In 1589, Galileo is said to have dropped two cannon balls of different masses from the Leaning Tower of Pisa to prove that they would both touch the ground at the same time. He wanted to disprove Aristotle’s theory of gravity which stated that heavier objects fell faster than lighter objects.
Now, if we were to drop a cannonball made of iron and a bird’s feather from a building at the same time, one would fall straight to the ground and faster while the other will float in the air and glide down slowly because of atmospheric air resistance.
However, as Commander David Scott of Apollo 13 demonstrated, if we performed the same experiment on Moon where there is no air resistance, they both do hit the ground at the same time. In other words, no matter what the physical properties (how massive or what it’s made of) are, if two objects experience free fall, they will both accelerate at the same rate, thus reaching the ground at the same time.
Einstein and Free Fall
One important part of Einstein’s theory of relativity is based on the above-mentioned universality of free fall. Einstein’s theory proves and explains that if two people are in a free-falling elevator, they will not know that they are free falling because they (as well as all objects in the elevator) are accelerating down at the same rate and hence, relatively observing, they feel they are not in free fall.
Some scientists have challenged that Einstein’s theory of relativity as it pertains to free fall and uniform acceleration will not hold true in case of extreme conditions. Most relativity tests are done with our sun and the stars, which have finite mass. Until now, scientists have never been able to fully test this presumption.
But a recent discovery of a triple star system named PSR J0337+1715, located about 4,200 light-years from Earth provided a perfect laboratory scenario for an experiment to be conducted to test Einstein’s theory of relativity in extreme gravity.
A "Live" Test of Relativity
The triple star system consists of three end-of-life stars with extremely dense masses. A central neutron star orbited closely by a companion white dwarf star and both of which are in turn orbited by another white dwarf star. This system allows one to investigate how the outer dwarf star’s gravitational pull influences both the neutron star and its partner white dwarf star.
Another beauty of this is that the neutron star can be measured by its signature pulses of radio waves that hit earth at exact time intervals (it is like a giant space clock with tick-tocking pluses that hit earth in equal intervals and thus tracked accurately by radio astronomy) and its companion white dwarf (which orbits it) can be measured by optical observations using doppler. If the pulsar and the inner white dwarf fall differently towards the outer white dwarf, then the pulses of the neutron star would arrive at a different time than expected.
So, for six years, Anne Archibald, a postdoctoral researcher at the University of Amsterdam conducted this experiment with her team using several advanced telescopes around the world. It was essential that they have three cross checks to get accurate results. They concluded that they did not detect a difference in acceleration of the neutron star and the inner White dwarf star toward the outer dwarf star. If there was any measurable difference, it was less than "three parts in a million", thus proving that Einstein’s theory of general relativity and equivalence of free fall holds true even in extreme gravitational conditions.
Here is to another feather (and a cannonball) in the cap of the Theory of Relativity! The video below explains the basics of free fall.
Sources: Space.com, Nature, Scientific American