Optics of the Event Horizon Telescope

Gravitational bending of light rays near objects of size approaching 2MG/c2 (gravitational radius or ‘event horizon’) is known to be severe.  Light rays circle indefinitely at 3MG/c2  as Charles Darwin (the grandson) described in 1958.  Gravitational lensing causes more distant objects to appear as arcs or multiple images.  As collapsing stars take infinite time to reach 2MG/c2, Trevor Marshall considered light paths (geodesics) from objects sized 2-3MG/c2, as a spherical collapsar or supermassive galactic centre.   The figure indicates how a distant observer sees the rear side of the collapsar, with the full image appearing to have a size 2.6 times larger.

From Marshall (2016) showing light rays from an object slightly larger than (1.1 times) the gravitational radius or ‘event horizon’. Rays from the front make it appear 1.5 times larger, but there are also rays from the rear giving a less bright halo.

Light reaches the exterior from a surface light cone greater than 68.3o (90o in the 3MG/c2 limit), not an infinitesimal light cone as generally assumed. Scattered reflected light or thermal emission from the collapsar would be detectable.  Unlike from a hypothetical ‘black hole’, Marshall pointed out, the collapsar is not absolutely ‘black’, though it blanks out background radiation.  The M87 galactic centre has a bright accretion disc, giving greater problems in detecting the interior object; our galactic centre (Sgr A*) has only a very weak disc so processing to reveal a non-black centre is more feasible.

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1 Response to Optics of the Event Horizon Telescope

  1. sobercitizen says:

    The M87 observation needs to be put in perspective. The central dark sphere has a radius of 1.8×10^13 metres and a mass of 1.2×10^40 kg, so its average density is a very moderate 0.5 kg/m^3, which may be compared with about 5000 for our planet Earth and about 150 for the Sun. No observational evidence exists for the alleged black-hole singularity at its centre, and indeed we (Trevor Marshall and Max Wallis) made a plausible calculation indicating that most of the mass is concentrated near its surface (ResearchGate entry, https://tinyurl.com/Super-neutron-star-mergers). The ray tracing of the accretion disk is, of course, independent of the interior mass distribution.

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