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Global Science TV: Scientists detect an ‘impossible’ black hole

Astronomers from the LIGO and Virgo Scientific Collaboration have observed two monster black holes colliding to form an even bigger one. Here is what that means.
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In this episode, Nuala Hafner interviews Professor Susan Scott from the Department of Quantum Science Physics Education Centre at the Australian National University.


Nuala Hafner: Warning, this video is rather mind-boggling, even for space nerds. Gravitational wave scientists have observed the collision of the heaviest system of two black holes ever detected.

Professor Susan Scott: The amazing thing about this discovery is that at least two out of the three black holes involved are what we call impossible black holes.

Nuala Hafner: Yes, we’re talking things that shouldn’t exist. I warned you it was mind-boggling. Now, when Susan Scott mentions three black holes, she’s referring to the two involved in the initial collision and then a third one created as a result of that collision. So, let’s start with the first two. Both were massive, but one impossibly so. Well, so we thought, weighing in at about 85 times the mass of the sun. That falls squarely into a forbidden range known as the upper black hole mass gap. Sorry to labour this point, but anything within this range should be impossible. That’s because stellar black holes are formed by the collapse of a star. But stars with masses in that forbidden range undergo a process called pair instability, resulting in them being blown apart.

Professor Susan Scott: So if it wasn’t created through the collapse of a star, how did it form?

Nuala Hafner: A very good question. Black holes are sometimes described as vacuum cleaners of the universe. So one theory is that our impossible black hole had sucked up other black holes. Professor Scott compared it to a Russian doll.

Professor Susan Scott: If you like, this black hole contains a number of smaller black holes.

Nuala Hafner: But let’s get to the collision. It was so violent that it created gravitational waves, as in ripples in space time, and that’s what this signal represents. It was detected by the LIGO and Virgo observatories on May 21st, last year. But the collision itself happened when the universe was about seven billion years old, around half its present age. Just consider for a moment how extraordinary that notion is. We’re talking a seven-billion-year-old echo of a very distant collision between black holes. Before the remarkable sensitivity of LIGO, we would never have heard it above the other vibrations of our planet.

Professor Susan Scott: The collision formed a larger black hole weighing a whopping 142 times the mass of the sun.

Nuala Hafner: And that makes it the largest black hole ever observed through gravitational wave detections by far. It’s what’s known as an intermediate-mass black hole, or IMBH. They’re classed as those with a mass of 100 to 100,000 solar masses. So that makes them heavier than stellar black holes but lighter than the supermassive ones often located at the centre of galaxies. Scientists aren’t yet sure how supermassive black holes are formed, but they’re pretty excited to confirm that this IMBH was created by the merger of those two relatively smaller black holes.

Professor Susan Scott: We were just so excited to achieve the first direct observation of an intermediate-mass black hole in this mass range. It was incredible.

Nuala Hafner: Gravitational waves can teach us so much. It’s amazing to think that it was only in late 2015 that the first waves were detected. That discovery by LIGO is considered one of science’s greatest achievements. Our understanding of the universe has taken some big leaps since then. And we can to expect some even bigger ones ahead as the sensitivity of the detectors keeps improving. And that means we need to keep rethinking what’s impossible.

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