That’s another profound question, and it touches on active areas of theoretical physics.
Suppose the only thing in the Universe is a single black hole. Can we meaningfully say whether it is spinning?
In general relativity, an isolated black hole is completely described by just three quantities:
- Mass
- Electric charge
- Angular momentum (spin)
A spinning black hole is described by the Kerr solution, while a non-spinning one is described by the Schwarzschild solution.
The catch is this: how would you tell the difference if there were literally nothing else in the Universe?
Normally, we detect a black hole’s spin by its effects:
- It drags spacetime around with it (frame dragging).
- It changes the orbits of nearby matter.
- It affects light passing close by.
- It influences gravitational waves during mergers.
But in your thought experiment there are:
- no stars,
- no photons,
- no test particles,
- no observer.
There is nothing on which the black hole can exert these effects.
This raises an important distinction between having a property and being able to measure it.
According to general relativity, the Kerr and Schwarzschild solutions are mathematically different, so the theory says the black hole either has angular momentum or it doesn’t.
However, from an operational point of view, if there is absolutely nothing else in existence, there is no conceivable experiment that could distinguish the two. The spin would be unobservable.
Some physicists, particularly those with a relational view of physics, would argue that if no observation can ever distinguish between the two cases, then the distinction may not correspond to a physically meaningful difference. Others would say the property still exists as part of the geometry of spacetime, even if nothing can reveal it.
There’s another subtlety. In general relativity, spin is defined relative to the asymptotic structure of spacetime—essentially by looking at the geometry far away from the black hole. In a universe containing only one black hole, there is still spacetime extending to infinity, so the mathematics can still define angular momentum. But whether that mathematical definition corresponds to something physically meaningful without any observers or interactions is a deep philosophical question.
Your question is closely related to ideas behind Mach’s principle, which asks whether concepts like rotation have meaning without the rest of the Universe. Ernst Mach argued that rotation should only be meaningful relative to the distribution of matter in the Universe. Albert Einstein was strongly influenced by this idea when developing general relativity, although the finished theory doesn’t fully implement Mach’s principle.
So your thought experiment isn’t just philosophy—it highlights a genuine tension between:
- the mathematical description of spacetime, which says the black hole can possess spin, and
- the operational meaning of physical quantities, which asks how such a property could ever be observed if nothing else exists.
It’s exactly the sort of question that theoretical physicists still debate when discussing the foundations of space, time and gravity.