If one drops a marble and a bowling ball from the 44th floor of a high-rise in Manhattan, they would fall at the same time.

The same principle holds true even for all objects regardless of their mass, according to the principle of gravity in Einstein's theory of relativity. This is called the Strong Equivalence Principle, which states that all objects fall at the same rate notwithstanding their mass or composition.

This has long been proven in heavenly objects in the solar system. The Earth and Jupiter, for example, both "fall" at the same rate toward the sun even though they have very different masses. Astronomer Dave Scott famously demonstrated this with a hammer and feather falling at the same rate on the moon.

Even in the most extreme conditions, the principle stands. This has been proven by a team of international researchers after rigorous observations of the behavior of a rare star system just 4,200 light-years away from Earth.

Rare Triple Star System Proves Equivalence Principle

The National Science Foundation's Green Bank Telescope in West Virginia first spotted the star system just 4,200 light-years from Earth in the constellation Taurus. Scientists have christened it the PSR J0337+1715 star system, which consists of a neutron star in a 1.6-day orbit around a white dwarf. Both are also in a 327-day orbit around a second white dwarf farther away.

A neutron star is the remains of a star after it has exploded and collapsed in on itself. It is typically never bigger than a city on Earth, but it contains the same amount of mass as the sun. A tablespoon of a neutron star is about as heavy as Mount Everest.

Because of its extreme density, a neutron star has a strong gravitational field, making it one of the most extreme environments to test Einstein's principle of gravity. What raises the stakes even higher is the existence of two white dwarfs.

White dwarfs are small stars about the size of a planet. A white dwarf is a star that has used up all its fuel that only a hot core remains. White dwarfs typically have a mass one-fifth of the sun.

Because they do not have a gravitational force as strong as neutron stars, the researchers found it intriguing to find two white dwarfs in the same vicinity as a neutron star. Not a lot of objects can survive the explosive death of a star.

"This is a unique star system," says co-author Ryan Lynch of the Green Bank Observatory in West Virginia. "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."

Studying A Neutron Star's Radio Pulses

In a new paper published in the journal Nature, the researchers have found that the inner stars accelerated at the same speed, lending the most accurate evidence yet for gravity as Einstein described it. They have also observed that the second white dwarf did not affect the movement of the inner stars in any way.

The researchers were able to come to this conclusion by studying the movements of the neutron star. When a neutron star rotates, it becomes a pulsar. This radiates radio waves, X-rays, and even visible lights as it spins.

This pulsar, in particular, spins at a very rapid rate of 366 rotations per second. Along with each rotation, the pulsar sends pulses of radio waves that can be detected on Earth by using sophisticated radio equipment.

Over a span of six years, the researchers made observations of the movement of the pulsar using the Westerbork Synthesis Radio Telescope in the Netherlands, the Arecibo Observatory in Puerto Rico, and the Green Bank Telescope.

"We can account for every single pulse of the neutron star since we began our observations," said principle author Anne Archibald of the University of Amsterdam and the Netherlands Institute for Radio Astronomy. "We can tell its location to within a few hundred meters. That is really a precise track of where the neutron star has been and where it is going."

As the pulsar spins more rapidly, it sends more pulses that make a more precise tracking of its location. If it accelerates at a different rate than the white dwarf, the researchers would have seen pulses arriving at times different from what they expected. This was not the case.

In fact, the difference in the rate of acceleration between the pulsar and the white dwarf is so small that it is almost impossible to detect. The researchers say the difference is not more than three parts in 1 million.

Alternative Theories To Gravity

Einstein described gravity as a curve in space-time that objects follow as they "fall" towards each other. This can be demonstrated in the curved orbit of the moon around the Earth and the planets around the sun.

However, some experts are not convinced by the idea of gravity being a curve. This is why they have proposed alternative theories that may explain how gravity behaves in extreme conditions.

The latest research, however, makes it that much harder to disprove Einstein's predictions. The researchers admit that their findings are not indisputable proof of Einstein's gravity. Objects at the very, very small levels, for instance, have yet to reveal how they behave with gravity.

"We've done better with this system than previous tests by a factor of 10," says co-author and physicist David Kaplan of University of Wisconsin-Milwaukee. "But it's not an ironclad answer. Reconciling gravity with quantum mechanics is still unresolved."

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