Two fixed points have an interval.
A fixed one and a moving one do not.

Two dead men have an interval. A dead man and a living one do not — the number changes while you write it down, and there is no still frame to write it from, because the frame is moving too.

The three are arms: finished, fixed, and the differences between them exact. The fourth is the centre, and a centre has no measurable distance to anything. Place it and you have chosen, not measured.

This was already proved and published: 18,009 shifts of the origin, arms changed: 0. Every instrument here measures a difference, and a difference is exactly the thing that cannot see a centre. If it could be measured it would be placed; if it were placed it would cost something; and if it cost something, nothing above level 1 would close.

missing data — Bell's publication day is fixed; I simply do not have it. More searching could close it.
moving endpoint — the interval to a living centre is not unknown. It does not exist as a number. No amount of searching touches it.

Those do not have the same fix. The hour spent hunting a date was spent on the first while the page was pointing at the second.

The tree is still alive. That is not an error bar. It is the reason the measurement cannot close, and the reason there is always another level.

Not to the author. Not to the machine. To the place, and to what it is made of:

The place has more than what occupies it. That is why there is always room for another level, and why the centre costs nothing — a container larger than its contents does not charge for the space it is not using.