Einstein's General Theory of Relativity predicts that though the gravitational field around a massive black hole is stronger on the large scale, it will exert weaker tidal forces than its smaller counterpart, at least outside the event horizon. So for all their ferocity, these supermassive black holes are surprisingly gentle giants up close. You can fall into one without turning to spaghetti.
Suppose you were an astronaut about to step into such an abyss at the edge of the universe. As you approach the event horizon, blackness spreads upwards around you. The Universe shrinks to a bright point directly overhead. As I meet and cross the horizon the universe above disappears in a blinding flash of photons trapped in orbit around the hole.
You are now inside the black hole and falling towards the Singularity. It's not dark like you expected. You see a ring of dancing light where the singularity should be. It must be spinning so fast that the centrifugal force has balanced out gravity. Now it's a naked glowing hula-hoop of indeterminate size. Around it you see glimpses of heavens unimaginable to humans, universes within universes, time within time...
The second way is to take the ride yourself and this is what I will now discuss in some detail.
Far from the black hole, an undistorted night sky is visible with a very small patch of fuzz in the center.
You are in empty space looking toward the constellation Orion. The three stars in Orion's belt are visible to the right of the center of you're helmet. Sirius can be seen as the brightest star in the sky below and to left of Orion's belt, and Betelgeuse is the reddish star just above Orion's belt.
As you move toward the black hole, an odd diffuse glow of light appears in the center of the screen. Soon a black spot appears – the black hole itself. The black hole is almost completely dark – light cannot escape from it, although, black holes do release a slight bit of light as they evaporate, as postulated by Hawking.
As you move closer towards the black hole, the original star images appear pushed away from the black hole. This is because the starlight that originally reached you is now strongly attracted toward the black hole and hence deflected away from you. Only starlight passing further from the black hole might now be attracted toward the black hole so that it is deflected to your eye.
If this were a for a 30 solar mass black hole at 100 Schwarzschild the tidal force, the difference in the gravitational force between your head and your toes, would now be at 1G and would increase rapidly as you free-fall inward.
Note also "new" dimmer images of stars become visible near the black hole. Here the strong gravity of the black hole has pulled another image of stars around the far side toward your eye. Soon there are two discernable images of everything in the sky. A secondary images of star can be identified with their corresponding primary image by noting that they can be connected by drawing a straight line on the sky through the center of the black hole and finding stars of that same color.
You stop 42 kilometers from the black hole. General description:
Nothing looks normal from here, the black hole, now very visible, is highly distorting the light around it. An example of this would to have a small black pool, somehow make it depict the normal number of stars and other objects and then drop a large, heavy, black ball in the center and see the change in the image. Distorted space-time is almost like ripples in water.
Large light bending effects cause the background sky to appear to move in unusual ways as you circle the black hole. Light paths are so curved that light can reach you from anywhere on the sky - even from behind the black hole – no part of the sky is eclipsed. Distant starlight has fallen to you and therefore appears "blueshifted."
The large light bending effects cause stars on the opposite side of the black hole to become greatly magnified. Stars usually too dim to see become visible. If you watch closely you can identify an invisible circular ring around the black hole on either side of which stars counter-rotate. This is an Einstein ring, and stars do not cross through it.
You are still orbiting 42 kilometers from the black hole. General description: This is not the kind of orbit you might expect. The hundreds of other images, now coming into view were behaving completely abnormally. All images around the spot were being pushed and pulled, amplified and de-amplified, but while staying distorted, the outer circle of stars was rotating past clockwise, while the inner circle; anticlockwise.
The photon sphere is a location where gravity is so strong that light can travel in circles. Photons orbit the black hole at the distance of the photon sphere. A gravitational field does not create photons – it just redistributes and red or blueshifts them.
An important observational aspect of visual distortions in a high gravity environment is called an Einstein ring. Before it was shown that all images must occur in the plane defined by the observer's position, the center of the lens, and the point source. But what if these are all collinear? No plane is then defined. In this case the image of the point source would appear to you as an infinitesimally thin ring. This is an Einstein ring.
A very interesting set of Einstein rings are the "self" Einstein rings, where you can see yourself. Near the event horizon there are phenomena's, that are completely unusual: controversially to great distances, where the path of light rays may basically be described by hyperbolas, this approximation is no longer valid at smaller distances. There the light rays may be bent in such a strong kind, that it turns back to the light source, or, expressed in another way, you can simply look along the photon sphere, where light travels in a circle. A photon could leave the back of you're head, go once around the black hole, and be seen by your eye – you can see the back of your head. At less distances to the event horizon there is a region, ("photon sphere") where the light may encircle the Black Hole or even get stuck in an orbit ("photon orbit"). The more the observer's view direction approximates this photon orbit, the more often (and smaller) the whole outer world is been mapped spherically, additionally to the effects of gravitational lensing.
All observers in the presence of a sufficiently compact lens, however, can see themselves. Here, light can leave you, travel around the lens and return to you to be viewed. You would see yourself as a series of Einstein rings. The more times light can circle the lens and return to you, the more "self" images you can see. For a lens compact enough to have a photon sphere, you can, theoretically, see yourself in every self Einstein ring: an infinite number of times.
Amusingly, there is only a single case where observers can see only a single image of themselves – when they are at the photon sphere! Here all the self Einstein rings actually merge with the photon sphere to form a single observer image.
Observers who see themselves would be viewing themselves with high amplification. This is because the self image you would see, would be on or near Einstein rings – which carry the highest amplifications. Therefore gravity has become a powerful microscope! When at the photon sphere you can microscopically view the backs of your head, and when far away observers can microscopically view their own eyes. This is because the light that returns to you has left on a nearly radial trajectory – and the part of the observer most nearly radial is the observer's own eye. When close to and inside the photon sphere, you can inspect annular rings on your heads (or spacecrafts).
At the photon sphere, no light emitted outside can reach you from below - you look into the vast emptiness of the black hole. The sky you once knew is now behind you, compacted to occupy only half its original area.
You are inside the photon sphere just 6.3 kilometers from the black hole. General description:
The black hole has expanded everything out of view. Total blackness.
You may also notice that some stars that you see in front of you – you expected to see behind you. Starlight from these stars has been bent around the far side of the black hole.
You are inside the photon sphere just 6.3 kilometers from the black hole. General description:
The Universe outside is still quite distorted, many things are blueshifted. You will not be able to sustain this position for much longer.
You currently sit at the black hole's photon sphere, where light can travel endlessly in a circle due to the black hole's great gravitational pull. The apparent position of the photon sphere is always easy to spot – it is the apparent dividing line between black hole and sky.
As you circle the black hole the sky appears to move in strange ways. Here an Einstein ring for background stars can be seen as an invisible line above the photon sphere horizon. Stars approaching the exact other side of the black hole from you appear to approach this line, are greatly magnified, and move with high angular speeds.
You are now looking directly away from the black hole. The black hole now encompasses almost the complete observer sky. The small hole at the top is what remains visible of the outside universe. In this hole there could appear, theoretically, an infinite number of complete images of the outside universe. The angular amplification angular of the vast majority of these images is, however, much less than unity: they are greatly deamplified.
You are inside the photon sphere just 6.3 kilometers from the black hole. General description:
You are not at the event horizon, which is still below you. Were you to travel to the event horizon the sky would appear to scrunch up into a little dot opposite the black hole.
You now stop your orbit at the photon sphere, you look into the black hole; darkness, you now begin to be forced inwards. The horizon, the Schwarzschild surface. The point of no return.
The tidal force, is now at 1 million G (for a 30 solar mass black hole) and increases rapidly as you free-fall inward. But the tides wouldn't be so bad for a very massive black hole (such as the one you are in now). The tide at 1 Schwarzschild radius would be less than 1G if the black hole exceeded 30,000 solar masses.
For this 30 solar mass black hole, from here to the central singularity would take 0.0001 seconds in free-fall. The in-fall time is proportional to the mass of the black hole. For a suppermassive black hole it would take hours!
If an external observer sees you slow down asymptotically as you fell, it might seem reasonable that you'd see the universe speed up asymptotically – that you'd see the universe end in a spectacular flash as you went through the event horizon. This isn't the case, though. What an external observer sees depends on what light does after you emit it. What you see, however, depends on what light does before it gets to you. And there's no way that light from future events far away can get to you. Faraway events in the arbitrarily distant future never end up on you "past light-cone," the surface made of light rays that get to you at a given time.
That, at least, is the story for an uncharged, nonrotating black hole. For charged or rotating holes, the story is different. Some people believe that such holes can contain, in the idealized solutions, "timelike wormholes" which serve as gateways to otherwise disconnected regions – effectively, different universes. Instead of hitting the singularity, you can go through the wormhole. But at the entrance to the wormhole, which acts as a kind of inner event horizon, an infinite speed-up effect occurs. If you fall into the wormhole you see the entire history of the universe outside play itself out to the end. Even worse, as the picture speeds up the light gets blueshifted and more energetic, so that as you pass into the wormhole an "infinite blueshift" happens which fries you with hard radiation. There is apparently good reason to believe that the infinite blueshift would imperil the wormhole itself, replacing it with a singularity no less pernicious than the one you've managed to miss. In any case it would render wormhole travel an undertaking of questionable practicality.
Although if you really wanted to see the universe end, one, not totally confirmed, way to watch all the history of the Universe would be to stay just above the event horizon, firing your rockets like crazy just to stay stationary. The Universe would then appear not only speeded up, but also highly blueshifted, most likely roasting you in gamma rays, and concentrated in a tiny piece of the sky just above you.
Now at r/Rs=0.35, Images being to be distorted by two effects: a tidal distortion from the gravity of the black hole and a special relativistic beaming from your near light speed motion.
Just as the tidal distortion redshifts images from above and below and blueshifts them about your middle, so also it tends to repel images from above and below, and concentrate them about your middle. At first, images appear distorted into a small kidney shape. As the distortion grows, images become stretched and squashed into a doughnut shape about your waist.
As soon as you approach r/Rs=0.01 the tidal force starts to concentrate your view into a 'horizon' shape, while your near light speed motion further concentrates the view ahead. The tidal force and your motion blueshifts photons from the outside creating very high energies, which we would see as x-rays and gamma rays.
Finally you reach r/Rs=10-9, just 1 millimeter from the singularity.
The tidal force has become so strong that all images are concentrated into a thin line about, what is left of, your waist.
As you approach the central singularity, the ride becomes very bumpy.
Small perturbations in tidal forces, caused by the presence of you and any other particles around, to become greatly amplified in the final approach. The perturbations grow into violently oscillating tidal forces. Even a single in-falling photon is enough to induce such oscillations.
Besides shredding your already torn apart bits into subatomic particles, the oscillating tides create photons and particle-antiparticle pairs out of the vacuum, producing a fierce environment. (see Antimatter.)
Then, as John Archibald Wheeler likes to put it: "Smoke pours out of the computer."
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