Why half life

The graph shown in Figure 1 is a visual representation of these equations above. It is important to note that regardless of the actual length of the half-life whether it is millions of years or a few nanoseconds the shape of the graph will be the same. Knowledge of half lives is part of how geologists date rocks with radioisotopic dating.

Fossil Fuels. Nuclear Fuels. Acid Rain. Climate Change. Climate Feedback. Ocean Acidification. Rising Sea Level. The most obvious instance is drugs; the half-life is the time it takes for their effect to halve, or for half of the substance to leave the body. The half-life of caffeine is around 6 hours, but as with most biological half-lives numerous factors can alter that number. People with compromised liver function or certain genes will take longer to metabolize caffeine.

Consumption of grapefruit juice has been shown in some studies to slow caffeine metabolism. It takes around 24 hours for a dose of caffeine to fully leave the body.

The half-lives of drugs vary from a few seconds to several weeks. To complicate matters, biological half-lives vary for different parts of the body. Lead has a half-life of around a month in the blood, but a decade in bone. Plutonium in bone has a half-life of a century — more than double the time for the liver. Marketers refer to the half-life of a campaign — the time taken to receive half the total responses.

Unsurprisingly, this time varies among media. A paper catalog may have a half-life of about three weeks, whereas a tweet might have a half-life of a few minutes. Calculating this time is important for establishing how frequently a message should be sent. According to Arbesman, information has a predictable half-life: the time taken for half of it to be replaced or disproved.

Over time, one group of facts replaces another. As our tools and knowledge become more advanced, we can discover more — sometimes new things that contradict what we thought we knew, sometimes nuances about old things. The rate of these discoveries varies.

Our body of engineering knowledge changes more slowly, for example, than does our body of psychological knowledge. Arbesman studied the nature of facts. The field was born in , when mathematician Derek J. Price noted something surprising: the sizes of the books fit an exponential curve.

His curiosity piqued, he began to see whether the same curve applied to science as a whole. Price established that the quantity of scientific data available was doubling every 15 years. This meant that some of the information had to be rendered obsolete with time. Scientometrics shows us that facts are always changing, and much of what we know is or soon will be incorrect. Indeed, much of the available published research, however often it is cited, has never been reproduced and cannot be considered true.

Many researchers are incentivized to find results that will please those giving them funding. Intense competition makes it essential to find new information, even if it is found in a dubious manner.

Yet we all have a tendency to turn a blind eye when beliefs we hold dear are disproved and to pay attention only to information confirming our existing opinions.

As an example, Arbesman points to the number of chromosomes in a human cell. Up until , 48 was the accepted number that medical students were taught. In , it had been declared an established fact by a leading cytologist. They declared the true number to be During their research, Tjio and Levan could never find the number of chromosomes they expected.

Discussing the problem with their peers, they discovered they were not alone. Plenty of other researchers found themselves two chromosomes short of the expected Many researchers even abandoned their work because of this perceived error.

But Tjio and Levan were right for now, anyway. As Arbesman puts it, facts change incessantly. Many of us have seen the ironic in hindsight doctor-endorsed cigarette ads from the past.

A glance at a newspaper will doubtless reveal that meat or butter or sugar has gone from deadly to saintly, or vice versa. We forget that laughable, erroneous beliefs people once held are not necessarily any different from those we now hold. The people who believed that the earth was the center of the universe, or that some animals appeared out of nowhere or that the earth was flat, were not stupid.

They just believed facts that have since decayed. Arbesman gives the example of a dermatology test that had the same question two years running, with a different answer each time. This is unsurprising considering the speed at which our world is changing. Our world seems to be in constant flux. With our knowledge changing all the time, even the most informed people can barely keep up. All this change may seem random and overwhelming Dinosaurs have feathers? When did that happen? This order is regular and systematic and is one that can be described by science and mathematics.

The order Arbesman describes mimics the decay of radioactive elements. Whenever new information is discovered, we can be sure it will break down and be proved wrong at some point.

If we zoom out and look at a particular body of knowledge, the random decay becomes orderly. Through probabilistic thinking, we can predict the half-life of a group of facts with the same certainty with which we can predict the half-life of a radioactive atom.

The problem is that we rarely consider the half-life of information. Many people assume that whatever they learned in school remains true years or decades later. Medical students who learned in university that cells have 48 chromosomes would not learn later in life that this is wrong unless they made an effort to do so. OK, so we know that our knowledge will decay.

What do we do with this information? Arbesman says,. We would end up going a little crazy as we frantically tried to keep up with the ever changing facts around us, forever living on some sort of informational treadmill. It takes another two days for the count rate to halve again, this time from 40 to Note that this second two days does not see the count drop to zero, only that it halves again. A third, two-day period from four days to six days sees the count rate halving again from 20 down to This process continues and although the count rate might get very small, it does not drop to zero completely.

The half-life of radioactive carbon is 5, years. If a sample of a tree for example contains 64 grams g of radioactive carbon after 5, years it will contain 32 g, after another 5, years that will have halved again to 16 g. It should also be possible to state how much of a sample remains or what the activity or count should become after a given length of time.

This could be stated as a fraction, decimal or ratio. For example the amount of a sample remaining after four half-lives could be expressed as:.