Often, when I return to Hamburg, I see the ghost of my German grandmother, walking the streets where she spent her youth during the heyday of the Third Reich. I sometimes see her promenading along the path that runs along the River Alster, or window-shopping on Jungfernsteig in a fur coat that's seen better days. She still looks the same after all these years - tall, robust and indestructible, more shapely in her dotage than most women half her age. For a moment I'm transfixed, torn between terror and excitement. But then I take a closer look and I sense there's something not quite right about her. And then I put on my glasses and I realise that it isn't my grandma after all.
She grew up in Hamburg during the heady chaos of the Weimar Republic, in a big old house beside the Alster. She lived there with her mother, stepfather and a small entourage of servants. Her stepfather was a rich industrialist who somehow hung on to his money during the inflation of the 1920s. His chauffeur used to drive my grandma to school. This was the house where she raised her children, too - my aunt and uncle, Marion and Michael, and my father - and whenever I'm in Hamburg I go back there, to try to imagine the woman she used to be, and the life she left behind.
I always take the same way there, by ferry across the Außenalster - that huge bulge in the River Alster that's like a city-centre lake. I get off at Rotherbaum and walk past rows of palatial villas to Heimhuderstraße, where my grandma lived before the war. As in all posh streets, the first thing that hits you is the silence. It's a weekday and, as always, there are no cars on the road, no pedestrians on the pavement.
My grandma's house, and the house next door, are different from the others. The other houses are whitewashed, like most big houses here in Hamburg. Hers is plain red-brick and so's the one next door. With their wide empty windows and blank wooden doors framed by superfluous Doric columns, they look more like mausoleums than private houses. Like an abandoned film set, they look realistic yet unreal.
The house next door to my grandma's used to belong to a German Jewish doctor and his family. He treated my aunt and uncle for minor ailments when they were small. But then one day an ambulance arrived and took him away. After he'd gone, the word went round that he'd been taken to a concentration camp. Nobody knew why. Then one day he reappeared, looking a lot older than before. He didn't say anything. He simply packed up and moved away. Nobody knew where he'd gone, and nobody saw him again. No one spoke about him either. People knew it was best not to talk about such things. And so, like her neighbours, my grandma did nothing. Well, what would I have done? Nothing, I suppose. Maybe if she'd been childless, she might have done more to help, but her husband was away, and her children were her first priority. Parenthood makes fellow-travellers of us all.
The next inhabitant of the house was the local police chief. God knows what he'd done to land such a plum job. Naturally, he was a dedicated Nazi. He flew a swastika flag from the pole in his garden. Every morning his children would raise this banner and salute it. Then one day they came and knocked on my grandma's door. "Excuse me," they asked my grandma. "Please can Marion and Michael come out to play?"
Such a friendship was fraught with danger, not because my grandma was a committed anti-Nazi - she wasn't - but because children are born informers. Like most of her peers, my grandma was politically apathetic. Her remarks about Hitler were merely patronising, rather than downright rude. Yet everyone had heard of someone who'd been sent to a concentration camp for less, sometimes condemned by their own children. What's more, my grandma's maid was now courting a local Brownshirt, so her private life was now public. Everything she said was on the record, so she said nothing at all.
On 28 July 1943, the RAF launched a huge air raid on Hamburg. Code-named Operation Gomorrah, it killed 40,000 people in one night, more than the entire Blitz in Britain. My grandma was lucky. A bomb landed in the back garden and blew out the windows, but that was all. Still, it was too close for comfort, so she went to stay with her sister in Dresden. Dresden was beyond the range of British bombers. She reckoned she'd be safer there.
Grandma's luck held. She survived the bombing of Dresden, too. When she returned to Hamburg after the war, her husband was a POW, and her house - and the house next door - had been occupied by British soldiers. She fell in love with one of them and followed him back to Britain. That was the end of her life in Hamburg. She never saw my grandfather again.
Today, my grandma's house has been divided into flats. There's an underground garage that wasn't here last time I came, and an avant-garde sculpture in the front garden, like an enormous crushed tin can. I walk down the side alley, to try to find the bomb crater in the back garden, but halfway there my courage fails and I hurry back again. I think of ringing one of the doorbells but then I decide against it. What would I say? What I really want to know is what happened here before the war, but naturally that's the last thing most Germans want to talk about. They're like old lags who've done their time; you can hardly blame them. The house next door now adjoins the Danish consulate. "Private", reads a forbidding sign in smart brass letters on the door.
At the end of the road is the church where my grandmother and grandfather were married. It's grandiose neo-Gothic, like St Pancras Station in miniature. Outside is a dark metal plaque, and though my German is pretty lousy, I can just about make out what it says. "In 1933, nearly 20,000 Jews lived in Hamburg. In 1945, there were 945. When they were hunted down, deprived of their rights, and then forgotten, when their synagogues were destroyed, we did nothing. We beg their forgiveness, and shalom."