Making Connections with Gravitational Waves

MergingBlackHoles_V2Numerical simulations of the gravitational waves emitted by the inspiral and merger of two black holes. The colored contours around each black hole represent the amplitude of the gravitational radiation; the blue lines represent the orbits of the black holes and the green arrows represent their spins. Source: NASA/Ames Research Center/C. Henze Last week, scientists announced an important new discovery: the first observation of "gravitational waves,” a phenomenon predicted 100 years ago by Albert Einstein’s general theory of relativity, but never directly verified by experiments until now. On Thursday, Dr. Dave Morgan, director of Innovation Lab @Ross, spoke to Ross Upper School students at a special Community Meeting, sharing information about the discovery and performing some demonstrations to explain what it all means.

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Einstein’s theory of general relativity tells us that gravity is caused by a distortion or “curvature” of space and time. But it also predicts that spacetime can twist, vibrate, and distort in other ways. To demonstrate the effect of waves on spacetime, Dr. Morgan first had the students on the bleachers do “the wave,” a familiar pastime at sporting events. He explained that a wave passes through a medium: in this case, the medium was the students; while surfing, the wave passes through the medium of water; sound waves pass through the medium of air; and in the case of gravitational waves, the medium is space. He then used a piece of stretchy fabric, held by four volunteers, to represent the curvature of spacetime, placing a series of balls on it to show how the gravity of objects distorts what starts out as a plane. These physical representations of complex concepts helped students to grasp the mechanics of seemingly perplexing scientific principles.

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Dr. Morgan went on to explain that a gravitational wave is a distortion or “ripple” in spacetime caused by a violent event in space. This ripple moves at the speed of light, and causes space to stretch and compress as it travels across the cosmos. In fact, he explained, a gravitational wave could move through the medium of the students themselves, causing the students to stretch taller and compress shorter in turn—but that the amount of stretch would be less than a nanometer, and thus imperceptible to the human eye. In the case of the gravitational wave detected by the current experiment, called LIGO, the event that caused the ripple seems to be the collision of two large black holes, each one around 30 times as massive as our Sun.

LIGO is short for Laser Interferometer Gravitational-Wave Observatory. An interferometer is a device that splits a beam of light and sends it along two perpendicular paths. The two beams bounce off of mirrors and are recombined. Deviations in the recombined light waves can be used to measure very small differences in the distance traveled by the two beams. In the LIGO experiment, when a gravitational wave passes by, the path of one light beam is stretched while the other one is squished. This difference in the beams is very tiny—much smaller than the size of an atom. But sophisticated data analysis allowed scientists to detect this tiny “wobble” in spacetime as it jiggled LIGO’s mirrors.

Dr. Morgan also made the point that while the discovery of evidence of gravitational waves is notable and interesting, it is important to realize that the discovery is confirmation of a theory that has been an integral part of scientific understanding for a hundred years. He followed up by saying, “This is how science works. Even when we are almost certain a theory is correct, we have to check out all of its predictions, even if it’s hard, and even if it takes a really long time. Scientists have been checking off Einstein’s predictions for over a century now. LIGO’s discovery is just one more success in a long line of successes for the theory of general relativity, which is one of the most well-tested theories in all of science.

Dr. Morgan continued, “A lot of interest in this discovery comes from the fact that it involves the ideas of Albert Einstein, who is a sort of ‘heroic’ figure in science—a pop culture icon in addition to being an important physicist.” He asked students to mentally picture Einstein, and then to reflect on whether they could picture other scientists as well. He pointed out the posters, coffee mugs, T-shirts, and even his own socks that featured Einstein, explaining, “Einstein’s ideas are fascinating, and they sound like science fiction, and they really excite people—which is great for physics, because it gets people’s attention and generates a lot of interest.”

Click here to hear the "chirp" sound detected by the LIGO experiment, and to link to the New York Times article about the discovery.