## Spinning Black Hole's Edge Rotates At Nearly the Speed of Light 227

astroengine writes

*"Astronomers have directly measured the spin of a black hole for the first time by detecting the mind-bending relativistic effects that warp space-time at the very edge of its event horizon. By monitoring X-ray emissions from iron ions (iron atoms with some electrons missing) trapped in the black hole's accretion disk, the rapidly-rotating inner edge of the disk of hot material has provided direct information about how fast the black hole is spinning. Astronomers used NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) — that was launched into Earth orbit in June 2012 — and the European observatory XMM-Newton measured X-ray radiation as a tool to directly infer the spin of NGC 1365's black hole. 'What excites me is the fact that we are able to do this for the very massive black holes at the centers of galaxies but we can also make the same measurement for black holes in our galaxy ... black holes that resulted from the explosion of a star ... The fact we can extend this from billions of solar masses to 10 solar masses is pretty cool,' Fiona Harrison, professor of physics and astronomy at the California Institute of Technology, Pasadena, Calif., and principal investigator of the NuSTAR mission, told Discovery News."*
## Re:know your audience (Score:2, Interesting)

i love how this summary explains what an ion is, but assumes i know the definitions of black hole, x-ray, and solar mass. great writing, folks!

I love how that part of the summary is plagiarized from the page one of the first article linked and the link takes you to page two!

## Re:Spin equal to mass? (Score:4, Interesting)

Its local gravity is determined by its rest mass not its relativistic mass.

No. Gravity is determined by the stress-energy tensor, and the energy component is total energy, aka relativistic mass (literally, they're the same thing). Relativistic mass is the gravitational mass is also the inertial mass.

A proton's mass -- the ratio between its acceleration and the force exerted by an electric field -- is much higher than the intrinsic mass of the quarks that make it up. It's the kinetic energy of those quarks held together by the Strong Nuclear Force that gives a proton 90% of its mass. The Higgs Field only explains that last 10%.

Similarly the gravity of the sun is far greater than just the intrinsic mass of the quarks and electrons inside it. It's the sum of all energy in the sun.

If you an accelerate an object it gains energy, and therefore (E=mc^2) relativistic mass, and also therefore increased gravity.

Oh, and yes, this means photons have gravity. Not are affected by gravity (though of course they are) but exert it.

## Re:Seems obvious to a naive engineer! (Score:5, Interesting)

In my limited understanding of these things, (mostly from articles meant for mass consumption, not scholarly journal papers), I imagine a black hole to be so massive not even light can escape its gravitational pull. Which technically means the escape velocity is the speed of light. So anything at the event horizon should be at the speed of light. This is of course, a naive view. The escape velocity is based on Newtonian, not Relativistic, physics.

In Einstein's theory of general relativity, the Newtonian concept of mass doesn't really exist, being spread out over the Einstein curvature tensor on one side of the general relativity equation and the stress energy tensor on the other. Calculating the radius of a gravitational field where the escape velocity is equal to c is straight forward in both Newtonian mechanics and general relativity, and produce the same value, the so called event horizon (Scharzschild radius, technically) but something interesting happens when the gravitational field is generated by a rotating object -- it drags spacetime around with it. [wikipedia.org] This would cause the orbital plane of an object to precess, something that Newtonian mechanics completely misses but was predicted nearly a century ago when people first started exploring solutions to Einstein's equations. Being able to directly arrive at the rotational rates of a wide variety of blackholes (which is what this announcement is all about) means that both frame-dragging and the no-hair conjecture concerning the characterizability of blackholes with just three Newtonian values -- mass, charge, and angular momentum -- can in principle now be studied more rigorously.

## Re:WRONG! (Score:5, Interesting)

Light has momentum (which "require" mass in more classical thinking). Light is "moved" by gravity (which indicates mass)

Also light has energy which

ismass in Relativistic thinking, and is moved by (and moves other things by) gravity which is due to it's energy (same as mass).This is confusing because people think of "mass" as the things photons don't have and matter does (which is true if we mean intrinsic mass), but also think of "mass" as the thing which effects/is affected by gravity and makes objects resist acceleration, when that's actually the relativistic mass (= energy).

It's both a particle and a wave, thus *is* a particle.

A photon is a quantum mechanical particle, which is a thingie which behaves kinda like a classical particle and kinda like a classical wave but not exactly like either.

However the key thing about quantum mechanics is that stuff is quantized... like particles are. So we call them particles. There is no misconception in doing so.