The Current Turmoil
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
which shows up about halfway through the
evaporation of a black hole.
Until recently, many scientists satisfied
their frustration with the information paradox by thinking of the inside and
outside of a black hole as two different realms that cannot communicate.
However, in search of equations to describe the Complementarity Principle, the
AMPS – Almheiri, Marolf, Polchinski and Sully, discovered that this solution
contains a self-contradiction. They
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 from 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 to 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
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’.
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 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.
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?