The short answer:
Gravitational waves are an important prediction of Einstein's general theory of relativity, published a century ago. Because this theory changed the way we understand the nature of space, time, and gravity, it also fundamentally changed our perception of how we fit into the universe, both as a species and as individuals. Last week's announcement of the first direct detection of gravitational waves reaffirms Einstein's incredible achievement, further demonstrating the power that we have as human beings when we put our minds to constructive rather than destructive purposes.
The longer Q & A:
Q: What are gravitational waves?
Einstein's theory of relativity tells us that space and time are intertwined as a four-dimensional spacetime. Spacetime has a structure that can vary from place to place (and time to time), much as a two-dimensional surface -- like a rubber sheet or the surface of a pond -- can have varying bumps, dips, and ripples. The structure of spacetime is shaped by the gravity of the objects within it, so if these objects undergo certain types of movement or change, they can cause a change in the structure of spacetime around them. According to general relativity, this change then propagates outward through the universe like ripples on a pond, and it is these spacetime ripples that we call gravitational waves.
Q: What do gravitational waves do that allowed them to be detected?
Gravitational waves travel outward from their source at the speed of light (a prediction confirmed by the new discovery), slightly distorting space as they pass through it-- with the key word being "slightly." The changes predicted to occur as gravitational waves pass by are so small that Einstein himself doubted that we'd ever be able to detect them. But advances in science and technology proved Einstein too pessimistic in this case, since they have now been observed with the detectors known as LIGO, short for Laser Interferometer Gravitational-Wave Observatory.
Q: How did LIGO detect them?
There are two LIGO detectors, one in Louisiana and the other in Washington State. Each consists of an L-shaped pair of four-kilometer-long arms with mirrors at their ends. (Yes, you read correctly: four kilometers long! These are large scientific instruments.) In essence, LIGO measured very slight changes in the lengths of the arms as the gravitational waves passed through. However, the changes in length are smaller than the size of an atomic nucleus, so they cannot be measured with rulers. Instead, they are measured with what physicists call an interferometer (the '"I" in LIGO); the LIGO interferometers use lasers in a way that allows the detectors to notice when lengths change even by incredibly tiny amounts.
This photograph shows the LIGO detector in Washington State.
Q: What makes scientists think the tiny changes they observed are really from gravitational waves and not some artifact?
With such tiny measurements being made, great care must be taken to be sure the changes are due to gravitational waves, rather than, say, vibrations on Earth that might affect the lasers. This is why there are two LIGO detectors: The instruments in Louisiana and Washington both measured the same signal, at times separated only by the light-travel time between the two locations. That rules out a cause that is local to either observatory. Even so, the reason the scientists waited months to announce the discovery was so they could carefully rule out other potential signal sources. The result is that the scientists are now much more than 99% confident that the detection was real.
Q: Can the measurement be repeated to be even more certain that it is real?
Gravitational waves travel at the speed of light, so this particular signal has now passed us by and cannot be rechecked. However, if it was real, we should expect to pick up signals from other similar events in the near future. If we don't, then we might be concerned. But we almost certainly will; in fact, there may already have been additional detections for which the data are still being analyzed.
Q: Why do scientists think the signal came from the merger of two black holes?
The equations of general relativity allow scientists to calculate the precise characteristics -- for example, the frequency and amplitude -- of the gravitational waves produced by different types of events in the universe. In this particular case, the signal that was observed matches the one expected from the merger of a pair of black holes. The black holes were orbiting each other, presumably because they were the remains of a binary star system in which both stars were massive and became black holes when they died. They merged because orbiting black holes represent a type of mass movement that emits strong gravitational waves, and these waves carry energy away, causing the orbits to decay until the two black holes eventually merge. It was the final moments of the orbital decay and the subsequent merger that produced the strong gravitational waves detected by LIGO.
These three panels are based on a supercomputer simulation of the final moments before a pair of black holes merge. Gravitational waves are depicted in red, orange, and yellow, in order of increasing intensity. (Illustration is from The Cosmic Perspective, 8th Edition, based on the NASA simulation at http://www.nasa.gov/centers/goddard/universe/gwave.html.)
Q: Did scientists really hear a "chirp" sound from the gravitational waves?
Gravitational waves are not a form of sound. However, like sound waves, they cause vibrations in the material they pass through, and it turns out that the frequencies (that is, the number of vibrations each second) of many gravitational waves happen to be the same as the frequencies of sound waves audible to the human hear. For that reason, it is possible to create sound waves with the same frequencies as the gravitational waves, and in this case the sound happens to be somewhat like a bird's chirp. Bottom line: Gravitational waves do not produce sound, but we can artificially create sounds with the same wave pattern and that can be useful when trying to interpret the gravitational wave signal.
Q: Is this the first evidence for gravitational waves?
No - we already had very strong evidence that gravitational waves existed, but only indirectly. Recall from our earlier question that two orbiting black holes lose energy to gravitational waves, causing them to gradually spiral into each other. The same type of "in spiral" also occurs with orbiting neutron stars, and the decay of the orbits of neutron star pairs had previously been measured and found to be in precise agreement with the prediction of general relativity. Indeed, the 1993 Nobel Prize in Physics was awarded (to Russell Hulse and Joseph Taylor) for this indirect discovery of gravitational waves. But direct detection is even better, as we'll discuss next, and it seems likely that this detection will generate another Nobel Prize.
Q: Why is this discovery important to science?
The direct detection of gravitational waves once again validates Einstein's general theory of relativity but, as above, we already had strong evidence that these waves existed. So the more important part of this discovery is that direct detection opens up an entirely new way of studying the universe, allowing us to "observe" events that are invisible to ordinary telescopes and that probe some of the most exotic objects in the universe, including black holes. (Note: Scientists also expect to be able to detect evidence of gravitational waves from the Big Bang, and you may recall that such a detection was announced in 2014, but then retracted once scientists realize they may have been seeing an artifact from cosmic dust rather than a signal from the early universe. To learn more about this, see my video at https://youtu.be/mpaV7CzPtQ0.)
Q: Why is this discovery important to me?
Einstein's discovery of relativity is a fantastic example of what human beings are capable of when we put our minds to work in positive ways. This new discovery extends and amplifies that message. While relativity was in many ways the achievement of a single human being (Einstein), the detection of gravitation waves is a triumph of team work, with scientists and engineers from many nations and cultures all working together to make a major discovery. There's also teamwork in how it was funded, in that it was funded by government which, when it works well, is a form of teamwork involving entire nations. I hope that this discovery will therefore inspire each and every one of us to do our own parts in working together to build a better future for ourselves and for our children and grandchildren.
Q: How can I learn more?
- This 10-minute video from PBS gives a great explanation of the new discovery and what it means.
- The LIGO project has a great set of short pages that provide a general overview of the science and technology.
- Slate blogger Phil Plait (aka "the bad astronomer") always does a great job in keeping you up to date on new discoveries in space science, and he did a great post on this one.
- I enjoyed this article by Lawrence Krauss article in The New York Times.
- And if you want to learn more about relativity in general, I hope you will check out my book What is Relativity?, as well as my lectures that you can view at: www.bigkidscience.com/relativity-tour