Category: Science

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

Guest Post: John Preskill on Individual Quantum Systems

By Sean Carroll | October 10, 2012 6:12 pm

In the last post I suggested that nobody should come to these parts looking for insight into the kind of work that was just rewarded with the 2012 Nobel Prize in Physics. How wrong I was! True, you shouldn’t look to me for such things, but we were able to borrow an expert from a neighboring blog to help us out. John Preskill is the Richard P. Feynman Professor of Theoretical Physics (not a bad title) here at Caltech. He was a leader in quantum field theory for a long time, before getting interested in quantum information theory and becoming a leader in that. He is part of Caltech’s Institute for Quantum Information and Matter, which has started a fantastic new blog called Quantum Frontiers. This is a cross-post between that blog and ours, but you should certainly be checking out Quantum Frontiers on a regular basis.

When I went to school in the 20th century, “quantum measurements” in the laboratory were typically performed on ensembles of similarly prepared systems. In the 21st century, it is becoming increasingly routine to perform quantum measurements on single atoms, photons, electrons, or phonons. The 2012 Nobel Prize in Physics recognizes two of the heros who led these revolutionary advances, Serge Haroche and Dave Wineland. Good summaries of their outstanding achievements can be found at the Nobel Prize site, and at Physics Today.

Serge Haroche developed cavity quantum electrodynamics in the microwave regime. Among other impressive accomplishments, his group has performed “nondemolition” measurements of the number of photons stored in a cavity (that is, the photons can be counted without any of the photons being absorbed). The measurement is done by preparing a Rubidium atom in a superposition of two quantum states. As the Rb atom traverses the cavity, the energy splitting of these two states is slightly perturbed by the cavity’s quantized electromagnetic field, resulting in a detectable phase shift that depends on the number of photons present. (Caltech’s Jeff Kimble, the Director of IQIM, has pioneered the development of analogous capabilities for optical photons.)

Dave Wineland developed the technology for trapping individual atomic ions or small groups of ions using electromagnetic fields, and controlling the ions with laser light. His group performed the first demonstration of a coherent quantum logic gate, and they have remained at the forefront of quantum information processing ever since. They pioneered and mastered the trick of manipulating the internal quantum states of the ions by exploiting the coupling between these states and the quantized vibrational modes (phonons) of the trapped ions. They have also used quantum logic to realize the world’s most accurate clock (17 decimal places of accuracy), which exploits the frequency stability of an aluminum ion by transferring its quantum state to a magnesium ion that can be more easily detected with lasers. This clock is sensitive enough to detect the slowing of time due to the gravitational red shift when lowered by 30 cm in the earth’s gravitational field.

With his signature mustache and self-effacing manner, Dave Wineland is not only one of the world’s greatest experimental physicists, but also one of the nicest. His brilliant experiments and crystal clear talks have inspired countless physicists working in quantum science, not just ion trappers but also those using a wide variety of other experimental platforms.

Dave has spent most of his career at the National Institute of Standards and Technology (NIST) in Boulder, Colorado. I once heard Dave say that he liked working at NIST because “in 30 years nobody told me what to do.” I don’t know whether that is literally true, but if it is even partially true it may help to explain why Dave joins three other NIST-affiliated physicists who have received Nobel Prizes: Bill Phillips, Eric Cornell, and “Jan” Hall.

I don’t know Serge Haroche very well, but I once spent a delightful evening sitting next to him at dinner in an excellent French restaurant in Leiden. The occasion, almost exactly 10 years ago, was a Symposium to celebrate the 100th anniversary of H. A. Lorentz’s Nobel Prize in Physics, and the dinner guests (there were about 20 of us) included the head of the Royal Dutch Academy of Sciences and the Rector Magnificus of the University of Leiden (which I suppose is what we in the US would call the “President”). I was invited because I happened to be a visiting professor in Leiden at the time, but I had not anticipated such a classy gathering, so had not brought a jacket or tie. When I realized what I had gotten myself into I rushed to a nearby store and picked up a tie and a black V-neck sweater to pull over my levis, but I was under-dressed to put it mildly. Looking back, I don’t understand why I was not more embarrassed.

Anyway, among other things we discussed, Serge filled me in on the responsibilities of a Professor at the College de France. It’s a great honor, but also a challenge, because each year one must lecture on fresh material, without repeating any topic from lectures in previous years. In 2001 he had taught quantum computing using my online lecture notes, so I was pleased to hear that I had eased his burden, at least for one year.

On another memorable occasion, Serge and I both appeared in a panel discussion at a conference on quantum computing in 1996, at the Institute for Theoretical Physics (now the KITP) in Santa Barbara. Serge and a colleague had published a pessimistic article in Physics Today: Quantum computing: dream or nightmare? In his remarks for the panel, he repeated this theme, warning that overcoming the damaging effects of decoherence (uncontrolled interactions with the environment which make quantum systems behave classically, and which Serge had studied experimentally in great detail) is a far more daunting task than theorists imagined. I struck a more optimistic note, hoping that the (then) recently discovered principles of quantum error correction might be the sword that could slay the dragon. I’m not sure how Haroche feels about this issue now. Wineland, too, has often cautioned that the quest for large-scale quantum computers will be a long and difficult struggle.

This exchange provided me with an opportunity to engage in some cringe-worthy rhetorical excess when I wrote up a version of my remarks. Having (apparently) not learned my lesson, I’ll quote the concluding paragraph, which somehow seems appropriate as we celebrate Haroche’s and Wineland’s well earned prizes:

“Serge Haroche, while a leader at the frontier of experimental quantum computing, continues to deride the vision of practical quantum computers as an impossible dream that can come to fruition only in the wake of some as yet unglimpsed revolution in physics. As everyone at this meeting knows well, building a quantum computer will be an enormous technical challenge, and perhaps the naysayers will be vindicated in the end. Surely, their skepticism is reasonable. But to me, quantum computing is not an impossible dream; it is a possible dream. It is a dream that can be held without flouting the laws of physics as currently understood. It is a dream that can stimulate an enormously productive collaboration of experimenters and theorists seeking deep insights into the nature of decoherence. It is a dream that can be pursued by responsible scientists determined to explore, without prejudice, the potential of a fascinating and powerful new idea. It is a dream that could change the world. So let us dream.”

CATEGORIZED UNDER: Guest Post, Science, Top Posts

Nobel Prize to Haroche and Wineland

By Sean Carroll | October 9, 2012 8:32 am

Nobody comes to these parts (at least, they shouldn’t) looking for insight into atomic physics, quantum optics, and related fields, but hearty congratulations to Serge Haroche and David Wineland for sharing this year’s Nobel Prize in Physics. Here are helpful stories by Alex Witze and Dennis Overbye.

One way of thinking about their accomplishments is to say that they’ve managed to manipulate particles one at a time: Haroche with individual photons, and Wineland with trapped ions. But what’s really exciting is that they are able to study intrinsically quantum-mechanical properties of the particles. For a long time, quantum mechanics could be treated as a black box. You had an atomic nucleus sitting there quietly, not really deviating from your classical intuition, and then some quantum magic would occur, and now you have several decay products flying away. The remoteness of the quantum effects themselves is what has enabled physicists to get away for so long using quantum mechanics without really understanding it. (Thereby enabling such monstrosities as the “Copenhagen interpretation” of quantum mechanics, and its unholy offspring “shut up and calculate.”)

These days, in contrast, we can no longer refuse to take quantum mechanics seriously. The experimentalists have brought it up close and personal, in your face. We’re using it to build things in ways we wouldn’t have imagined in the bad old days. This prize is a great tribute to physicists who are dragging us, kicking and screaming, into a quantum-mechanical reality.


Digging Up the Early Universe

By Sean Carroll | October 2, 2012 8:31 am

I wrote another column for Discover (the actual magazine), which is now available online. It’s about how far back in cosmological time we can push our knowledge on the basis of actual data, not mere theory.

Of course we literally look back in time every time we peer into a telescope, since it takes time for light to travel to us from distant objects. But there’s an earliest moment we can possibly see using light — the moment of recombination, about 380,000 years after the Big Bang, when electrons hooked up with protons and other nuclei to form atoms. Earlier than that, the electrons were floating around freely, bumping into photons, and generally making the universe opaque.

So we have to be a bit more clever. And we have been: using the fact that the early universe was a nuclear fusion reactor, and observing the surviving abundances of light elements to pin down what conditions were like at that time. This technique gets us within seconds of the Big Bang. But if things break just right — the dark matter turns out to be a weakly-interacting particle, whose properties we can study here on Earth — we might be able to push the data-informed era much earlier back than that.

Think about what that means: Sitting here on Earth, cosmologists extrapolated our understanding back 13.7 billion years, to a few seconds after the universe began. We used that understanding to make predictions about the current universe—and we were right. We may not know for sure whether it will rain tomorrow, but we do know exactly how protons and neutrons bounced around like Super Balls in the nuclear inferno of the Big Bang. This will surely go down as one of the most impressive accomplishments of the human intellect.

And yet cosmologists want to do better still. The goal is to discover relics that predate even Big Bang Nucleosynthesis. At the moment that’s not quite possible, but there is one promising candidate: dark matter, the dense but unseen stuff that holds galaxies together.

Roughly speaking, if we get lucky, we could learn about conditions in the universe about 1/10,000th of a second after the Big Bang. We’d like to go even much earlier than that, but let’s not forget to be impressed at how well we’ve already done.


You are getting sleeeeeeepy…..

By Julianne Dalcanton | October 1, 2012 1:33 pm

Ikeguchi Laboratories has posted one of the most fantastic “physics in action” videos I’ve seen in a long time:

The concept is simple — 32 metronomes on a table, all set to the same tempo, but started at slightly different times. But here’s the fun bit — although they begin “out of phase“, after about 2 minutes, they all lock onto the same phase and synchronize! (Well, almost all — there’s a rebel on the far right that takes an extra minute to get with the program).

So what’s going on? The key is that the metronomes are not on a solid table, but instead are on a slightly flexible platform hanging from a string. Thus, as a metronome’s pendulum rod changes direction, it imparts a small force to the platform, which leads to small motions in the platform. The moving platform then gives small nudges back to the metronomes. These forces will tend to push the other metronomes to speed up or slow down to match the timing of the original metronome, bringing the metronomes “in phase”.

Now the really fun bit (for me at least), was watching exactly how this played out in practice. If you watch the video closely, you’ll see that the synchronization does not happen all at once, nor does it happen randomly. Instead, the synchronization tends to take place first in pairs, with adjacent metronomes locking onto the same phase. This behavior makes a lot of sense, because the strongest forces on a metronome will initially be from its nearest neighbors, at least until enough metronomes are in phase that you start getting a large scale coherent swaying of the platform (which starts to happen about a minute in, becoming increasingly strong during the next minute). The pairs are also more likely to be oriented along the rows (side-by-side), reflecting the direction in which the metronomes cause the platform to move.

The other phenomena you can notice is that adjacent pairs will frequently spend quite a bit of time “180 degrees out of phase” (i.e., showing totally opposite behavior from the neighboring pair — going “tick” exactly when the other goes “tock”). If you watch one of these sets, after happily going along for a while, the equilibrium will shift, and the pairs start to change their tempo. The relative phases will shift, and will gradually drag the pair that’s 180 degrees out of phase back in line with the rest of them, before reverting back to the natural frequency of the metronome. This behavior is probably clearest in the “rebel on the right”, which is in a quasi-stable equilibrium, and spends an extra minute beating a syncopated tempo.

So, lots of interesting stuff to see in a remarkably simple set-up!

(Ed: In comments, Andy Rundquist linked to a post of his analyzing an earlier metronome video, along with bonus Mathematica code, along with a link to a much older publication about modeling the system.)

(Ed: Aaaaand now Paul Gribble (in comments) has just checked relevant Python code into Github.  Y’all are nuts.  My kind of nuts, but nuts.)


Guest Post: Joe Polchinski on Black Holes, Complementarity, and Firewalls

By Sean Carroll | September 27, 2012 2:55 pm

If you happen to have been following developments in quantum gravity/string theory this year, you know that quite a bit of excitement sprang up over the summer, centered around the idea of “firewalls.” The idea is that an observer falling into a black hole, contrary to everything you would read in a general relativity textbook, really would notice something when they crossed the event horizon. In fact, they would notice that they are being incinerated by a blast of Hawking radiation: the firewall.

This claim is a daring one, which is currently very much up in the air within the community. It stems not from general relativity itself, or even quantum field theory in a curved spacetime, but from attempts to simultaneously satisfy the demands of quantum mechanics and the aspiration that black holes don’t destroy information. Given the controversial (and extremely important) nature of the debate, we’re thrilled to have Joe Polchinski provide a guest post that helps explain what’s going on. Joe has guest-blogged for us before, of course, and he was a co-author with Ahmed Almheiri, Donald Marolf, and James Sully on the paper that started the new controversy. The dust hasn’t yet settled, but this is an important issue that will hopefully teach us something new about quantum gravity.


Thought experiments have played a large role in figuring out the laws of physics. Even for electromagnetism, where most of the laws were found experimentally, Maxwell needed a thought experiment to complete the equations. For the unification of quantum mechanics and gravity, where the phenomena take place in extreme regimes, they are even more crucial. Addressing this need, Stephen Hawking’s 1976 paper “Breakdown of Predictability in Gravitational Collapse” presented one of the great thought experiments in the history of physics. Read More

CATEGORIZED UNDER: Guest Post, Science, Top Posts

Why String Theory?

By Sean Carroll | September 21, 2012 8:41 am

Breathless press reports notwithstanding, string theory is very far from being dead. If you’re interested in what it is and what’s going on within the field, I can recommend a new website called Why String Theory? (And of course, accompanying twitter feed @WhyStringTheory.) It was set up by Oxford undergraduates Charlotte Mason and Edward Hughes, working under Joseph Conlon. It’s a very engaging and professional-looking site, featuring a great deal of explanatory material.

Developing pedagogical sites like this is a great project for undergrads; the only looming issue is keeping the site going once the students move on to bigger and better things. Hopefully this one is kept up — I think an initial surge of interest has already been taxing the poor web server.

CATEGORIZED UNDER: Internet, Science

Time Talk

By Sean Carroll | September 18, 2012 2:14 pm

With The Particle at the End of the Universe scheduled to come out in November, most of the popular-level talks I’ll be giving in the near future will have to do with the LHC and the Higgs boson — and quantum field theory, as part of my secret agenda to get QFT accepted as part of the mainstream pop-sci vocabulary. So I’ll be giving fewer talks about the arrow of time, at least near-term. I thought I’d commemorate the occasion by sharing the slides I used for a recent version of this talk: “The Origin of the Universe and the Arrow of Time.” Not that I’m by any stretch done talking about it, but hopefully the next time the occasion arises I’ll have the energy to make up new slides from scratch.


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