Oct 072013


Physicists have long thought that the singularities associated with gravity (like the inside of a black hole) should vanish in a quantum theory of gravity.

It now appears that this may indeed be the case. Researchers in Uruguay and Louisiana have just published a description of a quantum black hole using loop quantum gravity in which the predictions of physics-ending singularities vanish, and are replaced by bridges to another universe.

Singularities, such as the infinitely strong crushing forces at the center of a black hole, in a physical theory are bad. What they tell you is that your description of the universe fails miserably to explain what happens as you approach the singularity. Tricks can sometimes resolve what appears to be singular behavior, but essential singularities are signs of a failure of the physical description itself.

General relativity has been summed up by the late John Wheeler’s phrase: “Spacetime tells matter how to move, matter tells spacetime how to curve.” Relativity is riddled with essential singularities, because gravity is both attractive and nonlinear – curvature in the presence of mass tends to lead to more curvature, eventually leading to trouble.

The result is rather similar to a PA system on the verge of producing a feedback whistle. If you whisper into the microphone (small gravitational fields) the positive feedback isn’t enough to send the PA into oscillation, but talking at a normal volume (larger gravitational fields) produces that horrible howl. Whispering is the comparable to the familiar actions of gravity that keep the planets and stars in their courses. The howl is the process that eventually leads to a singularity as the end result of gravitational collapse.

Let’s follow this analogy a bit further. On a PA system, the volume of the feedback is limited by the power capacity of the amplifier, so it can’t reach truly destructive levels (other than to our eardrums.) However, gravity as described by general relativity doesn’t have such a limit. Since gravity is always attractive, and eventually becomes stronger than all the (known) forces that normally give volume to matter, there is nothing to keep gravitational collapse from proceeding until the curvature of the spacetime tends toward infinity – i.e. a singularity.

Remember that this is the prediction of the classical theory of gravity, general relativity. Classical physical theories contain no fundamental limitation on mass-energy density or on the size of spacetime curvature. While this may be (and probably is) incorrect, we rarely run into a problem caused by this error, so have largely ignored the problem for centuries.

Then along came gravitational collapse and black holes. First proposed by geologist John Mitchell in 1783, a black hole is a region of spacetime from which gravity prevents anything, even light, from escaping.

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