# What is time dilation? Is it possible for an astronaut to travel 2,000 light years in a lifetime?

Time dilation, also known as gravitational time dilation, is a concept in physics that relates to the passage of time. General relativity is the cause of this change—simply speaking, this concept can be explained by pointing out how a clock that operates in outer space will move faster than any clock that’s on the planet Earth.

There is also a time dilation that’s explained through special relativity. This concept states that fast objects will move through time more slowly than slower ones. Satellites that are close to Earth, such as the International Space Station, can orbit our planet quite quickly. This slows down their passage through time. Its time dilation with respect to gravity is not at the same level as time dilation with respect to speed. A clock on the ISS, then, will move more slowly.

If something is in geostationary orbit, it will move more slowly and is also located further away from Earth. This means that the clocks in that state will move more quickly than the ISS. Since GPS satellites know about both forms of time dilation, they can work to guide us on Earth.

Physicists and astronauts have conducted experiments which give supportive evidence of time dilation (both kinds). So can an astronaut travel around 2,000 light years during his lifetime? At a glance, this doesn’t seem possible. A light year is a measure of distance and denotes how far light can travel in the span of one year. Since light travels at 671000000 miles or 1080000000 kilometers per hour, a light year is quite a long distance. Even one light year isn’t possible for a human to travel in a lifetime if we don’t think about time dilation. However, with a closer look at the concept of time dilation according to the theory of relativity, this practice might actually become possible.

Einstein’s theory of relativity tells us that the pace of time becomes slower and slower for an object approaching the speed of light, as compared with time’s rate of passage for a stationery observer. If the speed of light can be ever achieved, time would come to a complete standstill. If an object could go fast, it could cover a vast distance while, for a man flying along with it, no time would elapse – neither for his watch nor for his heartbeat, which controls his life span.

A meson (an unstable subatomic particle), when traveling at a velocity close to the speed of light, has a clearly longer decay time than its 2.1 microseconds ‘half-time’ at lower speed – when an Earth-fixed observer does the timing. But if the observer were flying along with the meson, the half-time of 2.1 microseconds would not seem to be affected by the particle’s speed, since the observer’s watch would be subjected to the same time dilation as the meson itself.

This effect makes it possible for a stellar astronaut to travel from the Earth to a fixed star 1,000 light-years away in what he would think was 13.2 years. If he didn’t spend any additional time at his destination, he would thus have been away from the Earth for 26.4 years. The trouble is that, during his absence, more than 2,000 years would have elapsed on Earth. Thus, upon return, he might wind up in a zoo or under observation in a lab.

Thus, theoretically, it is possible for an astronaut to travel 2,000 light years within the span of his lifetime. ‘Time dilation’ would help the astronaut to stay young and alive for the whole round trip.

The strange phenomenon called time dilation is a hard pill to swallow. This might be because the flow of time appears to us completely unaffected by physical conditions. Whether we sleep or work, sit at a desk or in a speeding jetliner, our wrist watch seems to tick away at the same pace. So does our heart. However, the fact is that this cherished piece of ‘everybody experience’ is valid only in the realm of relatively low velocities in which we slowpokes live.

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