What You Would Perceive As You Entered a Black Hole | Dr. Brian Greene

I think what we'll do now, if it's okay with you, is um turn to string theory. And I mean, I'm so ignorant about string theory that it's kind of a miracle, and I guess so I can I'm going to start by asking you some basic questions. I guess the first question, you know, you touch upon this in the second edition of your 1999 book, which is um, The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for Ultimate Theory. You've just updated that, and uh, you open the book by explaining, I suppose, at least part of the problem that string theories hypothetically pose to solve.

And at least to some degree, that's the uh lack of unity between the theories of general relativity and the theories of the quantum physicists. So maybe you could explain to us first what it means that those theories aren't unified, like what that means, yeah what that means in the scientific realm, but also what it means practically. And then walk us through how String Theory—what string theory is and how it constitutes a potential solution to that, uh, to that conundrum.

Yeah, so the two big discoveries of the 20th century are Einstein's general theory of relativity, which describes the force of gravity, and as we discussed before, the force of gravity matters when things are big—stars and galaxies in the whole universe. In that domain, Einstein's ideas have been tested, and they do an incredible job of explaining things that we see in the heavens.

The other big development, which we've spent some time already talking about, is quantum physics, which describes the small things—molecules, atoms, subatomic particles. And in that domain, quantum physics has itself been tested to incredible precision, and it works. The crazy thing is, in any situation where you need to put quantum physics and general relativity together, when you need to use the equations in tandem, you get nonsensical results. You get results like infinity is the only answer that you ever get for any question that you pose.

Now, you might say, well, do you ever need to put them together? Quantum mechanics is small; general relativity is big. Those seem pretty separate. But there are extreme realms, like the center of a black hole, where a lot of mass has been crushed to a very small size—big mass, general relativity, small size, quantum physics—or the Big Bang, the entire observable universe crushed to a very small size, a lot of mass energy, small size. Again, you need general relativity and quantum mechanics.

In those extreme realms, you find that the equations simply fall apart. The laws of general relativity and the laws of quantum physics do not play too well together. They are ferocious antagonists, and that's the problem that we've been trying to fix. It produces these mathematical absurdities that you've been describing and interferes with our understanding.

I guess there's the aesthetic problem too, which is that we're possessed by the strong intimation that all forms of descriptive knowledge should unify, at least not exist in contradiction to one another. That seems, well, it seems to violate our understanding of what understanding itself is.

And so that's okay, okay. And so, well, can you give us a maybe a more tangible indication of what sort of absurdities might emerge in the conceptual realm when you're dealing with something like the situation that obtains in a black hole? You said that the equations will produce references to infinity continually, which seem to be non-helpful, but that's still pretty abstract for people who aren't mathematically oriented. Is there a way of simplifying that so that it's more graspable?

Yeah, so for imagine—you jump into a black hole. Inadvisable, but imagine you do it. We know that as you get closer and closer to the center of the black hole, things will start to feel uncomfortable. If you jumped in feet first, your feet are going to be pulled more strongly than your head, so your body is going to stretch. It's going to spaghettify, we call it. Ultimately, it's going to be pulled apart into its constituents, and those constituents are then going to fall toward the center.

And the deep question is, what finally happens when you actually reach the center? If you ask any physicist today for the answer, they would be forced to tell you—if they're honest—we don't know. We just don't know what happens at the center of the black hole. Some ideas are it's a portal to another universe. That's a wild sci-fi sounding idea; it’d be wonderful if it's the case, but we just don't know. Some people think it's a location where time comes to an end—it's just where there's no notion of time any further. So these are the things that we just don't know how to answer.

So I have a question about that too. Well, correct me if I'm wrong about this, but my understanding is that as something falls into a black hole—and this is from the perspective of an external observer—as it falls, it's transforming more and more slowly, and that that transformation decreases in speed until it's really at a standstill. And so, I'm wondering if that's the case and that there's that time dilation that's accompanied by descent into a black hole.

I'm trying to put like vague imaginations here—imagining you have something that's very, very dense at the center of that, and you also have this time dilation process. And you have the idea that at some distant point in the future, everything's going to come back together in a big crunch, at least that's one of the hypotheses. Is there any difference between the destination point when a given body is falling into a given black hole and the Big Crunch itself? Like is that destination point the same destination point? I mean, that would account to some degree for the kind of infinite density at the center of the black hole, and it seems to make sense if time is dilating to that degree.

That's a good question! And certainly, you're right from the standpoint of an outside observer watching say you jump into a black hole—they will see you move slower and slower as you reach the event horizon, the edge of the black hole. In fact, they will see you ultimately come to a standstill right at the event horizon itself. But the amazing thing is from your perspective, you will fall right through that event horizon. You will go right to the center, and it will happen in finite time. It will not happen in some long cosmological time; it'll happen in finite time.

And so it's not as though the center of the black hole is the Big Crunch, if there is such a thing for the universe. But it is the case that we believe that if we could answer the question of what happens at the center of a black hole, we would then be able to answer the question of what happens at the Big Crunch or what happens at the Big Bang because we face exactly the same issue.

If you weren't so interested in black holes but you were interested in how the universe got started, again, ask any physicist what really happened at the moment of the Big Bang—time zero itself. If the physicist is straightforward and honest, they'll say we don't know for exactly the same reason—that's a realm where the density is so high that you need general relativity, where quantum physics is vital because it's so small, and the equations break down. The equations are the only tool that we have to gain insight into realms that we can't literally visit, and that's the issue that we're trying to fix.

Okay, so why does it matter that from your perspective you would continue falling at a finite time with regard to the question of whether what's at the bottom of the black hole is the eventual aggregation of all matter? Because looking at it from the outside, as that falling entity grinds to a halt, there's an infinite duration of time that's now involved in the process, and in that infinite duration of time, if the Big Crunch models are correct, that Big Crunch is eventually going to occur. So I don't understand why they don't—yeah, why they don't necessarily dovetail, let's say, or converge.

So our goal is to be able to explain the happenings in the universe from any and all perspectives. The one thing Einstein taught us is that different perspectives can tell very different stories about the universe. But our goal is to be able to understand all those stories. We want to chronicle all the narratives, if you will, that could be told about the universe. And so you’re right from the standpoint of the outside observer; the chronicle you're telling is correct: infinite time. But we also want to know the chronicle from the person that could fall in as well.