Astrophysicists confirm the universality of free fall

Hyperaxion June 10, 2020 10:53 pm

Scientists have been able to prove the universality of free fall, a cornerstone of Einstein’s Theory of General Relativity.

An international group of astronomers, led by researchers from the University of Manchester, in England, managed to confirm a concept of physics essential to the Theory of General Relativity, developed by Albert Einstein: the “universality of free fall”. The finding occurred thanks to the observation of two pulsars, and the results were published in the scientific journal Astronomy and Astrophysics on Wednesday (10).

Astrophysicists confirm the universality of free fall
(Credit: Guillaume Voisin / The University of Manchester).

The principle of universality of free fall states that two bodies that are in the same gravitational field suffer the same acceleration – regardless of their composition. For example, if you take a book and a pen, position them at the same height and release them at the same time, you will find that, even though one object is much heavier than the other, both will touch the floor at almost the same time.

In this example, the friction between the objects and the air around them can be disregarded because the fall distance is very small, but the same is not true for larger scales. If you could repeat this experiment with, say, a plane and a tennis ball, the result would not be the same, because gravity would not be the only force acting on the objects (besides friction, the wind itself can interfere with the fall).

Still, it is this principle that explains why two objects in a vacuum, where there is no resistance whatsoever, undergo the same acceleration and therefore fall at the same speed. This was demonstrated for the first time by Galileo Galilei, who, in a famous episode, would have thrown objects of different masses from the top of the Tower of Pisa to see if they both reached the ground simultaneously.

This concept is also at the heart of the General Theory of Relativity developed by Albert Einstein over a hundred years ago. However, the ideas of the German physicist have been put in check with the advances of science in the last century, mainly because his theory is not entirely consistent with quantum mechanics and some findings about dark matter.

But the new study led by Manchester University astronomers corroborates Einstein’s ideas. Observations of Pulsar J0337+1715, which is a super-dense neutron star (its core is 1.44 times the mass of the Sun and only a few kilometers in diameter), show that the object orbits two white dwarfs with much weaker gravitational fields.

“The pulsar emits a beam of radio waves which sweeps across space. At each turn this creates a flash of radio light which is recorded with high accuracy by Nançay’s radio telescope. As the pulsar moves on its orbit, the light arrival time at Earth is shifted,” said Guillaume Voisin, one of the researchers, in a statement. “It is the accurate measurement and mathematical modelling, down to a nanosecond accuracy, of these times of arrival that allows scientists to infer with exquisite precision the motion of the star.”

But, how do these measurements prove the universality of free fall? As Voisin explains, it is the configuration of this system that is so important. According to him, the two white dwarfs are “falling” towards the pulsar as they orbit it. The neutron star, in turn, is responsible for the gravity that acts on the other two stars.

In the Solar System, previous experiments have already proved that the Moon and the Earth are identically affected by the Sun’s gravitational field, as predicted by the universality of free fall. However, it is known that some deviations can occur for self-gravitating objects, such as neutron stars, whose mass is made up significantly of their own gravitational energy.

According to the scientists, the new research fills the gap left by observations in the Solar System, where no object is strongly self-gravitating. The team demonstrated that the gravitational field confirms the predictions of general relativity – and this is the most accurate confirmation ever obtained that the universality of freefall is a valid concept.

“[Observing the pulsar] has allowed to perform a stellar version of Galileo’s famous experiment from Pisa’s tower,” compared Voisin. “Two bodies of different compositions fall with the same acceleration in the gravitational field of a third one.”

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