happen if you fell into a black hole?
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.
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 even light is not 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 center of a very massive star collapses in upon itself.
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
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.
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’.
spaghettification 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’ 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
Complementarity: Saving Quantum Theory
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.
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.
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 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 from just above the event horizon, where it is
described by two-dimensional equations that do not include gravity at all.
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 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.
Until recently, many scientists satisfied
their frustration with the information paradox with the aid of the
Complementarity Principle. However, in search of equations to describe this
idea, 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. However, before we explain their argument, let’s first talk
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 of
the entangled particle pair would destroy the entanglement between them. 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. So, is the paradox back? Only for a while.
The AMPS took the idea
about entangled pairs across the event horizon even further and got rid of the
idea of spaghettification.
let’s talk more entanglement. articles can be entangled but entanglement itself
does not require the presence of particles to exist. In fact, even empty space
is entangled. The quantum properties of empty space tell us that it is not
really empty. Quantum fluctuations rive rise to particles that constantly pop
in and out of existence.
dividing empty space into two halves.
though, according to the principle of monogamy, entanglement cannot occur
between more than two systems, 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 so much force that they collapse into black
holes. We would then end up with two black holes that are entangled with each other.
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
which shows up about halfway through the
evaporation of a black hole.
? Entanglement is not restricted to small systems;
? Entanglement does not require the presence of
? If a black hole is created, entangled with another system,
a ‘firewall’ will occur, causing the whole interior of the black hole to be
wiped out, creating ‘nothingness’;
? About half way through the evaporation of a black
hole, a ‘firewall’ will occur, contradicting the principle of complementarity.
For over a
century, the two planks of modern physics, the theories of relativity and
quantum mechanics have been the best …, but nevertheless they face
The Current Turmoil