The most relevant evidence that would have refuted the theory of gravity before it was proven to be a law would be:
Experiments demonstrating that objects do not accelerate towards the Earth when dropped.
This would directly contradict the fundamental premise of gravitational theory, which states that objects are attracted to one another, causing them to accelerate towards the Earth when dropped. If experiments consistently showed that objects did not fall or accelerate towards the Earth as predicted, it would suggest that the theory of gravity is incorrect.
The other options would not necessarily provide definitive evidence against the theory of gravity:
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Observations showing that objects of different masses fall at the same rate in a vacuum actually support the theory of gravity, as it aligns with the predictions made by gravitational theory.
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Discoveries that gravitational forces only act on objects in space, not on Earth would need to be framed in a broader context; gravitational theory does not assert a distinction in how forces act based on location but rather that they affect all mass irrespective of location.
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Evidence that the gravitational force is stronger at higher altitudes than at sea level would contradict current understanding, which shows that gravity decreases with altitude; however, it wouldn’t refute gravity itself but rather necessitate a revision of the understanding of how it varies with altitude.
In summary, the strongest refutation of gravity would come from demonstrable evidence that objects do not experience the expected gravitational acceleration when dropped.