Concept
Spacetime
The four-dimensional structure in which space and time are not separate. Different observers can disagree about duration, distance, and which distant events are happening now, while agreeing on the laws.
Illustration made for this site. Not a photograph of the claim.
In general relativity, mass and energy curve this structure, and that curvature is what we call gravity. Atomic clocks measure the effect over a millimetre of height. Spacetime is physical. It is not, in Einstein’s writing, a medium you harvest, or a plane you enter by changing a psychological frequency.