仅利用引力测定物体质量：是否存在不依赖电磁力的方法？

Hey there! You’ve hit on a really insightful point—nearly all our daily mass/weight measurements do rely on normal forces, which at their core are electromagnetic (thanks to the electron repulsion between atoms in surfaces). But absolutely, there are ways to measure mass that don’t involve electromagnetic interactions whatsoever. Let’s break down the most prominent ones:

1. Gravitational Interaction-Based Methods #

These leverage the universal gravitational force, which is entirely separate from electromagnetism:

• Cavendish Torsion Balance: This is the classic experiment to measure the gravitational constant ( G ), but it can also calculate the mass of small objects. By observing the twist of a thin wire caused by the gravitational pull between large lead spheres and smaller test masses, you can derive the gravitational force magnitude. Plugging this into Newton’s law of universal gravitation lets you solve for the unknown mass—no electromagnetic forces involved (as long as you use non-magnetic materials to avoid interference).
• Celestial Orbital Calculations: For astronomical bodies, we use orbital parameters (like orbital radius and orbital period) alongside Kepler’s third law and Newton’s gravitation equations to compute mass. For example, measuring Earth’s orbit around the Sun lets us calculate the Sun’s mass, or tracking the Moon’s orbit gives us Earth’s mass. This only relies on gravitational interactions, with zero electromagnetic input.

2. Inertial Mass Measurements Using Nuclear/Weak Interactions #

These methods use conservation laws and non-electromagnetic fundamental forces:

• Collision-Based Mass Spectrometry (Non-Electromagnetic): While most common spectrometers use electromagnetic deflection, time-of-flight (TOF) mass spec or collision-based methods don’t. For instance, firing a particle of known energy and mass at an unknown particle, then measuring the momentum of the collision products, lets you use conservation of momentum and energy to solve for the unknown mass. The interaction here is governed by nuclear strong force, not electromagnetism.
• Mass-Energy Equivalence (( E=mc² )) for Radioactive Decay: When a radioactive particle decays, we can measure the total kinetic energy of its decay products. Using Einstein’s mass-energy equivalence, we can reverse-calculate the mass lost during decay (and thus the original particle’s mass). This process relies on weak or strong nuclear interactions, not electromagnetic forces.

3. Neutron Interferometry #

Neutrons have no electric charge, so they don’t interact via electromagnetism. By using a neutron interferometer, we can measure how a neutron’s wave phase shifts in a gravitational field. This not only tests the equivalence principle but also allows us to determine the neutron’s inertial and gravitational mass—all without touching electromagnetic forces.

It’s worth noting these methods are mostly used in labs or astronomical contexts because electromagnetic-based measurements are far more practical for everyday use.

内容的提问来源于stack exchange，提问作者Suhrid Mulay