I wanted to expand on Mark's comments about Masi Oka's character on Heroes. Not so much about the performance or the plot, but the physics of it. (Spoilers ahead).

When Hiro gets sent back to the present just as he's about to kiss Charlie, it threw me for a loop. It seemed like such a cheap plot device: don't let Hiro fix everything just by going back in time. It bothered me. I kept pondering it.

Hiro describes his power as the ability to "bend time and space". That got me thinking about The Elegant Universe, a PBS series about string theory (watchable online). Einstein's theory of general relativity describes gravity as essentially being a bending/warping of space and time.

Einstein spent his last years trying to find a way to combine two of the fundamental forces of the universe, gravity and electromagnetism, under one unified theory. He did not succeed.

Gravity and electromagnetism may have some common underlying structure, but scientists are not sure how. For one thing, electromagnetism is way, way, way stronger than gravity. That may surprise some people, but think of it this way: if gravity had the same strength as magnetism, you'd need a magnet at least the size of the earth to lift a paper clip off the ground. Instead, all you need is a magnet the size of a dime.

So here's where Hiro's inability to control his power surprises me. To Hiro, time and space behave not like gravity, but like a magnet. It seems polarized somehow. You can take two magnets, use one to move the other, to push it one direction, or pull it in another. But if you want to take the north side of one magnet, and push it against the north side of another magnet, the magnets will push back. The more you try to push two poles of the same kind together, the stronger it repels you in the other direction.

So when Hiro tries to kiss Charlie, he is trying to change some thing that doesn't want to be changed, like two north poles being pushed together. The more he tries to push those things together, the stronger he gets repelled back to where he came from.

Here's the physics question that arises from this: if gravity and electromagnetism can be unified, why doesn't gravity have the same behavior as electromagnetism, namely, polarization. Magnets have north and south poles; atomic particles have positive and negative charges. Yet gravity does not seem to have any sort of polarity. There's no opposite of gravity: in our universe, gravity only seems to pull, never push. Why not?

Perhaps gravity is polarized, but we've never noticed. If Einstein could have figured out a way to combine gravity with magnetism, he would have died a happy man. Hiro accidentally discovers just such a way, but to him, it's a bitter disappointment. Truth and knowledge are not always happy concepts.