## Abstract

In this paper, we cover introductory general relativity and compute the Ricci scalar of the (2 + 1)d Schwarzschild metric. We review concepts of the black hole information paradox; the firewall solution and black hole complementarity including their breakdown. We then discuss islands as a new promising solution to the black hole information paradox. Finally, we address the difference between intrinsically flat spacetimes and Ricci flatness.

## Introduction

This paper explores the fundamental aspects of general relativity (GR) and the intricate details of the black hole information paradox. We begin by providing a broad context for the importance of these topics in modern physics. Following this overview, the paper is structured to progressively delve into the theoretical frameworks, specific computations, and recent advancements in addressing the information paradox.

### Broader Context: Why General Relativity and the Information Paradox?

GR, proposed by Einstein in 1915, revolutionized our understanding of gravity by describing it not as a force but as the curvature of spacetime caused by mass and energy. This theory has been pivotal in explaining a wide range of astronomical phenomena and remains a cornerstone of modern physics. One of the intriguing consequences of GR is the prediction of black holesâ€”regions of spacetime with gravitational fields so intense that nothing, not even light, can escape from them.

The black hole information paradox arises from a conflict between GR and quantum mechanics. According to quantum mechanics, information about the physical state of a system cannot be destroyed. However, GR suggests that information that falls into a black hole could be lost forever when the black hole evaporates, as described by Hawking radiation. This paradox challenges our fundamental understanding of the laws of physics and has significant implications for the development of a unified theory of quantum gravity. Solving the information paradox would not only resolve a major conflict between two foundational theories of physics but also better our understanding of black hole thermodynamics, quantum field theory in curved spacetime, and the nature of spacetime itself. It could provide crucial insights into the behavior of information at the quantum level and potentially lead to breakthroughs in quantum computing and encryption.

### General Relativity

GR, contrary to the Newtonian framework it has largely replaced, posits that the presence of mass-energy changes the geometry of spacetime, and this change in geometry is what we perceive as gravity. One of the central pieces of GR is the equivalence principle, which states that the effects of gravity are locally indistinguishable from acceleration. This principle suggests that inertial motion and gravitational motion are fundamentally the same, forming the foundation of GR.

In GR, the path of a freely falling object is called a geodesic, which represents the shortest distance between two points in curved spacetime, analogous to a straight line in flat spacetime. Geodesics illustrate how objects move under the influence of gravity alone, following the curvature of spacetime. The curvature of spacetime is mathematically described by Einstein’s field equations, a set of ten interrelated differential equations that specify how the geometry of spacetime is influenced by the presence of mass and energy. These equations are central to GR, allowing us to predict phenomena such as the bending of light by gravity (gravitational lensing), the precise orbits of planets, and the expansion of the universe.

Einsteinâ€™s field equations are complex, but their essence can be understood as relating the curvature of spacetime (described by the Einstein tensor) to the energy and momentum of whatever matter and radiation are present (described by the stress-energy tensor). This relationship implies that matter tells spacetime how to curve, and curved spacetime tells matter how to move. Understanding these concepts is essential for comprehending the profound implications of GR, including the nature of black holes and the dynamics of the universe.

GR has successfully predicted numerous phenomena and has been substantiated through a range of empirical observations. One of the most notable predictions is the accurate calculations of the orbits of Pluto and Mercury. Unlike Newtonian mechanics, which struggled with precision in predicting the movements of distant celestial bodies, GR provided a more accurate model that matched observations. Shapiro et al.^{1} made relativistic corrections in 1968, which were essential in accounting for the slight deviations observed in planetsâ€™ orbits. This success highlighted the theory’s ability to explain even the subtle deviations in Plutoâ€™s and Mercuryâ€™s pathsâ€”a significant triumph for physics and physicists.

During the 1919 solar eclipse, GRâ€™s prediction of the bending of light by gravity was confirmed by Dyson et al.^{2}, as light from stars was observed to be deflected when passing near the Sun. Gravitational redshift was experimentally validated by Pound and Rebka^{3} in 1960 through measurement of the frequency shift of gamma-ray photons in Earth’s gravitational field. Ashby^{4} in 2002 wrote of adjustments needed for GPS satellites due to relativistic time dilation, both from their high velocities and weaker gravitational fields in orbit, an effect predicted by Einstein when he first proposed GR. Finally, the direct detection of gravitational waves by the LIGO and Virgo collaborations^{5} validated the prediction that accelerating masses, such as colliding black holes, produce these waves.

### Metric Tensors

The metric tensor, , is a fundamental construct in GR that quantitatively models the structure of spacetime. It encapsulates the manner in which spacetime intervalsâ€”comprising distances and anglesâ€”are influenced by the presence of mass and energy. This tensorial representation serves as the mathematical framework that correlates the curvature of spacetime with the gravitational influence exerted by objects within it.

The metric tensor yields insights into the gravitational fields surrounding an isolated, non-rotating, and spherically symmetric body. This particular solution to Einsteinâ€™s field equations characterizes the gravitational field external to such a mass distribution, frequently typified by bodies such as inert stars or black holes. The Schwarzschild metric is the spherically symmetric black hole vacuum solution of Einsteinâ€™s field equations and takes the form^{6}

(1)

where ( f(r) ) is the blackening factor given by

(2)

is the Schwarzschild radius in full units, is the differential time element, is the differential radial coordinate, and is the square of the radial coordinate times the differential solid angle.

### Background: The Black Hole Information Paradox

Recent breakthroughs in theoretical physics have brought scientists closer to solving the black hole information paradox. These breakthroughs suggest that information does not vanish inside a black hole; instead, it escapes over time. These findings, inspired by string theory but standing independently, propose that information escapes through gravityâ€™s ordinary operation combined with quantum influences. This challenges previous assumptions and suggests that black holes do not destroy information, as they were once thought to. While this progress is significant, it hasnâ€™t yet provided a complete solution, leaving questions about the broader context of quantum gravity theory and the specifics of information escape still open.

The black hole information loss paradox may be closer to resolution. This paradox involves the apparent disappearance of information inside black holes. Recent research on “entanglement islands” suggests that information might not be lost within black holes. However, some gaps in this solution remain. Another theory, “holography of information,” argues that the paradox can be resolved by taking gravity more seriously. It proposes that all quantum information is recorded at the boundary of the universe. Debate continues between these two approaches.

Black holes vary in size and were first theorized by Schwarzschild alongside Einsteinâ€™s relativity. Hawkingâ€™s discovery of black hole evaporation implies their eventual disappearance. Einsteinâ€™s theory of relativity causes time and space warping near black holes. The event horizon marks the boundary where nothing escapes their gravitational pull. Black holes are detected indirectly, impact nearby objects, and efficiently convert mass into energy through blackbody radiation. The existence of wormholes within black holes remains a debated topic in the scientific community.

**Complementarity**

Black hole complementarity is a theoretical framework proposed by Leonard Susskind in 1993^{7}. It suggests that different observers, such as an observer falling into a black hole and an external observer, can have differing descriptions of the same event due to the effects of GR and quantum mechanics. Essentially, it implies that information falling into a black hole is not lost from the perspective of an outside observer. Instead, it is encoded on the event horizon and eventually released as Hawking radiation.

**Firewalls**

Firewalls emerged as a potential solution due to work by Almheiri et al.^{8} Firewalls propose that the region near the event horizon of a black hole is not the smooth, empty space suggested by classical GR. Instead, it is a highly energetic and chaotic region filled with extremely high-energy particles and radiation. This idea challenges the notion of black hole complementarity, suggesting that information falling into a black hole is immediately incinerated upon crossing the event horizon, leading to a paradox known as the black hole firewall paradox.

**The Death of the Theories**

Both black hole complementarity and firewalls emerged as potential solutions to the information paradox associated with black holes. The information paradox arises from the apparent conflict between the principles of quantum mechanics, which imply that information cannot be lost, and the classical theory of black holes, which suggests that information is lost beyond the event horizon. However, neither black hole complementarity nor firewalls have become widely accepted as definitive solutions for several reasons. The first is the lack of resolution provided by the theories. Neither concept provides a clear and indisputable solution to the information paradox. They offer different perspectives on the problem but do not fully reconcile the conflict between quantum mechanics and GR as discussed by Marolf and Polchinski^{9} in 2013.The second is the challenge of proving the theories. Both ideas involve complex theoretical constructs and assumptions, making them difficult to test or verify through empirical observations. This lack of empirical support has led many physicists to view them as unproven conjectures rather than established solutions, as noted by Polchinski^{10} in 2016 and Harlow^{11} in 2016. The third is the emergence of new theories and the procurement of new data. Over time, researchers have developed alternative approaches to addressing the information paradox, such as the “” conjecture introduced by Maldacena and Susskind11 in 2013, which suggests that wormholes (Einstein-Rosen bridges) could play a role in preserving information. These newer ideas have led to a shift in focus away from black hole complementarity and firewalls. Fourth, and finally, is their incompatibility with quantum mechanics. Firewalls, in particular, have faced criticism for their apparent violation of fundamental principles of quantum mechanics, such as unitarity and the equivalence principle. These violations have raised concerns about their theoretical consistency, as discussed by Almheiri et al.^{8} in 2013 and Marolf and Polchinski^{9} in 2013.

**Islands**

Islands have emerged in place of other theories that have failed, such as those discussed in section 1.3, as a promising solution to the information paradox. Papers like that by Hashimoto et al.^{12} suggest that, for Schwarzschild black holes, the island solution fits the Page curve well. Such a finding suggests that black hole islands may be a helpful theory for describing Schwarzschild black holes and thus may be a potential solution to the black hole information paradox.

According to the island proposal^{13},

(3)

where denotes the entanglement entropy of the black hole, indicates the entropy of the island region, and indicates the entropy of the rest of the universe.

The entropy of a defined region, in a pure state, equals the entropy of the rest of the universe (the complementary region), so one can simplify (1.3) and consider only the entropy of the boundary region. The sum of the entropy from the matter fields and the entangling region is the generalized entropy. If one were to set , then the generalized entropy can be written as

(4)

where represents the area contribution and the matter contribution. Therefore, the entanglement entropy can be represented by the generalized entropy using the equation

(5)

To determine the location of , one must look for the value for which minimizes â€“ that is, the minimum of the extrema. One can do so by finding the derivative of (1.4):

(6)

From this result, one can see that, logically,

(7)

In this section, we show that in the case of a universe with (2+1) dimensions: the island region is defined as spanning from , which represents the singularity of the black hole, to some point . We then show that the boundary region can be represented by the space from to . When one chooses the location of the point , the island rule will determine the location of based on where is placed.

**Outline of the Remainder of the Paper**

In section 2, we detail symbolic computations, including the calculation of Christoffel symbols, the Riemann curvature tensor, the Ricci tensor, and the Ricci scalar for the Schwarzschild metric. In section 3, we provide a brief summary of the paper and a discussion on the differences between intrinsically flat spacetimes and Ricci flatness, and their implications in the context of GR and black hole physics.

## Computing the Einstein tensor of the Schwarzschild metric

The Schwarzschild metric describes the geometry of spacetime around a non-rotating, spherically symmetric mass. It characterizes how the presence of mass warps the fabric of spacetime, influencing the motion of objects in its vicinity. The metric is defined by a set of equations that govern the structure of spacetime, incorporating a radial coordinate to measure the distance from the central mass and a time coordinate. The metric predicts the existence of black holes through solutions that indicate a point of such strong gravity that not even light can escapeâ€“the event horizon of a black hole. The Schwarzschild metric provides a mathematical framework for understanding the gravitational effects of massive objects on the structure of spacetime.

Understanding the geometric properties of spacetime around black holes is essential when discussing the black hole information paradox and the island theory. Not only are the computations of the Christoffel symbols, the Riemann curvature tensor, the Ricci tensor, and the Ricci scalar mathematical exercises, but they are also crucial to determining how GR defines the curvature of spacetime.

Semi-classical physics describes islands, within which quantum fields are placed on a curved background. Islands are central to recent developments in solving the black hole information paradox. Solutions to Einstein’s field equations, such as the Schwarzschild metric, provide this curved background. Solving these equations in a vacuum around a spherically symmetric non-rotating mass yields the Schwarzschild metric, which is central to explaining the structure of spacetime around black holes.

We obtain insights into the effects of mass and energy on spacetime curvature by computing the Christoffel symbols, Riemann curvature tensor, Ricci tensor, and Ricci scalar for the Schwarzschild metric. The geometric properties of black holes and their surrounding regionsâ€”including the “island” regions that are relevant to the black hole information paradoxâ€”are better interpreted in terms of these computations. Therefore, these quantities assume a significant role in bridging the gap between the quantum mechanical aspects of the island theory and the geometric aspects of GR.

### Computing the Christoffel symbols

The Christoffel symbol is given by

(8)

While , , and are accounted for in the indices attached to the Christoffel symbol itself, appears to be alone on the right side of (2.1). This is because is a dummy variable over which we should sum, as there exists an implicit sum, as per the Einstein summation convention, which states that any repeated index is summed over. Such an operation can be simplified, however, by understanding that the metric is diagonal, and thus . For the Christoffel symbol in (2+1) dimensions, there exists a total of five non-zero components. The Christoffel symbols that correspond to such non-zero components are: , , , , and .

To calculate the component, the indices must first be substituted into their corresponding locations in (2.1):

(9)

Since the metric is symmetric, the indices in the metric, as well as the lower indices in the Christoffel symbols, are interchangeable, in that . Thus, the equation can be simplified before anything is substituted in:

(10)

Using the metric, and , and thus . Substituting these values,

(11)

Simplifying once again,

(12)

If one were to follow the above steps for the other four non-zero Christoffel symbols in (2+1) dimensions, one finds

(13)

(14)

(15)

(16)

### Computing the Riemann curvature tensor

The Riemann curvature tensor is given by

(17)

where ( k ) is a dummy variable which we again should sum over. There exist six non-zero components of the Riemann curvature tensor in (2+1) dimensions. These six components are: , , , , , and .

To calculate the component, the indices must first be substituted into their corresponding locations in (2.10):

(18)

In (2+1) dimensions, Christoffel symbols will never exhibit dependence. Therefore, the equation can be simplified:

(19)

Because is a dummy variable that we sum over, we can simply look to see which values of will cause the Christoffel symbols, and thus their products, to be non-zero. For , the first product is non-zero. For , the second product is non-zero. Substituting into the equation the non-zero values for the Christoffel symbols with these values of ,

(20)

. Simplifying, we obtain

(21)

If one were to follow the above steps for the other five non-zero Riemann curvature tensors in (2+1) dimensions, one finds

(22)

(23)

(24)

(25)

(26)

### Computing the Ricci tensor

The Ricci tensor is given by

(27)

where and are dummy variables which we again should sum over. There exist three non-zero components of the Ricci tensor in (2+1) dimensions. These three components are: , , and .

To calculate the component, the indices must first be substituted into their corresponding locations in (2.20):

(28)

Because and are dummy variables that we sum over, we can simply look to see which values of and will cause the Christoffel symbols, and thus their products, to be non-zero. If one were to follow the substitutions and simplifications for the three non-zero Ricci tensors in (2+1) dimensions, one finds

(29)

(30)

(31)

### Computing the Ricci scalar

The Ricci scalar is given by

(32)

For the three-dimensional Schwarzschild metric we get

(33)

Substituting in the metric components and the Ricci tensor components (2.22)-(2.24) we get

(34)

Finally, substituting in the blackening factor (1.2) gives

(35)

Notice that the Ricci scalar is zero for this Schwarzschild metric, as expected.

## Discussion

In this paper, we started with a Schwarzschild metric in (2+1) dimensions, and we computed the Christoffel symbols, the Riemann curvature tensor, the Ricci tensor, and finally the Ricci scalar.

For the Schwarzschild metric, the Ricci scalar vanishes. This does, of course, not mean that spacetime is flat. Intrinsically flat spacetime refers to a scenario where the curvature is entirely due to coordinate choices and not an inherent property of spacetime itself. This implies that parallel lines remain parallel and do not converge or diverge, akin to the geometric principles of Euclidean space. An example of intrinsically flat spacetime is Minkowski spacetime in special relativity. Ricci flat spacetime is a more specific condition related to the Ricci curvature tensor in Riemannian geometry. A spacetime is Ricci flat if the Ricci curvature tensor is zero , indicating the absence of certain gravitational effects. Confusion often arises between intrinsically flat and Ricci-flat spacetimes. The former emphasizes coordinate-independent geometric flatness, while the latter specifically addresses the absence of gravitational effects described by the Ricci curvature tensor. While intrinsically flat spacetime is a global geometric property, Ricci flatness has a local interpretation, focusing on specific regions where the gravitational effects are null.

## References

- I. Shapiro, G. Pettengill, M. Ash, M. Stone, W. Smith, R. Ingalls, R. Brockelman, Fourth test of general relativity: preliminary results,
*Physical Review Letters***21**, 1265-1269 (1968). [↩] - F. Dyson, A. Eddington, C. Davidson, A determination of the deflection of light by the sun’s gravitational field, from observations made at the total eclipse of may 29, 1919,
*Philosophical Transactions of the Royal Society of London A***220**, 291-333 (1920). [↩] - R. Pound, G. Rebka, Apparent weight of photons,
*Physical Review Letters***4**, 337-341 (1960). [↩] - N. Ashby, Relativity and the global positioning system,
*Physics Today***55**(5), 41-47 (2002). [↩] - B. Abbott, et al. (LIGO Scientific Collaboration and Virgo Collaboration), Observation of gravitational waves from a binary black hole merger,
*Physical Review Letters***116**, 1-16 (2016). [↩] - This metric is in natural units, i.e. . [↩]
- L. Susskind, L. Thorlacius, J. Uglum, The stretched horizon and black hole complementarity,
*Physical Review D***48**, 3743â€“3761 (1993). [↩] - A. Almheiri, D. Marolf, J. Polchinski, J. Sully, Black holes: complementarity or firewalls?,
*Journal of High Energy Physics*(2013). [↩] [↩] - D. Marolf, J. Polchinski, Gauge/gravity duality and the black hole interior, 2013. [↩] [↩]
- J. Polchinski, The black hole information problem, 2016. [↩]
- D. Harlow, Jerusalem lectures on black holes and quantum information,
*Reviews of Modern Physics***88**(1), 2016. [↩] - K. Hashimoto, N. Iizuka, Y. Matsuo, Islands in schwarzschild black holes,
*Journal of High Energy Physics*(2020). [↩] - A. Almheiri, R. Mahajan, J. Maldacena, Islands outside the horizon, 2023. [↩]