Yes, according to Einstein's theory of general relativity, free fall is considered an inertial motion, and therefore a local inertial frame of reference. This is different from classical mechanics where gravity is considered a force causing acceleration.
Understanding Inertial Frames
Before discussing free fall, let's define what an inertial frame is:
- Inertial Frame Definition: An inertial frame of reference is one in which an object not subjected to any net external force will either remain at rest or continue to move at a constant velocity in a straight line. This is essentially Newton's first law of motion.
Free Fall in Classical Mechanics
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Gravity as a Force: In classical mechanics, free fall is considered accelerated motion due to the force of gravity. An object falls towards the Earth due to this force.
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Non-Inertial Perspective: Because of this acceleration, an object in free fall in classical mechanics is not considered to be in an inertial frame.
Einstein's View: Free Fall as Inertial Motion
Einstein's theory of general relativity changes our understanding of gravity:
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Gravity as Curvature: Instead of a force, Einstein views gravity as the curvature of spacetime caused by mass and energy.
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Free Fall as "Following the Curvature": Objects in free fall are not being "pulled" by a force; instead, they are following the natural curvature of spacetime. This means that they are traveling along a geodesic (the path of least resistance) and are considered to be in inertial motion.
As stated in the reference: "free-fall (def) is actually inertial motion (def)" This differs from classical mechanics.
Examples and Insights
Here are some ways to further understand this:
- Floating Objects: Consider two objects floating inside a freely falling elevator. Relative to the elevator, they do not accelerate towards each other. Instead, they remain at rest or move at constant relative velocities. This absence of acceleration demonstrates the inertial nature within the falling system.
- Local vs. Global: It's crucial to remember that this inertial frame is only local. This means that if you observe the falling elevator from afar, it is clearly accelerating, but within the elevator (and in the immediate surrounding space), the objects inside are essentially experiencing weightlessness, characteristic of an inertial frame.
- Differences from a Stationary Frame: When you are standing on the ground, you are not in an inertial frame, because you are experiencing the force of gravity pushing upwards.
- Practical Implications: Understanding freefall as an inertial frame is essential in areas such as space travel, where artificial gravity is sometimes created via rotation.
Summary
| Classical Mechanics | General Relativity | |
|---|---|---|
| Cause of Free Fall | Force of Gravity | Curvature of Spacetime |
| Inertial Frame in Free Fall? | No | Locally, Yes |
| Example | Object falls due to force of gravity | Following the curves of spacetime |
Conclusion
In summary, according to general relativity, free fall is an inertial frame of reference locally. This is because objects in free fall are not experiencing a force but rather following the curvature of spacetime.
