The world has made a colossal cultural mistake that shames our generation, an American law professor said in a lecture at Cambridge University last week. Every thinking person – according to Jamie Boyle, of Duke University, North Carolina – should know something about this mistake just as they must now know something about the environment.

So what is this "colossal cultural mistake"? It has its roots in what may be one of the dullest subjects on earth – copyright law. A 1976 law set US copyright at 28 years, after which copyright holders had to renew it. Around 85% didn't renew because the "cultural object" no longer had commercial value. (This would certainly apply to every word I've ever written, including my books that are now available for 1p through AbeBooks.) These "cultural objects" – books, poems, films, music, images – now belonged to the world, and libraries could reproduce them for everybody for free.

Now, however, the United States has extended copyright to life plus 70 years, you don't have to renew, and everything – including presumably the scrawlings in my diaries – is now copyrighted automatically, without any need to apply. Then the law is covered by strict liability, meaning that it's heavily weighted to anybody suing for breach of copyright. To cap it all, the law has been backdated to 1923, meaning that virtually all 20th century cultural objects are locked up.

Copyright holders can, of course, waive copyright, but the problem for libraries is, first, the expense of tracking down copyright holders (85% of whom, we know, would probably give up copyright); and, second, many of the holders simply cannot be found.

The result may be that libraries – which Boyle declared as "radical institutions" that have for centuries made cultural objects available to all human beings – will become mausoleums, full only of objects. They will not be able to digitise and distribute free to everybody everywhere the cultural objects of most of the 20th century, the century that formed our world.

Why have we made such a colossal mistake? Boyle – whose excellent book Public Domain: Enclosing the Commons of the Mind, is available for free – saw three causes, the first of which he called "cultural agoraphobia" (fear of openness) and illustrated with two thought experiments. Imagine yourself a policymaker in the early 1990s who had to decide which of two global networks to implement. The first would be available to everybody but tightly controlled with only approved people and organisations – like the BBC, the Guardian, and the government – able to place material on it. The alternative network would allow everybody to publish anything and anything could be copied and sent to everybody everywhere.

We can immediately imagine the second network would be full of pornography, spam, propaganda, lies and illegal copying. It's unthinkable that anybody would want to conduct business on such a network. Most people, argued Boyle, would opt for the first network because we are good at seeing what could go wrong but poor at imagining the extraordinary flowering that is the internet.

In a second thought experiment, imagine that it's five years ago and you are responsible for developing the most comprehensive and up-to-the-minute encyclopedia the world has ever seen. One strategy is to create a global company, employ the brightest people available, check every fact produced, and implement the most rigorous editorial controls. A second option is to "just create a website and let anybody put up anything". Again, we'd mostly have opted for the first strategy, and the world wouldn't have Wikipedia.

So cultural agoraphobia is one cause of this draconian copyright law. Another is the obvious idea that the easier something is to copy the tighter the need to be the controls. Before the intervention of the printing press we didn't need copyright as it might take a monk six months to copy the Bible. Now something can be copied at virtually zero cost and made available to everybody with access to the internet, so businesses want tight controls. But wouldn't they be better off, asked Boyle, with a market of a billion people and 20% illegally copying than a market of a million where copying was heavily punished?

The third cause, argued Boyle, is that – in contrast, to decisions on, say, pharmaceuticals – law on copyright is made without evidence. Anecdotes of the losses from illegal copying are showered on legislators but little is heard of the huge losses of "lost sharing".

So what might be the answer? Boyle – rightly for a lawyer – dismissed my suggestion of mass illegality rather like mass trespassing. A better strategy would be legal reform, perhaps re-establishing the requirement for renewal; businesses with valuable objects would certainly renew.

With such reform we could create "a digital library of Alexandria", with free global access to everything that is no longer being commercially exploited. Without it we may deny access to the cultural achievements of the 20th century to most people.