I take you on a journey into a black hole. Black holes are super-massive objects in our universe that have such a strong gravitational pull that not even light can escape. So, we cannot observe what happens inside from the outside. Get your towel and brace yourself for a ride in a spaceship to experience the inside.
First, we need to pick a suitable black hole for our journey. To experience the inside, we need to pass what is called the event horizon. It is not a physical barrier; it cannot have cracks as depicted in one Star Trek episode; it is the point of no return, an imaginary surface around the black hole at which light can move only along the surface or inside it, but not outside. The closer we approach to the hole the stronger the gravitational pull. Tidal forces, the difference in gravitational force between front and back of the spaceship, could brake our ship. So, we need to be careful to choose a suitably-sized hole.
Our galaxy, the Milky Way, has a gigantic black hole at its core, Sagittarius A*, with a mass estimated to be about 4 million times larger than our sun. How do we know the mass? In the black hole’s vicinity, we can observe a star, measure its orbital speed and distance, and together with the known gravitational constant (measured on Earth) have all the ingredients to compute the mass pulling at the orbiting star. At the event horizon of this black hole, the gravitational pull is extreme, half a million times stronger than what we experience on Earth. But if we fall at this acceleration together with the spacecraft, we would experience weightlessness like inside the International Space Station - not too bad. The only forces we would experience are the tidal forces. Our black hole, however, is huge: with a radius of the event horizon of about 6 million miles, the tidal forces are minuscule. For smaller black holes, though, they might be deadly.
At this extreme acceleration of our spacecraft, we will approach the event horizon at breathtaking speeds - hardly enough time to enjoy the transition into the black hole. If we want a ride at lower acceleration, we need to pick an even larger hole. The radius of the event horizon scales proportionally to the mass of the black hole. Fortunately, the gravitational acceleration decreases inverse proportionally with the square of the radius. As a result, for every ten-fold increase in mass, we get one tenth of the acceleration at the event horizon. For example, if we pick a black hole the mass of the Milky Way, estimated to be about a trillion times more than our sun’s mass, 250,000 times more than Sagittarius A*, our acceleration would be that of a sports car at peak acceleration - sufficiently low to allow an arbitrarily slow travel speed.
Finally, our journey through the event horizon should be a pleasant ride. Still something unusual happens. Space-time curves as we approach the event horizon. When we stretch out our arm toward the black hole, the blood will take longer to flow back to our heart. When we move the fingers, their motion will appear to be slower than what we commanded them to be. Moreover, the nerve bundles in our arm will return delayed sensations from our fingers. When we look past the fingers to the front of the spaceship, the wall colors appear to have a reddish tint.
We pass the event horizon, but do not notice any difference. Through the rear window, we see the familiar star pattern, a bit distorted on the sides, and through the front window only blackness. As we gaze outside, a crew mate joins us. She says she will get a coffee from the front of the spaceship. We watch her drift away. She appears to move amazingly slow: we are waiting a full hour until she reaches the front. There, her movements are extremely slow, picking up a space coffee pouch in slow motion.
She returns after another hour, but says she was only gone for a few minutes. In disbelief, we compare our watches. Indeed, her watch advanced only three minutes. Strangely, our times passed at different speeds depending on how we moved relative to the black hole. Our crew mate did not experience the slowness herself. In contrast, she observed us wiggling around and moving our hands at unusually fast speed.
Such time phenomena actually also happen on Earth: a person in a valley will experience time passing slower compared to a person on a mountain, but the effect is tiny and barely noticeable (only with super-accurate clocks). At the event horizon and inside a black hole, however, the effects can be so strong that we notice differences even within a spaceship.
Once our spaceship passes the event horizon, it cannot stop anymore. It will steadily move closer and closer to the center of the black hole. At the slowest possible speed towards the center, even stranger things happen inside the spacecraft.
Anything can move only towards the front of the spaceship or to the sides, but not towards the back. If we stretch out our hand to the front, we cannot see it anymore. In fact, we cannot see anything in front of us, only blackness. Unfortunately, this situation has a catastrophic effect on our brain since communication between large parts of the brain shuts down; communication can happen only on two-dimensional brain slices - likely, insufficient for conscious thought. So, don’t drive the spaceship at the slowest possible speed! Reaching the slowest possible speed is also tough on the engines. The lowest limit of the speed can be reached only by massless particles like light, and the closer we get to the lowest limit, the more force we require - we are fighting the space-time suction of the black hole.
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