Why Do Scientists Measure Things by Half-Life?

istock.com/SPmemory
istock.com/SPmemory

Reader @Procrustes tweeted at us to ask: “Why do scientists measure things like radioactive elements in half-life? Why not just measure the whole life?”

If you’re not familiar with the term “half-life,” maybe you’ve heard one of your nerd friends use it. If they weren’t complaining about a guy named Gabe and ranting about steam and a valve, they were probably using it in reference to radiometric dating, a technique that uses measurement of radioactive decay to figure out the age of archaeological artifacts and dinosaur fossils.

Decay and Dating

At the center of every atom is a dense region called a nucleus, which consists of protons and neutrons. In some atoms, the forces in the nucleus are balanced and the nucleus is stable. In others, the forces are unbalanced and the nucleus has an excess of internal energy; it’s unstable, or radioactive. These unstable atoms essentially self-destruct because of the imbalance and break down, or decay. When they do this, they lose energy by emitting energetic subatomic particles (radiation).

These particles can be detected, typically with a Geiger counter. In the case of radiocarbon dating, a common dating method for organic matter that uses carbon-14 (an isotope, or variant, of the element carbon) to estimate age, one radioactive “beta particle” is produced for every carbon-14 atom that decays. By comparing the normal abundance of carbon-14 in a living creature (which is the same concentration in the atmosphere) with the amount left in the material being dated, based on the known decay rate, scientists can figure out roughly how long ago whatever they’re looking at was still alive.

Half-life steps onto the scene in the decay process. While the lifespan of any individual atom is random and unpredictable, the probability of decay is constant. You can’t predict when an unstable atom will break down, but if you have a group of them, you can predict how long it will take. Atoms that have an equal probability of decaying will do so at an exponential rate. That is, the rate of decay will slow in proportion to the amount of radioactive material you have.

“Many will disappear early on in the process but some will last for much longer time periods,” says Dr. Michael Dee, a researcher at Oxford University’s radiocarbon lab. “It’s a bit like putting (a lot) of coins out in the rain. Although they all have an equal probability of being hit by raindrops, many will be struck immediately and others will remain dry, perhaps for an extended period of time.”

It’s easy misinterpret half-life to mean “one half of the time it takes for whatever atoms you’re looking at to decay,” but it actually means “the length of time it takes for one half of the atoms you’re looking at to decay.” The measurement is useful in radiometric dating, says Dee, because exponential decay means “it doesn’t matter how much radioactive material you have, the time taken until half of it is gone [the half-life] is always the same.”

The whole life of the material, on the other hand, would be equal to the lifespan of the last atom in the group to decay. Since an atom’s lifespan is random, inestimable and essentially infinite, the whole life would be, too. It winds up being a not-very-useful measurement. “It’s a bit like one coin sitting out in the rain,” says Dee. “And never getting hit, ever.”

How Is a Sunscreen's SPF Calculated?

Rawpixel/iStock via Getty Images
Rawpixel/iStock via Getty Images

I’m a pale person. A very pale person. Which means that during these hot summer months, I carry sunscreen with me at all times, and apply it liberally. But I’ve never really understood what those SPF numbers meant, so I asked some sun care to break it down for me—and to tell me how to best apply the stuff so that I can make it through the summer without looking like a lobster.

Soaking up the sun ... safely

SPF stands for Sun Protection Factor, and it indicates a sunscreen’s ability to block UVB rays. The concept was pioneered at the Coppertone Solar Research Center in 1972; in 1978, the FDA published an SPF method based on Coppertone’s system, according to Dr. David Leffell, chief of Dermatologic Surgery and Cutaneous Oncology at Yale.

The numbers themselves stand for the approximate measure of time a person who has applied the sunscreen can stay out in the sun without getting burned. Say you get burned after 20 minutes in the sun without sunscreen; if properly applied (and reapplied), SPF 30 will allow you to stay in the sun 30 times longer without burning than if you were wearing no protection at all. So, theoretically, you should have approximately 600 minutes, or 10 hours, in the sun. But it’s not an exact science because the amount of UV light that reaches us depends on a number of factors, including cloud cover, the time of day, and the reflection of UV rays off the ground, so it’s generally recommended that you reapply sunscreen every two hours (or even sooner).

What gives a sunscreen a higher SPF comes down to the product’s formulation. “It’s possible that an SPF 50 might contain slightly more of one or more sunscreen active ingredients to achieve that higher SPF,” Dr. Patricia Agin, president of Agin Suncare Consulting, says. “But it’s also possible that the SPF 50 might contain an additional active ingredient to help boost the SPF performance to SPF 50.”

No matter what SPF your sunscreen is, you’ll still get a burn if it’s not properly applied. So let’s go over how to do that.

How to apply sunscreen

First, make sure you have a water-resistant, broad spectrum sunscreen—which means that it protects against both UVB and UVA radiation—with an SPF of at least 30. “Typically, you don’t have to buy sunscreen that has an SPF higher than that unless you have very sun sensitive skin,” Leffell says. “That’s a very small percentage of the population.” (Redheads, people with light eyes, and those who turn pink after just a few minutes in the sun—you’ll want to load up on SPF above 30.)

Twenty minutes before you go out to the beach or the pool, begin to apply your sunscreen in an even coat. “Don’t apply it like icing on a cake,” Leffell says. “I see these patients and they’ve got the tops of their ears covered with thick, unevenly applied sunscreen, and that’s not a good sign.” Sunscreen sprays will easily give you that even coat you need.

Whether you’re using lotion or a spray, when it comes time to apply, Leffell recommends starting with your scalp and face, even if you plan on wearing a hat. “Make sure you’ve covered the ears and nose and under the eyes,” Leffell says. “Then, I would move down to the shoulders, and make sure that someone can apply the sunscreen on your back beyond the reach of your hands.”

Other areas that are important that you may forget to cover, but shouldn’t, are the tops of your feet, the backs of your hands, and your chest. “We see it all the time now—the v of the chest in women has become a socially and aesthetically huge issue when they are 50 and beyond. Because even though they can treat their faces with all sorts of cosmetics and procedures, the chest is much harder, and they are stuck with the face of a 40-year-old and the chest of a 60-year-old. You want to avoid that using sunscreen.”

Another important thing to keep in mind: Water-resistant doesn’t mean waterproof. “I always tell patients to reapply every couple of hours while you’re active outdoors," Leffell says, "and always reapply when you come out of the water or if you’ve been sweating a lot, regardless of whether the label says water resistant."

Determining whether or not you’ve succeeded in properly applying your sunscreen is easy: “You know you’re applying your sunscreen properly if, after the first time you’ve used it, you haven’t gotten a burn,” Leffell says.

Agin has a caveat, though: "It’s not a good idea to think of sunscreens only as a way to extend your time in the sun," she says. "One must also understand that even before becoming sunburned, your skin is receiving UV exposure that causes other damage to the skin. At the end of the 600 minutes, you will have accrued enough UV to cause a sunburn—one Minimal Erythema Dose or MED—but there is pre-MED damage done to skin cells’ DNA and to the skin’s supporting structure of collagen and elastin that is not visible and happens even before you sunburn. These types of damage can occur without sunburning. So you can’t measure all the damage done to your skin by only being concerned about sunburn."

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

An earlier version of this post ran in 2014.

What's the Difference Between Ice Cream and Gelato?

iStock/Getty Images/zoff-photo
iStock/Getty Images/zoff-photo

'Tis the season for beach reads, tan lines, and ice-cold desserts. You know it's summer when going to the local ice cream or gelato shop becomes part of your daily routine. But, what exactly is the difference between these two frozen treats?

One of the key differences between the two is butterfat. While ice cream's main ingredients include milk, cream, sugar, and egg yolks, the secret to making gelato is to use much less cream and sometimes little to no egg yolk. This leads to a much smaller percentage of butterfat in gelato. The FDA rules say that ice cream cannot contain less than 10 percent milkfat (though it can go as high as 25 percent) while gelato, much like soft serve, stays in the 4- to 9-percent range.

The churning method for both also differs, which affects the treat's density. Ice cream is churned at a much faster pace, leading to more air being whipped into the mixture. Ice cream's higher butterfat content comes into play here—due to all of that milkfat, the mix absorbs the air more readily. Gelato, on the other hand, is churned at a slower pace and absorbs far less air, creating a much denser dessert.

You also might have noticed that the serving style for the two treats aren't the same, either. In order to get those perfectly stacked ice cream scoops on a cone, buckets of ice cream must be stored at around 0°F to maintain its consistency, while the softer gelato is stored at a warmer 10°F to 22°F. Ice cream is then scooped into fairly uniform balls with the round ice cream scooper, whereas a spade or paddle is best for molding gelato into mound in a cup or a cone.

You can't really go wrong with either gelato or ice cream on a sweltering summer day, but there is one more difference to keep in mind while you debate which to get: taste. If you want a bolder flavor, you'll want to go with gelato. Because of the density of the cream and because there's less butterfat to coat your taste buds, gelato can seem to have more intensity to its flavors.

Have you got a Big Question you'd like us to answer? If so, send it to bigquestions@mentalfloss.com.

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