Is spacetime orientation a convention?

[vanhees71]

Yes, but the fact that one cannot be continuously transformed into the other is not. So there are two orientations, which we can label however we want, and given any physical object, we can check which orientation it can be continuously transformed into and which one it can't. The result of this check is independent of any coordinate choice or any choice of labeling of orientations; that is the sense in which a physical object has an orientation that is not a convention. (The only complication here is that some objects, for example a perfect sphere, will not have an orientation in this sense.)

My point was that you need a reference, which you can choose by convention. An objective one, transportable to any place and valid at any time, according to the HEP Standard Model, is the left-handedness (right-handedness) of neutrinos (anti-neutrinos). Then you can define of any (chiral) object which orientation it has relative to the so defined properties "left" and "right". That's indeed independent of the choice of coordinates.

 

[PeterDonis]

you need a reference, which you can choose by convention

Yes, agreed.

An objective one, transportable to any place and valid at any time, according to the HEP Standard Model, is the left-handedness (right-handedness) of neutrinos (anti-neutrinos)

Yes, but this one also has a stronger property: you don't have to transport any specific neutrinos from one region of spacetime to another. Any neutrinos anywhere will serve as a reference, and they will all agree.

But you could also construct, for example, a right-handed set of rulers on Earth (right-handed by our conventions), and then transport it to Mars, or Alpha Centauri, or the Andromeda Galaxy, and use it there. The rulers can serve equally well as an objective reference for handedness; but you have to transport that specific set, unlike the neutrinos. And you can't guarantee that this set of rulers will match, for example, a set constructed by the aliens in the Andromeda Galaxy as their reference.