Today’s physics describes nature from the perspective of one observer at a time. In special (SR) and general relativity (GR), his proper space d1, d2, d3 and his proper time τ are set equal to coordinate space x1, x2, x3 and coordinate time t, respectively. These x1, x2, x3, t span “coordinate spacetime”. Here I show: There is a physical issue in SR/GR, and there is a more powerful concept of spacetime. (1) Rulers and clocks measure di and τ. They are not aware of an observer’s constructs xi and t. Thus, they cannot display xi' and t', which are calculated in SR/GR. (2) SR/GR work well in many respects, but we can describe nature also from the perspective of each observed object. Its d1, d2, d3 and τ span “proper spacetime”, which is equal to 4D Euclidean space (ES) if we interpret cτ as d4. In Euclidean relativity (ER), all energy is moving through ES at the speed of light c. Each observer’s reality is created by projecting ES orthogonally to his proper space d1, d2, d3 and to his proper time τ=d4/c. These four axes are set equal to x1, x2, x3, t in SR/GR and reassembled to a non-Euclidean spacetime. However, the SO(4) symmetry of ES is not compatible with the formation of waves. This is not an issue because waves are a special feature of coordinate spacetime. Only the SO(4) symmetry tells us: What I deem wave, deems itself matter. ER solves 15 fundamental mysteries, such as time’s arrow, the Hubble tension, dark energy, the wave–particle duality, and non-locality. I conclude that proper spacetime improves our understanding of cosmology and quantum mechanics.