One of the 20th century's key encounters took place in 1919, when Henry Crouch visited Albert Einstein at the latter's home in Berne, Switzerland. The great scientist's key paper, "Cosmological Considerations on the General Theory of Relativity", had been published in 1917 and, with the war over and the world exhausted by fighting, people were ready to embrace something new, peaceful and pure: the idea of super-intelligence.

Einstein's reputation had spread. He was the mad-haired genius, the Berne patents clerk who was solving the mysteries of the universe, the good German, the ultimate rationalist. Everyone wanted to know how he had overthrown the laws of classic Newtonian science. So the New York Times sent Henry Crouch to interview him. But Crouch wasn't the paper's science correspondent; he was their golfing correspondent and hadn't much of a clue about relativity or quantum mechanics. He did have a nice line in myth-making. And he assured readers that Einstein had written a book (actually a paper) which "only 12 men in the world could comprehend".

It was nonsense, but potent nonsense, and the idea of a tiny elite of super-brains who could grasp the secret knowledge of the universe stuck in the popular imagination. Even scientists seemed to buy it. Sir Arthur Eddington, a British astronomer, was asked by a journalist if it was true that he was one of only three people in the world who understood Einstein's theories. He considered for a moment and replied: "I'm trying to think who the third person is..."

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We all think we're pretty clever. Look at us. We can fill in a tax return, follow a recipe, change a plug and negotiate a roadmap without a sat-nav. Though we're hazy about the exact benefits of the AV system or the reason for the synchronicity of the North African uprisings, we understand lots of stuff. We can comprehend conceptual art, once the relation of the cow's head, the flies and the zapper has been explained to us, we know Freudian psychology is about sex and verbal slippage, we appreciate that fundamentalism can be about Christianity as well as Islam and we're pretty sure chaos theory involves butterflies.

Encouraged by these gobbets of knowledge in our mental baggage, we imagine that, if it came down to it, there are few subjects that would baffle us, if we applied our brains to them. We all, however absurdly, have a tiny voice of conceit at the back of our heads that says: if only 12 people in the world are smart enough to understand this, I'm probably one of them. This impulse has spawned a whole industry of visual and literal elucidation, of books and magazines and TV and radio programmes. They're characterised by two things: they deal in subjects that hardly existed 50 or 100 years ago, from game theory to poststructuralism; and the explanations are getting shorter, simpler and coarser all the time. We're spoilt for choice when it comes to the Brisk Introduction to the Enormous Subject. Wherever we look, a freelancing cultural historian or television academic is offering to explain some arcane philosophical, historical or biological theory to us in a matter of minutes. Forget the days of Get Rich Quick. We're the generation of Get Knowledgeable Quick.

We can blame Einstein. In the 18th and 19th centuries it was generally accepted that scientific innovation or discovery could be explained in a way that ordinary mortals could understand. Modern readers of Darwin and Freud remark how they presented their earth-shaking findings in prose that might come from a novel. With the arrival of quantum mechanics, however, science split into two – the stuff that could be read and appreciated by hoi polloi, and the stuff that only hoi aristoi could make head or tail of. Einstein became a byword for Opaque Genius, Brilliant Unintelligibility – a challenge for soi-disant intellectuals everywhere.

In May 1920, endowed by a patron of the arts and sciences called Eugene Higgins, Scientific American magazine ran a competition offering $5,000 for the best explanation of the General Theory in under 3,000 words. It was a popular challenge. "I'm the only one in my entire circle of friends who is not entering," Einstein commented, wryly. "I don't believe I could do it." The winning explainer, by a pleasing coincidence, was a clerk in a patents office, just like Einstein.

We've become suckers for a brain-challenge. Tell a man or woman that a subject – astrophysics, say, or differential calculus – is beyond their grasp and they'll beat a path to the door of anyone who offers to enlighten them. Hence the rise in the 1988 bestseller charts of Stephen Hawking's A Brief History of Time. It's one of the least-read works in publishing history – but millions, when told it held the key to the "incomprehensible" subject of cosmology, rushed to buy it. The key word in the title that drew them wasn't "History" or "Time". It was "Brief".

We're gluttons for brevity. We think small is beautiful. Hungry for knowledge but time-poor, we embrace offers of learning without tears, the no-study-needed degree, the qualification without the three years' hard grind. I was first aware of how this appetite was being fed in the early 1970s, when the Fontana Modern Masters series was launched. These tiny "pocket guides" led readers through 90 or 100 pages of packed and allusive text and introduced them to the worlds of Camus, Joyce, Orwell, Freud, Gandhi, Wittgenstein and Yeats, along with less canonical but trendier thinkers such as Che Guevara, Marcuse, Levi-Strauss and Frantz Fanon. The series stretched on into the 1980s and 1990s with 57 titles.

Its general editor was Frank Kermode, the literary critic, who wrote: "By Modern Masters we mean the men who have changed and are changing the life and thought of our age. Everybody wants to know who they are and what they say, but hitherto it has often been very difficult to find out. This series makes it easy. Each volume is clear, concise and authoritative. Nothing else can offer, in such an acceptable form, an assured grasp of these revolutionary thinkers."

You could hear snorts of derision from academics at "but hitherto it has often been very difficult to find out". It damn well should be difficult, they said, to apprehend the subtleties of these complex men and women, and it should take years of study rather than a glance at a booklet. Damning with faint praise, The Times Literary Supplement welcomed the first 10 books as, "just what is needed by the so-called 'general reader' in search of a guide to intellectual currents," while subtly conveying the impression that the "general reader" was a trend-chasing dilettante or an exceptionally idle student.

As a student myself, I found them of variable quality: John Gross's tiny book on Joyce was good on the life and the reputation, but trying to say anything meaningful about Ulysses or Finnegans Wake in a dozen pages each was a fool's errand; Al Alvarez's introduction to Beckett was insightful about the great man's plays, but disastrously thin about the prose of the author's all-important Trilogy. Both authors conveyed, however, a palpable excitement in dealing with such literary titans in a small compass.

A modern equivalent of the Fontana books is Radio 4's In Our Time, which Melvyn Bragg has co-edited and presented since October 1998. The programme celebrated its 500th broadcast last month, with Lord Bragg's donnish guests ruminating on Free Will. A week earlier, Bragg and the Astronomer General, Martin Rees, chewed over the Age of the Universe. A week later he and some chin-stroking medievalists considered the influence of Middle Ages universities on European intellectual life. The show's title reflects its original intention – to consider the key ideas of the 20th century, and give each a 45-minute airing with expert commentators – but has become increasingly meaningless, as Melvyn and Co have introduced listeners to Pliny the Elder and Egyptology. Their subjects range from the social implications of Ageing to the controversial nature of Zero.

That crazily wide range is what seems to attract the show's two million listeners, many of whom, according to the comments on the In Our Time website, have listened avidly to every instalment. But can they (Can anyone? Can Lord Bragg?) possibly grasp the nub of every subject? Or are we kidding ourselves?

I tuned in last week to a discussion of the neutrino, a subject of which I had no prior knowledge; I imagined the word referred to some brand of modern trouser. The show began promisingly. Melvyn Bragg explained that neutrinos are tiny organisms sent from the sun, that pass through our bodies, and through the whole mass of the earth, in their billions, every second. An Oxford particle physicist called Frank Close produced some fine poetic metaphors (the only way of describing science to the layman): how neutrinos are: "Near to nothing... They possess no charge and no mass, and pass through the earth like a bullet through fog. They move through the universe like spectators. Sixty million pass through our eyeballs every second without our knowing."

After five minutes, through, a familiar response kicked in. A mention of "electrons emitted in beta decay", followed by Melvyn's invitation to "develop" this or that increasingly recondite area of subatomic theory, and I was lost. Despite a bravura description by Professor Susan Cartwright about what happens when a particle travels at more than the speed of light in water, I was hopelessly at sea. So, I suspect, was Melvyn, plus a fair percentage of the listeners. Did I – did we? – really imagine we'd end the programme fully briefed about the role of the neutrino in the physical world? But I doubt if anyone switched off. To adapt Sir Thomas Beecham's observation about music: the British may not understand science, but they love the noise it makes.

Front-runners in the current field of miniaturised explanation are the Very Short Introduction guides published by Oxford University Press. Introduced in 1995, they're immensely successful, and sell in 25 countries. And they cover 200 subjects in an average of 120 pages. Two hundred subjects! I bet you didn't think the General Reader cared about so many topics. But he does. He or she can now buy read-in-one-evening guides to, say, Advertising, Anarchism, the Dead Sea Scrolls, Medical Ethics and the Laws of Thermodynamics, digesting them at a rate of one a day.

Are they any good at bringing their subjects to life? I must admit: absolutely yes. The authors are mostly experts in their field, and seem to relish the challenge of bringing clarity and user-friendliness to their subject. The most recent productions from OUP are Beauty by Roger Scruton, English Literature by Jonathan Bate and Genius by Andrew Robinson.

Scruton adopts a sternly philosophical approach to beauty, teasing out questions of morality and aesthetics, exploring the interface between human beauty, sexual desire and reverence for the sacred, before writing, tantalisingly, on page 162, "The reader will have noticed that I have not said what Beauty is..." Robinson takes a directly ad hominem approach, approaching a definition of the over-used and misunderstood word "genius" by mulling over the exact achievement, and the reputation, of Da Vinci, Mozart, Shakespeare, Van Gogh and others, and asking vital questions about whether perspiration or inspiration contributes more to the Eureka moment.

Jonathan Bate, faced with the awesome task of cramming the whole of Eng Lit, from Anglo Saxon poetry to Derek Walcott, into 160 pages, sidles into the subject crabwise via children's books, later devoting one chapter to poetry, another to the novel; he picks illustrative examples from ancient and modern, as if the key authors were all present in some bookish Valhalla, being introduced to the reader as a gang rather than a succession of individuals. Their generic titles notwithstanding, these Very Short Introductions are very substantial contributions in their own right, by academics who have had clarity and focus thrust upon them.

A more radical approach to the keep-it-simple introduction is the series of Graphic Guides published by Icon Books under the general editorship of Richard Appignanesi, who also co-authored several. From Aesthetics to Wittgenstein, the 60 titles feature graphic-novel or comic-strip stills, doctored photographs with speech bubbles, talking heads and limpid quotations from classics put into the mouths of cartoon figures.

It's a lot more sophisticated than it sounds, and you can learn much from the visuals that you might not learn from slabs of prose. The trouble is, the presentation is so frantic, it pre-supposes a student with the attention span of a grasshopper. Page after page, you're bombarded by visual stimuli, random voices, nudging little jokes, digressions – anything to keep your attention without requiring you to read more than two or three actual sentences. The subjects that benefit most from this treatment are those that already involve some degree of visual reference. The books on Postmodernism and Media Studies, with their clever tweaking of artworks or newspaper images, are a lot of fun. When dealing with something more complex, however – say, Relativity – the authors throw the kitchen sink at the reader. Monkeys, birds, insects and a brace of genial observers called Bob and Alice are drafted in to explain quantum theory. They leave the general reader more baffled than ever.

We reach the final frontier of reductionism with a new initiative from the same Icon publishing house: 30-Second Books. Three titles are already published in large-format, sensible-looking hard covers, like downbeat car manuals: 30-Second Philosophies, 30-Second Economics and 30-Second Theories, with 30-Second Psychology still to come. Their unique selling proposition is that they offer to explain the knottiest scientific or economic theories, or philosophies, to the general reader "in just half a minute, using nothing more than two pages, 300 words and one picture".

So, in 30-Second Theories, you get Kant's Categorical Imperative in 300 words (starting, "Consistency is at the heart of morality. If I think that I deserve a certain sort of treatment, then others in my situation are entitled to that treatment too...") and Descartes's Mind-Body Problem and Plato's Cave and Godel's Theorem. Bish, bosh, zing zing. The vital element missing from these basic elucidations is any encouragement for the reader to think about how true or otherwise they are, to grasp their meaning by testing examples and overcoming logical cruxes. Otherwise, you might just as well have the words embroidered onto a T-shirt to be worn by the reader as his only engagement with the wisdom of the past.

In days gone by, you could spend a whole term, or university year, wrestling with Hume's Problem of Induction, or Popper's Conjectures. Now they're available to you in half a minute. Inevitably, Einstein's Theory of Relativity gets the half-minute treatment, reducing the huge oratorio of his epoch-making discovery to the equivalent of a pop single (which misleadingly talks about time travel).

We have come a long way since Scientific American magazine wondered if anyone could explain it in 3,000 words, and Einstein doubted it was possible. It's hard to know how much more compressed these brief introductions to complex thought can become, without offering the reader nothing more than a dictionary definition or a flickering image on YouTube. That's what the Enlightenment has become in the 21st century. Where once we knew a gratifying amount about a few subjects, we're now in danger of knowing a few seconds' worth of information about everything.