On Physics: Religion, Science & Extra Dimensions

In her new book, Knocking on Heaven's Door, Harvard University theorist Lisa Randall explores how physics may transform our understanding of the fundamental nature of the world. She thinks an extra dimension may exist close to our familiar reality, hidden except for a bizarre sapping of the strength of gravity as we see it. She also ponders the makeup of dark matter, unseen particles that have shaped the growth of the entire cosmos. These ideas, once the sole province of fiction writers, face real tests in a new generation of experiments. Sensitive detectors now sniff for dark matter, while the most complex scientific machine ever created, the Large Hadron Collider (LHC), beneath the border of Switzerland and France, smashes subatomic particles into one another at almost the speed of light.

What were your main goals for your new book?

One goal was to describe the science I'm interested in today: the physics happening at the LHC and searches for dark matter. But I also wanted to clarify the nature of science: what it means to be right and wrong, what it means to make measurements, and the roles of uncertainty, risk and creativity.

Do you feel the physics community is on the verge of finding something remarkable?

I certainly hope so. We have a good chance [with the LHC] to see the Higgs particle, which tells us how elementary particles acquire mass. Other deep issues include space-time symmetry and whether there are extra dimensions. We really do have a chance of making inroads on those subjects. There are a lot of bizarre ideas here, from string theory to a "brane" of extra dimensions right next to our own.

Why should we regard these ideas as more than fanciful constructs?

I'm certainly not asking anyone to take on faith any of the ideas that I present. That's part of the point of the book: science proceeds, and we systematically end up with new ideas and explanations, going from the human scales we're very familiar with to scales that are so remote it's hard to have intuition about them. Science is a self-correcting process, too, something that I expect will happen with the recent announcement of neutrinos that may move faster than the speed of light.

Can you describe the essence of your idea about extra dimensions?

There could be more to the universe than the three dimensions we are familiar with. They are hidden from us in some way, perhaps because they're tiny or warped. But even if they're invisible, they could affect what we actually observe in the universe. There are lots of things we cannot see with the naked eye that turn out to be based in reality. Extra dimensions could be relevant to one of the questions we're trying to answer at the LHC: how particles get their mass, and why they have the masses that they do, which are far smaller than physicists would expect them to be. So our idea is there's an extra dimension that's so warped, the masses would be big in one place and small in another. In other words, gravity could be weaker in one place and stronger in another. If so, it could be a natural explanation both for why particles masses are what they are, and why gravity is so much weaker than the other elementary forces we observe. This extra dimension could be separated from ours by a million trillion trillionth of a centimeter.

Is this a parallel yet inaccessible universe?

It interacts with our dimensions only via gravity. And gravity is extremely weak. An elementary particle at ordinary energies exerts negligible gravitational force. But at the LHC, if this idea is right, we would see evidence of this extra dimension. Particles could carry momentum into the extra dimension, and that could actually be observable.

But it's not something you think of as a "parallel universe?"

Technically, yes, it could exist parallel to our universe. But it's not just a carbon copy of our universe, which a lot of people think of when they hear that phrase. If physicists do find solid evidence of extra dimensions, how would that affect our view of the universe and our place in it?
You can have very exotic underlying phenomena, but they still would be consistent with the ordinary rules we're familiar with. At some level, it doesn't change anything. However, it means that at some deep underlying level, there's a much richer universe out there. It's just a wonderful thing to know what our universe is made of.

You describe the LHC as a "stupendous achievement."

Technologically, it's a tour de force. The fact that this thing works is amazing. We're looking for very rare events, so you need a very precise, very well understood machine to make them and detectors to understand what you see. You need an extreme amount of energy focused in a very tiny region to make these collisions happen, allowing the subcomponents of protons—quarks and gluons—to collide directly. And when they do, they can make new forms of heavier matter.Many people feared the LHC would produce a planet-devouring black hole. Scientists took it very seriously, and they ruled out this possibility not only theoretically, but also by looking at collisions of cosmic rays that create this same type of energy. We live in a world where there are many risks, and it's high time we start taking seriously which ones we should be worried about. Physicists showed this particular one is not a risk.

You offer a forthright discussion about religion and its compatibility with science. Why did you decide to broach that subject?

I almost had to in a book titled Knocking on Heaven's Door. But there is real confusion about what it means to be right and wrong—the difference between what spiritual beliefs are and what science is. I felt that if I was going to explain science, it was important to explain those distinctions. I wanted to take seriously the different views of the universe that people have, but to say there really are differences.

You wrote: "The religious part of your brain cannot act at the same time as the scientific one. They are simply incompatible.”

When I say they are incompatible, I mean something very specific: A spiritual belief based on something that isn't based on actual material or cause and effect—the ways we understand scientifically—is just different than science. It's a very specific statement.

When you speak to public audiences, which popular misconception about physics strikes you the most?

You're trying to get me in trouble! It's probably the over-application of quantum mechanics. People think it explains things that it can't. There are a lot of mysteries about quantum mechanics, but they mostly arise in very detailed measurements in controlled settings.

You describe the LHC's giant detectors as works of art. Is probing the nature of the universe just as much an aesthetic endeavor as a scientific one?

Art and science do appeal to some of the same creative instincts. There's an appreciation of something larger than ourselves, which I think both art and science address. However, you can have a beautiful idea in science, and it can be just wrong—not because it's mathematically inconsistent, but because it's not realized in the world.

-Robert Irion (“Opening Strange Portals in Physics,” Smithsonian magazine, September 2011. Image from The Twilight Zone, Spiral Staircase.)

The Dark Matters: Colliding & Annihilating In Space...

"Is this the dark side speaking?

A concatenation of puzzling results from an alphabet soup of satellites and experiments has led a growing number of astronomers and physicists to suspect that they are getting signals from a shadow universe of dark matter that makes up a quarter of creation but has eluded direct detection until now. Maybe...

“Nobody really knows what’s going on,” said Gordon Kane, a theorist at the University of Michigan. Physicists caution that there could still be a relatively simple astronomical explanation for the recent observations. But the nature of this dark matter is one of the burning issues of science. Identifying it would point the way to a deeper understanding of the laws of nature and the Einsteinian dream of a unified theory of physics.

The last few weeks have seen a blizzard of papers trying to explain the observations in terms of things like “minimal dark matter” or “exciting dark matter,” or “hidden valley” theory, and to suggest how to look for them in particle accelerators like the Large Hadron Collider, set to begin operation again outside Geneva next summer. “It could be deliriously exciting, an incredibly cool story,” said Nima Arkani-Hamed of the Institute for Advanced Study in Princeton, N.J., who has been churning out papers with his colleagues. “Anomalies in the sky tell you what to look for in the collider.”

On Thursday, a team of astrophysicists working on one of the experiments reported in the journal Nature that a cosmic ray detector onboard a balloon flying around the South Pole had recorded an excess number of high-energy electrons and their antimatter opposites, positrons, sailing through local space. The particles, they conceded, could have been created by a previously undiscovered pulsar, the magnetized spinning remnant of a supernova explosion, blasting nearby space with electric and magnetic fields. But, they say, a better and more enticing explanation for the excess is that the particles are being spit out of the fireballs created by dark matter particles colliding and annihilating one another in space.

“We cannot disprove that the signal could come from an astrophysical object. We also cannot eliminate a dark matter annihilation explanation based upon current data,” said John P. Wefel of Louisiana State University, the leader of the team, adding, “Whichever way it goes, for us it is exciting.”

The results came on the heels of a report earlier this fall from Pamela, a satellite built by Italian, German, Russian and Swedish scientists to study cosmic rays. Pamela scientists reported in talks and a paper posted on the Internet that the satellite had recorded an excess of high-energy positrons. This, they said, “may constitute the first indirect evidence of dark matter particle annihilations,” or a nearby pulsar.

Antimatter is rare in the universe, and so looking for it is a good way of hunting for exotic phenomena like dark matter. Another indication that something funny is happening on the dark side of the universe is evident in maps of the cosmic background radiation left over from the Big Bang. Those maps, produced most recently this year by the Wilkinson Microwave Anisotropy Probe satellite, show a haze of what seem to be charged particles hovering around the Milky Way galaxy, according to an analysis by Douglas Finkbeiner of the Harvard-Smithsonian Center for Astrophysics.

Adding to the mix and mystery, the European Space Agency’s Integral satellite detected gamma rays emanating from the center of the Milky Way, suggesting the presence of positrons there, but with much lower energies than Pamela and Dr. Wefel’s experiments have seen. What all this adds up to, or indeed whether it all adds up to anything at all, depends on which observations you trust and your theoretical presumptions about particle physics and the nature of dark matter. Moreover, efforts to calculate the background level of high-energy particles in the galaxy are beset with messy uncertainties. “The dark matter signal is easy to calculate,” Dr. Kane said. “The background is much harder.”

Dark matter has teased and obsessed astronomers since the 1930s, when the Caltech astronomer Fritz Zwicky deduced that some invisible “missing mass” was required to supply the gravitational glue to hold clusters of galaxies together. The idea became respectable in the 1970s when Vera C. Rubin of the Carnegie Institution of Washington and her collaborators found from studying the motions of stars that most galaxies seemed to be surrounded by halos of dark matter.

The stakes for dark matter go beyond cosmology. The most favored candidates for its identity come from a theory called supersymmetry, which unifies three of the four known forces of nature mathematically and posits the existence of a realm of as-yet-undiscovered particles. They would be so-called wimps — weakly interacting massive particles — which feel gravity and little else, and could drift through the Earth like wind through a screen door. Such particles left over from the Big Bang could form a shadow universe clumping together into dark clouds that then attract ordinary matter.

The discovery of a supersymmetric particle would also be a boost for string theory, the controversial “theory of everything,” and would explicate the nature of a quarter of the universe. But until now, the dark matter particles have mostly eluded direct detection in the laboratory, the exception being a controversial underground experiment called Dama/Libra, for Dark Matter/Large Sodium Iodide Bulk for Rare Processes, under the Italian Alps, where scientists claimed in April to have seen a seasonal effect of a “dark matter wind” as the Earth goes around its orbit.

The sky could be a different story. Dark matter particles floating in the halos around galaxies would occasionally collide and annihilate one another in tiny fireballs of radiation and lighter particles.

Dr. Wefel and his colleagues have been chasing sparks in the sky since 2000, when they flew an instrument known as ATIC, for Advanced Thin Ionization Calorimeter, around Antarctica on a balloon at an altitude of 23 miles, looking for high-energy particles known as cosmic rays raining from space.

The Nature paper includes data from the first two balloon flights. It shows a bump, over theoretical calculations of cosmic ray intensities, at energies of 500 billion to 800 billion electron volts, a measure of both energy and mass in physics. One way to explain that energy bump would be by the disintegration or annihilation of a very massive dark particle. A proton by comparison is about one billion electron volts. Dr. Wefel noted, however, that according to most models, a pulsar could generate particles with even more energy, up to trillions of volts, whereas the bump in the ATIC data seems to fall off at around 800 billion electron volts. The ATIC results, he said, dovetail nicely with those from Pamela, which recorded a rising number of positrons relative to electrons, but only up to energies of about 200 billion electron volts.

Reached in China, where he was attending a workshop, Neal Weiner of New York University, who is working with Dr. Arkani-Hamed on dark matter models, said he was plotting ATIC data gleaned from the Web and Pamela data on the same graph to see how they fit, which was apparently very well. But Piergiorgio Picozza, a professor at the University of Rome and the Pamela spokesman, said in an e-mail message that it was too soon to say the experiments agreed. That will depend on more data now being analyzed to learn whether Pamela continues to see more positrons as the energy rises.

Moreover, as Dr. Kane pointed out, Pamela carries a magnet that allows it to distinguish electrons from positrons — being oppositely charged, they bend in opposite directions going through the magnetic field. But the ATIC instrument did not include a magnet and so cannot be sure that it was seeing any positrons at all: no antimatter, no exotic dark matter, at least at those high energies. But if he is right, Dr. Wefel said that the ATIC data favored something even more exotic than supersymmetry, namely a particle that is lost in the fifth dimension. String theory predicts that there are at least six dimensions beyond our simple grasp, wrapped up so tightly we cannot see them or park in them. A particle in one of these dimensions would not appear to us directly. You could think of it as a hamster running around on a wheel in its cage. We cannot see the hamster or the cage, but we can sort of feel the impact of the hamster running; according to Einsteinian relativity, its momentum in the extra dimension would register as mass in our own space-time. Such particles are called Kaluza-Klein particles, after Theodor Kaluza and Oscar Klein, theorists who suggested such an extra-dimensional framework in the 1920s to unify Einstein’s general theory of relativity and electromagnetism.

Dr. Wefel’s particle would have a mass of around 620 billion electron volts. “That’s the one that seems to fit the best,” he said in an interview. The emergence of a sharp edge in the data, he said, “would be a smoking gun” for such a strange particle. But Dr. Arkani-Hamed said that Kaluza-Klein particles would not annihilate one another at a fast enough rate to explain the strength of the ATIC signal, nor other anomalies like the microwave haze. He and his colleagues, including Dr. Weiner, Dr. Finkbeiner and Tracy Slatyer, also of Harvard, drawing on work by Matthew Strassler of Rutgers, have tried to connect all the dots with a new brand of dark matter, in which there are not only dark particles but also a “dark force” between them.

That theory was called “a delightful castle in the sky” by Dr. Kane, who said he was glad it kept Dr. Arkani-Hamed and his colleagues busy and diverted them from competing with him. Dr. Kane and his colleagues favor a 200 billion-electron-volt supersymmetric particle known as a wino as the dark matter culprit, in which case the Pamela bump would not extend to higher energies. Dr. Wefel said he had not kept up with all the theorizing. “I’m just waiting for one of these modelers to say here is the data, here is the model,” he said. “Fit it out. I’m not sure I’ve seen it yet.” Dr. Picozza said that it was the job of theorists to come up with models and that they were proliferating. “At the end of the story only one will be accepted from the scientific community, but now it is too early."

Sorting all this out will take time, but not forever. “With so many experiments, we will soon know so much more about all of this,” Dr. Weiner said. “In a year or two, we’ll either not be talking about this idea at all, or it will be all we’re talking about.”

- Dennis Overbye (A Whisper, Perhaps, From the Universe’s Dark Side, NY Times, 11.25.08. Image: -Loomis Dean, "Shadows Of The Junior Astronomy Club Peering Through A Telescope, Greeley,CO, Life Magazine, 1955 ).