The discussion centers around whether light from a lensed object takes longer to reach observers compared to light from an unlensed object at the same distance, particularly focusing on the implications of gravitational lensing and the Shapiro delay effect.
Participants generally agree that light from a lensed object takes longer to reach us, but there is disagreement regarding the primary cause of this delay, with some emphasizing the curved path and others focusing on the Shapiro delay effect.
The discussion includes assumptions about gravitational potentials and the nature of light propagation in these contexts, which may not be fully resolved.
scupydog said:Hi. Does the light from a lensed object take longer to reach us, than the light from an unlensed object at the same distance.
I mean because the light from a lensed object has a curved path.
Thx. Dave...
Chronos said:Correct. For discussion see
http://arxiv.org/abs/astro-ph/9606001
Lectures on Gravitational Lensing
authors: Narayan, Bartelmann
Jonathan Scott said:Yes, it takes longer to reach us but this is normally mainly because of the Shapiro delay effect rather than simply the curved path. Basically, light slows down slightly in a lower gravitational potential, closer to a massive object or system, as if it were in a medium with a refractive index slightly greater than 1. The amount of deflection relates to the gravitational field (how rapidly that potential changes with sideways displacement along the light path) but the amount of time delay mainly relates to the potential along the path.