Science Questions with Surprising Answers
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Dr. Christopher S. Baird

What makes radioactive atoms get old so quickly and decay?

Category: Physics      Published: March 13, 2015

radioactive decay
Artistic illustration of radioactive beta decay. Public Domain Image, source: Christopher S. Baird.

Atoms don't age. Atoms radioactively decay when a lower-energy nuclear configuration exists to which they can transition. The actual decay event of an individual atom happens randomly and is not the result of the atom getting old or changing through time.

The phrases "getting old" or "aging" are rather vague and could refer to a lot of things. For biological organisms and mechanical devices, "aging" usually refers to the progression of complex internal processes. A single atom does not have any internal biological or mechanical systems, and therefore does not age in this way. There is no clock inside an atom telling it that it is now a minute older. For other objects, "aging" refers to the wearing down or corrosion of the object because of repeated use or exposure to the environment. Atoms are too simple to wear down, corrode, or steadily change. No matter what reasonable definition we use for the word "aging", individual atoms don't do it. Note that aging is different from experiencing time. Everything, including atoms, experiences time. An atom can sit at on my desk on Tuesday and then fall off and sit on the carpet on Wednesday, because it experiences time. However, an isolated atom does not deterministically change from one day to the next. (An atom's electrons and nucleons can be excited, but these excited particles quickly relax back down to the ground state. Therefore, excitations do not fundamentally change the atom. Also, an atom's nucleus can change via nuclear reactions, but these changes are random rather than the result of aging.)

If atoms don't age, how do radioactive atoms know when to decay? How can we possibly say that a radioactive isotope has a lifetime if it does not age? The answer is that radioactive atoms don't know when to decay. In fact, an individual radioactive atom does not decay at a particular, predictable time. It's not like an atom has an internal clock ticking away telling it when it's time to fall apart. Rather, an atom decays at a random time, completely independent of how long it has been in existence. Radioactive decay is governed by random, statistical effects and not by internal deterministic machinery. A particular radioactive atom can and will decay at any time. The "lifetime" of a radioactive isotope is not a description of how long a single atom will survive before decaying. Rather, it is a description of the average amount of time it takes for a significant portion of a group of radioactive atoms to decay. A characteristic lifetime does not come about by the progression of internal machinery, but by the statistical behavior of a large group of atoms governed by probability.

An analogy may be helpful. A standard six-sided die will show a single number between "1" and "6" when rolled. Let us agree that when we roll a "6", we smash the die to pieces and the game is over for that particular die. We begin rolling the die and get a "3", and then a "1" and then a "5". Next we roll a "6" and destroy the die as agreed upon. Since the die was destroyed after four rolls, we say that this particular die had an individual lifetime of four rolls. Now we get a new die and repeat the game. For this die, we roll a "2", then a "1", then "4", "3", "1", "5", and then finally a "6". This die therefore had an individual lifetime of seven rolls. When we repeat this game for many dice, we discover that the individual lifetime of a particular die can be anything from one roll to hundreds of rolls. However, if we average over thousands of individual lifetimes, we find that the dice consistently have an average lifetime of about six rolls. Since an individual die has no internal machinery telling it to show a "6" after a certain number of rolls, the individual lifetime of a die is completely random. However, since the random events are governed by probabilities, we can experimentally find a fixed characteristic average lifetime of a group of dice by averaging over a large ensemble of dice. We can also mathematically find the average lifetime by calculating probabilities. For the die, there are six possible outcomes to a single roll, each with equal probability of occurring. Therefore, the probability of rolling a "6" and destroying the die is 1 out of 6 for every roll. For this reason, we expect it to take six rolls on average to roll a 6 and destroy the die, which is just what we found experimentally. The dice do not have a predictable average lifetime because they age, but because they experience probabilistic events. In the same way, atoms do not age and yet we can identify a meaningful decay lifetime because of the probabilities.

Topics: aging, atom, atoms, decay, life, lifetime, probability, radioactive decay, radioactivity