Contents
The laws of Physics Topics are used to explain everything from the smallest subatomic particles to the largest galaxies.
What are the 4 Fundamental Interactions?
Scopes And Excitements of Physics
The conquest of physics is often compared to is climbing up a mountain. We climb up to get a better view and a better realisation of the universe around us. The farther we climb, the greater is our view and we acquire a more refined knowledge. Sometimes the old path is abandoned, temporarily or permanently, and a new path is invented to climb up to a higher level and to have a better view of the realities of nature. A generally accepted viewpoint is that the peak of the mountain is far above the highest point we could reach so far. The peak, the ultimate truth regarding nature, is probably far beyond our present-day conceptions.
As a consequence, the scopes of physics, and of science in general, and the corresponding excitements are almost limitless, paving the way for continuous quests for the truths of the universe. Physics, and science in general, is the only subject that has the humility to declare openly that it does not know every truth of nature, and perhaps would not be able to know this within any conceivable period in the future. This realisation is actually the main strength of science-the future is absolutely open for exciting and novel concepts and explorations, and for the development of knowledge.
Nature of Physical Laws
Physics is concerned with the study of matter and energy in transit. The theoretical structure of physics in the present times is as follows:
Classical physics: Before the beginning of the 20th century, it was convincingly established that,
- matter is composed of particles and obeys Newton’s laws of motion, and
- radiant energy is composed of waves and obeys Maxwell’s electromagnetic field theory. These two constitute what is known today as classical physics.
This is extremely successful till date, except in the domains of
- particle speed compared to that of light (3 × 108m ᐧ s-1) and
- particle size of the order of 10-10 m or less.
Theory of relativity: When the particle speed rises to nearly the speed of light, Newton’s laws are no longer obeyed. Einstein’s theory of relativity can successfully explain particle behaviours in that domain. However, this theory is not just a modification of the classical one; it introduces some revolu-tionary concepts, particularly on space and time.
Quantum mechanics: Classical physics fails to describe the motion of microscopic particles, of diameter 10-10 m or less. Heisenberg, Schrodinger, de Broglie, Dirac and others developed quantum mechanics, that can successfully describe this domain. In contradiction to the classical concept, it establishes the dual nature of both matter and radiation; each of them behaves sometimes as composition of particles, and at other times, of waves.
Quantum field theory: This encompasses the theories of relativity and quantum mechanics, and hence, describes the behaviours of high-speed microscopic particles.
Conservation Principles
A few inherent symmetries of nature led the physicists to accept some conservation principles, and to analyse the physical world on the basis of these principles. A physical quantity is said to be conserved when it can change its manifestation only, but can never be created or destroyed. The most important of these principles are:
i) Conservation of mass-energy: The symmetry of nature with respect to translation of time is called homogeneity of time and it leads to the law of conservation of energy. Earlier, the conservation of mass and the conservation of energy constituted two separate principles. The theory of relativity established that mass and energy are interconvertible, and this concept led to the principle of conservation of mass-energy.
ii) Conservation of linear momentum: Laws of nature take the same form everywhere in the universe i.e., there is no preferred location in the universe. This symmetry of the laws of nature with respect to displacement or translation in space is called homogeneity of space and gives rise to the law of conservation of linear momentum.
iii) Conservation of angular momentum: Isotropy of space (i.e., there is no preferred direction in space) gives rise to conservation of angular momentum.
iv) Conservation of charge: Charged particles can be created but only in pairs of equal and opposite charge such that the total amount of charge remains the same. A few other conservation principles are also in use.
Four Basic Interactions
Only four types of interactions among matter and energy exist in nature and each of them is mediated by the exchange of a particle called mediator or exchange particle.
i) Gravitational interaction: This is the attractive interaction between two masses. It obeys inverse square law. So its range is infinite theoretically, although it may not be detectable beyond a large but finite distance. It is the weakest force in nature.
ii) Electromagnetic interaction: This is the attractive or repulsive interaction between two electrostatic charges, or between two magnetic poles. The range is similar to that of gravitational interaction.
iii) Strong or nuclear interaction: It is the strong attractive force which is responsible for holding neutrons and protons together inside atomic nucleus. It is non central and non conservative force. It is a short range force that operates only over the size of the nucleus (~10-14 m) .Beyond this, the interaction goes to zero. It does not obey inverse square law.
iv) Weak interaction: An example of this interaction is that of an electron with a proton or a neutron within nuclear dimensions as in a beta decay. Its range is also very short-it ceases beyond about 10-15 m.
The following table gives a summary of the four fundamental forces in order of increasing strength.
Table-1
Einstein conceived a dream that all the four interactions are different manifestations of a single ultimate natural interaction. This came from his strong belief that the nature has some form of ultimate overall symmetry. The consequent but so far undiscovered concept is known to physicists as the unified field theory. The quest for this theory is on. In spite of some partial successes, the gravitational interaction in particular escaped to show any substantial link with the other three interactions.