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.

What Do Those Recycling Symbols and Codes Mean?

iStock.com/ChrisSteer
iStock.com/ChrisSteer

Earth Day is here again, serving as an annual reminder of the need to reduce, reuse, and recycle our way to a better planet.

When it comes to the last part of that familiar three-”R” mantra, most people know enough to separate certain items from the rest of their garbage, but much of our modern recycling routine remains a mystery. From the recycling symbol itself to what those numbers on plastic containers actually mean, there's a lot you can learn from your trash before it becomes someone else's treasure.

An International Symbol With An Earthy Origin

The universal recycling symbol—three folded arrows that form a triangle, with the head of one arrow pointing to the tail of the next—was created in 1970 by University of Southern California student Gary Anderson as part of a contest tied to the very first Earth Day. Each arrow of the design represents one of the steps in the recycling process: collecting the recyclable goods after use, breaking them down and reforming them, and then packaging new products in the containers.

Originally designed as an inverted triangle, the symbol was later rotated to the pyramid-like orientation commonly used now.

The Number Game

The American Society of Plastics Industry first began using numbers inside the recycling symbols on plastic containers in 1988 as a way to assist with sorting them. The "Resin Identification Code" uses seven numbers to identify the type of synthetic material used to manufacture the container, with the higher numbers representing less commonly used plastics.

Here's a primer on each of the codes:

1. Polyethylene Terephthalate (PETE/PET)
Usually accompanied by the letters "PETE" or "PET," this resin is generally used for soda bottles and other containers for edible and non-edible goods. When it's not being used to manufacture containers, you might recognize it by another name: polyester. (Yes, it's the stuff that insulates your jackets.) It's also one of the most widely accepted forms of plastic in curbside recycling programs, though the amount of useable material available for new products after breaking down this plastic is relatively small.

2. High Density Polyethylene (HDPE)
The second most widely used resin for plastic bottles, HDPE is a stiff, strong material with a high resistance to chemicals, which has made it the go-to plastic for food items like milk and juice, as well as household cleaners and trash bags. It's also easy to break down in the recycling process and easy to reform, making it one of the most efficient consumer plastics. Most curbside recycling programs have no problem with accepting products made from this plastic.

3. Polyvinyl Chloride (PVC)
First discovered in the 19th century, PVC is commonly used in building materials today—especially pipes and plumbing material—due to its strength and chemical resistance (although it's occasionally used for some household products). It has a nasty habit of releasing highly carcinogenic toxins into the atmosphere when it's burned, so recycling is a significantly less appealing option for PVC disposal, and it's usually not accepted by curbside recycling programs.

4. Low Density Polyethylene (LDPE)
This plastic is becoming more common today, especially for manufacturing squeeze bottles and grocery bags. Plastics made from LDPE are usually very strong, and they're regularly used as sealants because of this quality. While they weren't included in curbside recycling programs at first, plastics made from LDPE are now becoming more commonly accepted.

5. Polypropylene (PP)
Regarded as one of the “safest” plastics produced today, PP is generally used for squeezable bottles, bottle caps, and straws. Along with LDPE, it's also used for food-storage containers that can be reused over time. It has an extremely high melting point, so it's one of the best consumer plastics for items that will be exposed to heat. Like LDPE, it's becoming more common for curbside recycling programs to accept items made from this plastic.

6. Polystyrene (PS)
More commonly known as styrofoam, this type of plastic is not only notoriously difficult to recycle, but it's also been shown to leach dangerous toxins over time into anything packaged in it—and even greater amounts of toxins when it's burned. This is the resin usually found in disposable serving trays, egg cartons, and cups, and it's rarely accept by curbside recycling programs due to the danger it poses and the difficulty of recycling it. Basically, this is the worst of the bunch.

7. Everything Else
There are countless other plastics, but very few of them are easily recycled in curbside programs, making this category the catch-all for everything that could conceivably be broken down and reformed, but might be better off reused or reformed in some way that doesn't require a chemical process. This category encompasses everything from bulletproof material to those large water jugs on office coolers, and is rarely included in curbside recycling programs.

Safety In Numbers

For anyone wondering which plastics are safe to reuse in their current form, it's widely accepted that HDPE (2), LDPE (4), and PE (5) can be reused multiple times for edible items, as they're generally resistant to chemicals, haven't been shown to degrade, and don't leach dangerous substances into their contents.

This story first ran in 2013.

What Do the Terms on Energy-Saving Light Bulbs Mean?

Scott Olson, Getty Images
Scott Olson, Getty Images

There's a reason your parents used to scold you for not turning off a light when you left a room. According to the U.S. Department of Energy, an average American household uses up to 5 percent of their total energy expenditure on lighting. Living rooms get flooded with light. Dining rooms and dens are full of lighting accents. Motion lights, hallway lights, bathroom vanity lights, lamps—we like our lives to be nice and bright.

Fortunately, energy-saving lighting sources have largely replaced the conventional incandescent bulbs that once used up a substantial amount of power. Those bulbs heated up a coil, or filament, of tungsten wire that gave off light. Roughly 90 percent of the energy they passed on was in the form of heat, which siphoned off energy and kept utility bills inflated. Today's bulbs brighten without the waste. That's the good news. The bad? The varieties of bulbs can be confusing. If you've ever been lost in the fixtures section of the hardware store, here's a quick primer on what these terms mean.

Halogen Incandescent:

These are incandescent light bulbs that contain a halogen gas-filled capsule around the filament to help increase energy efficiency. While cheaper to operate than a conventional incandescent bulb—they use 25 to 30 percent less energy—they don't produce as much of a cost savings as other options. On the plus side, they reach full brightness immediately. Other choices may take time to warm up.

Compact Florescent Lamp (CFL):

When you see a coiled light bulb, it’s likely to be a CFL, which is simply a downsized version of the tubular florescent lighting seen in commercial spaces. Instead of an electric current traveling through a filament like in an incandescent bulb, the current goes through a tube containing argon and mercury vapor. The resulting ultraviolet light activates phosphor inside the tube, which emits light. It uses one-third of the energy of a halogen incandescent. The downside? They can take a little time to warm up, especially if used outdoors. They also contain mercury, a potential health hazard if the bulb breaks. (See the "mercury" entry below.)

Light Emitting Diode (LED):

This type of bulb uses a semiconductor to convert electricity into light. In addition to being energy-efficient, they usually last eight to 25 times longer than halogen incandescent bulbs and four to eight times longer than CFLs—perhaps as long as 18 to 46 years. You'll probably pay more up front, but the expense is offset by their durability. Most LEDs are compatible with dimming switches, too. Most CFLs aren't, so if that's important to you, you'll want to stick with LED.

Energy Star:

A bulb with an Energy Star label was evaluated by a third party to make sure its energy-saving claims are accurate, and they'll typically have a longer warranty than bulbs without the endorsement.

But what about the "nutritional label" style information box that appears on light bulb packaging? Let's take a closer look.

An example of a label that appears on energy-efficient light bulb packaging is pictured
Federal Trade Commission

Brightness:

You have probably inferred that brightness refers to the light output given off by a bulb. This is measured in lumens and rounded off to the nearest five. (A bulb will never be 822 lumens. It's 820.) The higher the number, the brighter the bulb. Since you're probably used to shopping by wattage, consider that a bulb with 800 lumens is roughly the equivalent of a 60-watt incandescent. A 1100 lumen bulb will resemble a 75-watt bulb.

Estimated Energy Cost:

This is a rough estimate of much it will cost an average household to operate the bulb. What's average? The wattage of the bulb is calculated with three hours of daily operation at a cost of 11 cents per kilowatt. Your actual cost will go up or down whether you use it more or less or pay your energy supplier a different amount.

Life:

This is how long the bulb is expected to last based on the same usage estimated for the energy cost and rounded to the nearest tenth of a year.

Light Appearance:

This refers to the color temperature of the bulb measured in Kelvin, a temperature scale measuring light color. The range is from 2600 K (yellow and warm) to 6600 K (blue and cool). Bright white is about 3500 K. You should probably avoid anything above 3000 K for any interior room.

Energy Used:

This is how much energy the bulb will require and is measured in watts. The lower the wattage, the cheaper it costs to operate. This is where the energy savings materializes, as a 10-watt LED bulb may give off as much light as an old 60-watt incandescent.

Color Rendering Index (CRI):

It's not on all bulb packaging, but if you see it, it refers to how accurate colors will appear under the bulb's light on a scale of 0 to 100. Halogen incandescent bulbs score high. CFLs and LEDs aren't quite as accurate, though they may still get the job done. Try to get a high CRI if you'll be using the bulbs in a bathroom, as skin tone can appear off with lower CRI numbers.

Mercury:

You might see some CFL bulb packaging with a mercury disclosure. This isn't an issue if the bulb remains intact, but if it breaks, it might release potentially hazardous mercury vapor and the introduce the very small possibility of mercury poisoning. Avoid using CFL bulbs in kids' rooms if there's potential for knocking over a lamp or light. Broken bulbs that contain mercury should be cleaned up by following Environmental Protection Agency guidelines—picked up with tape, not vacuumed—and disposed of properly. Old bulbs should be recycled.

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