On clear nights our north woods cabin is embraced by spangled darkness. I spend many hours prowling the constellations with a telescope, following visual paths into the universe. It's pleasing to understand that this enabling darkness is also a matter of human survival.
A telescope is a time machine. The light penetrating the lens of my 4-inch refractor delivers ancient messages. If I focus on M31, the Andromeda Galaxy, I'm seeing that whirlpool of stars not as it exists now, but as it appeared 2.5 million years ago. That's how long its light traveled to reach us across the chasm of intergalactic space—at a velocity of 186,000 miles per second.
The glowing oval in the eyepiece is a view that no longer pertains to our time. Yet we may safely assume M31 is still there. And under a dark rural sky, without optical aid, you can easily spot M31, a small hazy smudge near a fourth-magnitude star in the constellation of Andromeda. It's as far as you can see with your naked eye.
An observer somewhere in M31, peering at our galaxy, would see a structure that appears similar to what we see in a view of Andromeda. It's exciting to realize that M31 is speeding toward our Milky Way galaxy at 250,000 miles per hour. But even at that velocity, which would zip us to the moon in just under an hour, it will be several billion years before the two galaxies collide, blending the energy and gravity waves of 300 billion stars. The fate of our solar system will be up for grabs. It might be hurled into benign space or incinerated by the radiation of exploding stars. We can speculate about such events due to an epoch-making telescopic observation in the relatively recent past.
During predawn hours on Oct. 6, 1923, astronomer Edwin Hubble photographed M31 with a 100-inch reflecting telescope at Mount Wilson in California. In the upper right corner of the resulting photographic plate he scribbled "Var!" in red ink. That stood for "variable star," or more specifically, Cepheid variable, a type of sun that regularly brightens and dims in a predictable manner. The longer it takes the star to pulsate through its period, the greater its intrinsic brightness. Using the inverse square law, an astronomer can use this cycle as a rough measure of distance, by comparing apparent luminosity with true luminosity and doing some arithmetic.
When Hubble identified such a beacon in the Andromeda Galaxy, he calculated that it was more than a million light-years away. That was an astounding discovery because until then the general consensus was that our Milky Way galaxy was the extent of the universe. Astronomers thought our galaxy contained all the observed nebulae, or celestial clouds such as M31. Hubble's photograph was the seed of a paradigm shift. The acknowledged size of the universe—and its age—increased by orders of magnitude: It became clear that many of the nebulae were separate galaxies, some of them much larger than our own.
When I lift my face from the eyepiece of the telescope, I see a small, forested patch of northern Minnesota. This sylvan nook of boreal ecosystem, whose waters drain north toward Hudson Bay, is complex and fascinating in itself, but a tiny facet of our reality. The Earth circles the sun at a distance of 93 million miles, or eight light-minutes. That is, if the sun suddenly winked out, we wouldn't know it for 480 seconds.
The sun and the rest of the planets in our solar system are situated in one of the spiral arms of the Milky Way galaxy, about 28,000 light-years from the galactic core. Our solar system, whipping along at 140 miles per second, completes an orbit around this center once every 230 million years.
The sun is one of approximately 200 billion stars in the Milky Way. The nearest star to our own is about four light-years away: If we could fly there in an airliner at 600 miles per hour, it would take 4.5 million years to arrive. So although our galaxy is packed with about 30 times more stars than there are humans on our planet, the stars are extremely far apart.
If you are beyond urban light pollution on a clear, moonless night, your unaided eyes can see about 2,000 stars. But why so few? Given that the universe holds 100 billion galaxies, why is the natural night sky of the Earth so dark? Why doesn't our sky blaze with the hot, white light of innumerable alien suns?
Consider again my patch of forest. The individual trees average 6 to 12 inches in diameter at breast height, spaced roughly 5 to 15 feet apart. This density of woods sprawls far enough in every direction from our cabin that if I shot an arrow that whistled a quarter-mile, it would certainly strike a tree. That trunk might be 20 feet out or 200 feet out, but from the perspective of the arrow, our cabin is surrounded by a solid wall of wood.
And so it is with the stars. If you consider simple line of sight, the sky should be brighter than the disk of the sun. Professor Edward Harrison writes: "A line extended from the eye in any direction eventually terminates at a point on the surface of a star, and this occurs regardless of whether stars are distributed uniformly or clustered into galaxies." Darkness at night should be impossible. Just within the cup of the Big Dipper—which takes up 0.24 percent of the visible sky—a large telescope will reveal over 1 million galaxies. The sun, at 0.20 degrees, is 103,000 times smaller than the visible sky. In other words, 103,000 times the radiation of the sun should be apparent in the sky: Our planet should be fried in a brilliant conflagration.
Why isn't it? This question is called Olbers' Paradox, after Heinrich Olbers (1758–1840), a German physician and amateur astronomer. He clearly defined the problem in 1823, exactly 100 years before Hubble made his zeitgeist-altering observations. Several explanations had been offered over the previous centuries to resolve the puzzle, including English mathematician Thomas Digge's opinion that many stars were just too weak to be seen, or the idea of the great German astronomer Johannes Kepler that the night sky was dark because there simply weren't that many stars and they were "enclosed and circumscribed as by a wall or vault."
The first satisfactory explanation was provided by Edgar Allan Poe in 1848, in a piece titled Eureka: A Prose Poem. After outlining the line-of-sight conundrum, he wrote: "The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids [darkness] which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all." In other words, we see a finite distance in a cosmos where stars have been burning for a finite period. Or the universe is still too young for light from the most distant stars and galaxies to reach us. Recall that as you gaze into astronomical distances, you are peering back into time. For as Poe also noted in his essay, "Space and Duration are one." We must speak of spacetime.
Poe was anticipating the discoveries of Hubble, whose work helped demonstrate the universe (that is, space) is expanding. In this scenario, light from receding stars and galaxies "redshifts" to longer wavelengths and loses energy, appearing dimmer to our eyes.
The character of this cosmic fabric allows us not only to study the stars, but also to actually exist on the surface of our planet. Night is a necessary gift—not only for cosmological revelation, but also for survival. Darkness, our occasional repository of nightmares, is also our living space. In that sense, it's a natural resource. Next time you are in a region of Minnesota where the night sky is truly dark, be aware that such shadow is as valuable as clean air, clean water, healthy forest, and wildlife.