Spaghettification

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The tidal forces acting on a spherical body in a larger body's gravitational field. The larger body is to the bottom of the sphere.
Click on this image for a fuller explanation.

In astrophysics, the process of spaghettification is the stretching of objects into long thin shapes (rather like spaghetti) as they approach the gravitational singularity of a black hole. It is caused by extreme tidal forces. This stretching is so extreme that no object can withstand it, no matter how strong its components are. The word spaghettification comes from an example given by Stephen Hawking in his book A Brief History of Time, wherein he describes the plight of a fictional astronaut who, passing within a black hole's event horizon, is "stretched like spaghetti" by the gravitational gradient from head to toe.

Spaghettification is caused by the differences in gravitational forces acting on the four objects. Each object follows a slightly different path.
See also an animated version.

To understand this process, consider the rather more tame example shown in the diagram on the left. Four objects are allowed to fall towards a large mass, such as a planet or a star. Each object accelerates "straight down", towards the centre of the planet. Therefore, widely spaced objects will follow trajectories that tend to converge. This causes the left hand and right hand bodies to squash together. On the other hand, the top and bottom bodies fall at different rates, because the force of gravity falls off with distance. The body nearer the planet is pulled harder than the one that is further away, and so the top and bottom objects are in effect pulled apart. The net result of these two sets of effects is to distort the diamond shape into a longer and thinner form. (See the animation.) A rigid object will resist distortion, because internal forces will act against the tidal forces. However, whenever the tidal forces become large enough to overcome these internal forces, a rigid body will be forced to change its shape.

Tidal forces for a planet or ordinary star are relatively weak. This is because they fall off with approximately the cube of the distance. If an object moves twice as far away, the tidal force will be one eighth as strong. For comparison, the gravitational force falls off as the square of the distance; therefore the force is reduced to one-quarter strength at double distance. Objects falling towards an ordinary massive object will hit the surface before the tidal forces get strong enough to overcome the internal forces holding a body together.

In the case of a black hole, there is no surface to halt the fall. As objects fall into a black hole, the tidal forces become stronger and stronger until nothing can resist them. The infalling bodies are stretched into thin streams of matter. Eventually, close to the singularity, the forces become large enough to tear molecules apart. For this reason, it would be impossible for a human to survive entering a singularity. The point at which these tidal forces become fatal depends on the size of the black hole. For a very large black hole, such as those found at the center of galaxies, this point will lie well inside the event horizon, so an astronaut may cross the event horizon without noticing any squashing and pulling whatsoever (although it's only a matter of time, because once inside an event horizon, there is no getting out again). For small black holes whose Schwarzschild radius is much closer to the singularity, the tidal effects may become fatal long before the astronaut even reaches the event horizon.

[edit] References

1. ^  Hawking, Stephen (1988). A Brief History of Time. Bantam Books. ISBN 0-553-38016-8.