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ch2829_print - Special Relativity and Quantum Physics 1 The...

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Special Relativity and Quantum Physics 1 – The Principle of Relativity Although Newtonian mechanics works very well at low speeds, it fails when applied to particles whose speeds approach that of light. Experimentally, the predictions of Newtonian mechanics at high speeds can be tested by accelerating an electron through a large electric po- tential difference. For example, an electron can be accelerated to v = 0 . 99 c by using a potential difference of several million volts. According to Newtonian mechanics, the speed should be doubled to v = 1 . 98 c , if the potential difference is increased by a factor 4. However, experiments show that the speed of the electron remains lower than the speed of light, no matter how large the potential differ- ence is. Dr.D.Wackeroth Spring 2005 PHY102A Special Relativity and Quantum Physics Albert Einstein’s theory of special relativity describes this phenomenon correctly. It is based on two postulates: 1. The laws of physics are the same in all inertial reference frames . 2. The speed of light (in vacuum) has always the same value of c , and is independent of the motion of the observer or of the light source. To describe a physical event, it is necessary to specify a frame of refer- ence , i.e. a coordinate system and a clock. Reference frames in which Newton’s first law (law of inertia) is valid are called inertial frames . Since the laws of physics are the same in all inertial reference frames, there is no preferred frame and there is no experiment that can distin- guish between different inertial frames. Dr.D.Wackeroth Spring 2005 PHY102A Special Relativity and Quantum Physics 2 – The Speed of Light There is a serious contradiction between the Newtonian addition law for velocities and the fact that the speed of light is the same for all observers. Suppose a light pulse with speed v = c is sent out by an observer in a truck moving with speed v relative to the ground. According to Newtonian addition of velocities, the light pulse has a speed v + v > c relative to a stationary observer (?). Therefore, either (1) the law for addition of velocities is wrong, or Dr.D.Wackeroth Spring 2005 PHY102A Special Relativity and Quantum Physics (2) the laws of electricity and magnetism are not the same in all inertial frames. If (1) is correct, notions of absolute time and length are incorrect. If (2) is correct, a preferred reference frame must exist. In the 19th century it was thought that electromagnetic waves require a medium to propagate, the so-called ether , which was to be present everywhere, even in empty space. If the ether actually existed, a preferred (“absolute”) frame would ex- ist. If (!) the Sun is assumed to be at rest in the ether, and v is the velocity of the Earth with respect to the “ether”, the speed of light would then be c + v “downwind”, and c - v “upwind”.
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