Are solid objects really “solid”?

In this video, the host investigates how quickly a push at one end of a solid steel bar makes the other end respond. Through a tabletop experiment and material-science reasoning, the talk shows that force propagation is not instantaneous and is governed by the bar's elastic properties and the speed of sound in the material. By comparing models from rigid-body approximations to real wave propagation, the video reveals why even a simple, solid object behaves like a medium that carries a compression wave. The measured delay in a 91 cm bar is about 180 microseconds, matching the extensional speed of sound in steel and highlighting the importance of wave dynamics in everyday objects.

Understanding the core question: does a solid bar respond instantly?

The video opens with a thought-provoking question about whether pushing one end of a steel bar is felt at the other end immediately. The concept scales from a small object to a hypothetical 300,000-kilometer bar and then tests competing physics models. The takeaway is that no real object is perfectly rigid; force propagation occurs through a compressive wave that travels at a finite speed inside the material. This section emphasizes that physics models are approximations, and the choice of model determines what you predict and how you test it, especially when reconciling with relativity and other principles.

"No physical object is truly perfectly rigid; a compressive wave has to propagate" - Derek-Muller

From models to materials: how physics approximates a bar

Between the extremes of a fully quantum mechanical description and a simple rigid-body picture lies a spectrum of useful approximations. The video explains why solving the exact quantum state for a macroscopic bar is impractical and how engineers rely on Newtonian mechanics for whole objects or wave-based descriptions for internal propagation. The extensional speed of sound, determined by the bar’s stiffness, density, and geometry, becomes central. The spring-like bonds inside the metal behave as an interconnected network, so a disturbance travels as a compression wave rather than instantaneously shifting every atom at once. This section bridges everyday intuition with material physics and foreshadows the bench-top results.

"The rigid body approximation can be convenient, but it ignores the speed of sound in the material" - Derek-Muller

The experiment: setup, method, and what the data shows

The presenter describes bench-top experiments designed to time the propagation of force along a bar. After iterations with different sensors, the final setup uses a metal-to-metal impulse to start timing and a sensor to stop it as the bar pushes into the sensor. The key finding is a delay of about 180 microseconds for a 91-centimeter bar, corresponding to a compressive wave speed near 6,000 meters per second in steel. The data demonstrate that different parts of the bar move at different times as the wave travels, and they reveal practical measurement challenges such as sensor squashing and waveform artifacts that can complicate interpretation.

"The observed delay in the steel bar was about 180 microseconds over 91 cm, matching the extensional speed of sound in steel" - Derek-Muller

Material physics: the extensional speed of sound and why thin bars behave differently

The final section ties the observed delay to material properties through the extensional speed of sound, which in slender bars is governed by density, stiffness (Young’s modulus), and Poisson’s ratio. In a near-one-dimensional transmission, the wave speed depends on averages over the bar, not the full three-dimensional bulk speed of sound. The explanation shows why the 1D model is apt for thin bars and how geometry and alloy composition can influence measured speeds. The talk also clarifies that while a 3D cube would support different wave modes, the experiment’s slender geometry yields the extensional wave speed consistent with the measured data.

"The speed of sound in a bar comes from the extensional wave and the bar's geometry, density, and Young's modulus, including Poisson's ratio effects" - Derek-Muller