Reconsidering maximum luminosity
Abstract
The suggestion that there is a maximum luminosity (maximum power) in nature has a long and somewhat convoluted history. Though this idea is commonly attributed to Freeman Dyson, he was actually much more circumspect in his views. What is certainly true is that dimensional analysis shows that the speed of light and Newton’s constant of gravitation can be combined to define a quantity P∗=c5GNP∗=c5GN with the dimensions of luminosity (equivalently, power). Then in any physical situation, we must have Pphysical=℘P∗Pphysical=℘P∗, where the quantity ℘℘ is some dimensionless function of dimensionless parameters. This led some authors to suggest a maximum luminosity/maximum power conjecture. Working within the framework of standard general relativity, we will re-assess this conjecture, paying particular attention to the extent to which various examples and counter-examples are physically reasonable. We focus specifically on Vaidya spacetimes, and on an evaporating version of Schwarzschild’s constant density star. For both of these spacetimes, luminosity can be arbitrarily large. We argue that any luminosity bound must depend on delicate internal features of the radiating object.
This essay received an Honorable Mention in the 2021 Essay Competition of the Gravity Research Foundation.
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