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Doesn't an event horizon imply a gravitational pull so strong not even light can escape? Isn't the effect of gravity the same at the event horizon for all sizes

Doesn't an event horizon imply a gravitational pull so strong not even light can escape? Isn't the effect of gravity the same at the event horizon for all sizes of a black hole? I thought the size only affected the "acceleration" of the gravitational pull based on the distance from the singularity? A supermassive black hole means you will take longer to be spaghettified, but it will still happen and it will happen before you cross the horizon, won't it?

radioactivist 6y ago
A uniform force doesn't do anything harmful to you. You get stretched into spaghetti by the difference in force between the ends of your body. In large black holes that difference can be small even as you pass through the horizon (where the total force is large).
gizmo686 6y ago
In some sense, there is no such thing as a gravitational pull. What there is is a curvature of space-time that causes an apparent force in some coordinate systems [0]. For a sufficiently large black hole, gravity is effectively constant in the vicinity of the event horizon; which means there exists a coordinate system where the space around (a small region of) the event horizon is essentially flat space. From the perspective of an outside observer, you would appear to flatten as you approach the event horizon, but this is a consequence of the coordinate system. You don't feel any effect at that point. The reason light cannot escape from past the event horizon is that that is the point where space-time is so warped that what you consider a perfectly normal time axis is, from the perspective of an outside observer, now pointing in a spatial dimension directly towards the center of the black hole. In contrast, regardless of how small you are, there is some point where you do experience tidal forces and feel yourself getting spaghetti. For a small black hole, this is before you pass the horizon, for a large black hole, it is after. [0] The distinguishing feature between a real force like gravity, and a truly fictitious force like the centrifugal force, is that for the later, you can construct a coordinate system where the force disappears at all locations. For a real force, you can construct a coordinate system where the force goes away at any given point; but there will always be some point with an apparent force.

radioactivist 6y ago

A uniform force doesn't do anything harmful to you. You get stretched into spaghetti by the difference in force between the ends of your body. In large black holes that difference can be small even as you pass through the horizon (where the total force is large).

gizmo686 6y ago

In some sense, there is no such thing as a gravitational pull. What there is is a curvature of space-time that causes an apparent force in some coordinate systems [0]. For a sufficiently large black hole, gravity is effectively constant in the vicinity of the event horizon; which means there exists a coordinate system where the space around (a small region of) the event horizon is essentially flat space. From the perspective of an outside observer, you would appear to flatten as you approach the event horizon, but this is a consequence of the coordinate system. You don't feel any effect at that point. The reason light cannot escape from past the event horizon is that that is the point where space-time is so warped that what you consider a perfectly normal time axis is, from the perspective of an outside observer, now pointing in a spatial dimension directly towards the center of the black hole. In contrast, regardless of how small you are, there is some point where you do experience tidal forces and feel yourself getting spaghetti. For a small black hole, this is before you pass the horizon, for a large black hole, it is after. [0] The distinguishing feature between a real force like gravity, and a truly fictitious force like the centrifugal force, is that for the later, you can construct a coordinate system where the force disappears at all locations. For a real force, you can construct a coordinate system where the force goes away at any given point; but there will always be some point with an apparent force.