Loss of energy by light as it passes through the universe

 Does Light Lose Energy as it passes through the Universe?

As a wave of light travels through the universe, does it lose energy? For example, what is the wavelength of 450 nm (blue) light after traveling a trillion (1,000,000,000,000) km in the universe? The speed of light is the fastest anything can travel. What happens to a photon from a galaxy 25 million light years away on its journey toward Earth? The answer involves time dilation. Light, whether from a star or your flashlight, travels at 186,000 miles per second. The anisotropies of the Cosmic microwave background (CMB) as observed by Planck. The CMB is a snapshot of the oldest light in our Universe, imprinted on the sky when the Universe was just 380,000 years old. The loss of energy is usually officially termed a cosmological redshift, and it’s an interesting combination of the way that light moves through space, and the nature of our Universe’s expansion.

My telescope, set up for astrophotography in my light-polluted San Diego backyard, was pointed at a galaxy unfathomably far from Earth. Space photo streamed to my tablet. It sparkled on the screen in front of me. "That's the Pinwheel galaxy,". 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 (about 150 quintillion miles) to get to my telescope. 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 km's) per second, or almost 6 trillion miles per year (9.6 trillion km's). 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.

Light behaves both as a particle and as a wave. Depending on the situation, it can be easier to talk about photons the light particle, or light waves- for travelling the vast distances of space, either works. However, light’s energy is very tightly tied to one of its more wave-like properties, its wavelength. Wavelength measures the distances between peaks, and can be used to measure the distances between ocean waves, or it can be used to very precisely measure the colour of light reaching your eyes or a camera. The shorter the wavelength of light, the bluer the colour. The bluer the light, the more energy it has, and things like gamma rays and X-rays have more energy still than anything our eyes can detect. As incredibly fast as that is, space is incredibly spread out. Light from the Sun, which is 93 million miles (about 150 million km's) 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 km's). 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.

 So let’s imagine that we have a space three feet long, and a flexible spring, almost three feet long. To reach from one edge of our space to the other, we’ll have to stretch the spring a little, but the distance between the coils won’t be very large. If we started to extend the spring, and suddenly found that our space had doubled in size, we’d have to stretch the spring much further, and the distance between coils would be much greater. With those enormous distances in mind, consider this question: How can light travel across the universe and not slowly lose energy? Actually, some light does lose energy. This happens when it bounces off something, such as interstellar dust, and is scattered about. But 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.

This is fundamentally what happens to light, as it travels through an expanding universe. The universe as a whole is expanding, meaning that the space between many galaxies is increasing. As light travels away from a galaxy, the Universe is continually expanding, meaning that the distance the light needs to travel is continually increasing as well. As space stretches out underneath a beam of light, its wavelength increases, and its energy decreases. Measuring this loss of energy is one of the main ways that distance is now measured in the Universe. This metric works well because we have a good sense (from other measurements) of how fast the Universe has been expanding, and what the energy loss should be for light which began its journey at an earlier time. Here's another concept: Picture yourself as an astronaut on board the International Space Station. You're orbiting at 17,000 miles (about 27,000 km's) per hour. Compared with someone on Earth, your wristwatch will tick 0.01 seconds slower over one year. That's an example of time dilation, time moving at different speeds under different conditions. If you're moving really fast, or close to a large gravitational field, your clock will tick more slowly than someone moving slower than you, or who is further from a large gravitational field. To say it succinctly, time is relative. Even astronauts aboard the International Space Station experience time dilation, although the effect is extremely small.    

Now consider that light is inextricably connected to time. Picture sitting on a photon, a fundamental particle of light; here, you'd experience maximum time dilation. Everyone on Earth would clock you at the speed of light, but from your reference frame, time would completely stop. That's because the "clocks" measuring time are in two different places going vastly different speeds: the photon moving at the speed of light, and the comparatively slowpoke speed of Earth going around the Sun. What's more, 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 our 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.

However, a trillion km's, on an astronomical standard, is still relatively small. A trillion km's is roughly a tenth of a light year (about five weeks of light travel time). This distance is sufficiently small that a more useful unit is the astronomical unit (au) which measures the distance between the Earth and the Sun. A trillion km's is about 10,000 au. On the scale of our solar system, this would stretch from the Sun to not quite out into the Oort cloud. This distance would put you way past Pluto, which orbits our sun at around 40 au from the Sun, and well past any hypothetical Planet 9, which is supposed to hang out around 200 au from the Sun. But from our perspective on Earth, the photon left the galaxy 25 million years ago and travelled 25 million light years across space until it landed on my tablet. As distant as these measures are, this measures only our very closest cosmic neighbourhood, and in this regime, space is not expanding. Everything within our Galaxy is bound gravitationally to each other much more tightly than the expansion of the Universe can pull apart. The expansion is not so rapid that it is able to shear the Galaxy apart. Light’s loss of energy really only comes into play at much larger scales, far beyond the nearest galaxies.