Einstein went on to develop this insight into his now-famous elevator thought experiment, demonstrating that any measurement of the laws of physics within a small contained space such as an elevator could not discern whether the elevator is static, cradled in a strong gravitational field, or rapidly accelerating upward across space—when the frames of reference are mutually accelerating, any general theory of relativity would implicitly also be a theory of gravity. To the person undergoing it, any rapid ascent to new heights of experience can feel equally imperceptible from the interiority of
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