If anti-gravity existed, the book that explains it would be impossible to put down. Unfortunately, anti-gravity does not exist. Or does it? It is not a settled question, and there is a sense in which it will never be. Nonetheless, that does not matter much. How can that be? Keep reading!
There are two ingredients at play in this: theory and evidence. And their connection is more complicated than one may initially think. Let me start with theory. Gravity is responsible for stuff falling on the ground, as well as for planets moving in the sky. Scientific theories have been proposed to account for these phenomena: Newton’s theory of gravity first and Einstein’s general relativity later. Newton’s gravity is a force that acts instantaneously to pull bodies closer in virtue of their mass. In other words two massive bodies, no matter how distant, feel each other’s presence instantly and tend to get together.
Here now comes evidence. Newton’s theory has been very successful. The theory predicted, for instance, the Halley comet to be seen again in 1758. One may even think that our best scientific theories are definitely proven by experiments - Newton’s theory successfully predicted the return of the Halley comet, therefore Newton’s theory is true. Right? Well, no. It does not logically follow that Newton’s theory is true even if all experiments come out as predicted. It like someone concluding that it is snowing right now by starting with the consideration that if it's snowing then the streets will be covered with snow, and then observing that the streets are now covered with snow. This is unwarranted: it takes snow a very long time to melt, so snow could have fallen earlier in the day. Similarly, the Halley comet return is a good indication of the past success of Newton’s theory, but does not provide any guarantee of its future success.
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