Allan Adams “became a physicist to understand how the world works at its most fundamental level.” Adams, a professor of theoretical physics at MIT, points out that the things we’ve learned so far are pretty incredible: We know the universe began just 13.8 billion years ago, and have a pretty good idea how it’s going to end. We have a recipe for all the particles in the universe.

But to Adams, the most spectacular idea in modern physics is something called holography. He explains, “It reinterprets the structure of space and time, challenges our very notions of what it means to be fundamental.”

The story starts with black holes. By definition, black holes are things from which nothing can escape — and they aren’t just figments of our imagination, they really exist. Now, if you look straight at a black hole, what would you see? You’d think there would be nothing — the whole point is that nothing gets out. But it turns out to be more complicated.

To help think about it, imagine a fish swimming up a waterfall. As the fish gets closer to the waterfall it has to swim faster and faster. Eventually it hits a point of no return, where it can’t swim fast enough to escape. This is the horizon. No fish that falls through the horizon will make it back.

But what happens beyond? If you were a fish upstream, you would never know, since no fish that goes through ever returns. “Does the waterfall continue forever? Does the fish get torn apart? Is there another pond on the other side?”

This is a perfect metaphor for black holes, but what’s flowing into the black hole is space itself. As you fall into a black hole, eventually you reach a point where even the speed of light isn’t enough to escape. But, asks Adams, “What happens behind the horizon of the black hole?” It turns out you can’t just calculate the answer, the math breaks down — “You get things like: The probability that I’ll get pulled into the black hole and turned into a giraffe is 300%. That’s nonsense.”

But maybe we can see what’s happening as something falls in? So strap a flashlight to the fish and throw it in. As it gets closer, the light has to work harder, so they take longer and longer and are weaker and weaker. Eventually the light is just stopped. We never see anything fall into a black hole, it’s just smeared on the surface.

The amazing part, though, is that according to the fish nothing interesting happens. It just keeps going and sailing through the horizon, and the flashlight keeps going. So there are two descriptions, a fish flying through three dimensions, or a fish smeared on the surface of a black hole. “That’s kind of problematic,” says Adams, “Who are you going to believe, me or the fish?” The utterly remarkable answer is that both are right. The two descriptions turn out to be mathematically identical. Completely the same. But it turns out to be true. A description of a universe in three dimensions with gravity is identical to a universe in two dimensions without gravity. “That might sound strange, but to a trained theoretical physicist who hears it for the first time, it sounds like the ravings of a complete lunatic.”

It turns out to get even stranger yet. The surface of the black hole turns out to behave like a quantum liquid, and that behavior shows up in all sorts of other places: in the fireball after the Big Bang, in cold gases of atoms, in superconductors. All these phenomena are related.

But the thing that really, really thrills Adams the most is what it does to our understanding of what the world is and what it does. We have an intuitive sense that the components, particles interacting with forces moving through three dimensions, are fundamental. But it turns out that maybe they’re not fundamental at all. There is no single answer to the basic question “How many dimensions are there?” It turns out it depends on how you ask the question — not randomly or any which way, but there isn’t just one answer, and that’s a profound change to how we view the world.

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## commented on Mar 20 2014

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