Image courtesy of Dana Berry/NASA
How soon might we see hard evidence of gravitational waves from violent events like colliding black holes?
LIGO is a several-stage project. We upgrade the detectors to better and better sensitivity. We are now operating our first detectors, completing the first long search. It’s possible, but not probable, that we already have gravitational waves in the can, that we will see them as we complete the data analysis. In Advanced LIGO, which will begin its searches early in the coming decade, we expect to see a rich plethora of different types of waves, with signals coming in every day or week.
Can you describe briefly how you are going to be able to detect gravitational waves?
When gravitational waves reach the earth, the waves stretch and squeeze space. This is a tiny stretch and squeeze. Far too small to detect with ordinary human senses. We are attempting to detect the gravitational waves by hanging two huge mirrors from wires, each pair of mirrors about two and a half miles apart, and as the waves pass, the mirrors ride on that stretching and squeezing space so they are pushed apart and pulled together, back and forth. We monitor the oscillating separation between the mirrors using a laser beam. Even though these are 25-pound mirrors, their motions are so exquisitely small that they are perturbed quantum-mechanically by our monitoring. And so we have had to devise ways to get a gravity wave signal into these mirrors and through them, without it being contaminated by the quantum jiggling motion of the mirrors. Normally, you only see quantum-mechanical jiggling when you look at objects the size of atoms and molecules, but we are moving into a domain where we see the centers of mass of these big mirrors jiggle quantum-mechanically. We will soon be measuring the motions of the mirrors with a precision that is about the same as the width of the mirrors’ quantum-mechanical wave function, which means that we will see the mirrors behave quantum-mechanically. We will see these human-sized objects behave like atoms behave and molecules behave, which has never, ever been done before.
How is this possible?
My superb experimentalist colleagues are able to do it because they are making such exquisitely accurate measurements. The motions that they are now able to see are at a level of about 1/100,000,000 the size of an atom. The surfaces of the mirrors have bumps and valleys that are the size of a few atoms. And we’re measuring them to an accuracy of almost a billionth of the size of the bumps and valleys. And so you might say: How is it possible to measure that level? The answer is the laser beam is big—something like four inches across—and it averages over huge, huge numbers of these bumps and valleys, and it averages over time. We’re looking for the motion of the centers of the mirrors as they move back and forth about 100 times a second. But the atoms inside the surface of the mirror are oscillating thermally at somewhere around one trillion oscillation per second. So, we average over an enormous number of thermal oscillations and an enormous numbers of atoms—the laser beam does that automatically. And it thereby can actually be sensitive only to the exquisitely tiny average motion of the whole mirror, the so-called center of mass motion.
Besides black holes, what other kinds of objects that are made from warped space-time and create gravity waves?
Well, a neutron star is an example. It’s made partially from nuclear matter and partially from warped space and time. We hope to watch a black hole tear a neutron star apart. We will see the dynamical behavior of the warped space and time around these two objects as one destroys the other. Another example is something called a cosmic string. These are hypothetical cracks in the fabric of space that are thought to have been created at the very beginning of the universe, by the inflationary expansion of fundamental strings—the objects that string theorists tell us everything is made from. Cosmic strings are like cracks in the fabric of space. The geometry around a string is not like that of a flat sheet paper. Instead, space is warped in a conical sort of a way; the circumference around the string is less than pi times diameter. The core of the string is made of fields that have enormous energy that do this warping, and the core has enormous tension, like the tension of a violin string. If you pluck a violin string, waves go traveling along it. Similarly if a cosmic string gets plucked, oscillations travel along it at very high speed—at the speed of light—and they produce gravitational waves as they travel.
Cosmic strings are an idea that comes from string theory, the most studied area of theoretical physics. This area is still a long way from being a coherent unification of quantum and classical physics. Do you think string theory is exciting enough to merit the amount of attention it’s gotten?
Absolutely. I think there’s no question. It shows many signs of being on the right track toward a correct quantum theory of gravity. It has given rise to a number of very important ideas that have a good shot at being correct, such as higher dimensions, such as the possibility of forming mini-black holes at the LHC [Large Hadron Collider, a new particle accelerator that may be up and running next year], and thereby probing higher dimensions. String theory is now beginning to make concrete, observational predictions which will be tested. Claims that it is just theorists playing mental masturbation are, I think, nonsense.
