Kurzgesagt’s latest video offers a clear and engaging explanation of why practical time travel remains impossible, focusing on the concept of our constant movement through spacetime as described by special relativity.
In our universe, everything travels at the speed of light through four-dimensional spacetime. Our movement through spacetime is a combination of our velocities in the spatial and temporal dimensions. Since we cannot remain stationary, even at rest in space, we are continually advancing into the future.
While it is possible to alter your rate of progression through time by adjusting your speed through space—moving faster in space slows your passage through time, and moving slower in space speeds it up—the total speed through spacetime remains constant at the speed of light.
As a commenter succinctly noted, “Your speed is constant. So the faster you move through the spatial dimensions, the slower you move through the time dimension, and vice versa.” This trade-off underscores the fundamental challenge of achieving useful time travel.
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