Posted on by Tammy Plotner

Pardon Me, But Your Black Hole Is Leaking…

Gaia BH1 is a Sun-like star co-orbiting with a black hole estimated at 10 times the Sun's mass. Credit: ESO/L. Calcada

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Yes. We thought we knew everything there was to know about black holes. We know they are massive and compact. We know they possess a gravity so intense that it even bends “space time”. We know they won’t even allow light to escape. But what we weren’t really prepared for is that our human line of reasoning might be wrong. Black holes might consume everything… But they leak information.

Thanks to a new study done by Professor Samuel Braunstein and Dr Manas Patra of the University of York, we just might need to realign our way of thinking about black holes and one of the most fundamental forces of Nature – gravity. Professor Braunstein says: “Our results didn’t need the details of a black hole’s curved space geometry. That lends support to recent proposals that space, time and even gravity itself may be emergent properties within a deeper theory. Our work subtly changes those proposals, by identifying quantum information theory as the likely candidate for the source of an emergent theory of gravity.”

Are your quantum mechanics a bit rusty? Then blame a few holes in these theories. “This vision was motivated in part by Jacobson’s 1995 surprise result that the Einstein equations of gravity follow from the thermodynamic properties of event horizons.” says the team. “Taking a first tentative step in such a program, we derive the evaporation rate (or radiation spectrum) from black hole event horizons in a spacetime-free manner. Our result relies on a Hilbert space description of black hole evaporation, symmetries therein which follow from the inherent high dimensionality of black holes, global conservation of the no-hair quantities, and the existence of Penrose processes. Our analysis is not wedded to standard general relativity and so should apply to extended gravity theories where we find that the black hole area must be replaced by some other property in any generalized area theorem.”

Like your elderly neighbor whose curtains twitch each time you take your telescope into the yard at night and hastens to grab the telephone to tell other neighbors, information can leak from a black hole. The neighbor knows you’re out there… And soon enough, the rest of the neighbors know as well. Professor Braunstein says: “Our results actually extend the predictions made by well-established techniques that rely on a detailed knowledge of space time and black hole geometry.”

Dr Patra adds: “We cannot claim to have proven that escape from a black hole is truly possible, but that is the most straight-forward interpretation of our results. Indeed, our results suggest that quantum information theory will play a key role in a future theory combining quantum mechanics and gravity.”

For Further Reading: Black Hole Evaporation Rates without Spacetime. Original News Source: University of York News Release.

Posted on by Matt Williams

What is Fermi Energy?

When it comes to physics, the concept of energy is a tricky thing, subject to many different meanings and dependent on many possible contexts. For example, when speaking of atoms and particles, energy comes in several forms, such as electrical energy, heat energy, and light energy.

But when one gets into the field of quantum mechanics, a far more complex and treacherous realm, things get even trickier. In this realm, scientists rely on concepts such as Fermi Energy, a concept that usually refers to the energy of the highest occupied quantum state in a system of fermions at absolute zero temperature.

Fermions:

Fermions take their name from famed 20th century Italian physicist Enrico Fermi. These are subatomic particles that are usually associated with matter, whereas subatomic particles like bosons are force carriers (associated with gravity, nuclear forces, electromagnetism, etc.) These particles (which can take the form of electrons, protons and neutrons) obey the Pauli Exclusion Principle, which states that no two fermions can occupy the same (one-particle) quantum state.

Neils Bohr's model a nitrogen atom. Credit: britannica.com
Neils Bohr’s model a nitrogen atom. Credit: britannica.com

In a system containing many fermions (like electrons in a metal), each fermion will have a different set of quantum numbers. Fermi energy, as a concept, is important in determining the electrical and thermal properties of solids. The value of the Fermi level at absolute zero (-273.15 °C) is called the Fermi energy and is a constant for each solid. The Fermi level changes as the solid is warmed and as electrons are added to or withdrawn from the solid.

Calculating Fermi Energy:

To determine the lowest energy a system of fermions can have (aka. it’s lowest possible Fermi energy), we first group the states into sets with equal energy, and order these sets by increasing energy. Starting with an empty system, we then add particles one at a time, consecutively filling up the unoccupied quantum states with the lowest energy.

When all the particles have been put in, the Fermi energy is the energy of the highest occupied state. What this means is that even if we have extracted all possible energy from a metal by cooling it to near absolute zero temperature (0 kelvin), the electrons in the metal are still moving around. The fastest ones are moving at a velocity corresponding to a kinetic energy equal to the Fermi energy.

Bosons, fermions and other particles after a collsion. Credit: CERN
Image showing bosons, fermions and other particles created by a high-energy collision. Credit: CERN

Applications:

The Fermi energy is one of the important concepts of condensed matter physics. It is used, for example, to describe metals, insulators, and semiconductors. It is a very important quantity in the physics of superconductors, in the physics of quantum liquids like low temperature helium (both normal and superfluid 3He), and it is quite important to nuclear physics and to understand the stability of white dwarf stars against gravitational collapse.

Confusingly, the term “Fermi energy” is often used to describe a different but closely-related concept, the Fermi level (also called chemical potential). The Fermi energy and chemical potential are the same at absolute zero, but differ at other temperatures.

We have written many interesting articles about quantum physics here at Universe Today. Here’s What is the Bohr Atomic Model?, Quantum Entanglement Explained, What is the Electron Cloud Model, What is the Double Slit Experiment?, What is Loop Quantum Gravity? and Unifying the Quantum Principle – Flowing Along in Four Dimensions.

If you’d like more info on Fermi Energy, check out these articles from Hyperphysics and Science World.

We’ve also recorded an entire episode of Astronomy Cast all about Quantum Mechanics. Listen here, Episode 138: Quantum Mechanics.

Sources:

Posted on by Jean Tate

What is Schrodinger’s Cat?

Schrodinger’s cat is named after Erwin Schrödinger, a physicist from Austria who made substantial contributions to the development of quantum mechanics in the 1930s (he won a Nobel Prize for some of this work, in 1933). Apart from the poor cat (more later), his name is forever associated with quantum mechanics via the Schrödinger equation, which every physics student has to grapple with.

Schrodinger’s cat is actually a thought experiment (Gedankenexperiment) – and the cat may not have been Erwin’s, but his wife’s, or one of his lovers’ (Erwin had an unconventional lifestyle) – designed to test a really weird implication of the physics he and other physicists was developing at the time. It was motivated by a 1935 paper by Einstein, Podolsky, and Rosen; this paper is the source of the famous EPR paradox.

In the thought experiment, Schrodinger’s cat is placed inside a box containing a piece of radioactive material, and a Geiger counter wired to a flask of poison in such a way that if the Geiger counter detects a decay, then the flask is smashed, the poison gas released, and the cat dies (fun piece of trivia: an animal rights group accused physicists of cruelty to animals, based on a distorted version of this thought experiment! though maybe that’s just an urban legend). The half-life of the radioactive material is an hour, so after an hour, there is a 50% probability that the cat is dead, and an equal probability that it is alive. In quantum mechanics, these two states are superposed (a technical term), and the cat is neither dead nor alive, or half-dead and half-alive, or … which is really, really weird.

Now the theory – quantum mechanics – has been tested perhaps more thoroughly than any other theory in physics, and it seems to describe how the universe behaves with extraordinary accuracy. And the theory says that when the box is opened – to see if the cat is dead, alive, half-dead and half-alive, or anything else – the wavefunction (describing the cat, Geiger counter, etc) collapses, or decoheres, or that the states are no longer entangled (all technical terms), and we see only a dead cat or cat very much alive.

There are several ways to get your mind around what’s going on – or several interpretations (you guessed it, yet another technical term!) – with names like Copenhagen interpretation, many worlds interpretation, etc, but the key thing is that the theory is mute on the interpretations … it simply says you can calculate stuff using the equations, and what your calculations show is what you’ll see, in any experiment.

Fast forward to some time after Schrödinger – and Einstein, Podolsky, and Rosen – had died, and we find that tests of the EPR paradox were proposed, then conducted, and the universe does indeed seem to behave just like schrodinger’s cat! In fact, the results from these experimental tests are used for a kind of uncrackable cryptography, and the basis for a revolutionary kind of computer.

Keen to learn more? Try these: Schrödinger’s Rainbow is a slideshow review of the general topic (California Institute of Technology; caution, 3MB PDF file!); Schrodinger’s cat comes into view, a news story on a macroscopic demonstration; and Schrödinger’s Cat (University of Houston).

Schrodinger’s cat is indirectly referenced in several Astronomy Cast episodes, among them Quantum Mechanics, and Entanglement; check them out!

Sources: Cornell University, Wikipedia