Let’s consider an accelerating frame of reference again. Think of the elevator scenario. This time, the ball is stationary in the lab frame. In the elevator’s frame of reference, the ball (alongside anything stationary in the lab frame) would appear to be accelerating downwards at the rate of . It is as if the ball is being affected by a force. This force is dubbed the pseudo-force, as it does not really exist in an inertial frame of reference.
This pseudo-force causes uniform acceleration across the frame of reference. Note that a uniform gravitational field has a similar effect of accelerating everything equally. In fact, the effect of pseudo-force and uniform gravitational field are indistinguishable.
Allow me to introduce a simple and relatable analogy: I kidnap you and put you in a box with no window. You feel a uniform gravitational field of . Can you tell whether you are on earth, or you are attached to a silent rocket running with the acceleration of ? You can’t.
Fig 02.01 Box.excalidraw
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An important implication from this principle is that if there exists a uniform gravitational field, then there exists a frame of reference in which the field is nullified.
For example, in the elevator scenario I mentioned prior, the gravitational field is in the elevator frame. The frame of reference that would nullify this field is simply the lab frame.
In the case of true gravity, no such frame of reference exist. True gravity can also be identified by the effect of tidal force, where objects can be deformed due to different magnitudes of force in different positions.
Fig 02.02 Tidal Forces.excalidraw
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