2016 2016: Q&A on the discovery of gravitational waves

Physics professors Robert Fisher and Gaurav Khanna offer their reaction to this discovery being heralded as opening a new window to our universe.

Today the National Science Foundation gathered scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration (LSC) to announce the first discovery of gravitational waves by merging black holes. The historic news comes 100 years after Albert Einstein predicted the existence of gravitational waves. UMass Dartmouth physics professors Robert Fisher and Gaurav Khanna offer their reaction to this discovery being heralded as opening a new window to our universe.

What are gravitational waves? Why are they so hard to detect?

GK: Gravitational waves are traveling waves in space-time itself that were predicted by Einstein in 1916 as one of the consequences of his newly formulated general relativity theory.  They are often referred to as “ripples in the fabric of space-time” and are analogous to the ripples formed on the surface of a pond if the water surface is disturbed. These waves travel at the speed of light.

Gravitational waves are extremely weak. Even when emitted by violent astrophysical events — like the merger of two highly energetic black holes, they are so weak that they require extremely high precision measurement technology for detection on Earth — that technology simply didn’t exist until very recently. The LIGO instrument that has just successfully made this first direct detection, is quite simply a marvel of science and engineering.

To get a sense of the precision measurement needed, for successful detection one needs to be able to make a measurement of the movement of a macroscopic test mass by less than one-thousandth of the size of the nucleus of an atom.

It's been said that gravitational waves can open a new window to our universe? How so?

RF: Black holes have captivated both the public and scientists alike for decades. Nonetheless, the evidence in support of their existence has all been gathered far away from the boundary of the black holes, beyond which nothing -- not even light -- may return. By directly revealing the final moments in the mergers of black holes, the detection of gravitational waves will provide "smoking gun" evidence that black holes do indeed exist.

Moreover, the types of black hole mergers being discussed by the LIGO team are hundreds of times more energetic than even the most energetic stellar explosions which astronomers have seen in their conventional telescopes. The amount of energy released during the final 'death throes' of such black hole mergers is absolutely astonishing -- thousands of times greater than the entire energy output of our sun over its entire lifetime. However, this enormous amount of energy is radiated away in gravitational waves which are pure distortions of space-time, and can only be detected by a gravitational wave telescope like LIGO.

GK: Astronomers have been using light to observe the universe for centuries. It is quite remarkable how much we have learned via this single medium for observation. Gravitational waves offer a completely new view onto the universe. It is sometimes described as being able to “hear” the universe in addition to being able the “see” it. It is therefore akin to opening up a completely new sensory system for making observations.

In particular, it is worth pointing out that there are a number of limitations of light-based astronomy. For example, light is very easy to distort or disrupt. So, light coming from long distances could be highly corrupted due to other astrophysical objects or gases/dust that just got in the way to Earth. However, gravitational waves can essentially pass through anything (stars, planets, etc.) completely undisturbed. In that sense, they are nearly the perfect medium for astronomical observation. In addition, they also carry somewhat different information on the astrophysical system they are emitted from -- for example, they have direct information on the "bulk" properties of the system, like its mass and spin. These can be rather challenging to infer from more traditional light-based observations.

Of course the best case scenario would be to observe an astrophysical system using both, gravitational waves and light!

How will this discovery aid in our understanding of black holes and other similar phenomena?

RF: The region outside of black holes may have not be entirely black. If you've seen the film Interstellar, you will recall the beautifully-portrayed visualizations of the gaseous disk surrounding the fictional supermassive black hole Gargantua. Merging black holes detected by LIGO may also be surrounded by similar gaseous disks, powering energetic processes which are observable to astronomers working with conventional telescopes on the Earth. 

Indeed, the LIGO scientists are working closely with a worldwide network of astronomers, sending out rapid alerts whenever a possible gravitational wave signal is detected. These astronomers are poised at a moment's notice to deploy their expansive array of instruments spanning the globe and Earth orbit to search for counterparts to gravitational wave signals. From these combined observations, scientists may soon be able to crack a number of outstanding mysteries surrounding black holes, merging neutron stars, exploding stars, and much more.

GK: The merger of two black holes is amongst the strongest sources of gravitational waves -- no doubt that is why the waves that LIGO has successfully detected is from such an event. Information carried by these waves can tell us a lot about the system they are coming from. That information would help us not only verify our theories but could also offer new understanding of these rather mysterious systems. For a start, through the detection announced today, we are now 100 percent certain that black holes actually do exist. The signal detected by LIGO could only have arisen from a binary black hole system. 

How will this help us better understand gravity?

GK: Einstein's general relativity theory is our current best theory of gravity. Despite the fact that theory is a century old now, it’s still going strong i.e. no deviations have ever been found and the theory has passed many, many rigorous experimental and observational tests. However, one limitation of the tests performed in the past is that they were mostly so-called "weak field" tests i.e. Einstein's theory of relativity had not been rigorously tested when the effects of gravity are very strong. The reason is that the tests mostly relied on what experiments one could do on Earth or careful observations in our solar system.

However, this first direct detection of gravitational waves changes all that. Now we have clear evidence of black holes and the gravitational waves emitted by a binary black hole system. That completely vindicates Einstein's theory even in the strongest of gravitation related-phenomena. However, there is a lot more to learn about our universe from these waves and LIGO and one never knows what Nature has in store for us.

About Robert Fisher

Dr. Robert Fisher earned his B.S. in physics with honors from Caltech in 1994. He received his Ph.D. in physics from the University of California at Berkeley in 2002, where he received a NASA Graduate Research Fellowship. He was subsequently a postdoctoral research scholar at Lawrence Livermore National Laboratory and research scientist at the Department of Energy Advanced Simulation and Computing Flash Center in the Department of Astronomy and Astrophysics at the University of Chicago. The primary theme of Dr. Fisher's research is the fundamental physics of turbulent flows, and its application to the two endpoints of stellar evolution—star formation and supernovae—using a combination of theoretical and computational techniques.

About Gaurav Khanna

Dr. Khanna works on a variety of challenging problems in theoretical and computational astrophysics. His primary research work is related to the physics of binary black hole systems using perturbation theory and estimation of the properties of the emitted gravitational radiation. He has published more than sixty research papers in top international journals and secured more than a million-dollars in research funding to date. Dr. Khanna obtained his doctoral degree from Penn State University in 2000 and his undergraduate degree from Indian Institute of Technology, Kanpur (India) in 1995. His research has been supported by grants from the University of Massachusetts, National Science Foundation (NSF), Glaser Trust of New York, Foundational Questions Institute (FQXi), Apple Inc., SCEA (Sony) and the TeraGrid.

...scientists may soon be able to crack a number of outstanding mysteries surrounding black holes, merging neutron stars, exploding stars, and much more.