How Light Travels Across the Universe Without Losing Energy

One evening in my light-polluted San Diego backyard, I had my telescope set up for astrophotography and aimed it at an incredibly distant galaxy. As the first image appeared on my tablet screen, my wife, Cristina, came over.

“That’s the Pinwheel Galaxy,” I told her—named for its spiral shape and home to roughly a trillion stars.

The light from this galaxy had traveled an astonishing 25 million years—approximately 150 quintillion miles—to finally reach my telescope.

Cristina asked a question that sparked a deeper conversation: “Doesn’t light get tired on such a long trip?”

That curiosity opened the door to an important and fascinating question: Why doesn’t light lose energy over time?

Understanding the Nature of Light

As an astrophysicist, one of the most counterintuitive things I learned is how light behaves. Light is electromagnetic radiation—essentially a pairing of electric and magnetic waves moving through space. It has no mass, and that’s key. Any object with mass, whether tiny or massive, is limited in how fast it can travel.

Because light has no mass, it travels at the universe’s speed limit: 186,000 miles per second (300,000 kilometers per second), which adds up to nearly 6 trillion miles a year. That’s so fast a light particle could circle Earth more than twice in the time it takes to blink.

Still, even at that speed, cosmic distances are vast. Light from the Sun takes just over eight minutes to reach Earth. The nearest star beyond our Sun, Alpha Centauri, is so far that its light takes four years to get here—that’s why we say it’s “four light-years away.”

Given those distances, Cristina’s question becomes even more intriguing: how does light travel for millions of years without losing energy?

Does Light Ever Lose Energy?

Some light does lose energy—when it scatters after hitting interstellar dust, for example. But in most cases, light travels through space uninterrupted. Since space is almost entirely empty, there’s nothing to slow it down. As long as light doesn’t interact with matter, it keeps going at full speed, retaining its energy.

Time and Light: A Strange Relationship

Let’s consider time. Imagine you’re an astronaut aboard the International Space Station, moving at 17,000 miles per hour. Your watch would tick slightly slower—by about 0.01 seconds per year—compared to someone on Earth. That’s time dilation: time flows at different rates depending on speed and gravity.

Now imagine you’re riding a photon, the basic particle of light. From your perspective, time doesn’t just slow down—it stops entirely. Time for the photon is frozen, even though observers on Earth see it moving at light speed.

Also, space itself behaves differently at such speeds. When moving close to the speed of light, distances contract in the direction of motion. For the photon, its journey from the galaxy to Earth happens in an instant. Space compresses, and the trip becomes infinitely short and fast.

The Illusion of Distance

That brings us back to the Pinwheel Galaxy. From the photon’s viewpoint, the moment it was emitted by a star and the moment it hit my telescope occurred simultaneously. Its journey of 25 million light-years didn’t seem like a journey at all—it was instantaneous.

From Earth, we perceive that light traveling for millions of years across space before hitting my telescope. But to the photon, it experienced no time or distance at all.

And on that cool spring evening, as the light from a galaxy 25 million years away lit up a single pixel on my screen, it sparked a simple yet profound question—and a beautiful moment of cosmic wonder shared between a curious wife and her science-loving husband.

Read the original article on: science Alert

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