GRAVITY-WAVE TEST
The strings in string theory are so tiny—about a billionth of a billionth the size of a proton—they can be conjured up only in our imagination. The smallness of the strings means we should look for evidence of them shortly after the Big Bang, when the entire universe was extremely small. The vibration of strings in that early era should have created ripples in gravity, or gravitational waves, that resonated across the universe at the speed of light. String theory predicts the frequencies of such waves. If we observe gravity waves and find that their frequency does not match what string theory predicts, the whole idea would be thrown into doubt.
Nobody has yet detected a gravitational wave, but not for a lack of trying. The new Laser Interferometer Gravitational Wave Observatory, housed in two sprawling facilities in Louisiana and Washington State, went online in 2002. Scientists are still calibrating the equipment and increasing its sensitivity; they are hopeful that, in the coming years, the observatory will detect gravitational waves for the first time.
In about eight years, NASA and the European Space Agency plan to launch the Laser Interferometer Space Antenna, called LISA. It consists of three satellites orbiting the sun. They will be linked by three laser beams, forming a triangle of light whose sides are each 3 million miles long. The satellites are designed to detect a change in their spacing as small as one-tenth the diameter of a single atom. In theory, a gravity wave passing by would change the contours of space between the satellites, altering how the laser beams combine with each other in a measurable way.
Gravity waves should be generated by many sources, including colliding black holes and exploding stars, but LISA should also be able to detect waves created immediately after the birth of the cosmos. Earlier satellites such as the Wilkinson Microwave Anisotropy Probe detected microwave energy left over from the Big Bang, showing what the infant universe looked like when it was roughly 300,000 years old. LISA should be able to peer back in time much earlier—to one-trillionth of a second after the Big Bang.
Results from LISA might allow physicists to distinguish between different theories about what happened immediately after, and even before, the moment when the universe went bang. A leading cosmological model, known as inflation, predicts that our universe is just one part of a greater multiverse and that our Big Bang may have been one of many. In this model, our universe expanded extremely rapidly during the first fraction of a second of its existence. Another theory, rooted in string theory, envisions a scenario in which the Big Bang occurred as a result of the collision between two parallel universes floating in higher-dimensional space.
These theories may seem fantastic, but they each predict a specific pattern of gravity waves emitted from the Big Bang. LISA might be able to distinguish between some of them, offering an empirical test of conditions that existed when the universe began 13.7 billion years ago. Even if LISA is not sensitive enough to perform this test, then the betting among physicists is that its successors will be. If the signals LISA and its successors pick up are those expected by string theorists, they will verify that some version of string theory is the correct quantum theory of gravity.
PARTICLE-ACCELERATOR TEST
Impatient physicists may not have to wait for LISA to find out whether string theorists are on the right track. In just two years, the world’s most powerful particle accelerator, the Large Hadron Collider, will begin operation outside Geneva. It will smash high-energy protons into one another in a scenario somewhat analogous to shooting two watches out of cannons at each other to find out what they are made of. By sorting through the debris momentarily created by the colliding protons, string theorists hope to find massive particles that have never been seen before.
According to string theory, familiar particles such as protons, neutrons, and electrons represent the lowest vibration mode of a string—the lowest octave, in a sense. Other, higher-pitched vibration modes should produce related but substantially more massive families of particles, dubbed superparticles, or sparticles. String theory predicts that all subatomic particles have such partners. For example, the electron should have a superpartner dubbed the selectron, while each quark has a superpartner called a squark. No one has yet detected a sparticle, perhaps because existing particle accelerators are too feeble.
Some physicists expect the Large Hadron Collider to be powerful enough to reveal sparticles. The heart of the collider is a 17-mile-long circular tunnel straddling the border of France and Switzerland. There, two beams of protons will circulate in opposite directions. When engineers flip a switch in 2007, a 12,000-ampere pulse of electrical power will slam down huge coils of electromagnets, creating fields 100,000 times more powerful than Earth’s. The magnets will bend particles along a circular path as they accelerate to 99.999999 percent the speed of light and attain energies approaching 14 trillion electron volts, trillions of times more powerful than the energy released by dynamite.
Before the Large Hadron Collider goes hunting for sparticles, it will first test the boundaries of the standard model of particle physics, the reigning theory of how subatomic particles behave (see “Catch Me if You Can” by Karen Wright, Discover, July 2005). The standard model is perhaps the most successful quantum theory, explaining every subatomic interaction witnessed so far, but it merely whets the appetite of string theorists. They believe the standard model is contrived, ugly, and incomplete because it contains at least 19 adjustable parameters, three near-identical copies of subatomic particles, and no description of gravity.
Superstring theory holds that the standard model describes only the lowest vibration modes of the strings. In this view, the standard model does a good job describing the world we know, yet it is unfinished. Nevertheless, the standard model has worked as a viable theory for decades. The discovery of sparticles would mark its first failure to adequately explain the tiny quantum world and would unleash an avalanche of new tests by experimental particle physicists, who sometimes deride string theory as too abstract. Sparticles would not, however, seal the deal on string theory. Some physics theories explain the existence of sparticle-like particles without resorting to strings.
The Large Hadron Collider could support string theory in other ways. For instance, it might create miniature black holes predicted by one version of the theory; these in turn would produce telltale showers of subatomic particles as they disintegrated. (Physicists say the black holes are so small they pose no danger of swallowing up Switzerland and the rest of Earth.) The collider may also be powerful enough to test one of the most bizarre predictions of string theory—that there are many dimensions out there. Recent versions of string theory predict there are actually seven spatial dimensions beyond the three that we can sense. Collisions at the Large Hadron Collider might be able to knock subatomic particles into one of the other dimensions, batting them right out of our three-dimensional ballpark. The missing mass and energy, or the decay products of the higher-dimension particles themselves, could then be detected by the Large Hadron Collider’s sensors.
