Venturing Past the Horizon, A Theoretical Description

Most people probably don’t care about what happens once you cross the event horizon. After all, once you’ve overstepped that thin line, there’s nothing in the universe we can do to get you back. However, say you did overstep that line, what would you see? What might you feel? And what might it look like from the outside.

Say we’ve got four (very) brave adventurers, Joe, Jamie, Jerry and Jonny. Our adventurers are equipped with the latest sci-fi-nonsense tech, strong enough to be nigh on invincible.

Currently they are drifting slowly towards the singularity. Through the glowing plasma of the accretion disk, the event horizon comes into view. One side seem slightly reddish, and the other blueish due to the spin of the black hole compressing and stretching the wavelengths of the emitted light. It seems that you are looking through a fisheye lens, and stars you thought were to your sides now appear in front of you. But you can’t miss it, the gaping hole in the universe, the maw of the all-consuming singularity, wrapped in the red-hot disk of spinning plasma of the accretion disk, which appears to bend over the top of the singularity.

For a while now, the group has felt a force on them, pushing them from their path, first they assumed it was friction from the plasma, but it is something entirely different; the group has entered the ergosphere. The ergosphere is a region around the black hole, where nothing can remain still, this is due to the singularity in the centre spinning at enormous speeds, and with it’s colossal mass, dragging the fabric of the universe, to create gravitational waves which are detectable from millions of miles away, but completely overwhelming this close.

As they approach one thing the group notices as they stare at the black hole is that the stars around them seem to have become bluer than before. Even if they were stop in place, the odd rendering of the universe stays the same, and it is consistent all around. This is because the gravity near the black hole is so strong, time slows down relative to the outside. This means that when light comes nearer the black hole, the wavelength shortens, blue shifting the light to create the effect they see.

Suddenly, overcome with existential dread, Joe decides to stop, accelerating backwards to halt his descent into the limits of mathematics and reasoning. Here, the narrative splits. To Joe, his comrades whom he deserted seem to slow down, as the light takes longer to read him from closer to the black hole. Then they seem to compress, like an image compressed too much on one axis, and slowly, their images turn red, as the light reflected off them in their last moments is redshifted until it is no longer visible. And so; Joe is now alone.

Continuing from the perspective of the other group, they can now see a second disc, what once seemed to be just a part of the accretion disk, but after taking a closer look with their high-powered camera, they realise that it is a reflection of the universe. The light from behind them had been orbiting the black hole, and flung back towards their origins, creating the illusion of a second disk. The region around which this phenomenon can occur is called the photon sphere.

As they approach the horizon, they think they can just about make out images of objects which had passed through here before, the light reflected and emitted off them slow to escape, only now reaching human eyes after struggling against the tides of gravity. After dropping his flashlight, Jonny accelerates ahead to try and catch it, in doing so, passing the event horizon. As he passes, his legs seem to stretch out, like ink on the surface of water, but this is nothing but an illusion, the same effect that Joe will perceive later. This, however, is a special case that not everyone will experience, since the event horizon is the boundary at which light cannot escape. This means that any object which emits or reflects light at this point should have the photons trapped at that surface, and since Jonny just recently dropped his flashlight past it, there are still some unshifted photons hovering around the horizon. To his surprise, however, Jonny can still see the stars, and more surprisingly, his surroundings. Whilst it is true that the curvature of spacetime now no longer allows light to escape, the curvature of spacetime in a localised region can always be approximated to be flat, and so light can still travel relative to other objects within this limited region.

Jamie and Jerry head towards the horizon. Jerry realising his terrible mistake, starts accelerating away from the singularity, trying to stay as close to the horizon as possible, in an attempt to stay alive as long as possible. Jamie decides to do nothing, and let nature take its course, whilst Jonny continues accelerating towards the singularity.

Here, things get weird. Within this supermassive black hole, Jamie perceives that it takes him 25 hours to reach the singularity, Jonny perceives 21 and Jerry 15. Of course, these numbers aren’t exact, and won’t always be in the same proportions for a group in the same situation as theirs. But it illustrates an interesting point; near a black hole straight lines might not be straight. This is an extremely tough concept to visualise; so I will simplify it.

Space is curved, it dips around mass to form a pitted landscape. Now imagine a cloth, and on this cloth, there are a lot of protruding lines perpendicular to the curve. Then imagine 90-degree cones coming out from these lines. The image should look something like this:

Source:http://212.201.48.1/course/fall02/c210101/students/BlackHoles/Black%20holes%20and%20Schwartzschild%20geometry_files/image030.gif

The diagram is a representation of our universe, but “flattened”. Everything you see here is moved down one dimension (this is not to say time is a dimension). The fabric of the universe bends around objects with mass (albeit, not normally to the degree shown in the diagram), and objects which move closest to it will move along the geodesic. This will involve them moving inwards, towards the object. This model describes gravity not as a force as such, more that opposing ‘gravity’ to take the less optimal route requires an opposing force. Now, what about the cones? To explain this, imagine the ‘fabric’ having an identical copy stacked on top of it, such that they fit together perfectly. Now you can move this copy up and down, and you might notice that the cones make an intersection with that moving plane. The intersection that they make is the area in space which light can move to, given a certain amount of time (which is represented by the new plane moving up and down). What is so important about this is the fact that nothing can move faster than the speed of light, so the intersection also represents the area in which any non-light object can move to given a certain time.

Now, in this imaginary world, you may have noticed something. For cones near a black hole, the intersection always moves down. The cones are so tilted that even light must be pulled further and further in. Some physicists even interpret the extreme angle of the cone as having the time and space ‘dimensions’ of the universe flipped. Moving forward is as inevitable as moving forward in time, and the more you struggle, the more you move, the faster your time goes.

However, the strangeness doesn’t end there; you will never see anyone else cross the singularity. The effect is very much similar to the effect where objects crossing the event horizon seem to slow down until they freeze. The same happens here. The light reflected off a descending object will be slowed down, such that it only reaches observers within the black hole later. However, unlike the event horizon, even a person right above the singularity will still perceive the object in front of him to still be falling, even though logic dictates that it should have long fallen into the singularity. If we take the viewpoint of Jamie just before his death, he will see Jonny far down below him, and Jerry far above him, even though they both perceive a shorter time of descent than Jamie does. Jerry will see Jonny and Jamie below him, and Jonny will see Jamie and Jerry above him.

To fall into a black hole is the most twisted fate the universe can grant. It is the only place in the universe where you are truly alone; nobody can see your death, nobody can prevent your death, and you will always die alone.