# Why will data from a black hole solve the gravity problem?

In Interstellar, the team hopes that data collected from the inside of a black hole will solve “the gravity problem” (presumably the question of how to get a massive space station to escape the Earth’s atmosphere, explained in more detail in this question).

Two related questions on this:

1. How can “data” alone solve this scientific problem? Is it that they have too many possibilities to manually test, and their question is just “which is the right one?” Are there any real-world historical examples of good data leading to a breakthrough on achieving what was previously thought to be impossible?

2. Why does the data have to come from a black hole? Isn’t this counter-intuitive, because a black hole is the one place where gravity doesn’t behave as it does on Earth, which is where the problem applies?

• @InfinityWar Given your 50k+ rep, I find it hard to believe you're not yet familiar with StackExchange's "Be Nice" policy... – b1nary.atr0phy May 9 '18 at 6:15

This is explained in more detail in the book The Science of Interstellar by physicist Kip Thorne, who came up with the original script treatment and was a consultant on the movie. The explanation is a bit involved, but feel free to skip to the last paragraph of this answer for a short summary.

The main idea about new physics in the movie was that gravity was affected by events happening in a higher spatial dimension, "the bulk". This idea was based on some real theoretical physics models involving branes (higher-dimensional analogues of strings in string theory), specifically models involving a large extra dimension where our 3D universe is a 3-brane sitting in a higher space dimension (which physicists do refer to as 'the bulk'). As Thorne explains in chapter 23, these models suggest that while most particles emitted in our 3-brane stay confined to it, gravitons can escape. If they were able to radiate off arbitrarily far away, that would mean the strength of the gravitational force would fall off with the cube of distance rather than the square as we observe. But in the Randall-Sundrum model, there is a type of strong spacetime warping, "Anti-deSitter warping", in the region of the bulk near our 3-brane, known as the "AdS layer", which causes most gravitons leaving our 3-brane to travel on bent paths that are confined to this region. This confinement means that gravity does continue to obey an inverse-square law at large distances (by an argument I won't try to reproduce here, but see either chapter 23 of Thornes' book or Lisa Randall's own book Warped Passages for more info), though at very short distances it could deviate from this. And because warping of spacetime in the AdS layer affects the behavior of gravity in our 3-brane, apparently this means that fluctuations in this warping could change the behavior of gravity. As Thorne says in this chapter:

By confining gravity, the AdS layer regulates its strength. In Interstellar we see gravity's strength fluctuate, perhaps due to fluctuations in the AdS layer. These fluctuations—gravitational anomalies—play a huge role in Interstellar.

Thorne goes on to say in chapter 24 that in Intersellar's universe "gravitational anomalies" have been found in the gravitational tidal forces surrounding the Earth, and in chapter 25 he goes into detail about Professor Brand's proposed explanation for these anomalies, involving higher-dimensonal sources in the bulk, according to his own "science extrapolation" of what was going on in the movie (even if this wasn't spelled out onscreen):

In my extrapolation, it does not take long for the Professor to convince himself that the anomalies are due to gravity from the fifth dimension. From the bulk. Why?

The sudden changes in tidal gravity have no apparent source in our four-dimensional universe. For example, in my extrapolation the Professor's team sees the tidal gravity above an oil deposit switch, in just a few minutes, from the pattern we expect (top picture in Figure 25.2) to a radically different pattern (bottom picture). The oil has not moved. The rocks have not shifted. Nothing in our four-dimensional universe has changed except the tidal gravity. These sudden changes must have a source. If the source is not in our universe, on our brane, then there is only one other place it can be, the Professor reasons: in the bulk.

He goes on to say that for technical reasons the Professor rejects the idea that it's just a matter of a higher-dimensional object in the bulk passing near our brane and exerting gravitational forces on it. Instead, the Professor concludes that the most likely explanation is that there are "bulk fields" which consist of collections of lines of force ('What kind of force lines, the Professor doesn't know') that generate tidal gravity when they pass through our brane. Thorne also says if our universe has only 3 spatial dimensions, then relativity says the gravitational constant G must be constant, but "if the bulk does exist, then the relativistic laws allow this G to change. It might, the Professor speculates, be controlled by the bulk fields. It probably is controlled by the bulk fields, he thinks." One of the anomalies Thorne imagines in the world of the film was a small but measurable change in the relative strength of the Earth's gravitational pull at different points on its surface, and the Professor thinks the best explanation for this was not just a change in tidal forces, but an actual variation in the value of G at different points in the space occupied by the Earth.

The equations on the Professor's blackboard were meant be "a mathematical description of the bulk fields and how they might generate anomalies" and control the value of G (along with two other applications he discusses: holding the wormhole open, and protecting our universe from collisions with other 'confining' branes). Note that he was originally not trying to find a full theory of quantum gravity, see the part I bolded in the quote below:

In creating this mathematics, the Professor was guided by the trove of observational data his team was collecting (Chapter 24), and by Einstein's relativistic laws of physics in five dimensions.

The Professor embodied all his insights in a single equation, THE equation, which he wrote on one of the sixteen blackboards in his office (Figure 25.7). Cooper sees the equation on his first visit to NASA, and the equation is still there thirty years later, when Murph has grown up to become a brilliant physicist in her own right, and is helping the Professor try to solve it.

This equation is called an "Action." There is a well-known (to physicists) mathematical procedure to begin with such an Action, and from it deduce all the nonquantum physical laws. The Professor's equation, in effect, is the mother of all nonquantum laws. But for it to give birth to the right laws—the laws that predict correctly how the anomalies are produced, how the wormhole his held open, how G is controlled, and how our universe is protected—the equation must have precisely the correct mathematical form. The Professor doesn't know the correct form. He is guessing. His is an educated guess, but a guess nevertheless.

His equation contains lots of guessing: guesses for things called "U(Q), H_ij (Q^2), W_ij, and M (standard model fields)" on his blackboard (Figure 25.7). In effect, these are guesses for the nature of the bulk fields' force lines, and how they influence our brane, and how fields in our brane influence them.

...

When the Professor and his team speak of "solving the equation," in my extrapolation they mean two things. First, figure out the right forms for all these things they are guessing: "U(Q), H_ij(Q^2), W_ij, and M (standard model fields)." Second (following the well-known procedure), deduce, from his equation, everything he wants to know about our universe, about the anomalies, and most important, about how to control the anomalies so as to lift colonies off the Earth.

When characters in the movie speak of "solving gravity," they mean the same thing.

In the movie, when the Professor is very old, we see him and grown-up Murph trying to solve his equation by iterations. On a blackboard, they make a list of guesses for the unknown things (guesses that I wrote on the board just before the scene was filmed; Figures 25.8 and 25.9). Then, in my extrapolation, Murph inserted each guess into a huge computer program they've written. The program computes the physical laws for that guess, and those laws' predictions for how the gravitational anomalies behave.

In my extrapolation, none of the guesses predicts anomalies that look anything like the observations. But in the movie, the Professor and Murph keep trying. They keep iterating: making a guess, computing the consequences, abandoning the guess, and going on to the next guess, one guess after another after another, until exhaustion sets in. Then they begin again the next day.

A bit later in the movie, when the Professor is on his deathbed, he confesses to Murph: "I lied, Murph. I lied to you." It is a poignant scene. Murph infers that he knew something was wrong with his equation, knew from the outset. And Dr. Mann tells the Professor's daughter as much in an equally poignant scene on Mann's planet.

But in fact—Murph realizes, soon after the Professor's death—"His solution was correct. He'd had it for years. It's half the answer." The other half can be found inside a black hole. In a black hole's singularity.

In chapter 25, Thorn explains how, in the movie, they could solve the equation if they did have a full theory of quantum gravity--they could discard the "quantum fluctuations" and be left with the non-quantum theory of the higher-dimensional fields that the Professor was looking for. They hope that data from the black hole could help them find a theory of quantum gravity:

If Professor Brand could discover the quantum gravity laws for the bulk as well as our brane, they by discarding those laws' fluctuations, he could deduce the precise form of his equation (Chapter 25). And that precise form would tell him the origin of the gravitational anomalies and how to control the anomalies—how to employ them (he hopes) to lift colonies off Earth.

In my extrapolation of the movie, the Professor knows this. And he also knows a place where the quantum gravity laws can be learned: inside singularities.

In real life, most physicists expect that a theory of quantum gravity would probably only diverge noticeably from the predictions of our best classical theory of gravity (Einstein's theory of general relativity) at the Planck scale, involving both very high energy densities and very short distances and times. The only places where you can get such intense concentrations of energy are under circumstances where general relativity would predict an infinite concentration of mass/energy, i.e. where it predicts singularities. It's under these extreme circumstances that physicists think quantum gravity effects would become significant.

One other subtlety--they actually aren't talking about the singularity at the black hole's center! As Thorne explains, attempts to model realistic rotating black holes by physicist suggest the presence of two other singularities, caused by the fact that waves falling into the black hole (gravitional waves, electromagnetic waves etc.) get infinitely blueshifted (meaning a localized region of infinite--or Planck-scale, in quantum gravity--energy density) well before they reach the center of the black hole. I discussed this in more detail in an answer here, but the basic idea is that if you fall into a black hole, depending on your trajectory you can pass through either an "infalling singularity" caused by matter/energy/gravitational waves that fell in after you, or an "outflying singularity" (discovered as a theoretical possibility only recently, in this 2012 paper) caused by matter/energy/gravitational waves that fell in before you, but where "a tiny fraction of that stuff is scattered back upwards towards you, scattered by the hole's warpage of space and time ... The upscattered stuff gets compressed, by the black hole's extreme slowing of time, into a thin layer rather like a sonic boom". Both these singularities may be "gentle" singularities in the sense that although the tidal forces go to infinity as you pass through them, the tidal forces only become really large for such a brief period of time that they may not rip you apart (as they would if they lasted for longer, see spaghettification). And in chapter 28 Thorne explains the idea is that Cooper did manage to pass through the outflying singularity before being scooped out of the black hole by the tesseract, and thus TARS was able to obtain the needed information about quantum gravity.

So that's the long answer to your question—probing the singularity would allow them to discover the correct theory of quantum gravity, which would allow them to find the correct form of the Professor's nonquantum equation after discarding the quantum fluctuations, which would allow them to understand the anomalies and control the local value of the gravitational constant G.

• Wow, detailed answer, thank you! Seems like a great book, I'll have to check it out. – jamaicanworm Nov 20 '14 at 2:00
• Meanwhile @Hypnosifl is raking in the rep from all the Interstellar questions :) – Möoz Apr 21 '15 at 22:49
1. The "gravity problem" is implied to be the search for a theory of quantum gravity. The details are omitted, but one candidate has long been string theory. And from Wikipedia:

As presently understood, however, string theory admits a very large number (10500 by some estimates) of consistent vacua, comprising the so-called "string landscape". Sorting through this large family of solutions remains a major challenge.

sounds like your idea of "too many possibilities to manually test" is not far off!

As for a real-world example: most drug research. We can theoretically synthesize all possible molecules. But with complex molecules like proteins, the possibilities are virtually endless. How do we find among those endless possibilities e.g. something that kills cancer cells? One way is to look at what proteins all kinds of plants or animals produce, since they presumably derive some sort of advantage from those. There are estimates that nearly half of all newly-discovered drugs are found that way.

2. Well, they actually wanted to find a way to make gravity not behave like it usually does on earth!

• Great answer, thanks! In your drug research example, what would be the analogue of "the molecules that kill cancer cells" to the gravity problem? The angle/acceleration of launching the space station? The bending of spacetime (somehow) around the space station? Just trying to visualize what this would look like of it were a scene they included in the movie (which I really think they should have included)... – jamaicanworm Nov 19 '14 at 19:42
• @jamaicanworm: I guess it would have to be some way of neutralizing or controlling gravity. But I think it's better that they didn't include it, it wouldn't have fit into the arc of suspense except as a flashback. – Michael Borgwardt Nov 20 '14 at 9:00