The Long Earth by Terry Pratchett, Stephen Baxter Page A

stepwise worlds, West 1 and East 1, the space the cellar occupied was solid ground, so you couldn’t step straight in. You’d either have to dig a parallel hole, or else come in at ground level and fight your way down. Either way, the element of surprise was lost. By the following morning the belowground facilities in all the police stations, even in the Capitol building, were being fitted out as refuges.
    ‘You weren’t the only one to figure that out, Jansson. But you were
among
the first, even nationally. I hear the President herself sleeps in some bunker under the White House now.’
    ‘Glad to hear it, sir.’
    ‘Yeah, yeah. Then it started getting more exotic.’
    ‘Odd to hear you using that term without the word “dancer” attached to it, sir.’
    ‘Don’t push it, Jansson.’ He showed her reports of various religious nuts. Apocalypse-minded types had gone flooding into the ‘new Edens’, believing their sudden ‘appearance’ was a sign of the End of Days. One Christian sect believed Christ must have survived the crucifixion and stepped out of the tomb by the time His disciples came looking for His body – and, it wasn’t much of a leap to conclude, He was probably still out there in the Long Earth somewhere. All this presented public order challenges for the police.
    Clichy pushed away the laptop and massaged the top of his fleshy nose. ‘Who am I, Stephen Hawking? My brain is boiling. Whereas you, on the other hand, Officer Jansson, have taken to all this like a pig to shit.’
    ‘I wouldn’t say that, sir—’
    ‘Just tell it to me the way you understand it. The most basic problem – I mean for Madison’s finest – is that we can’t carry through our pieces intact. Right?’
    ‘Not even a Glock, no, because of the steel parts. No metallic iron can be carried over, sir. Or steel. You can take through whatever you can carry, save for that. I mean, there’s iron ore in the other worlds, and you can dig it up and process it and manufacture iron over
there
, but you can’t carry that stepwise either.’
    ‘So you need to build a forge on every world you settle.’
    ‘Yes, sir.’
    ‘I’ll tell you what puzzles even a doofus like me, Jansson. I thought we all got iron in our blood, or something. How come that doesn’t get left behind?’
    ‘In your blood, the iron’s chemically bound up in organic molecules . Inside your haemoglobin, one molecule at a time. Iron molecules can go over if they are in chemical compounds like that, just not in the form of metal. Why, rust can be carried over, because that’s a compound of iron with water and oxygen. You can’t take your piece over, sir, except for all the rust on the shaft.’
    He eyed her. ‘That isn’t some kind of lewd remark, is it, Jansson?’
    ‘Wouldn’t dream of it, sir.’
    ‘Anybody know why things are this way?’
    ‘No, sir.’ She had been following the discussions, as best she could. Some physicists had pointed out that iron nuclei were the most stable in nature; iron was the end result of the complicated fusion processes that went on at the heart of the sun. Maybe its reluctance to travel between the worlds was something to do with that. Maybe stepping between the worlds was something like quantum tunnelling, a low-probability passage between energy states. As iron had the most stable atomic nucleus, maybe it lacked the energy to escape from its energy well on Datum Earth … Or maybe it was magnetism. Or something. Nobody
knew
.
    Clichy, a practical man, just nodded. ‘At least we know the rules. Kind of a gut-wrench for most Americans to have gun control suddenly imposed on ’em, however. What else? There are no people in these other worlds, right? I mean, except for the ones who leaked over from ours.’
    ‘That’s right, sir. Well, as far as anybody knows. There has been no systematic exploration even of the nearby worlds, not yet. You can’t know what’s hiding over the next ridge. But they have sent up a