Black hole collides with a neutron star

What happens when a black hole collides with a neutron star?
03 September 2019

Interview with 

Joris van Heijningen, UWA


An artist's impression of a black hole


In 2015, the LIGO/VIRGO collaboration announced the first ever detection of gravitational waves, and won the Nobel Prize. Gravitational waves are ripples in the very fabric of space and time that are created when heavy objects move in certain ways. If you rotate your hands around each other, right now, you’re actually producing gravitational waves, they are just so tiny that they could never be detected. In order to produce detectable waves, the sources need to be much, much bigger. But LIGO didn’t stop after those observations in 2015, and, to tell us about some fascinating new gravitational wave results, Ben McAllister spoke to Joris van Heijningen, from the ARC Centre of Excellence for Gravitational Wave Discovery, OzGrav, at the University of Western Australia. Joris didn’t work on the new result directly, but he does work with LIGO/VIRGO... 

Ben - It’s September 2015 and two laser beams are travelling down two perpendicular four kilometre long tunnels. The lasers are making extremely precise measurements of the distances between two sets of mirrors, one mirror at either end of each of the tunnels. Why are they doing this? Well, that's a fair question. If a gravitational wave, a ripple in the fabric of space and time, were to travel through those tunnels, it would ever so slightly change their length as it travelled by, thus changing the distance between the mirrors, and scientists would be able to detect it. And that's exactly what happened. A long time ago, 1.3 billion years to be precise, in a galaxy far far away, two black holes each weighing tens of times the mass of the sun were rapidly rotating around each other, moving at half the speed of light. Then they collided and merged. The resulting gravitational waves travelled at the speed of light for 1.3 billion years to be detected by LIGO with their tunnels and lasers.

Now let's back up for a minute. Just in case you don't have a degree in astrophysics, I asked Joris van Heijningen what exactly is a black hole.

Joris - So a black hole is a result of a collapsing star.

Ben - And these aren't just any collapsed star corpses, these star corpses are so dense that nothing can escape their gravitational pull, not even light. Well, nothing except gravitational waves, that is. Since the initial observation of black holes colliding with each other, the team hasn't looked back. They've since observed more collisions between black holes, as well as collisions between interesting astronomical structures known as neutron stars.

Joris - The neutron star is also products of a dying star. The star explodes and all matter collapses down to something that is under very high pressure, all the protons and electrons smash together into neutrons to form a neutron star.

Ben - One of the reasons these results are interesting is because we don't really know a lot about what neutron stars are made up of and these observations may help us learn more.

Joris - what scientists call “the neutron star equation of state” is one of the holy grails in astronomy these days. This equation of states basically tells you what the inside of a neutron star is made up of.

Ben - But it isn't all just black holes merging with black holes and neutron stars merging with neutron stars. The LIGO-Virgo team recently announced that they believe they've spotted something different for the first time ever. Something which actually occurred 900 million light years away, 900 million odd years ago.

Joris - On the 14th of August, just before midnight European time, we saw a signal that may be a neutron star-black hole event. It was very loud. So we're pretty certain that this signal is real. However we are not certain what's the lighter object is.

Ben - Okay. But to clarify, it's a collision between two objects, one that we think is a black hole and one that we think is a neutron star.

Joris - Yes that's correct.

Ben - Frequently, gravitational wave observations are accompanied by corresponding signals of light like for example when they first observed two neutron stars colliding back in 2017. But as the LIGO-Virgo team found with this observation that isn't always the case.

Joris - The August 2017 binary neutron star merger gave lights across the spectrum. So far we haven't seen any light. If that turns out not to be the case when astronomers stop looking there it could mean two things. It could mean that the neutron star was gobbled up a whole, a little bit like a pacman, or it could be that the lighter object was actually also a black hole.

Ben - With observations like these, we're now entering what some scientists are calling “the era of gravitational wave astronomy”, where we're able to use detectors like LIGO and Virgo to detect enormous universe shaking events like collisions of black holes and then tell traditional astronomers with regular telescopes where to point those telescopes in order to pick up the corresponding light signals.

Joris - We are just having our detectors in observation mode. And then it's just waiting for the signal to come.

Ben - They're expecting many more observations in the future and more results like these ones could help us uncover the mysterious nature of neutron stars. Despite the fact that these gravitational wave observations sometimes don't give off any light at all, the future looks bright for LIGO-Virgo.

Joris - We will continue measuring gravitational waves. One thing that we still want to measure is a supernova in order to understand more about these events. And we need more of these binary neutron star events to determine this equation of state.

Ben - With gravitational wave detectors only getting more and more sensitive, we're sure to learn plenty more incredible things about the universe!


So, reading between the lines of what Joris said, is it now accepted that gravity and gravitational waves travel FASTER than the speed of light? Or is he suggesting that there is some kind of gravitational mechanism where the speed of the photons produced by the event are retarded (held back, impeded?) by the accompanying gravitational incident that allows the gravitational wave to reach us ahead of the visible spectacle?

Hi Roy,

Ben here (from this interview). This is a fantastic question!

Indeed, the fact that the light from this supposed black hole - neutron star merger hasn't been observed yet is a bit of a smoking gun, but the team isn't currently proposing that this is due to a difference in the speed of light and the speed of gravity.

We have seen good agreement between light and gravity 'speeds' in previous observations, and it is currently accepted that they travel at the same speed (as is expected from the theory).

Rather, the team is thinking that perhaps the reason we haven't seen any light from this merger could be one of a few things:

1. As you suggest, there is some effect which prevents the light from reaching us, perhaps due to the neutron star (and all of the light) being "gobbled up" completely by the black hole.

2. The smaller object in the collision wasn't a neutron star after all, but rather another, small black hole.

Only further study will get to the bottom of this one!

I hope this answers your question.


Hi Ben

Thanks for the reply - that makes a good deal of sense and if the theory that black holes have a gravity well that is so deep even light cannot escape it is correct, then an effect that holds the light back would seem to the most logical

Thanks for the clarification!!


Really enjoying the podcast!
Continue provoking a rational attitude. You have allies in Mexico!

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