Time travel has been one of the most astounding dreams one would want to experience. We have seen it in the movies how spaceships use wormholes to travel across vast distances in a short time. Will this be possible ever in reality? Albert Einstein was one of the greatest scientists who was successful in deriving the theory of relativity.
Since the theory of relativity explains a concept called as time dilation, there is quite a possibility that we could in the future indeed travel in the future without ageing. (Just as speed of light in a vacuum, time slows down as you approach celeritas).
Before we understand the connection between light weakening and time, let us take an argument that confirms the link between light and gravity. Einstein himself arrived at the idea that gravity affects light by an entirely different line of reasoning, in 1907. By this stage of his career, his genius had been recognized by the scientific community, but he was still employed at the Swiss Patent Office; he did not yet have a university position. He had, however, been awarded a Ph.D. by the University of Zurich for a research project in statistical mechanics.
It seems his employment duties were undemanding enough for Einstein to have plenty of time to sit and think about the nature of the physical universe. In 1907, whilst contemplating the mysteries of gravitation, he came up with an elegant line of reasoning that typifies the way in which he was able to draw profound conclusions about the world based on pure thought. Like Galileo three centuries earlier, Einstein began his deliberations on gravitation by comparing a gravitational force to an acceleration.
First, he imagined what an acceleration felt like. In the final version of the argument, he used the example of an elevator suddenly starting up. We are all familiar with the way that accelerated motion produces “g-forces”—that is, it feels just like gravity. An upwardly accelerating elevator presses you towards the floor, adding to your weight, whereas a downwardly accelerating elevator “leaves your stomach behind” as it temporarily reduces your weight. Another example of acceleration mimicking gravity is rotation.
In Stanley Kubrick’s film 2001: A Space Odyssey, the space station is shaped like a wheel, and slowly turns to create “artificial gravity” along its rim. Although Galileo and Newton were aware of the close link between acceleration and gravitation, they regarded it as an incidental feature of nature. Einstein elevated it to a fundamental principle, which he called the “principle of equivalence,” which states that, in the immediate proximity of an accelerating system, the acceleration is physically equivalent to a gravitational force.
The next step of Einstein’s argument was to note that motion has an effect on light, the Doppler Effect. A good example of the auditory Doppler effect at work is when a police car with siren blaring rushes by. The pitch of the siren falls suddenly as the car passes (wee-wee-wee- wee-wow-wow-wow . . .). This happens because the onrushing car compresses the sound waves before it, boosting their frequency. Conversely, when the car is receding, the waves coming back at you get stretched to a lower frequency.
The same thing happens with light waves: light from an approaching source suffers a frequency rise, whereas light from a receding source suffers a frequency fall (only a very small change for everyday speeds). Because the frequency of light is related to its color, the Doppler shift for light amounts to a color shift. The long-wavelength end of the visible spectrum is red, the short wavelength is blue, so an approaching source is blue-shifted, a receding source red-shifted. The Doppler Effect applies to all electromagnetic waves; it is, for example, employed in police radar traps to spot speeding motorists.
By splicing the principle of equivalence and the Doppler Effect, Einstein ingeniously deduced that gravity affects light. Imagine accelerating away from a source of light. As your speed rises, so the light will become more red-shifted by the Doppler Effect. Therefore, reasoned Einstein, it should also become red-shifted by a gravitational field because an acceleration mimics a gravitational field and should produce equivalent physical effects. Using his special theory of relativity, he was able to arrive at the formula that describes the magnitude of the gravitational, red-shift effect.
It is this red shift which saves us from the perpetuum mobile paradox because there is a relationship between the frequency of light and the energy of the corresponding photons. In fact, these two quantities are in direct proportion. Thus, if light is red-shifted, the photon energy is reduced; so, in the conveyor system, the photons arriving at the top of the device will indeed be enfeebled, and unable to excite the atoms there.
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