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Why do clocks run slower in stronger gravitational fields?

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Brian Greene sits in a living room, holding a pen, with handwritten equations m = E/c^2 and E = mc^2 on a whiteboard beside him, highlighting c^2 as the speed of light. — relativity Brian Greene kicking off his *Your Daily Equation* video series with Albert Einstein's famous equation E = mc².

In general relativity, clocks run slower in stronger gravitational fields because gravity is a geometric phenomenon arising from the curvature of space-time caused by the presence of mass.12

This effect, known as gravitational time dilation, is a fundamental prediction of Einstein’s theory. As a clock approaches a massive body, the curvature of space-time alters the rate at which time passes relative to an observer in a weaker gravitational field. This phenomenon is not merely a mechanical error but a physical reality of the space-time continuum.345

Experimental evidence confirms this effect, such as through the gravitational redshift of light, where light vibrating in a strong gravitational field appears to have a lower frequency and longer wavelength. Additionally, highly precise measurements—including those using the Mössbauer effect and comparisons of atomic clocks at different altitudes—have verified that time indeed passes more slowly closer to a massive object like Earth.3564