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2 results

  • 3 - Spacetime, Curvature and Gravity

  • Luciano Rezzolla, Goethe-Universität Frankfurt Am Main
  • Book:
    The Irresistible Attraction of Gravity
    Published online:
    02 March 2023
    Print publication:
    02 March 2023, pp 25-48

  • Summary

    The end result of Einstein’s revolutionary vision is that gravity is simply the manifestation of the curvature of spacetime. This is a concept that has a deep significance and is at the heart of the Einstein field equations. This Chapter will explain why we need to introduce the idea of 'spacetime' and how we can define the concept of spacetime curvature in this description. Starting from the example of a spacetime empty of matter – that is, a flat spacetime – we will move to the example of a spacetime containing matter and energy – that is, a curved spacetime. This chapter will explain why we find the description of gravity proposed by Newton very reasonable and why we have trouble appreciating the new vision proposed by Einstein. We will contrast the two descriptions with a simple example and show how the very same physical phenomenon – the orbit of the Earth around the Sun – can be seen with very different explanations by Newton and Einstein.

  • Sensitivity to full-field visual movement compatible with head rotation: Variations with eye-in-head position

  • Laurence R. Harris, Lori A. Lott
  • Journal:
    Visual Neuroscience / Volume 13 / Issue 2 / March 1996
    Published online by Cambridge University Press:
    02 June 2009, pp. 277-282

  • Variations in velocity detection thresholds for full-field visual rotation about various axes are compatible with a simple channel-based system for coding the axis and velocity of the rotation (Harris & Lott, 1995). The present paper looks at the frame of reference for this system. The head-centered, craniotopic reference system and the retinal-based, retinotopic reference systems were separated by using eccentric eye positions. We measured the threshold for detecting full-field visual rotation about a selection of axes in the sagittal plane with the eyes held either 22½ degs up, straight ahead or 22½ degs down in the head. The characteristic features of the variation in detection thresholds did not stay stable in craniotopic coordinates but moved with the eyes and were constant in retinotopic coordinates. This suggests that the coding of head rotation by the visual system is in retinotopic coordinates.