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Hawking radiation and black hole evaporation

Hawking radiation is named after physicist Stephen Hawking who in 1974 provided a theoretical argument for the existence of thermal radiation emitted by black holes. The existence of Hawking radiation, now commonly accepted among physicists, presents a very significant logical problem for those who additionally believe that gravity affects time and that light and matter can pass through the event horizon of a black hole. Specifically, in addition to completely evaporating a black hole before light or matter could reach the black hole’s event horizon, Hawking radiation, intensified by the acceleration of time, will destructively irradiate anything drawing near to the event horizon.

To understand the logical implications of Hawking radiation, consider again the adventures of Bob and Betty, introduced in the paper The time barrier that prevents formation of black holes. As set out in that paper, Bob and Betty had different perceptions of time as Bob approached a very highly compacted mass whose escape velocity measured from the surface was just under the speed of light. From the perspective of Betty, Bob’s journey took so long that, as a result of the second law of thermodynamics, he disintegrated before reaching the surface. From Bob’s perspective the passage of time was accelerated as he approached the compacted mass so that the journey was over in a flash as he instantly disintegrated upon drawing near to the surface. Let’s imagine this time that Betty observes and records on her video camera Bob’s descent as he attempts to penetrate the event horizon of a black hole.

As Bob gets close to the event horizon, Betty notes that his progress seems to slow down. Finally just before he arrives, it seems like he stops entirely. This happens because of the effect gravity has on the passage of time, making the clock Bob uses to measure time run at a different rate than the clock Betty uses to measure time.

As Betty keeps her vigil and diligently records Bob’s progress, she notices the slow leakage of Hawking radiation from the black hole. Some of this radiation*, emitted through the event horizon, passes by Bob and thus keeps Betty apprised of Bob’s position. Occasionally the radiation from the black hole impacts Bob, but from Betty’s perspective the rate of radiation emission is so slow it has almost no affect on Bob. Betty is very tenacious and her video camera has a lot of memory so she keeps recording until Hawking radiation completely evaporates the black hole.

As was the case in their previous adventure, Betty views Bob’s journey from Bob's perspective of time by playing her recording of  Bob’s journey at an increasingly faster speed. When Betty speeds up playback, she discovers that what appeared to her to be very slow evaporation of a black hole, happens much faster as the recording plays back at faster speeds. When she speeds up the recording so much that the evaporation of the black hole happens almost instantaneously she begins to understand what Bob experienced as he drew near to the event horizon where the passage of time accellerates almost ifinitely. That is, from the perspective of time at the event horizon, all the matter within the black hole instantaneously evaporated in a blazing inferno of unimaginable intensity. As Bob drew near the event horizon, he was fried!

As Bob was hurtling towards his demise, the Hawking radiation he encountered along the way provided a way for him to measure the acceleration of time he experienced on the journey. When he started from Betty’s location, the Hawking radiation was a mere trickle. However, as he approached the event horizon, the rate the black hole evaporated increased with a resulting higher density of Hawking radiation, indicating an acceleration in the passage of time. Very near the event horizon, the density of radiation approached infinity corresponding to the almost infinite acceleration of the passage of time.

Betty’s ability to observe Hawking radiation pass by Bob before it reached her, confirms that it is logically impossible for Bob to slip past the event horizon before the black hole evaporates. As the black hole evaporated, the Hawking radiation carried information from Bob to Betty indicating to Betty that Bob was still outside the black hole when the radiation passed by Bob. The Hawking radiation will continue to carry this information about Bob’s location until the radiation annihilates Bob, or if Bob were somehow hardy enough to withstand the bombardment of radiation, until the black hole completely evaporates.

For a more rigorous consideration of the subject matter see Five fallacies used to link black holes to Einstein’s relativistic space-time

Copyrighted Article--by Doug Weller--used with permission

*As Bob draws very near to the event horizon, much of the Hawking radiation may first appear at a radial location outside Bob, thereby not providing information to Betty about Bob's position.

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