Eddie makes great pizzas but Jeremy thinks they stay in the oven just a little too long. As he crunched an extra-crispy wedge-edge he mused, “Gravity aside, I wonder what it’d be like to land on a black hole. I bet it’d be real slippery if it’s as smooth as Mr Moire says.”

Jennie cut in. “Don’t be daft, lad. Everyone’s read about the spaceman sliding through the event horizon unaware until it’s too late. Someone far away sees the bloke’s spacetime getting all distorted but in his local frame of reference everything’s right as rain. Right, Sy?”

“As rain, Jennie, if all you’re concerned about is relativity. But Spaceman Jeremy has lots of other things to be concerned about on his way to the event horizon. Which he couldn’t stand on anyway.”

“Why not, Mr Moire? I mean, I said ‘gravity aside’ so I ought to be able to stand up.”

“Nothing to stand on, Jeremy. It’d be like trying to stand on Earth’s orbit.”

“Pull the other one, Sy. How can they be alike?”

“Both of them are mathematical constructs rather than physical objects. An orbit is an imaginary line that depicts planet or satellite locations. An event horizon is an imaginary figure enclosing a region with such intense spacetime curvature that time points inward. They’re abstract objects, not concrete ones. But let’s get back to Jeremy’s black hole evaporation quest. He’ll have to pass three perils.”

“Ooo, a Quest with Perils — loverly. What are the Perils then?”

“The Roche Radius, the Photon Sphere and the Firewall. Got your armor on, Jeremy?”

“Ready, Mr Moire.”

“Stand up. The Roche effect is all about gravitational discrepancy between two points. The two meter distance between your head and feet isn’t enough for a perceptible difference in downward pull. However, when we deal with astronomical distances the differences can get significant. For instance, ocean water on the day side of Earth is closer to the Sun and experiences a stronger sunward pull than water on the night side.”

“Ah, so that’s why we get tides.”

“Right. Sit, sit, sit. So in 1849 Édouard Roche wondered how close two objects could get until tidal forces pulled one of them apart. He supposed the two objects were both just balls of rocks or fluid held together by gravity. Applying Newton’s Laws and some approximations he got a formula for threshold distance in terms of the big guy’s mass and the little guy’s density. Suppose you’re held together only by gravity and you’re nearing the Sun feet-first. Its mass is 2×10^{30} kg/m³. Even including your space armor, your average density is about 1.5 kg/m³. According to Roche’s formula, if you got closer than 8.6×10^{6} kilometers your feet would break away and fall into the Sun before the rest of you would. Oh, that distance is about 1/7 the radius of Mercury’s orbit so it’s pretty close in.”

“But we’re talking black holes here. What if the Sun collapses to a black hole?”

“Surprisingly, it’s exactly the same distance. The primary’s operative property is its mass, not its diameter. Good thing Jeremy’s really held together by atomic and molecular electromagnetism, which is much stronger than gravity. Which brings us to his second Peril, the dreaded Photon Sphere.”

“Should I shudder, Sy?”

“Go ahead, Jennie. The Sphere is another mathematical object, not something physical you’d collide with, Jeremy. It’s a zero-thickness shell representing where electromagnetic waves can orbit a massive object like a black hole or a neutron star. Waves can penetrate the shell easily in either direction, but if one happens to fly in exactly along a tangent, it’s trapped on the Sphere.”

“That’s photons. Why is it a peril to me?”

“Remember that electromagnetism that holds you together? Photons carry that force. Granted, in a molecule they’re standing waves rather than the free waves we see with. The math is impossible, but here’s the Peril. Suppose one of your particularly important molecules happens to lie tangent to the Sphere while you’re traversing it. Suddenly, the forces holding that molecule together fly away from you at the speed of light. And that disruption inexorably travels along your body as you proceed on your Quest.”

[*both shudder*]

~~ Rich Olcott