Artist's
impression of an active black hole. (Mark Garlick/Science Photo Library/Getty
Images) |

Given that the first direct detections of black holes occurred only this century, humanity can be forgiven for not knowing a few facts about these fascinating cosmic objects.

We don't
even know what we don't know, according to a recent discovery. A pair of
physicists discovered that black holes exert pressure on the space surrounding
them while calculating equations for quantum gravity corrections for the
entropy of a black hole.

To be
fair, it's not much pressure – but it's a fascinating result that backs up
Stephen Hawking's prediction that black holes emit radiation and hence have a
temperature, as well as the ability to shrink over time in the absence of
accretion.

Physicist
and astronomer Xavier Calmet of the University of Sussex in the United Kingdom
remarked, "Our finding that Schwarzschild black holes have a pressure as
well as a temperature is even more thrilling considering that it was a complete
surprise."

When just
general relativity is considered, it can be shown that black holes have a
singularity in their centres where the laws of physics as we know them must
break down.

When
quantum field theory is merged into general relativity, it is hoped that a new
description of black holes would emerge.

Calmet and
his University of Sussex colleague, physicist and astronomer Folkert Kuipers,
were using quantum field theory calculations to try to explore the event
horizon of a black hole when they discovered their finding.

They were
attempting to comprehend the oscillations at a black hole's event horizon that
correct its entropy, a measure of the movement from order to disorder.

Calmet and
Kuipers kept coming across an additional figure in their equations while
completing these calculations, but it took them a while to realise what they
were looking at – pressure.

After
months of wrangling with the mystery result in our calculations, we finally
realised that the black hole we were researching had a pressure, which was
exhilarating, according to Kuipers.

It's
unknown what's creating the pressure, and it's quite little, according to the
team's calculations. Furthermore, it's negative – -2E-46bar for a black hole
the mass of the Sun, vs 1bar at sea level for Earth.

This
indicates that the black hole would be shrinking rather than growing, as the
name implies. That's in line with Hawking's prediction, however it's impossible
to say how negative pressure connects to Hawking radiation at this time, or
even if the two phenomena are related.

However,
the discovery could have important ramifications for our efforts to reconcile
general relativity and quantum physics (at macro scales) (which operates on
extremely small scales).

This
project is supposed to need the use of black holes. The black hole singularity
is a one-dimensional location of extremely high density where general
relativity fails, but the gravitational field around it can only be represented
relativistically.

Understanding
how the two regimes interact could potentially aid in the solution of a
particularly difficult black hole problem. Information lost beyond a black
hole, according to general relativity, could be lost forever. It isn't possible
according to quantum mechanics. This is the black hole information dilemma,
which could be solved by mathematically probing the space-time around a black
hole.

Our
research is a step in the right direction, according to Calmet, and while the
pressure exerted by the black hole under study is negligible, the fact that it
exists opens up a slew of new possibilities in astronomy, particle physics, and
quantum physics.

Originally
published By Science Alert.

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