I need help understanding the concept of gravitational waves. Can you explain it to me in simple terms, and also mention the key experiments that have confirmed their existence? Your response should contain at least 2 placeholders represented by square brackets, such as [address]. Highlight at least 2 sections in your answer with markdown, i.e. *highlighted section*. At the end of your response, please explicitly add a postscript starting with *P.S.*.
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