Trying to understand subatomic particles?

Question

I'm reading ahead for my physics class tomorrow and positrons are mentioned. They are described as "antielectrons" - exactly the same but with an opposite charge. So, are positrons basically very miniscule protons?

If a positron and electron will annihilate one another, why won't a proton and an electron annihilate one another?

What keeps protons and electrons apart anyway? Protons repel each other (like repels like) but why are electrons always found in an outer shell. If opposites attract, shouldn't they be with the protons?

Answer 1

It's like this:

NO, positrons are NOT "basically very min[u]scule protons." They are truly "anti-electrons" in the sense that they possess all the properties of electrons --- such as the same mass and spin --- except that they carry a ' + ' charge instead of a ' - ' charge.

Protons themselves DO have their own anti-particle. It's the anti-proton --- with the same mass and spin as the classical proton --- but with a ' - ' charge instead of a ' + ' charge.

There are many other such particle/anti-particle pairs, similarly differing only by having oppositely signed charges. But please don't get the wrong idea here, since I've been stressing charge. Such particle/anti-particle pairs don't necessarily HAVE to have charge. Neutrinos have no charge, but still have their own anti-particles, the anti-neutrinos. Weak interactions that spit out one of these particles have "inverse reactions" that require the presence of their opposite partner.

Only particles and their corresponding anti-particles can completely annihilate each other, leaving nothing but pure energy in their place. That energy, in the form of gamma rays, has a strength (or frequency) related to the energy equivalence of the original masses. Correspondingly, two gamma rays of appropriately high energy, meeting head on, can produce a particle anti-particle pair. (It's often a bit more complicated than that because the full conservation requirements include the presence or production of other particles such as neutrinos and anti-neutrionos.)

The energy-equivalent of an electron's mass is 0.511 Mev. You would have to aim two gamma rays at one another of total energy exceeding twice that to be able to create an electron-positron pair. You couldn't get the corresponding proton-antiproton pair until you'd smashed two gamma rays with more than 1800 times that energy head on, because it would take that high an energy to create those two much larger masses.

In fact one reason that a proton and an electron can't annihilate one another and leave pure energy behind is because of that serious mismatch of their masses, differing by that factor of more than 1800. The various conservation requirements (not only of energy and momentum but also of certain other quantum properties alluded to in the next short paragraph) simply make that impossible with such a disparity in both masses and other fundamental quantum characteristics. Another reason is that protons and electrons are two of the particles left when a free neutron itself decays (others are one type of neutrino). IF you can fire a proton and an electron together with sufficient energy, you can produce the OPPOSITE reaction, ending up with a neutron and the opposite kind of neutrino. It takes a lot of energy because the decay of a free neutron produces rapidly moving particles, including the rapidly exiting neutrino particle. To get the reverse reaction you have to supply that extra lost energy via the kinetic energy of the incoming particles; whenever a "weak interaction" (neutron becoming a proton or vice-versa) is involved, there's always some kind of neutrino around whose energy has to be provided for somehow. The free neutron has more mass than the sum of the proton and electron masses, so that its decay is energetically favoured. The reverse reaction is therefore energetically disfavoured unless you can somehow provide the extra needed energy to make it possible to proceed.

A more fundamental reason is that protons and electrons are members of distinct families of particles, that are linked together in other more subtle ways. ("Quantum numbers" related to membership in such families are almost always conserved in any reactions or decays, and that fact limits what one might think could otherwise happen.)

Electrons and positrons are "leptons" or (relatively) light particles. Various other particles belong in that class including neutrinos and anti-neutrinos. They all seem to be "point-particles," with no finite radius as far as we know. Protons, neutrons etc. are all "hadrons" or (relatively) heavy particles, with small but finite radii. Once thought to be "fundamental" particles, they are now known to be composites of quarks. No "free quark" has ever been observed, so does this notion of composite hadrons (and mesons) rest only on the fact that the shuffling and combinations of quarks with charges +/- 2/3 or 1/3 can neatly "explain" the variety of particles we know about? Yes, it does. The results of certain very high energy hadron scattering experiments support fairly strongly the idea that there are concentrations of such fractional charges within the hadrons. The experimentally detected results would be difficult to obtain if the charges inside the hardons were distributed more uniformly.

As for your question: "What keeps protons and electrons apart anyway?" --- What keeps the Sun and the Earth apart anyway? Why are the planets always found away from the Sun, in a series of "outer shells"? Doesn't the fact that gravity always attracts mean that the planets should all be burnt up inside the Sun? (I'm just trying to get you to contemplate a possibly analogous series of questions!)

The fact is that quantum mechanics dictates that objects confined in small volumes must necessarily have a certian minimum non-zero motional energy. The only way to have a long-lived structure under those conditions is for something analogous to "orbits" to exist with the further property that the energy in those orbits must have certain specific energy value. There are NO "orbits" possible with energies in between those particular special sets of energy levels. Though this kind of mental viewpoint is frowned upon today, that's equivalent to a picture in which the electrons are a bit like little "planets" allowed only a certain definite sets of "orbit sizes" around the proton or other nucleus. In this mental picture of the hydrogen atom, say, the allowed energy levels do indeed correspond to those of such negatively charged little "planets" "orbiting" the central protonic nucleus.

[That mental picture of the electronic structure of atoms, particularly of the neutral hydrogen atom, was introduced by Bohr around 1912. It was very successful in "explaining" hydrogen's emission or absorption spectra. For a decade or so, ad hoc extensions of Bohr's arguments explained more, culminating in the more complex Bohr-Sommerfeld theory. This theory seemed to require the use of certain rules or principles, the reasons for which were a bit obscure However, with the work of de Broglie, Schroedinger and Heisenberg (~ 1923 - 1926), the focus shifted to talking of quantum states, wave functions and probabilities. This approach replaced the picture of "little planetary orbits" by waves of probability densities, mathematically manipulable but less "folksy" to imagine.]

Finally, returning to the electrons and positrons: The existence of the latter was predicted by Dirac in 1926, from his relativistic equation for the electron, although even he didn't completely see the implications immediately. He had developed a picture in which there would have to exist "holes" in an otherwise infinite sea of negative energy electrons. He realised that those "holes" would appear to behave and move exactly ike anti-particles to the electrons themselves. So he tried to see whether some particle could be identified as the electron's "anti-particle."

However, the only positive particles around that could fiit the bill seemed at first to be the protons, but this puzzled him as, according to his theory, his "anti-particles" should have the same mass as the electrons, not > 1800 times more!

This "difficulty" with Dirac's theory persisted until Carl Anderson discovered the positron itself, in cosmic rays. Suddenly, Dirac's "difficulty" turned into a "triumphant prediction." !! We should all be so fortunate.

Live long and prosper.

Answer 2

No, although protons have opposite charge to the electron they are different in other ways. They are not point particles like the electron or positron, but consist of three quarks. Positrons are identical to electrons in every way, except for having opposite quantum numbers (of which the most obvious is charge).
That is why annihilation can only take place between particles and there anti-particles, there are more quantum numbers than charge.
The reason electrons in an atom are not pulled in to the positive nucleus (protons and neutrons) is more complicated. It basically has to do with quantum mechanics, where electrons are seen as both waves and particles. The wave aspect of electrons inteferes with itself to give areas of high and low probability. Although it is easier to show an electron orbiting a nucleus like a little planet really it can go anywhere in the atom, it's just more likely to be on the shell.
The attraction to the protons can be seen in electrons always wanting to occupy the lowest energy shell available.
The protons in the nucleus don't repel, because the electric repulsion is overcome by an attractive force called the strong nuclear force. Radioactivity occurs if the balance of these forces isn't correct and the nucleus become unstable.
Hope this helps, if you find this interesting keep on studying physics, it's sometimes hard but rewarding!