Journey into a Schwarzschild black hole.
The simplest kind of black hole is a Schwarzschild black hole, which has mass yet no electric charge or spin. This black hole geometry was discovered by Karl Schwarzschild in 1915, shortly after Einstein presented his final theory of General Relativity. The gifs above are created from a simulation depicting what you would theoretically see if you traveled towards a black hole, against a panorama of our Milky Way.
First of all, as you approach, you clearly see gravitational lensing taking place, with the black hole bending light around it. It appears to ‘repel’ the Milky Way radially, which then stretches the image transversely. The sections closer to the black hole experience greater ‘repulsion’, so the image appears to be compressed radially.
You then take note of the Einstein Ring seen around the black hole, occurring because of the bright objects lying directly behind it. Due to the aforementioned gravitational lensing, the light from these bright objects is bent around the black hole and forms this ring.
Fortunately (or maybe unfortunately), as you get closer, the trajectory of your journey does not have enough angular momentum to go into an unstable circular orbit. If you had slightly more, you would find yourself orbiting this black hole, which would, in fact, make for a fairly nice view. However, you carry on travelling towards the center.
Next, you swiftly pass through the photon sphere,where light rays can orbit the black hole in unstable circular orbits. However, you do not see anything of particular interest, but are more concerned with your forthcoming fall through the horizon.
As you travel, you would not know at what point you fell through the black hole’s horizon. However, as you do pass through it unaware, it apparently splits in two, explained nicely by these Penrose diagrams (if you have the chance to give them a quick glance over whilst you’re hurtling towards your inevitable death). Here, space is falling faster than light, meaning you are carried inexorably inward.
Anyone who happens to be watching your spectacular journey would see you as fairly dim and red. This effect is due to red shift, with anything falling past the black hole’s horizon appearing this way to an observer outside of this point.
As you get closer and closer to the center, the black hole’s tidal forces begin to wear on you. Presuming you are travelling feet first, you feel a greater force of gravity in your lower half than up by your head. Due to these forces, you are stretched vertically and crushed horizontally; this is known as spaghettification. These forces also mean that your view of the Universe beyond is blue shifted and bright around your waist, but red shifted and dim above that; a strange sight.
Despite having been utterly torn apart from the tidal forces, a tenth of a second later you reach the black hole’s singularity, the center point of infinite curvature. Here, space and time as you know them come to an end, and so does your exciting journey.
It must be remembered that real black holes are probably much more complicated than Schwarzschild black holes; they likely spin and are not isolated, so a journey into a normal black hole could be slightly different adventure.