Throughout the years predictions for the technology of the future have abounded in everything: from reports by government think-tanks to science-fiction.
Military: Ray guns
Unstuck in time, Billy Pilgrim finally came unstuck in the mid-1970s when he was assassinated by ray gun in Kurt Vonnegut's 'Slaughterhouse 5'. But ray guns - or 'directed energy weapons', according to current military jargon - have failed to live up to the destructive image they acquired in sci-fi from the 1950s onwards. To some extent, sci-fi has caught up. A running theme in the 'Stargate' TV series is that, despite seeming primitive, bullets sometimes turn out to be more effective than the energy weapons used by more sophisticated enemies.
Although it is now possible to build lasers powerful enough to destroy missiles - the dream of Ronald Reagan's Star Wars programme - they require massive amounts of energy. So, the direction for many of these projects is concentrate on using microwaves and RF beams to confuse the electronics in enemy systems, where the dispersion problem can work in favour of the energy weapon's wielder.
Space: Martian colonies
Remember George Bush Senior? In July 1989 he marked Apollo 11's 20th anniversary by announcing 'a journey into tomorrow, a manned mission to Mars'. Nasa wrote the infamous '90-Day Report', so-called because that was how long it took to make Bush wish he'd never mentioned Mars.
The mission was costed at $200bn, and everyone apart from die-hard space cadets soon lost interest. Robert Zubrin, a former Lockheed Martin engineer, has shown that a mission can be achieved for far less, but in our cash-strapped era no one has made a compelling case for sending people rather than robots.
Despite countless engineering proposals for settlements on the Moon and Mars, no one's solved the fundamental problem: what's the payback for the expense? The discovery of life on Mars would help, but the Red Planet stubbornly refuses to deliver the hard evidence that might justify a human visit, let alone a permanent colony.
Society: The paperless office
In 1975, Business Week magazine predicted 'the paperless office' by 1990, with all records retrieved and displayed electronically. The masses of data, generated by digital money and other forms of legally significant transactions, still call for a vast amount of paper backup. All this and Hello magazine, too.
Despite iPads, average global paper use is 48kg per person. Americans leaf through 330kg each. Digital archiving media swiftly become obsolete. A sheet of paper, properly stored, lasts indefinitely.
Our best hope for a paperless world is to dump everything in the Cloud, so that 'someone else' always handles data migrations from old hardware to new. Electromagnetic pulses from a nuclear war could wipe everything in a flash, so to speak. Perhaps those paper printouts are a good idea after all.
Want to be rebuilt but better than before? Sci-fi argues for the mechanical option: from Steve Austin's powerful bionic legs to the complete rebuild of Alex Murphy in 'Robocop'. Even in a technologically advanced society able to clone complete humans, such as that depicted in 'Star Wars', the Skywalkers receive mechanical replacements for their lost limbs. But few artificial materials compete in strength and weight with biological alternatives. Although advanced prostheses are beginning to be fitted to people who have lost limbs, these are more likely to be temporary fixes until the technology to grow biological replacements develops.
Entertainment: 3D displays
Remember that moment in 'Star Wars' where Luke Skywalker inadvertently sees a 3D message intended for Obi Wan Kenobi? A pint-sized Princess Leia calls for help. Admit it, you know the scene. In real life, the best 3D systems depend on special glasses or headsets, while the 'no glasses' benefit of a lenticular 3D screen is all too obviously limited by the edges of the display. And TV companies such as the BBC'and ESPN have given up even on those for the moment.
There are ways in which you can get the''Star Wars'-style display, but they are very limited. Companies such as Heliodisplay and Perspecta beam miniature images onto a controlled haze of'oil or water droplets. Other systems, such as one recently developed at MIT, can generate holographic videos that appear above a reflective surface, but we cannot yet throw convincing illusions of depth and solidity into thin air.
Transport: Flying cars
In countless futuristic visions of city life, flying cars nip neatly overhead as as if there's no possible doubt about the matter. Surely our urban transport infrastructure must one day reach as high as all those skyscrapers?
Californian company Moller International's M400 SkyCar uses four ducted fan blades, each driven by a compact ethanol rotary engine. The car looks good in photos but makes an unconvincing racket lifting off on its short, wobbly flights.
Depressing power to weight ratios limit what we can achieve without the aerodynamic help of wings. In the absence of featherlight cars held aloft by anti-gravity beams, we'll be taking the Tube for a while yet.
Fashion: Smart, shiny clothes
There's a good reason film directors require people of the future to dress up in shiny silver costumes - it makes the future look different. But people are far more conservative than costume designers. No matter how often the fashion industry reinvents itself, formal dress for men hasn't changed much since the invention of the lounge suit and neck-tie in the 19th century. Women's outfits have changed more but, aside from a night out clubbing,'people tend to avoid uncomfortable plastics unlike their futuristic counterparts.
It's not just the style. For the past two decades, engineers have been working on smart clothes. But, aside from a bit of clubwear novelty, no one has come up with a convincing reason for stitching LEDs and other circuitry into clothes - none of which yet survive a cold-cycle machine wash.
Nutrition: Food pills
In the shiny environment of futuristic sci-fi, food is a messy business, best cleaned up by turning the experience of eating into the purely functional exercise of swallowing a couple of pills that somehow taste like a proper meal. The appeal of the pill was arguably greatest at the point where heavily processed food was new and promised much greater convenience over traditional post-war stodge. As the public fell out of love with TV dinners and artificial additives, the idea that people of the future eat ordinary food came back into fashion. By the time of 'Star Trek Voyager', writers began to realise that it was probably going to take far less energy to put the kettle on rather than let anyone order 'Earl Grey ' hot' from a modified matter transporter.
Energy: Limitless power
What could be a better, cheaper power source than water - or at least two of the isotopes of hydrogen found in water? With a fusion power plant we could harness the power of the sun directly, using the energy released by fusing two nuclei into one of helium. The sun manages it through its immense gravitational force. We need other means to do the same, and that's where the trouble starts. In principle, a second's worth of sustained fusion could run a city for a day with almost no pollution, but getting to that point is taking a very long time and it could be decades before we see fusion releasing more energy than is needed to stimulate and safely contain the reaction. Even then, the cost of maintaining and repairing the reactor may turn out to be the major obstacle and far outweigh the advantage of using such a cheap fuel.
Physics: Defying relativity
If faster-than-light or time travel are possible, you have to ask the question: "Why aren't they here yet?" Although Einstein's theory of relativity has yet to be fully reconciled with those of quantum mechanics, attempts to see if it is possible to overcome its limits have so far failed. A recent experiment with metamaterials hinted strongly that the arrow of time points in one direction and an aberrant result from the Large Hadron Collider that suggested some particles travelled faster than light turned out to be exactly that. We probably won't have to worry about paradoxes, but space travel is likely to remain extremely tedious.