this post was submitted on 20 Feb 2025
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If the escape velocity is above 1c, isn't it a black hole?
Yes. 1/2 c is super fast still and gravity that strong would have effects like noticably bending outgoing light.
Neutron stars are right on the verge of becoming black holes and are incredible.
And 1/2c is a pretty middle of the road escape velocity for a neutron star.
The lightest known neutron star, at 1.4 solar masses has an escape velocity of right around 1/4c, while the heaviest at 2.35 solar masses is 3/4c.
All of which assumes the neutron star isn't spinning. Equatorial bulging caused by the rotation reduces the escape velocity at the equator relative to the poles and depending on whether or not you launch with the direction of the rotation you might be able to subtract the rotational velocity from your escape velocity.
As an example, in the case of that 2.35 solar mass neutron star, it has a rotational velocity of approximately 0.24c. So of you launch with the rotation you get an escape velocity of 0.5c, whereas if you launch against it you're looking at more like 0.98c.
So technically there could be an object that is mostly a black hole, except on the equator?