Present at the Future

by Ira Flatow

  “You would expect, if you just did a back-of-the-envelope estimate, that the dark energy would be many, many orders of magnitude larger, and the mystery of why it is as small as it is, is really the bone in the throat of today’s fundamental physics. No theory of physics has so far explained this remarkable fact. It’s not mysterious that it should be there. In fact, any reasonable theory predicts it should be there, but it predicts vastly too much of it.”

  It’s easier to eliminate what the dark energy is not rather than to surmise what dark energy is. Dark energy is not anything we know of today.

  “Dark energy is not particles,” says Weinberg. “That’s the dark matter. The dark energy is something smooth. The dark energy is something with negative pressure. And that doesn’t come out of any kind of particles. It comes out of a condition of space itself. It’s an energy in space itself.”

  There’s a concept that’s hard to wrap your mind around: Empty space is not so empty after all. It has energy. And that energy has a force to it. Not an attractive force such as gravity, but a repulsive force that is causing the universe to expand. And if this repulsive force is not astounding enough, consider this bit of information: The dark energy did not show up until a few billion years ago. “The amazing thing is that it wasn’t there from the beginning.” If it had been, says Weinberg, “the universe would have expanded too rapidly to ever form galaxies. Which means no stars, or planets, or life. We wouldn’t have been here.” And if that idea is not enough to make your hair hurt, here’s the kicker: The repulsive energy did not reveal itself until a few billion years ago. Yes, even though the universe is about 14 billion years old, the dark energy did not show up until the universe was about 8 billion years old.

  “On the theoretical side,” says Turner, “we’re very, very excited because this is a big problem, and theorists like big problems. And big problems require new ideas, ideas that at the time may seem crazy. And so both sides, both the astronomers telling us about the acceleration and the theorists making up ideas, are having a great time. What’s really exciting is that it’s a game that we can all play. We don’t have a clue as to what’s causing the universe to accelerate, but we know it’s extremely important and may provide a key to understanding how the forces and particles of nature are unified.”

  THE SEARCH FOR MORE DIMENSIONS

  “Some of the solutions that have been suggested involve the fact that maybe the universe has more than four dimensions, and maybe out there, in those other dimensions, things are happening that are influencing the four dimensions that we’re privy to, and that maybe the speeding up of the expansion has something to do with the existence of other dimensions,” says Dr. Lisa Randall.

  We’re all comfortable with the idea that the world we live in has at most four dimensions: three spatial and one time. Randall has been thinking about a universe with as many as nine extra dimensions, hidden dimensions that we can’t perceive. She is a professor of theoretical physics at Harvard University and the author of Warped Passages: Unraveling the Mysteries of the Universe’s Hidden Dimensions.

  “One of the things that makes it difficult to get it across is that you can’t picture it. We are not physiologically designed to picture more than three dimensions. It doesn’t mean they’re not there, but we certainly can’t just picture them very simply. Really, the right way to understand it is with words or equations. It’s hard to understand it with pictures, because we just can’t see them.”

  So the process of thinking about extra dimensions becomes a mind game, a “thought problem,” as Einstein used to call it.

  “Sometimes I’ll just sit on my sofa and think. Sometimes I’ll just talk to other people and exchange ideas. A lot of the time we’ll have the blackboard filled up with equations or pictures, where we try to have some back-and-forth about ideas, or sit in a coffee shop. Sometimes I will just sit down and work out the equations, sometimes you’ll sort of talk to the equations. You’ll try to figure out, what are they trying to tell you? Or you might have some idea of where you’re going when you use your equations, so where is this all heading?”

  The search for evidence of these dimensions has united the worlds of the very big and the very small, those seeking to understand the universe on a cosmological scale and those seeking to reveal the unseeable, subatomic world of particle physics. Both worlds are looking for evidence of these extra dimensions to satisfy their own hunger to explain how the universe was formed and where it is going.

  “We have quantum mechanics, which describes things with small scales, atomic scales,” says Dr. Randall. “We have general relativity, the theory of gravity, that describes things on big distance scales. And these theories work fine, but at some infinitesimal distance, not a distance we’re going to experience—ten to the minus thirty-three centimeters—the theories are incompatible. That tells us that there’s something wrong with the theory, and we’d really like to have a theory that can describe everything.”

  Ah, yes. The quest for a theory of everything. A theory that would unite all the forces and matter in nature. One of the great problems still challenging physics is trying to unite gravity, which Einstein described as curved space and works over long distances, as Dr. Randall says, with the other forces of nature, which are described as quantum particles—subatomic particles such as quarks, bosons, leptons, and neutrinos. The key to answering this problem is understanding why gravity is so weak compared with the other forces in nature.

  “It is kind of remarkable that you can pick up a paper clip with a magnet when the entire Earth is pulling against it. And from the point of view of an elementary particle, gravity is just completely negligible, compared to the other forces. In fact, it’s very hard to test gravity because the other forces swamp it so much. And it’s even worse than that because in order to unite gravity and quantum physics, “we actually have to introduce such a big fudge in this area that we know there’s something else. And that’s why this question is really driving a lot of research today: How do you unite the two?”

  Can you find particles that describe gravity? The answer to that problem, so far, has eluded just about everyone, from Einstein on. Scientists have been looking for this “theory of everything” for quite a while. “We haven’t yet worked it out,” quips Randall. “We don’t even really know, in many ways, what the theory is.”

  STRING THEORY

  Randall is referring to one of the most widely talked about theories of everything: string theory. It’s the idea that instead of tiny “particles” being the building blocks of everything we see and feel, the universe can be explained by tiny, unseeable “strings” that when “plucked,” vibrate in so many different ways that they create the forces and stuff of nature, even solve the gravity–quantum problem. We’ll explain this in greater detail in the next section. But for now, suffice it to say that string theory requires the existence of all those added dimensions. Perhaps 11 of them or more. And yet even with those extra dimensions, says Randall, string theory does not tie up the loose ends of a theory of everything.

  “In fact, one thing we’ve found is it’s not even just a theory of strings; it has other exotic objects called branes, which are membranelike objects. So the theory clearly has a richness to it, but we don’t know exactly where it’s leading us.”

  “I think it’s important for people to realize that not only has this idea [extra dimensions] cropped up many times,” says Lawrence Krauss, author of Hiding in the Mirror: The Mysterious Allure of Extra Dimensions, from Plato to String Theory and Beyond, “but that in some sense, we’re kind of hard-wired to want there to be more out there than we can see.”

  Krauss is not convinced that string theory will be successful or even necessary to solve problems in physics. Or even that extra dimensions may be necessary to explain our existence.

  “The world is a terrifying place, and I think when humans first evolved, it was clearly terrifying, and the hope that there was some better place where things might be fa
irer was certainly a part of it. And this is a time when science is again looking at this intensively, and serious people are saying there may be ten, eleven, twenty-six dimensions. But maybe the fact that it keeps cropping up even in science has more to do with our psyche than with the universe.”

  Randall disagrees, “We know ideas like extra dimensions and these things called branes (strings that are stretched long and flat) might actually be a part of our universe, and so you can’t just decree that we’re not going to think about it. On the other hand, we do want to make connections to the world. So really what we want are two simultaneous directions. We know this theoretical problem of trying to reconcile gravity and quantum mechanics is there, but we also know that there are phenomena that we don’t understand, even at observable scales, which is why we want to build accelerators like the Large Hadron Collider [LHC] that will be at CERN [European Organization for Nuclear Research] in Geneva.”

  Randall is optimistic that experiments, like those to be conducted at the LHC in 2008, may answer some of the unknowns. The LHC is the world’s biggest atom smasher, or as they say today, particle accelerator, built near Geneva, Switzerland. In it, two beams of subatomic protons will race in opposite directions, underground, in a 27-kilometer-circumference circular tunnel. Gaining enough energy, they will smash into each other at tremendous speeds, in the hopes of producing even more subatomic particles that may answer some of the riddles of nature, such as “Can we find evidence of these extra dimensions?”

  Experiments at CERN will look for evidence of “supersymmetry.” Supersymmetry is a very popular idea suggested by the unification of the forces of nature. It says that for every particle there is a supersymmetry partner. If tests at CERN find evidence of these supersymmetry partners, then it adds weight to the validity of string theory.

  “It doesn’t mean we’ll directly test string theory,” says Randall, “but perhaps we can test ideas that come out of string theory. I think it will give us ideas of where to head with string theory. If we discover double the number of particles, that would tell us something about that symmetry holding to very low energies. If we discover extra dimensions, it would tell us something about what those dimensions look like. So I doubt very much it will tell us whether string theory is right, but I think it could tell us things that might guide string theory research in the future.”

  NOT YOUR FATHER’S TELESCOPE

  While the particle physicists are keeping their fingers crossed on finding evidence of supersymmetry at CERN, cosmologists are betting that they can beat the particle physicists at their own game by using their telescopes to discover evidence of supersymmetry in nature’s own particle colliders: the black holes, energetic hot stars, galaxies, and supernovae of deep space. Even the echoes of the big bang that still reverberate around the universe—the dark microwave background radiation—offers clues.

  “One thing we would all probably agree on is that there are deep connections between the elementary particles and the universe,” says Turner. “When we talk about the birth of the universe, we need ideas from the elementary particle physicist, and then we need to turn those ideas around and see how they can be tested with the sky today. And when we look at the solutions to the big questions involving dark matter, we hope to produce [it] at an accelerator. So it’s not just telescopes. It’s also accelerators.”

  Unlike theorists who hang out by their equation-filled blackboards and think heavy thoughts, the experimentalists actually do the heavy lifting. They must search for evidence in the cosmos that would back up the repulsive nature that the theorists are predicting. “The observers now have the ability to probe the acceleration of the universe,” says Turner.

  Some of the tools astronomers are using are the “way back” machines of astronomy: telescopes. In practical usage, a telescope is really a time machine, says Turner. “It allows you to look out in the universe, and as you look out, you look back in time. And so you can compare what the expansion rate is today with what it was back then.”

  But the telescopes they use are not the kind you use in your backyard to look at the moon. “Some of them will use X-ray telescopes.” Some will use telescopes that can see gravity acting as a lens to bend light around galaxies. “Some of them are going to use the microwave background to get at this question of acceleration.”

  The microwave background radiation is the “echo” left over from the big bang. A NASA satellite, the WMAP (Wilkinson Microwave Anisotropy Probe), hovering in space between the Earth and the sun, has been able to peer back in time to almost the beginning of our universe, detect this very faint microwave remnant, and produce a detailed picture of its early history. “It amazes me that we can say anything about what transpired within the first trillionth of a second of the universe, but we can,” said Charles L. Bennett, WMAP principal investigator and a professor in the Henry A. Rowland Department of Physics and Astronomy at Johns Hopkins University. “We have never before been able to understand the infant universe with such precision.”

  So far, in a world where no one knows just what the dark matter or energy is, Dr. Neta Bahcall, professor of astrophysics at Princeton University, says the observations are “remarkable,” because they are all consistent. “All the different observations from different methods—from weighing the universe to measuring distant supernova, to the microwave background radiation—all of those very different methods yield the same result. They all show that we made dark matter in the same amount as we expected. They all are consistent with having some amount of dark energy, and that’s why it is so exciting. If you talk to the astronomers, we are just jumping for joy, and really jumping in our seats because it is a very exciting time to see that the data is all getting together so beautifully. What is a dark matter? What is a dark energy? What does it all mean for physics? We’re going to have answers in the next ten years or so.”

  DARK MATTER: AXIONS, WIMPS, AND MACHOS?

  Some of those answers may come not from looking up into the heavens but from looking down into the basements of giant particle accelerators. In their search for the solutions to the super-big questions of the universe, physicists are probing the super-tiny world of the atom, looking for the particles, about whose existence they can only speculate.

  Among the most popular candidates are axions—cold particles predicted to have been created in abundance during the big bang. Axions were named after a laundry product by a copredictor of the particle’s existence, Dr. Frank Wilczek. He believed that their presence would help “clean up” some of the problems in theoretical physics. Wimps are another creative solution to the dark energy problem. The term is short for “weakly interacting massive particles.” Wimps theoretically cannot be seen directly because they do not interact electromagnetically or with atomic nuclei. But because they are cold, massive particles that would tend to clump together, wimps fit the bill for cold dark matter. All we have to do is prove they exist.

  Finally, of course, if you have wimps, then you need their alter ego: machos, “massive compact halo objects.” (Those physicists are such cards!) A macho would be a clump of normal atomic matter, such as protons or neutrons, that float unseen through the halos around galaxies. They might be black holes, neutron stars, brown dwarfs…yada yada yada. You get the idea; the possibilities are many.

  “With all of the instruments we have, two satellites to map the echo of the big bang, the Hubble Space Telescope, the Chandra X-ray Observatory, and NASA’s next-generation space telescope [the James Webb Space Telescope] that will have ten times the light-collecting power,” Turner thinks that cosmologists will have the riddle of the dark matter and energy solved within a decade. “Some of them will use X-ray telescopes. Some of them will use telescopes in a very intriguing way where they look at how the distribution of matter between us and the distant universe is bent or gravitationally lensed. Some of them are going to use the microwave background to get at this question of acceleration. I can’t predict the exact date, but in the next ten
years we’re actually going to be able to answer these difficult questions.”

  “As we use this high technology to zero in on every little detail of the matter in the universe, we’ll see that infrastructure in stunning detail, and that will reflect the type of dark matter,” says Dr. Andreas Albrecht, professor of physics at the University of California, Davis.

  “An important point to make is that the problem with dark matter is not a particle physics problem,” says Dr. Bernard Sadoulet, professor of physics and a director of the Center for Particle Astrophysics at the University of California, Berkeley. “It is an astrophysical problem, which may point to questions of deep solutions for the particle physics problem. But it’s really the merging of these two fields, of cosmology and particle physics.”

  “Discovering supersymmetry has been the holy grail for a number of years of the particle physicists,” notes Turner, “and it looks like the cosmologists—I’ve got my fingers crossed here—might beat them to it, because it might be that our galaxy is held together by super-symmetric particles, and so I’m putting my money on people like Bernard, that they’re actually going to beat the LHC to finding supersymmetry. But as Sadoulet says, in the end, both the astrophysical evidence and the evidence in the laboratory will make it a much richer picture.”

  This is a remarkable idea: that the largest things we can imagine—galaxies stretching millions of light-years—started out as small, subatomic particles. “And even more remarkable than that,” says Turner, is that “we can test it. And the answer seems to be written across the microwave sky that the biggest things did evolve from the smallest things.”

  CHAPTER EIGHT

  STRING THEORY: WE HAVE A PROBLEM

  String theorists don’t make predictions, they make excuses.