Reconsidering maximum luminosity

TL;DR

This paper reevaluates the idea of a universal maximum luminosity in nature, analyzing specific spacetime models within general relativity to show that luminosity can be arbitrarily large, challenging the conjecture of a fixed maximum power.

Contribution

It critically examines the maximum luminosity conjecture using detailed models in general relativity, demonstrating that luminosity bounds depend on internal object features.

Findings

Luminosity can be arbitrarily large in Vaidya spacetimes.

An evaporating star model shows no fixed luminosity limit.

Luminosity bounds are sensitive to internal structure details.

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_* = c^5/G_N with the dimensions of luminosity (equivalently, power). Then in any physical situation we must have P_{physical} =$\wp$ P_*, where the quantity $\wp$ is some dimensionless function of dimensionless parameters. This has lead 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.