physics4me

physicsgg

Posts Tagged ‘BLACK HOLES

Researchers observe stationary Hawking radiation in an analog black hole

leave a comment »

Black holes are regions in space where gravity is very strong—so strong that nothing that enters them can escape, including light. Theoretical predictions suggest that there is a radius surrounding black holes known as the event horizon. Once something passes the event horizon, it can no longer escape a black hole, as gravity becomes stronger as it approaches its center.

Theoretical physicist Stephen Hawking predicted that while nothing can escape from within them, black holes spontaneously emit a limited amount of light, which is known as Hawking radiation. According to his predictions, this radiation is spontaneous (i.e., it arises from nothing) and stationary (i.e., its intensity does not change much over time).

Researchers at Technion- Israel Institute of Technology have recently carried out a study aimed at testing Hawking’s theoretical predictions. More specifically, they examined whether the equivalent of Hawking radiation in an “artificial black hole” created in a laboratory setting was stationary.

“If you go inside the event horizon, there’s no way to get out, even for light,” Jeff Steinhauer, one of the researchers who carried out the study, told Phys.org. “Hawking radiation starts just outside the event horizon, where light can barely escape. That is really weird because there’s nothing there; it’s empty space. Yet this radiation starts from nothing, comes out, and goes towards Earth.”

The artificial black hole created by Steinhauer and his colleagues was approximately 0.1 millimeters long and was made of a gas composed of 8000 rubidium atoms, which is a relatively low number of atoms. Every time the researchers took a picture of it, the black hole was destroyed. To observe its evolution over time, they thus had to produce the black hole, take a picture of it and then create another one. This process was repeated many times, for months.

The Hawking radiation emitted by this analog black hole is made of sound waves, rather than light waves. The rubidium atoms flow faster than the speed of sound, so sound waves cannot reach the event horizon and escape from the black hole. Outside of the event horizon, however, the gas flows slowly, so sound waves can move freely.

“The rubidium is flowing fast, faster than the speed of sound, and that means that sound cannot go against the flow,” Steinhauer explained. “Let’s say you were trying to swim against the current. If this current is going faster than you can swim, then you can’t move forward, you are pushed back because the flow is moving too fast and in the opposite direction, so you’re stuck. That’s what being stuck in a black hole and trying to reach the event horizon from inside would be like.”

According to Hawking’s predictions, the radiation emitted by black holes is spontaneous. In one of their previous studies, Steinhauer and his colleagues were able to confirm this prediction in their artificial black hole. In their new study, they set out to investigate whether the radiation emitted by their black hole is also stationary (i.e., if it remains constant over time).

“A black hole is supposed to radiate like a black body, which is essentially a warm object that emits a constant infrared radiation (i.e., black body radiation),” Steinhauer said. “Hawking suggested that black holes are just like regular stars, which radiate a certain type of radiation all the time, constantly. That’s what we wanted to confirm in our study, and we did.”

Hawking radiation is composed of pairs of photons (i.e., light particles): one emerging from a black hole and another falling back into it. When trying to identify the Hawking radiation emitted by the analog black hole they created, Steinhauer and his colleagues thus looked for similar pairs of sound waves, one coming out of the black hole and one moving into it. Once they identified these pairs of sound waves, the researchers tried to determine whether there were so-called correlations between them.

“We had to collect a lot of data to see these correlations,” Steinhauer said. “We thus took 97,000 repetitions of the experiment; a total of 124 days of continuous measurement.”

Overall, the findings appear to confirm that the radiation emitted by black holes is stationary, as predicted by Hawking. While these findings apply primarily to the analog black hole they created, theoretical studies could help to confirm if they can also be applied to real black holes.

“Our study also raises important questions, because we observed the entire lifetime of the analog black hole, which means that we also saw how the Hawking radiation started,” Steinhauer said. “In future studies, one could try to compare our results with predictions of what would happen in a real black hole, to see if ‘real’ Hawking radiation starts from nothing and then builds up, as we observed.”

At some point during the researchers’ experiments, the radiation surrounding their analog black hole became very strong, as the black hole formed what is known as an ‘inner horizon.” In addition to the event horizon, Einstein’s theory of general relativity predicts the existence of an inner horizon, a radius inside black holes that delineates a further region closer to its center.

In the region inside the inner horizon the gravitational pull is far lower, thus objects are able to move around freely and are no longer pulled towards the center of the black hole. Yet they are still unable to leave the black hole, as they cannot pass through the inner horizon in the opposite direction (i.e., heading toward the event horizon).

“Essentially, the event horizon is a black hole’s outer sphere, and inside it, there’s a small sphere called the inner horizon,” Steinhauer said. “If you fall through the inner horizon, then you’re still stuck in the black hole, but at least you don’t feel the weird physics of being in a black hole. You’d be in a more ‘normal’ environment, as the pull of gravity would be lower, so you wouldn’t feel it anymore.”

Some physicists have predicted that when an analog black hole forms an inner horizon, the radiation it emits becomes stronger. Interestingly, this is exactly what happened in the analog black hole created by the researchers at Technion. This study could thus inspire other physicists to investigate the effect of the formation of an inner horizon on the intensity of a black hole’s Hawking radiation.

https://phys.org/news/2021-02-stationary-hawking-analog-black-hole.html

Written by physicsgg

February 20, 2021 at 8:13 am

Posted in RELATIVITY

Tagged with ,

Hawking for beginners

leave a comment »

A dimensional analysis activity to perform in the classroom
Jorge Pinochet
In this paper we present a simple dimensional analysis exercise that allows us to derive the equation for the Hawking temperature of a black hole. The exercise is intended for high school students, and it is developed from a chapter of Stephen Hawking’s bestseller A Brief History of Time.
Read more at https://arxiv.org/pdf/2004.11850.pdf

Click to access 2004.11850.pdf

Written by physicsgg

April 28, 2020 at 8:39 pm

Comments on magnetic black holes

leave a comment »


Juan Maldacena
We discuss aspects of magnetically charged black holes in the Standard Model. For a range of charges, we argue that the electroweak symmetry is restored in the near horizon region. The extent of this phase can be macroscopic. If Q is the integer magnetic charge, the fermions lead to order Q massless two dimensional fermions moving along the magnetic field lines. These greatly enhance Hawking radiation effects.

Read more at https://arxiv.org/pdf/2004.06084.pdf

Click to access 2004.06084.pdf

Written by physicsgg

April 15, 2020 at 9:36 am

Posted in High Energy Physics

Tagged with

THAT Black Hole picture

leave a comment »

Mike Merrifield, Omar Almaini and Meghan Gray react to the much-anticipated black hole photo from the Event Horizon Telescope Collaboration.

Written by physicsgg

April 11, 2019 at 6:19 am

Posted in ASTRONOMY, ASTROPHYSICS

Tagged with

Black Hole Entropy is Thermodynamic Entropy

leave a comment »

Schematic illustration of a black hole Carnot cycle. The system consists of a black hole and a photon gas, enclosed in a box. The size of the black hole is proportional to the temperature of the system, i.e. small is hot and large is cold.

Carina E. A. Prunkl, Christopher G. Timpson
The comparison of geometrical properties of black holes with classical thermodynamic variables reveals surprising parallels between the laws of black hole mechanics and the laws of thermodynamics. Since Hawking’s discovery that black holes when coupled to quantum matter fields emit radiation at a temperature proportional to their surface gravity, the idea that black holes are genuine thermodynamic objects with a well-defined thermodynamic entropy has become more and more popular. Surprisingly, arguments that justify this assumption are both sparse and rarely convincing. Most of them rely on an information-theoretic interpretation of entropy, which in itself is a highly debated topic in the philosophy of physics. We discuss some of the pertinent arguments that aim at establishing the identity of black hole surface area (times a constant) and thermodynamic entropy and show why these arguments are not satisfactory. We then present a simple model of a Black Hole Carnot cycle to establish that black hole entropy is genuine thermodynamic entropy which does not require an information-theoretic interpretation.
Read more at https://arxiv.org/pdf/1903.06276.pdf

Written by physicsgg

March 19, 2019 at 7:21 pm

Black Hole as Extreme Particle Accelerator

leave a comment »

Life of the jet set. This simulation follows along in a “co-moving” reference frame with a fixed set of particles as they are blasted out of an active galactic nucleus (AGN). The magnetic field lines they experience change as they move from a smoother region (left) to a region with a kink instability (right).  [Credit: E. P. Alves et al., Phys. Rev. Lett. (2018)]

Efficient Nonthermal Particle Acceleration by the Kink Instability in Relativistic Jets

E. Paulo Alves, Jonathan Zrake, Frederico Fiuza
Relativistic magnetized jets from active galaxies are among the most powerful cosmic accelerators, but their particle acceleration mechanisms remain a mystery. We present a new acceleration mechanism associated with the development of the helical kink instability in relativistic jets, which leads to the efficient conversion of the jet’s magnetic energy into nonthermal particles. Large-scale three-dimensional ab initio simulations reveal that the formation of highly tangled magnetic fields and a large-scale inductive electric field throughout the kink-unstable region promotes rapid energization of the particles. The energy distribution of the accelerated particles develops a well-defined power-law tail extending to the radiation-reaction limited energy in the case of leptons, and to the confinement energy of the jet in the case of ions. When applied to the conditions of well-studied bright knots in jets from active galaxies, this mechanism can account for the spectrum of synchrotron and inverse Compton radiating particles, and offers a viable means of accelerating ultra-high-energy cosmic rays to 1020 eV.

Read more at https://physics.aps.org/articles/v11/130 and https://arxiv.org/pdf/1810.05154.pdf

Written by physicsgg

December 16, 2018 at 9:25 pm

The black hole fifty years after: Genesis of the name

leave a comment »

Ann Ewing’s article in 1964 where the term Black Hole is published for the first time

Carlos A. R. Herdeiro, José P. S. Lemos
Black holes are extreme spacetime deformations where even light is imprisoned. There is an extensive astrophysical evidence for the real and abundant existence of these prisons of matter and light in the Universe. Mathematically, black holes are described by solutions of the field equations of the theory of general relativity, the first of which was published in 1916 by Karl Schwarzschild.
Another highly relevant solution, representing a rotating black hole, was found by Roy Kerr in 1963. It was only much after the publication of the Schwarzschild solution, however, that the term black hole was employed to describe these objects. Who invented it?
Conventional wisdom attributes the origin of the term to the prominent North American physicist John Wheeler who first adopted it in a general audience article published in 1968. This, however, is just one side of a story that begins two hundred years before in an Indian prison colloquially known as the Black Hole of Calcutta.
Robert Dicke, also a distinguished physicist and colleague of Wheeler at Princeton University, aware of the prison’s tragedy began, around 1960, to compare gravitationally completely collapsed stars to the black hole of Calcutta. The whole account thus suggests reconsidering who indeed coined the name black hole and commends acknowledging its definitive birth to a partnership between Wheeler and Dicke.
Read more at https://arxiv.org/pdf/1811.06587.pdf

Written by physicsgg

November 20, 2018 at 2:52 pm

What Is a Black Hole?

leave a comment »

Erik Curiel
Although black holes are objects of central importance across many fields of physics, there is no agreed upon definition for them, a fact that does not seem to be widely recognized. Physicists in different fields conceive of and reason about them in radically different, and often conflicting, ways. All those ways, however, seem sound in the relevant contexts. After examining and comparing many of the definitions used in practice, I consider the problems that the lack of a universally accepted definition leads to, and discuss whether one is in fact needed for progress in the physics of black holes. I conclude that, within reasonable bounds, the profusion of different definitions is in fact a virtue, making the investigation of black holes possible and fruitful in all the many different kinds of problems about them that physicists consider, although one must take care in trying to translate results between fields.
Read more at https://arxiv.org/pdf/1808.01507.pdf

Written by physicsgg

August 19, 2018 at 10:06 am