##### Hawking radiation mimicked in the lab

Recently scientists have come closer than ever before to creating a laboratory-scale imitation of a black hole that emits Hawking radiation, the particles predicted to escape black holes due to quantum mechanical effects.

The black hole analogue, reported in Nature Physics1, was created by trapping sound waves using an ultra cold fluid. Such objects could one day help resolve the so-called black hole ‘information paradox’ – the question of whether information that falls into a black hole disappears forever.

The physicist Stephen Hawking stunned cosmologists 40 years ago when he announced that black holes are not totally black, calculating that a tiny amount of radiation would be able to escape the pull of a black hole. This raised the tantalising question of whether information might escape too, encoded within the radiation.

Hawking radiation relies on a basic tenet of quantum theory — large fluctuations in energy can occur for brief moments of time. That means the vacuum of space is not empty but seethes with particles and their antimatter equivalents. Particle-antiparticle pairs continually pop into existence only to then annihilate each other. But something special occurs when pairs of particles emerge near the event horizon — the boundary between a black hole, whose gravity is so strong that it warps space-time, and the rest of the Universe. The particle-antiparticle pair separates, and the member of the pair closest to the event horizon falls into the black hole while the other one escapes.

Hawking radiation, the result of attempts to combine quantum theory with general relativity, comprises these escaping particles, but physicists have yet to detect it being emitted from an astrophysical black hole. Another way to test Hawking’s theory would be to simulate an event horizon in the laboratory.

To this end, Jeff Steinhauer, a physicist at the Technion-Israel Institute of Technology in Haifa, used a collection of rubidium atoms chilled to less than 1-billionth of a degree above absolute zero. At such temperatures, the atoms are tightly packed and behave as a single, fluid quantum object and so can be easily manipulated. The cold temperature also ensures that the fluid, known as a Bose-Einstein condensate, provides a silent medium for the passage of sound waves that arise from quantum fluctuations.

Using laser light, Steinhauer manipulated the fluid to flow faster than the speed of sound. Like a swimmer battling a strong current, sound waves travelling against the direction of the fluid become ‘trapped’. The condensate thus becomes a stand-in for the gravitational event horizon.

Pairs of sound waves pop in and out of existence in a laboratory vacuum, mimicking particle-antiparticle pairs in the vacuum of space. Those that form astride this sonic event horizon become the equivalent of Hawking radiation. To amplify these sound waves enough for his detectors to pick them up, Steinhauer established a second sonic event horizon inside the first, adjusting the fluid so that sound waves could not pass this second event horizon, and are bounced back. As the soundwaves repeatedly strike the outer horizon, they create more pairs of soundwaves, amplifying the Hawking radiation to detectable levels.

Some researchers say it’s still not clear how closely this laboratory model, which took Steinhauer five years to perfect, mimics Hawking radiation. The amplification in Steinhauer’s model allows him to detect only one frequency of the radiation, so he cannot be sure it has Hawking’s predicted intensity at different frequencies that true Hawking radiation would have.

Steinhauer is now working to develop the technology to study his artificial black hole without having to amplify the sonic radiation. This could allow him to use his ‘Hawking radiation’ to explore the information paradox.

It might also help physicists in their question to reconcile quantum theory with gravity, the only force in nature that has not been accommodated within quantum mechanics. Because Hawking radiation draws on both quantum mechanics and general relativity, it is a first step in addressing how to marry the two – and an artificial black hole might provide an opportunity to study how this might be done.

Experimental physicist Daniele Faccio of Heriot-Watt University in Edinburgh calls the work “possibly the most robust and clear-cut evidence” that laboratory models can emulate phenomena at the interface between general relativity and quantum mechanics. In 2010, Faccio and his colleagues reported that they had detected an analogue of Hawking radiation3, but the team has since acknowledged they had seen a different phenomenon.

However Physicist Ted Jacobson of the University of the Maryland in College Park, who suggested in 1999 that analogue radiation could be seen in the laboratory4, says that the possibility of gleaning new insights about black holes from the sonic experiment remains “far fetched”, for now. For Jacobson, the value of the experiment lies in exploring the physics of ultracold atoms.

But even if the sonic radiation as it stands is not a perfect match, William Unruh, a theoretical physicist at the University of British Columbia in Vancouver points out that “it is the closest anyone has come” to detecting Hawking radiation. “I find it a very exciting and interesting experiment,’ he says.

#### Ok so lets start the explanation, What is Hawking Radiation and how is it related to black Holes?

The answer briefly is that; Hawking radiation (sometimes also called Bekenstein-Hawking radiation) is a theoretical prediction from British physicist Stephen Hawking, which explains thermal properties relating to black hole.

Normally, a black hole is considered to draw all matter and energy in the surrounding region into it, as a result of the intense gravitational fields. However, in 1972 the Israeli physicist Jacob Bekenstein suggested that black holes should have a well-defined entropy, and initiated the development of black hole thermodynamics, including the emission of energy.

In 1974, British physicist Stephen Hawking worked out the exact theoretical model for how a black hole could emit black body radiation.

In a simplified version of the explanation, Hawking predicted that energy fluctuations from the vacuum causes the generation of particle-antiparticle pairs of virtual particles near the event horizon of the black hole. One of the particles falls into the black hole while the other escapes, before they have an opportunity to annihilate each other. The net result is that, to someone viewing the black hole, it would appear that a particle had been emitted.

Since the particle that is emitted has positive energy, the particle that gets absorbed by the black hole has a negative energy relative to the outside universe. This results in the black hole losing energy, and thus mass (because E = mc2).

Smaller primordial black holes can actually emit more energy than they absorb, which results in them losing net mass. Larger black holes, such as those that are one solar mass, absorb more cosmic radiation than they emit through Hawking radiation.

Hawking radiation was one of the first theoretical predictions which provided insight into how gravity can relate to other forms of energy, which is a necessary part of any theory of quantum gravity.

Though Hawking radiation is generally accepted by the scientific community, there is still some controversy associated with it. There are some concerns that it ultimately results in information being lost, which makes physicists uncomfortable. Alternately, those who don’t actually believe that black holes themselves exist are similarly reluctant to accept that they absorb particles.

Now lets be honest, this is a very very brief explanation of what Hawking’s Radiation is, so if you are interested you can read further for a LOT more information.

Hawking radiation is black body radiation that is predicted to be released by black holes, due to quantum effects near the event horizon.

It is named after the physicist Stephen Hawking, who provided a theoretical argument for its existence in 1974, and sometimes also after Jacob Bekenstein, who predicted that black holes should have a finite, non-zero temperature and entropy.

Hawking’s work followed his visit to Moscow in 1973 where the Soviet scientists Yakov Zeldovich and Alexei Starobinsky showed him that according to the quantum mechanical uncertainty principle, rotating black holes should create and emit particles. Hawking radiation reduces the mass and the energy of the black hole and is therefore also known as black hole evaporation. Because of this, black holes that lose more mass than they gain through other means are expected to shrink and ultimately vanish. Micro black holes (MBHs) are predicted to be larger net emitters of radiation than larger black holes and should shrink and dissipate faster.

In September 2010, a signal that is closely related to black hole Hawking radiation (see analog gravity) was claimed to have been observed in a laboratory experiment involving optical light pulses. However, the results remain unverified and debatable. Other projects have been launched to look for this radiation within the framework of analog gravity. In June 2008, NASA launched the Fermi space telescope, which will search for the terminal gamma-ray flashes expected from evaporating primordial black holes. In the event that speculative large extra dimension theories are correct, CERN’s Large Hadron Collider may be able to create micro black holes and observe their evaporation

Black holes are sites of immense gravitational attraction. Classically, the gravitation is so powerful that nothing, not even electromagnetic radiation (including light), can escape from the black hole. It is yet unknown how gravity can be incorporated into quantum mechanics, nevertheless, far from the black hole the gravitational effects can be weak enough for calculations to be reliably performed in the framework of quantum field theory in curved spacetime. Hawking showed that quantum effects allow black holes to emit exact black body radiation, which is the average thermal radiation emitted by an idealized thermal source known as a black body. The electromagnetic radiation is as if it were emitted by a black body with a temperature that is inversely proportional to the black hole’s mass.

Physical insight into the process may be gained by imagining that particle-antiparticle radiation is emitted from just beyond the event horizon. This radiation does not come directly from the black hole itself, but rather is a result of virtual particles being “boosted” by the black hole’s gravitation into becoming real particles. As the particle-antiparticle pair was produced by the black hole’s gravitational energy, the escape of one of the particles takes away some of the mass of the black hole.

A slightly more precise, but still much simplified, view of the process is that vacuum fluctuations cause a particle-antiparticle pair to appear close to the event horizon of a black hole. One of the pair falls into the black hole while the other escapes. In order to preserve total energy, the particle that fell into the black hole must have had a negative energy (with respect to an observer far away from the black hole). By this process, the black hole loses mass, and, to an outside observer, it would appear that the black hole has just emitted a particle. In another model, the process is a quantum tunnelling effect, whereby particle-antiparticle pairs will form from the vacuum, and one will tunnel outside the event horizon.

An important difference between the black hole radiation as computed by Hawking and thermal radiation emitted from a black body is that the latter is statistical in nature, and only its average satisfies what is known as Planck’s law of black body radiation, while the former fits the data better. Thus thermal radiation contains information about the body that emitted it, while Hawking radiation seems to contain no such information, and depends only on the mass, angular momentum, and charge of the black hole (the no-hair theorem). This leads to the black hole information paradox.

However, according to the conjectured gauge-gravity duality (also known as the AdS/CFT correspondence), black holes in certain cases (and perhaps in general) are equivalent to solutions of quantum field theory at a non-zero temperature. This means that no information loss is expected in black holes (since the theory permits no such loss) and the radiation emitted by a black hole is probably the usual thermal radiation. If this is correct, then Hawking’s original calculation should be corrected, though it is not known how (see below).

A black hole of one solar mass has a temperature of only 60 nanokelvin (60 billionths of a kelvin); in fact, such a black hole would absorb far more cosmic microwave background radiation than it emits. A black hole of 4.5 × 1022 kg (about the mass of the Moon) would be in equilibrium at 2.7 kelvin, absorbing as much radiation as it emits. Yet smaller primordial black holes would emit more than they absorb and thereby lose mass.

Now there is a huge amount of more information in relation to this, that I just can not fit onto this post; however, although some of it can be VERY confusing, it can also help explain a lot of the reasoning and the overall understanding of the aforementioned. Therefore if you do need more information or are studying this subject, I would seriously consider reading the rest of the article produced on the wiki page here. This being said, I personally found it a lot easier and quicker to grasp by first watching the some documentaries about the subject first, but hey that’s just my study preference 🙂

##### Stephen Hawking – A Brief History of mine

##### Black Holes – An Introduction

##### Stephen Hawking

##### PDF Documents

Hawking Radiation of Black Holes

Hawking Radiation and Black Hole Evaporation

Hawking Radiation and Black Hole Thermodynamics

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Main article “Hawking radiation mimicked in the lab” Written by: Ron Cowen

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