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Light years help give an idea of distances through space and time. When we look at a star 100 light years away, that 100 light years not only gives an idea of the immense distance to the object but will also tell us that what we see is light from 100 years in the past.

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Ultimately, the answer boils down to convenience. When we want to describe the distance between here and, for instance, the star Sirius (the brightest star in our night sky), it would be a little cumbersome to write $\ell=8.13\times10^{18}\,{\rm cm}$ any time we want to write its distance from us. And really this goes for any astronomical object: they're ...

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Whenever you see things at a distance, your perceptions are of things that happened in the past. You can see it at a football game, for example, where someone kicking the ball is seen very much before it is heard (sound is slower than light). Light only has a finite speed, too; and light turns out to be the fastest thing there is. A light year is the ...

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Purely historical and convenience reasons, people standardized measures that were more obvious from real life (remember, quantum mechanics and relativity didn't exist when SI was drafted). It's actually arbitrary how many quantities you define as fundamental (and associate them with base units), and it doesn't matter which ones are they. In quantum ...

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Temperature Amount of a substance Luminous intensity are pretty much bogus fundamental units. The unit temperature is just an expression of the Boltzmann constant (or you could say the converse, that the Boltzmann constant is not fundamental as it is merely an expression of the anthropocentric and arbitrary unit temperature). The unit energy will be ...

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One Joule (the unit of enery) equals one kgm^2/s^2. So you see, a unit of energy can be expressed in terms of units of mass, distance, and time. The people who chose the SI units could, for example, have made the Joule an SI base unit and defined the unit of mass in terms of distance, time, and energy (kg=Js^2/m^2), but then we still have the same number of ...

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why have these been chosen as the fundamental units? Courtesy of the National Institute of Standards and Technology (NIST), we have some historical context. It basically boils down to wanting to have absolute measurements with respect to units of mass, length, and time. These few were originally chosen because these form a set of mutually independent ...

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