Why Is It So Dark in Outer Space?

iStock/blackdovfx
iStock/blackdovfx

by Kenny Hemphill

Interstellar. Gravity. 2001: A Space Odyssey. Even Star Wars. They all have one thing in common: Beyond the lights of their spacecrafts, and aside from the faint needlepoint glow of distant stars, space is oil-slick dark.

Why that should be so is a question scientists have been asking for more than 400 years. Everyone from Johannes Kepler to Edmond Halley has had a go at trying to figure it out. But it was German astronomer Heinrich Wilhelm Olbers who gave his name to the paradox of the dark sky. Olbers wondered: If the universe is infinite, and there are an infinite number of infinitely old stars, why isn't the light from those stars visible from Earth? If it were, the night sky would be bright, not dark.

By the end of the 19th century, the idea of an infinite universe had been largely abandoned—something which was anticipated by Edgar Allan Poe in his 1848 essay, Eureka, where he wrote:

"Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy—since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids 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, Olbers' Paradox is resolved with the assumption that the universe has a finite age (something which is supported by the Big Bang Theory), that the speed of light is finite, and thus the observable universe has a horizon beyond which we can't see the stars. Fifty years later, Lord Kelvin used math to prove that in a finite universe, or one in which stars were born and died, the night sky should be dark.

There are other contributing factors to the darkness out there. Cosmic expansion over billions of years means that the energy from the radiation which was emitted following the Big Bang has been red-shifted, or reduced to the low temperature of microwaves. That puts it beyond the visible spectrum. And other radiation in space—infrared and ultra-violet light, radio waves, and X-rays—are all invisible to the human eye. If we could see them, space would seem a little less dark.

Universe Today has another explanation: "Space is black to our perception because there are few molecules of matter that can reflect or scatter light like our atmosphere on Earth. Since light goes in a straight line it seems to be absorbed by the void and vacuum of space. Otherwise space would look similar to the sky on Earth."

Think of a flashlight in a dark room. Look directly at the bulb and you see its light. Point it at furniture or a wall, and you see the light reflected. If there was nothing to reflect it, you wouldn't see any light at all. Which is exactly what happens in space.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

How Did 6 Feet Become the Standard Grave Depth?

iStock
iStock

It all started with the plague: The origins of “six feet under” come from a 1665 outbreak in England. As the disease swept the country, the mayor of London literally laid down the law about how to deal with the bodies to avoid further infections. Among his specifications—made in “Orders Conceived and Published by the Lord Mayor and Aldermen of the City of London, Concerning the Infection of the Plague”—was that “all the graves shall be at least six feet deep.”

The law eventually fell out of favor both in England and its colonies. Modern American burial laws vary from state to state, though many states simply require a minimum of 18 inches of soil on top of the casket or burial vault (or two feet of soil if the body is not enclosed in anything). Given an 18-inch dirt buffer and the height of the average casket (which appears to be approximately 30 inches), a grave as shallow as four feet would be fine.

A typical modern burial involves a body pumped full of chemical preservatives sealed inside a sturdy metal casket, which is itself sealed inside a steel or cement burial vault. It’s less of a hospitable environment for microbes than the grave used to be. For untypical burials, though—where the body isn’t embalmed, a vault isn’t used, or the casket is wood instead of metal or is foregone entirely—even these less strict burial standards provide a measure of safety and comfort. Without any protection, and subjected to a few years of soil erosion, the bones of the dearly departed could inconveniently and unexpectedly surface or get too close to the living, scaring people and acting as disease vectors. The minimum depth helps keep the dead down where they belong.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

This article originally appeared in 2012.

What's the Difference Between Apple Juice and Apple Cider?

iStock/Alter_photo
iStock/Alter_photo

In a time before pumpkin spice went overboard with its marketing, people associated fall with fresh apples. Crisp and fresh, they practically beg to be crushed and pulped into liquid. But what’s the difference between apple juice and apple cider?

According to the state of Massachusetts, home to a variety of apple-picking destinations, both apple juice and apple cider are fruit beverages. But apple cider is raw, unfiltered juice—the pulp and sediment are intact. To make cider, the apples are ground into an applesauce-like consistency, then wrapped in cloth. A machine squeezes the layers and strains out the juice into cold tanks. That’s the cider that ends up on store shelves.

Apple juice, on the other hand, takes things a step further—removing solids and pasteurizing the liquid to lengthen its shelf life. It’s typically sweeter, possibly with added sugar, and may lack the stronger flavor of its relatively unprocessed counterpart. It’s also often lighter in color, since the remaining sediment of cider can give it a cloudy appearance.

But that’s just the Massachusetts standard. Each state allows for a slight variation in what companies are allowed to call apple cider versus apple juice. The cider may be pasteurized, or the cider and juice may actually be more or less identical. One company, Martinelli’s, states in its company FAQ that their two drinks are the same in every way except the label: "Both are 100 percent pure juice from U.S. grown fresh apples. We continue to offer the cider label since some consumers simply prefer the traditional name for apple juice."

The US Apple Association, a nonprofit trade organization that represents growers nationwide, indicates that apple juice can be made from concentrate, which is why you might see water as the first ingredient on the label. Generally, cider is the hard stuff: Crushed apples with minimal processing. Because it can ferment, it's usually found refrigerated. Apple juice can often be found elsewhere in stores, where it can remain stable.

Which you should buy comes down to personal preference. Typically, though, recipes calling for apple cider should use apple cider. Processed juice may be too sweet an ingredient. And you can always try making a pumpkin spice hot apple cider, although we may stop talking to you if you do.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

SECTIONS

arrow
LIVE SMARTER