Every now and then you come across a technology, or a mooted technology, that sounds so far-fetched, so outlandish that it belongs with proper flying cars and the paperless office. Things that will never happen.The idea of “printing” objects, so-called 3D printing once seemed pretty outlandish but it has already made the voyage from science fiction to startup. According to a recent report by analysts McKinsey & Company, “Disruptive technologies: Advances that will transform life, business, and the global economy,” 3D printing will have a direct economic impact of between $230 billion and $550 billion a year in 2025.But if you think 3D printing is disruptive, then what about a technology that could in the view of one of its main evangelists “make the world editable.” That is disruption.That technology is 4D printing — 3D printing but with designs that continue to evolve after manufacture. That really does sound far-fetched. But then a rocket was never supposed to be able to leave the Earth’s atmosphere and four-member guitar bands were never going to be big.According to Jeff Kowalski, chief technology officer for Autodesk, Inc., the very earliest building blocks for that future are in place today.Before we stray into to that extra dimension, what do we mean by 3D printing? Additive printing, as it sometimes known, is the opposite of conventional, subtractive, production. Say you want to make a metal part. Normally, you start with a large block of metal and mill, drill and grind bits off. To 3D-print that same part you start with a metal powder and, in much the same way as a bubble jet printer builds up a 2D image one row at a time, a 3D printer builds up the part one layer at a time. Using fusion techniques the metal powder becomes a solid.