The Tantalizing Quest For Gravity Waves
Jason Lederman
at 11:28 AM Feb 12 2016
The Tantalizing Quest For Gravity Waves
Laser antenna
Bill Bourne/ Popular Science archives

Today, February 11, 2020, LIGO scientists announced they had detected gravitational waves in September 2015—the first direct evidence of the cosmic inflation that created our universe. "The Tantalizing Quest For Gravity Waves," written by Arthur Fisher and originally published in the April 1981 issue of Popular Science, explores the international effort to detect these ripples in space-time.

In the vast reaches of the cosmos, cataclysms are a commonplace: Something momentous is always happening. Perhaps the blazing death of an exhausted sun, or the collision of two black holes, or a warble deep inside a neutron star. Such an event spews out a torrent of radiation bearing huge amounts of energy. The energy rushes through space, blankets our solar system, sweeps through the Earth . . . and no one notices.

But there is a small band of experimenters, perhaps 20 groups worldwide, scattered from California to Canton, determined that some day they will notice. Pushed to the edge of contemporary technology and beyond, battling the apparent limits of natural law itself, they are developing what will be the most sensitive antennas ever built. And eventually, they are sure, they will detect these maddeningly intangible phenomena—gravity waves.

Even though gravity waves (more formally called gravitational radiation) have never been directly detected, virtually the entire scientific community is convinced they exist. This assurance stems, in part, from the bedrock on which gravity-wave notions are founded: Albert Einstein's theory of general relativity, which, though still being tested, remains untoppled. Says Caltech astrophysicist Kip Thorne, "I don't know of any respectable expert in gravitational theory who has any doubt that gravity waves exist. The only way we could be mistaken would be if Einstein's general relativity theory were wrong and if all the competing theories were also wrong, because they also predict gravity waves."

In 1916, Einstein predicted that when matter accelerated in a suitable way, the moving mass would launch ripples in the invisible mesh of space-time, tugging momentarily at each point in the universal sea as they passed by. The ripples—gravity waves—would carry energy and travel at the speed of light.

In many ways, this prediction was analogous to one made by James Clerk Maxwell, the brilliant British physicist who died in the year of Einstein's birth—1879. Maxwell stated that the acceleration of an electric charge would produce electromagnetic radiation—a whole gamut of waves, including light, that would all travel at the same constant velocity. His ideas were ridiculed by many of his contemporaries. But a mere decade after his death, he was vindicated when Heinrich Hertz both generated and detected radio waves in the laboratory.

Why, then, more than 60 years after Einstein's bold forecast, has no one seen a gravity wave? Why, despite incredible obstacles, are physicists still seeking them in a kind of modern quest for the Holy Grail, one of the most exciting in the whole history of science?

To find out, I visited experimenters who are building gravity-wave detectors and theoreticians whose esoteric calculations guide them. In the process, I learned about the problems, and how the attempts to solve them are already producing useful spinoffs. And I learned about the ultimate payoff if the quest is successful: a new and potent tool for penetrating, for the first time, what one physicist has called "the most overwhelming events in the universe."

A kiss blown across the Pacific

The fundamental problem in gravity-wave detection is that gravity as a force is feeble in the extreme, some 40 orders of magnitude weaker than the electromagnetic force. (That's 1040, or a 1 followed by 40 zeros.)

Partly for this reason, and partly because of other properties of gravity waves, they interact with matter very weakly, making their passage almost imperceptible. And unlike the dipole radiation of electromagnetism, gravitational radiation is quadrupole.

The fundamental problem in gravity-wave detection is that gravity as a force is feeble in the extreme.

If a gravity wave generated, for example, by a supernova in our galaxy passed through the page you are now reading, the quadrupole effect would first make the length expand and the width contract (or vice versa), and then the reverse. But the amount of energy deposited in the page would be so infinitesimal that the change in dimension would be less than the diameter of a proton. Trying to detect a gravity wave, then, is like standing in the surf at Big Sur and listening for a kiss blown across the Pacific.

As for generating detectable waves on Earth, a la Hertz, theoreticians long ago dismissed the possibility. "Sure, you make gravity waves every time you wave your fist," says Rainer Weiss, a professor of physics at MIT. "But anything you will ever be able to detect must be made by massive bodies moving very fast. That means events in space." Astrophysicists have worked up whole catalogs of such events, each associated with gravity waves of different energy, different characteristic frequencies, and different probabilities of occurrence. They include the supposed continuous background gravitational radiation of the "big bang" that began the universe, and periodic events like the regular pulses of radiation emitted by pulsars and binary systems consisting of superdense objects. And then there are the singular events: the births of black holes in globular clusters, galactic nuclei, and quasars; neutron-star quakes; and supernovas.

Probably the prime candidate for detection is what William Fairbank, professor of physics at Stanford University, calls "the most dramatic event in the history of the universe"—a supernova. As a star such as our sun ages, it converts parts of its mass into nuclear energy, perhaps one percent in five billion years. "The only reason a large star like the sun doesn't collapse," explains Fairbank, "is because the very high temperature in its core generates enough pressure to withstand gravitational forces. But as it cools from burning its fuel, the gravitational forces begin to overcome the electrical forces that keep its particles apart. It collapses faster and faster, and if it's a supernova, the star's outer shell blasts off. In the last thousandth of a second, it collapses to a neutron star, and if the original star exceeded three solar masses, maybe to a black hole."

One way of characterizing the energy of a gravity wave is the strain it induces in any matter it impinges on. If the mass has a dimension of a given length, then the strain equals the change in that length (produced by the gravity wave) divided by the length. Gravity waves have very, very tiny strains. A supernova occurring in our galaxy might produce a strain on Earth that would shrink or elongate a 100-cm-long detector only one one-hundredth the diameter of an atomic nucleus. (That is 10-15 cm, and physicists would label the strain as 10-17.) To the credit of tireless experimenters, there are detectors capable of sensing that iota of a minim of a scruple.

But there is a catch: Based on observations of other galaxies, a supernova can be expected to occur in the dense center of any given galaxy roughly about once in 30 years. That is a depressingly long interval. Over and over again, the scientists I spoke to despaired of doing meaningful work if it had to depend on such a rara avis. Professor David Douglass of the University of Rochester told me: "To build an experiment to detect an event once every 30 years—maybe—is not a very satisfying occupation. It's hardly a very good Ph.D. project for a graduate assistant; it's not even a good career project—you might be unlucky."

Gravity waves: powerful astronomical tools?

What if we don't confine ourselves to events in our own galaxy, but look farther afield? Instead of the "hopelessly rare" (in the words of one researcher) supernova in our galaxy, what if we looked for them in a really large arena— the Virgo cluster, which has some 2,500 galaxies, where supernovas ought to be popping from once every few days to once a month or so? That's Catch-222. The Virgo cluster is about 1,000 times farther away than the center of our own galaxy. So a supernova event from the cluster would dispatch gravity waves whose effect on Earth would be some million times weaker (1,000 times 1,000, according to the inverse-square law governing all radiative energy). And that means building a detector a million times more sensitive. "There is no field of science," says Ronald Drever of Caltech and the University of Glasgow, Scotland, "where such enormous increases in sensitivity are needed as they are here, in gravity-wave detection." Trying to detect a supernova in a distant galaxy means having to measure a displacement one-millionth the size of an atomic nucleus.

Paradoxically, it is this very quality that gives gravity waves the ability to be, as Kip Thorne says, "a very powerful tool for astronomy. True, they go through a gravity-wave detector with impunity. But that means the gravity waves generated during the birth of a black hole can also get away through all the surrounding matter with impunity." And neither light, nor gamma rays, nor radio waves can. During a supernova we can see the exploding shell via showers of electromagnetic radiation, but only hours or days after the initial massive implosion—the gravitational collapse. During the collapse, while a neutron star or black hole is being formed, nothing but gravity waves (and, theoretically, neutrinos) can escape.


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