Albert Einstein is dubbed one of the greatest thinkers in recorded history. His ground-breaking theory of general relativity has revolutionized our understanding of the universe more than we could have imagined. Simply put it, he’s a genius, nonetheless. General relativity has been applied to the deep realms of classic physics, with astronomy being the hallmark of his theory.
But here’s the catch: was Einstein right about the universe? Of course, he was, and has been hailed till date. However, several scientists have been testing his ideas for decades hoping to find flaws in his theory. And well…, it seems Mr Relativity has always been right.
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A Relativity Test For Relativity
In a 16-year long experiment that was conducted to challenge general relativity, a team of international researchers examined pulsars — a type of neutron star — with seven radio telescopes scattered across the world. For over 100 years since Albert Einstein presented his theory, scientists all around continue in their efforts to find flaws in general relativity.
These scientists aren’t being pessimistic, but finding any deviations in the theory would constitute either a major discovery that would open up frontiers in classical physics, or alter the laws of physics once and for all.
“We study a system of compact stars that is an unrivaled laboratory to test gravity theories in the presence of very strong gravitational fields,” says Michael Kramer, a physicist at the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, Germany, and the team’s led researcher, in a statement.
“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.”
As he explained, these observations were not only in agreement with relativity, “but we were also able to see effects that could not be studied before.” For their experiment, the team observed a system of pulsars they discovered as far back as 2003. This system became a cosmic laboratory known as “Double Pulsar,” which consists of two active radio pulsars that orbits each other in just 147 minutes at a velocity of roughly 1 million kilometers per hour. One of them rotated about 44 times per second, and the other was quite young and had a rotation period of 2.8 seconds.
“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,” says Ingrid Stairs from the University of British Columbia in Vancouver, as she gives an example of their experiment.
“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 that we can detect. Never before has such an experiment been conducted at such a high spacetime curvature.”
Energy Equals Mass Times The Speed Of Light Squared
The system, as depicted in the illustration, are two “edge-on” active radio pulsars as seen from Earth, an indication of the orbital plane relative to our line of sight roughly 0.6 degrees. The researchers measured — with a precision of one parts in a million(!) — that there is a change in its orbital orientation.
This is a well-known relativistic effect that’s 140,000 times stronger. What they also realized is that, at that level of precision, they only needed to consider the impact of the rotation of the pulsar around the spacetime that “dragged along” the spinning pulsar.
As Dick Manchester from the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia’s national science agency, illustrates:
“Such fast orbital motion of compact objects like these — they are about 30 percent more massive than the Sun but only about 24 kilometers across — allows us to test many different predictions of general relativity — seven in total! Apart from gravitational waves, our precision allows us to probe the effects of light propagation, such as the so-called “Shapiro delay” and light-bending.”
“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! While this seems a lot, it is only a tiny fraction — 3 parts in a thousand billion billion(!) — of the mass of the pulsar per second.”
Another led author of the study, Norbert Wex from the MPIfR, also explains:
“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.”
Relatively, Relativity Wins
“It is the combination of different complementary observing techniques that adds to the extreme value of the experiment.” says Adam Deller from Swinburne University in Australia, as he sheds more light on his part of the experiment. “In the past similar studies were often hampered by the limited knowledge of the distance of such systems.”
The researchers were able to combine a series of pulsar timing techniques together with careful interferometer measurements of the system to determine its distance under high image resolution. They had a resulting value of 2,400 lightyears with only 8 percent margin for error.
“We gathered all possible information on the system and we derived a perfectly consistent picture,” says Bill Coles from University of California San Diego, in agreement with the experiment conducted, “involving physics from many different areas, such as nuclear physics, gravity, interstellar medium, plasma physics and more. This is quite extraordinary.”
“Our results are nicely complementary to other experimental studies which test gravity in other conditions or see different effects, like gravitational wave detectors or the Event Horizon Telescope,” says Paulo Freire, another team member also at MPIfR. “They also complement other pulsar experiments, like our timing experiment with the pulsar in a stellar triple system, which has provided an independent (and superb) test of the universality of free fall.”
“Our work has shown the way such experiments need to be conducted and which subtle effects now need to be taken into account,” as Kramer concludes, “and, maybe, we will find a deviation from general relativity one day.”
Source: Max Planck Institute for Radio Astronomy
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Written by: Nana Kwadwo, Sat, Jan 01, 2022.
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