What are Gravitational Waves?


Earlier in the year, physicists made headlines claiming that they may have detected gravitational waves—only to announce, some time later, that it had just been dust.

How could such a blunder be made? How could their detection equipment not immediately distinguish between a fundamental discovery and some specks of debris? Turns out: pretty easily.

Gravity is the force that affects us every second of every day, even if we’ve forgotten to notice it, but it’s by far the weakest of the four fundamental forces (gravity, electromagnetism, the weak nuclear force, and the strong nuclear force). You can test this easily at home—pick up a piece of metal with a magnet, or some scraps of paper with a negatively charged comb.

As well as being the weakest, gravity is perhaps the least understood. Two fundamental ideas have been proposed: Newton explained that gravity is an attraction between two bodies, then Einstein’s General Relativity modified this explanation, postulating that matter actually causes warps in space-time, like balls sitting on a rubber sheet, and this distortion is felt as gravitational influence.

As an object with mass moves, the curvature of space-time changes accordingly to remain ‘around’ the object. If an object accelerates, it can cause ripples in the curvature of space-time—but only if its motion isn’t perfectly spherically symmetric. Consider a supernova: if the star was exactly spherical, when it explodes it will not produce a gravitational wave, but a star that is even slightly asymmetrical will. These disturbances propagate outwards as a wave. (Check out some potential sources of gravitational waves here.)

We’re familiar with the idea that the electromagnetic force travels as a continuous wave, with an electric and magnetic component propagating at right angles to each other, transporting energy as electromagnetic radiation.


(Image Credit)

In a similar way, we expect gravity to be expressed as a gravitational wave, though these waves aren’t oscillations of electric and magnetic fields: they’re oscillations of space-time. These waves would have a maximum speed of c (the speed of light), so technically, if our Sun suddenly disappeared, the Earth would keep happily orbiting for another 8 minutes, until the gravitational ‘information’ reached us.

But here’s the issue: though gravitational waves are supported by mathematics, we haven’t actually been able to observe any yet. Their intensity drops off as it get further away from its source, so by the time they reach Earth, they are predicted to be very small, with frequencies in the range of 10-16 to 104 Hertz. For decades, researchers have built and worked on ever-more-sensitive detectors, but since gravitational waves are weak, there is a huge amount of interference to be weeded out, and no definite detections have been made yet. The European Space Agency is currently developing a space-based gravitational waved observatory called LISA (Laser Interferometer Space Antenna), which would eliminate a lot of interference, but unfortunately may not be launched for decades.

Detecting gravitational waves would be revolutionary, not just because it would confirm theories about general relativity and the nature of gravity, but because gravitational waves can penetrate parts of space that electromagnetic waves can’t. In the future, we could use gravitational wave ‘telescopes’ instead of optical and radio telescopes, and have a new ‘view’ of exotic objects like black holes and of times near the very beginning of the universe.

As for the scientists whose breakthrough turned out to be dust—well, what can you expect from a telescope named BICEP?


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