Stephen Hawking's "How to build a time machine" (non-fiction, free)

In Mail Online. [via Boing Boing]

He discusses 3 things in currently known physics that could be used to time travel to future (but not to past):

1. "wormholes" - tiny theoretical constructs at quantum level that are thought to link two desperate places in time & space. They're unstable, & involve paradoxes.
2. Black holes. Really big ones are far away, & they don't move us too far in time really fast.
3. The most practical method: If you can somehow travel very very fast, so time slows down for you.

Quotes.

"sadly, it looks like time travel to the past is never going to happen."

"This doesn't make all time travel impossible. I do believe in time travel. Time travel to the future." "time is like a river in another way. It flows at different speeds in different places and that is the key to travelling into the future."

"time runs faster in space than it does down on Earth. Inside each spacecraft [of GPS system] is a very precise clock. But despite being so accurate, they all gain around a third of a billionth of a second every day."

"The problem doesn't lie with the clocks. They run fast because time itself runs faster in space than it does down below. And the reason for this extraordinary e ffect is the mass of the Earth. Einstein realised that matter drags on time and slows it down like the slow part of a river. The heavier the object, the more it drags on time. And this startling reality is what opens the door to the possibility of time travel to the future."

"A black hole like this one [at Galactic center] has a dramatic effect on time, slowing it down far more than anything else in the galaxy. That makes it a natural time machine."

"I like to imagine how a spaceship might be able to take advantage of this phenomenon, by orbiting it. If a space agency were controlling the mission from Earth they'd observe that each full orbit took 16 minutes. But for the brave people on board, ... time would be slowed down by half. For every 16-minute orbit, they'd only experience eight minutes of time."

"Around and around they'd go, experiencing just half the time of everyone far away from the black hole... Imagine they circled the black hole for five of their years. Ten years would pass elsewhere."

Related: Time travel fiction.