National Geographic | Ker Than | 2011 May 26
Mini black holes that look like atoms could pass through Earth dailyBlack holes smaller than atoms pass unnoticed through planet, study suggests.
Like cosmic ghosts, miniature black holes may be zipping harmlessly through Earth on a daily basis, a new study suggests.
The new theory rebuts doomsday scenarios in which powerful atom-smashing machines such as the Large Hadron Collider spawn black holes that swallow the planet.
Instead, the study authors think that tiny black holes would behave very differently from their larger brethren in deep space, called astrophysical or stellar-mass black holes.
Despite having roughly the mass of a thousand sedans, a mini black hole would be smaller than an atom. At that size the black hole wouldn't swallow much matter and would instead mostly trap atoms and some larger molecules into circling orbits—in much the same way that protons in atoms capture and bind electrons.
The study authors therefore call mini black holes with orbiting material Gravitational Equivalents of an Atom, or GEAs.
"GEAs would not cause any damage to you," said study co-author Aaron VanDevender, a researcher at biotechnology firm Halcyon Molecular in Redwood City, California. "An atom bound to the GEA might get stripped off and collide into you, but you wouldn't notice. It's a very small amount of energy."
Universe Seeded With Mini Black Holes
Stellar-mass black holes are thought to form when giant, dying stars collapse, leaving corpses that are so dense not even light can escape their gravitational pull.
Scientists think multiple stellar-mass black holes can merge to form supermassive black holes, which are found in the hearts of large galaxies, including our own Milky Way.
While we can't see a black hole itself, scientists can see the light from superheated material spiraling into the black hole, creating what's known as an accretion disk.
Meanwhile, theory predicts that an abundance of tiny black holes were created shortly after the beginning of the universe, as very dense matter was expanding and cooling.
(Related: "Immaculate Black Holes Found Near Universe's Conception")
This primordial matter was not evenly distributed throughout the early cosmos, so some regions of space were denser than others, VanDevender said.
"Because of random variations in the density [of matter], some of those chunks happened to form black holes in the beginning," he said.
According to physicist Stephen Hawking, smaller black holes should actually lose mass in the form of radiation and should ultimately evaporate.
But this so-called Hawking radiation has never been observed, so the new study assumes that tiny primordial black holes continue to exist throughout the universe.
Based on their calculations, VanDevender and his father, J. Pace VanDevender of Sandia National Laboratories in Albuquerque, New Mexico, estimate that one or two of these mini black holes passes through Earth every day.
Mini Black Holes Too Small to Devour Much
According to the new study, published online this month on arXiv.org, the main behavioral difference between small and large black holes is what happens at the so-called event horizon, the closest an object can get to a black hole before it becomes impossible to escape.
The larger and more massive a black hole is, the wider its event horizon.
"We think of gravity as always being an attractive force, and in the case of very large black holes, that attraction is so great that it's going to pull everything into it," Aaron VanDevender said. "But in those cases, you're pulling it into a very large event horizon. You have a very large space to absorb things into."
(Related: "Huge Black Hole Found in Dwarf Galaxy")
By contrast, the event horizon for a mini black hole is smaller even than the diameter of an atom. This means that a mini black hole can zip through an entire planet and still have very little chance of veering close enough to an atom for it to pass the event horizon.
When a mini black hole does attract a particle, it will most likely circle the black hole far from the event horizon and not be absorbed, the theory states.
"In the GEA case, atoms don't fall into the event horizon for the same reason that electrons don't fall into a nucleus," VanDevender explained.
According to quantum mechanics, electrons don't have well-defined orbits around atoms, as the planets do around the sun. Instead the particles exist in a kind of cloud of possibilities around the nucleus. The most stable—and thus the most likely—orbit for an electron is not too close and not too far from a nucleus.
Similarly, "although a mini black hole attracts atoms using gravity ... the effect that prevents the mini black hole from absorbing its bound atoms is quantum mechanical."
Very rarely, an atom or molecule will get close enough to a mini black hole to be devoured. But the VanDevenders calculate that it would take much longer than the age of the universe for a mini black hole to swallow all the atoms in the Earth.
(Related: "Einstein Theories Confirmed by NASA Gravity Probe")
Atoms Unstable Around Tiny Black Holes?
Massimo Ricotti, an astronomer at the University of Maryland, agrees that it would be very improbable for a mini black hole to gravitationally capture an atom.
"It's very hard to accrete on tiny black holes, because they're so small," said Ricotti, who was not involved in the study. "Even if they're moving through a solid body, most of the time they find themselves almost in a vacuum, given their small size."
(Related: "Proton Smaller Than Thought—May Rewrite Laws of Physics")
Ricotti is skeptical, however, about whether atoms that do get captured can form stable orbits around a mini black hole, creating a GEA.
One reason is that the orbiting atoms would likely be superheated due to the intense gravity and would develop electrical charges. The charged particles would emit electromagnetic radiation, draining energy from the particles and ultimately causing them to spiral into the black hole.
"Certainly GEAs would be interesting objects if they exist," Ricotti added.
But "I would like to better understand some issues related to the stability of the GEA and the mechanisms by which [an atom] gets accreted in the first place."
PhysOrg | Lisa Zyga | 2011 May 13
Mini Black Holes Could Form Gravitational AtomsIn a new study, scientists have proposed that mini black holes may interact with matter very differently than previously thought. If the proposal is correct, it would mean that the time it would take for a mini black hole to swallow the Earth would be many orders of magnitude longer than the age of the Universe.Similar to how electrons orbit an atomic nucleus
without collapsing inward, mini black holes below
a certain mass may cause surrounding matter
to orbit without falling into the black hole.
Credit: Halfdan/Wikimedia Commons
In their paper, which is posted at arXiv.org, Aaron P. VanDevender from Halcyon Molecular in Redwood City, California, and J. Pace VanDevender from Sandia National Laboratories in Albuquerque, New Mexico, wanted to find a way to detect the mini black holes that are thought to exist in nature. Their calculations suggest that mini black holes may be passing through the Earth on a daily basis, and pose a very minimal threat to the planet.
Orbiting matter
Mini black holes are different than the ordinary astrophysical black hole in terms of how they’re formed and their size. Whereas astrophysical black holes are formed by the collapse of giant stars, mini black holes are thought to have formed during the Big Bang, which is why they’re also called primordial black holes. And while an astrophysical black hole has a minimum mass of 1030 kg, the mass of mini black holes range from the tiny Planck mass to trillions of kilograms or more, but are still much smaller than astrophysical black holes. (Although physics should allow for black holes of all sizes, scientists don't yet know of any mechanism that could produce objects in the intermediate range.) The expected mass of laboratory-produced mini black holes is on the small side, about 10-23 kg. Because of their extreme density, even the most massive mini black hole is microscopic in size.
The conventional view of a black hole is one of an object that is so dense that its powerful gravity pulls in all nearby matter past a critical point called the event horizon, from where it cannot escape. But the VanDevenders have suggested that something different happens with mini black holes with masses below 1012 kg. Instead of absorbing matter, these mini black holes may gravitationally bind matter, so that matter orbits the black holes at a certain distance. Because matter atoms orbiting a black hole due to gravity are reminiscent of the way that electrons orbit a nucleus due to electrostatic forces - both without collapsing inward - the physicists call this theoretical system the Gravitational Equivalent of an Atom (GEA).
Although this may seem purely theoretical, the concept could provide a way to test the current theory of how mini black holes age and die, called quantum evaporation. In this process, mini black holes lose mass until they eventually disappear. As they lose mass, they should produce X-rays. However, attempts to observe the X-ray signature of the final stages of evaporation have so far been unsuccessful. This lack of evidence suggests that either mini black holes were not created in large numbers as predicted, or that they do not evaporate.
Assuming the latter explanation, the VanDevenders propose that, instead of searching only for evaporation effects, researchers should search for evidence of the actual existence of the mini black holes, as well. If their theory of mini black holes as GEAs is correct, then the gravitationally bound matter in a GEA should produce emissions that could be detected with current detectors, even though the chance of detecting these emissions would be slim.
“Quantum evaporation has been a major cornerstone of quantum gravity theories for three decades, yet it has never been experimentally confirmed,” Aaron VanDevender told PhysOrg.com. “Our study asks, ‘what if small back holes do not evaporate?’ We have shown that if they do not evaporate, they may interact with matter and be detected. If we are able to observe such objects, it will have a significant impact on our understanding of black hole evaporation, and quantum gravity in general.”
How a GEA works
In their paper, the researchers mathematically describe how a black hole can exist on Earth without consuming all of the surrounding mass. Such a mini black hole has constraints on its Schwarzschild radius, which is the closest an object can be to a black hole before it is absorbed, never to escape. Any object smaller than its Schwarzschild radius is a black hole. But because mini black holes with masses below 1012 kg are so small, they can have a Schwarzschild radius that is much smaller than the orbit of the gravitationally bound matter particles. As long as these matter particles stay beyond the mini black hole’s Schwarzschild radius, they will orbit rather than be absorbed. (Black holes with masses of 1012 kg have a Schwarzschild radius that equals the ground state radius at which the nearest matter particles orbit, so this mass is the upper limit for a GEA.) The researchers compare the GEA’s risk of collapse with that of real atoms.
“The concern that a terrestrial GEA might absorb the earth is similar to the early 20th century expectation that electrons orbiting a nucleus should radiate their energy away and fall into the nucleus,” the researchers wrote in their study. “Since the electron energy levels are quantized and the expectation value of the radius of the ground state is much larger than the radius of the nucleus, the probability of an electron being captured by the nucleus is vanishingly small. Similarly, particles of mass m are unlikely to fall into the black hole at the center of a GEA; however, those few that do could, in principle, provide energy for observable emissions.”
Up close
The scientists calculated that mini black holes with a mass of about 100,000 kg may be of particular interest, since they could be candidates for dark matter. They estimated that, if dark matter is composed primarily of mini black holes and is evenly distributed throughout the galaxy, then about 40 million kg of mini black holes should pass through the Earth every year. The researchers calculated that about 400 mini black holes per year could be detectable through their strong electromagnetic emissions from their gravitationally bound matter.
If a particle on Earth approaches a GEA while it’s passing through the planet, the particle could either scatter off, be captured in orbit, or strip an already bound particle off. Due to the mini black hole’s high velocity compared to the binding energy required to capture a particle, the researchers predict that the mini black hole would quickly be stripped of its particles as it passes through the Earth. Therefore, the search for the emissions should be focused on space-based sources.
“It would be difficult, but not impossible [to detect one of the mini black holes passing through the Earth],” Aaron VanDevender said. “The available power of a GEA to emit detectable radiation is small but not negligible. It would likely be substantially easier to observe a GEA in orbit around the Earth, rather than one that is passing through at a tremendous velocity. Also, the larger GEA will likely be much easier to detect, so it is worth focusing our observational efforts on objects in the range of 104 to 106 kg.”
The researchers also noted that black holes created at the LHC would be too small and not have sufficient binding energy to bind matter into quantum orbitals that might emit detectable radiation.
In any case, according to this theory, mini black holes of any size would not absorb large amounts of matter very quickly. The scientists calculated that, for a black hole with a mass of 1 kg, it would take 1033 years to swallow the Earth. For comparison, the Universe is about 13.7 x 109 years old. And for smaller black holes like those that might be formed at the LHC, the time it would take to absorb the Earth would be even longer.
Technology Review | The Physics arXiv Blog | kfc | 2011 May 06
Structure and Mass Absorption of Hypothetical Terrestrial Black Holes - AP VanDevender, JP VanDevenderTiny black holes may be capable of capturing particles around them, forming the gravitational equivalent of atoms
There's a significant difference between astrophysical black holes and primordial ones. The former occur when huge stars collapse to create a region of space in which gravity is so strong that nothing can escape (which is why they are black).
And they are huge. The one sitting at the centre of our galaxy is thought to be about 4 million times more massive than the Sun.
By contrast, primordial black holes are tiny, with masses measured in tonnes. Astrophysicists believe these objects must have formed in great numbers during the Big Bang. They also think that primordial black holes slowly evaporate, finally disappearing in a puff of powerful gamma rays.
However, nobody has conclusively seen the death of a primordial black hole, leaving open the possibility that something else may be going on.
So today, Pace VanDevender at Sandia National Labs and Aaron VanDevender put forward an alternative idea. Perhaps primordial black holes don't evaporate. Instead, these objects interact with nearby particles to form the gravitational equivalent of atoms.
That's an interesting idea (and certainly no crazier than cosmologist usually contemplate). Normally, gravity is so weak that it can effectively be ignored at the scale of atoms. But that's not the case for mini black holes, which should generate forces capable of trapping atoms into orbit around them.
That immediately raises the big fear associated with black holes: that they consume all matter in their path while growing rapidly into planet-eating monsters. Won't these mini black holes simply suck any nearby atoms into oblivion?
The VanDevenders say this is unlikely. And they make a pretty convincing stab at explaining why. Their argument is similar to that which Planck and others used to develop the theory of the atom early in the last century.
The problem then was that, in classical theories, an electron orbiting an atom ought to spiral into the nucleus. So in theory, atoms shouldn't exist.
The new theory of quantum mechanics solved this by introducing the idea of quantisation in which the probability of the electron being absorbed by the nucleus is not impossible but vanishingly small.
The VanDevenders say a similar situation ought to exist for primordial black holes, provided they are small enough. These objects must have a gravitational field powerful enough to attract objects such as neutral atoms into orbit around them. But they must also have a radius that is so small that the chances of the orbiting atom encountering the black hole is vanishingly small.
The VanDevenders say that this should be true for black holes with a mass significantly smaller than a few hundred billion kilograms. And they go on to give a detailed study of some of the properties of these gravitational atoms.
For example, some black holes will be so small that the thermal energy of nearby particles will easily overcome the gravitational attraction. These black holes will scatter matter but cannot bind it into shells. Apparently, the black holes that might be formed in experiments like the LHC fall into this category.
Larger mini black holes of about 10 to1000 tonnes, however, can trap neutral atoms and so ought to be be surrounded by shells of atoms such as silicon or iron.
These objects ought to be detectable as they strike the Earth. The VanDevenders calculate that such a gravitational atom would be stripped of its orbiting atoms as it passed through Earth, creating radiofrequency emissions.
"Therefore, a search for electromagnetic signals from gravitational equivalent atoms should focus on fast moving, unidentifified rf sources in the space surrounding Earth," they say.
That's something we could look for now with relative ease. There may even be extant data that could place limits on the possibility that gravitational atoms exist.
Probably worth somebody taking a look.
- arXiv.org > gr-qc > arXiv:1105.0265 > 02 May 2011
The prospect of mini black holes, either primordial or in planned experiments at the Large Hadron Collider, interacting with the earth motivate us to examine how they may be detected and the scope of their impact on the earth. We propose that the more massive of these objects may gravitationally bind matter without significant absorption. Since the wave functions of gravitationally bound atoms orbiting a black hole are analogous to those of electrons around a nucleus, we call such an object the Gravitationally Equivalent of an Atom (GEA). Mini black holes are expected to lose mass through quantum evaporation, which has become well accepted on purely theoretical grounds. Since all attempts to directly observe x-rays from an evaporating black hole have failed, we examine the possibility of the inverse test: search for extant mini black holes by looking for emissions from matter bound in a GEA. If quantum evaporation does not occur, then miniature black holes left over from the early universe may be stable, contribute to dark matter, and in principle be detectable through emissions associated with the bound matter. We show that small black holes-with masses below 1012 kg-can bind matter without readily absorbing it into the black hole but the emissions are too weak to be detected from earth.