How Far Is A Light Year in Miles Or Kilometers?

by Elizabeth Howell on April 22, 2014

The NASA/ESA Hubble Space Telescope has captured this vivid image of spiral galaxy Messier 77 — a galaxy in the constellation of Cetus, some 45 million light-years away from us. The streaks of red and blue in the image highlight pockets of star formation along the pinwheeling arms, with dark dust lanes stretching across the galaxy’s starry centre. The galaxy belongs to a class of galaxies known as Seyfert galaxies, which have highly ionised gas surrounding an intensely active centre. Credit: NASA, ESA & A. van der Hoeven

The NASA/ESA Hubble Space Telescope has captured this vivid image of spiral galaxy Messier 77 — a galaxy in the constellation of Cetus, some 45 million light-years away from us. The streaks of red and blue in the image highlight pockets of star formation along the pinwheeling arms, with dark dust lanes stretching across the galaxy’s starry centre. The galaxy belongs to a class of galaxies known as Seyfert galaxies, which have highly ionised gas surrounding an intensely active centre. Credit: NASA, ESA & A. van der Hoeven

We hope you have a big suitcase if you’re planning a trip across the cosmos. At the speed of light — a speed that no technology yet can achieve — it would take you about four years to get to the Alpha Centauri star system, the nearest group to our own. Zipping to the nearest galaxy, the Canis Major Dwarf Galaxy, would take you about 25,000 light-years.

As the phrase “light year” implies, that’s the distance that light travels in a year. It’s as fast as anything can travel, at least as best as we can understand it. Sometimes referred to as a “cosmic speed limit“, going as fast as light (or even faster) is impossible for a physical object, according to Albert Einstein’s measurements a century ago. As you get faster and faster, he showed, your mass increases to infinity. But there’s still discussion around that.

We can also use light to refer to distances in our solar system. The sun is about eight light-minutes away, and the moon about a light-second. The light you see from the outer planets arrived a few hours ago. This presents interesting challenges for communication; imagine a colony on Mars trying to communicate with Earth, when radio transmissions (travelling at the speed of light) take an average of 20 minutes to arrive!

NASA's Mars rover Curiosity took this self-portrait, composed of more than 50 images using its robotic arm-mounted MAHLI camera, on Feb. 3, 2013. The image shows Curiosity at the John Klein drill site. A drill hole is visible at bottom left.  Credit: NASA / JPL / MSSS / Marco Di Lorenzo / Ken Kremer- kenkremer.com

NASA’s Mars rover Curiosity took this self-portrait, composed of more than 50 images using its robotic arm-mounted MAHLI camera, on Feb. 3, 2013. The image shows Curiosity at the John Klein drill site. A drill hole is visible at bottom left. Credit: NASA / JPL / MSSS / Marco Di Lorenzo / Ken Kremer- kenkremer.com

In any case, it’s clear a light-year is a vast expanse. Over a year, light travels 5.87849981 x 1012 miles (roughly 9.4605284 x 1012 kilometers.) Written out, that’s 5,878,499,810,000 miles  or 9,460,528,400,000 kilometers.

Can anything travel faster than light? Physicists have been puzzling over at least one possible loophole for decades — the idea of quantum entanglement (which Einstein famously referred to as “spooky action at a distance.”) It describes a phenomenon where after a photon (light bit) splits into two and becomes entangled, it appears the pair can communicate instantaneously — no matter how far their separation. (Here’s a video that explains how it works.)

Sometimes people think that because galaxies are moving faster than the speed of light away from each other, that breaks the speed limit. It doesn’t, however. That’s because the galaxies are moving with the expansion of the universe. It’s a function of the universe’s expansion and not items travelling within the universe.

Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.

Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.

Is it possible at all for humans to achieve warp drive, like you see in Star Trek? A 2009 page from NASA said scientists aren’t sure about that. “We are at the point where we know what we do know and know what we don’t, but do not know for sure if faster than light travel is possible,” NASA stated. You can read more about the research at this page.

While “light-year” is a convenient way for people to remember how far away things are, astronomers usually use another unit called the parsec (which is about 3.26 light years). As “Noisy Astronomer” Nicole Gugliucci says, parsecs are “a unit of distance that makes more physical sense with respect to how we measure distances.” Very simply put, it refers to how stars shift from the perspective of Earth as we move in our orbit, but to get the full story you should read this past article on parsecs from Universe Today founder Fraser Cain.

For more resources on light speed and light years, check out the below!

Movie: Approaching Light Speed (Astronomy Picture of the Day)
Is There Anything Faster Than Light? (How Stuff Works)
A Slower Speed Of Light (a game from the Massachusetts Institute of Technology)
The Speed Of Light (how it was discovered; text by Museum Victoria)
How Big Is The Universe? (London’s Science Museum)

About 

Elizabeth Howell is the senior writer at Universe Today. She also works for Space.com, Space Exploration Network, the NASA Lunar Science Institute, NASA Astrobiology Magazine and LiveScience, among others. Career highlights include watching three shuttle launches, and going on a two-week simulated Mars expedition in rural Utah. You can follow her on Twitter @howellspace or contact her at her website.

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