People often ask me: “What is spacetime? The easy answer is that it’s the "fabric of space.” That means little; space isn’t a fabric, and doesn’t act like one. But it does bend, and ripple, and stretch, and vibrate.
What is spacetime, really? It’s a coordinate system, a kind of conceptual grid. It’s a way of accounting for the dimensions of the Universe, where in addition to up and down, left and right, forward and backward, we also have time—the future and the past. The most revolutionary idea Einstein proposed was that physics only makes sense when space and time are inextricably linked together, where the way we travel through one changes the way we travel through the other. And gravity, a property of anything with mass or energy, changes both.
The most extreme example of this is a black hole. Approach a black hole and, aside from ultimately being torn limb from limb by the steeply climbing gravitational field, you won’t notice much changing. But relative to someone keeping a safe distance, your wristwatch, heart rate, thought processes, and every other aspect of your existence, will have slowed to a standstill.
By the time you cross the event horizon, marking the point of no return, you will appear to the rest of the Universe as a slowly fading, darkening image of yourself. And if you look out at the Universe you’ve left behind, you’ll see it whirling away in fast-forward, charging on into the future without you. The rest of your journey, into the singularity at the center of the abyss, will be yours alone to see.
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