain (using the anthropic principle or simple probability arguments) why we observe one set of laws rather than some other. Not that we have, or even seem very close, but the scientific agenda is clear.

So how could we answer Feynman’s question today? What do we not consider part of physics, which someday we might?

CATEGORIZED UNDER: Science, Time, Top Posts

Moving Naturalism Forward

By Sean Carroll | October 11, 2012 11:09 am

I’m very excited about a workshop I’ll be at later this month: Moving Naturalism Forward. By “naturalism” we mean the simple idea that the natural world, obeying natural laws, is all there is. No supernatural realm, spirits, or ineffable dualistic essences affecting what happens in the universe. Clearly the idea is closely related to atheism (I can’t imagine anyone is both a naturalist and a theist), but the focus is on understanding how the world actually does work rather than just rejecting one set of ideas.

Once you accept that we live in a self-contained universe governed by impersonal laws of nature, the hard work has just begun, as we are faced with a daunting list of challenges. The naturalist worldview comes into conflict with our “folk” understanding of human life in multiple ways, and we need to figure out what can be salvaged and what has to go. We’ve identified these particular issues for discussion:

  • Free will. If people are collections of atoms obeying the laws of physics, is it sensible to say that they make choices?
  • Morality. What is the origin of right and wrong? Are there objective standards?
  • Meaning. Why live? Is there a rational justification for finding meaning in human existence?
  • Purpose. Do teleological concepts play a useful role in our description of natural phenomena?
  • Epistemology. Is science unique as a method for discovering true knowledge?
  • Emergence. Does reductionism provide the best path to understanding complex systems, or do different levels of description have autonomous existence?
  • Consciousness. How do the phenomena of consciousness arise from the collective behavior of inanimate matter?
  • Evolution. Can the ideas of natural selection be usefully extended to areas outside of biology, or can evolution be subsumed within a more general theory of complex systems?
  • Determinism. To what extent is the future determined given quantum uncertainty and chaos theory, and does it matter?

(Massimo Pigliucci has already started blogging about some of the questions we’ll be discussing.)

To hash all this out, we’re collecting a small, interdisciplinary group of people to share different perspectives and see whether we can’t agree on some central claims. We have an amazing collection of people Read More

CATEGORIZED UNDER: Humanity, Philosophy, Science, Top Posts

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