back to article Two white dwarfs and superdense star. Yup, IDEAL for gravity lab in the sky - boffins

Boffins have discovered a unique triple star system that could help them to probe one of the most fundamental physics problems of all: the true nature of gravity itself. Millisecond pulsar, left foreground, is orbited by a hot white dwarf star, centre, both of which are orbited by another, more-distant and cooler white dwarf, …


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  1. Phil O'Sophical Silver badge

    the story of Galileo dropping two balls of different weights from the Leaning Tower of Pisa

    I prefer the Apollo 15 version, where one of the astronauts dropped a feather and a hammer together, on the moon. Both, or course, hit the ground at the same time.

    1. Nigel 11

      Here on Earth, if you put the feather on top of the hammer and drop in still air, the feather will fall with the hammer. The hammer pushes the air out of the way, and gravity does the rest.

      I'd be very surprised if Galileo hadn't tried this, thought he was probably smart enough to realise that the limitations of this particular experiment would be siezed upon by his enemies.

      1. Anonymous Coward
        Anonymous Coward

        IIRC, the weight vs feather story is apocryphal. Gallileo knew very well that a feather would be blown around, he used 2 spherical weights of identical size, but of 2 different materials (lead and copper? Can't remember).

        1. Annihilator

          I thought it was already known that it only worked for spherical chickens in a vacuum?

          1. VinceH

            "I thought it was already known that it only worked for spherical chickens fired from a vacuum cleaner?"


        2. Miek

          "Gallileo knew very well that a feather would be blown around" -- it's about air resistance not being "blown around by the wind", for further reading look up "Terminal Velocity"

        3. Anonymous Coward
          Anonymous Coward


          As I recall our lecturer, and my visit to the Science Museum in Florence (Museo Galileo, Piazza dè Giudici, 1, 50122 Florence, Italy, vaut un detour) Galileo did not drop weights because it is very difficult to compare the arrival of two falling objects. He used an inclined plane to roll them down, and timed them with the help of a musical friend who could keep strict tempo.

          Galileo got the "right" answer but was also wrong because he didn't realise that his masses were acquiring rotational kinetic energy as they fell, and so if they had been inhomogeneous (one, say, being a hollow sphere and the other being solid) they would have reached the bottom at different times. Also, he could not have calculated g from his experiment.

          I mention this not simply because I am an obnoxious smartass, but because it shows how our understanding of something that Newton turned into an apparently simple equation is constantly evolving - and indeed this latest experiment still presumably isn't going to be treating mass from its quantum mechanical, Higgs field point of view. There's an awful lot more synthesis to come.

          1. Nigel 11

            Re: Galileo

            I'd be most surprised if he didn't start by dropping weights. It's not hard to distinguish clunk! from clunk-clunk!, where the impacts are separated by as little as 50ms (maybe less). Then he thought "that's interesting". Then he'd have looked for a way to make the experiment more accurate, and got lucky by using solid rather than hollow spheres on an inclined plane. Right result, but missing a large chunk of reasoning about then-unknown rotational kinetic energy and the importance of the spheres being homogeneous.

  2. Gordon 10

    Ransom Archibald and Stairs

    Should set up in business as a Law firm or debt collection agency

  3. Anonymous Coward
    Anonymous Coward

    I love it when you humans look up at the infinite and think something is unique.

    Because, as the bard said, 'space is big'.

  4. Destroy All Monsters Silver badge

    "Best hope of seeing quantum theory in action"

    That is actually your nearest high-school lab.

    What you hope to see here is some deviation from the predictions of GR. This may or may not indicate something about Quantume Gravity if they occur. But probably not.

    1. Sander van der Wal


      You nearest window will do just fine. After it gets dark. You can see through it and see your face being reflected. At the same time. Amazing, isn't it? Some photons pass through and some bounce back. Quantum in action.

      1. Anonymous Coward
        Anonymous Coward


        "Quantum in action."

        You do realize that your cat (Quantum ?) will be half reflected, half seen, in your window.In the half-dark, Photon(your neighbour's cat) will be also half seen, half reflected and , more than half starved........

  5. I ain't Spartacus Gold badge

    It ain't as if it's rocket science or nuffin'?

    Why's this so hard?

    Simply measure the radio waves the pulsar is emitting and test them. If it's outputting Radio 4, then this is a rational universe, and Einstein was on the money. If' however it's outputting TalkSport - then it's all quantum.

    Unless there's a White Van Man theory of cosmology...

  6. Tom 7

    Where General Relativity breaks down...

    The trouble with that is the only way we have of measuring is with things that filter only General Relativity behaviour type things through.

    They seem to be off to an interesting start - using the Strong Equivalence Principle to calculate the positions of the stars - and then any error in the SEP will show up as a ??

    Its been tested to 1 in 10**14 so far...

    1. Ugotta B. Kiddingme

      Re: Where General Relativity breaks down...

      My own experiments indicate general relativity breaks down 2-4 hours after Christmas dinner. By that time, I've had quite enough of my relatives and want them out of my house.

  7. Klaxon

    Circular reasoning?

    So, they've constructed a model of the system using the pulsar timing data, and they are going to test their model, using... the pulsar timing data?

    1. Francis Boyle Silver badge

      Not sure if you're simply joking

      but this is standard science stuff. The model is constructed using the theory to be tested. If the predictions of the model check out, score one for the theory. If not, more interesting science stuff.

  8. Aaiieeee


    I don't get how a pulsar giving off radio waves can be 'measured' that results in giving the relative mass and distances of the two other stars? I struggle to understand how they can get so much information from a radio signal and then use it for scientific basis? (Genuine questions)

    1. Bluewhelk

      Re: how?

      Timing variations in the pulsar pulses give you its speed (sort like Doppler effect). As time pieces pulsars are very stable so any changes are due to relative motion over short time scales.

      Integrate speed to get size of orbit (simplistic analogy I'll admit)

      Keplers laws then give you relative masses (possibly need relativistic corrections?).

    2. Nigel 11

      Re: how?

      I haven't worked out the details (which will be very complex, probably involving serious supercomputer resources to model) but there will be gravitational perturbations of each orbit by the other bodies. Here in the solar system, the existence and position of Pluto was deduced and calculated by very careful observation of the orbits of Uranus and Neptune. Once they knew where to look, they pointed a powerful telescope at that patch of sky, and there was Pluto.

      For this system we can test GR rather than simple Newtonian gravity, because the bodies are very dense and close. It tests the equivalence princible, because a Neutron star is made of Neutrons and a White Dwarf is made of ordinary atomic nuclei (ie with protons, and electrons between the nuclei). Short of a black hole, that's about as much different as two astronomical masses can be. (*)

      I'll have to file this system along with Zircons dating of the age of the Earth. $DEITY is being extremely kind to us, providing us with the means to scientifically investigate questions we might have thought were forever beyond our grasp.

      (*) I've always wondered whether bosons (commonly neutrons and protons) and fermions (commonly electrons) are gravitationally equivalent. Is there any way to probe this, even conceptually, given that the electromagnetic interactions of electrical charges are about 10^40 times greater than any gravitational effects?

      1. Anonymous Coward
        Anonymous Coward

        Re: how?- @Nigel 11

        Except, perhaps unfortunate example, that there wasn't Pluto, and it has taken many years and much astronomical wailing and gnashing of teeth to get accepted the fact that the little rock that was actually found and called "Pluto" was not the explanation of the disturbances of the orbits of Uranus and Neptune. The history of how its size was initially estimated at the uppermost limit of the possible range to try and make it big enough, only to have the estimate whittled away as better observations occurred, is a wonderful story of the self-delusion of astronomers, and the politicisation of science (Pluto was the only planet to be discovered by an American, and resistance to an attack of realism and its downgrading came largely from the US. If the discoverer had been Russian, Chinese or indeed from any large country, it would have been the same story; this isn't a knock-the-US rant, and the US has, since the 1930s, achieved so much in astronomy that worrying about the status of a poxy little lump of rock seems utterly pointless.)

        The difference between the announcement of the discovery of Pluto and the relatively cautious and low-key announcement of the Higgs discovery is evidence that we've advanced quite a lot in the proper analysis of experimental results; any sensationalism nowadays is usually journalistic rather than chauvinistic.

        1. asdf

          Re: how?- @Nigel 11

          >is usually journalistic rather than chauvinistic.

          Don't you mean jingoistic? (maybe not but I love that word).

      2. Lars Silver badge

        Re: how?

        Pluto is not such a good example. Borrowing from the Wikipedia:

        In 1978, the discovery of Pluto's moon Charon allowed the measurement of Pluto's mass for the first time. Its mass, roughly 0.2% that of Earth, was far too small to account for the discrepancies in the orbit of Uranus. Subsequent searches for an alternative Planet X, notably by Robert Sutton Harrington,[48] failed. In 1992, Myles Standish used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5%, to recalculate its gravitational effect on Uranus. With the new figures added in, the discrepancies, and with them the need for a Planet X, vanished.[49] Today, the majority of scientists agree that Planet X, as Lowell defined it, does not exist.[50] Lowell had made a prediction of Planet X's position in 1915 that was fairly close to Pluto's position at that time; Ernest W. Brown concluded almost immediately that this was a coincidence,[51] a view still held today.[49]

      3. John 62

        Re: how? (@Nigel 11)

        Your asterisk is almost right.

        Bosons are things like photons, things that can pass through each other without interacting (i.e. two or more bosons can occupy the same space).

        Electrons, protons and neutrons are all fermions, things that interact with each other and cannot pass through each other (i.e. two or more fermions cannot occupy the same space).

  9. Roger Jenkins


    "Finding a deviation from the Strong Equivalence Principle would indicate a breakdown of General Relativity and would point us toward a new, correct theory of gravity."

    I get from that that the opinion is that the 'old' theory of gravity is incorrect. I thought the point of the experiment was to test if one or the other is correct, or at least to point to one being 'more' correct than the other.

    1. Bluewhelk

      Re: bias?

      I think it's more like where you can use Newtonian gravity to calculate the solar system interactions but this becomes inaccurate when you try to follow Mercury over a longish period where you need to use general relativity. Thus it is said that Newtonian model of gravity breaks down when influenced by a large mass (like the sun here).

      Here they are trying to find circumstances where the predictions given by general relativity start to break down. We already know it's incomplete because it doesn't work at quantum scales for example.

      The trick is to be able to test the theories and if it fails it can then point out which newer theory has a better fit. The problem is that you need extreme examples to push the boundaries of general relativity, this system is fairly extreme and as it contains a quasar it contains an accurate clock that we can take readings from.

      It doesn't really mean the old theories are wrong but just incomplete. Like we can still use Newtonian gravity so long as we understand where the limits are.

      1. John Brookes

        Re: bias?

        Correction: pulsar, not quasar.

        Otherwise, agreed

  10. Xamol

    Equivalence Principle

    Just a thought/question...

    Since Galileo we've been limited to releasing relatively small objects (balls, feathers and hammers etc) close to large objects (earth and moon) in order to observe the effects of gravity. That these observations reveal no difference in behaviour due to the smaller objects size or mass may be a limitation of the experiment. Couldn't size or mass be a factor weighted by the relative size or mass compared to the other object(s)?

    Is this one reason why they're trying to find solar system sized experiments to observe?

    1. Anonymous Coward
      Anonymous Coward

      Re: Equivalence Principle

      "Since Galileo we've been limited to releasing relatively small objects (balls, feathers and hammers etc) close to large objects (earth and moon) in order to observe the effects of gravity."

      Not really, our understanding of gravity has been tested against larger objects than that, such as planets, stars and galaxies.

      They already know that General Relativity can't work in every situation, particularly in the extremes of the theory. In particular, where particularly "deep" gravity wells exist. It breaks down completely in black holes for example, and neutron stars closely approach this limit. By being able to study this type of system, they're able to measure how far reality differs from theory and arm them with more knowledge to work towards modifying or replacing GR.

    2. Nigel 11

      Re: Equivalence Principle

      Good question.

      The gravitational interaction of two human-sized objects has been observed, and Newton's law of gravitation (inverse-squares) confirmed for masses of the order of kilogrammes at distances of the order of a meter. The experiments are very hard, and the accuracy isn't great. Some alternative theories have proposed that gravitation breaks down on milli- or micro-meter scales, and of course at much smaller scales still we get into quantum effects. These aren't yet testable.

      Other theories posit that inverse-squares gravitation breaks down on the scale of a galaxy and above, and attempt to explain away dark matter and dark energy in terms of a different law of gravitation on very large scales. So far, such attempts have not been very successful in doing away with the need for dark matter and energy to explain the observations.

      The equivalence principle is much better-tested for terrestrial masses, than Newton's law itself. But all the masses tested were made of conventional atoms. Perhaps fortunately, we don't have neutronium or large masses of antimatter to play with.

      AFAIK the orbits of the Voyager spacecraft are still observed to be not quite as expected. Tantalizing, but they weren't ever designed as gravity probes, and their deviation from predicted orbit is just within the likely errors of observations.

    3. DropBear

      Re: Equivalence Principle

      "Since Galileo we've been limited to releasing relatively small objects (balls, feathers and hammers etc) close to large objects (earth and moon) in order to observe the effects of gravity" - strange, I thought that the masses used in ye olde Cavendish experiment (aka "weighing the earth", straight to within 1% of currently accepted density) were remarkably comparable...

  11. Vociferous


    "When a massive star explodes as a supernova and its remains collapse into a superdense neutron star, some of its mass is converted into gravitational binding energy that holds the dense star together. The Strong Equivalence Principle says that this binding energy will still react gravitationally as if it were mass. Virtually all alternatives to General Relativity hold that it will not."

    General relativity holds that energy exerts gravity? Am I reading this correct?

    1. Chemist

      Re: Waitwhat

      "General relativity holds that energy exerts gravity? Am I reading this correct?"

      Simple answer seems to be yes ! Energy is one of the factors in the Einstein gravity equations along with mass The curvature of spacetime (=gravity) depends on its mass & energy content.

      Mind, what do I know, I'm a chemist. Einstein : "The trouble with chemistry is that it is too difficult for chemists" !

      1. Vociferous

        Re: Waitwhat

        > Simple answer seems to be yes !

        But gravity is exchange of momentum via virtual photons, the thought of ?photons? exchanging momentum via virtual photons breaks my brain!

        1. Lapun Mankimasta

          Re: Waitwhat

          Not really, it's no different from the exchange of energy quanta between a photon and an electron, the theory of which being the reason why Einstein won the Nobel Prize for Physics way back when.

        2. Chemist

          Re: Waitwhat

          "But gravity is exchange of momentum via virtual photons"

          Never come across gravity being described that way only the other fundamental forces. AFAIK the fundamentals of GR are that the 'force' of gravity is a result of spacetime itself being curved by mass/energy.

          As there is no accepted quantum gravity theory AFAIK, the exchange of virtual particles ( which does derive from quantum theory) in a gravity-inducing sense is speculation.

    2. Bluewhelk

      Re: Waitwhat

      Pretty much.

      Mass and energy are equivalent from E = m c^2

      I think high energy physicists regularly interchange the two.

      For sane amounts of energy you won't get much mass increase but in these types of systems I guess it'll be significant.

    3. dan1980

      Re: Waitwhat

      Remember, also, that - as per the equation - mass and energy are equivalent at a truly mind-boggling ratio and so you would need a lot of energy to equal the gravitational attraction of relatively small amounts of mass.

      Hence, presumably, the necessity of such precise measurements.

  12. Martin Budden Silver badge

    is it really only a three-body system?

    If there's also a planet or two involved, wouldn't that mess with the results?

    1. MrDamage Silver badge

      Re: is it really only a three-body system?

      Yes and no.

      Any extra, unexplained "wobble" amongst the 3 primary bodies being studied will quickly lead to fursther studies in an effort to find out the cause of it, and thus the discovery of yet another exoplanet. Once verified and mass calculated, its figures will be entered into the overall equation in order to more finely tune the model being worked on.

      1. Martin Budden Silver badge

        Re: is it really only a three-body system?

        Which brings us back to "circular reasoning" as mentioned above ;-)

  13. John Brown (no body) Silver badge

    Is it just me...

    ... or does that look more like Red Dwarf in the top right corner?

    "Millisecond pulsar, left foreground, is orbited by a hot white dwarf star, centre, both of which are orbited by another, more-distant and cooler white dwarf, top right."

    1. MrDamage Silver badge

      Re: Is it just me...

      Its a hipster white dwarf. Being a white white dwarf is just too mainstream.

  14. Long John Brass

    Musta been one helluva place to live

    In it's hey day

    Am I right in thinking ...

    Neutron star -- was blue giant?

    Hot white dwarf -- Recently deceased G type/Red giant?

    Cooler white dwarf -- Smaller g type

    All in an orbit of about earth norm?

    Sunsets there must have been epic!

  15. Calidore

    Galileo's original experiment...

    ...used a witch and a duck, but was invalidated when it was discovered that they actually weigh the same.

    1. DropBear

      Re: Galileo's original experiment...

      Of course they do! Why, just what were they expecting the maximum takeoff weight of a standard-issue single-seat broom to be? That of a fully grown adult?!?

  16. Dodgy Geezer Silver badge

    I forsee an interstellar legal battle here...

    "This triple system gives us a natural cosmic laboratory far better than anything found before for learning exactly how such three-body systems work ...

    Have they considered the possibility that it IS a cosmic laboratory, but that it ISN'T natural?

    If someone else has put that together to demo three-body mathematics, they might be a bit pissed off at us using it on the side, and come after us for part of the set-up costs.

    Then again, if it's just some junior grade alien's homework, maybe we can fob them off with some Earth-specific teenage present. Perhaps a night out in Cardiff with Justin Bieber or Momoiro Clover Z, or both, according to choice...

    1. Toltec

      Re: I forsee an interstellar legal battle here...

      Would all radio telescopes capable of receiving the signal have to pay or just the ones using the information it contains.

      Would Capita get the contract from the Galactic Broadcasting Corporation to administer it?

  17. lunatik96

    prove Einstein wrong?

    They been trying to do this for quite a while. In the words of Sponge Bob, Good luck with that.

  18. Michael H.F. Wilkinson

    Interesting stuff

    Also good to see the old Westerbork Synthesis Radio Telescope still going strong after so many years (first light was in 1970). It has undergone some major upgrades, of course.

    As for quantum gravity, I am always a bit uncertain about that

    Sorry, I'll get me coat

  19. Ashbert

    Can we test Special Relativity too?

    Special Relativity has been far less tested than Quantum Mechanics. Even Einstein's own suggested platform-and-traincar experiment, which can now be done in table-top form, has not been done.

    Time dilation and length contraction are the two central consequences Special Relativity. Forget testing the accuracy of the Length Contraction equation, it has not even been tested that length contraction happens. To put it another way, length contraction has never been observed, and over a hundred years have passed since it was theorized.

    General Relativity's failure to explain the observed universe is well known. The evidence against General Relativity is not just in the behavior of galaxies but Earth-side too in the behavior of various launched space-crafts. Dark matter was created to explain failures of General Relativity .

    Billions of dollars are being allocated and are being spent to search for Dark Matter. If Special Relativity fails an experiment then no Dark Matter or other such creation can save it. And General Relativity is built on the assumption that Special Relativity is true. It might be wise to spend a tiny amount to at least confirm Special Relativity.

    It is good to see that individuals are now committing personal money to help bring about experimental testing of Special Relativity. The owner of below site has committed $7 million.

    Ashish Sirohi

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