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.

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What's the Difference Between Straw and Hay?

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iStock.com/dusipuffi

The words straw and hay are often used interchangeably, and it's easy to see why: They're both dry, grassy, and easy to find on farms in the fall. But the two terms actual describe different materials, and once you know what to look for, it's easy to tell the difference between them.

Hay refers to grasses and some legumes such as alfalfa that are grown for use as animal feed. The full plant is harvested—including the heads, leaves, and stems—dried, and typically stored in bales. Hay is what livestock like cattle eat when there isn't enough pasture to go around, or when the weather gets too cold for them to graze. The baled hay most non-farmers are familiar with is dry and yellow, but high-quality hay has more of a greenish hue.

The biggest difference between straw and hay is that straw is the byproduct of crops, not the crop itself. When a plant, such as wheat or barley, has been stripped of its seeds or grains, the stalk is sometimes saved and dried to make straw. This part of the plant is lacking in nutrients, which means it doesn't make great animal fodder. But farmers have found other uses for the material throughout history: It what's used to weave baskets, thatch roofs, and stuff mattresses.

Today, straw is commonly used to decorate pumpkin-picking farms. It's easy to identify (if it's being used in a way that would be wasteful if it were food, chances are it's straw), but even the farms themselves can confuse the two terms. Every hayride you've ever taken, for example, was most likely a straw-ride.

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 and Why Did Silent Letters Emerge in English?

iStock/Bychykhin_Olexandr
iStock/Bychykhin_Olexandr

Kory Stamper:

The easy answer is “"because English can’t leave well enough alone."

When we first started speaking English around 600 AD, it was totally phonetic: every letter had a sound, and we sounded every letter in a word. But English—and England itself—were influenced quite a bit by the French, who conquered the island in 1066 and held it for a long time. And then later by Dutch and Flemish printers, who were basically the main publishers in England for a solid two centuries, and then by further trading contact with just about every continent on the planet. And while we’re shaking hands and stealing language from every single people-group we meet, different parts of the language started changing at uneven rates.

By the 1400s, English started to lose its phonetic-ness: the way we articulated vowels in words like “loud” changed slowly but dramatically, and that had an effect on the rest of the word. (This is called “The Great Vowel Shift,” and it took place over a few hundred years.) Somewhere in the middle of the GVS, though, English spelling became fixed primarily because of the printing press and the easy distribution/availability of printed materials. In short: we have silent letters because the spelling of words stopped changing to match their pronunciations.

This post originally appeared on Quora. Click here to view.

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