The stadium -- three-quarters empty, one-quarter filled with substitutes, coaches and media at the sort of population density you might find in Alaska -- must have made it worse. Angry boos and vicious abuse, anguished howls and despairing cries... however unpleasant, they tell you you're alive. They're the slap to the face or pinch of the cheek that generates pain and adrenaline to shake the numbness and activate whatever fight is left. The surreal near-emptiness of the Estadio da Luz must have sown some seed of doubt in the minds of Barcelona players Friday as they went 4-1 down with just over half an hour gone against Bayern Munich in their Champions League quarterfinal .