Artist’s impression of the Double Pulsar system and its effect on spacetime. The spacetime curvature (shown in the grid at the bottom) is highest near the pulsars. As they orbit one another, these deformations propagate away at the speed of light as gravity waves, carrying away orbital energy. By counting each time the pulsed beam of radio emission sweeps over the Earth, we can track the slowly shrinking orbit. Image credit: M. Kramer / MPIfR
For over 100 years, Einstein’s general theory of relativity has been our best description of how the force of gravity acts in the Universe.
General
relativity is not only very precise, but ask any astronomer about the theory
and they’ll probably also define it as “beautiful”. But it has a dark side too:
a vital conflict with our other great physical theory, quantum mechanics.
General
relativity works very well at large scales in the Universe, but quantum
mechanics deals with the microscopic realm of atoms and fundamental particles.
To solve this conflict, we need to see general relativity pressed to its
limits: extremely intense gravitational forces at work on small scales.
We considered
a pair of stars called the Double Pulsar which provide just such a scenario.
After 16 years of observations, we have found no flaws in Einstein’s theory.
Pulsars:
nature’s gravity labs
In the
early 2000s, astrophysicists at CSIRO’s Parkes radio telescope, Murriyang, in
New South Wales discovered a double pulsar system about 2,400 light years away
that provides a perfect opportunity to study general relativity under extreme
conditions.
Imagine a
star 500,000 times as heavy as Earth, yet only 20 km across. This ultra-dense
“neutron star” spins 50 times a second, busting out an intense beam of radio
waves that our telescopes detect as a faint blip whenever it sweeps over Earth.
There are more than 3,000 such “pulsars” in the Milky Way Galaxy, but this one
is unique because it revolves in an orbit around a similarly extreme companion
star every 2.5 hours.
According
to Einstein’s General Relativity, the immense accelerations in the Double
Pulsar system strain the fabric of space-time, giving out gravitational ripples
away at the speed of light that slowly sap the system of orbital energy.
This gradual
loss of energy makes the stars’ orbit drift ever closer together. In 85 million
years’ time, they are destined to merge in a remarkable cosmic pile-up that
will enrich the surroundings with a hefty dose of precious metals.
Using
stars as clocks
Working
with an international team of astrophysicists led by Michael Kramer of the Max
Planck Institute for Radio Astronomy in Germany, we have used this “pulsar
timing” practice to study the Double Pulsar ever since its discovery.
Data from
five other radio telescopes across the world, we demonstrated the exact arrival
times of more than 20 billion of these clock ticks over a 16-year period.
The Parkes
64-metre diameter radio telescope, located in Central NSW, Australia, was used
to observe the pulsed radio emission. Image credit: Shaun Amy/CSIRO
The VLBA
has such high resolution it could spot a human hair from 10km away! Using it,
we were able to observe a teeny tiny wobble in the obvious position of the
Double Pulsar every year, which results from Earth’s movement around the Sun.
And since
the size of the wobble depends on the distance to the source, we could indicate
that the system is 2,400 light years from the Earth. This delivered the last
puzzle piece we wanted to put Einstein to the test.
Finding
Einstein’s fingerprints in our data
Combining
these painstaking figures allows us to accurately track the orbits of each
pulsar. Our standard was Isaac Newton’s simpler model of gravity, which
predated Einstein by several centuries: every deviation offered another test.
These
“post-Newtonian” effects – things that are irrelevant when considering an apple
falling from a tree, but obvious in more extreme conditions – can be compared
against the predictions of general relativity and other theories of gravity.
One of the
effect is the loss of energy due to gravitational waves mentioned above.
Another is the “Lense-Thirring effect” or “relativistic frame-dragging”, in
which the spinning pulsars drag space-time itself around with them as they
move.
References:
https://au.news.yahoo.com/counted-20-billion-ticks-extreme-190729319.html
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