An international research team have conducted a 16-year long experiment with the aim to challenge Einstein’s theory of general relativity.
A research team from the University of East Anglia and the
University of Manchester have collaboratively utilised seven radio telescopes
across the globe to investigate a pair of extreme stars called pulsars. They
used them in order to challenge Einstein’s most renowned theory of relativity,
with some of the most rigorous tests yet.
The study reveals new relativistic effects that have been
observed for the first time. Dr Robert Ferdman, from UEA’s School of Physics,
said: “As spectacularly successful as Einstein’s theory of general relativity
has proven to be, we know that is not the final word in gravitational theory.”
Einstein’s theory of general relativity
More than 100 years later, scientists around the world
continue their efforts to discover weaknesses in his theory. General relativity
is not compatible with the other fundamental forces described by quantum
mechanics. Therefore, it is important to place the most stringent tests upon
general relativity to determine how and when the theory breaks down.
“Apulsaris a highly magnetised rotating compact star that
emits beams of electromagnetic radiation out of its magnetic poles,” explained
Dr Ferdman. “They weigh more than our sun, but they are only about 15 miles
across, so they are incredibly dense objects that produce radio beams that
sweep the sky like a lighthouse. We studied a double pulsar, which was
discovered by members of the team in 2003 and presents the most precise
laboratory we currently have to test Einstein’s theory. Of course, his theory
was conceived when neither these types of extreme stars, nor the techniques used
to study them, could be imagined.”
Double pulsar analysis
The double pulsar, as its name suggests, consists of two
pulsars which orbit each other in 147 minutes with velocities of about 1
million km/h. One pulsar spins very fast at about 44 times a second. The
companion is young and has a rotation period of 2.8 seconds. It is their motion
around each other that can be used as a near perfect gravity laboratory.
Seven sensitive radio telescopes were used in the
researchers experiment to observe this double pulsar, in Australia, the US,
France, Germany, the Netherlands and in the UK with the Lovell Radio Telescope.
“We studied a system of compact stars that is an unrivalled
laboratory to test gravity theories in the presence of very strong
gravitational fields,” said Professor Kramer. “To our delight we were able to
test a cornerstone of Einstein’s theory, the energy carried by gravitational
waves, with a precision that is 25 times better than with the Nobel-Prize
winning Hulse-Taylor pulsar, and 1000 times better than currently possible with
gravitational wave detectors.” He explained that the observations were not only
in agreement with the theory, but they also highlighted our ability to now
discover effects that we were unable to in the past.
The results of the discovery
Professor Benjamin Stappers, from the University of
Manchester, explained: “The discovery of the double pulsar system presented us
with the only known instance of two cosmic clocks which allow precise
measurement of the structure and evolution of an intense gravitational field.
The Lovell Telescope at the Jodrell Bank Observatory has been monitoring it
every couple of weeks since then. This long baseline of high quality and
frequent observations provided an excellent data set to be combined with those
from observatories around the world.”
“We follow the propagation of radio photons emitted from a cosmic
lighthouse, a pulsar, and track their motion in the strong gravitational field
of a companion pulsar,” further explained Professor Ingrid Stairs from the
University of British Columbia at Vancouver. “We see for the first time how the
light is not only delayed due to a strong curvature of spacetime around the
companion, but also that the light is deflected by a small angle of 0.04
degrees. Never before has such an experiment been conducted at such a high
spacetime curvature.”
Such a fast orbital motion of compact objects such as these,
that are about 30% more massive than the Sun, allows us to test seven different
predictions of general relativity. “Apart from gravitational waves and light
propagation, our precision also allows us to measure the effect of time
dilation that makes clocks run slower in gravitational fields,” explained
Professor Dick Manchester from Australia’s national science agency, CSIRO. “We
even need to take Einstein’s famous equation E = mc2 into account when
considering the effect of the electromagnetic radiation emitted by the
fast-spinning pulsar on the orbital motion. This radiation corresponds to a
mass loss of 8 million tonnes per second.”
The researchers also measured, with a precision of 1 part in
a million, that the orbit changes its orientation. This relativistic effect is
also well-known from the orbit of Mercury, but here it is 140,000 times
stronger. As a result, they realised that at this level of precision they also
need to consider the impact of the pulsar’s rotation on the surrounding
spacetime, which is dragged along with the spinning pulsar.
Precision tracking of the rotations of the neutron star
Dr Norbert Wex from the MPIfR, another main author of the
study, said: “Physicists call this the Lense-Thirring effect or frame-dragging.
In our experiment it means that we need to consider the internal structure of a
pulsar as a neutron star. Hence, our measurements allow us for the first time
to use the precision tracking of the rotations of the neutron star, a technique
that we call pulsar timing to provide constraints on the extension of a neutron
star.”
The technique of pulsar timing was combined with careful
interferometric measurements of the system to determine its distance with high
resolution imaging, resulting in a value of 2400 light years with only an 8%
error margin. Team member Prof Adam Deller, from Swinburne University in
Australia, who was responsible for this part of the experiment, said: “It is
the combination of different complementary observing techniques that adds to
the extreme value of the experiment. In the past similar studies were often
hampered by the limited knowledge of the distance of such systems.”
However, this is not the case here, as in addition to pulsar
timing and interferometry, the information gained from the effects of the
interstellar medium were also carefully considered. Professor Bill Coles from
the University of California San Diego agreed: “We gathered all possible
information on the system and we derived a perfectly consistent picture,
involving physics from many different areas, such as nuclear physics, gravity,
interstellar medium, plasma physics and more.”
“We have reached a level of precision that is unprecedented.
Future experiments with even bigger telescopes can and will go still further,”
Prof Kramer added. “Our work has shown the way such experiments need to be
conducted and which subtle effects now need to be taken into account. And,
maybe, we will find a deviation from general relativity one day.”
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