# Why is the discovery of merging neutron stars important?

I'm fairly certain people here will have heard about it, already, but apparently, two supernova leftovers clashed some 130 million years ago and some billion billion kilometres away ...

What I haven't heard yet, however, is why we should care.

I mean sure, it's an interesting phenomenon and measuring it can't have been easy.

But now that we've heard it ... what changes?

I'll admit it, I don't know particularly much about astronomy, but I'm curious:

What's the significance of having achieved this? Why does it matter whether or not we know?

• From Veritasium's First Ever Light & Gravitational Wave Cosmic Event! I would say: the way the discovery went shows human ingenuity.
– user1569
Oct 16 '17 at 20:43
• Reminds me that we can't build a Doomsday Machine without research. People tried to sue the LHC to stop it wiping us all out (rejected by court). But really it was true - we just want to destroy the universe. :-) Oct 17 '17 at 1:18
• Gamma ray bursts are a plausible extinction event - learning everything we can about them is the first step in understanding the mechanism and eventually predicting them. Any plausible protection is unfortunately still in the sci-fi domain. Oct 17 '17 at 7:11
• When Einstein published general relativity theory in 1915 possibly many contemporaries wondered "what's the deal? Can't we just use Newtonian physics?". Fast forward 2017, everyone takes for granted having in pockets devices that tell exact location wherever on globe, technology made possible by Einstein discovery. Oct 17 '17 at 10:51
• Oct 17 '17 at 13:43

## 2 Answers

Reasons why this is important:

It is the first simultaneous detection of a gravitational wave and electromagnetic signal, and the strongest GW signal yet in terms of signal to noise (Abbott et al. 2017a). It spectacularly corroborates the reality of the GW detection technology and analysis. The progenitor has been unambiguously located in a (relatively) nearby galaxy (Soares-Santos et al. 2017), allowing a host of other telescopes to obtain detailed measurements.

It shows that GWs travel at the speed of light, a further verification of Einstein's General Relativity (Abbott et al. 2017b).

It shows that most of the very heavy elements such as gold, platinum, osmium etc. are plausibly produced by merging neutron stars and constrains the rate of such mergers in the local universe (e.g. Chornock et al. 2017; Tanvir et al. 2017).

It shows that short gamma ray bursts — some of the most energetic explosions in the universe — can be caused by neutron star mergers (e.g. Savchenko et al. 2017; Goldstein et al. 2017).

It is the closest detected short gamma ray burst (with a known distance). That the progenitor has also been characterised allows a closer investigation of the interesting physics underlying the ejection and jet mechanisms thought to be responsible for the gamma rays and later X-ray and radio emission (e.g. Margutti et al. 2017; Alexander et al. 2017).

It provides observational constraints on how matter behaves at extremely high densities, testing our understanding of fundamental physics to its limits — for example, the details of the gravitational wave signal moments before merger are diagnostic of the interior conditions of neutron stars at densities of $\sim 10^{18}$ kg/m$^3$ (Hinderer et al. 2010; Postnikov et al. 2010).

It provides an independent way of measuring the expansion of the universe. Merging binary gravitational wave sources are known as "standard sirens", because the distance to the GW source pops straight out of the analysis and can be compared with the redshift of the identified host galaxy (Abbott et al. 2017c). The result agrees with measurements made using the cosmic microwave background and the distance-redshift relation calibrated by other means, verifying our estimation of distances, at least in the local universe.

Finally, this event will turn out to be important because it was lucky; in the sense that the source was detected well-inside the sensitivity horizon of LIGO (Abbott et al. 2017a). The detection itself, was not unexpected given the rates predicted based on studying the neutron star binary systems in our own Galaxy (e.g. Kim et al. 2015), but the fact that it was so close — within the closest 5% of the sensitive survey volume where it could have been detected — is fortunate.

In the end, if someone thinks none of the above is interesting or important, then nothing I can write will convince them otherwise. The vast majority of people I speak to are curious and fascinated to find out about our cosmic origins and how the universe works.

• Now that's the kind of answer I was hoping for. Thank you. For the record, I wasn't saying it's not interesting, just that the media telling us little more than "scientists heard explosion from the past, yay", doesn't immediately impress on your average Joe why anybody would care. Oct 16 '17 at 21:55
• @User1291 I did assume (incorrectly). Small edit made. Oct 16 '17 at 22:38
• One question: does this show that all short GRBs are caused by neutron star mergers, or only some of them? Oct 17 '17 at 3:51
• @jamesqf It shows that merging neutron stars can produce a sGRB. So, at least some are caused by merging neutron stars. Oct 17 '17 at 5:50
• @jamesqf You need to be careful not to make a corellation fallacy here. A short GRB was observed. It neither mean that all sGRBs are caused by neutron star mergers nor that all neutron star mergers will cause a sGRB. It however does show that a sGRB can be generated and probably might be generated by such an event regularly considering the energy released, but we won't know with reasonable certainty until we've observed more such events. Oct 17 '17 at 7:24

Because its awesome (SMBC)

So this guy called Copernicus suggested that the Earth orbits the Sun (not the other way round) - What changes?

This guy Newton had a theory for how a mass responds to force, and how gravity works - So what?

Another guy called Maxwell had this idea of how light could actually be waves of electromagnetic fields - does this matter?

A guy called Monet decided to paint some pictures of some waterlilies. Who cares?

Last February, some guys from Denver carried a ball over a line more often than some guys from Carolina carried the ball over another line. So what?

It is worth finding things out because it means finding things out. It is worth understanding our world because it is there to be understood. Discovery is its own reward. It is not measured in by £ or \$.

The observations of GW170817 show that heavy elements are created by the merger of neutron stars. Heavy elements like gold, platinum on Earth were likely created in a neutron star merger in The Milky Way, billions of years ago, that enriched the interstellar dust.

If it doesn't matter to you that's fine. If Monet's Waterlilies or the Superbowl leave you cold, that's no problem either. But not everything of value is of use.

• Last February, some guys from Denver carried a ball over a line more often than some guys from Carolina carried the ball over another line. So what? I'm totally asking that one on Sports SE... Oct 17 '17 at 4:18
• I didn't realise Rugby was so popular in Denver
– user2374
Oct 20 '17 at 8:03
• "Discovery is its own reward" but not when there's a limited amount of money, and lots of hands trying to grab at it. Oct 21 '17 at 23:35