Stretching fabric over a frame to create a “spacetime” surface lets students see how mass curves space and guides motion. By placing heavy and light objects on the fabric and rolling marbles, the class can model orbits, accretion, tides, and even visualize gravitational waves.

1. Build the frame: assemble a circular or square PVC frame on legs tall enough for students to see underneath; tighten joints so the surface will be drum-taut.
2. Stretch the “spacetime” sheet: pull a large piece of spandex over the frame and clamp or tie it so the surface is uniformly tight.
3. Set the scale: place a heavy mass (e.g., a bowling ball) at the center to represent a star or planet and observe the curved “well.”
4. Show straight-line motion on curved space: roll a marble far from the mass; note how its path bends toward the center.
5. Demonstrate stable and unstable orbits: roll marbles tangentially at different speeds and distances; compare long-lasting orbits, spirals inward, and escape trajectories.
6. Model accretion and solar system formation: launch more marbles from different directions; discuss collisions, energy loss, and why remaining orbits tend to align.
7. Compare masses: swap the central mass for lighter or heavier objects; observe how curvature and orbital speeds change.
8. Explore eclipses and transits: roll a “moon” marble around a “planet” (smaller central mass) and view alignments as it passes in front of or behind the planet relative to an observer’s line.
9. Show tides and the Roche idea: place two small clusters of beads connected by a light string near the central mass; watch how differential pull stretches and can break them apart.

Spandex Gravity Well (+instructions to make your own!) - UToledoPhysAstro:

Gravity Visualized - apbiolghs:

📄 Gravity Visualized - The Physics Well: https://the-physics-well.net/gravity-visualized/

• Let every student launch marbles simultaneously, using two colors in opposite directions to dramatize collisions and net rotation.
• Project a grid (overhead or taped lines) on the fabric to make curvature and geodesics easier to discuss and measure.
• Use different fabrics (thicker or thinner spandex) to compare how “stiffness” affects curvature and orbital longevity.
• Add a ring of low walls around the rim or a soft catch net to keep marbles from rolling away during public shows.

• Keep fingers clear of pinch points when assembling PVC and stretching fabric.
• Place a soft mat under the frame and use a rim or catch net to prevent marbles from becoming floor hazards.
• Use eye protection when operating power tools (e.g., drill for wave demo) and keep long hair and cords clear of moving parts.
• Ensure the frame is stable and cannot tip when heavy masses are added; do not overload the fabric beyond its rated stretch.
• Supervise younger students closely; only the presenter should place or remove large central masses.

• Why do marbles curve toward the central mass even when you roll them “straight”? (Because they follow straight paths on a curved surface, which appear curved in ordinary space.)
• How does increasing the central mass change the orbits? (It deepens the well, requiring higher speeds for stable orbits and causing stronger deflection.)
• Why do many marbles end up orbiting in the same direction after many collisions? (Collisions dissipate energy and redistribute angular momentum, favoring a common rotation.)
• What real phenomena are not captured by this model? (True general relativity is 3D+time, not a 2D sheet; friction and downward gravity are artifacts; there is no negative mass or light-speed limit.)
• How do the “tidal stretching” and the Roche idea appear in this model? (The gradient in curvature pulls more strongly on the near side than the far side, stretching and potentially disrupting weakly bound objects.)