Re: Question about black holes and neutron stars

Jimmy wrote:
You have a neutron star teetering, it is literally ready to collapse,
but is just lacking a little bit of mass.
Neutron star falls t word a black hole accelerating it a significant
portion of light speed. Relativity takes over, neutron stars mass
increases and it collapse into a black hole.

First let's discuss the neutron star alone, far from the black hole.

The motion of an object has no effect on its mass. Mass is an intrinsic property of an object like a neutron star. Just think about it -- if you were in a rocket ship passing that neutron star, it could have an enormous velocity relative to you. But what effect ON THE STAR is there from you looking at it from a rocket ship? -- NONE.

Do not be fooled by people who discuss "relativistic mass"
as if it were mass. It is really energy, not mass; it does
change with an object's velocity, and is not intrinsic to
an object. But the subject is much more complicated than
this brief note.

Bottom line: the motion of the neutron star cannot possibly turn it into a black hole. It will not collapse into a black hole without an increase in its intrinsic mass density, which is unchanged by mere motion.



But falling toward a black hole has subtle implications. As the star approaches the black hole's horizon, but still outside it, a new horizon forms inside the star and expands at the local speed of light to meet the horizon of the black hole; these horizons distort towards each other at the local speed of light, until they coalesce into a single horizon enclosing both objects. The two horizons meet before any portion of the star enters the original horizon, so there is a period of time in which the neutron star has not yet collapsed, but it has been surrounded by its own horizon. So while motion cannot convert an object into a black hole, approaching another black hole can do so (in the sense of having its own horizon); once that happens it is impossible for the object to escape from the black hole. Eventually the neutron star will become part of the black hole, collapsing into it ("eventually" can be quite soon -- a few microseconds for a few-solar-mass black hole).

Note I assumed the neutron star does not change shape while
approaching the black hole. This is unrealistic. Most
likely, the neutron star would be pulled apart by tidal
forces before reaching the original horizon. Each piece
would get its own horizon as it approached the black hole
horizon, while still outside.

I'm pretty sure my description here is accurate, but it's based on heuristics, not actual computations (which are incredibly difficult for this, and are not within my expertise).


Tom Roberts
.

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