In this third and last part of the article dedicated to black holes, I will describe what would happen to someone traveling through a black hole.
Gravitational time dilation and gravitational redshift
First, I will start by describing two important effects: gravitational time dilation and gravitational redshift (I will spare you the equations describing these effects, unless someone asks for further details).
Gravitational time dilation is the effect of time passing by at different rates in regions of different gravitational potential; the lower the gravitational potential (which actually means stronger gravity, or greater acceleration – in virtue to the equivalence principle that states that all accelerated reference frames are physically equivalent to a gravitational field of the same strength), the more slowly time passes by. The effect was predicted by Einstein’s general relativity, and has been confirmed by many tests.
Gravitational redshift is the change in the wavelength of electromagnetic radiation in a gravitational field predicted by general relativity. A photon with a given wavelength originating from a source placed in a region of stronger gravitational field will be found to be of longer wavelength when received by an observer in a region of weaker gravitational field: the wavelength is shifted towards the red end of the electromagnetic spectrum.
What would happen if I fell into a black hole?
Now that the gravitational time dilation and gravitational redshift have been introduced, here comes the fun (well, maybe not that much…) part.
What would happen if you fell into a black hole? First, let’s describe what you would feel. Away from the black hole, you do not feel anything at all, you feel weightless (just like an astronaut in Earth orbit); as you get closer to the black hole (because you do want to go into that black hole), you start to feel the gravitational forces. Let’s imagine that your feet are closer to the center of the hole than your head; the gravitational pull gets stronger and stronger as you get closer to the center, so your feet feel a stronger pull than your head does. This will stretch your body and the gravitational pull will eventually rip you apart.
Now imagine that you are falling in a black hole that is big enough for you to cross the event horizon before being turned into a spaghetti noodle (the effect I just described is actually called spaghettification, or noodle effect). You might now be wondering what is going to happen when crossing the horizon: this might sound surprising, but nothing particular happens. You can still see what is going on outside the black hole (light coming from the outside can still reach you – on the other hand, you are now invisible to anyone outside, as the light you are emitting cannot escape from the black hole). The bad news is that since you crossed the horizon, you will not be able to escape the black hole, and you are doomed to the inevitable end: hit the singularity (and incidentally, die). The interesting part is that if someone is standing at a safe distance of the same black hole, things will look quite different from their point of view.
What would another person (at a safe distance) see when looking at me falling into the black hole?
As you get closer and closer to the horizon, this person will see you move more and more slowly. Actually, no matter how long this person would wait, they would never see you reaching the event horizon. This is due to gravitational time dilation: the light you are emitting takes longer and longer to reach that person, and the radiation you emit right as you cross the event horizon will hover there forever and never reach them.
In fact, you will become invisible to that person before too much time has passed. As everything is slowing down when you approach the event horizon, the light you are emitting will become redder and dimmer, because of gravitational redshift. Eventually, the photons you are emitting will no longer be visible light, but infrared radiation, then radio waves.
