To date, Einstein’s theory of general relativity may have stood up to test after test, but that doesn’t mean it’s infallible – or that scientists should stop trying to test it. Every time the theory holds, we learn something valuable about the Universe.
In 2012, the discovery of a new star system showed promise as a new testing ground. And now it, too, has been proven by an international team of researchers to fall right in line with Einstein’s theory.
Called PSR J0337+1715, and located some 4,200 light-years away in the constellation of Taurus, it’s a triple system, and its interest as a relativity test harks back to Galileo in the 16th century, when he demonstrated the equivalence principle that’s fundamental to general relativity.
It’s a famous concept, although historians mostly agree it was a thought experiment and didn’t actually take place. But the story goes that Galileo dropped two balls made of differing materials off the Leaning Tower of Pisa, and observed them reaching the bottom at the same time.
What this proved – simplified and formalised in Isaac Newton’s law of universal gravitation – is that the acceleration of a mass due to gravitation is independent of the mass itself.
This was famously proven to dramatic effect in 1971, when astronaut Dave Scott simultaneously dropped a hammer and a feather while standing on the Moon. Without air resistance to slow the feather, the two items dropped to the Moon’s surface at the same speed.
OK, back to PSR J0337+1715. These previous tests have successfully demonstrated a weak version of the equivalence principle. But, according to general relativity, it should scale up even at tremendous masses, and even in the three-dimensional gravitational field of space.
And this is where general relativity is an outlier.
“Every other theory of gravity besides general relativity basically predicts that the strong equivalence principle fails at some level,” said researcher Scott Ransom of the National Radio Astronomy Observatory, speaking to New Scientist in July of last year.
The system consists of three dead, or end-of-life, stars. Two of them are white dwarfs – small, very dense, very hot remnants of stellar cores, left behind after a red giant collapses.
The third is a pulsar, a rapidly rotating, extremely dense neutron star pulsing with a beam of electromagnetic radiation with incredible regularity as it spins on its axis, like a very fast cosmic lighthouse. It’s also much heavier than the white dwarfs that accompany it.
Because the neutron star’s pulses are so regular – the period between flashes is just 2.73 milliseconds – astronomers can use any variations in the timing to precisely gauge its orbit. If the flashes slow down or speed up, that means the star is moving in relation to Earth.
This is how the white dwarfs were discovered. The three stars gravitationally tug on each other, which skews the pulsar’s orbit. The neutron star and one of the white dwarfs are quite close to one another, while the second white dwarf is farther away.
According to the strong equivalence principle, it’s not just the materials that should accelerate at the same rate – it’s also the energy bound up in gravitational fields. So high-mass bodies should “fall” at the same rate as low-mass bodies.
If you think of the neutron star as the hammer and the inner white dwarf as the feather in Scott’s demonstration, it’s a decent analog for a strong equivalence principle test. All three stars are “falling” around each other’s gravitational field.
If the pulsar were to move faster than the inner white dwarf towards the outer white dwarf, its orbit would become more elliptical.
As it turns out, this did not happen.
The inner white dwarf and the pulsar had matching accelerations, to within 0.16 thousandths of a percent of each other – leaving us with an epic, huge-scale demonstration of the equivalence principle. And once again Einstein’s work has stood its ground.