<text><span class="style10">uantum Theory and Relativity (4 of 4)Special relativity</span><span class="style7"></span><span class="style26">Inertial frames</span><span class="style7">. Physical laws such as Newton's laws of mechanics are stated with respect to some </span><span class="style26">frame of reference</span><span class="style7"> that allows physical quantities such as velocity and acceleration to be defined. A frame of reference is called </span><span class="style26">inertial</span><span class="style7"> if it is unaccelerated and it does not contain a gravitational field.</span><span class="style26">Einstein's relativity principle</span><span class="style7">. In 1905 Einstein stated that all inertial frames are equally good for carrying out experiments. This assumption, coupled with the evidence that the speed of light is the same in all frames, led Einstein to develop the theory of special relativity. This theory has been extensively tested using particle accelerators, where electrons or protons travel at speeds within a fraction of 1% of the speed of light. The masses of such particles measured by an observer in the laboratory in which the particles are traveling are higher than the masses measured by an observer at rest with respect to the particles.</span><span class="style26">Time</span><span class="style7">. The classical view of time is that if two events take place simultaneously with reference to one frame then they must also occur simultaneously within another frame. In terms of special relativity, however, two events that occur simultaneously in one frame may not be seen as simultaneous in another frame moving relative to the first. The sequence of cause and effect in related events is not, however, affected. Light plays a special role in synchronizing clocks in different frames because it has the same speed in all frames. In the classical view all observers have the same time scale, whereas in special relativity every inertial observer requires an individual time scale.</span><span class="style26">Space-time</span><span class="style7">. An important feature of special relativity is that time and space have to be considered as unified and not as two separate things. This means that time is related to the frame of reference in which it is being measured. This is a different view of space to that of Newton.</span><span class="style26">Length contraction</span><span class="style7">. The equations of special relativity lead to the very simple prediction that the length of a moving body in the direction of its motion measured in another frame is reduced by a factor dependent on its velocity with respect to the observer. What this means is that a car traveling very fast on a motorway would be measured by a stationary observer to be slightly shorter and heavier than usual, although the driver would not determine any difference. The length of a body is greatest when it is measured in a frame traveling </span><span class="style26">with</span><span class="style7"> the body; as the speed of the body </span><span class="style26">relative to</span><span class="style7"> the frame of reference approaches the speed of light the measured length approaches zero.</span><span class="style26">Time dilation</span><span class="style7">. A similar effect happens to moving clocks (which can be any regularly occurring phenomenon, such as the vibration of atoms - the basis of atomic clocks - or the decay of particles). A clock moving with a uniform velocity in one frame is measured as running slow in another frame. Its fastest rate is in its own frame, and at speeds - relative to the observer - approaching the speed of light the clock rate approaches zero.</span><span class="style26">Paradox of reality</span><span class="style7">? Both of the above effects of length and time contraction have been the inspiration of numerous `paradoxes' (and some science-fiction writing), and have been criticized on such grounds. But this simply goes to show that our `common-sense' view of the world is rooted in frames that travel with respect to the observer at tiny speeds compared to that of light, and is just as inappropriate in describing these phenomena as it is in describing the quantum effects of the atomic world. Time dilation has been measured experimentally both with decaying particles and with actual macroscopic clocks. In all cases the effects predicted by special relativity were encountered.</span><span class="style10">General relativity</span><span class="style7">This is an extension of the theory of special relativity to include gravitational fields and accelerating reference frames. Gravitational fields arise because of the distortions of space-time in the vicinity of large masses, and space-time is no longer thought of as having an existence in dependent of the mass in the universe. Rather, space-time, mass and gravity are interdependent. The concept of `curved space-time' was put forward by Einstein in his general theory of relativity. The motion of astronomical bodies is controlled by this deformation or curvature of space and time close to large masses. Light is also bent by the gravitational fields of large masses. Light rays have been observed to bend as they pass close to the Sun, so providing experimental verification of Einstein's theories.PH</span><span class="style10">THE SPEED OF LIGHT</span><span class="style7">All experimental evidence suggests that the speed of light in a vacuum is a constant independent of the speed of the observer. The speed of light (c) is approximately 3 x 10 to the power of 8 m s-1(300 000 km or 186 000 mi per second).The most important feature is that although this is an enormous speed (a typical aircraft travels at about 250 m s-1 ), it is </span><span class="style26">finite</span><span class="style7">. It is also a </span><span class="style26">limiting </span><span class="style7">speed - nothing material travels faster than light (or electromagnetic radiation in general) in a vacuum.</span><span class="style10">E = mc to the power of 2</span><span class="style7">The gain in mass that occurs in a body moving at high speed led Einstein to conclude that the energy (E) and mass (m) of a body are equivalent. He derived a formula to relate them - the well-known E = mc to the power of 2, where c is the speed of light. Perhaps the most elegant verification of the theory of equivalence is shown by the annihilation at rest of an electron and a positron into two gamma rays, each with an energy equal to the particle rest mass.</span></text>
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<text><span class="style10">. Relative time</span><span class="style7">. Supernova A occurs 120 light years from Earth (i.e. light takes 120 years to travel from Supernova A to Earth), and Supernova B is 200 light years from Earth. From Earth the two events are observed simultaneously in 1920, even though Supernova B occurred 80 years before Supernova A. From Planet X, in contrast, the two events are not observed simultaneously. Planet X is only 40 light years from Supernova B, but 150 light years from Supernova A. Therefore, on Planet X, Supernova B is observed in 1760, while Supernova A is not seen until 1950.</span></text>
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<text>ΓÇó TIMEΓÇó MOTION AND FORCEΓÇó WAVE THEORYΓÇó ELECTROMAGNETISMΓÇó ATOMS AND SUBATOMIC PARTICLESΓÇó ELEMENTS AND THE PERIODIC TABLE</text>
