Not much in science is more of a mind-bender than thinking about the size and fate of the known universe (except for quantum mechanics and string theory, which also has a lot to do with the size and fate of the universe, albeit on the opposite end of the size spectrum). When we first developed theories about the universe, the model which resulted depicted all of space as static and unchanging, infinite in depth in any direction. Then Einstein posited general relativity and suddenly a whole host of universes were theoretically possible: static, dynamic, infinite, and finite.

1. Einstein showed that light travels in bundles called quanta, which are at
the heart of the light-emitting diode. When electrons in a semiconductor-based diode move from one side to another, they shift to a less excited state, releasing energy in the form of photons. Channel these, and you get a bright, long-lasting light source.

2. In 1917 Einstein demonstrated that when a photon comes into contact with an atom, it can trigger a chain-reaction release of additional photons from

If we wish to travel through time, we must be able to control it, to mold it to our own desires and specifications. This is possible only through Einstein's theory of relativity. Instead of imagining space as an immutable structure, part of Einstein's genius was realizing that space and time are interlinked in a single, pliable framework called space-time. Both space and time can be distorted, sometimes dramatically. Time travel requires nothing more than exploiting those distortions to create a path through time that ends earlier-or much later-than it begins.

When H.G. Wells sent the hero of The Time Machine into what Wells called "futurity," it was on a grim 30-million-year round-trip to pretty much the end of Earth time, when the last, poorest excuses for life were flopping around like squid under a darkening sun. Wells wasn't the first writer to imagine time travel, but he advanced the idea that a machine, rather than an angel or a bonk on the head, could accomplish it, and he pushed his machine to the limit.