Why Do Particles Decay?

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1. Aug 7, 2015

Ryan Reed

What is so unstable about one particle that it only lasts a billionth of a second? Opposed to another particle that lasts much longer. What is so different between the two and why are there "timers" set when these particles created?

2. Aug 8, 2015

Staff: Mentor

Quantum field theory. Sorry, I don't think there is an easier accurate answer. It is possible to predict the lifetime, but we could live in a different universe with a different set of laws where the lifetimes would be different.

There are no timers set. The lifetime is just the expectation value, particles can decay earlier or later. They have the same probability to decay for each interval in time.

3. Aug 8, 2015

Staff: Mentor

More explicitly, suppose that a particular kind of particle has a probability of 10% of decaying within one second. It doesn't matter whether that second starts at the instant the particle is created, or ten seconds later, or an hour later, or a day later, etc., provided that the particle actually exists (has not yet decayed) at the beginning of that second.

The particle doesn't "remember" how long it has lived so far.

Last edited: Aug 8, 2015
4. Aug 8, 2015

Demystifier

The ultimate cause of all instability in quantum physics is uncertainty of energy. When a physical system has a certain energy, then the probability does not depend on time so the system is stable. When the energy is not certain, then the probability does depend on time, and the system "wants" to settle down into a stable state with certain energy. That's the simplest non-technical but reasonably accurate and informal explanation I am aware of.

5. Aug 8, 2015

Avodyne

To add to that: stable particles are all stable because there is some conservation law that prevents them from decaying. For example, an electron is the lightest charged particle. Since charge is conserved, there is nothing for an electron to decay to.

6. Dec 9, 2015

Dirk Pons

This *why* and the related *how* question is a difficult question. From within quantum mechanics the nearest explanation is quantum field theory. However that does not actually answer the question which is more of an ontological nature. QM is premised on particles being 0-D points with intrinsic variables, and these quantum numbers are mathematical constructs rather than physical attributes. Consequently it is unrealistic to expect explanations based on physical realism, and indeed many interpretations of QM deny physical realism altogether. So this means there is no physical answer to the questions from within QM, and possibly there never will be.

Nonetheless QM is not the only theory of physics. There are also the string theories and non-local hidden-variable (NLHV) theories, both groups of which allow particles to have internal structure. The question is more tractable within those other frameworks. A number of journal papers address this issue and qualitative explanations are available.

The explanations are given in terms of the internal rearrangement of the particle under conservation and energy laws, and the perturbation of random vacuum fluctuations, and the resulting emergence of new particle identities. The result is that the state of the particle and its external environment (bond or not, perturbation by photons, fields, or other particles) determines the hazard rate. This is the probability of the particle failing in the next time interval. Different particles have different hazard rates, and therefore show different decay half-lives. It is not helpful to think of it as involving deterministic timers. Better to think of it as the particle needing to remanufacture itself because its existing state (structure and energy) is unsuitable to the dynamic and uncertain set of external perturbations that it experiences. Related to the question about the lifetimes of particles, is the especially tough question about the stability, instability, and non-existence of nuclides, which can be addressed by NLHV theory. Why do adjacent isotopes differ so greatly in these respects? Why the different lifetimes? Why are the drip lines where they are? Why the stable isotopes and isotones? Why the gaps in the stable series? These issues are inadequately explained by empirical considerations of binding energy, or semi-empirical mass formula (SEMF), or by quantum theory.

Consequently the answer to the question depends on one's philosophical position on what comprises a sufficient theory of physics. If empiricism is the objective then quantum mechanics will give the necessary mathematical formulation of the overall probabilities of the decay transitions, and allow quantification of the effect. But don't expect explanations based on physical realism from QM because it's not that type of theory. If you wish to have physical realism then the NLHV theories may give you greater satisfaction as they can already explain these effects though as yet only qualitatively.

7. Dec 10, 2015

Staff: Mentor

Standard quantum mechanics allows to make quantitative predictions. They sometimes have large uncertainties, but qualitative descriptions are pointless. Qualitatively, you can go to space by jumping up. Quantitatively, you cannot.