Physicists Just Accidentally Made a New Discovery About Black Holes


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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