Scientists work on the boundaries of
the unknown, where every new piece of knowledge
forms a path into a void of uncertainty. And nothing is more uncertain– or potentially enlightening–
than a paradox. Throughout history, paradoxes have threatened to
undermine everything we know, and just as often, they’ve reshaped our
understanding of the world. Today, one of the biggest paradoxes in
the universe threatens to unravel the fields of general
relativity and quantum mechanics: the black hole information paradox. To understand this paradox, we first need to define what we mean
by "information." Typically, the information we talk about
is visible to the naked eye. For example, this kind of information tells us that
an apple is red, round, and shiny. But physicists are more concerned with
quantum information. This refers to the quantum properties of
all the particles that make up that apple, such as their position, velocity and spin. Every object in the universe is composed of particles with unique
quantum properties. This idea is evoked most significantly
in a vital law of physics: the total amount of quantum information
in the Universe must be conserved. Even if you destroy an object beyond
recognition, its quantum information is never
permanently deleted. And theoretically, knowledge of that
information would allow us to recreate the object
from its particle components. Conservation of information isn’t just an
arbitrary rule, but a mathematical necessity, upon which
much of modern science is built. But around black holes,
those foundations get shaken. When an apple enters a black hole, it seems as though it leaves the universe, and all its quantum information
becomes irretrievably lost. However, this doesn’t immediately
break the laws of physics. The information is out of sight, but it might still exist within the
black hole’s mysterious void. Alternatively, some theories suggest that information doesn’t even make it
inside the black hole at all. Seen from outside, it’s as if the apple’s
quantum information is encoded on the surface layer of the
black hole, called the event horizon. As the black hole’s mass increases, the surface of the event horizon
increases as well. So it’s possible that as a black hole
swallows an object, it also grows large enough to conserve
the object’s quantum information. But whether information is conserved
inside the black hole or on its surface, the laws of physics remain intact– until you account for Hawking Radiation. Discovered by Stephen Hawking in 1974, this phenomenon shows that black
holes are gradually evaporating. Over incredibly long periods of time black holes lose mass as they shed
particles away from their event horizons. Critically, it seems as though the
evaporating particles are unrelated to the information
the black hole encodes– suggesting that a black hole and all the
quantum information it contains could be completely erased. Does that quantum information
truly disappear? If not, where does it go? While the evaporation process
would take an incredibly long time, the questions it raises for physics
are far more urgent. The destruction of information would force us to rewrite some of our most
fundamental scientific paradigms. But fortunately, in science, every paradox is an opportunity
for new discoveries. Researchers are investigating a broad
range of possible solutions to the Information Paradox. Some have theorized that information actually is encoded
in the escaping radiation, in some way we can’t yet understand. Others have suggested the paradox is
just a misunderstanding of how general relativity and
quantum field theory interact. Respectively, these two theories describe the largest
and smallest physical phenomena, and they’re notoriously
difficult to combine. Some researchers argue that a solution
to this and many other paradoxes will come naturally with a “unified
theory of everything.” But perhaps the most mind-bending
theory to come from exploring this paradox is the holographic principle. Expanding on the idea that the 2D
surface of an event horizon can store quantum information, this principle suggests that the very
boundary of the observable universe is also a 2D surface encoded
with information about real, 3D objects. If this is true, it’s possible that
reality as we know it is just a holographic
projection of that information. If proven, any of these theories would
open up new questions to explore, while still preserving our current
models of the universe. But it’s also possible that those
models are wrong! Either way, this paradox has already
helped us take another step into the unknown.
