Life would go out in a 'fraction of a second' (that was the phrase), but all night he had been realizing that time depends on clocks and the passage of light. There were no clocks and the light wouldn't change. Nobody really knew how long a second of pain could be. It might last a whole purgatory--or for ever.
The sense of unhappiness is so much easier to convey than that of happiness. In misery we seem aware of our own existence, even though it may be in the form of a monstrous egotism: this pain of mine is individual, this nerve that winces belongs to me and to no other. But happiness annihilates us: we lose our identity.