Special Relativity, Uncertainty Principle, Radiation, and Interactions
Special Relativity
A fundamental problem in physics in the late nineteenth century was that the laws of electromagnetism varied by changing the reference system, violating the principle of relativity of Galileo, which was the basis of Newton’s mechanics. Thus, observers in relative motion would get different results when studying electromagnetic phenomena. In 1905, Einstein reconciled the two theories (mechanical and electromagnetism) with his Special Theory of Relativity, which is based on the following two postulates:
- Principle of Relativity: All laws of physics are the same in inertial reference systems (i.e., for different observers).
- Principle of Constancy of the Speed of Light: The speed of light in a vacuum is a universal constant.
Einstein’s theory leads to some conclusions that would require changing conceptions of classical space, time, mass, and energy:
- Space and time are not absolute: different inertial observers measure different time intervals for the same event and different lengths for the same object.
- No body can travel faster than the speed of light in a vacuum.
- Mass and energy are equivalent and can be transformed into each other according to the equation E = mc2.
Photon Concept
To explain certain phenomena of emission and absorption of light by matter, including the photoelectric effect, Einstein returned to the particle theory of the nature of light. He assumed that the energy of electromagnetic radiation was not continuous but discrete, so that an electromagnetic wave of frequency f could be considered to consist of quanta or particles which travel at the speed of light, each of which has an energy E = hf (where h is Planck’s constant) and a momentum p = h / λ. These were later called photons. Einstein’s theory does not invalidate the electromagnetic theory of light. Modern physics has had to introduce wave-particle duality, admitting that light has both wave and particle qualities. When light interacts with matter, it behaves like a stream of particles (photons) with energy and momentum; when it spreads or suffers diffraction or interference, light behaves like a wave characterized by its wavelength and frequency. Later, de Broglie proposed for reasons of symmetry that the field also presents wave-particle duality, so that any particle has an associated wave. The associated wavelength is very small at macroscopic scales, so the wave character of matter appears only at the microscopic level.
Uncertainty Principle
The Heisenberg Uncertainty Principle states that certain pairs of physical quantities of an object, such as position and velocity, or energy and time, cannot be known with arbitrary precision simultaneously. Formally stated as: The product of the uncertainties in measuring the position and momentum of a particle is at least equal to Planck’s constant divided by 2π. This implies that the greater the accuracy with which one measures the position, the lower the precision of momentum, and vice versa. The principle does not preclude the accurate measurement of a variable separately, but both simultaneously. The Uncertainty Principle sets a fundamental limit that has nothing to do with technical limitations of the measuring instrument or with experimental errors, but is inherent to nature. It is a consequence of wave-particle duality and the inevitable interaction between the observer and the observed phenomenon. As h has a very small value, the Uncertainty Principle is not a significant limit to the measures on a macroscopic scale, where the uncertainties when measuring the position and momentum are much greater than Planck’s constant.
Radiation Types
There are three types of radiation that differ by the type of particles emitted, by their power, and by their penetration in this area:
Alpha Radiation
It consists of alpha particles, which are helium nuclei consisting of two protons and two neutrons. They occur when a parent nucleus disintegrates into a daughter nucleus that has two protons and two neutrons less. Alpha particles have a positive electrical charge and penetrate very little in the field.
Beta Radiation
It also consists of particles, electrons in this case. These electrons are derived from the decay of neutrons in the nucleus: a neutron of a parent nucleus originates an electron, a proton, and an uncharged particle called an antineutrino. The daughter nucleus has, therefore, one more proton and one less neutron. Beta radiation has a negative charge, and its penetration power is greater than that of alpha particles.
Gamma Radiation
It is electromagnetic in nature and is composed of photons. It occurs because nuclei can be in different energy states. When a nucleus changes from an excited state to a lower energy state, it emits a photon of high frequency. As photons have no charge, gamma radiation does not suffer diversion through an electric or magnetic field. Gamma radiation is more pervasive.
Fundamental Interactions
All forces of nature are reduced to four fundamental interactions: strong nuclear, weak nuclear, electromagnetic, and gravitational.
Strong Nuclear Force
It is the most intense. It is a very short-range force (not seen outside the nucleus). It holds together the protons and neutrons that make up the nucleus of atoms. Nuclei would not be stable if there were not this force, which is stronger than the electrostatic repulsion between protons.
Electromagnetic Force
This is the second in intensity. It is far-reaching. It acts on electrically charged particles and can be attractive or repulsive. It is responsible for atoms and molecules of matter being linked.
Weak Nuclear Force
It is the third in intensity. Like the strong nuclear force, it is very short-range. It is the cause of some nuclear reactions and beta radiation.
Gravitational Force
It’s the weakest of all. It occurs between all bodies. It is always attractive and far-reaching. It is responsible for the movement of heavenly bodies, that bodies fall, tides, etc.