Albert Einstein’s theory of gravity, postulated more than a century ago as the General Theory of Relativity, is here to stay and remains to this date “a reasonable way to understand the universe,” states Discover Magazine.
This is because Einstein’s principle of gravity has just been proven right by yet another major science experiment, making it increasingly difficult for alternative gravity theories to demonstrate their case.
Just two weeks after another study confirmed that Einstein’s theory of relativity still stands even in far-away galaxies — producing the same type of gravitational lensing as the predicted, the Inquisitr reported — a new paper concludes once again that Einstein was right.
Published on July 4 in the journal Nature, this new study focused on another prediction of General Relativity known as the “Strong Equivalence Principle.”
Feather, Hammer, Neutron Star
This principle states that gravity affects all objects the same way, even the most massive of them, which should fall at the same rate as lighter objects in the absence of air, under the pull of gravitational forces.
While this might be a little surprising, the equivalence principle was tested numerous times on Earth — and even on the moon in 1971, during the Apollo 15 mission, notes Science Daily — each time with the same result: that all objects fall the same, regardless of their mass and weight, if you take air resistance out of the equation.
“Einstein’s theory of gravity […] is based on the universality of free fall, which specifies that all objects accelerate identically in an external gravitational field,” the authors write in their paper.
While it remains true that gravity makes heavier objects accelerate faster, the idea is that the more massive an object is, the harder it becomes for it to accelerate, meaning that the two effects cancel each other out, explains Discover Magazine.
This is why the famous hammer and feather experiment performed on the moon by Apollo 15 astronaut David Scott saw the two objects hit the lunar dirt at the same time, notes Space.com.
Now, almost 80 years later, a study led by researchers from the University of Amsterdam in the Netherlands have proven that even extremely massive objects, such as superdense neutron stars, fall just like a feather. Here’s how they did it.
The opportunity to test Einstein’s theory of gravity on extremely massive objects arose in 2011, with the discovery of a distant triple star system called PSR J0337+1715, detected 4,200 light-years from our planet, in the direction of the Taurus constellation.
Made up of two white dwarf stars, this system also includes a pulsar — a fast-spinning, superdense neutron star, as reported by the Inquisitr. This particular pulsar, which weighs 1.4 times the mass of our sun but is only about the size of Amsterdam, the capital city of the Netherlands, finds itself in close orbit with one of the white dwarfs — a star of considerably lower mass, of only 0.2 solar masses, but of bigger proportions, roughly the size of Earth.
The pair is engaged in a 1.6-day orbit, while also orbiting together around the other, more distant white dwarf every 327 days.
According to study co-author Ryan Lynch, from the Green Bank Observatory in West Virginia, these singular conditions offer the perfect staging area for the ultimate gravity experiment.
“This is a unique star system. We don’t know of any others quite like it. That makes it a one-of-a-kind laboratory for putting Einstein’s theories to the test.”
The team has kept their eyes on the pulsar for about six years now, observing it with three giant telescopes: the National Science Foundation’s (NSF) Green Bank Telescope (GBT), which incidentally discovered the triple star system, the Westerbork Synthesis Radio Telescope in the Netherlands, and the NSF’s Arecibo Observatory in Puerto Rico.
The Strong Equivalence Principle ‘Is Strong With This One’
The GBT alone has spent more than 400 hours looking at the pulsar, carefully monitoring the radio waves emitted toward Earth by the superdense neutron star as it rotates 366 times per second. Because the star spins at incredibly fast rates, the continuous radiation coming from it appears as a string of regular pulses as seen from Earth — hence the name “pulsar.”
“We can account for every single pulse of the neutron star since we began our observations,” says lead study author Anne Archibald, from the University of Amsterdam and the Netherlands Institute for Radio Astronomy. “We can tell its location to within a few hundred meters. That is a really precise track of where the neutron star has been and where it is going.”
The six-year-long study of these pulses has shown that the distant white dwarf has the same gravitational effect on the pair of stars orbiting it, despite their obvious difference in mass.
In other words, both the heavy pulsar and its lighter white dwarf companion maintain the same path around the outer star of the system, which proves that the strong equivalence principle applies in their case.
If this principle were incorrect, the pulsar and the white dwarf would accelerate at different rates, translating in a distortion in the pulsar’s radiation, which would be detected at a different rhythm by our telescopes.
Yet, as it turns out, the telescopes didn’t pick up any difference in the acceleration of the two stars, confirming the strong equivalence principle and proving that Einstein’s gravity theory is correct.
“If there is a difference, it is no more than three parts in a million,” notes study co-author Nina Gusinskaia, also from the University of Amsterdam.
Since the team’s measurements are 10 times more accurate than any other gravity test ever performed, they pretty much shatter alternative gravity theories, which propose that heavily compact objects (just like the pulsar in PSR J0337+1715) should fall differently from objects of lesser mass (such as its white dwarf companion), due to their gravitational binding energy — the gravitational force that holds them together.