≡ Menu

Probing General Relativity with Neutron Stars

Another of those ‘new eras’ I talked about in yesterday’s post is involved in the latest news on gravitational waves. Let’s not forget that it was 50 years ago — on November 28, 1967 — that Jocelyn Bell Burnell and Antony Hewish observed the first pulsar, now known to be a neutron star. It made the news at the time because the pulses, separated by 1.33 seconds, raised a SETI possibility, leading to the playful designation LGM-1 (‘little green men’) for the discovery.

We’ve learned a lot about pulsars emitting beams at various wavelengths since then and the SETI connection is gone, but before I leave the past, it’s also worth recognizing that our old friend Fritz Zwicky, working with Walter Baade, first proposed the existence of neutron stars in 1934. The scientists believed that a dense star made of neutrons could result from a supernova explosion, and here we might think of the Crab pulsar at the center of the Crab Nebula, an object whose description fits the pioneering work of Zwicky and Baade, and also tracks the work of Franco Pacini, who posited that a rotating neutron star in a magnetic field would emit radiation. Likewise a pioneer, Pacini suggested this before pulsars had been discovered.

Writing about all this takes me back to reading Larry Niven’s story ‘Neutron Star,’ available in the collection by the same name, when it first ran in a 1966 issue of IF. Those were interesting days for IF, but I better cut that further digression off at the source — more about the magazine in a future post. ‘Neutron Star’ is the story where Beowulf Shaeffer, a familiar character in Larry’s Known Space stories, first appears. If you want to see a neutron star up close and learn what its tidal forces can do, you can’t beat Niven’s tale.

Typically, a neutron star will get up to two solar masses while showing a diameter of a mere 20 kilometers, telling us it’s made of a kind of dense matter about which we have much to learn. Scientists at the Max Planck Institutes for Gravitational Physics and for Radio Astronomy have been looking at two major tools for studying the intense gravity associated with neutron stars: Pulsar timing and gravitational wave observations like the recent GW170817 event.

What’s interesting here is that neutron stars can help us investigate theories of gravity in which the gravitational fields associated with their interactions may differ from what General Relativity predicts. Lijing Shao, lead author of the paper coming out of this work, notes that alternate theories of gravity will produce different predictions on the behavior of binary neutron star systems. The paper gives us a glimpse of the kind of analyses gravitational wave astronomy will enable.

“The gravitational acceleration at a neutron star’s surface is about 2×1011 times that of the Earth which makes them excellent objects to study Einstein’s general relativity and alternative theories in the strong-field regime. In a systematic investigation with pulsar timing technologies, we were able to put constraints on a class of alternative gravity theories showing for the first time in detail how they depend on the physics of the extremely dense matter they contain.”

All this emerges as the “equation of state” of neutron stars. Equations of state relate pressure, volume and temperature in a particular substance that is in thermodynamic equilibrium. Shao and colleagues have been studying possible equations of state for five binary pulsar systems, each of which contains a neutron star and a white dwarf. They believe that gravitational wave detectors will soon become sensitive enough to investigate alternate gravitational theories.

Image: The constraints on deviations from general relativity set by pulsar timing leave a gap between about 1.6 – 1.7 solar masses. Gravitational wave observations of binary neutron stars of the appropriate mass could fill this gap and thus further constrain alternative theories of gravity. Credit and Copyright: L. Shao (Max Planck Institute for Gravitational Physics & Max Planck Institute for Radio Astronomy), N. Sennett, A. Buonanno (Max Planck Institute for Gravitational Physics).

Co-author Alessandra Buonanno, who is also director of the Astrophysical and Cosmological Relativity division at the Institute in Potsdam, puts the matter this way:

“The LIGO-Virgo detectors may soon discover binary neutron star systems with suitable masses that could improve the constraints set by binary-pulsar tests for certain equations of state and thus put Einstein’s general relativity and alternative theories to a qualitatively new test.”

Thus we’re pushing into the early days of gravitational wave astronomy by looking at conditions in ultra-strong gravitational fields, with an eye toward yet another way of probing General Relativity. Improving the analyses already made through pulsar timing alone, gravitational waves will help us probe deeper, especially as we begin to get better mass measurements for known pulsars, where in many cases the uncertainties are large. As the paper notes:

Our comparisons between binary pulsars and GWs made use of the current limits of the former and the expected limits of the latter. It shows that advanced and next-generation ground-based GW detectors have potential to further improve the current limits set by pulsar timing.

The paper is Shao et al., “Constraining nonperturbative strong-field effects in scalar-tensor gravity by combining pulsar timing and laser-interferometer gravitational-wave detectors,” accepted at Physical Review X (preprint).


{ 15 comments… add one }
  • Geoffrey Hillend October 25, 2017, 16:34

    There is nothing wrong with testing general relativity, however, it’s predictions of gravitational fields have never been shown to be inaccurate. I don’t think they ever will. I have to disagree with the idea that there can be or will be any deviation of gravitational fields in binary neutrons stars of what general relativity predicts. General relativity is the principle of physical reality itself on the large scales so that it is absolute and invariant.

    • Michael Simmons October 25, 2017, 21:22

      >I don’t think they ever will
      ok, let’s not bother….
      Seriously we know something is wrong with our understanding. Dark Matter, Dark energy and lots of other things aren’t explained by any current theory.
      People can theorize all they want but the experimental proof is the only thing that makes it real. And the only way to do that is to examine the universe in increasing detail and in new ways.
      Hopefully, at some point, gravitational detectors will detect a deviation of some kind and that will lead to a new understanding.

      • Michael October 26, 2017, 11:35

        I have absolutely no doubt that General Relativity is a special case of (tbd)

    • David Herne October 25, 2017, 21:42

      I wondered this too. A discussion of alternative theories sometime in the future would be interesting.

    • Project Studio October 25, 2017, 23:20

      I think using has more to do with gaining a better understanding of the nature of space-time, and its underlying field properties. For example, understandings that would interest me include whether something like “negative mass” is actually possible, or if space can be curved in such a way as to mimic the faster-than-light expansion of space theorised in Inflation Theory. Also related to Inflation theory, what type of “space” is, in fact, is our space? Many kinds are theorised and practical extra-galactic laboratories may be able to tell us the answers.

      • Project Studio October 25, 2017, 23:21

        “I think using [neutron stars to test GRT]…”

    • David Herne October 26, 2017, 9:25

      To be clear, my response was in reply to Geoffrey’s post. Another similar line of thinking – https://www.forbes.com/sites/startswithabang/2017/10/25/merging-neutron-stars-deliver-deathblow-to-dark-matter-and-dark-energy-alternatives/#22d993643b8c – Quoting Ethan Siegel ‘But modifying gravity, either to account for dark matter or dark energy (much less both), is a game you have to play very carefully. Einstein’s theory of General Relativity has already been tested quite rigorously, and its predictions have been borne out every time. If you modify gravity, you’re altering that theory, so you have to do it in a way that doesn’t impinge upon the observations and measurements that have already taken place.’

  • Jim Early October 26, 2017, 9:45

    I attended an astronomy seminar on neutron stars at MIT in 1962. At the time neutron stars and black holes were considered entertaining theoretical speculations. Nobody at the seminar had any ideas on how to experimentally test the concepts which seemed closer to science fiction than physics.

    One year later the initial observations were made of the red shift of a quasar which was eventually recognized to be a super-massive black hole. Only four years later the pulsar observation was made which was eventually was recognized to be a neutron star.

    Perhaps LIGO will surprise us with one of these unanticipated observations that make physics so interesting.

    • ljk October 26, 2017, 15:53

      Pulsars were actually discovered a few months before their “official” discovery in 1967 by a USAF officer who was originally looking for Soviet missile launches.


      1967 was a good year for finding exotic cosmic objects via the military, as gamma ray bursts (GRB) were first found then by the Vela satellites which were used for testing nuclear bomb tests detections, including the far side of the Moon where it was suggested that the Soviets might use the 2,160 mile wide rocky bulk of Earth’s natural satellite to hide such blasts.


      Makes one wonder what else the military found during the Cold War that was either ignored or covered up.

  • Adam Byrne October 26, 2017, 16:04

    Why do neutron stars end up in pairs so frequently? What is the process behind that? You would think that being rare and far apart they wouldn’t have a great chance of pairing up.

    • Pasander October 29, 2017, 6:04

      Apparently, stars are often formed as binary or multiple star systems. I wonder how many of today’s singles are actually ejects from a multiple star system. The most massive components of a multiple star system would also naturally be the most likely to stay when the lightweights of the group are kicked out.


      It is estimated that approximately one third of the star systems in the Milky Way are binary or multiple, with the remaining two thirds being single stars.[69] The overall multiplicity frequency of ordinary stars is a monotonically increasing function of stellar mass. That is, the likelihood of being in a binary or a multi-star system steadily increases as the mass of the components increase.[68]

  • ljk November 29, 2017, 17:33

    Little Green Men? Pulsars Presented a Mystery 50 Years Ago

    By Calla Cofield, Space.com Senior Writer | November 28, 2017 07:21 am ET

    Fifty years ago this month, a small group of astronomers made a revolutionary cosmic discovery — explaining a phenomenon that they initially thought might come from an intelligent alien civilization.

    In November 1967, Jocelyn Bell (now Dame Jocelyn Bell Burnell), a graduate student at Cambridge University in England, made what turned out to be the first detection of a pulsar — an incredibly dense ball of material formed when a massive star runs out of fuel and collapses in on itself.

    In the time since the discovery of pulsars, the objects have provided insight about the life cycle of stars and extreme states of matter, and provided evidence that supports Albert Einstein’s theory of gravity.

    There are currently efforts underway to use pulsars to detect gravitational waves, or ripples in the fabric of the universe, and another to use pulsars as part of a space-based navigation system.

    Full article here:


    To quote:

    Bell Burnell was in charge of operating the telescope and analyzing the data, according to an article she wrote for Cosmic Search Magazine in the 1970s. Using this technique, Bell Burnell spotted an object that appeared to be flickering every 1.3 seconds; this pattern repeated for days on end. The object didn’t match the profile of a quasar. The signal conflicted with the generally chaotic nature of most cosmic phenomenon, the researchers would later explain. In addition, the light was of a very specific radio frequency, whereas most natural sources typically radiate across a wider range.

    For those reasons, Bell Burnell, Hewish and some other members of the astronomy department had to acknowledge that they might have found an artificially created signal — something emitted by an intelligence species. Burnell even labeled the first pulsar LGM1, which stood for “little green men 1.”

    A second discovery

    Bell Burnell would later report that Hewitt called a meeting without her, in which he discussed with other members of the department how they should handle presenting their results to the world. While their fellow scientists might practice restraint and skepticism, it was likely that the possible detection of an intelligent alien civilization could create chaos among the public, the scientists said. The press would very likely blow the story out of proportion and descend on the Cambridge researchers. According to Hewitt, one person even suggested (perhaps only partly joking) that they burn their data and forget the whole thing.

    Years later, Burnell wrote that she was rather annoyed at the appearance of the strange signal for another reason. As a graduate student, she was trying to get her thesis work done before her funding ran out, but work on the pulsar was taking away from her primary pursuit.

    “Here I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us,” she wrote in the article for Cosmic Search Magazine.

    But then, Bell Burnell resolved the problem. She went back through some of the data from the radio array and found what looked like a similar, regularly repeating signal, this one coming from an entirely different part of the galaxy. That second signal indicated that this was a family of objects, rather than a single civilization trying to make contact.

    “It finally scotched the little green men hypothesis,” Bell Burnell said in the a BBC documentary filmed in 2010. “Because it’s highly unlikely there’s two lots of little green men, on opposite sides of the universe, both deciding to signal to a rather inconspicuous planet, Earth, at the same time, using a daft technique and a rather commonplace frequency.”

    “It had to be some new kind of star, not seen before,” she said. “And that then cleared the way for us publishing, going public.”

    In 1974, the Nobel Prize in Physics was awarded to Hewish, along with radio astronomer Martin Ryle, “for their pioneering research in radio astrophysics: Ryle for his observations and inventions, in particular of the aperture-synthesis technique, and Hewish for his decisive role in the discovery of pulsars.” The omission of Bell Burnell’s name as a contributor to the pulsar discovery has stirred controversy among scientists and members of the public, though Bell Burnell has not publicly contested the Nobel committee’s decision.

  • ljk November 30, 2017, 11:04

    Pulsars were discovered 50 years ago

    By EarthSky Voices in Space | November 30, 2017

    In 1967, while helping analyze data from a new telescope, Cambridge student Jocelyn Bell observed a bit of “scruff” – the first evidence of a pulsar. The discovery changed our view of the universe.


    To quote:

    Pulsars also offer promise as a navigation system for guiding craft travelling through deep space. In 2016 China launched a satellite, XPNAV-1, carrying a navigation system that uses periodic X-ray signals from certain pulsars.

Leave a Comment