The primary pragmatic obstacle to human expansion into space is unquestionably economic. The technological issues, even the issues of countering the physical stresses on the human body, CAN be overcome- if a practical motive exists to do so. In this sense, economics is not just about money per se but also about resources. In order for a realistic effort to be mounted to put human beings into space long-term in a viable manner, as opposed to things like the space station or former manned moon missions both of which do not rise much if at all beyond the level of an extremely expensive publicity stunt, a WHY has to be provided that appeals to more people than the starry-eyed idealists. Light propulsion has been proposed as a small part of the answer in that, if it can be practical, this type of technology would allow for extremely low-cost travel between planets or other bodies in space. This hypothetical technology does not itself provide a reason WHY to occupy space directly, but it would lower the cost to profit ratio so that an economic motivation for human occupation of space becomes more possible. The idea is that although for example spending $10 to earn $5 may not be worth it, if one can by reducing transportation costs make it a case of spending $4 to earn $5, then this would be a worthwhile investment.
The idea of light propulsion is to adapt techniques of sailing to space travel. In lieu of ordinary wind to push a sail, one would use the so-called “solar wind”. This term refers to the flow of light [and also other particles] outward from the sun. On the everyday scale here on Earth, the wavelike properties of light tend to dominate for most practical purposes, apart from devices using some sort of lens. Nevertheless, light also has particle-like properties. In the archetypal quantum duality, light is both made up of particles called photons and yet also photons of light themselves- singly or collectively- also have wavelike properties. That is how light is diffracted from a lamp or an electric torch. Yet, a small pressure results when light is shined onto an object because the shining of the light is literally bombarding that surface with particles- photons. On Earth or in any other strong gravity well, light pressure as this effect is called is negligibly small, although it is a real measurable effect if one uses precise enough tools. In principle at least, in space IF a ship were made light enough and equipped with surfaces big enough, that tiny light pressure might be enough to push along a space-ship, so the theory goes.
This is where one next encounters economic issues, concomitant with the physical issues. First, at the orbit of the Earth, from which any ship humans put into space at this point or in the foreseeable future would have to start, the power per area due to light pressure would be a minuscule approximately 8.32 micro-watts per square meter. One would need a sail several kilometers across, length and width, to move a mass comparable to a small container of potato salad. No material durable enough to make a solar sail from and yet light enough not to be rendered useless by its own mass yet exists. Of course, the farther one gets away from the sun, the more the power available from the sun to use. A second practical issue becomes return voyages, meaning sun-ward voyages. The solar wind never changes direction; it is always directed away from the sun. If one could use a solar sail to fly to the orbit of Mars, one would have to sail against the solar wind to come back to Earth.
This is not to say the idea of light propulsion will always be useless. Yet, realistically speaking, bright ideas are needed to make it practical- especially if one wishes to build ships to transport humans and other cargo.
