One of the things predicted by the theory of relativity is the "universality of free fall" – basically, any two objects in a vacuum will fall at the same rate. Whether or not this applied to massive objects like planets and stars has long been debated, but now astronomers have used the Arecibo Observatory to show that the phenomenon does hold true, meaning Einstein was right yet again.

Here on Earth, all objects will fall to the ground at a steady speed of 9.8 m/s2. In practice, of course, that sounds ridiculous – if you drop a bowling ball and a feather off a building, the bowling ball will make a beeline for the ground while the feather will float around and take a much more leisurely journey. That difference is due to external factors like wind and air resistance, and the rule becomes much clearer when you take those objects into a vacuum chamber – or the Moon.

What hasn't been clear is whether this phenomenon would apply to objects with extreme gravity, with some alternate theories of gravity having been put forward for these much-larger scales. To find out, a team of researchers used the radio-radar telescope at Arecibo in Puerto Rico.

The "natural laboratory" that they focused on was a triple star system known as PSR J0337+1715. About 4,200 light-years from Earth, this system is made up of a neutron star that orbits a white dwarf once every 1.6 days, while these two orbit another white dwarf star every 327 days.

This triple star system provides a great test bed for the theory of relativity. If Einstein's ideas apply to massive objects, the white dwarf/neutron star binary should fall towards the other white dwarf at the same rate. The team used Arecibo to focus on the neutron star, then analyzed the data to study the system as a whole.

"We were able to measure this by looking at the neutron star alone," says Anne Archibald, lead scientist on the study. "The neutron star, a millisecond pulsar, behaves like a clock: it rotates 366 times per second, and beams of radio waves rotate along. They sweep over the earth at regular intervals, like a cosmic lighthouse. We have used these radio pulses to track the position of the neutron star. The high-quality data we have from Arecibo allowed us to divide the data up into subsections and compare them with each other, allowing us to resolve an apparent conflict with Einstein's gravity."

Sure enough, the team found that the two objects are falling at the same rate, "differ(ing) fractionally by no more than 2.6 × 10−6."

"Thanks to the power of Arecibo and in conjunction with the Westerbork Synthesis Radio Telescope here in the Netherlands and the Green Bank Telescope in West Virginia we were able to show that Einstein's theory of relativity holds water even when it comes to neutron stars and dwarf stars," says Archibald.

The research was published in the journal Nature.