I know now how a time machine will be invented.
The first element is technology like Microsoft's Photosynth, an application that takes multiple digital photographs, then analyzes and combines them to create a three dimensional model of a scene that you can immerse yourself and move around in. With enough photographs you should theoretically be able to create a complete encapsulation of an entire scene, where you can seamlessly view anything you want from any angle.
A good example of this is the inauguration of Barack Obama last January, where CNN asked viewers at the scene to contribute photographs of the moment Obama was sworn in as president. The hundreds of photos were fed into Photosynth to create a complete snapshot of the inauguration, allowing immersion into the scene so you can view the event from thousands of angles.
Now, combine this technology with another element: Google Street View. As you may or may not know, Google's been going crazy putting cameras on top of cars, driving through the roads of the world's major cities, taking 360-degree photographs of our whole world, and two years later I'm still frickin' astounded by it-- I can view anything I want in the streets of Atlanta, New York, Sydney, Paris, Tokyo, London, and hundreds of other cities on our Earth.
Another element is the recent introduction of historical satellite data on Google Earth. Not only is it possible to view satellite data of all the world's major cities, but you can now move through time to see what the looked like in the past (depending on the availability of data, of course). With a few clicks of a mouse you can view the transition of a barren plot of land to a construction site to the glittering Mall of Asia. As time goes on, historical data will only become more available and frequent.
Expand that idea to Street View. It's only a matter of time before it will become necessary for Google to update the snapshots of street views, leaving them with historical data that will inevitably be available to allow us to see cities change and develop over time.
Why not take this concept further and predict that a time will come that public photography and data storage will become so inexpensive that we will have video cameras constantly running with overlapping coverage available in city streets. Is the idea so far fetched? London already has hundreds of thousands of video cameras watching their streets as a potential deterrent to crime. We may eventually reach the point that people realize that making the video coverage available is no more invasive to privacy than what we currently have with street view.
And if widespread public video camera footage can be combined with the image analysis technology of Photosynth, we can create an immersive three dimensional world that encapsulates not only a moment in time but a span of time in history, allowing us to practically travel through time to the moment in history of our choosing.
Whenever I'm showing off Google Street View to people, I like to amaze them my checking the views of houses I've been to in the US, and then I zoom in on the door and say "Ok, so now let's take a look inside the house..."
For a second they gasp and actually believe that such a thing is possible.
We'll get there...