Re: What is mechanism of conversion of matter into energy?

Hi Hemant,

I'm not sure exactly how to answer this question. You don't need a particular mechanism to convert matter into energy. The principle that explains how matter and energy are equivalent is Special Relativity. You can catalog all the forms that matter and energy take, and all of the mechanisms by which they interact, using something called Quantum Field Theory. There's one particular set of quantum field theories that seems to describe (almost) everything we know about matter and energy; this theory is called the Standard Model of particle physics.

Quantum field theory tells us, roughly speaking, that anything can decay into anything else. There are only two caveats. Universally, heavier things have to decay into lighter things; otherwise you violate the law of conservation of energy. (If the initial particle is heavier than the sum of the final particles, energy is still conserved; the leftover mass-energy of the decaying particle winds up as kinetic energy of the "daughters". There's no way for a light particle to decay into a heavy particle.) Going into more detail---some of it specific to the Standard Model---the initial and final states have to have the same total charge, spin, baryon number, and lepton number. If you can meet those criteria, the decay can and will occur ... eventually. In some cases you may have to wait a long time.

Why do all these decays happen? Imagine that your particle is a constantly quivering, morphing blob; imagine that each possible decay mode (as well as the "null" decay mode, not decaying at all) is a template which the blob can either match or not match. As the blob quivers around, maybe most of the time it is a sphere (say, matching the null-decay template) but sometimes wanders into the shape of a dumbell, or a football, or a saucer (corresponding to three different allowable decays). If the blob matches the dumbell-template better than the sphere-template, the particle will decay via that mode, rather than persisting. To translate this into the language of quantum mechanics, the "blob" is a wavefunction, and the "templates" are different eigenfunctions that the wavefunction may project onto. This sort of projection is a very general property of quantum mechanical systems.

A few examples may help:

1. A free neutron can decay into a proton, electron, and electron neutrino. There's no other imagineable list of decay products that does not violate one of the conservation laws. (You might legally have, in addition to n --> p e nu, things like n --> p e nu photon, or n --> p e nu nu nu. Indeed, these ought to occur, but rarely.)
2. A heavy particle, like a kaon, has dozens or hundreds of options for decay paths. The list of possible decays is like a laundry list of every particle lighter than the kaon itself. Each allowable decay is one that conforms to the conservation laws; you could dream up hundreds of other decays that violate the laws (say k+ --> e+ photon, which violates lepton number conservation, or k+ --> pi0 pi0, which violates charge conservation), but noone has ever seen one of these actually occur. But heavy particles like this illustrate the idea: if it's not specifically forbidden, it will happen sometimes.
3. A proton can't decay at all. Protons have a baryon number B=+1, and there is no lighter particle with a nonzero baryon number. Thus, the proton has "nowhere to go" and does not decay. You start with a proton with a certain mass (E=mc^2 determines its rest energy); you end with a proton with the same mass, therefore the same energy; therefore you didn't extract any energy, you didn't convert any mass-energy to another type. This is why you don't see the rest energy very often in daily life; it can only do something useful if a particle has decayed or changed somehow.
4. Something like a TNT molecule can "decay"; it can fall apart from its high-energy state, C6H2(NO2)3CH3, into the decomposed state 6CO + 2.5 H2 + 1.5 N2 + C. This violates no conservation laws, and releases a tiny tiny fraction of the TNT molecule's mass-energy (usually thought of as "chemical binding energy") into the kinetic energy of the products. (The spontaneous decay is very slow, but it can be "triggered" to go quickly in an explosion.)

You asked about nuclear decays. Well, a nucleus is a large cluster of protons and neutrons. It has a "baryon number" equal to its (integer) atomic mass number; since it is forbidden to change the baryon number, it has to decay into some other combination of protons and neutrons with the same baryon number. Moreover, the end-products of the decay have to be lighter, added together, than the parent nucleus. If you can find some combination of decay products that fit this criterion, the decay is allowed. Sometimes there is no allowable decay mode at all (for example, for 56-Fe nuclei, or H, or 4-He); usually there are only one or two possibilities (a beta decay channel, or alpha emission). Sometimes there are dozens of different possibilities, as for spontaneous fission of 238-U. Sometimes there's an allowable decay---for example, spontaneous fission of xenon---which, it turns out, is just so slow-moving that we'll never see it. (In my quivering-blob analogy, imagine that the decay template is very, very specific, and very very different than the blob's usual ways of morphing; it may take an incredibly long wait before the blob stumbles across a decay configuration.) The "binding energy" is just a convenient way of saying whether the daughter nucleus is lighter than the parents, and by how much.

I hope this helps. Perhaps you were looking for a more specific Standard Model answer; I could talk about the Strong Force, Weak Force, and Electromagnetism for hours; I could talk about the details of nuclear shell structure, tunneling, and liquid drop models. But the mere fact that particles decay can be understood in simple quantum mechanics. You only need the Standard Model if you want to know the probability associated with each template/decay, or if you want to write down the wavefunction and see the eigenvalues. I can recommend the David Griffiths textbook "Introduction to Elementary Particles" to get started on this. If you want to play around with some particle masses and decays, start at the Particle Data Group web page.

-Ben