nterstellar, the main character Cooper escapes from a black hole to see his daughter Murph in her final days. Some have argued that the movie is so scientific that it should be taught in schools. In reality, many scientists believe that anything sent into a black hole would be destroyed. But a new study suggests that this might not be the case after all.

The research
says that, rather than being devoured, a person falling into a black hole would
actually be absorbed into a hologram – without even noticing. The paper challenges
a rival theory stating that anybody falling into a black hole hits a “firewall”
and is immediately destroyed.

**Hawking’s black holes**

Forty years
ago Stephen Hawking shocked the scientific establishment with his discovery
that black holes aren’t really black. Classical physics implies that anything
falling through the horizon of a black hole can never escape. But Hawking
showed that black holes continually emit radiation once quantum effects are
taken into account. Unfortunately, for typical astrophysical black holes, the
temperature of this radiation is far lower than that of the cosmic microwave background, meaning detecting them is beyond current technology.

Hawking’s
calculations are perplexing. If a black hole continually emits radiation, it
will continually lose mass – eventually evaporating. Hawking realized that this
implied a paradox: if a black hole can evaporate, the information about it will
be lost forever. This means that even if we could measure the radiation from a
black hole we could never figure out it was originally formed. This violates an
important rule of quantum mechanics that states information cannot be lost or created.

Another way
to look at this is that Hawking radiation poses a problem with determinism for
black holes. Determinism implies that the state of the universe at any given
time is uniquely determined from its state at any other time. This is how we
can trace its evolution both astronomically and mathematically though quantum
mechanics.

Lots of
theories but only one way to find out for sure. NASA/Flickr, CC BY-SA

This means
that the loss of determinism would have to arise from reconciling quantum mechanics with Einstein’s theory of gravity – a notoriously hard problem and ultimate
goal for many physicists. Black hole physics provides a test for any potential
quantum gravity theory. Whatever your theory is, it must explain what happens
to the information recording a black hole’s history.

It took two
decades for scientists to come up with a solution. They suggested that the
information stored in a black hole is proportional to its surface area (in two
dimensions) rather than its volume (in three dimensions). This could be
explained by quantum gravity, where the three dimensions of space could be
reconstructed from a two-dimensional world without gravity – much like a
hologram. Shortly afterwards, string theory, the most studied theory of quantum
gravity, was shown to be holographic in this way.

Using
holography we can describe the evaporation of the black hole in the
two-dimensional world without gravity, for which the usual rules of quantum
mechanics apply. This process is deterministic, with small imperfections in the
radiation encoding the history of the black hole. So holography tells us that
information is not lost in black holes, but tracking down the flaw in Hawking’s
original arguments has been surprisingly hard.

**Fuzzballs versus firewalls**

But exactly what the black holes described by quantum theory look like is harder to work out. In 2003, Samir Mathur proposed that black holes are in fact fuzzballs, in which there is no sharp horizon. Quantum fluctuations around the horizon region records the information about the hole’s history and thus Mathur’s proposal resolves the information loss paradox. However the idea has been criticised since it implies that somebody falling into a fuzzball has a very different experience to somebody falling into a black hole descried by Einstein’s theory of general relativity.

The general
relativity description of black holes suggests that once you go past the event
horizon, the surface of a black hole, you can go deeper and deeper. As you do,
space and time become warped until they reach a point called the “singularity”
at which point the laws of physics cease to exist. (Although in reality, you
would be die pretty early on on this journey as you are pulled apart by intense tidal forces).

In Mathur’s
universe, however, there is nothing beyond the fuzzy event horizon. Currently,
a rival theory in quantum gravity is that anybody falling into a black hole
hits a “firewall” and is immediately destroyed. The firewall proposal has been
criticized since (like fuzzballs) firewalls have drastically different
behavior at the horizon than general relativity black holes.

But Mathur
argues that to an outside observer, somebody falling into a fuzzball looks
almost the same as somebody falling into an Einstein black hole, even though
those falling in have very different experiences. Others working on firewalls
and fuzzballs may well feel that these arguments rely on properties of the
example he used. Mathur used an explicit description of a very special kind of
fuzzball to make his arguments. Such special fuzzballs are probably very
different to the fuzzballs needed to describe realistic astrophysical black
holes.

The debate
about what actually happens when one falls into a black hole will probably
continue for some time to come. The key question is to understand is not that
the horizon region is reconstructed as a hologram – but exactly how this
happens.

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