In the movie Interstellar,
the main character Cooper escapes from a black hole in time 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 probably 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 actuallybe 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 blackholes 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 withEinstein’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, wasshown 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 andtime 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 tidalforces).

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 andtime 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 tidalforces).

In Mathur’s universe, however,
there is nothing beyond the fuzzy event horizon. Currently, a rival theory inquantum 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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