Black holes found to be energy efficient

[JME2]

From Yahoo

A new study finds that the supermassive black holes at the hearts of some galaxies are the most fuel efficient engines in the universe.

"If you could make a car engine that was as efficient as one of these black holes, you could get about a billion miles out of a gallon of gas," said study team leader Steve Allen of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University. "In anyone's book, that would be pretty green."

The finding, made using NASA’s Chandra X-ray Observatory and announced in a media teleconference today, is giving scientists insights into how supermassive black holes generate energy and how they affect the galaxies where they make their homes.

Black holes are regions of space where gravity is so strong that matter and light can't escape once they pass the event horizon, a spherical boundary surrounding the black hole.

However, inflowing matter that hasn't yet passed this point of no return can—through friction and interaction with the black hole's strong magnetic field—release energy in the form of either diffuse light or focused jets of energy.

"Once gas comes within a distance about a million times larger than the event horizon of the black hole, it becomes gravitationally captured," Allen explained. "At this point the gas becomes fuel for the black hole engine."

The new study looked at nine supermassive black holes at the centers of elliptical galaxies; each one was about a billion times more massive than our Sun. The black holes were relatively old and generated much less energy than the fiercely luminous and rapidly growing supermassive black holes known as “quasars."

The researchers found that these "quiet" black holes released about 1,000 times more energy as jets than as light. The reasons for this are still unclear.

"That's a mystery, how these black holes selectively put that much energy into the jets without producing much light," study team member Christopher Reynolds from the University of Maryland told SPACE.com.

Most of the energy in the jets is being emitted as radio waves, but in at least one of the black holes studied, the energy was in the form of more energetic X-rays.

"The energy in these jets is absolutely huge, about a trillion trillion trillion watts," Allen said.

As they race outwards from their parent black holes at nearly light speed, the jets carve out enormous cavities, or "bubbles," in the surrounding gas environment; some of these bubbles can be tens of thousands of light years across.


Bubbles can also form in the aftermath of stellar explosions called supernovas; our own solar system is enveloped by such a structure, called the "Local Bubble," which was formed during an explosion long ago.


The researchers used these bubbles to figure out the fuel efficiency of the black holes. Using Chandra images, they first calculated how much fuel in the form of gas was available to each black hole. They then estimated the power required to produce the bubbles that were observed.

The finding could have implications for other types of black holes as well, including much smaller, stellar-mass black holes, the researchers say.

"We already knew that powerful quasars are very efficient at making light. Now we know that black holes in elliptical galaxies are efficient as well," said Kim Weaver, a researcher at NASA's Goddard Space Flight Center who was not involved in the study. "This suggests that being green is a trait that all black holes may have in common."

The scientists think the supermassive black holes are green in another way, too. The energy that each black hole emits as jets warms the surrounding environment. This prevents gas from cooling and coalescing into billons of new stars and places an upper limit on how large a galaxy can grow.

"In an environmental sense, the black holes are actually preventing galactic sprawl from taking over the neighborhood," Weaver said.

The study will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

[Nieztchean Uber-Amoeba]

Cool. Yet more evidence that the Romulan Star Empire is the most intelligent race in the Trek-verse.

[SyntaxVorlon]

I'm pretty sure we've known this for a while, matter falling into the BH's gravwell turns 25% of its mass into energy, compared to 1-5% tops for fusion reactions.

[Sriad]

I'm pretty sure we've known this for a while, matter falling into the BH's gravwell turns 25% of its mass into energy, compared to 1-5% tops for fusion reactions.

I've seen numbers up in the 40% range for the theoretical top. But yea, short of antimatter or quantum BH hawking radiation, just plain chucking stuff at a black hole is as efficient energy production as you can get.

[kheegster]

I'm pretty sure we've known this for a while, matter falling into the BH's gravwell turns 25% of its mass into energy, compared to 1-5% tops for fusion reactions.

I believe the earlier figures were from theoretical arguments based on the accretion theory, while this observational study actually used indirect methods to measure the amount of material surrounding the SMBH and comparing it with the observed luminosity.

[wilfulton]

In retrospect, how much energy does a hole need to function, regardless of what color it is?

[SyntaxVorlon]

In retrospect, how much energy does a hole need to function, regardless of what color it is?

Your death will be long and painful. You will survive initial spagettification.

[kc8tbe]

I'm disappointed that the article didn't mention Hawking radiation. So what if black holes spew 40% of their mass-energy input away as radiation? They eventually radiate away 100% of their mass-energy input as energy in the form of Hawking radiation! I mean, first law of thermodynamics -- duh!

It is interesting that the black holes studied radiate so much energy via the jet mechanism as opposed to some other mechanism. It is also interesting that the black holes only seem to eat only 60% of the mass that comes their way, radiating the rest away as energy before it crosses the event horizon. Hopefully this will lead to a new discovery that will expand the horizons of physics.

But seriously, comparing the "efficiency" of a black hole to a heat engine is silly. It's like comparing lightening to windmills. If we could harness the lightening in thunderstorms we would have a nearly inexhaustable supply of energy right in our backyards. But we can't, so we build lower-efficiency windmills instead.

[Neko_Oni]

It is a rather silly comparison. After all the holes convert mass-energy while a motor converts chemical-energy. I bet a modern motor which got the same efficiency but using the fuel's mass energy instead of chemical energy we could get a crazy number of miles per gallon, although it would also put out so much waste heat it would kill the passengers, everyone around it and then promptly melt.

[kheegster]

I'm disappointed that the article didn't mention Hawking radiation. So what if black holes spew 40% of their mass-energy input away as radiation? They eventually radiate away 100% of their mass-energy input as energy in the form of Hawking radiation! I mean, first law of thermodynamics -- duh!

Hawking radiation is an extremely slow and inefficient process, and wouldn't significantly affect the mass of anything but the smallest micro-black holes, certainly with a supermassive black hole it can effectively be ignored.

It is interesting that the black holes studied radiate so much energy via the jet mechanism as opposed to some other mechanism.

As mentioned in the article, not all SMBHs have jets, nor are they always the dominant radiative mechanism. There's a whole zoo of AGN types ranging from Seyfert galaxy cores, quasars to radio galaxies, characterised by their radiation mechanisms.

It is also interesting that the black holes only seem to eat only 60% of the mass that comes their way, radiating the rest away as energy before it crosses the event horizon. Hopefully this will lead to a new discovery that will expand the horizons of physics.

There are never any 100% efficient processes in nature...40% efficiency is about as good as it gets. The reason for this most likely lies in the details of the system, rather than in any fundamental new physics.

[kc8tbe]

Hawking radiation is an extremely slow and inefficient process, and wouldn't significantly affect the mass of anything but the smallest micro-black holes, certainly with a supermassive black hole it can effectively be ignored.

True, Hawking radiation is terribly relevant on the short-term cosmic scale because it is comparatively slow for large black holes. However if you were going to use a black hole to power your car, you would probably use a micro-black hole by feeding it matter and using the Hawking radiation to drive a heat engine, kind of like Mr. Fusion.

The problem with this is that you would have to feed the micro-black hole matter at an unreasonable rate (if your car were dense enough, it would eat your car on the timescale of minutes) and you would have to figure out a way to radiate away all the waste heat.