Whenever classical musicians and rock music fans meet, the conversation turns to those four Liverpudlians and how good they really were. Do we only like the Beatles because they were "our music"? Or will they last in the way that Mozart and Beethoven have lasted?
One hundred years from now Beatles songs may be so well known that every child will learn them as nursery rhymes, and most people will have forgotten who wrote them. They will have become sufficiently entrenched in popular culture that it will seem as if they've always existed, like Oh Susannah, This Land Is Your Land, and Frère Jacques.
Why can we listen to certain songs across a lifetime and still find pleasure in them? Great songs activate deep-rooted neural networks in our brains that encode the rules and syntax of our culture's music. Through a lifetime of listening, we have learned what is essentially a complex calculation of statistical probabilities of what chord is likely to follow what, and how melodies are formed. Skilful composers play with these expectations, meeting and violating them in interesting ways. In my laboratory we've found that listening to a familiar song that you like activates the same parts of the brain as sex or opiates do. But there is no one song that does this for everyone; musical taste is both variable and subjective.
Today the Beatles catalogue is cross-culturally loved - the product of a six-year burst of creativity unparalleled in modern music. The Beatles incorporated classical elements into rock music so seamlessly that it is easy to forget that string quartets and Bach-like countermelodies and bass lines (not to mention plagal cadences) did not always populate pop music. Music changed more between 1963 and 1969 than it has in the 37 years since, with the Beatles among the architects of that change. Paul McCartney may be the closest thing our generation has produced to Schubert - a master of melody, writing songs that seem to have been there all along.
Most people don't realize that the well-known tunes Ave Maria and Serenade were written by Schubert (or that his Moment Musical in F so resembles Martha My Dear). McCartney writes with similar universality. His Yesterday has been recorded by more musicians than any other song in history. Its stepwise melody is deceptively complex, drawing from outside the diatonic scale so smoothly that anyone can sing it, yet few can explain what it is that McCartney has done. (And the odd seven-bar phrases hark back to an old Haydn trick of asymmetric phrases in his minuets.)
The timelessness of Beatles melodies was brought home to me by Les Boréades, who have recorded three CDs of Beatles music arranged for and played on baroque instruments. These give the sense that you're hearing Bach or Vivaldi, and for a moment it is possible to forget that you're listening to Beatles songs. Stripped of their 60s production, and all their personal and social associations, you can hear something revelatory: the songs themselves, the intricate and beautiful interplay between rhythm, harmony and melody.
On the bus to my office, the radio played And I Love Her and a Portuguese immigrant my grandmother's age sang along. How many people can hum even two bars of Beethoven's Fourth, or Mozart's 30th? I recently played one minute of these to an audience of 700 people - professional musicians included - but not one recognised these pieces. Then I played a half-second of two Beatles songs - a fraction of the first "aah" of Eleanor Rigby and the guitar chord that opens A Hard Day's Night - and virtually everyone shouted out the song names, more than could recognise the Mona Lisa.
To a neuroscientist, the Beatles' longevity can be explained by the fact that their music creates subtle and rewarding schematic violations of popular musical forms, causing a symphony of neural firings from the cerebellum to the prefrontal cortex. To a musician, each listening showcases subtle nuances not heard before, details of arrangement and intricacy that slowly reveal themselves across hundreds or thousands of listenings. I have to admit, they're getting better all the time.
· Daniel J Levitin, a former record producer, is professor of psychology and music at McGill University in Montreal; his book, This Is Your Brain on Music, will be published by Grove Atlantic in November