What Is Time Dilation?
Einstein’s theory of special relativity, developed in 1905, establishes a relationship between space and time for objects moving at a constant speed in a straight line. In short, in states that as an object approaches the speed of light, its mass moves towards infinity, ultimately preventing it from surpassing the velocity at which light travels.
An implication which will be the subject of this article, is the subsequent dilation of time, the difference in elapsed time that two clocks will have measured depending on the circumstances in which the time on the said clock evolves.
When an object reaches relativistic speeds, or in other words, speeds high enough for the desired effect to be observed in a considerable manner, an observer sees the second clock ticking slower than that of his own reference frame. As such, no time can be said to be wrong, as it is a relative rather than an absolute concept.
Time dilation, however, finds two different causes within the theory of relativity, explaining this relative stretching of time. The first one which will be explored is known as velocity time dilation and the second as gravitational time dilation.
Velocity Time Dilation
In the case where clocks have a velocity relative to each other, if we restrict one of the systems to be an inertial frame of reference, one upon which is acting a zero net force, any clock moving faster than rest in the initial frame of reference, will be seen to tick slower than the reference clock. As the velocity gap grows larger and larger, the time dilation between both systems becomes greater.
Virtually, as the velocity of an object approaches the speed of light, its rate of time approaches zero. However, as massless particles travel at the speed of light, their mass remains by definition unaffected and null, the implication being that they are hence unaffected by the passage of time.
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Experimental Confirmation of Velocity Time Dilation
In order to confirm these speculations, physicists were to put in place an experiment comparing two clocks moving at relative speed: one stationary, and one moving at a constant rate. However, they had to create a ‘clock’, a mechanism of reference, which could be accelerated relatively easily within our current scientific capacities.
This was achieved using the Experimental Storage Ring, a circular particle accelerator at the GSI Helmholtz Centre for Heavy Ion Research in Germany. They set the ticking of the clock to be the transitions of electrons between different energy levels within lithium ions. They accelerated some to a third of the speed of light while keeping some at rest. The frequency of these transitions was seen to differ depending on the velocity of the ion, adapting themselves to its internal clock, its own relative perception of time.
This experiment was not the first but is to this date one of the most precise conducted in the laboratory. However, time dilation has now also been proved in practice thanks to GPS satellites.
Technology today severely restricts us in terms of what we are able to experience with respect to velocity time dilation. However, astronauts that enter the ISS for long enough are still subject to minuscule but quantifiable time dilation. As they travel at speeds as high as 7700 meters per second, every human that spends a year up there will be 0.01 seconds younger than the average human which will have remained on Earth.
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Gravitational Time Dilation
If two clocks are now situated in two locations separated by a difference in gravitational potential, the same phenomenon will occur. The difference of elapsed time is due to two observers or systems located next to gravitating bodies of different masses or simply non-equal distances from a single gravitating mass. As such, a clock located closer to a massive body (lower gravitational potential) will run slower than one situated further away (larger gravitational potential).
Experimental Confirmation of Velocity Time Dilation
The speculations of gravitational time dilation have been confirmed by establishing a simple difference in altitude and thus gravitational potential, between two atomic clocks (an extremely precise timekeeping device based on electron transition frequency). The result of this procedure was, as expected, a slight difference in elapsed time. The results do however remain negligible when experiments are Earth-bound, as much larger distances would be required to observe greater discrepancies.
This, combined with the postulate of special relativity explains the concept of time in black holes. As stated, as an object approaches the black hole, of theoretically infinite mass at the singularity, gravity becomes stronger and stronger. As such, the breakdown of time at the singularity is explained by the fact that the gravitational potential at the singularity would be of virtually zero. However, as phenomena occurring beyond the event horizon are imperceptible by any outside observers, these predictions remain theoretical.
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