If this was a film, this would constitute a brutal jump cut, I guess.

That the brutality should come from the same source as last week’s acoustic sounds (even with the salt and vinegar of John Lennon’s voice on You’ve Got to Hide Your Love Away) makes it all the more thrilling.

We have now arrived in the deep sixties, that moment in pop culture where the battle lines are being drawn up.

Again, you might say. Ten years earlier, after all,  rock’n’roll was seen as an assault on convention. But its agenda was purely sonic. In the sixties the assault was on culture and politics and sexuality and anything else that was going.

The sixties youthquake was marked by the hugeness of its ambitions (and when it failed to match them the distrust of the baby boomer generation from those of us who came after was all the greater. Because it had aimed high and fallen short).

And that radical, at times revolutionary, ambition was encoded in the music. Nowhere more than here.

Tomorrow Never Knows appears at the end of the Beatles’s Revolver album and marks a step-change in their sonic palette.

It was inspired by John Lennon’s introduction to the hallucinogenic LSD. Lennon used Timothy Leary’s book The Psychedelic Experience as a guide and it was Leary’s paraphrasing of The Tibetan Book of the Dead that gives the song it’s opening line: “Turn off your mind, relax and float downstream.”

If we now challenge Leary’s nostrums about the drug - and Lennon struggled for years with bad trips and the impact they had on his psyche in the late sixties - there’s no question that Lennon’s experience opened him up to new sounds.

You can argue, though, that some of them were already in play in the Beatles soundscape. The influence of Indian music had already made an impact on Harrison, and McCartney’s interest in the avant garde presumably helped nose the band towards musique concrete (the tape loops that constitute the most extraordinary elements of the track were recorded by McCartney at home and then overdubbed by the band in the studio).

For Beatles expert Ian MacDonald, though, it’s the adoption of the Indian drone that marks Tomorrow Never Knows out. MacDonald argues that only experimental composer La Monte Young had heard the possibilities of the drone before the band.

But it was the Beatles that made it matter, bringing it into, in MacDonald’s words, “First World culture”, and in so doing, challenging “not only seven centuries of Western music, but the operating premise of Western civilization itself”.

That’s an immense claim, but it’s typical of the era (albeit that MacDonald was writing nearly 30 years later). And even now, nearly 50 years on, it’s impossible to deny what MacDonald describes as the “seething dazzle” of the result.

What marks The Beatles out is that exhilarating sense of adventure. If the Rolling Stones were darker and dirtier than their contemporaries in the years that followed, they never strayed too far from the basic building blocks of rock ’n’ roll - R&B and the Blues (although there was the odd English madrigal influence, I suppose).

The Beatles, by contrast, were by 1966 beginning to take on 20th century music in its entirety, looping the past into the present and taking both forward. The Beatles reinvented what was possible. That revolutionary enough for you?

Other contenders

River Deep - Mountain High, Ike & Tina Turner

Got to Get You Into My Life, The Beatles

Paint It Black, The Rolling Stones

Eleanor Rigby, The Beatles

Under My Thumb, The Rolling Stones

Tell It Like It Is, Aaron Neville

Last Train to Clarksville, The Monkees

God Only Knows, The Beach Boys

And the best-selling UK single of 1966: Green Green Grass of Home, Tom Jones