Travelling at the speed of light
Can we live for hundreds of years? Travel at the speed of light? Terraform Mars or other planets? If you read science fiction or science, you will have encountered and mused about these kinds of questions. None of these are possible today but could they be possible in the future or are they mere fantasy?
David Deutsch gives us a framework for separating the achievable from the impossible this in Chapter 3 of the The Beginning of Infinity (page 55-56)
The increasingly intimate connection between explaining the world and controlling it is no accident, but is part of the deeper structure of the world. Consider the set of all conceivable transformations of physical objects. Some of those (like faster-than-light communication) never happen because they are forbidden by laws of nature; some (like the formation of stars out of primordial hydrogen) happen spontaneously; and some (such as converting air and water into trees, or converting raw materials into a radio telescope) are possible but only happen when the requisite knowledge is present - for instance, embodied in our genes or brains. But those are the only possibilities. That is to say, every putative physical transformation, to be performed in a given time with given resources or under any other conditions, is either:
impossible because it is forbidden by the laws of nature
achievable given the right knowledge
That momentous dichotomy exists because if there were transformations that technology could never achieve regardless of what knowledge was brought to bear, then this fact would itself be a testable regularity in nature. But all regularities in nature have explanations, so the explanation of that regularity would itself be a law of nature, or a consequence of one. And so, again everything that is not forbidden by laws of nature is achievable, given the right knowledge.