Universe Today
In 1969, Roger Penrose (yes, that Penrose, the one who would later win a Nobel Prize) worked out a way to steal energy from a black hole. It's easy! Fly a spacecraft close to a spinning black hole, drop a payload at just the right angle, and you come back with more energy than you started with. The black hole gets slightly smaller. Nothing escapes the event horizon, no laws are broken, and yet the universe just handed you free energy.
Ah yes, the land of black holes, where nothing makes sense and everything we thought we knew turns out to be not quite right. But that's okay. We're going to take our time with this one. We're going to unpack how the process works, why it doesn't break any laws of physics, and hopefully, by the end, the universe will make a little more sense.
And to get started we need to introduce a rather beefy physics concept. It's called frame dragging. It's real, it's fun, and most importantly, it's what we're here to talk about.
To get to frame dragging, we first need to change how we think about spacetime. We need to...level up.
The level 1 way most of us imagine space is basically Newton's version: an empty stage. Objects sit in it, move through it, and exert forces on each other across it, but the space itself is a passive backdrop, a giant three-dimensional room that doesn't care what's inside. In this picture space has no properties of its own. It doesn't stretch, it doesn't push back, it doesn't do anything. It's just where stuff happens. It exists, and that's enough for us to get physics done.
Of course, this level-1 picture was blown up by Einstein's general relativity, which is our level 2 way of imagining space. Spacetime is not a passive stage. It's a physical thing with a shape. Mass and energy bend that shape, and the bent shape is what we experience as gravity. Picture one seamless four-dimensional thing that can be stretched, curved, twisted, and, spoiler alert, dragged (not much of a spoiler, since I already told you this series is about frame dragging). Spacetime is an object in its own right, imbued with physical existence just like particles and forces and fields are. It's dynamic. It's alive.
A Horseshoe Einstein Ring, imaged by Hubble. The gravity of a foreground galaxy bends the spacetime around it, warping the light of a more distant galaxy into a near-perfect ring. Credit: NASA/ESA Hubble.
But that's still only level-2 thinking. It's good enough for most work in general relativity, but not for where we're going today. So let's move to level 3.
Because spacetime has shape and behavior, we can treat it as a kind of fluid. Now, this is going to be mostly analogy, but also not 100 percent. General relativity gives us a lot of freedom in how we describe spacetime and its interactions with matter. For example, you can imagine the space around a black hole as fixed, with things falling into it: eventually the gravity gets so strong, the walls of the well so steep, that nothing can escape. But you can also imagine that same space as flowing toward the black hole, like water into a sinkhole, and as you approach the event horizon space flows faster and faster, until you try to leave and find you have to push against a current moving faster than light. Two completely different pictures, but a single unified mathematical structure underneath. This happens all the time in physics, when we have equivalent descriptions of the same phenomenon. It's just especially fun with general relativity.
Now, when I say let's treat spacetime like a fluid, I don't mean a real fluid made of molecules. I mean a substance with its own local geometry that responds to what mass and energy are doing inside it. When something moves through spacetime, it interacts with the geometry, like motion through water: you push the water, the water pushes back on you. It's a two-way dialogue. And when something spins, it doesn't just churn the air or the water around it. It churns the geometry of spacetime itself.
And churning fluids have a very curious property.
Think of stirring honey with a spoon. The honey right next to the spoon moves fastest. A little farther out, it rotates more slowly. Farther still, it barely moves at all. Just by rotating, the spoon has set up a slow, decaying swirl in the fluid around it.
General relativity says a spinning mass does exactly the same thing to spacetime. Spacetime itself picks up a slow rotation, strongest near the object, fading with distance. This is frame dragging: mass drags spacetime, and a spinning mass drags it in a rotational pattern.
In this level-3 view, spacetime is a fluid participant, not a stage. It has local flow. It responds to what mass is doing, and then it goes on to do its own thing. The part we care about is that spinning things generate swirls. There is genuinely a sense in which spacetime gets carried along by what's inside it.
Of course, this is only a model to help guide us through the math, so let's not get carried away. Spacetime is not made of anything the way the ocean is made of something. It isn't molecules with pressure and viscosity. It has no temperature, it doesn't slosh, and if you set up a swirl there's no friction to slow it down over time. The swirl persists as long as the spinning source does. It's a geometric effect, not a mechanical one.
But it is a real effect. Spacetime is real, it's a thing, even though it's only made of itself, and that thing responds to the motion of what's inside it, and the response persists. Which means that even with all those caveats, our level-3 thinking is going to help us understand how we can pull energy out of black holes.
In Part 2, we track frame dragging down to an absurdly tiny number, and follow the decades-long effort to measure it right here at Earth.
