Slice of Light

A light-year is the distance light travels in one year. In the vast-ness of space beyond the solar system, astronomers use the light-year as a basic unit of distance.

Star Words

A light-year is the distance light travels in one year. In the vast-ness of space beyond the solar system, astronomers use the light-year as a basic unit of distance.

The universe is ruled by the tyranny of distance. That is, the universe is so vast, that we are able to see many things that we will never be able to visit. Light is able to travel at extraordinary speeds (about 984,000,000 feet, or 300,000,000 meters, every second), but the light that we now see from many objects in the sky left those sources thousands, millions, or even billions of years ago. It is possible, for example, to see the Andromeda galaxy, even with the naked eye, but will we ever travel there?

Well, Andromeda is about two million light-years away, and a light-year is the distance light travels in one year—about 9,461,000,000,000,000 meters (some 6 trillion miles). Now, light can travel that far every year, so to get the distance to Andromeda, you multiply the velocity of light (6 trillion miles in a year) by the amount of time it took the light to get here (2 million years), and you get a lot of miles—approximately a 1 with 19 zeroes after it. Another way to think about these unbelievable distances: If you could travel at the speed of light (an impossibility, according to Einstein's theory of relativity, as we will see in later chapters), it would still take you two million years to reach Andromeda.

But we can't travel at anywhere near the speed of light. Right now, the fastest rockets are capable of doing 30,000 miles per hour (48,000 km/h). Maybe— someday—technology will enable us at least to approach the speed of light, but that still means a trip of two million years up and two million back. All of recorded history consumes no more than 5,500 years, and civilization, perhaps 10,000 years.

Why not go faster than the speed of light? In Chapter 7, "Over the Rainbow," we'll see that, according to our understanding of space and time, the speed of light is an absolute speed limit, which cannot be exceeded.

So revel in the fact that, on a clear night, you are able to gaze at the Andromeda galaxy, an object so distant that no human being will likely ever visit it.

Electromagnetic radiation transfers energy and information from one place to another, even in the vacuum of space. The energy is carried in the form of rapidly fluctuating electric and magnetic fields and includes visible light in addition to the similar but less familiar radio, infrared, ultra-violet, x-ray, and gamma ray radiation.

Star Words

Electromagnetic radiation transfers energy and information from one place to another, even in the vacuum of space. The energy is carried in the form of rapidly fluctuating electric and magnetic fields and includes visible light in addition to the similar but less familiar radio, infrared, ultra-violet, x-ray, and gamma ray radiation.

The Andromeda galaxy, as photographed by the Yerkes Observatory, 1930s.

(Image from arttoday.com)

Space ships may be severely limited as to how fast they can travel, but as we've said, the information conveyed by electromagnetic radiation can travel at the speed of light. The information from Andromeda, it is true, is not exactly recent news by the time we get it. In fact, the photons that we are receiving from Andromeda left that galaxy long before Homo sapiens walked the earth. But everything we know about Andromeda and almost all other celestial bodies (aside from the few solar system objects we have visited with probes or landers), we know by analyzing their electromagnetic radiation: radio, infrared, and ultraviolet radiation, as well as x-rays and gamma rays and what we call light.

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