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Πέμπτη, 28 Σεπτεμβρίου 2017

The Odd Case of Quantum Black Holes

The Odd Case of Quantum Black Holes


Black Holes can be considered as some of the most mysterious yet fascinating objects in the universe. As they are usually portrayed in science fiction these astronomical riddles are the omnivores of the universe, consuming everything, even light, that dares to approach their observable boundary, aka the ”Event Horizon”, never to be seen again. In principle nothing can escape from them. Yet, as science has often done in the past, this consensus has been partially disproven by recent developments in quantum mechanics, as in its place arose a new problem also known as the information paradox which still to this day remains unresolved.

Introduction




1.1 The creation of a Black Hole

To understand what the problem is we must first take closer look to how black holes are formed. According to Einstein’s general theory of relativity if an object is compressed enough it can carve a region in space-time from which nothing can escape. But wait Jason! you might remark; Black holes are just stars that have died right? Well yes and no. Black Holes can indeed be massive stars that have ”died”, or to phrase it better stars that have went through a process called gravitational collapse in which the star literally contracts onto itself under the influence of its own gravity (why that happens now is an entirely different matter in and off itself and because I don’t want to blabber about something unrelated to the original subject of this article you can read more about it in this article by NASA:
 https://map.gsfc.nasa.gov/universe/rel stars.html),
 but giant stars are not the only things that can turn into black holes.

 Literally any object that is compressed down to what is called the Schwarzschild radius can turn into a black hole. The Schwarzschild radius is simply the radius of a sphere from which no information, no light, no particle can escape so that we can measure it (or so we thought, you’ll see what I mean later). The surface of that sphere is what we call the Event Horizon that I mentioned in the abstract. If the sun where to turn into a black hole it would have to be squeezed in a sphere with radius of about three kilometers! In the case that you would want to derive the Schwarzschild radius of anything you can simply use this equation: 








The only problem with creating a black hole is the amount of energy required to contract an object within such a small space, since the force to counteract the repulsive quantum forces between its subatomic particles is too great. Such energy is only observed in the gravitational collapses of giant stars of about six or seven solar masses, making black hole candidates scarce in the present universe. But here’s the catch, it turns out stellar collapse is not the only way to create black holes.


1.2 Primordial Black Holes


We generally know that the universe is expanding, decreasing in density, constantly, therefore it is safe to assume that in the past the average density was way way higher than today and in fact so high as to exceed the nuclear density of 2.3 × 1017 kg/m3 in the first microsecond of the life of the universe. The highest value of density that the universe could have started with is the so known Planck density, about 1097 g/cm3 , a density so high that even the fabric of space-time would break down. At these conditions black holes could have formed as small as 10−35 m across, also known as the Planck length, and with a mass of 10−8 kg, dimensions comparable to elementary particles. For all this incredible information we have to thank Stephen Hawking and Bernard J. Carr for their derivation of this mechanism for creating black holes[2]. 4 The fact that black holes could be so small intrigued Hawking, making him ponder about the possible quantum effects that could come into play in such a small scale. Thus came his famous conclusion of black holes emitting particles rather than just swallowing them[3].




2 Hawking Radiation


2.1 Emission and Evaporation




How can a black hole emit particles and why should it do that? A strange question indeed and difficult to answer at that. To make sense of this situation we must first take a look at some of quantum mechanics most basic principles.

 2.1.1 Vacuum Fluctuation



Heisenberg’s uncertainty principle states that one cannot know in great detail and in the same time the exact position and speed of a particle, or - from a more mathematical viewpoint - the product of the uncertainty of both position and momentum should always be greater than a certain value as seen in this inequality:

 ∆x∆p ≥ ¯h /2 

Where ¯h is the reduced Planck constant ( h /2π or 1.054×10−34m2kg/s). 

This relation has also been derived[4] in a energy/time form:

∆E∆t ≥ ¯h /2 

which means that in any point in space there must always be a minimal change in energy no matter how small or ”empty” this space is. Such a realization is very important because it necessitates that a vacuum can never truly be a vacuum, as in containing nothing, meaning that a small amount of energy should be created and later destroyed in order to fill this empty space. Wow slow down there! What about the conservation of energy? Energy cannot be created nor destroyed! Worry not, everything is fine since Quantum Mechanics show us that there is no violation of the law when we measure very short instances of time, thus the uncertainty of energy is incredibly high.


2.1.2 Particle Pairs



Many of you might be baffled by the fact that particles can both appear from and disappear into nothing but stay with me since this is a very important property of particles and it directly leads us to why black holes emit matter.
 Einstein has generously provided us with a very important equation, the famous 

E = mc2 , 

that shows us that energy and mass are just two faces of the same coin. Therefore if energy can pop into existence as the energy/time uncertainty principle dictates, then mass can too. If in a small space where one would consider to be a true vacuum suddenly appears a particle for a short moment and if that particle happens to disappear after this time has passed then there is a change in the energy in the space and so the uncertainty principle is satisfied. This happens all the time in space, constantly particles pop in existence only to disappear a moment later, filling space with energy fluctuations. An important requirement for such an event to unfold is that not only one particle appears but actually two, one made of matter and one of antimatter. That way particles would annihilate each other after a small amount of time. These particles are known as virtual particles. Also we must clarify that one of two particles has negative energy thus (stay with me) negative mass. This will be important in the next part of the article.



2.1.3 Radiation and Black Hole Temperature


If virtual particles come into existence extremely close to the surface of a black hole, even closer than the photon sphere (a spherical cortex on which light literally can orbit the black hole), then there is a chance that the particle pair will not annihilate itself but rather the particles will part ways, one escaping the black hole and the other falling into it. If the second one happens to be the one with negative mass then the net mass of the black hole decreases, and since we cannot see the negative mass particle as it has fallen into the hole we would only see the bizarre effect of the black hole shrinking accompanied by an emission of a particle than in actuality was just the part of the pair that survived. Some of you may point out the fact that in the grand scheme of things, there should be an equal chance of both the negative mass particle and the positive mass particle to fall into the black hole. That is not the case and the reasoning for the phenomenon takes us back to the world of Thermodynamics. The 6 second law of Thermodynamics states that the entropy of an isolated system (Entropy being the measure of disorder in the universe [5]) must always increase or stay at an equilibrium. Thus in a statistical view of the system there is a higher chance that the negative mass particle will plunge into the black hole, as this action will increase the entropy of both the system and consequently the universe.




Nevertheless, what we must take from all this is that we see that the black hole has the ability to radiate particles in a way very reminiscent of to what we know as temperature. Hawking in his work to study the effects of quantum mechanics on the surface of a black hole he derived a formula[3] also that shows that the temperature of a black hole is inversely proportional to the mass of the black hole, meaning a small black hole emits more energy than a big one. The formula he derived is this:









which shows us that its proportional to the hole’s mass. As the hole decreases in size it evaporates energy faster and in greater amounts until it gets so small that the amount of energy that needs to be released is so great and it has to be released in such a small amount of time that the hole literally explodes with more power than a million-megaton nuclear bomb! Of course these formulas are only approximations since a lot more things come into play when dealing with real life black holes like the radiation from the Cosmic Microwave Background or random matter falling into the black hole.




2.2 Leading to the information paradox


Hawking’s work is incredibly important and praise worthy as he connected three seemingly unrelated areas of physics, these being Relativity, Quantum Mechanics and Thermodynamics. Unfortunately this is the part that I introduce the problem that I mentioned in the abstract, the infamous information paradox, that took the physics society by storm.



3 The Information Paradox


3.1 Relativity versus Quantum Mechanics


3.1.1 The essence of information 



 In physics the term information is not really something tangible, but it must not be mistaken for not being well defined. Physical information can be defined as the complete wavefunction of a particle or just all of the properties of the particle (these being charge, spin, mass etc.). This definition can be expanded to an arrangement of particles, denoting to the way that the individual particles are connected and interact with each other.
 A very good example of this way of thinking has been recently introduced by the YouTube channel Kurzgesagt in their recent video ”Why Black Holes Could Delete The Universe The Information Paradox”[6]. In their video they say that if you arrange a bunch of carbon atoms in a certain way you will get coal but if you arrange them in a different way you get diamond, therefore information is just a property of the arrangement of these atoms. A very important law that also has to be stated is the conservation of information, that is that information cannot be destroyed (some- 8 thing that was derived by the quantum field theory and Liouville’s theorem). It can be separated to small pieces that are very hard to measure accurately (like burning a piece of paper and then trying to reconstruct the original by measuring every single change that underwent with every single molecule), or it can be stored somewhere that is not accessible by the laws of physics i.e. the surface of a black hole.




3.1.2 The contradiction



The paradox that arose by the conjecture of hawking radiation stems from the fact that information is ”lost” when falling into a black hole according to relativity. But if the black hole evaporates its mass away completely what happens to the mass inside? Information cannot be destroyed according to quantum mechanics but that is what would happen if a black hole where to evaporate completely. What is going on here? Hawking strongly believed in his theory and supported it firmly, suggesting that information is indeed lost. A conviction of this scale is frightening since it can mean that our current understanding of physics is so deeply mistaken that we would need to scrap all of the efforts of thousands of physicists across history out of the window and force us to start over.




3.2 Other Solutions


Only recently physicists have come up with possible ways to cut this gordian knot, with propositions like the black hole leaving a remnant of information after its death or creating an entirely new universe briefly before its death to store the information or it just leaks out the information over time. Still all of those hypotheses are yet to be proved since we have not yet seen a black hole evaporate, as we do not have the proper equipment for such a feat in our current technological state.



 References


[1] M.L. Kutner. Astronomy: A Physical Perspective. Cambridge University Press, 2003. 
[2] B. J. Carr and S. W. Hawking. Black holes in the early Universe. Monthly Notices of the Royal Astronomical Society, 168:399–416, aug 1974.
 [3] Stephen W Hawking. Particle creation by black holes. Communications in mathematical physics, 43(3):199–220, 1975.
 [4] J S Briggs. A derivation of the time-energy uncertainty relation. Journal of Physics: Conference Series, 99(1):012002, 2008.
 [5] J. Gribbin, M. Gribbin, and J. Gribbin. Q is for Quantum: An Encyclopedia of Particle Physics. Touchstone, 2000. 
[6] Kurzgesagt In a Nutshell. Why Black Holes Could Delete The Universe The Information Paradox. youtu.be/yWO-cvGETRQ, August 2017. 




Jason A. Andronis 

Physics Department, University of Crete


 September 24, 2017 



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