Explained: light’s ability to travel vast cosmic distances without losing energy

How can light travel across the universe and not slowly lose energy? An astrophysicist explains.

My telescope, set up for astrophotography in my light-polluted San Diego backyard, was pointed at a galaxy unfathomably far from Earth. My wife, Cristina, walked up just as the first space photo streamed to my tablet. It sparkled on the screen in front of us.

“That’s the Pinwheel galaxy,” I said. The name is derived from its shape – albeit this pinwheel contains about a trillion stars. The light from the Pinwheel travelled for 25 million years across the universe to get to my telescope.

My wife wondered: “Doesn’t light get tired during such a long journey?”

Her curiosity triggered a thought-provoking conversation about light. Ultimately, why doesn’t light wear out and lose energy over time?

I am an astrophysicist, and one of the first things I learned in my studies is how light often behaves in ways that defy our intuitions.

Light is electromagnetic radiation: basically, an electric wave and a magnetic wave coupled together and traveling through space-time. It has no mass. That point is critical because the mass of an object, whether a speck of dust or a spaceship, limits the top speed it can travel through space.

But because light is massless, it’s able to reach the maximum speed limit in a vacuum – about 300,000 km per second, or almost 9.6 trillion km per year. Nothing traveling through space is faster. To put that into perspective: In the time it takes you to blink your eyes, a particle of light travels around the circumference of the earth more than twice.