What is spacetime?
In OPH, spacetime is an effective description observers agree on: a shared account of positions, durations and which events can influence which. It is reconstructed from compatible records and their comparisons. Donald Hoffman's desktop-interface metaphor is useful here. A file's icon has a position on a screen, but that position does not reveal the arrangement of the computer's circuits. Likewise, an experienced spatial layout can guide action without mirroring the underlying mechanism. OPH gives the comparison a constructive target: recover geometry, physical clocks and field behavior from the observer process. Agreement means that observers' accounts translate consistently where they overlap, rather than that everyone sees the same picture. The metaphor illustrates an interpretation; the physical claim requires a reconstruction that reproduces actual ruler, clock and signal measurements.
See also What is space?, What is time?, How can continuous space and smooth motion emerge from discrete records?, How can two different descriptions of nature both be right?
Sources: Donald Hoffman: The Origin of Time in Conscious Agents; OPH: observer spacetime and conditional Einstein reconstruction
Also in Chapter 22: Why Do All the Clocks Agree?, Chapter 15: Why Does Space Have Three Directions?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 17: Why Is There a Speed Limit?watch a universe make its own particles