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!
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
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.
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’.
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.
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.
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
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
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’.
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’.
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
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.
happen if you fell into a black hole?