Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets.
How to measure the spin of a black hole: This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.
Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM- Newton telescope, there were two competing models to explain why this peak might not appear to be sharp. The "rotation" model shown at top held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole. The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.
This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X- ray light, with energies between 3 and 70 keV.
This image taken by the ultraviolet-light monitoring camera on the European Space Agency's (ESA's) XMM-Newton telescope shows the beautiful spiral arms of the galaxy NGC1365. Copious high-energy X-ray emission is emitted by the host galaxy, and by many background sources. The large regions observed by previous satellites contain so much of this background emission that the radiation from the central black hole is mixed and diluted into it. NuSTAR, NASA's newest X-ray observatory, is able to isolate the emission from the black hole, allowing a far more precise analysis of its properties.
What XMM-Newton saw: The solid lines show two theoretical models that explain the low-energy X-ray emission seen from the galaxy NGC 1365 by the European Space Agency's XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the measurements from XMM-Newton, which are explained equally well by both models.
Two X-ray observatories are better than one: NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has helped to show, for the first time, that the spin rates of black holes can be measured conclusively. It did this, together with the European Space Agency's XMM-Newton, by ruling out the possibility that obscuring clouds were partially blocking X-ray right coming from black holes. The solid lines show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.
In an effort to help solve the black hole information paradox that has immersed theoretical physics in an ocean of soul searching for the past two years, two researchers have thrown their hats into the ring with a novel solution: Lasers.
So what the heck have lasers got to do with black holes? Technically, we’re not talking about the little flashy devices you use to keep your cat entertained, we’re talking about the underlying physics that produces laser light and applying it to information that falls into a black hole.
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. In its simplest form, laser light is generated inside a cavity where a photon will interact with a material, simulating it and amplifying the output. The process cascades to generate a beam of collimated and coherent light that is very useful for communications, industry and entertainment.
“Basically, it works like a copy machine: you throw something into the machine, and two identical somethings come out,” said Chris Adami, a physics professor at Michigan State University. If you apply this mechanism to an event horizon as matter falls into a black hole, according to Adami, we may have some kind of solution to the black hole “firewall paradox.”
Adami co-wrote a paper based on this research with Greg Ver Steeg of University of Southern California, Los Angeles, that was published in the April 7 edition of the journal Classical and Quantum Gravity.
The firewall is an uneasy solution to decades of debate in the physics world around information falling into a black hole.
In the 1970s, renowned black hole physicist Stephen Hawking made the realization that black holes aren’t totally black. Taking his cue from quantum physics, pairs of virtual particles are predicted to pop into existence, annihilate with each other and then disappear. But right at the edge of an event horizon — the point at which spacetime warping around a black hole is so extreme even light cannot escape the black hole’s grasp — one of these virtual particles may become trapped and pulled away from being annihilated by its partner, making the escaping particle “real,” leaching a tiny amount of mass from the black hole.
This slight “evaporation” of mass became known as Hawking radiation and completely revolutionized our perspective on black holes — depending on their mass, they will evaporate and eventually disappear. Black holes aren’t as permanent we once thought.
The formulation of Hawking radiation heralded the beginning of some hefty theoretical hurt and it all boiled down to how black holes deal with information. The information contained within the matter falling into a black hole is ultimately destroyed by its total evaporation out of existence — a scenario that violates our physical understanding of how the universe works. Is information really being destroyed or is it somehow being conserved in a way don’t yet understand?
A recent development to the last few decades of debate (involving Hawking and other key physicists) came in 2012 when physicists led by Joseph Polchinski of the University of California in Santa Barbara published their research on the information paradox. If indeed black holes do not destroy information, there must be a raging inferno just inside the black hole’s event horizon called a “firewall.”
Earlier this year, Hawking stepped in arguing that the firewall is not needed and advocated a “chaos wall” that instead jumbles up the information (thereby not violating quantum rules) and varies the location of the event horizon depending on the information that is falling in. In this scenario, argued Hawking, an event horizon (in its classical sense) does not exist — it should be replaced by an “apparent horizon.”
This certainly isn’t a “win” for Hawking or anyone else, it’s just another idea that has been thrown into the pot in an effort to find some balance between apparently conflicting theories that govern information getting consumed by a black hole.
But say if this whole thing has been overcomplicated? Perhaps a mechanism has been overlooked? This is where Adami’s stimulated emission of radiation idea comes in.
“If you throw information at a black hole, just before it is swallowed, the black hole first makes a copy that is left outside. This copying mechanism was discovered by Albert Einstein in 1917, and without it, physics cannot be consistent,” said Adami in a news release.
“In my view Chris Adami has correctly identified the solution to the so-called black hole information paradox,” said Paul Davies, theoretical physicist at Arizona State University. “Ironically, it has been hiding in plain sight for years.”
As matter falls into a black hole’s event horizon, Adami thinks radiation is generated via stimulated emission, retaining a copy of the in-falling information. This radiation will be distinct from the spontaneous Hawking radiation also being generated.
“Stimulated emission of radiation is precisely the process of copying information: one particle comes in, two leave with the same exact set of quantum numbers,” write Adami and Ver Steeg in their publication.
However, information cannot be copied perfectly in the quantum world (a concept known as the no-cloning theorem), “but it turns out that the spontaneous emission of radiation (the Hawking radiation of the black hole) conveniently prevents perfect cloning by supplying the necessary minimum amount of noise.”
The researchers state that this research doesn’t directly address the information that falls beyond the black hole’s event horizon, however, as stimulated emission would occur at the boundary of the black hole’s event horizon, but it could be a potential answer to the information paradox.
“Stephen Hawking’s wonderful theory is now complete in my opinion. The hole in the black hole theory is plugged, and I can now sleep at night,” added Adami.
Although this may not tame the conflicting ideas about how black holes may (or may not) conserve information, it is a surprisingly elegant theory to a horribly complex problem that uses the established mechanism of stimulated emission to transport information away from a black hole.
So the next question would be: How could we go about detecting this stimulated emission if it does exist?