How far can you travel without leaving your home? - Fabio Pacucci and Lindsay DeMarchi
Happy 100th birthday! Your favorite granddaughter (shh, don’t tell the others) created a surprise: a holographic map displaying everywhere you’ve traveled—not just on Earth, but through the universe! While you haven’t literally been to space, you've lived on a spinning rock hurtling around a sun, whizzing through a galaxy, and the journey only gets wilder from there.
You’ve made some real progress in the grand scheme of things. But... how much, exactly? Your atlas starts on the planet’s surface. Over the course of your life, you’ve walked about 120,000 kilometers—the equivalent of three trips around the globe. Daily commutes and international travel add a few more. This may seem remarkable until you factor in your pirouette around the planet each day due to its rotation.
The distance traveled this way differs from person to person—those close to the poles trace a smaller circle than those at the equator. Living halfway between them, you’ve picked up 30,000 kilometers every day without shifting a muscle. Except, your motion isn’t perfectly circular. It's a curlicue. As the Earth elliptically orbits the Sun, there you go, adding roughly another 940 million kilometers every year. But it doesn’t end there.
Our entire solar system is contained within the heliosphere, a bubble of charged particles emitted by the Sun. That bubble orbits the Milky Way’s center, which harbors a supermassive black hole, at a speed of about 200 kilometers per second. One full orbit takes 230 million years—meaning we've aged little more than one galactic year since the first dinosaurs. In your 100 years, you’ve witnessed four ten millionths of one rotation. That’s still 600 billion kilometers, or 2,200 round trips between the Earth and Sun.
Our galaxy and over 100 neighbors together constitute “The Local Group.” The Milky Way and Andromeda are hurtling towards each other at 125 kilometers per second and will collide in about 4.5 billion years. The Local Group is a speck within the Virgo Supercluster, which is itself just one of many lobes of the Laniakea Supercluster that contains over 100,000 galaxies. This supercluster has a mysterious gravitational center called the Great Attractor.
Because these enormous masses all gravitationally tug on each other, our galaxy’s motion is much more helter-skelter than a clean, circular orbit. In your lifetime, you’ve traveled 2 trillion kilometers at about 600 kilometers per second relative to the Great Attractor. The Laniakea Supercluster is, you guessed it, moving with respect to everything else in the universe.
At every step thus far, your granddaughter has used central reference points to describe your relative motion. But the universe has no center. Instead, astronomers use the Cosmic Microwave Background, or CMB, an echo of the early universe, which involves low-energy photons bouncing around everywhere in all directions, all the time.
Imagine standing somewhere where the wind blows toward you at the same speed from every direction. If you started running any which way, the wind in your face would have a higher speed, while the wind at your back would be gentler. The CMB is like that—any direction we travel, the CMB appears more energetic or blue-shifted, while behind us it appears red-shifted. By measuring the degree and direction of that shift, we can determine where we’re going and how fast, relative to the CMB.
And the answer is: 630 kilometers per second towards the Great Attractor. To recap: you’re spiraling around a sun circling a supermassive black hole, hurtling towards another galaxy, chaotically weaving around a supercluster, and barreling out into the great expanse. Yet you’ve felt none of that; tucked as you are into your planetary spaceship by gravity’s embrace.
If you drew a straight line from the point where you were born to where you are today, it would measure about one fifth of a light year! That may not sound like much, but neither does hiking through a park or sitting through a sunset. There’s wonder to be found at every point of our all too brief journeys.