The iPad is probably the right answer, assuming you had time to prepare. (Be sure to bring a charger!) The tablet would be shockingly small, but it has a nice, big screen--big enough for a bunch of dumbstruck 50s denizens to crowd around. It also has the capacity to hold a staggering amount of content. No, you couldn't surf the Web in 1956, but plenty of apps work just fine without a network connection, and of course you could pre-load it with music, movies, TV shows, and books from the future. That one iPad could change the entire timeline!
What if you didn't have time to prepare? What if you were to blink and be sent back in time by a Weeping Angel? Well, then you'd be left with the iPhone in your pocket. Still an impressive product, but I'd recommend immediately putting it in Airplane Mode and start figuring out how to charge the thing. And if you're using iTunes Match, you'd miss the chance to play the Beastie Boys to a young and impressionable Paul McCartney.
Speaking of angels, we're probably deep in head-of-a-pin territory here. But consider in turn what we might make of the technology of 2070.
Arthur C. Clarke famously said that any sufficiently advanced technology is indistinguishable from magic. It'd be great if every technology company aspired to creating products that would seem magical even to contemporary consumers. (Is it any surprise that Apple uses that very word to describe its own products?)
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