Event Horizons as the Boundary of the Universe

Approaching the Event Horizon

    What happens to an object as it approaches the event horizon of a black hole? Even light cannot escape from the event horizon of a black hole, so we can conclude that gravity is pulling in objects at or faster than the speed of light. So an object falling into a black hole would move at a faster and faster speed approaching the speed of light. Remember that as an object approaches the speed of light, the fabric of space-time becomes distorted in a way that the properties of space-time is changed for that object. This is because its effective mass becomes infinite as the object approaches infinite speed. In the previous section, we talked about how time dilation and length contraction affects observers in different inertial frames, but how do these phenomena of special relativity apply to objects affected by the powerful gravitation force near a black hole?

    Imagine we are observing an object falling into a black hole. As the object falls into the black hole, tidal forces will rip the object apart so that it stretches out like a piece of spaghetti in a process aptly named spaghettification. This is a purely physical process and not the kind of change we are interested in. We want to know the relativistic changes that are happening to the object. Let's take a closer look at how special relativity affects objects very close to the event horizon of a black hole.

Perspective of an Outside Observer

    An object approaching the event horizon would appear to be slowing down as it approaches the black hole instead of speeding up as we would expect. This is because of time dilation, which slows the passing of time for an object moving close to the speed of light with respect to an outside observer. The object eventually would slow down until it stops moving when it reaches the event horizon because that is when it will be moving at the speed of light. An outside observer will never see the object passing the event horizon, firstly because light cannot escape, but even if it could, the time dilation of the object would become infinite meaning it would take an infinite amount of time for it to cross the event horizon.

    Second, the object would appear to be contracting or becoming flattened in the direction of its motion. This flattening only occurs in one direction so that when the object reaches the event horizon, it essentially becomes a two dimensional object from the perspective of an outside observer.

Perspective of an Observer Falling into a Black Hole

    It becomes even more interesting when we take the perspective of the object falling into a black hole. As you fall into a black hole, you would not notice any of the time dilation or length contraction that an outside observer would see in you. For you, time would pass at the same rate as it always has been, so that you would fall towards the black hole, pass the event horizon, and hit the singularity in almost no time at all. But when you observe the rest of the universe outside the black hole, time would speed up faster and faster as you approach the event horizon. Then the universe would go infinitely into the future when you reach the event horizon.

    You would also not experience any length contraction on yourself. You would still have the same 3 dimensional shape you always did (except for any spaghettified parts). But as you get closer to the event horizon, everything in the universe will become contracted until it becomes a tiny point above you. Essentially, the infinite universe turns into a singularity when you hit the event horizon.

Event Horizon as the Boundary of the Universe

    Think about what we have talked about so far. When you approach the event horizon of a black hole, you are actually approaching the limit of space-time. What this means is that when you reach the event horizon of a black hole, you are also reaching the end of space-time. We have this notion of a boundary as something that is outside and beyond, but the universe is something that is infinite in both space and time! In order to find the boundary of something that is infinite, you must approach the infinite itself.