Einstein's special relativity produces time dilation and length contraction and establishes a cosmic speed limit: nothing can exceed the speed of light. That limit makes rapid interstellar travel and round-trip communications across light-years effectively impractical. A commonly offered explanation cites increasing mass with speed, implying infinite mass and energy at light speed, but that mass-based account is misleading in detail. Relativity nevertheless forbids objects with non-zero mass from attaining light speed and forbids massless particles from exceeding it. The popular mass-increase narrative does not constitute the precise technical reason for the universal speed limit.
The idea that there is a maximum speed is pretty counterintuitive; after all, in everyday experience, you can make a car go faster simply by stepping harder on the gas or upgrading to a sports car. In rocketry, you can just let the rocket fire longer. So why is it that we can't move faster than the speed of light?
And this is kind of a satisfying and intuitive answer. It's harder to push more massive objects and, therefore, if the mass of an object gets heavier, you have to work harder to go faster. And, if the mass of an object becomes infinite near the speed of light, then it would take an infinite amount of energy to push it even faster.
While this answer is satisfying and intuitive, it's also wrong-at least in detail. Now before anyone decides to quote me as saying that Einstein's theory of relativity is wrong, don't. Relativity does indeed state that an object with non-zero mass cannot go at the speed of light, and even massless objects cannot go faster than light. So, this mass misstatement is no help to those erstwhile interstellar explorers.
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