Undestanding the true nature of black holes

Posted on January 25, 2024 • 8 minutes • 1681 words • Other languages: Русский

Was J. Robert Oppenheimer a victim of a black hole’s allure? Recent cosmology publications might lead one to think so, given their omission of his name in their indexes. These books extensively explore the intricate theories and math of black holes but fail to acknowledge him. Nonetheless, it was Oppenheimer, the renowned leader of the Los Alamos group responsible for the Hiroshima and Nagasaki atomic bombings, who initially proposed the concept of these peculiar cosmic phenomena as a direct consequence of Einstein’s theory of relativity. Towards 1938’s close, in collaboration with George Volkoff, Oppenheimer deduced the sizes and masses of neutron stars. This research led him to believe in the inevitable collapse of massive stars at their life’s end. He pondered over the aftermath of such a collapse.

Oppenheimer recruited a talented and independent-thinking student from the California Institute of Technology, Hartland Snyder, to assist with the complex mathematical equations. Kip Thorne, a leading black hole expert, detailed Oppenheimer’s contributions in his 1994 book “Black Holes and Time Warps,” despite being a protégé of Oppenheimer’s rival, John A. Wheeler. Thorne acknowledged the challenging calculations Snyder undertook under the supervision of both Oppenheimer and Richard Tolman. These problems remained unsolved until supercomputers emerged in the 1980s. Thorne noted the necessity of creating a simplified model of the imploding star to make any headway and apply physical laws. Snyder impressively formulated and resolved these equations. Through these formulas, physicists could understand various perspectives of the implosion, whether from outside, inside, or the star’s surface.

Many in the physics community struggled to grasp the equations' implications. From an external viewpoint, the implosion appeared eternally paused, yet to an observer on the star, this halt was unnoticeable. This dual perception suggested an unprecedented distortion of time, far beyond Einstein’s time warping theory or the observer effect in quantum mechanics and Heisenberg’s uncertainty principle. While these concepts were accepted at the subatomic level, applying them on a larger scale was too extreme for most American physicists.

The 1939 paper by Oppenheimer and Snyder was not the first to broach its subject. A decade earlier, Subrahmanyan Chandrasekhar, a young physicist, hypothesized that stars with cores exceeding 1.4 times the mass of the sun couldn’t form into white dwarfs, but would instead keep collapsing under their own gravity. Around the same time, Russian physicist Lev Davidovich Landau reached a similar conclusion. Both were later awarded the 1983 Nobel Prize in Physics for their pioneering work. The substantial delay in receiving this recognition indicates the groundbreaking nature of their theories.

In 1928, Sir Arthur Eddington, renowned for validating Einstein’s space warping theory during the 1919 solar eclipse, vehemently opposed Chandrasekhar’s ideas, calling for a natural law to prevent such star behavior. Similarly, the Oppenheimer/Snyder paper initially faced skepticism from John A. Wheeler and his peers in the U.S. The onset of World War II then shifted American physicists' focus to atomic bomb development.

Post-war, tensions rose between Oppenheimer and Wheeler, both at Princeton’s Institute for Advanced Study, especially over the hydrogen bomb’s creation. Oppenheimer initially resisted it on moral and practical grounds, later accepting only the practicality. Wheeler, a key figure in the hydrogen bomb’s development, contrasted sharply with Oppenheimer’s stance. During the 1950s, amidst McCarthyism, Oppenheimer’s security clearance was controversially revoked, casting a long shadow over his career and possibly contributing to his underrepresentation in black hole discussions.

Wheeler’s eventual full embrace of black hole theory, even coining the term in 1969, overshadowed Oppenheimer’s contributions. Interestingly, “Star Trek” in 1967 used the term “black star,” nearly pre-empting Wheeler’s nomenclature, as noted by Laurence M. Krauss in “The Physics of Star Trek.”

Public fascination with black holes has been significant, possibly due to Wheeler’s evocative naming and the mysterious nature of these cosmic entities. Unlike other stellar objects, black holes captivate the public imagination, akin to the historical allure of comets. This interest might stem from the inherent challenge in comprehending black holes, rendering them a mysterious canvas for individual interpretation.

Kip Thorne, in his 1994 book, expanded on the traditional definition of black holes, describing them as entities where matter can enter but never escape. Despite his caution, Thorne’s discussions suggest even more bizarre aspects of black holes, reflecting the ongoing perplexity they pose to physicists.

Let’s consider a fundamental question: What is the size range of a black hole?

In theory, any object can transform into a black hole. This includes celestial bodies like stars and moons, man-made structures, animals, individuals, or even small objects. The key requirement is compressing the object sufficiently to intensify its gravitational pull to the extent that it warps space and traps light, resulting in a black hole. For perspective, a human-turned-black hole would be incredibly tiny, billions of times smaller than an electron. If Earth were to become a black hole, its size would shrink to smaller than a Ping-Pong ball. The Sun, as a black hole, would have a radius of approximately 2.4 kilometers (about a mile and a half).

However, realistically speaking, neither the Sun nor any of us are likely to become black holes due to insufficient mass. Certain stars, though, are massive enough to eventually transform into black holes. Timothy Ferris, in “The Whole Shebang,” elucidates that a star’s existence is a tug-of-war between gravity, which works to condense the star, and the heat from the core, which pushes outward. Stars experience fluctuations due to this interplay of opposing forces, regulated by a delicate feedback loop involving heat and gravity. This mechanism allows stars like our Sun, currently halfway through its 10 billion-year lifespan, to persist. The rate of fuel consumption in a star’s core accelerates with its mass, leading to shorter lifespans for larger stars.

When a star’s balance between heat and gravity falters, collapse becomes unavoidable. Stars up to 1.4 times the mass of our Sun end up as white dwarfs, Earth-sized but incredibly dense, limited by the Pauli exclusion principle which governs electron behavior. Larger stars undergo further collapse, often leading to neutron stars, typically less than 16.1 kilometers in diameter. These stars, packed with neutrally charged subatomic particles, can spin rapidly, up to a thousand times per second. If they possess a magnetic field, they emit strong, pulsing radio signals, earning them the name ‘pulsars.’

Larger stars, due to their immense mass, can exceed the thresholds that normally prevent further collapse in white dwarfs or neutron stars, leading to the formation of black holes. The defining trait of a black hole is its gravitational pull so strong that not even light can escape. Near a black hole, the boundary known as the event horizon marks where traditional gravitational principles are superseded by those unique to black holes. Essentially, black holes are singularities, regions where standard physical laws don’t apply. Theories about the internal mechanics of black holes vary widely, ranging from the stretching of objects into elongated forms to hypothetical travel to alternate universes. Despite numerous theories and complex equations, the true nature of what happens inside a black hole remains unknown.

The notion of black holes gained credibility after John Wheeler, a renowned physicist, endorsed the concept. This led to a surge in theoretical exploration by leading cosmologists, especially throughout the 1970s to the 1990s. Despite a plethora of theories, empirical evidence of black holes was lacking.

The challenge for astronomers is the inherent invisibility of black holes; their presence is inferred through the impact on surrounding celestial bodies. Technological advancements, particularly improvements to the Hubble Space Telescope in 1994 and the development of X-ray telescopes, have enabled more direct observations. By the late 1990s and early 2000s, accumulating evidence supported many black hole theories, leading most cosmologists to accept their existence. New discoveries often lead to as many questions as answers.

Cyg X-1, identified as a potential black hole in 1974, was part of a binary system with unique characteristics. One star was visible optically but not in X-rays, suggesting it orbited a massive, unseen companion. This companion, too dense to be a neutron star, was suspected to be a black hole. This hypothesis led to a friendly bet between Kip Thorne and Stephen Hawking. By the 1990s, accumulating evidence strengthened Thorne’s position, leading to Hawking’s playful concession.

Cyg X-1 was later confirmed as a black hole through combined optical and X-ray evidence. Additionally, in the late 1990s, observational data began to indicate two distinct types of black holes: those similar in mass to binary stars like Cyg X-1, and supermassive black holes with masses billions of times that of the sun. These supermassive black holes were frequently found at the centers of galaxies, identified by the motion of gas disks influenced by their gravitational pull.

Research indicated that larger galaxies typically housed larger black holes at their centers. Moreover, these supermassive black holes were predominantly found in elliptically shaped galaxies with a dense central cluster of stars. Conversely, galaxies lacking such a bulge seemed devoid of black holes. The Milky Way, with its modest central bulge, contains smaller black holes, each with a mass a few times that of the sun. In any case, the mass of a black hole tends to be about 0.2% of its galaxy’s central bulge’s mass.

This data has led cosmologists to theorize that black holes might be central to galaxy formation. Douglas Richstone from the University of Michigan, upon discovering three supermassive black holes, suggested in January 2000 a mutual regulation between the mass of black holes and their host galaxies. This idea of large-scale mutual influence, akin to the quantum-level interaction among electrons, both baffles and intrigues researchers. The debate over whether black holes or galaxies form first, or if their development is interdependent, remains ongoing.

In 1939, the Oppenheimer/Snyder paper proposing black holes faced skepticism from leading cosmologists. Over time, the scientific community gradually accepted the existence of black holes, a view cemented by the late 1990s with evidence from the Hubble telescope. Despite this acceptance, black holes continue to be a source of new enigmas in the cosmological field, promising both increased understanding and further complexities in unraveling the universe’s workings.