Tuesday, March 24, 2015

"The Happiest Thought of My Life": 100 Years of General Relativity

One hundred years ago this year, Einstein first published his theory of general relativity. This theory fundamentally changed how physicists think about the universe and is thoroughly embedded in the everyday technologies we rely on here on Earth.

In tribute, the talent behind Science Magazine created a wonderful interactive comic that describes the key idea behind general relativity — that space and time warp around massive objects, leading to the "force" we call gravity. Click on this image to view the comic, or follow the link here.

Click to view an interactive comic on general relativity, featuring superhero Einstein!
Credit: Screenshot from Science Magazine's comic.

"The happiest thought of my life"

Soon after Einstein published his theory of special relativity in 1905, he started trying to incorporate gravity into a more general theory. He envisioned a universe where gravity was not a force but a geometric property of curved spacetime, warped around massive objects. Einstein famously called this "the happiest thought of my life". John Wheeler, renowned black hole physicist, later summed up general relativity as "spacetime tells matter how to move; matter tells spacetime how to curve". 
But it took years for Einstein to fully develop the equations that describe general relativity, and he often sought the advice of more experienced mathematicians including his colleague Marcel Grossmann
Credit: Screenshot from Science Magazine's general relativity comic. 

In November 1915, Einstein presented his mathematical theory of general relativity to the Prussian Academy of Sciences. The following year he slightly amended some of the equations and published the full version of the theory, along with three experimental tests that could prove its accuracy: the mystery of Mercury's orbital precession, the curving of light around a massive object, and the gravitationally-induced redshift of light.
All three predictions have since been confirmed, as well as many other successful tests of this now famous theory. A classic example is the operation of the global positioning system or GPS.

We'd all be lost without general relativity

Simulation of 24 GPS satellites and their
visibility from a fixed point on Earth.
Credit: El pak / Public domain
Without general relativity, our smartphones would be incapable of accurately locating us on a map. The GPS receiver in our smartphones collects the exact time and position from each satellite in view (about 6-12 in most cases, with a minimum of 4), and uses these measurements to determine its current position on Earth. Any errors in timing translate to errors in position that can quickly accumulate over time and render the GPS system useless.

Without the effects of Einstein's special and general relativity carefully accounted for, the GPS satellite clocks would be faster by about 38 microseconds per day compared to ground-based clocks, according to Ohio State astronomy lecturer, Richard Pogge.

This is the combination of two effects. Special relativity says that the satellite clocks will run slow by 7 microseconds per day because of their relative motion to the ground. General relativity, on the other hand, says that clocks closer to a massive object like the Earth tick more slowly than the further-away satellite clocks, by a difference of 45 microseconds per day. The combination of the two effects means that the satellite clocks run fast by 38 microseconds per day.

In order to accurately pinpoint a receiver to within a few meters, nanosecond timing accuracy is required, so the 38 microseconds difference would quickly lead to large errors in position.

Instead the engineers of GPS accounted for relativity by slowing down the satellite clocks in order to correct for the relativistic effects and equipping the satellites with computers that can calculate the full relativistic equations in real-time, keeping us pinpointed on the map.

Credit: Screenshot from Science Magazine's general relativity comic.

So enjoy this quick and fun general relativity comic and remember that everything is relative!

The comic was created for a special issue Science Magazine by writer Adrian Cho, artist Nguyên Khôi Nguyễn, developer Jon Dang, and project manager Martyn Green.

By Tamela Maciel, also known as "pendulum"

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