The Surfer's Guide to Gravitational Waves

In a nutshell: Gravitational waves are ripples in the fabric of space time produced by violent events, like merging together two black holes or the explosion of a massive star. Unlike light (electromagnetic waves) gravitational waves are not absorbed or altered by intervening material, so they are very clean proxies of the physical process that produced them. They are expected to travel at the speed of light and, if detected, they could give precious information about the cataclysmic processes that originated them and the very nature of gravity. That’s why the direct detection of gravitational waves is such an important endeavor. Definitely worthy of a Nobel prize in physics.

Left: ’AL’ Einstein on vacation (Credits: Rick Rietveld)
Top Right: The Earth’s mass changes the curvature of space-time. Bottom Right: Gravitational waves generated by merging black holes (Credits: NASA/LIGO)

General relativity is a theory of gravity, where gravity emerges as a particular geometrical property of space-time. This geometrical property is called curvature. Similarly to a large sheet that warps under the weight of an object, mass and energy bend space-time and create curvature. In the absence of mass and energy the space-time is flat, and objects move in straight lines. Around a large body like the Earth, objects move following the curvature produced by the mass of the planet. Which means if you throw a stone, it eventually curves and falls to the ground, and does not move in a straight line. The elegant movements of celestial bodies in the cosmos are mostly orchestrated by mass and energy diligently following the (usually) gentle hills of space-time. Since moving mass and energy affect the curvature as well, the resulting dance is a complex, ever-changing choreography. The laws of this dance can be written in a very elegant form, called the Einstein’s Equation (\ref{einstein}), maybe one of the most beautiful equations in physics. \[\label{einstein} G_{\mu\nu}+\Lambda T_{\mu\nu}=\frac{8\pi G}{c^4}T_{\mu\nu}\]