SCIENCE DESK · VERIFIED PHYSICAL EFFECT

Does time really pass at different speeds?

Yes. Two accurate clocks can record different amounts of time when they move differently or sit at different heights.

Your own clock still feels normal to you. The difference appears when you compare clocks that have followed different paths or spent time at different gravitational potentials. Scientists call this time dilation.

What changes a clock’s rate?

Motion and gravity both matter. A clock moving relative to an observer ticks more slowly in that observer’s frame. A stationary clock higher above Earth ticks slightly faster than one lower down. These effects are small in everyday life, but accurate clocks can measure them. NASA explains the two effects using airplanes and satellites.

Where do we use this?

GPS satellites carry atomic clocks. Their speed makes their clocks lose about 7 microseconds per day compared with clocks on Earth. Their higher position makes them gain about 45 microseconds. Together, that is a gain of about 38 microseconds per day. GPS timing accounts for this difference. NIST describes these measurements and corrections.

Does that mean we can visit yesterday?

No. Measured differences in elapsed time do not show that people can travel into the past. A fictional time machine can follow rules that have not been demonstrated in nature.

Published September 29, 2026. Sources: National Aeronautics and Space Administration (NASA), “Is Time Travel Possible?”; National Institute of Standards and Technology (NIST), “Putting Einstein to the Test With the World’s Most Accurate Clocks,” February 25, 2026.