Does Sound Travel Faster or Slower in Space?

iStock/BlackJack3D
iStock/BlackJack3D

Viktor T. Toth:

It is often said that sound doesn’t travel in space. And it is true … in empty space. Sound is pressure waves, that is, propagating changes in pressure. In the absence of pressure, there can be no pressure waves, so there is no sound.

But space is is not completely empty and not completely devoid of pressure. Hence, it carries sound. But not in a manner that would match our everyday experience.

For instance, if you were to put a speaker in interstellar space, its membrane may be moving back and forth, but it would be exceedingly rare for it to hit even a single atom or molecule. Hence, it would fail to transfer any noticeable sound energy to the thin interstellar medium. Even the somewhat denser interplanetary medium is too rarefied for sound to transfer efficiently from human scale objects; this is why astronauts cannot yell to each other during spacewalks. And just as it is impossible to transfer normal sound energy to this medium, it will also not transmit it efficiently, since its atoms and molecules are too far apart, and they just don’t bounce into each other that often. Any “normal” sound is attenuated to nothingness.

However, if you were to make your speaker a million times bigger, and let its membrane move a million times more slowly, it would be able to transfer sound energy more efficiently even to that thin medium. And that energy would propagate in the form of (tiny) changes in the (already very tiny) pressure of the interstellar medium, i.e., it would be sound.

So yes, sound can travel in the intergalactic, interstellar, interplanetary medium, and very, very low frequency sound (many octaves below anything you could possibly hear) plays an important role in the formation of structures (galaxies, solar systems). In fact, this is the mechanism through which a contracting cloud of gas can shed its excess kinetic energy and turn into something compact, such as a star.

How fast do such sounds travel, you ask? Why, there is no set speed. The general rule is that for a so-called perfect fluid (a medium that is characterized by its density and pressure, but has no viscosity or stresses) the square of the speed of sound is the ratio of the medium’s pressure to its energy density. The speed of sound, therefore, can be anything between 0 (for a pressureless medium, which does not carry sound) to the speed of light divided by the square root of three (for a very hot, so-called ultrarelativistic gas).

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

Grocery Stores vs. Supermarkets: What’s the Difference?

gpointstudio/iStock via Getty Images
gpointstudio/iStock via Getty Images

These days, people across the country are constantly engaging in regional term debates like soda versus pop and fireflies versus lightning bugs. Since these inconsistencies are so common, you might have thought the only difference between a grocery store and a supermarket was whether the person who mentioned one was from Ohio or Texas. In reality, there are distinctions between the stores themselves.

To start, grocery stores have been around for much longer than supermarkets. Back when every town had a bakery, a butcher shop, a greengrocery, and more, the grocery store offered townspeople an efficient shopping experience with myriad food products in one place. John Stranger, vice president group supervisor of the food-related creative agency EvansHardy+Young, explained to Reader’s Digest that the grocer would usually collect the goods for the patron, too. This process might sound familiar if you’ve watched old films or television shows, in which characters often just hand over their shopping lists to the person behind the counter. While our grocery store runs may not be quite so personal today, the contents of grocery stores remain relatively similar: Food, drinks, and some household products.

Supermarkets, on the other hand, have taken the idea of a one-stop shop to another level, carrying a much more expansive array of foodstuffs as well as home goods, clothing, baby products, and even appliances. This is where it gets a little tricky—because supermarkets carry many of the same products as superstores, the next biggest fish in the food store chain, which are also sometimes referred to as hypermarkets.

According to The Houston Chronicle, supermarkets and superstores both order inventory in bulk and usually belong to large chains, whereas grocery stores order products on an as-needed basis and are often independently owned. Superstores, however, are significantly larger than either grocery stores or supermarkets, and they typically look more like warehouses. It’s not an exact science, and some people might have conflicting opinions about how to categorize specific stores. For example, Walmart has a line of Walmart Neighborhood Markets, which its website describes as “smaller-footprint option[s] for communities in need of a pharmacy, affordable groceries, and merchandise.” They’re not independently owned, but they do sound like grocery stores, especially compared to Walmart’s everything-under-the-sun superstore model.

Knowing the correct store terms might not always matter in casual conversation, but it could affect your credit card rewards earnings. American Express, for example, offers additional rewards on supermarket purchases, and it has a specific list of stores that qualify as supermarkets, including Gristedes, Shoprite, Stop & Shop, and Whole Foods. Target and Walmart, on the other hand, are both considered superstores, so you won’t earn bonuses on those purchases.

And, since grocery shopping at any type of store can sometimes seem like a competitive sport, here’s the ideal time to go.

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What Does CPR Stand For?

undefined undefined/iStock via Getty Images
undefined undefined/iStock via Getty Images

The life-saving technique known as CPR stands for cardiopulmonary resuscitation. It's a method that allows oxygenated blood to temporarily circulate throughout the body of a person whose heart has stopped. When the heart ceases beating during cardiac arrest, lungs stop receiving oxygen. Without oxygen, nerve cells start to die within minutes; it can take just four to six minutes for an oxygen-deprived person to sustain permanent brain damage or die.

The cardio part of the phrase refers to the heart, the muscular organ that pumps blood through the body's circulatory system. Pulmonary involves the lungs. People take approximately 15 to 20 breaths per minute, and with each breath you take, your lungs fill with oxygen. Resuscitation means bringing something back to consciousness, or from the brink of death.

We have two physicians, Peter Safar and James Elam, to thank for developing mouth-to-mouth resuscitation in the mid-1950s. In 1957, the American military adopted their CPR method for reviving soldiers. In 1960, the American Heart Association integrated chest compressions, which keep the blood circulating.

Doctors, nurses, dentists, first responders, lifeguards, and some teachers are required to be certified in CPR. But because approximately 85 percent of cardiac arrests occur at home, it’s smart for the average person to know how to perform it, too. In school, you were probably taught CPR by the traditional method of giving 100 to 120 chest compressions per minute (play the Bee Gees’ "Stayin’ Alive" in your head to keep the beat) and mouth-to-mouth resuscitation. Today, the American Heart Association recommends that average people learn hands-only CPR, which simply involves chest compressions. The organization has found that people can be reluctant to administer mouth-to-mouth CPR in an emergency because they're afraid of doing it wrong or injuring the patient. With hands-only CPR, bystanders feel less anxiety and more willingness to jump in. The AHA also notes that hands-only CPR can be just as effective in saving a life. (And any CPR is better than none at all.)

But how many people actually know CPR?

In 2018, a Cleveland Clinic survey found that 54 percent of Americans said they knew CPR, but only one in six people knew that bystander CPR requires only chest compressions. Only 11 percent of people knew the correct pace for compressions. Again, singing "Stayin' Alive" to yourself is one way to remember the pace—though being a fan of The Office can apparently help, too (as one lucky life-saver recently discovered).

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