Why do particles have "half lives"?

Question

Radioactive substances, and even neutrons and many other sub-atomic particles will break down in half lives.

Some atoms break down, but others don't. Why don't the just all break down at the same time?

What determines which atoms break down and which do not?

If it is not known, are there any theories?

BTW - Please, no "laws of chance" or random. Random only means you don't which particle will decay, that is a given.

If I have 10 to the 24th neutrons, with a half life of 5 minutes or so, that are formed at the same time, they should all decay at the same time. Statistics and the laws of probabilty and chance are very valid laws, but they do not address the reason why neutrons decay before others.

Oh, and for that first answer, radioactive hydrogen (aka Tritium), decays with a half life also. I have heard that all atoms have a half life, it is just that the half life of some is so long as to be immeasurable.

Answer 1

If you listen to a geiger counter it crackles rather than ticks like a clock because radioactive decay is in fact a random process. But as you suggest that does not explain the process. You probably have heard of the straw that broke the camels back. In the nucleus of a radioactive atom there is an imbalance that threatens to break up the atom. Sometimes the imbalance is severe and the half life is measured (on average) in seconds and at other times the imbalance is more like a straw on a camel and the particle is relatively stable and may have a half life of a thousand or million of years. In either case after one half life is used up, only half of the atoms under question remain (on average). In a reactor, neutrons can enter a uranium atom and increase its instability (too many particles including the sum of protons and neutrons) and the atom will soon fission, producing energy plus two new atoms that are likely unstable and radioactive. The new atoms may have to eject an electron or a gamma ray to become more stable but often there are many decay events needed to reach a highly stable condition and each process has its own half life.

When an atom fissions, the two new atoms weigh less than the original atom and the difference (mass discrepancy) is thought of as the "glue" that held the atom together which is converted to energy that flings the atoms apart or perhaps generates a gamma ray. Perhaps an unstable atom has too many protons and neutrons and possibly too much glue and must decay to set things right. The process depends entirely on the nucleus of the atoms because heating or cooling or ionizing the atoms, etc. which affects only its electrons has no effect on the half life.

Answer 2

You asked not to use a probability explanation, but that is impossible because the concept of HALF LIFE is a probability measure itself. That would be like asking: "why do people have average height, but please, don't say it's because of a random distribution of heights".

It's a measure of "rate", the rate in which particles break down, like evaporation rate. When I read "[ ] particles will break down in half lives", it is not clear if you understand the concept (I'm sorry, maybe you just expressed yourself in an unfortunate way). I liked the 5 yo boys example, but would make a small change: they don't come back from the rest room, so that after the 45 min, you only have half of the initial group; and 45 min after that, only a quarter; after 45 more minutes, only an eighth and so on. This measure is not an attempt to describe any individual 5 yo boy, only 5 yo boys in general, i.e, hal-life is not applicable to particles but to an amount of that particle.

In relation to the last part (last two questions) I suppose it has to do with quantum physics and will leave it at that.

Answer 3

In the case of atomic decay, half life is a result of the instability of the nucleus, which is dependent on the probabilistic nature of quantum mechanics. However, the half-life itself has nothing to do with probabilities; you can have a half-life for any quantity that decays. You can apply the half-life measurement to the charge on a leaky capacitor, for example, or the charge on a battery. It is simply the time it takes for the measured quantity to reduce to 50% of it's starting value. If the decay is exponential (like the capacitor charge or radioactive decay) the starting point does not matter; it will always reduce to 50% of starting value in the same time. That is why the half-life is a useful measurement; at any point, you can tell what it will be later no matter when you start measuring. While quantum mechanics determines the nature of the decay, it relates to half-life only in that the resulting decay is exponential.

Answer 4

The straight answer is that quantum mechanicl phenomena are governed ONLY by laws of probability. There is NO underlying physical mechanism which, if you could understand it, would convert it to a classical and predictable format. Bells theorem pretty much put an end to this possibility of "hidden variables", and it has been confirmed by experiment.

During a half-life increment of time, each particle has to flip its own coin to see if it should or should not decay during that time increment. That is the bottom rung of our reality. If you don't like it, well , AE didn't like it either.

Answer 5

some of the smaller atoms, like hydrogen, have stronger bonds because they arent as big. their small size makes it much more difficult for them to break apart, most radioactive particles are fairly large