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How to Read a Pulsar Map

Voyager's Golden Record is a 12-inch gold-plated copper disk, encoded with music, sounds and images from Earth. Its aluminum cover is engraved with instructions, and a unique galactic map.

Astronomer and astrophysicist Frank Drake designed the map, working with fellow astronomer Carl Sagan and artist and writer Linda Salzman Sagan. The starburst-like diagram is called a pulsar map, because it shows the location of our sun relative to known pulsars.

Pulsars are the rapidly spinning remains of dying stars—the leftover cores of supernova explosions. They're only about 12 to 15 miles in diameter, but most contain more than twice the mass of our sun.

Their rapid spin and intense magnetic fields cause the pulsars to emit narrow beams of light, which flash like the beam from a lighthouse every time they pass across our field of view.

Each pulsar has its own signature pulse rate, making them easy to identify, and ideal as reference points on a map.

Frank Drake used 14 pulsars to create a map with our sun at the center. Each pulsar is connected to the sun by a solid line. The length of the line represents the pulsar's approximate relative distance from the sun.

Etched along each of the pulsar lines are vertical and horizontal dashes that represent a binary number than can, in turn, be converted into a decimal.

When multiplied by a known measure of time, that number reveals the frequency of the pulsar—how fast it spins and flashes.

On Earth, we’d use one second as the known unit of time, but Drake felt he needed a base unit that was more universal—one that intelligent life anywhere would be able to calculate, no matter what system of timekeeping they use.

He based his calculations on the hydrogen atom, and more specifically, on the time it takes for the spin of a hydrogen atom’s electron to change relative to its proton. It’s known as the hyperfine transition period of hydrogen, and he illustrated it on the map.

Why hydrogen? Because it's the most abundant element in the universe, increasing the chances that other intelligent beings would recognize it and know the length of its transition period (by our count, 0.7 billionths of a second.)

Assuming aliens know the hyperfine transition period, they could then calculate the exact frequency of Drake’s pulsars, and try to match them to their own observations.

Then, they could do that 13 more times—for each of the pulsars in Drake’s map.

But that’s just the beginning… Drake’s map is flat—two dimensional.  So they would still need to figure out where in 3D space the Pulsars are located.

That’s where the long, dash-free 15th line on the map comes in.

This line illustrates the approximate relative distance between our sun and the center of the galaxy (represented by the tick mark at the end of the line). It also establishes the 2-dimensional galactic plane—the plane in which the majority of our disc-shaped galaxy's mass lies.

The 14 pulsar lines also have tick marks, which, based on their distance from the end of their line, provide an estimation of how far off the galactic plane each pulsar is located. The closer to the end of the line the tick mark is, the closer to the galactic plane the pulsar is.

And that’s how an intelligent, spacefaring alien civilization can take Drake’s 2D map and turn it into a 3D one.

In 3D space, the flat, 2D map become most unhelpful.

But once the tick marks are taken into account, the lines to the pulsar maps fall into their correct 3D orientations, indicating where the pulsars actually are in relation to the center of the galaxy and our sun.

Even if the aliens can only locate a few of the 14 pulsars, they’d be able to triangulate our sun’s location and plot a course to our little corner of the Milky Way.

And as if this interstellar roadmap isn’t enough, there is one more bit of information cleverly hidden in its data.  The pulsar frequencies etched onto the golden record cover were calculated in the early 1970’s.  Over millions and billions of years, these frequencies change.

Drake predicted that any future aliens capable of a galactic road trip would know about this change, and could calculate its rate. Using that data to compare their current observations to the original ones on the map. They could even tell how long ago the Voyager spacecraft left Earth.

Back before laptops and cell phones and Google Maps, Frank Drake created an elegant diagram that provides galactic directions through space and time.

Can your GPS do that?

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