VISUAL DISTORTIONS NEAR A NEUTRON STAR AND BLACK HOLE

Robert J. Nemiroff
NASA / Goddard Space Flight Center
Code 668.1
Greenbelt, MD 20771

Published in The American Journal of Physics, 61, 619 (1993)

ABSTRACT

The visual distortion effects visible to an observer traveling around and descending to the surface of an extremely compact star are described. Specifically, trips to a "normal" neutron star with relatively weak surface gravity, an ultracompact neutron star with extremely high surface gravity, and a black hole are described. Concepts such as multiple imaging, red- and blue-shifting, conservation of surface brightness, the photon sphere, and the existence of multiple Einstein rings are discussed in terms of what the viewer would see. Computer generated, general relativistically correct illustrations highlighting the distortion effects are presented and discussed. A short movie (VHS) depicting many of these effects is available to those interested free of charge.

I. BACKGROUND

It is impossible for a human to travel very near a high gravity star which has a mass like that of the Sun. If, somehow, a person could survive the extremely harmful radiation that would be emitted on or near these objects, the high gravity itself would likely pose insurmountable problems. The person could not stand casually on the surface of such a star because the high surface gravity would tend to flatten them. (Lying down wouldn't help.) Were a person to orbit the star in a spaceship, however, the immense gravitational field would be overcome by a large outward centrifugal acceleration.[1] The problem in this case, however, is the extreme change in gravity between the head and toe of the person, the extreme tidal pull, would surely prove much more than annoying for any human[2].

Nevertheless it is informative and interesting to wonder what it would look like to visit such a high gravity environment. Significant speculations on this include popular science fiction stories such as those by Forward [3] and Niven [4]. A discussion (with cartoon drawings) of a trip to a black hole appears in Kaufmann's book ``The Cosmic Frontiers of General Relativity" [2]. A description of what hot spots on a high gravity neutron star would look like to an observer far away is given by Ftaclas, Kearney, and Pechenick, [5]. Other descriptions include what a typical neutron star would look like to a distant observer including a computer drawn wire mesh diagram [6], a description of the sky as seen from the vicinity of a black hole [7-9], a description of the image of a thin accretion disk around a black hole [10], a description of how the observer would see self-images near a black hole [11], and a short computer animated movie simulating a trip around a black hole while facing the constellation Orion by Palmer and Unruh [12]. In general, however, the professional science literature has focused mainly on mathematical detail rather than observable image distortions.

In this paper the visual aspects of a journey to several different types of high gravity stars will be discussed in some detail, along with computer generated illustrations highlighting the perceived visual distortions. The three types of stars that will be discussed are a) a "normal" neutron star having relatively weak surface gravity, b) a black hole, and c) an "ultracompact" neutron star [13] having relatively strong surface gravity. Here the speed of the traveler will always be considered small when compared to the local speed of light, so special relativistic effects will be ignored.

The paper is structured as follows: Section II discusses the physical principles and mathematics necessary to describe the perceived visual distortions. In Section III the types of visual distortions will be discussed generally. Section IV then proceeds to take the reader on a fantasy mission to these high gravity environments and describes what visual distortion effects the viewer would see. In Section V comments are made.

References
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