Oversimplification

For the imploding star, one feature was crucial above all others, Oppenheimer believed: gravity as described by Einstein’s general relativistic laws. It, and only it, must not be compromised when formulating a calculation that could be done. By contrast, the star’s spin and its nonspherical shape could be ignored; they might be crucially important for some imploding stars, but for stars that spin slowly, they probably would have no strong effect. 

Moreover, since (as Oppenheimer and Volkoff had shown) gravity could overwhelm all pressure in massive, dead stars, it seemed safe to pretend (incorrectly, of course) that the imploding star has no internal pressure whatsoever—neither thermal pressure, nor pressure arising from the electrons’ or neutrons’ claustrophobic degeneracy motions, nor pressure arising from the nuclear force. A real star, with its real pressure, might implode in a different manner from an idealized, pressureless star; but the differences of implosion should be only modest, not great, Oppenheimer’s intuition insisted.

Thus it was that Oppenheimer suggested to Snyder an idealized computational problem: Study, using the precise laws of general relativity, the implosion of a star that is idealized as precisely spherical, nonspinning, and nonradiating, a star with uniform density (the same near its surface as at its center) and with no internal pressure whatsoever.

Even with all these idealizations the calculation was exceedingly difficult. Fortunately, Richard Tolman was available in Pasadena for help. Leaning heavily on Tolman and Oppenheimer for advice, Snyder worked out the equations governing the entire implosion and he managed to solve them.