Creation and conservation of charge during electron-positron particle production from a photon

Production of electron and positron by a gamma ray (photon)

One of the facts you will learn in Physics 162 (or perhaps already know) is that an object in nature can have an electrical charge that causes the object to exert forces on other objects. Electrical charges have several interesting properties: the total charge of any isolated system is conserved (remains unchanged) as various events take place inside the isolated system, charge is quantized in integer multiples of the magnitude e of the charge of an electron, and the magnitude of an electrical charge does not change if the charge is moving. In contrast, a mass like the mass me of an electron does change with the speed of the mass (as first predicted by Einstein's theory of special relativity and later confirmed by many experiments), so mass is not conserved in an isolated system, e.g., a hotter block of metal has a slightly greater mass than a cooler block of metal because of the larger average speeds of the hotter nuclei. (This further implies that mass can not be quantized in integer multiples of some basic mass since mass changes continuously with its speed with respect to some observer.) Although mass is also a property of an object that causes that object to exert a force on other objects (through gravity), and although gravitational and electric fields seem similar (e.g., both satisfy inverse square laws, at least not near a neutron star or black hole), mass is somehow different from charge.

These properties of charge are illustrated in the above image, which shows the experimental creation and conservation of electrical charge. Here an invisible high-energy electrically-neutral light particle (photon) travels down from the top of the image and scatters off an invisible neutral hydrogen atom that is located near the middle of the figure, where the three curves meet at a cusp. The collision of the photon with the atom leads to four particles: a positively charged proton (not visible), a negatively charged electron knocked free from the hydrogen atom (this corresponds to the nearly straight track continuing downwards), and the creation of two new particles, a negatively charged electron and a positively charged antiparticle called a positron, whose paths trace out the two spirals.

Before the collision, the isolated system consisting of the liquid hydrogen in the bubble chamber was electrically neutral with a total charge of zero. After the collision and the creation of the electron-positron pair, the total electrical charge of this system remains zero since the created positron's charge has exactly the same magnitude e as the created electron's charge but is opposite in sign. How do we know this? Each of the three moving charged particles (the green arrow heads indicate the direction of motion) leave a wake of tiny bubbles of hydrogen gas as they plow through the super-heated liquid hydrogen in the bubble chamber, rendering their paths visible as continuous lines. An external magnetic field bends the paths of the electron and positron into oppositely oriented spirals whose geometric shapes can be used to deduce the sign and magnitude of their electrical charges, which turn out to be equal and opposite to the accuracy of this experiment. Note that the electron dislodged from the hydrogen atom (the middle curve moving downwards from the cusp toward the lower left) was moving too fast for the magnetic field to bends its path substantially.

Despite many advances in theoretical and experimental physics, including the development and confirmation of the Standard Model that explains a great diversity of experimental data concerning fundamental particles, many basic questions about electrical charge remain unanswered. We do not know why charge exists in the first place, nor why it has positive and negative values, nor why the fundamental unit of charge is the magnitude e of an electron, nor what determines the numerical value of e in some given set of units, nor why all charges are quantized in integer multiples of e, nor why the charge of a proton (a non-fundamental particle consisting of three quarks) is, within experimental error, identical to the magnitude of charge of an electron, which appears to be a point fundamental particle since no experiment has yet discovered any internal structure. However, we do understand from quantum field theory why charged antiparticles must have exactly the opposite charge and the same mass as the corresponding particle, and why all basic particles of a certain kind (electrons, positrons, protons) must be exactly identical.

Here are three questions about the above figure for you to think about:

  1. Can you tell from the above information which spiral track corresponds to the electron and which spiral track to the positron?
  2. Why did the positively charged ionized hydrogen atom (proton) not produce a track of bubbles?
  3. How did scientists deduce that the positron and electron were truly created out of nothing from the energy of the photon, as opposed to already existing inside the hydrogen atom or inside the photon and just being being knocked loose via the collision?


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