A key stumbling block was the question whether relativistic effects, like moving clocks running slowly, were only apparent, or were actually real, and Einstein had to fight to convince others of its reality. ( Read article in Physics Today on the history of the Doppler effect )ĭespite the generally positive reception of Einstein’s theory of special relativity, some of its consequences were anathema to many physicists at the time. This effect had not been predicted either by Christian Doppler (1803 – 1853) or by Woldemar Voigt (1850 – 1919). More significantly, it predicted a transverse Doppler effect for a source moving along a line perpendicular to the line of sight to an observer. Time dilation produced a correction to Doppler’s original expression for the longitudinal effect that became significant at speeds approaching the speed of light. This included the effects of time dilation in addition to the longitudinal effect of the source chasing the wave. In 1905 Einstein used his new theory of special relativity to predict observable consequences that included a general treatment of the relativistic Doppler effect. Einstein and the Transverse Doppler Effect He was aware himself that key experiments were required to prove that relativistic effects are real and not just apparent. Yet this understanding of relativistic effects took many years, even decades, to gain acceptance after Einstein proposed them. The relativistic effect of time dilation is “real”, not just apparent, regardless of whether they come back together to do the comparison. Yet when they get back together and, standing side-by-side, they compare their watches-the twin who went to Alpha Centauri is actually younger than the other, despite the paradox. Each twin sees the other twin’s clock running slowly because of relativistic time dilation. One of them rockets off to Alpha Centauri at relativistic speeds and returns while the other twin stays on Earth. Take, for instance, the classic twin paradox of relativity theory in which there are twins who wear identical high-precision wrist watches. One of the hardest aspects to grasp about relativity theory is the question of whether an event “look as if” it is doing something, or whether it “actually is” doing something.
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