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Comment by ben_w | Hacker News Reader

Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in.

A black hole happens when there is enough gravity that space gets pulled inwards somewhere, at at least the speed of light.

Gravity falls off with distance, and the distance where space is being pulled inwards at exactly the speed of light is called the "event horizon".

It has this name because speed of light is the speed of causality: events that happen further in, are "over the horizon" for you, they cannot causally influence you.

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> Normally people think of gravity as pulling on objects. You can instead think of it as pulling on the space those objects are in.

(Very uneducated person here) I’ve always wondered if large objects caused gravity, or if maybe large objects form in the places where there is a lot of gravity. This is probably elementary, but I’ve never looked in to it. Maybe today is the day!

If you want to take a very large-scale - if poetic - view of things, you could also (very arguably) say something like "Mass is Fate".

Mass represents a zone where probabilities want to be. The more that aggregate, the more they make other things want to glom on. With a high enough density, nothing that's nearby can glom to literally anywhere else, and there's your black hole. The Great Inevitable. In this space, there are no other possibilities. Very Demiurge-y.

That’s amazing

Both / they're related. More mass gravitates more.

Suppose you had an infinite universe that was filled with a cool gas of low uniform density. Then the gravitational field at any particular point would be 0, by Gauss' law.

But, if you wait a brief moment the gas will not stay uniform, because each atom of the gas will have some velocity. You'll observe fluctuations: places with small over-density and small under-density (compared to the average). The places with over-density will gravitate more than the average and places with under-density will gravitate less, and gravity will cause the gas to clump.

Wait a few billion years and some places will have amalgamated whole galaxies' worth of matter around them and other places will be empty.

Dark matter is mysterious enough to be compatible with both at the same time, I think.

(Is a collisionless gas really even an "object"?)

I tend to believe that dark matter is a name for something we don't understand better yet. I don't necessarily mean MOND or anything specifically, but dark matter is just a little too mysterious and convenient for my taste. Maybe in 100y we will laugh about the notion of something like an aether named dark matter, and see it as simply a model which was necessary, because we didn't get what really is going on.

I mean, it's kind of both: objects have gravity, which attract them together, which creates more gravity and larger objects. Stars are what happens when the rest of physics disagrees with this (and black holes are when gravity wins anyway).

Scientists have directly measured the gravity coming from big balls of lead. The gravity didn't make the scientist put the lead there, and the gravity wasn't there before the lead. The lead must have made the gravity.

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