Science

Greetings from Sihanoukville, Cambodia, or at least the waters immediately off. I’m here as part of Bright Horizons 19, a two-week cruise on the Holland American ship Vollendam, in collaboration with Scientific American. We started in Hong Kong and have been working our way south, stopping a few times in Vietnam, and after this we’ll briefly visit Thailand before finishing in Singapore. A fascinating, once-in-a-lifetime experience, even if two weeks is an amount of time I can’t honestly afford to be taking off. Been getting a touch of work done here and there, but not as much as I would have liked, in between dashes ashore to sample the local cuisine. Although the local cuisine has been pretty spectacular, I have to admit.

My job here is to give a few talks about physics and cosmology to the folks who signed up for the package — a public audience, but the kind of people whose idea of a good time while sailing the South China Sea is hearing talks about molecular biology or world history. Mostly my talks are variations of themes I’ve spoken on frequently before — the Higgs boson, the arrow of time, dark matter and dark energy. But to spice things up I decided to throw in something new, so I wrote up a talk on The Many Worlds of Quantum Mechanics.

And here it is — the slides, at least. The content is roughly based on my explanation in From Eternity to Here, with a few improvements thrown in.

Two basic goals here. One is to introduce QM to people who don’t know much more about it than a vague notion of “uncertainty” or “fluctuations.” And in particular, to focus on the conceptual foundations, rather than any of the other perfectly legitimate angles one could take: the historical development, the calculational basics, the experimental evidence, the role in modern technology, and so on. Hey, it’s my talk, I might as well concentrate on the parts I’m most fascinated by. So there’s a discussion of entanglement and decoherence that is a bit more specific and detailed than one would often get in a talk of this type, even if it is enlivened by silly pictures of cats and dogs.

The second goal was to give a subtle sales pitch for the Many-Worlds interpretation. Really more damage control than full-on hard sell; the very idea of many worlds is so crazy-sounding and counterintuitive that my job is more to let people know that it’s actually quite a natural implication of the formalism, rather than a bit of ad hoc nonsense tacked on by theorists who have become unmoored from reality. I’m happy to bring up the outstanding issues with the approach, but I do want people to know it should be taken seriously.

Comments welcome, especially since I’ve never tried this approach in a talk before. Of course by only seeing the slides you miss all the witty asides, but the basic substance should come through.

Every year we look forward to the Edge Annual Question, and as usual it’s a provocative one: “What scientific idea is ready for retirement?” Part of me agrees with Ian McEwan’s answer, which is to unask the question, and argue that nothing should be retired. Unasking is almost always the right response to questions that beg other questions, but there’s also an argument to be made in favor of playing along, so that’s what I did.

My answer was “Falsifiability.” More of a philosophical idea than a scientific one, but an idea that is bandied about by lazy scientists far more than it is invoked by careful philosophers. Thinking sensibly about the demarcation problem between science and non-science, especially these days, requires a bit more nuance than that.

Modern physics stretches into realms far removed from everyday experience, and sometimes the connection to experiment becomes tenuous at best. String theory and other approaches to quantum gravity involve phenomena that are likely to manifest themselves only at energies enormously higher than anything we have access to here on Earth. The cosmological multiverse and the many-worlds interpretation of quantum mechanics posit other realms that are impossible for us to access directly. Some scientists, leaning on Popper, have suggested that these theories are non-scientific because they are not falsifiable.

The truth is the opposite. Whether or not we can observe them directly, the entities involved in these theories are either real or they are not. Refusing to contemplate their possible existence on the grounds of some a priori principle, even though they might play a crucial role in how the world works, is as non-scientific as it gets.

I’m also partial to Alan Guth’s answer: “The universe began in a low-entropy state.” Of course we all know that our observable universe had a relatively low entropy at the Big Bang; Alan is making the point that the observable universe might not be the whole thing, and the Big Bang might not have been the beginning, so it’s completely possible that the universe as a whole was never in what one might call a “low-entropy” state. Instead, starting from a generic state, entropy could increase in both directions, leading to a two-sided arrow of time. This has been one of my favorite ideas for a while now, and Alan and I are writing a paper with Chien-Yao Tseng that examines toy models with such behavior.

Here are some other interesting/provocative answers, picked unsystematically out of over 100,000 words overall. Remember that the titles are what the person wants to retire, not something they’re in favor of.

• Danny Hillis, Cause and Effect.
• Daniel Dennett, The Hard Problem of Consciousness.
• Nina Jablonski, Race.
• Gary Marcus, Big Data.
• Steve Giddings, Spacetime.
• Buddhini Samarasinghe, Scientists Should Stick to Science.
• David Deutsch, Quantum Jumps.
• Rebecca Newberger Goldstein, Science Makes Philosophy Obsolete.
• Scott Atran, IQ.
• Amanda Gefter, Andrei Linde, and Seth Lloyd all suggest that we get rid of the idea of a unique universe, each from a slightly different perspective. Sorry, universe: the tide is turning against you.

Ari Buchalter is one of the many people who has successfully made the transition from graduate student and researcher in physics (Columbia PhD, Caltech postdoc) to the business world, where he is currently the CEO of MediaMath. But he never lost his interest in theoretical cosmology, which is completely appropriate — how our universe works is something everyone should be interested in, no matter what their day job might be.

In order to promote innovative thinking in cosmology (experimental as well as theoretical), Ari has founded the Buchalter Cosmology Prize, which was just announced at the meeting of the American Astronomical Society. It will be an annual award, given to the best cosmology papers to have appeared on the arxiv, as decided by a panel of esteemed judges. (I’m one of the esteemed judges, which is a mixed blessing — should be a lot of fun, but it means I can’t win.) Any PhD or current graduate student in physics or astronomy is eligible to submit papers for consideration; this year’s deadline is 30 September. The winner will walk away with $10,000, and even third place will bag you $2,500.

Currently, cosmology is in a situation where the dominant theoretical framework (Big Bang, Hubble expansion, dark energy and dark matter, possibly primordial inflation) is pretty darn good at fitting the data, but nevertheless has some worrisome conceptual issues. (Was there really inflation? Is there a multiverse? Is the dark energy a cosmological constant, and is the dark matter a WIMP? Why is the vacuum energy so small? Etc.) Hopefully a prize like this will help spur people to be just a tiny bit more bold and imaginative in tackling these issues than they would otherwise be.

It’s been hard to find time for blogging, but there’s one story I don’t want to let slip by before the end of the year: the observation by Ice Cube of neutrinos from beyond the Solar System.

It was my own bad sense of timing to blog about Ice Cube mere days before they announced this result — but just to mention the fun fact that they confirmed the existence of the Moon. And, like noticing the Moon, there’s a sense in which we shouldn’t be too surprised — we were pretty confident that neutrinos were in fact raining down upon us from the sky all the time. But that’s a bad attitude, because this is a big deal. It’s a new way of looking at the universe, and historically new ways of looking at the universe have always brought us surprises and new insights of one form or another.

The actual process by which Ice Cube determined that they had found cosmic neutrinos is a bit convoluted, so let’s go through it. For one thing, the detector doesn’t “see” neutrinos directly. It sees Cherenkov radiation, which is emitted when a charged particle moves through a medium at a speed faster than the velocity of light in that medium. (Nothing moves faster than light moves in vacuum, but the speed of light in ice is lower than in vacuum.) Neutrinos, you may have figured from the name, are neutral particles, not charged ones. So what you’re actually seeing are events where a neutrino bumps into one of the water molecules in the ice and creates some charged particles.

But most of the neutrinos you detect by this method are not really cosmic. They’re byproducts of cosmic rays — mostly charged particles flying through space at enormous energies, which smash into Earth’s atmosphere, creating neutrinos (and various other particles) along the way. So a cosmic ray interacts with the atmosphere, creating a neutrino, which then interacts with the ice to make charged particles we can observe. Ice Cube sees these “atmospheric neutrinos” all the time; indeed, it makes maps of them. And that’s great, and certainly helps teach us something about cosmic rays. But it would still be cool to find some neutrinos that have themselves made the long journey across the desolate cold of interstellar space. And that’s not easy; even if the detector finds some, they are likely to be swamped by the bountiful atmospheric beasts.

Enter Bert and Ernie.

Those are the colorful names given to two events observed over the last couple of years by Ice Cube. What makes them remarkable is their very high energies; over 30 trillion electron volts (TeV). (Francis Halzen, doyen of the experiment, “takes no responsibility” for the whimsical names.) That’s a lot more than you would expect from atmospheric neutrinos, but right in line for the most energetic cosmic neutrinos we predicted. But it’s only two events; the finding was announced earlier this year, but like good cautious scientists the collaboration didn’t quite say they were sure the events were cosmic in origin. (Note that a “cosmic neutrino” is one that traveled across the cosmos by itself, not one that was produced by a cosmic ray — sorry for the confusing nomenclature, it’s a cosmic world out there.)

Now we can do better. In November, right after my blog post about the Moon, Ice Cube announced that they had more data, and were able to identify another twenty-six events at very high energies. They put the confidence that these are truly cosmic neutrinos at four sigma — perhaps not quite the five-sigma gold standard we would like to reach, but pretty darn convincing (especially where anything astrophysical is concerned).

This result opens up a new era in astronomy. We can now look at the universe with neutrino eyes. Previously we had discovered neutrinos from the Sun, as well as the lucky few from Supernova 1987A, but now we apparently have a persistent source of these elusive particles from very far away. Perhaps from the center of our galaxy, or perhaps from hyper-energetic events in galaxies well outside our own. At the very least this kind of work should teach us something about the origin of cosmic rays themselves, and who knows what else.

I’m not sure whether to feel happy or sorry for Bert and Ernie themselves. Born in a cosmic cataclysm half a universe away, they sped through billions of miles of empty space, witnessing untold astronomical wonders, only to come crashing into the ice on a fairly run-of-the-mill planet. But at least they brought more than a little joy to the hearts of some curious scientists, which is more than most particles can say.