A long time ago, in a galaxy far, far away, two black holes spiraled towards each other, pulling closer and closer until they finally smashed together. This incredibly powerful collision unleashed ripples in the fabric of the universe that spread outwards at the speed of light. A billion years later, on September 14, 2015, they arrived at Earth and produced a faint signal at two of the most sensitive scientific instruments ever made. This detection is the result of a decades-long quest to know more about our universe, and opens up a new era in our ability to observe the cosmos.
These ripples — called gravitational waves — were predicted by Albert Einstein a century ago, a result of his theory of general relativity (the same theory that explains the motion of Mercury and makes your GPS work correctly, among other things). Like sound waves in the air, gravitational waves propagate away from a source — in this case, two black holes colliding. However, whereas sound waves are variations in air pressure, gravitational waves are wrinkles in space-time.
While hints and indirect evidence for the existence of gravitational waves had been seen before, no one had ever measured one directly — until now. Last week, scientists from the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that towards the end of last year, during engineering test runs before the start of the instrument’s official science observations, they had finally detected a gravitational wave.
Catching a Wave
When a gravitational wave passes through an object (and they pass through everything, which is important; but we’ll get to that later), the distortion in space-time jostles the particles within that object. While the particles themselves feel no force, the distances between them change as the space-time that they reside in is alternately stretched and squeezed by the passing wave.
Don’t worry though. As mildly terrifying as LIGO’s animation of the exaggerated effects of gravitational waves on Earth is, it’s important to remember that the stretching and squeezing effect from these waves is unimaginably tiny. The ratio of change in length to original length, known as strain, from gravitational waves caused by a distant black hole collision is on the order of 10-21 strain — that’s a decimal followed by 20 zeros, then a 1. Put another way, if a gravitational wave acted on the distance between the sun and the nearest star, Proxima Centauri, the change in distance would be about the width of a single strand of human hair.
But you know something amazing about humans? We can measure that.
Interferometry, My Dear Watson
To measure the tiniest of movements, the LIGO Collaboration built the most sensitive ruler in history. Two of them, in fact — one in Louisiana, and one in Washington. Each detector consists of two “arms,” each of which is four kilometers long. Together, they make up an interferometer, an extremely sensitive device that measures the difference between two distances. As this brief video from LIGO shows, an interferometer compares two distances by splitting a laser beam down two different paths, reflecting it back, and looking at interference between the two beams.
Light, along with all other forms of electromagnetic radiation, can be thought of as oscillations, or waves, in the strength of electric and magnetic fields. (We won’t get into the wave-particle duality in this post.) The distance between subsequent peaks in a plot of intensity over time is called the wavelength of that light, and is a characteristic that, among other things, determines how we perceive the color of that light.
Lasers are particularly useful because they have a tight beam and can have high spectral purity, meaning that the light within them has one very specific wavelength — the lasers used in LIGO, for example, have a wavelength of 1064 nanometers. When the laser beam is split, sent down each arm of the detector, bounced back, and recombined, the waves from each beam will be at a certain point along their cycle depending on the distance that they have traveled.
Since the split laser beams in an interferometer recombine at one point in space, and then travel together to a detector, the waves in the two beams will potentially be out of phase if one of the beams traveled farther than the other in the interim — meaning that the peaks of one no longer line up with the peaks of the other. This phase shift of one relative to the other causes interference, resulting in changes in the amplitude (the height of the peaks) of the recombined light wave, which is observed as changes in the brightness of the light.
Constructive interference happens when the peaks and valleys of the two combined waves are in the same position, and the amplitude of the resulting wave is at its highest. This results in bright light. Destructive interference, on the other hand, happens when the peaks of one wave align with the valleys of the other and they cancel out. This results in darkness. These are just the two extremes, of course; there are also an infinite number of positions in between that would result in a recombined wave of different amplitudes, as shown in the gif above. Since amplitude corresponds to brightness, this interference allows us to measure the difference in the distance traveled by the two light beams by measuring the brightness of the beam produced when they are recombined.
“We did it!”
Interferometry allows for extraordinarily precise measurements — in the case of LIGO, just enough for them to detect the effect of a passing gravitational wave. By making the arms of the detector so long, the designers of LIGO increased the scale on which a gravitational wave acts, thereby increasing the change in distance caused by the tiny strain of the gravitational wave and making it easier to measure.
When the wave passes through the detector, stretching and squeezing space-time as it goes, it changes the relative lengths of the arms of the LIGO detectors. This change in length changes the distance traveled by the laser beams going down each arm, which in turn changes the relative position of peaks and valleys in the waves of each light when they are recombined. The brightness of the recombined beam changes as the arm lengths change, and the passing wave has been measured.
The end result is a beautiful plot showing the signal at both detectors, as well as theoretical predictions, all in agreement. The detector in Louisiana and the detector in Washington both saw waves that increased in both amplitude and frequency before suddenly stopping, as was predicted. In the words of David Reitze, the Executive Director of LIGO, during the press conference announcing the finding: “Ladies and gentlemen, we have detected gravitational waves. We did it!”
Listening to the Universe
The detection of gravitational waves is a triumph of experimental science, and the culmination of decades of work by thousands of people around the globe. It confirms a century-old prediction, and helps us understand a little more about the inner workings of the universe around us. The pursuit of gravitational waves, like many great quests in the history of science and engineering, has certainly been what I think President Kennedy would call a challenge that “serve[s] to organize and measure the best of our energies and skills.”
But there’s more to this detection than the aspirational glory of scientific progress. The ability to detect a signal so tiny that its effect is a movement of less than one ten-thousandth of the width of a proton — a ripple in space-time that has traveled across the universe for a billion years — opens an incredible new era in our observation of the world around us.
For centuries, humans have looked at distant stars and with telescopes of increasing complexity. With each new invention, we could see a little farther, or perhaps peer into a whole new wavelength of the electromagnetic spectrum. Eventually, we spotted galaxies beyond our own, and came to realize that even the darkest parts of the sky are full of distant stars if we look hard enough. Each new way of looking showed us something we had not expected to see. Using our telescopes on the Earth and in space, we have seen some of the most beautiful and mysterious structures in our universe.
However, it’s important to remember that we are still simply seeing. As physicist Kip Thorne pointed out, “all previous windows through which astronomers have looked are electromagnetic.” These electromagnetic windows — which include the optical spectrum that we can perceive with our eyes — have shown us much about the universe, but they have their limitations. For example, vision can be blocked, and dust clouds or other collections of materials can sometimes obscure our vision. In addition, not all material in the universe radiates in a way that we can see.
However, gravitational waves are different. They are vibrations in the universe, the echoes of unimaginably energetic events. As mentioned before, they travel through all matter, so the cannot be blocked (at least, as far as we understand). They carry with them information about objects that we may not be able to see using our usual observational methods. Electromagnetic radiation may present the sights of the universe, but gravitational waves are its sounds. And, thanks to LIGO, you can hear them for yourself:
By detecting gravitational waves, we are for the first time listening to the universe around us. We have had our eyes open, but now we have finally opened our ears as well.