# Are there known objects rotating at close-to-light speeds?

For example, $$0.1c < v < c$$? Is this actually possible?

Background: is that true that the black hole in the center of the Milky Way rotates once per 20 minutes?

• The unit of the angular velocity is $\frac{1}{s}$, and not $\frac{m}{s}$. Please fix it. – peterh - Reinstate Monica Apr 12 '19 at 9:52
• I take it the velocity in question is at the "surface" of the object. – Steve Linton Apr 12 '19 at 10:40
• 10 km/s (to name an arbitrary value) is also sublight. Please edit your question so that title and content match. – user1569 Apr 12 '19 at 11:45
• Can you provide your source for the 20 minutes? Are you talking about things orbiting the BH at that period? Periodic variations in brightness? Spin measurements? What? – eshaya Apr 12 '19 at 19:30

A particle travelling in a circle of radius $$R$$ at velocity $$v$$ has an acceleration of $$v^2/R$$. So either $$R$$ is very large, or a very strong force is acting to stop the particles at the surface of the object flying away. We see no very large objects (galactic clusters, for instance) rotating this quickly, so we need to look for strong forces.
One option is gravity. We know of sub-millisecond pulsars, which we believe are neutron stars with a radius of perhaps 10 km, rotating 1000+ times per second. This gives a surface velocity at the equator of about $$60\,000 km/s$$ which is $$0.2c$$. So those meet your conditions.
At the other extreme, this stackexchange answer suggests that the electrons in a hydrogen atom rotate about the nucleus at something a bit less than $$0.01c$$. This scales up linearly with the charge on the nucleus, so the inner electrons in a lead or uranium atom are in some sense rotating at close to light-speed.