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# Lec26 - Special Relativity Special Relativity is the theory...

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Special Relativity Special Relativity is the theory that addresses the structure of space and time themselves, as the arena —or stage— on which physical interactions take place. As such, it is not a physical theory in itself, but rather the background for any other physical theory. Without Special Relativity, little of the rest of physics makes sense. One can think therefore of Special Relativity as the central organizing principle of physics. We start with the consideration of wave propagation: tension waves in strings require a medium (the string); sound waves require a medium (air), and water waves also require a medium (water). It was seeming natural to physicisits in the 19 th centurey to assume that any wave required a medium, such that wave behavior is the propagation of disturbances in that medium. After the discovery of electromagnetic waves, physicists therefore naturally assumed that there is also a medium in which these waves propagate, and they dubbed it the luminiferous ether , and started studying its properties. It was clear from the beginning that the luminiferous ether has some unusual properties. First, as light travels from very distant stars, it must permeate all space (otherwise we wouldn’t see those stars) and objects (as light can travel through materials, including non–visible light such as radio waves or x rays); second, it must by very thin —having in practice very low density and viscosity —so that as the Earth and other planets move through it, their motion is not impeded (otherwise, planetary orbits would decay becasue of friction due to collisions of planets with the ether); third, despite the above properties, the ether must also be very sti ff (because the speed of light is very high. Recall that the speed of sound waves is v = B/ ρ where B is the bulk modulus and ρ is the density. To have high v , the “elastic” property of the medium must be very high compared with the “inertial” property: the sti ff er the material, the higher the wave speed. Indeed, B = - Δ P/ ( Δ V/V ), meaning that to obtain a very high– B materials one

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