Implement Fixed Timestep for Physics

I need to implement a fixed timestep game loop for reliable physics simulation.

My game has: [describe physics, e.g., "platformer with jumping and gravity" or "ball physics with collisions"]

Create a game loop that:
1. Uses a fixed timestep for physics updates (e.g., 1/60th of a second)
2. Accumulates time and runs multiple physics steps if needed
3. Interpolates rendering between physics states for smooth visuals
4. Handles the "spiral of death" (when physics can't keep up)

My framework: [language/framework]

I'm learning about fixed vs variable timestep. Explain why physics needs fixed timestep but rendering can be variable, and what problems this solves.