Gravitational Lensing
Purpose
Marbles rolling across a stretched rubber sheet demonstrate how massive objects curve spacetime and deflect light paths. Students observe that deeper curvature (heavier central mass) produces stronger deflection, and that marble paths can cross on the far side of the mass — analogous to the multiple images seen in strong gravitational lensing.
Figure 1:
A glass marble at the center of the stretched sheet creates a depression that deflects rolling marbles, analogous to gravitational lensing by a massive object.
Figure 2:
Without a central mass, marbles roll on approximately straight paths across the flat sheet (some curvature is inevitable from imperfect tensioning).
Demo Apparatus
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