He is walking alone, he tells us, when suddenly he stops moving. In the field is a shepherd, his hand frozen in the act of putting a bit of bread into his mouth. Overhead a heron pauses in flight. The whole world stands still, holds its breath for a second; and Joseph knows that the birth has happened.
It's not a bad image for that event which Christians believe is the pivot on which history turns. And we might think about the way in which each recollection of the event has something of the same "stilling" effect. (In the Middle Ages, it was widely believed that you didn't get any older while you were at Mass; the same principle.) Something has been caught, arrested, God has put a finger on the spinning globe. Or even: God has made a protest against the idea that the world
in appetency, on its metalled ways
of time past and time future
(Eliot's words in "Burnt Norton").
Something steps in to change or challenge the automatic sequence of things. The philosopher Simone Weil touches on something similar when she says we are constrained by "gravity" in the spiritual world as well as in the material one. Whenever I act it sets up a chain of consequences which inevitably involve loss to someone else. So the other person learns to defend herself. I am hurt, so I will hurt someone else. The abused becomes the abuser. Your poverty is necessary for my welfare. Your debt reinforces my moral probity. Peace will dishonour the blood of our glorious dead. And so on - though, thank God, on Third World debt and in Northern Ireland we see some movement at last on this. But the basic point is that the "gravity" which affects all social transactions places us in a barely subdued state of war with other people.
All this can be stopped, says Weil - by a moment when someone chooses not to pass on the hurt but to "absorb" it; not passively, but in the most strenuous spiritual action possible. The "laws" of the spiritual realm turn out to be more breakable than physical gravity; but the breaking of them is supremely costly to the person who decides not to transmit the plague of violence and debt.
For the believer, the birth of Christ is something like that moment of "absorption", the breaking of a chain of gravity. God alone, ultimately, can absorb into the depth of divine life the violence of humankind so that nothing is passed on. That is why the life that begins in Bethlehem has to issue in a death utterly forlorn and unconsoled, a death that is like being pushed into the void. This is a life that creates a space instead of invading, a life that exists in and only in a letting-live. The word of Christ is a moment of silence in the chatter of competition and resentment, heroism, posturing and clinging to probity.
Unlike medieval Mass-goers, we are all too sure that we shall be older after Christmas and the Millennium. But if we are going to do more than treat the Millennium as an arbitrary marker for the passage of smooth undifferentiated time, if we are at all interested in what we are commemorating, we might think about what sort of silence, what sort of "absorption" is asked from us as individuals or as a society. The biblical idea of Jubilee which, on the matter of Third World Debt, has been fiercely and unanswerably pressed on us as the only possible moral content for the Millennium, is also about how our time can be punctured by moments that arrest the flow of "necessity" and which question the spiralling drive of wealth to rise further and further away from poverty. It is possible to stop. It is possible to flatten out the economic territory again. We need not be trapped by laws of our own making.
But deeper than this challenge lies the question that the events of Jesus still put to us. What, finally, does change the world or change the heart? Is there an inexhaustible life suffusing the world that we can - through the medium of this one biography, 2,000 years old - stop the wheel of necessity and draw human beings into its own action of gift and making space? Christians have so often been virtuosos of resentment and false heroism that it's hard to claim that a difference has truly been made.
But never mind - for a moment anyway - what's been made of it all. The fact is that people do still look up and glimpse against a darkening sky a bird frozen in flight. In this birth and this life, they see gravity suspended, the necessity of social transmissions and infections overtaken by gift, space and silence.
The Most Rev Rowan Williams is Archbishop of Wales