Among the more difficult problems that physics faces as it attempts to comprehend the universe is the issue of determining the speed of gravity. The most influential force shaping the universe, it is a heavy hitter in the largest scale, but it proves to be exceptionally elusive when dealing with sizes and distances available to the average physicist. And because it is extraordinarily weak in comparison to other forces, such as magnetism or electrical charge, it is impossible to isolate in the laboratory. As a result of this, all that we can learn about gravity must be learned through observation of astronomical phenomena.
Because of our limited ability to observe gravity in action, there are limits to our ability to observe its traits. For instance, there is some question as to just how quickly gravity travels and several experiments have been performed to attempt to determine the speed of gravity. However, because the positions of objects in space to not change very quickly relative to each other, determining the speed of gravity by observing its effects on objects in space requires very fine observations of objects over very long distances. Yet the speed of gravity, or the lack thereof, would have a profound effect upon our own understanding of the structure of galaxies and the universe.
The reason why the speed of gravity is important has first to do with the theory of general relativity. Among the many results of this theory, general relativity requires that nothing can move faster than light. The limit to how quickly information can be transferred from one place to another is the speed at which light can travel between those two places. Information can move slower than light, but no faster. So, according to general relativity, gravity can only move as fast as the speed of light, because the gravity coming from a mass transmits information about that object’s mass.
The problem with this idea is the already problematic set of objects known as black holes. This is because every black hole has an event horizon, which marks the boundary past which nothing can escape, not even light. Once anything, including light, crosses the event horizon that marks the boundary of the black hole, it stays inside the black hole forever.
The reason for this is the collapsed singularity at the center of the black hole. This object of something close to or equal to infinite density emits a gravitational field so strong that it can even pull in light. This means that no information can cross the event horizon because anything carrying information would need to be traveling faster than light in order to cross the event horizon. Thus, because nothing can travel faster than light, nothing can escape the black hole. But this begs the question of how gravity, if it travels at the speed of light, can escape from the black hole.
A black hole is created by a superdense singularity of infinite or nearly infinite density at its center. But because the singularity is inside the black hole, it should not be able to transmit anything, including information about its mass, outside the black hole that it forms. The only way for the black hole to emit mass information through gravity is for gravity to travel faster than light. And it cannot even be the little bit faster than light required to cross the event horizon, because the singularity is at the center of the black hole and the center of the black hole has even stronger gravity than the event horizon. Thus, for mass information to escape from a black hole, it would need to be moving much, much faster than light. Yet that, according to general relativity, is entirely impossible.
According to this description, let us take the simplest description of a black hole and see what would result. The simplest description, at least from my point of view, is a zero-dimensional singularity of infinite density. This is, of course, a star that had sufficient mass that, when its nuclear fuel ran out, the gravitational force that was both created by and acting upon the mass of the star caused the particles that composed it to be so attracted to each other that they could no longer occupy their own space in a way described by the Pauli Exclusion Principle. The minimum mass necessary for such a thing to happen is known as the Chandrasekhar Limit.
Let us now say that a star has just used up its fuel, exploded out into a supernova and the remaining mass has collapsed into a zero-dimensional singularity. This singularity should have a gravitational field. However, because the gravitational field is so strong, nothing moving slower than the speed of light can escape it. Yet gravity moves at the speed of light, so it cannot escape itself and the gravitational field collapses into the singularity too, creating a body of infinite density, but effectively no mass.
How can this be? The gravitational field of the body has collapsed, so it cannot emit information about itself to the outside universe. Nothing can be pulled into it or orbit around it, because there it cannot emit gravity from itself. Yet it cannot make its presence known through a collision with another body because it is zero-dimensional and there is nothing there to collide with anything. This black hole could literally pass through an electron without affecting it because there is nothing that can collide with the electron.
The black hole has no energy and no mass outside itself because that information collapses into the black hole along with everything else. Even if it were to be able to spin in order to create a magnetic field, we would need to accept the idea that magnetism moves faster than light in order for the magnetic field to be emitted from the black hole; and therefore, we would once again be in violation of the general theory of relativity in order to allow black holes to have magnetic fields outside themselves.
As a result of this, it seems that physicists need to determine which they would rather keep: black holes that can affect the universe around them or the speed limit set by light. Or so it would seem. In fact, there are other possibilities that are created by a gravity that travel at the speed of light.
A problem arises when one considers that gravity should collapse along with everything else that is collapsing into a black hole. The gravity is, essentially, pulling itself back into the center just as it is pulling everything else back in too. Many things can happen in this situation and we will consider each in turn.
The first case is, of course, the collapse toward a singularity. A zero dimensional nothing of infinite density that effectively does not exist because it cannot be observed directly or indirectly.
However, there is another possibility for the core of the black hole. This is what I shall refer to as a pseudo-singularity. This is an incredibly dense mass of matter that is not quite zero-dimensional, but rather manages to still have some shape and size. It is not quite a singularity, but it is, for all practical purposes, close enough in almost all but the most narrow cases.
This possibility could take a couple of different forms. First, there could be an exclusion principle more fundamental than the Pauli Exclusion Principle. Second, a singularity could be possible, but the particles are rotating about a center so close to the speed of light that they cannot fall toward the center -- because they would need to move faster than the speed of light to do so. However, the particles rotating around a center at nearly the speed of light would essentially act the same as they would if they were occupying the same space, so we can assume for now that the Pauli Exclusion Principle has broken down in a black hole.
In the case of the pseudo-singularity, the gravitational field of the black hole will still collapse, but the extent of its collapse has important repercussions for black holes.
We have already considered the case of a zero-dimensional singularity in our discussion. The second case we shall consider is the gravitational field of a pseudo-singularity collapsing to the edge of the pseudo-singularity. If the gravitational field collapses to the very edge of the pseudo-singularity, then the black hole will remain a nearly zero-dimensional object that will provide no gravity to the universe, but will be observable when it collides with a particle. What those effects might be would be hard to guess at offhand, but that effect is not currently our concern. It would, however, be an entertaining topic of debate for physicists during their lunch hours.
For our third case, let us consider what would happen if the gravitational field continues collapsing after the field has moved past the edges of the pseudo-singularity. The effects of this instance would be quite spectacular.
The only thing that was holding the pseudo-singularity together was the gravitational field emitted by the pseudo-singularity itself. However, once the gravitational field leaves the scene, the Pauli Exclusion Principle kicks into overdrive and every newly freed particle will get away from the other particles as fast as possible. Particles would then explode out at very near the speed of light as each goes in a separate direction and a massive amount of very high energy X-rays would be released in one spectacular burst. It is not hard to imagine this being the form of a supernova.
Interestingly, this case provides assistance to those who doubt the existence of black holes. If a star has a mass above the Chandrasekhar Limit, the gravitational field of the star would collapse in on itself even as the matter did the same. So any violation of the Pauli Exclusion Principle would be quickly solved by the removal of the cause of the violation. Even as the particles collapse together because of gravity, the gravity leaves and the particles separate with the most violent force in the current universe.
There is a fourth and final possibility that allows for both black holes that can exert gravity on the universe around them and a gravity that moves at the speed of light. This possibility is the oscillating black hole.
Consider a black hole with a pseudo-singularity at the center. Consider also that gravity should travel at the speed of light. Then imagine that the gravitational field should pull itself in past the edges of the pseudo-singularity as described in the previous case. And the same massive eruption of matter and high energy would occur, as before. However, as the gravitational field collapses, it also releases the restriction on the escape of mass information in the form of gravity. So, once the edges of the pseudo-singularity release their mass and energy tidal wave, the race is on between the mass-energy wave and the gravitational field.
Behind the mass-energy wave, a massive gravitational wave will also be released, and it will start chasing the released particles and energy toward the area where the final event horizon will be. Because the energy is traveling at the same speed as the gravitational wave -- the speed of light -- any energy that is released before the release of the full gravitational field will escape. However, because mass particles cannot actually reach the speed of light, the gravitational field will catch up to most, if not all, of them and pull them back into the black hole. Then the gravitational field will collapse under its own force and the process will repeat.
If black holes act in the oscillating manner described, a black hole should have a gravitational frequency. That is, it should emit regular gravitational waves that alternate with bursts of high energy X-rays. The peaks of the gravitational waves should peak at the X-ray troughs and the X-ray peaks should arrive at the gravitational wave troughs in a fairly regular pattern.
If black holes do, in fact, oscillate, we can assume that gravity does have a speed. And if oscillations of the gravitational waves are 180 degrees out of phase with the bursts of X-rays emitted by the black hole, then we can assume that gravity travels at the speed of light. If gravitational waves emitted by black holes do not have a relationship to high energy X-ray bursts, we can assume that gravity moves either faster or slower than light.
If the gravitational waves emitted by black holes have no correlation to X-ray bursts, then we must look at how the X-rays are being emitted. If black holes emit bursts of a significant amount of matter and X-rays, then we can assume that gravity moves slower than light, because the gravity would not have the ability to catch up with the matter ejected at nearly the speed of light. If black holes emit no X-ray bursts, or bursts that are weak to the point of almost not being there at all, then we can assume that gravity moves faster than light.
However, if black holes remain at a steady state, then we can gather that gravity travels much, much faster than the speed of light, or we have much yet to learn about gravity and the nature of the universe.