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Quest For Laws of Nature and Why We Care

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Prof. Tung-Mow Yan (ÃCªFZ ±Ð±Â)

## Abstract

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Since ancient times not only scientists and philosophers but also the general public have been interested in how the nature works. In this colloquium we will trace the experimental observations and theoretical developments that shape our understanding of natural phenomena. We now know that the fundamental constituents of matter are quarks and leptons. We may ask the question: could one anticipate the existence of these constituents well before their experimental discoveries? At first glance, the answer seems to be yes. According to the equipartition theorem we learned in elementary statistical mechanics, each degree of freedom contributes (1/2)kT to the total energy of unit mass. Thus, it appears that by a simple measurement of its specific heat, it will reveal all the degrees of freedom, and hence the existence of the basic constituents. Yet, no rational people believe this could be accomplished. This is known as Boltzman paradox. Its resolution came from quantum mechanics which states that energy levels are quantized and discrete. Therefore, at certain energy, degrees of freedom above that energy will be frozen and are unobservable. When quantum mechanics and special relativity are combined, we are automatically led to quantum field theory. Every reasonable quantum field theory Is only an effective theory which requires a cutoff to signify the energy scale of its applicability. For example, QED cutoff is about the electron mass or a few MeV, and electroweak interactions cutoff is about 100GeV. These cutoffs at different stages correspond to the resolving power required for the experiments to study the details at that energy scale. A key question facing the high energy particle physicists is whether there is another new scale for physics beyond the standard model, or there is a vast desert between TeV scale and Planck scale when gravity becomes important. Finally we discuss why we care about this long quest for the laws of nature. We will mention atomic bomb, nuclear power, GPS, MRI, Laser, Internet and others to show how basic sciences have unintended consequences, good or bad, that altered dramatically our daily lives.
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