• Question: My students are aware of the 2016 discovery of gravitational waves, but what do these waves tell us or is it more that now we’re able to detect them, we can observe the Universe better?

    Asked by Katie S to Meirin, Lucy, Harrison, David, Carsten, Anne on 8 Nov 2019.
    • Photo: David Ho

      David Ho answered on 8 Nov 2019: last edited 8 Nov 2019 10:22 am

      One of the things that the gravitational wave discovery by LIGO in 2016 did was confirm the predictions of Einstein’s General Theory of Relativity. Though there are many other good reasons to believe in the theory, it’s important to test all the predictions that a theory makes in case there are any differences between the theory and the experiment. In this case the theory and experiment lined up perfectly — while this might not seem so exciting, it was nonetheless still important to do the experiment!

      A more interesting consequence of gravitational wave detection was the detection of gravitational waves from the collision of two neutron stars in 2017. Unlike black hole collisions, which are much easier to detect (because they give out stronger gravitational waves), neutron star mergers also emit gamma rays that we can detect using more conventional equipment. The experiments LIGO and VIRGO found that the gamma rays and the gravitational waves arrived at pretty much exactly the same time. This means that gravitational waves and gamma rays (which are a higher-energy form of visible light) travel at the same speed. Differences in the speed of light and gravitational waves are predicted by a lot of theories that attempt to extend the theory of general relativity, so this observation rules a lot of them out.

      The other important consequence of the gravitational wave discovery is like you say, we can observe the universe better. Now that we can detect gravitational waves it’s like having another sense that we can use to get information about the universe around us: like we were deaf before and now we can hear. Scientists are working on building much more sensitive experiments that we can use to build a gravitational wave map of the entire visible universe.

    • Photo: Anne Green

      Anne Green answered on 8 Nov 2019:

      David’s answer is excellent, so I’ll just add one more example of how gravitational waves are helping us “observe the universe better”. Many of the gravitational wave “events” that the LIGO and Virgo collaborations have observed come from mergers of massive binary black holes (pairs of black holes that are orbiting each other due to gravity). These black holes weigh tens of times as much as the Sun. Astrophysicists have some ideas about how such big black holes could form and end up in binaries. However they weren’t expecting quite as many to be formed and then merge as LIGO and Virgo have observed. So these observations are providing a new probe of star formation and evolution. Some cosmologists have argued that these big black holes could be primordial black holes (black holes formed in the early Universe) rather than the ‘normal’ black holes which form at the end of the lifetime of massive stars. However personally I think that this is unlikely.

    • Photo: Harrison Prosper

      Harrison Prosper answered on 11 Nov 2019: last edited 11 Nov 2019 6:00 pm

      David and Anne have provided excellent answers, so I’ll take the opportunity to add another interesting aspect of gravitational waves. Gravitational waves have the property that they can travel enormous distances without being impeded by radiation and matter. Consequently, these waves could allow us to “see” the universe when it was younger than 300,000 years after the Big Bang (ABB). The time 300,000 ABB is when the universe became transparent to radiation and, in particular, to light. Because the universe was opaque to light before then, it is currently impossible for us to observe the universe earlier than that time. (Ironically, the opaque universe was brighter than the noon day Sun! Just like the Sun, light finds it very difficult to travel far when there is already a lot of light and charged particles present. For example, the light you received from the Sun today, took about 500 seconds to travel from the Sun’s photosphere (its “surface”) to the Earth, but the light (traveling at the speed of light!) took about a million years to get from where it was created at the Sun’s core to its photosphere.)

      Gravitational waves have no problem penetrating the opaque universe. Therefore, if these waves could be detected (which would require hugely more sensitive gravitational wave detectors than the current ones) we would be able to explore the universe when it was much younger than 300,000 ABB and, perhaps, even when it was a fraction of a second old. If that were possible, we would be able to test directly whether what we have learned at particle accelerators about the behavior of matter under extreme conditions really applies to the early universe. We assume it does apply, and that assumption agrees with all the post 300,000 ABB evidence we have, but currently we have no way of directly testing that assumption.

Comments

  • Photo: Susan

    Susan commented on :

    So far, observations of gravitational waves have told us a number of things:
    1. As David says, they are giving us new ways to test general relativity, such as checking that they travel (as Einstein predicted) at the speed of light.
    2. They are providing information on black holes of higher masses than we had previously observed using X-ray information. This has important consequences for our understanding of how very massive stars evolve: in particular, the very first LIGO event, which involved the coalescence of two black holes each weighing 30+ times as much as the Sun, was a surprise to stellar evolution theorists, who weren’t expecting black holes quite that massive. These observations have spurred a lot of work on the evolution of very massive stars – and very massive stars are important, because they help to create the heavy elements that make planets like the Earth (and its inhabitants) possible.
    3. One thing that David didn’t mention was that the neutron star merger in 2017 was associated with a short burst of gamma rays. This demonstrated that such short gamma-ray bursts really are consequences of neutron star mergers – this had been suspected for some time, but there was no actual evidence. I should say that we were incredibly lucky to get this: the event was about 10x closer than the next nearest short gamma-ray burst we have observed, and the gamma rays (which are emitted in a fairly narrow cone) almost – but not quite – missed us. The fact that LIGO/VIRGO localised this event well enough to allow its host galaxy to be identified also allowed us to observe the aftermath of the merger, a so-called kilonova, and spectroscopy of that event helped to show that neutron star mergers are sites of a particular form of heavy element production known as the r (for rapid) process. About half of the elements heavier than iron are made by the r process, so it’s important to our understanding of our origins that we know where these are made.