Thisfirewall idea really annoyed a bunch of physicists, including Stephen Hawking.He released a two-page statement that left the rest of the physics community quiteconfused. According to Hawking, both concepts about black holes –Complementarity and Firewall, are wrong; i.e. there are no singularities, butthat there are also no event horizons. Hawking proposed the idea of apparenthorizons, which store and scramble all matter and energy that they suck in.
Eventhough the information is scrambled, it is not irretrievable, meaning that thisidea has the potential to solve the information loss issue. However, without ablack hole interior, and specifically without a singularity togenerate a strong gravitational field, Hawking’s proposal still messes with thegeneral theory of relativity. The idea proposed by the AMPS is still veryproblematic. It essentially tells us that black holes nolonger have any centers. There is nothingbeyond the event horizon Firewall. It is the end of the Universe! And that isnot all! The rip in the fabric of spacetime throws into question Einstein’stheory of general relativity. The possibility of Firewall might mean that thegeneral theory of relativity would not only need a small modification, but amajor surgery.
So now the paradox is back! And worse than ever! The CurrentTurmoil This idea not only further disproved theComplementarity, but also entirely threw out the idea that black holes couldspaghettify you. Instead, of an in-falling observer smoothly crossing theharmless event horizon, they will find out there is no interior at all, andwill do so the hard way, by being burnt like a toast. But that wasnot the only issue with the Complementarity Principle. It turned out that anothersignificant problem shows up about halfway through the evaporation of a blackhole.
Halfway through its evaporation, the mass of a black hole will be thesame as the mass of the Hawking radiation that it had radiated. At that pointof the life of the black hole, it is perfectly entangled with its Hawkingradiation. This creates a problem. If our black hole is entangled with anothersystem, that would violate the monogamy principle of entanglement.
There arealready two entangled systems – one below and one above the event horizon, andthe Hawking radiation becomes a third. Inorder to deal with this violation of the monogamy principle, the entanglement across the eventhorizon would have to be destroyed, causing huge amounts of energy to bereleased across the event horizon. The AMPS called this dissipation of energy a’Firewall’. The Firewall would cause the whole interior of the black hole to bewiped out, creating a region of ‘nothingness’ beyond the event horizon. Accordingto the AMPS’s idea, matter does not get sucked into a black hole at all.
Instead, on hitting the event horizon, all matter ignites. All of theinformation about it would now finally be conserved. It might bounce offinto space, but as long it is not sucked past the event horizon, theinformation paradox is solved.
Now, let’s go back to the AMPS’s argument.Since the event horizon of a black hole is a smooth surface, with nodiscontinuities, it can simply be treated as empty space. But as we havealready seen, empty space is held together by entanglement, meaning that veryclose to the event horizon, systems below its surface will be entangled withsystems above it. The problem with the Complementarity Principle, as discoveredby the AMPS, involved throwing an entangled particle into a black hole, whilekeeping its twin outside. But that was no longer necessary, as it occurrednaturally on a much bigger scale. The AMPS realized that all systems just belowthe event horizon are entangled with those just above it. This brought furtherevidence that the Complementarity Principle was not the right solution to theinformation paradox.
But this is not the full story. In order tobe able to understand AMPS’s argument, we need to discuss the properties ofquantum entanglement a bit more. According to the principle of monogamy,entanglement cannot occur between more than two systems, but that does not meanthat it is restricted to small systems. On the contrary; imagine you had amachine that continuously produces pairs of entangled particles and is at thesame time connected to two boxes. Every time the machine produces a pair ofentangled particles, one of them goes into one of the boxes, while its twingoes to the other. As this process is repeated over and over again, the systemsin the two boxes will become more and more entangled with one another. Takingthis idea even further imagine we could hypothetically compress each of thesetwo systems with enough force to make them collapse into two black holes. Wewould end up with two entangled black holes and just as with an entangledparticle pair, the two black holes will have this property that we can findanything we want about one of them by making a measurement on the other.
Taking thisidea even further, let’s consider what would happen if we made a measurement onthis entangled system. Making a measurement on all space pockets across theboundary would destroy the entanglement between them. But it would also dosomething else. The measurement will create a huge amount of energy in theregion of the experiment.
This dissipation of energy will not only change thegeometry of space but will also rip it apart, resulting in a region of’nothingness’ in between. The idea of ‘nothingness’ does not imply the presenceof a vacuum, but rather a place where no quantum fluctuations can occur. Thisleads to the idea that space is held together by entanglement and destroyingthis entanglement, by making an observation, would ‘unzip it’.
Particlescan be entangled but entanglement itself does not necessary require thepresence of particles to exist. In fact, even empty space possesses a type ofentanglement. The quantum properties of empty space tell us that it is notactually empty. Quantum fluctuations give rise to particles that constantly pop in and out of existence.Imagine dividing empty space into two halves with an imaginary line. Suppose wecould divide the space near the boundary into small pockets. Investigatingthese space pockets across the boundary, we can see that they also possess atype of entanglement.
If we were to make a measurement on one of these pocketson the right-hand side of the boundary and found that it has no particles, wewould know that the corresponding space pocket on the left-hand side would alsohave no particles. Similarly, if a space pocket on the right-hand side of theboundary has a particle, then the corresponding space pocket on the left-handside would also contain a particle. So,is the paradox back? Well, only for a while. The AMPS took the idea about entangled systems across theevent horizon even further. But first, let’s talk more entanglement.
According to the Complementarity Principle,the regions inside and outside of the black hole can be thought of as twodifferent realms that cannot communicate, but if we tossed an entangledparticle inside a black hole, while keeping its twin outside, that would createa problem, simply because the very nature of entangled particles is to be ableto respond to one another. In this way, the AMPS totally threw away the idea ofthe two realms. Quantum entanglement is a quantum mechanicalphenomena that occurs when two particles are generated such that the quantumstate of one of the particles cannot be described independently of the other. Thetwo entangled particles are linked in such a way, that a change in theproperties of one of them will cause a change in those of the other, regardlessof the distance between them. In addition, making a measurement on theentangled particle pair would destroy the entanglement between them. However, in search of equations for complementaritythat could describe the evaporation process, the AMPS – Almheiri, Marolf, Polchinski and Sully, discovered that theComplementarity Principle contains a self-contradiction. They imagined whatwould happen if the two classes of observers, one outside and one inside of theblack hole, were replaced by a pair of entangled particles; i.
e. one of theentangled particles was tossed inside the black hole, while the other one waskept outside. To begin to understand their argument, we need to firstunderstand what entanglement is.TheFirewall This success of the holographic principle broughtmore faith into the Complementarity Principle idea and by 2005, Stephen Hawkinghad come to agree that black holes do not cause information to be destroyed andthat the general theory of relativity, rather than the quantum theory, needs tobe modified.
Remarkably, significant evidence emerged inthe late 1990s in support of the holographic principle. Theoretical physicistJuan Maldacena of Princeton University hypothesized that under the rightcircumstances, string theory is equivalent to a quantum theory but withoutgravity and with fewer dimensions. This solution to the information paradox requiresthat all events happening in the interior of a black hole can be described asthough they were just outside of the black hole. It involves ‘holography’, anidea that was developed by Gererd’t Hooft, a Dutch theoretical physicist andprofessor at Utrecht University, and further by Susskind. The idea is that theinformation about the 3D interior of a black hole, which is greatly affected bygravity, is stored in a 2D form just above the event horizon, where it isdescribed by two-dimensional equations that do not include gravity at all. Imagine two observers, Bob and Charlie thatare on a spaceship, orbiting a black hole. While Bob remains in the ship, Charlietakes a jump towards the black hole. As Charlie falls towards the singularity,the gravitational field he is in starts to get stronger and thus his clockstarts to run slower and slower compared to Bob’s clock.
Therefore, accordingto the Complementarity Principle, Bob will observe Charlie fall towards theblack hole, but then gradually slow down and accumulate at the surface of theevent horizon. Even though in Bob’s frame of reference Charlie does not fallthrough the event horizon of the black hole, does that mean that Charlie doesnot pass through it in his own reference frame? No! InCharlie’s reference frame, Charlie will pass through the event horizon and willcontinue falling towards the singularity of the black hole. The two observers, Bob and Charlie, would therefore seethe information in a different location, but since they cannot communicate, theprinciples of quantum theory are not violated and thus there is no paradox. This can further be explained with the aidof the special theory of relativity. Einstein’s gravitational time dilation hasshown that clocks run differently depending on the strength of thegravitational field they are in. Clocks that are in a stronger gravitationalfield will run slower than those in a weaker gravitational field. Therefore,clocks that are closer to the singularity of a black hole will run slower thanthose that are further away.
In search of a flaw in the general theoryof relativity, in 1992, Leonard Susskind,a professor of theoretical physics at Stanford University, and his youngerco-workers developed a proposal, called the ‘Complementarity Principle’. Itsuggested that the inside and outside of a black hole can be thought of as twodifferent realms and the position of the information depends on the point ofview of the observers. Observers that remain outside of the black hole wouldsee the information of everything that is falling into the black hole accumulateat the surface of the event horizon and then fly out in the Hawking radiation.However, observers that fall into the black hole would see the informationlocated inside it. Complementarity: Saving Quantum Theory The ‘information paradox’ has drawnattention to a potentially serious con?ict between quantum mechanics and thegeneral theory of relativity, leaning towards the idea that one, if not both,of the theories is incomplete.
This battle polarized the scientific community.Some scientists, such as Stephen Hawking believed that the quantum theory isincomplete and that it needs to be extended, just like Einstein extendedNewton’s laws of motion in his theory of relativity. However, others felt thatit was the general theory of relativity, not quantum theory, that needed to bechanged. The spaghesttification ideasatisfied scientists until the 1970s, when Hawking droppeda bombshell with the proposalthat black holesradiate particles.
The so-called Hawking radiation causes black holes to shrink in size and eventuallyevaporate completely. What has now become a widely accepted idea about the nature of blackholes raised a lot ofquestions, one of which still concerns physicists today – Where did theinformation go? If the information abouteverything that went into the black hole disappeared along with its evaporation, that would leadto the violation of one of the fundamental principles of quantum mechanics –information cannot be destroyed. Maybe theinformation came back out with the Hawking radiation? The problem is that theinformation in the black hole simplycannot get out due to the intense gravitational field it has to overcome to doso. One might argue that the problem could be solved if the information insidethe black hole is copied onto the Hawking radiation, but having copies ofinformation also disobeys the laws of quantum mechanics.
This gave rise to a paradox, that physicists refer toas ‘The Black Hole Information Paradox’. TheInformation Paradox So, whatwould happen if you fell into a black hole? For years scientists thought they knew how you wouldmeet your end. Imagine falling into the black hole feet first. As your feet arecloser to the singularity, they would feel a stronger gravitational force andwill thus start to move faster than the rest of your body, causing you to getstretched into a long noodle. Physicists call this process ‘spaghettification’. An analogyinspired by William G.
Unruh of the University of British Columbia, one of thepioneers in black hole quantum mechanics, helps to explain the significance ofthis pull. Imagine you are fish, swimming downstream a river that leads towardsa waterfall. If you are significantly far away from the cliff, you can easilyswim away to safety. But once you get far enough downstream, no matter how fastyou swim in the opposite direction, you cannot escape the pull of the water.For black holes, this ‘point of no return’ is called the event horizon and itis the place beyond which nothing, not even light can escape. For most of the past century,the scientific community thought that the extreme gravitational pull wouldcrush all the matter that made up the black hole into a one-dimensional point,called a singularity which is not only incredibly massive, but also incrediblydense.
The closer you are to this point, the stronger the gravitationalattraction is. To begin tounderstand this controversy, we need to first understand what a black hole is.A black hole isa region in space where the force of gravity is so strong that not even lightis able to escape. Although some black holes are thought to have formedin the early universe, soon after the big bang, most medium-sized black holes formwhen the centers of very massive stars collapse in upon themselves. One of the biggest paradoxes in physicstoday is one that sounds straight out of a science fiction novel. What wouldhappen if you fell into a black hole? Rest assured,the answer to this bizarre question is that you would die – that is not up fordiscussion.
But it is how exactly you would die that is keeping physicists upat night. There arecurrently two major theories fighting over this horrifying scenario and theoutcome of this battle could revolutionize the fundamental laws of our universe. What wouldhappen if you fell into a black hole?