Where were you when you heard that Zayn Malik had left One Direction? No, me neither, but as my teenage daughter reports, an entire generation of female youth appears to have been traumatised by the event. Not that she gives a monkey’s herself, of course, but she says that everyone she knows knows someone who knows someone who really cares, sometimes to the point of genuine distress. We can laugh, and indeed we have laughed, rather a lot, but for these sufferers, the flavour of life itself has been tainted and a Lake Baikal of tears has been shed. One Direction, once the perfect five, are now an eroded four. And the rest of the world looks on and wonders, which one will go next? And can it be soon, please?

For there is equilibrium in all things, and especially in boybands. I’m not sure when it became the norm for a boyband to be five, but it is and that’s the way it will stay. The Spice Girls, though not strictly boys, may have been the exemplar, as they clearly covered all the different marketing bases. When kids still formed bands in the old-fashioned way, it was because one person had an amp, and another had a bass guitar, and someone else wore good trousers. But when bands started to form without musicians in them, or anywhere near them if at all possible, the criteria for selection became more lax. The good trousers still count, but these days they are all wearing them.

So now that Zayn has left to spend more time with his hairstyle, will One Direction manage without him? What will have been lost? I’m sure they will manage, if only because of the unimpeded flow of cash still pouring in. And what will have been lost will be Zayn’s important contribution to the group, whatever that might have been.

But four is dangerous. Look at Take That. As four, they just about made sense. With their big middle-aged faces and shaggier hair they took up more room than they did when they were young, which made up for the absence of Robbie, who takes up more than enough room by himself, thank you very much. But now that Orange Jason has left — or was it Donald Howard? — they look vulnerable, like a trio of soon-to-be-unemployed Top Gear presenters. At the same time their records get fatter and more self-important, as though to deflect the question that can’t be asked: is there any purpose at all to your existence as a group? It can’t be asked because you can see the answer in their hollow, dead eyes.

It’s strange because four is the perfect number for bands made up of real musicians, and threes and twos can function perfectly adequately for years. The Police were the perfect three, held together by their profound mutual loathing. No one looks at the Pet Shop Boys and thinks, are you sure you don’t need someone else to tag along? But if One Direction lost one more member, I think the consensus would be that they were no longer quorate. In the end they are all going in one direction: out the door.

As so often, it was Stephen Hawking who provided much-needed reassurance. Asked in Australia last week about the cosmological effect of Zayn leaving One Direction, his advice to any heartbroken young girl was to ‘pay close attention to the study of theoretical physics. Because one day there may well be proof of multiple universes. It would not be beyond the realms of possibility that somewhere outside of our own universe lies another different universe. And in that universe, Zayn is still in One Direction.’

Where can he go? What can he do? When even Stephen Hawking knows who you are, there is no escape. But this does seem to me the most extreme manifestation yet of what pop historians of the future will call the Westlife Phenomenon. As connoisseurs we have long marvelled at the commercial success of Westlife, even though very few people can name any song they have recorded. One Direction have shown that this was no fluke. From now on, the global fame of a pop group will be seen to grow in inverse proportion to the memorability of their tunes. One Direction have achieved absolute fame, but only their core fans have any idea what their music sounds like. And it doesn’t matter. I think I’d resign from that band, if I had the chance.