Reactions from Other Astrophysicists

As Laguna noted, gravitational waves form just one of the pillars of the Center for Relativistic Astrophysics. David Ballantyne and Tamara Bogdanovic depend on light to study the universe, particularly focusing on black holes, galaxies, and stars. Instead of light, Ignacio Taboada observes particles: cosmic rays, neutrinos, and gamma rays.

Taboada: Until recently, and since the dawn of time, light had been humanity’s only astronomical messenger. Then researchers discovered neutrinos, which can be studied in particularly dense regions where light cannot escape. That’s because neutrinos are ghost-like: they can go through almost everything without being stopped. I’m a member of the IceCube collaboration, which has discovered astrophysical neutrinos that likely originated far away from our galaxy. Just like neutrinos, gravitational waves will allow us to peek deep into very violent phenomena where no light is emitted.

Map of sky, from Earth’s perspective, showing the probable locations of the binary black hole merger. (Credit: Shane Larson, Northwestern University) Click to enlarge.

Bogdanovic: Scientists studying gravitational waves are concerned about the waves black holes create as they draw closer to each other and collide. Astrophysicists like me fill in the spaces: where did the black holes come from, what pulled them together, and the kinds of galaxies they live in.

Ballantyne: Our field is often referred to as “traditional astronomy,” which is based on the astronomical techniques of the last 100 years or so. I focus on the interaction of radiation produced by growing black holes with gas and dust clouds in galaxies. We use all of this information to figure out the physical processes occurring around black holes and their role in the evolution of their host galaxies. Studying gravitational waves is examining the basic laws of the universe — it’s like looking at the notes of the symphony. Tamara and I try to understand how that symphony affects the audience.

Bogdanovic: That’s right. Gravitational waves tell us about the collision of two black holes. You learn about everything else around them by looking at the light emitted by the surrounding gas and stars in the host galaxy. When you put both light and gravitational waves together, you can place the merging black holes in a broader context of the universe.

Ballantyne: General relativity has been a good description of gravity — it has passed every test people have thrown at it. Although I’m not surprised that they’ve been detected, I’m excited because gravitational waves open a new window of the universe.

Bogdanovic: Each time one of these windows opens, it presents a brand new picture of the universe. Gravitational waves will inevitably bring confirmations of some previous expectations, as well as some surprises that will lead to new discoveries. I’m glad that it’s happening during my professional career because these types of developments don’t come around very often.

Refining Scientific Ideas Adds More Than Polish

Georgia Tech College of Sciences Dean Paul Goldbart is proud of his colleagues and the work they’ve done in the months and years leading to the announcement of gravitational waves. He’s the former chair of the School of Physics and calls the new findings awe-inspiring.

"Why the excitement about gravitational waves? Refining scientific ideas adds more than polish. It can bring us major advances, too. Think of a pair of electric charges exerting forces on one another. That’s Coulomb’s law, an example of action-at-a-distance, with like charges repelling and opposing ones attracting. Faraday’s field idea moved us to a new viewpoint in which charges cause electric fields that permeate space — stronger fields near them and weaker ones farther away — with the electric fields pushing charges. Now for the refinement. The field isn’t just a device to hide action-at-a-distance. It has its own mechanical reality, as we’ll see shortly. What about magnetic fields? They’re also caused by charge — not its presence but its motion (Ampère’s law) — and when magnetic fields change in time they cause electric fields (Faraday again).

James Clerk Maxwell (1831–1879). Date of photo unknown. Source: Wikimedia Commons.

Here’s where modern physics begins. Just over 150 years ago, reflecting on what was known about electricity and magnetism, James Clerk Maxwell divined a new piece of the puzzle: If changing magnetic fields cause electric fields, perhaps changing electric fields cause magnetic fields? They do! And with this masterstroke, Maxwell uncovered the nature of light: Changing electric fields cause changing magnetic fields cause changing electric fields … in self-sustaining patterns, launched but not maintained by moving charges, that run through space at a speed that Maxwell’s theory settles: the speed of light. Note the progression: action-at-a-distance, fields, electromagnetic radiation. And by surveying the sky at different wavelengths of this radiation — gamma rays, X-rays, light, microwave, radio — these messengers have brought us rich, detailed pictures of the cosmos and its astrophysical inhabitants, ever since the Big Bang.

Now ask what happens if we replace charge by mass. Newton taught us about the force called gravity, which acts between masses — action-at-a-distance again — and in the late 1700s, Laplace introduced the gravitational field concept. Then, in a dramatic refinement motivated by the equality of mass in inertia and gravity, Einstein conjured up his General Theory of Relativity, which holds that mass (and energy) cause space (and time) to warp, and that this warping has its own dynamical reality: Space and time are no longer stage, but actor, it’s sometimes said. And 100 years ago this year, Einstein could echo Maxwell and foresee the possibility of new waves — gravitational radiation shone out into the cosmos as wisp-like ripples not in but of space and time.

This is the elusive gravitational radiation, launched far away in the cosmos by mass in cataclysmic motion, that my Georgia Tech colleagues Laura Cadonati and Deirdre Shoemaker and the LIGO Collaboration seem very likely to have observed directly for the very first time, through a textbook interplay between deeply challenging experiment and cutting-edge computation. Today’s sensation is tomorrow’s calibration, and many physicists are expecting that, before long, we shall be hearing exciting new messages about the cosmos, whispered to us in the language of light’s gravitational cousin.”