Page 29 - EngineerIT January 2022

Figure 2: The seven radio telescopes used for observations of the double pulsar PSR J0737-3039. Clockwise from upper left: Effelsberg Radio Telescope
(Germany), Nançay Radio Telescope (NRT, France), Westerbork Synthesis Radio Telescope (WSRT, The Netherlands), Parkes Radio Telescope (Australia), Jodrell
Bank Telescope (UK), Very Long Baseline Array (VLBA, U.S.), Green Bank Telescope (GBT, U.S.).

of 2,8 seconds. It is their motion around each other which can be extreme value of the experiment. In the past similar studies were
used as a near perfect gravity laboratory. often hampered by the limited knowledge of the distance of such
Prof Dick Manchester from Australia’s national science agency, systems.” This is not the case here, where in addition to pulsar
CSIRO, illustrates: “Such fast orbital motion of compact objects like timing and interferometry also the information gained from effects
these - they are about 30% more massive than the Sun but only due to the interstellar medium were carefully taken into account.
about 24 km across - allows us to test many different predictions of Prof Bill Coles from the University of California San Diego agrees:
general relativity - seven in total! Apart from gravitational waves, our “We gathered all possible information on the system, and we
precision allows us to probe the effects of light propagation, such as derived a perfectly consistent picture, involving physics from many
the so-called “Shapiro delay” and light-bending. We also measure the different areas, such as nuclear physics, gravity, interstellar medium,
effect of “time dilation” that makes clocks run slower in gravitational plasma physics and more. This is quite extraordinary.”
fields. We even need to take Einstein’s famous equation E = mc into “Our results are nicely complementary to other experimental
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account when considering the effect of the electromagnetic radiation studies which test gravity in other conditions or see different
emitted by the fast-spinning pulsar on the orbital motion. This effects, like gravitational wave detectors or the Event Horizon
radiation corresponds to a mass loss of 8 million tonnes per second! Telescope. They also complement other pulsar experiments, like our
While this seems a lot, it is only a tiny fraction - 3 parts in a thousand timing experiment with the pulsar in a stellar triple system, which
billion billion - of the mass of the pulsar per second.” has provided an independent (and superb) test of the universality of
The researchers also measured - with a precision of 1 part in a free fall”, says Paulo Freire, also from MPIfR.
million - that the orbit changes its orientation, a relativistic effect Michael Kramer concludes: “We have reached a level of
also well-known from the orbit of Mercury, but here 140 000 times precision that is unprecedented. Future experiments with even
stronger. They realised that at this level of precision they also need bigger telescopes can and will go still further. Our work has shown
to consider the impact of the pulsar’s rotation on the surrounding the way such experiments need to be conducted and which subtle
spacetime, which is “dragged along” with the spinning pulsar. Dr effects now need to be taken into account. And, maybe, we will find
Norbert Wex from the MPIfR, another main author of the study, a deviation from general relativity one day…” n
explains: “Physicists call this the Lense-Thirring effect or frame-
dragging. In our experiment it means that we need to consider Radio pulsars – rapidly rotating highly magnetised neutron
the internal structure of a pulsar as a neutron star. Hence, our stars – are fascinating objects. Weighing more than our sun, yet
measurements allow us for the first time to use the precision tracking only about 24 km in diameter, these incredibly dense objects
of the rotations of the neutron star, a technique that we call pulsar produce radio beams that sweep the sky like a lighthouse. Since
timing to provide constraints on the extension of a neutron star.” their discovery by Jocelyn Bell-Burnell and Antony Hewish in
The technique of pulsar timing was combined with careful 1967, more than 3000 pulsars have been found. Pulsars provide
interferometric measurements of the system to determine its a wealth of information about neutron star physics, the Galactic
distance with high resolution imaging, resulting in a value of 2400 gravitational potential and magnetic field, the interstellar
light years with only 8% error margin. Team member Prof Adam medium, celestial mechanics, planetary physics and even
Deller, from Swinburne University in Australia and responsible for cosmology. They enable the strongest tests for predictions by
this part of the experiment, highlights: “It is the combination of gravity theories within extremely strong curved spacetimes.
different complementary observing techniques that adds to the

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