Women explore the frontiers of physics

Lisa Randall
String theory can make sense out of the tangles in our theories of the universes — if, that is, you can assume the universe has more than the three spatial dimensions and the one time dimension that we can perceive. Harvard theoretical physicist Lisa Randall is trying to figure out how to make sense out of string theory.

"If string theory has lots of dimensions, where are the other ones?" she asked.

One possibility is that the extra dimensions are rolled up into scales so incredibly compact that they can't be measured. Randall said she and her colleagues have proposed another possibility: an "infinite extra dimension" that blends in with the others except at very small scales.

The five-dimensional theory could answer a question that has bedeviled physicists since the days of Einstein: Why is gravity so much weaker than the other fundamental forces of physics? Perhaps it's because much of the gravity field leaks away into another realm, or "brane," via the extra dimension, Randall said.

Randall surveys the frontiers of physics in "Warped Passages," a book to be published later this year. "I didn't want to write a book about being a woman in physics," Randall said. "But it's fun, because sometimes there are analogies that just work better."

Univ. of Chicago Lisa Randall studies extradimensional cosmology.

For example, in the book she refers to the story of "The Princess and the Pea" when discussing the discovery of quarks, and talks about indirectly sensing the closeness of another brane-universe as if it were covered with perfume.

"Often, what's off-putting [to women in physics] is feeling like they have to suppress other interests or aspects of their personality," she said. "They have to do the physics, they have to do the mathematics. But they don't always feel as if they can indulge their other interests as well. It's important to realize that being good in physics and math doesn't mean you have to conform to a particular stereotype."

Licia Verde

Courtesy of Licia Verde Licia Verde is looking for "cosmic chronometers."

Italian-born Verde was on the team behind the highest-resolution picture ever made of the Big Bang's microwave afterglow, produced by the Wilkinson Microwave Anisotropy Probe, or WMAP. Now the University of Pennsylvania cosmologist is trying to build on those results and figure out how the universe's rate of expansion has changed over time.

To do that, she and her colleagues are using the light observed in certain kinds of galaxies as "cosmic chronometers," matching up their ages with their outward velocities. The technique could show how dark energy's effect has changed over the course of billions of years.

"It seems to work, but of course, the devil is in the details," she said. "Up to now, we haven't spent a penny. We would get this data for free as a byproduct of something else, so this is the perfect situation."

Eva Silverstein

Courtesy of Eva Silverstein Eva Silverstein studies space-time “in the spirit of Einstein.”

Silverstein is a 1999 MacArthur Fellow and a string theorist at Stanford who has joined with a variety of collaborators to study subjects ranging from dark energy and the accelerating universe to cosmic inflation and the fabric of space-time.

The concepts can be dizzying — including references to doughnut-hole "handles" in the space-time continuum that can appear and decay dynamically — but Silverstein said the fundamental questions go back to Einstein's day.

"We study the dynamics of space-time very much in the spirit of Einstein, extended to include string-theoretic and quantum corrections in the framework of string theory," she said. "For example, the topology-changing processes we study in string theory are very much in the spirit of relativity. ... We are still struggling with the physics of accelerated expansion and black holes, both of which are basic aspects of Einstein's theory."

Fotini Markopoulou-Kalamara

Perimeter Institute Fotini Markopoulou-Kalamara develops theories on loop quantum gravity.

Markopoulou-Kalamara, who was born in Greece but now works at the Perimeter Institute in Canada, has played a key role in developing an alternative to string theory, known as loop quantum gravity, or LQG.

Like string theory, LQG seeks to fulfill Einstein's dream of unifying quantum theory and general relativity. But unlike string theory, LQG doesn't dwell on extra rolled-up dimensions of space. Instead, it lays out a mathematical system of loops that interact to form "spin networks," the quantum foundations for the realities that each of us perceive. Markopoulou-Kalamara focuses on how spin networks reflecting the partial views of different observers can be combined to produce a shared perception of the universe.

LQG predicts that there should be some non-Einsteinian anomalies in how light photons travel, based on their energy — and so the theory's proponents hope that future results from NASA's Gamma-ray Large Area Space Telescope, due for launch in 2007, will show whether they're on the right track.

Wendy Freedman

Carnegie Observatories Wendy Freedman is director of the Carnegie Observatories.

Freedman is best-known for leading the effort to calculate the Hubble constant, which describes the universe's expansion rate. After eight years of work, Freedman and her colleagues concluded that the universe is expanding at a rate of 74 kilometers per second per megaparsec, with a 10 percent error margin. Astronomers used that number to estimate that the universe is 13 billion to 14 billion years old.

Today, Freedman is director of the Carnegie Observatories and project leader for the Giant Magellan Telescope, a huge installation that will be built in Chile and begin operations in 2016.

“The Giant Magellan Telescope will allow an unprecedented view of extrasolar planets as well as a window out to the largest scales and back to the earliest moments of the universe,” Freedman said last year.

Angela Olinto

Univ. of Chicago Angela Olinto traces the sources of cosmic rays.

Olinto says she's working with the biggest particle accelerator of them all: the flux of high-energy cosmic rays from far-flung regions of space. As a member of the science team for the international Pierre Auger Observatory, the University of Chicago astrophysicist studies particle phenomena that can reach energies 100 million times higher than those achievable by the Large Hadron Collider, which hasn't even started operations yet.

The Auger Observatory is an array of hundreds of monitoring stations, spread out over the plains of Argentina, that pick up the traces of cosmic-ray particles as they zoom right through Earth's path. Olinto is also helping draw up plans for another cosmic-ray observatory in the Northern Hemisphere.

What's the source of these "cosmic bullets"? They could come from huge black holes rotating around the centers of galaxies. "That's the more mundane possibility," Olinto said. "The more exotic ones would be dark matter decay or some sort of topological deformation in the early universe."

In addition to dark matter and the universe's beginnings, the cosmic-ray measurements provide yet another opportunity to check Einstein's century-old theories. "We are testing special relativity at a regime that no one else can reach," Olinto said.

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