China
Africa
Explore chapters and articles related to this topic
Introduction to Quantum Mechanics
Published in Caroline Desgranges, Jerome Delhommelle, A Mole of Chemistry, 2020
Caroline Desgranges, Jerome Delhommelle
However, special relativity does not apply any longer to objects that are accelerating or changing trajectories; this is the reason why, in 1915, Einstein develops a general theory of relativity. He determines that objects with a large mass, such as planets, cause a distortion in spacetime! This is manifested in the phenomenon of gravity. He also proposes the concept of time dilation which relies on both special and general relativity. There is a famous thought experiment known as the “twin paradox”. In this experiment, one of the twins stays on the Earth while the other embarks a spaceship to travel around the Earth. Because of the time-dilation effect, the time goes slower on the spaceship! Indeed, when the spaceship lands back on Earth, the twin who was on the spaceship has aged less than the twin who stayed on the Earth! This creates great excitement, and prompts the foremost intellectuals of the beginning of the 20th century to consider the following question: What is time? A famous meeting between Einstein and Bergson (1927 Nobel Prize in Literature) takes place in Paris in 1922. Bergson is convinced that Einstein’s theory is not consistent with our intuition of time, which is an intuition of duration. Bergson’s theory of duration is exposed in “Time and Free Will: An Essay on the Immediate Data of Consciousness” in which he studies the question: what is time? “If I glance over a road marked on the map and follow it up to a certain point, there is nothing to prevent my turning back and trying to find out whether it branches off anywhere. But time is not a line along which one can pass again.” He imagines the dialogue between defenders and opponents of free will: “The former reason thus: ‘The path is not yet traced out, therefore it may take any direction whatever.’ To which the answer is: ‘You forget that it is not possible to speak of a path till the action is performed: but then it will have been traced out.’ The latter say ‘The path has been traced out in such and such a way: therefore its possible direction was not any direction whatever, but only this one direction.’ To which the answer is: ‘Before the path was traced out there was no direction, either possible or impossible, for the very simple reason that there could not yet be any question of a path’.” After a 30-minute presentation by Bergson, Einstein replies: “There is no ‘philosopher’s time’ which is both psychological and physical; there exists only a psychological time which is different from the time of the physicist”!
Light propagation and local speed in the linear Sagnac effect
Published in Journal of Modern Optics, 2019
Gianfranco Spavieri, George T. Gillies, Espen Gaarder Haug, Arturo Sanchez
Concerning the interpretation of paradoxes – such as the ‘twin paradox’, which (as pointed out by Mansouri and Sexl (7)) is no longer a paradox when interpreted assuming absolute simultaneity – if absolute and Einstein synchronization are equivalent, it can be argued that the same paradox must be interpretable equally well by adopting either absolute or relative simultaneity. Similar considerations can be made in relation to any other paradox, such as the Selleri paradox (12) related to the Sagnac effect. The Sagnac effect and the Selleri paradox have been addressed by several authors (e.g. (14, 16, 19, 20, 30)) and most of them describe the problem from a single frame of reference, generally the laboratory frame (32), even adopting absolute synchronization (instead of relative) (14, 30) adhering to the view of conventionalism that absolute and Einstein synchronization are equivalent. In effect, as it occurs for other experiments supporting SR, the outcome of the Sagnac effect is synchronization-independent and, thus, foreseen by SR adopting either Einstein or absolute synchronization. Because of this, many interpretations of the Sagnac effect are missing this crucial point: the fact that different synchronizations provide the same round-trip result in the Sagnac effect, does not tell or reveal what the local speed of light is in every section of the light path. Therefore, these descriptions are not suitable for discriminating absolute from relative simultaneity and unable to verify the validity of Einstein's second postulate on the constancy of the speed of light.