Gravitational waves were detected for the first time this past February by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which consists of two widely separated installations within the United States — one in Livingston, Louisiana and one in Hanford, Washington. Before LIGO, there was no technology able to detect these vanishingly weak waves. Consequently, LIGO’s accomplishment has generated considerable excitement in the physics and astronomy communities. First, it confirmed the existence of gravity waves; a key prediction of Einstein’s 1915 theory of general relativity. Second, and remarkably, LIGO detected gravitational waves that were emitted during the final fraction of a second of the merger of two black holes, which were over a billion light-years away (one light-year is about 5.88 trillion miles)! What’s more, LIGO’s findings in fact proved the existence of black holes. Prior to LIGO’s achievement, the existence of black holes was widely accepted, but based only on indirect evidence.
Physicists and astronomers are also excited by the potential of gravitational-wave detectors to shed light on other basic concerns, such as determining whether gravitational waves travel at the speed of light—an important issue since it would answer whether gravity is transmitted by particles having no mass. These detectors may also enable astronomers to measure the rate at which the universe is expanding, and perhaps observe the effect of dark energy on space.
Most exciting perhaps, gravitational wave detectors may enable astronomers to see almost all the way back to the big bang. Until now, astronomers could only see as far back as 380,000 years after the big bang, when the universe became transparent to light and other electromagnetic radiation. However, gravitational waves would have traveled unhindered through the newborn universe. By scrutinizing gravitational waves from the infant universe, cosmologists hope to learn more about its beginning and, perhaps, even uncover evidence for the existence of other universes. Moreover, gravitational wave detectors might even lead to a “theory of everything (1).”
Scientists from other disciplines, as well as lay people, might very well marvel at the sheer ingenuity and persistence of the physicists and engineers who designed LIGO; a result of decades of instrument research and development. But first, here is a very brief account of gravity waves.
Einstein’s theory of general relativity predicts that matter emits gravity waves. These waves disturb the fabric of space, in fact causing the distances between objects to ebb and flow in an oscillatory manner. However, these oscillations are far too small to have been detected prior to LIGO.
Here is Lawrence M. Krauss’ account in the New York Times of the LIGO technical achievement (2). “To see these waves, the experimenters built two mammoth detectors, one in Washington State, the other in Louisiana, each consisting of two tunnels about 2.5 miles in length at right angles to each other. By shooting a laser beam down the length of each tunnel and timing how long it took for each to be reflected off a mirror at the far end, the experimenters could precisely measure the tunnels’ length. If a gravitational wave from a distant galaxy traverses the detectors at both locations roughly simultaneously, then at each location, the length of one arm would get smaller, while the length of the other arm would get longer, alternating back and forth …To detect the signal they observed they had to be able to measure a periodic difference in the length between the two tunnels by a distance of less than one ten-thousandth the size of a single proton. It is equivalent to measuring the distance between the earth and the nearest star with an accuracy of the width of a human hair….If the fact that this is possible doesn’t astonish, then read these statements again. This difference is so small that even the minuscule motion in the position of each mirror at the end of each tunnel because of quantum mechanical vibrations of the atoms in the mirror could have overwhelmed the signal. But scientists were able to resort to the most modern techniques in quantum optics to overcome this.” See Asides 1 and 2.
[Aside 1: Lawrence M. Krauss is a theoretical physicist and director of the Origins Project at Arizona State University. He is the author of “A Universe from Nothing: Why There is Something Rather than Nothing.”]
[Aside 2: Interestingly, the LIGO detectors had just been turned on for their first observing run when they discovered a clear signal emanating from the colliding black holes. Also, recall that these black holes were over a billion light-years away.]
Krauss later says, “Too often people ask, what’s the use of science like this, if it doesn’t produce faster cars or better toasters. But people rarely ask the same question about a Picasso painting or a Mozart symphony. Such pinnacles of human creativity change our perspective of our place in the universe. Science, like art, music and literature, has the capacity to amaze and excite, dazzle and bewilder. I would argue that it is that aspect of science — its cultural contribution, its humanity — that is perhaps its most important feature (2).”
Also, consider the following from an editorial in the New York Times. “The curiosity of our species knows no bounds; more remarkably, neither does our capacity for satisfying it. And that is truly wonderful in itself, even if it doesn’t lead to a better toaster (3).” See Aside 3.
[Aside 3: The development of LIGO was made possible by support from the National Science Foundation. “By coincidence, at about the same time that the LIGO discovery was announced, the U.S. House of Representatives passed a bill requiring that National Science Foundation grants be justified ‘in the national interest.’ It is doubtful that LIGO would have survived such political meddling (3).”]
- “The Theory of Everything,” Posted on the blog September 15, 2015.
- Lawrence M. Krauss, Finding Beauty in the Darkness, Opinion in Sunday Review, New York Times, February 14, 2016.
- The Editorial Board, New York Times, February 17, 2016