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If black holes are mass of the star concentrated on a single point then how can it be moving through a 3-dimensional space? How can they spin?

Qmechanic
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Kugelblitz
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3 Answers3

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Firstly, there is a considerable amount not known about Black Holes. As a physicist, when people say, that it's a singularity, what they mean is that there is no known physics that explains what goes on there. Our understanding of both Gravity and Quantum fields are valid upto an energy scale(that means upto some small length scale). Beyond that we'll require some new formulation.

Having said all this, a point particle can have spin(and can also move around in 3D space). The idea of quantum spin is not related to actual spinning around it's own axis(like earth's spin). It's a internal angular momentum with no classical analogue. In principle a point object can also have classical spin, just imagine that the axis of rotation doesn't go through the point but it is outside.

Ari
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  • I don't think "singularity" means what you said. I think that "singularity" comes from the math. There is a special locus (a single point if the BH has no angular momentum) in the space described by the theory where the math fails to produce a meaningful result. The fact that "there is no known physics that explains what goes on there" may mean that the theory is incomplete--that there are other terms we don't know about, maybe can't ever know about, that would avert the mathematical singularity. – Solomon Slow Mar 25 '20 at 11:22
  • But that's exactly what I said. The mathematics, which is based on current physical understanding, predicts a singularity. Which inevitably means that the current understanding fails there. Real world can't have singularities. – Ari Mar 25 '20 at 11:47
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If black holes are mass of the star concentrated on a single point

Black holes are a solution of General Relativity equations, which describes very large masses and the distortion they bring to space and time. It is a classical theory, and the mathematical form has a discontinuity at radius equal zero.

It is expected that quantization of gravity will bring the mathematical point to a "probability locus" in space time so there will be no longer a point where all mass is concentrated, but a bubble, similar to the bubble assumed in the mainstream cosmological model, the Big Bang, to avoid the same question. So, nature is quantum mechanical and once gravity is quantized definitively, there is no problem. At the moment astrophysics uses effective quantization of gravity where needed.

then how can it be moving through a 3-dimensional space? How can they spin?

As the other answer says, all elementary particles are point particles and most have a spin characterizing them. There are some off the main stream suggestions that try to characterize elementary particles as tiny black holes!

anna v
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Remember that we cannot see (or observe in any way) the singularity that may (or may not be) at the centre of a black hole. Anything inside the event horizon is causally cut off from the rest of the universe - it cannot affect outside observers like ourselves in any way.

From the point of view of an outside observer, we never actually see an object fall into a black hole - it will be smeared across the event horizon. Because of time dilation any photons that the objects emit when near the event horizon have their energy massively reduced by the time they reach an outside observer, so they are effectively invisible (although the accretion disk of gas rotating around the black hole may be very hot and bright in many wavelengths).

However, the angular momentum carried by these infalling objects cannnot disappear - it must be conserved. It is this angular momentum that we see as the "spin" of the black hole.

gandalf61
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