Enrico Fermi issued a statement that would go down in the annals of history: "Where is everybody?"
This became the basis of the Fermi Paradox, which refers to the disparity between high probability estimates for the existence of extraterrestrial intelligence (ETI) and the apparent lack of evidence.
Since Fermi's time, there have been several proposed resolutions to his question, which includes the very real possibility that interstellar colonization follows the basic rule of Percolation Theory.
One of the key assumptions behind the Fermi Paradox is that given the abundance of planets and the age of the Universe, an advanced exo-civilization should have colonized a significant portion of our galaxy by now.
This is certainly not without merit, considering that within the Milky Way galaxy alone (which is over 13.5 billion years old), there are an estimated 100 to 400 billion stars.
Another key assumption is that intelligent species will be motivated to colonize other star systems as part of some natural drive to explore and extend the reach of their civilization.
Last, but certainly not least, it assumes that interstellar space travel would be feasible and even practical for an advanced exo-civilization.
But this, in turn, comes down to the assumption that technological advances will provide solutions to the single-greatest challenge of interstellar travel.
In short, the amount of energy it would take for a spacecraft to travel from one star to another is prohibitively large, especially where large, crewed spacecraft would be concerned.
Relativity is a harsh mistress
In 1905, Einstein published his seminal paper in which he advanced his Special Theory of Relativity. This was Einstein's attempt to reconcile Newton's Laws of Motion with Maxwell's Equations of electromagnetism in order to explain the behavior of light.
This theory essentially states that the speed of light (in addition to being constant) is an absolute limit beyond which objects cannot travel.
This is summarized by the famous equation, E=mc2, which is otherwise known as the "mass-energy equivalence." Put simply, this formula describes the energy (E) of a particle in its rest frame as the product of mass (m) with the speed of light squared (c2) – approx. 300,000 km/s; 186,000 mi/s. A consequence of this is that as an object approaches the speed of light, its mass invariably increases.