Far out cosmology: light travels without energy loss (Introduction)

by David Turell , Monday, May 26, 2025, 21:03 (17 days ago) @ David Turell

Photons are massless:

https://www.sciencealert.com/light-travels-across-the-universe-without-losing-energy-bu...

"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 186,000 miles (300,000 kilometers) per second, or almost 6 trillion miles per year (9.6 trillion kilometers). 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.

"As incredibly fast as that is, space is incredibly spread out. Light from the Sun, which is 93 million miles (about 150 million kilometers) from Earth, takes just over eight minutes to reach us. In other words, the sunlight you see is eight minutes old.

"Alpha Centauri, the nearest star to us after the Sun, is 26 trillion miles away (about 41 trillion kilometers). So by the time you see it in the night sky, its light is just over four years old. Or, as astronomers say, it's four light years away.

***

"...most light just goes and goes, without colliding with anything. This is almost always the case because space is mostly empty – nothingness. So there's nothing in the way.

"When light travels unimpeded, it loses no energy. It can maintain that 186,000-mile-per-second speed forever.

***

"...when you're traveling at or close to the speed of light, the distance between where you are and where you're going gets shorter. That is, space itself becomes more compact in the direction of motion – so the faster you can go, the shorter your journey has to be. In other words, for the photon, space gets squished.

"Which brings us back to my picture of the Pinwheel galaxy. From the photon's perspective, a star within the galaxy emitted it, and then a single pixel in my backyard camera absorbed it, at exactly the same time. Because space is squished, to the photon the journey was infinitely fast and infinitely short, a tiny fraction of a second.

"But from our perspective on Earth, the photon left the galaxy 25 million years ago and traveled 25 million light years across space until it landed on my tablet in my backyard."

Comment: the universe is so large we must measure distance in light-years. The massless photon allows us to do that without varying the measurements of distance.