Falling into a supermassive black hole

Question

What could happen to the person to enter and hit the event horizon? Would it be possible to survive and or be taken to a different timeline or universe?

Answer 1

Introduction

Firstly, a warning: since no one ever entered a black hole, no one really knows what lies beyond the event horizon. In fact, there are a few lines of research that indicate that quantum effects might prevent the black hole from even forming. Hence, ultimately any answer to this question must make clear what assumptions are taken into account. I'll answer according to the predictions of General Relativity, which is one of the best theories ever developed by humankind in terms of agreement with experiment. Nevertheless, it does not take quantum effects into account and some scientists believe this might change things quite a bit.

Crossing the Event Horizon

That being said, assuming GR to hold well, you won't notice anything special when you cross the event horizon. The equivalence principle ensures that no local experiment will allow you to notice where the event horizon is, so you won't even know that you fell into the black hole to begin with. As for survival, you won't be crashed by gravity if the black hole is big enough. The larger the black hole, the smaller its surface gravity. For a Schwarzschild black hole (i.e., a black hole that has no charge and does not spin), the surface gravity is given by $$\kappa = \frac{c^4}{4 G M},$$ where $M$ is the black hole's mass, $G$ is Newton's constant, and $c$ is the speed of light. See also the last section of this answer for a few remarks on this.

While this might sound counterintuitive, Newtonian gravity already provides some intuition for it. I'll do a wrong calculation for the sake of illustrating what's going on, so don't take it too seriously. In any case, the correct, general relativistic calculation leads to the same results. Suppose you want to compute the gravitational acceleration at the event horizon. Newton's law reads $$F = \frac{GM m}{r^2},$$ and hence the gravitational acceleration $\kappa$ as a function of distance is $$\kappa = \frac{GM}{r^2}.$$ The horizon is located at the Schwarzschild radius, $r_S = \frac{2 G M}{c^2},$ which leads to $$\kappa_{\text{horizon}} = \frac{GM}{r_S^2} = \frac{GM c^4}{4 G^2 M^2} = \frac{c^4}{4 G M}.$$

As I said, this calculation is wrong because it is done with Newtonian gravity instead of working with General Relativity. In this particular case, the two approaches happen to give the same result and we can get a bit of intuition: while $M$ being larger makes the gravitational field more intense, it also makes the black hole bigger. While gravity grows linearly with mass, it decreases with the square of the distance. Since making a black hole bigger means making its event horizon further away, the net effect is that the gravitational field goes down.

As a consequence, for a sufficiently massive black hole, you can have its surface gravity at the event horizon be smaller than the gravitational field on your living room. Hence, yes, it is possible to cross the horizon without dying, as long as the black hole is large enough.

What's on the other side?

As to the question of whether you'd be taken to a different timeline or universe, it really depends on the precise meaning you give to those words. Once you go into the black hole, everything you knew is in your past. You are now in a region of spacetime from which you can't reach Earth again, for example. Since you can't go back to where you came from, it can be said in a certain sense you have entered a "new Universe", but it depends entirely on the specific meaning you give to the world "Universe".

There is no reason to believe you'll enter a different timeline or anything of the sort.

For this particular issue of "finding new Universes beyond a black hole", I suggest watching the excellent science communication video Mapping the Multiverse, from PBS Spacetime.

Dying at the Singularity

Black holes also come with a singularity, which is essentially a hole in spacetime. Particles that hit it simply are no longer in spacetime. This is frequently related to gravity being infinite at these points according to the predictions of General Relativity. Many (perhaps even most) physicists believe these are just cases of General Relativity predicting that it will stop working in situations of very strong gravity, when quantum effects need to be taken into account.

In either case, if you approach the singularity sufficiently, the gravitational field will be strong enough to kill you through spaghettification. In a Schwarzschild black hole (no charge, doesn't spin), it is impossible to avoid the singularity and you will hit it sooner or later. In other black holes (such as a charged or rotating black holes), it is possible to avoid the singularity by travelling in the right paths. However, this result (being able to avoid the singularities) holds for idealized black holes that should have always existed and are likely not physical. As of the moment in which I'm writing this answer, I am not sure of what is the current status of research with respect to physical black holes, which arise from the gravitational collapse of stars.

Summary

In short, someone who falls into the black hole wouldn't even notice it, according to General Relativity. It is possible to survive the crossing of the horizon it if you are going into a sufficiently big black hole, but there's quite some chance you'll die by hitting the black hole's singularity. There isn't really any reason to believe you'll go into a different timeline, and to say you are going into a different Universe depends on what you call another Universe.

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Remark on the Meaning of the Surface Gravity $\kappa$

As pointed out by safesphere in the comments, $\kappa$ is actually a redshifted measure of the acceleration needed to keep at a constant radial coordinate $r$. In fact, the acceleration to hover over the black hole at a certain height tends, as safesphere mentioned, to infinity as one gets closer and closer to the black hole (which can be thought of as a consequence of the fact that no amount of acceleration will get you out of the black hole).

I like to use the surface gravity to illustrate the fact that the gravitational effects are, in some sense, weak because I find the Newtonian analogy more intuitive, but safesphere's point regarding this is absolutely correct. A better statement is to say that the tidal forces near the horizon are very weak, i.e., your head and your feet will fall in roughly the same way close to the horizon, so your body won't be painfully stretched out until you die (that is, at least while you're not too close to the singularity). Another possible way of quantifying this would be to consider the Kretschmann scalar, which is another quantity that measures the "strength" of the gravitational field (in a yet different sense than the surface gravity). For the Schwarzschild black hole, it is given in terms of the radial coordinate $r$ by $$K = \frac{48 G^2 M^2}{c^4 r^6},$$ and hence at the event horizon ($r = r_S = \frac{2 G M}{c^2}$) it is given by $$K_{\text{horizon}} = \frac{3 c^8}{4 G^4 M^4},$$ and once again we have essentially the same effect.

Comment on the existence of "beyond the horizon" within GR

Safesphere also mentioned that GR does not predict the existence of whatever is beyond the horizon, and I should remark that that is a plausible point of view, although I'm not sure if a very common one. Indeed, we can't really falsify any predictions made about what happens beyond the horizon unless we actually go there, so it has a couple of difficulties. Nevertheless, as far as I understand, most physicists working in GR (at least of those I talked to) usually take the philosophical point of view that we should consider spacetime as being "as big as possible" (see, e.g., this question and its answer). My personal view is that the situation is similar to how we can't falsify any predictions about how Physics will work in December 2022 until we get there: technically this is correct, but I see no reason to question the knowledge we already have. There are, of course, counterarguments to this point of view, but my main point in this section is to make clear that there are a few different possible takes on what happens beyond the event horizon, if there is such a thing.