The Triple-Star test shows that Einstein was right, again

The following essay is reproduced with permission from The Conversation, an online publication covering the latest research

It may not be intuitive, but let fall a hammer and a feather and – In the absence of air resistance – they will hit the ground exactly at the same time. This is a key principle of physics known as the "free fall universal", which states that all objects accelerate identically in the same gravitational field. In fact, it's an important theme in Albert Einstein's immortal theory of general relativity, which describes how gravity works.

But although we know that this applies to hammers and feathers, stars. Now, a new study, published in Nature, tested the principle using a remarkably extreme astrophysical environment: a triple star system containing two white dwarfs and a pulsar (a rotating neutron star that emits radio waves) . These objects are the extremely dense remains of dead stars.

Spoiler Alert: It turns out that Einstein is always right, and it's getting harder and harder to prove him wrong.

But let's start with the basics. Hold an object in your hand. No matter what it is – the object will have some mass. We can think of this mass in two ways. Isaac Newton taught us that if we apply a force to a body, it will undergo an acceleration, and the size of this acceleration is directly proportional to the force applied – and inversely proportional to the mass itself. Give a push to a broken down car and it will not accelerate very quickly, but apply the same push to a shopping cart and you will send it to the driveway. When we think of the acceleration of an object due to a force exerted on it, we think of the "inertial mass" of the body.

Two objects having a mass are attracted to each other by the gravitational force. Thus, the object that you hold in your hand is attracted to Earth, and the size of the force that attracts it depends on the mass of the object. In this case, we think of the "gravitational mass".

If you dropped it, the object you are holding "would fall" freely – the force of gravity would accelerate it to the ground. The size of the force pulling the object depends on the gravitational mass, but the amount of acceleration depends on the inertial mass. But is there a difference between the two types of mass? To find out, one can write an equation of motion connecting the two types of mass: inertial mass of one side of the equation and gravitational mass of the other.

The equation predicts something that can be tested using an experiment: inertial mass is equivalent to the gravitational mass, so all objects should fall to Earth with an identical acceleration independently of of their mass. This often surprises people. This is what is called the "principle of equivalence".

Galileo first noticed that falling objects fall at the same rate, and you can do this experiment yourself by dropping two objects of different mass simultaneously. However, one problem on Earth is the presence of another force acting on falling bodies, called air resistance. If you drop a hammer and a feather, the feather will tend to drift gently to the ground, trolling – the objects are not strictly in free fall. But go to the moon and do this experiment, as did astronaut David Scott during Apollo 15, where there is no air resistance, and the principle of Equivalence is clear.

It is not known whether the theory describes gravity well in all situations . There is a lot at stake. If general relativity deteriorates for certain situations, then we would need a revised or modified gravity theory. In particular, scientists have wondered whether the universality of free fall applies to objects that have a strong "self-gravity" – an important gravitational field of their own. Indeed, some modified theories of gravity predict that the principle of equivalence might be violated for strongly self-gravitating bodies in free fall, while general relativity says that it should be universal.

Dance of stars

Through an extreme laboratory in space- a triple star system at 4,200 light-years – the new study has managed to test that. This name does not do justice: we are talking about two white dwarfs and a "millisecond" pulsar more massive (a neutron star turning about 366 times per second and emitting radio waves like a lighthouse). A white dwarf and the pulsar are in orbit every 1.6 days. In turn, they also rotate around the other white dwarf every 327 days

The pulsar-white dwarf pair can be considered free-falling to the other white dwarf because an orbit is just the case of free fall without ever reaching the ground, like the satellites around the Earth. Of course, the pulsar and the white dwarf are themselves very massive objects, with a strong self-gravity. General relativity predicts that the accelerations of the white dwarf and the pulsar, due to free fall to the outer white dwarf, should be identical – despite the differences in mass and self-gravity.

Combining observations covering six years of surveillance, astrophysicists have carefully modeled the orbits of the pair. They measured a parameter called Delta, which describes the fractional difference between the acceleration of the white dwarf and the more massive pulsar. If general relativity is verified, then Delta should be zero. The results indicate that, in measurement uncertainties, the difference in acceleration is indeed statistically consistent with zero – it can be said with 95% confidence that Delta is less than 0.0000026.

This new constraint is much better than anything that has been measured. It provides valuable new empirical evidence that general relativity remains our best functioning model of gravity, so it is unlikely that we need new or modified theories at this stage. This comes a few weeks after general relativity has been proven for the first time on a galactic scale.

Will we ever find a situation where general relativity collapses? In a way I hope, because it would reveal a new physics. But the continued success of general relativity, written for the first time a century ago, must surely be celebrated as one of the most incredible intellectual achievements of our kind.

This article was originally published on The Conversation. Read the original article.