This

firewall idea really annoyed a bunch of physicists, including Stephen Hawking.

He released a two-page statement that left the rest of the physics community quite

confused. According to Hawking, both concepts about black holes –

Complementarity and Firewall, are wrong; i.e. there are no singularities, but

that there are also no event horizons. Hawking proposed the idea of apparent

horizons, which store and scramble all matter and energy that they suck in. Even

though the information is scrambled, it is not irretrievable, meaning that this

idea has the potential to solve the information loss issue. However, without a

black hole interior, and specifically without a singularity to

generate a strong gravitational field, Hawking’s proposal still messes with the

general theory of relativity.

The idea proposed by the AMPS is still very

problematic. It essentially tells us that black holes no

longer have any centers. There is nothing

beyond the event horizon Firewall. It is the end of the Universe! And that is

not all! The rip in the fabric of spacetime throws into question Einstein’s

theory of general relativity. The possibility of Firewall might mean that the

general theory of relativity would not only need a small modification, but a

major surgery. So now the paradox is back! And worse than ever!

The Current

Turmoil

This idea not only further disproved the

Complementarity, but also entirely threw out the idea that black holes could

spaghettify you. Instead, of an in-falling observer smoothly crossing the

harmless event horizon, they will find out there is no interior at all, and

will do so the hard way, by being burnt like a toast.

But that was

not the only issue with the Complementarity Principle. It turned out that another

significant problem shows up about halfway through the evaporation of a black

hole. Halfway through its evaporation, the mass of a black hole will be the

same as the mass of the Hawking radiation that it had radiated. At that point

of the life of the black hole, it is perfectly entangled with its Hawking

radiation. This creates a problem. If our black hole is entangled with another

system, that would violate the monogamy principle of entanglement. There are

already two entangled systems – one below and one above the event horizon, and

the Hawking radiation becomes a third. In

order to deal with this violation of the monogamy principle, the entanglement across the event

horizon would have to be destroyed, causing huge amounts of energy to be

released 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 be

wiped out, creating a region of ‘nothingness’ beyond the event horizon. According

to 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 the

information about it would now finally be conserved. It might bounce off

into space, but as long it is not sucked past the event horizon, the

information 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 no

discontinuities, it can simply be treated as empty space. But as we have

already seen, empty space is held together by entanglement, meaning that very

close to the event horizon, systems below its surface will be entangled with

systems above it. The problem with the Complementarity Principle, as discovered

by the AMPS, involved throwing an entangled particle into a black hole, while

keeping its twin outside. But that was no longer necessary, as it occurred

naturally on a much bigger scale. The AMPS realized that all systems just below

the event horizon are entangled with those just above it. This brought further

evidence that the Complementarity Principle was not the right solution to the

information paradox.

But this is not the full story. In order to

be able to understand AMPS’s argument, we need to discuss the properties of

quantum entanglement a bit more. According to the principle of monogamy,

entanglement cannot occur between more than two systems, but that does not mean

that it is restricted to small systems. On the contrary; imagine you had a

machine that continuously produces pairs of entangled particles and is at the

same time connected to two boxes. Every time the machine produces a pair of

entangled particles, one of them goes into one of the boxes, while its twin

goes to the other. As this process is repeated over and over again, the systems

in the two boxes will become more and more entangled with one another. Taking

this idea even further imagine we could hypothetically compress each of these

two systems with enough force to make them collapse into two black holes. We

would end up with two entangled black holes and just as with an entangled

particle pair, the two black holes will have this property that we can find

anything we want about one of them by making a measurement on the other.

Taking this

idea even further, let’s consider what would happen if we made a measurement on

this entangled system. Making a measurement on all space pockets across the

boundary would destroy the entanglement between them. But it would also do

something else. The measurement will create a huge amount of energy in the

region of the experiment. This dissipation of energy will not only change the

geometry of space but will also rip it apart, resulting in a region of

‘nothingness’ in between. The idea of ‘nothingness’ does not imply the presence

of a vacuum, but rather a place where no quantum fluctuations can occur. This

leads to the idea that space is held together by entanglement and destroying

this entanglement, by making an observation, would ‘unzip it’.

Particles

can be entangled but entanglement itself does not necessary require the

presence of particles to exist. In fact, even empty space possesses a type of

entanglement. The quantum properties of empty space tell us that it is not

actually 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 we

could divide the space near the boundary into small pockets. Investigating

these space pockets across the boundary, we can see that they also possess a

type of entanglement. If we were to make a measurement on one of these pockets

on the right-hand side of the boundary and found that it has no particles, we

would know that the corresponding space pocket on the left-hand side would also

have no particles. Similarly, if a space pocket on the right-hand side of the

boundary has a particle, then the corresponding space pocket on the left-hand

side would also contain a particle.

So,

is the paradox back? Well, only for a while. The AMPS took the idea about entangled systems across the

event 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 two

different realms that cannot communicate, but if we tossed an entangled

particle inside a black hole, while keeping its twin outside, that would create

a problem, simply because the very nature of entangled particles is to be able

to respond to one another. In this way, the AMPS totally threw away the idea of

the two realms.

Quantum entanglement is a quantum mechanical

phenomena that occurs when two particles are generated such that the quantum

state of one of the particles cannot be described independently of the other. The

two entangled particles are linked in such a way, that a change in the

properties of one of them will cause a change in those of the other, regardless

of the distance between them. In addition, making a measurement on the

entangled particle pair would destroy the entanglement between them.

However, in search of equations for complementarity

that could describe the evaporation process, the AMPS – Almheiri, Marolf, Polchinski and Sully, discovered that the

Complementarity Principle contains a self-contradiction. They imagined what

would happen if the two classes of observers, one outside and one inside of the

black hole, were replaced by a pair of entangled particles; i.e. one of the

entangled particles was tossed inside the black hole, while the other one was

kept outside. To begin to understand their argument, we need to first

understand what entanglement is.

The

Firewall

This success of the holographic principle brought

more faith into the Complementarity Principle idea and by 2005, Stephen Hawking

had come to agree that black holes do not cause information to be destroyed and

that the general theory of relativity, rather than the quantum theory, needs to

be modified.

Remarkably, significant evidence emerged in

the late 1990s in support of the holographic principle. Theoretical physicist

Juan Maldacena of Princeton University hypothesized that under the right

circumstances, string theory is equivalent to a quantum theory but without

gravity and with fewer dimensions.

This solution to the information paradox requires

that all events happening in the interior of a black hole can be described as

though they were just outside of the black hole. It involves ‘holography’, an

idea that was developed by Gererd’t Hooft, a Dutch theoretical physicist and

professor at Utrecht University, and further by Susskind. The idea is that the

information about the 3D interior of a black hole, which is greatly affected by

gravity, is stored in a 2D form just above the event horizon, where it is

described by two-dimensional equations that do not include gravity at all.

Imagine two observers, Bob and Charlie that

are on a spaceship, orbiting a black hole. While Bob remains in the ship, Charlie

takes 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 clock

starts to run slower and slower compared to Bob’s clock. Therefore, according

to the Complementarity Principle, Bob will observe Charlie fall towards the

black hole, but then gradually slow down and accumulate at the surface of the

event horizon. Even though in Bob’s frame of reference Charlie does not fall

through the event horizon of the black hole, does that mean that Charlie does

not pass through it in his own reference frame? No! In

Charlie’s reference frame, Charlie will pass through the event horizon and will

continue falling towards the singularity of the black hole. The two observers, Bob and Charlie, would therefore see

the information in a different location, but since they cannot communicate, the

principles of quantum theory are not violated and thus there is no paradox.

This can further be explained with the aid

of the special theory of relativity. Einstein’s gravitational time dilation has

shown that clocks run differently depending on the strength of the

gravitational field they are in. Clocks that are in a stronger gravitational

field 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 than

those that are further away.

In search of a flaw in the general theory

of relativity, in 1992, Leonard Susskind,

a professor of theoretical physics at Stanford University, and his younger

co-workers developed a proposal, called the ‘Complementarity Principle’. It

suggested that the inside and outside of a black hole can be thought of as two

different realms and the position of the information depends on the point of

view of the observers. Observers that remain outside of the black hole would

see the information of everything that is falling into the black hole accumulate

at 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 information

located inside it.

Complementarity: Saving Quantum Theory

The ‘information paradox’ has drawn

attention to a potentially serious con?ict between quantum mechanics and the

general 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 is

incomplete and that it needs to be extended, just like Einstein extended

Newton’s laws of motion in his theory of relativity. However, others felt that

it was the general theory of relativity, not quantum theory, that needed to be

changed.

The spaghesttification idea

satisfied scientists until the 1970s, when Hawking dropped

a bombshell with the proposal

that black holes

radiate particles. The so-called Hawking radiation causes black holes to shrink in size and eventually

evaporate completely. What has now become a widely accepted idea about the nature of black

holes raised a lot of

questions, one of which still concerns physicists today – Where did the

information go? If the information about

everything that went into the black hole disappeared along with its evaporation, that would lead

to the violation of one of the fundamental principles of quantum mechanics –

information cannot be destroyed. Maybe the

information came back out with the Hawking radiation? The problem is that the

information in the black hole simply

cannot get out due to the intense gravitational field it has to overcome to do

so. One might argue that the problem could be solved if the information inside

the black hole is copied onto the Hawking radiation, but having copies of

information also disobeys the laws of quantum mechanics. This gave rise to a paradox, that physicists refer to

as ‘The Black Hole Information Paradox’.

The

Information Paradox

So, what

would happen if you fell into a black hole? For years scientists thought they knew how you would

meet your end. Imagine falling into the black hole feet first. As your feet are

closer to the singularity, they would feel a stronger gravitational force and

will thus start to move faster than the rest of your body, causing you to get

stretched into a long noodle. Physicists call this process ‘spaghettification’.

An analogy

inspired by William G. Unruh of the University of British Columbia, one of the

pioneers in black hole quantum mechanics, helps to explain the significance of

this pull. Imagine you are fish, swimming downstream a river that leads towards

a waterfall. If you are significantly far away from the cliff, you can easily

swim away to safety. But once you get far enough downstream, no matter how fast

you 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 it

is 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 would

crush 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 incredibly

dense. The closer you are to this point, the stronger the gravitational

attraction is.

To begin to

understand this controversy, we need to first understand what a black hole is.

A black hole is

a region in space where the force of gravity is so strong that not even light

is able to escape. Although some black holes are thought to have formed

in the early universe, soon after the big bang, most medium-sized black holes form

when the centers of very massive stars collapse in upon themselves.

One of the biggest paradoxes in physics

today is one that sounds straight out of a science fiction novel. What would

happen if you fell into a black hole? Rest assured,

the answer to this bizarre question is that you would die – that is not up for

discussion. But it is how exactly you would die that is keeping physicists up

at night. There are

currently two major theories fighting over this horrifying scenario and the

outcome of this battle could revolutionize the fundamental laws of our universe.

What would

happen if you fell into a black hole?