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According to Einstein's theory of relativity, nothing in the universe can exceed the speed of light (300,000 km/sec or 186,000 miles/second). This "universal speed limit" has all sorts of implications. One such implication is that each object in the universe has defined portions of the universe that it can and cannot influence within a given period of time.
Let us look at an example. On average, the sun is 150 million kilometers away from the earth, and it takes light about 8 minutes to make the trip. Therefore, even at the speed of light, I cannot make it to the surface of the sun in less than 8 minutes. Those areas that cannot be reached or influenced within a given period of time are said to be in the elsewhen.
Let us think about time in a linear fashion, with the future stretching out before us, and the past behind us. Those areas of the universe that we theoretically could have had an influence on are said to be within our past lightcone. Those areas that we can have an influence on (if we could travel at the speed of light) are said to be in our future lightcone. Anthing in the past or future lightcones are said to be in our "locality"; everything else is in the elsewhen.
If you are going to live to be 100 years old, there are many places in the universe that you could never get to even if you could travel at the universal speed limit and even if you devoted your entire life to the trip. In fact, even if you could live for thousands of years and could find a rocket that could travel at the speed of light, you would only be able to reach an infinitesimally small portion of the known universe.