‘Spacetime’ is essentially a mathematical model, the physical reality of it is… hard to understand. It’s a useful concept for doing things like relativity, but really, time and space are very different things.
The only other one I can think of off the top of my head is wave particle duality.
There has been a lot of debate over the course of science about whether light is made up of particles or of waves. Newton thought it was particles (corpuscules), whereas Huygens thought it was waves. Lots of scientists debated it and gave different evidence for either one, but no one ever really ‘won’.
The main evidence that light is a wave comes from the two-slit experiment. In this, light waves interfere and create dark patches as well as light patches.
However, Einstein (not just a pretty face) proved something called the photoelectric effect – something that can only be explained if light is made up of particles.
Now, this may seem a bit weird, but both of these experiments are true, and what they show is true. Light is a wave /and/ a particle. Only not at the same time.
In fact, light is both, and neither. ‘Waves’ and ‘particles’ are just ways of describing the properties of light. They are just models made up by the human brain. Which is somewhat mind bending!
Matter-energy: so when you have a nuclear reaction, this is because some mass (parts of the atom) are lost and turned into energy.
and a variant is,
position-momentum: due to Heisenburg’s uncertainty principle, the more accurately we know the position of something, the more uncertain we must be about its momentum so the two are intimately linked.
and now for some similar-ish ideas,
electric fields-magnetic fields: due to (special) relativity, in one frame of reference an electric field will appear as a magnetic field in another.
allotropes: you can get identical elements but with different numbers of neutrons so they appear identical chemically, but you can tell what has gone on by the differing quantities of each element’s allotrope/isotope. (Oh, do I mean isotope or allotrope?)
chirality: our bodies make essential proteins that have chirality or ‘handed-ness’, and all proteins are left-handed, and if we artificially make right-handed versions then we can’t use them—they go straight through us, whereas the food we eat contains left-handed versions.
Ice cream-hunger: when you are hungry, there is never enough ice-cream. 🙂