In astrophysics the process of spaghettification is the stretching of objects into long thin shapes rather like spaghetti as they approach the singularity of a black hole. It is caused by extreme tidal forces. This stretching is so extreme in fact that no body can withstand it no matter how tough a substance it is made of. 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.

To understand this process consider the rather more tame example shown in the diagram on the right. Four objects are allowed to fall towards a large mass such as a planet or a star. Each object accelerates "straight down". But straight down means 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. 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). Of course a rigid object will not distort because internal forces will act against the tidal forces. It's only when the tidal forces become large enough to overcome these internal forces that a rigid body will change its shape.

Tidal forces for a planet or ordinary star are comparatively weak. This is because they fall off approximately with the cube of the distance. So if you move twice as far away, the tidal force will be one eighth as strong. This is much more rapidly than the gravitation force itself falls off, which goes as the square of the 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 things fall into a black hole the tidal forces become stronger and stronger until nothing can resist them. The bodies are stretched into thin streams of matter. Eventually, close to the singularity, they become large enough to tear molecules apart (in theory at least, no one has actually tried it). For this reason, it would be impossible for a human to enter 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 the 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.