Highlights from Black Holes Week
Gravitational Lensing from Binary Black Holes
Just in time for it's self-proclaimed Black Hole Week, NASA released a neat animation for a pair of co-orbiting black holes. It's a visceral demonstration of Einstein's General Theory of relativity, which models spacetime itself with curved geometry.
The large mass of black holes distort spacetime so strongly that they act as lenses for the light coming in from behind them. They can magnify, rip and otherwise distort that light. This is perhaps the most dramatic departure from Newton's theory of Gravitation. The binary system modeled by NASA is a toy animation of two black holes orbiting in front of each other, demonstration the effect.
While we can't see black holes directly, the big ones typically come with a glowing accretion disk of hot, swirling gas that makes them hard to miss. The blue and orange colors in the animation are meant to represent those accretion disks.
A few years ago NASA's Event Horizon Telescope project coordinated an image of a black hole precisely in this context, the one at the center of the Galaxy known as M87. NPR also recently reported on an update to that image with polarization data (seen at top).
The strong gravitational lensing that causes these badly distorted effects are used by astrophysicists to assess the mass of the black hole. The black hole can also be used as a magnifying lens to see distant objects. By comparing how long different parts of the badly distorted light from the same object arrives on Earth, we can learn about more about how the universe is expanding.
"Weak" gravitational lensing has also been used as a sort of statistical effect to determine the masses of galaxies and other larger scale objects, albeit with less precision.
Seeing Extreme Magnetic Fields near the Event Horizon
The event horizon of a black hole is envelope of spacetime that surrounds it. and it is the point of no return. The equations of general relativity show that nothing can escape that boundary - anything that did would have to travel faster than the speed of light. Whence the name "Black Hole".
Lots of dust and gas and other space junk accumulate around the black hole, only to be sucked in. Just like the sun or our Earth's ionosphere, all the jostling of this space junk can generate large magnetic fields. With black holes, the magnetic fields near the event horizon get so powerful that a spinning black hole will eventually convert that hot plasma into powerful jets. Physics Today reports on recent calculations that are consistent with this phenomena for the supermassive black hole in the middle of that same black hole in the galaxy M87.
Observing Black Hole Details: Tidally Distorted Black Hole Dynamics
Stars aren't spheres. The Earth isn't spherical. Big astrophysical objects bulge, and tidal forces associated to orbital motion can be a big cause of that bulging.
For a long time it was thought that black holes don't bulge. It was thought that they are rigid objects, unlike planets and stars that are made up of atoms and molecules. These ideas were derived from calculations were done “statically” without any dependence on time. This sure makes the calculations easier, but doesn’t not reflect the real world.
With the rise of Gravitational Wave Observatories like LIGO and VIRGO, precision gravitational wave measurement means these details matter.
Sam Jarman reported in PhysicsWorld on a new, realistic simulation of a black hole interacting with a star in a dynamic fashion. The result? Black hole bulging! This bugling can induce tiny deviations in the emission of gravitational waves associated with such a pair, deviations that can now be observed, and therefore tested.
Physicists Alexandre Le Tiec and Marc Casals published this result under the curious title, Black Holes Fall in Love in PRL, but you can read the preprint on the arXiv.
