Could scientists create an expanding black hole?

Albert Einstein and the black holes

In his article from that year, Einstein attributed his renewed concerns about the Schwarzschild radius to discussions with the cosmologist Harold P. Robertson at Princeton University, New Jersey, and his assistant Peter G. Bergmann, now professor emeritus at Syracuse University (US state of New York) is. Einstein undoubtedly wanted to eradicate the Schwarzschild singularity once and for all. At the end of the article he wrote: "The main result of this investigation is a clear understanding why 'Schwarzschild singularities' do not exist in physical reality." That meant nothing other than that there could be no black holes.

As a justification, Einstein considered an ensemble of small particles that move on circular orbits under the influence of their mutual gravity - a kind of rotating globular star cluster model. Now he asked whether such a configuration could collapse into a stable star the size of the Schwarzschild radius due to its own gravity. He denied this, because the particles of the cluster would have to move faster than light, even with a slightly larger radius, in order to maintain the configuration.

Although Einstein's reasoning is coherent, his conclusion misses the topic: The fact that a collapsing star is unstable at the Schwarzschild radius does not mean anything, since the star - precisely because it continues to collapse - falls below the radius anyway. Incidentally, I was very touched by the fact that Einstein, who was then sixty at the time, gave tables in his article with numerical results that he must have obtained with a slide rule. Today the article is as obsolete as this instrument.

From neutrons to black holes

While Einstein at the Institute for Advanced Study in Princeton (New Jersey) argued in vain against the Schwarzschild singularity, Oppenheimer and his students created the modern theory of black holes in California (see "The Physicist J. Robert Oppenheimer" by John S. Rigden, Spectrum of Science, October 1995, page 44). Strangely enough, it starts with a completely wrong idea.

The British experimental physicist James Chadwick (1891 to 1974; Nobel Prize 1935) discovered the neutron in 1932, the electrically neutral building block of the atomic nucleus. Soon afterwards, the Swiss astrophysicist Fritz Zwicky (1898 to 1974) at the California Institute of Technology in Pasadena and, independently of him, the Soviet theorist Lew D. Landau (1908 to 1968; Nobel Prize 1962) began to consider whether neutrons might be another final stage of stellar evolution than in the form of white dwarfs. If the gravitational pressure is high enough, they argue, an electron could fuse with a proton to form a neutron inside the star. (Zwicky even correctly suspected that this process takes place in supernova explosions; such neutron stars have since been identified as pulsars.) At that time, the actual mechanism by which stars generate their energy was not yet known. (Although Hans Bethe and Carl Friedrich von Weizs├Ącker identified nuclear fusion as a stellar energy source in 1938, it took a few years before the new explanation was generally accepted.) One hypothesis was that a neutron star always sits in the center of normal stars - similar to what is assumed today that black holes provide the energy of the quasars.

This raised the question of what should correspond to the Chandrasekhar limit for such stars. This is much more difficult to answer than in the case of the white dwarfs, because neutrons interact with one another via the strong nuclear force, the nature of which is not yet fully understood even today. The mass limit at which gravity exceeds this force depends heavily on subtleties. Oppenheimer and his students Robert Serber and George M. Volkoff published two papers on this subject in 1938 and 1939, in which they came to the conclusion that the new mass limit was comparable to the Chandrasekhar mass for white dwarfs.

Oppenheimer now made exactly the same leap in thought as Eddington did earlier: What would happen if a star collapsed with a mass beyond both limits? Einstein's rejection of black holes, written in 1939, certainly didn't play a role, because the team in Pasadena was working at the same time and nearly 3,000 miles from Princeton.

In any case, Oppenheimer did not want to construct a stable star the size of the Schwarzschild radius, but wanted to find out what happens when the star is allowed to collapse through the Schwarzschild radius. He suggested to Snyder to work on the problem in detail, but - to simplify it - ignore the degeneracy pressure and the possible rotation of the star. Oppenheimer intuitively foresaw that these factors would not be decisive. (Many years later, a new generation of researchers challenged his assumptions using powerful computers; Snyder had only an old-fashioned mechanical calculating machine. But Oppenheimer was ultimately correct: his simplifications make no essential difference.) In this way, Snyder discovered that the description of a collapsing star depends crucially on the point of view of the observer.

Two views of a breakdown

Let us imagine a resting observer who is at a safe distance from the star. Let us also assume that there is a second observer on the star's surface who is carried away by the collapse and meanwhile is constantly sending light signals to his stationary colleague. He now realizes that the signals of the death-defying experimenter are shifting more and more to the red end of the spectrum. Since he interprets the signal frequency as a time measurement, he will come to the conclusion that the watch of the observer who is carried away by the collapse is going slower and slower.

The clock would even stop right at the Schwarzschild radius. The stationary observer must conclude from this that it takes an infinitely long time until the star shrinks to its Schwarzschild radius. What happens after that cannot be said because from the point of view of the stationary observer there is no after; for him, the collapse at the Schwarzschild radius is frozen, so to speak.

Accordingly, until December 1967, when physicist John A. Wheeler, who is now at Princeton University, coined the name black hole in a lecture, such objects were often called frozen stars in the professional world. This freezing is what the Schwarzschild singularity really means. As Oppenheimer and Snyder noted in their work, "[the collapsing star] seeks to shut itself off from any communication with a distant observer; only its gravitational field remains". But that means that a black hole has emerged.

But what will become of the observer on the star's surface who immediately suffers the collapse? According to Oppenheimer and Snyder, he experiences something completely different. For him, the Schwarzschild radius has no special meaning: he simply crosses it and, as a watch would indicate, moves towards the center of the star within a few hours. However, on the way he becomes the victim of gigantic tidal forces, which inevitably tear him and his watch apart.

In that year 1939, however, the Second World War broke out, the most devastating event on earth, and overshadowed all speculations about exotic singularities of space-time. Oppenheimer soon went into military research and built the most destructive weapon humans had ever devised. He never researched black holes again - and as far as I know, that also applies to Einstein.

After the war, in 1947, Oppenheimer became director of the Institute for Advanced Study at Princeton, where Einstein was still a professor. They talked occasionally; but we do not know if they ever discussed cosmic maelstrom. New advances in this area were not made until the 1960s, when people began to think again about the mysterious fate of the stars with the discovery of quasars, pulsars and compact X-ray sources.