请用英语简要解释"相对论"
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"Relativity" refers to the theories developed by Albert Einstein, including Special Relativity and General Relativity. Special Relativity deals with the relationship between space and time in the absence of gravitational effects. General Relativity extends this to include gravity as a curvature of spacetime caused by mass and energy.

Key Points:
1. Special Relativity (no gravity)
2. General Relativity (includes gravity)
3. Spacetime curvature by mass/energy
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