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Section 1.1 The World of Physics

Welcome to physics - the subject devoted to understanding the workings of nature at all scales from the world of subatomic particles to the entire universe. Over the past several centuries physicists have discovered a number of laws that have universal applicability. The laws of physics summarize the current understanding of the nature.

Figure 1.1.1. The image of the oldest recorded supernova RW86 observed by Chinese in 185 A.D. The bluish color are in the X-ray from NASA's Chandra X-ray Observatory and the European Space Agency's XMM-Newton Observatory and the yellow and red are infrared data from NASA's Spitzer Space Telescope, as well as NASA's Wide-Field Infrared Survey Explorer (WISE). The Chinese had recorded that the guest star remained in the sky for eight months.Credits: X-ray: NASA/CXC/SAO \amp ESA; Infared: NASA/JPL-Caltech/B. Williams (NCSU).

Expressing laws of physics mathematically allows one to investigate and deduce their implications more fully. Therefore, a solid training in mathematics is essential for a student to appreciate and understand physics. You should be very comfortable at least with algebra, plane geometry and trigonometry before you start your physics, and preferably also know some Calculus.

Laws of physics are not necessarily infallible. They are always tested against reproducible experiments or observations. This makes physics an empirical science, although physicists do spend much time developing models and theories. In physics, experiments and theories complement one another, and together, they provide the most powerful tools at our disposal for unlocking mysteries of nature.

Theories in physics range from the empirical to the abstract. Often a theory is simply a model or a mental image that helps grasp the experimental observations better. Theories may also suggest new experiments to render the physical situation clearer. Physicists also explore theories that seek to unearth the abstract principles that can explain a number of phenomena. Although the words - theory and model - are often used interchangeably, theories are usually understood to mean more general ideas while models usually refer to a particular phenomenon.

When many predictions of a general theory have been tested and found to be correct, the theory gains the status of a law of physics. There is a self-propelled process in the testing process. The testing of predictions of laws of physics is usually limited by development in technology, which itself is guided by a progress in physics, and/or the human ingenuity. With increased precision, or an access to some previously inaccessible physical conditions, or some hitherto unimagined experiment, the old accepted laws are often tested in new ways.

Sometimes, new tests show flaws in the accepted laws. Since many such exciting discoveries have been made in the past, physicists keep an open mind about their subject and treat the presently accepted laws as tentative. If a law is inconsistent with reliable observations, then either the law is modified or a new theory is invented to replace the old one. In this sense, physics is quite dynamic, changing continuously as our understanding of the world improves. Whether this process of correcting and replacing the old laws with new ones will ever end up with the eventual discovery of “the final laws” is unclear at this time since we do not know if such immutable final laws even exist.