Einstein and extreme gravity

16 June 2020

Interview with 

Ben McAllister


Artist concept of Gravity Probe B orbiting the Earth to measure space-time, a four-dimensional description of the universe including height, width, length, and time.


In 2015, theoretical physicist Albert Einstein’s work arguably faced one of its toughest tests when researchers went looking for the gravitational waves predicted by his theory of general relativity. Einstein passed with flying colours. And this week he’s got another feather in his cap, because scientists have proved right another prediction: that even under extremes of gravity, like that around massive stars, general relativity works correctly and explains how gravity behaves. To fill us in, here’s physicist Ben McAllister…

Ben - In the late 16th century, Italian physicist Galileo climbed the Leaning Tower of Pisa and dropped two rocks of different masses. He measured how long it took them to hit the ground. To most people's surprise, despite having different masses, the rocks hit the ground at the same time. This was one of the earliest observations of what we now call 'the equivalence principle'. The principle roughly states that things accelerate the same way in a gravitational field, regardless of their mass or what they're made of. So, whether we're talking about a falling rock or a falling meteor, they should both obey the rules identically. But when this pronouncement was made, we didn't know about any of the more exotic things out there in the universe, like black holes or neutron stars. These are cases of extreme gravity, with gravitational fields so strong and complex that they end up interacting with themselves in ways Galileo could never have even imagined when he dropped his stones.

Over the centuries, we eventually developed a theory of gravity which did allow us to comprehend those gravitationally-complex extreme cases: Einstein's theory of general relativity. This new theory of gravity suggested a possible extension of the equivalence principle, which we call 'the strong equivalence principle', and which states that even gravitational gargantuas like neutron stars should follow the rules and behave like Galileo's rocks. Nevertheless, this stronger version of the principle was initially just a theory and had yet to be tested. Fortunately, the universe gives us nice laboratories to do these tests. And recently a group of astronomers from around the world have used a radio telescope to perform the cosmic equivalent of Galileo's Pisa experiment.

They measured the motion of a fast- spinning neutron star known as a pulsar. This was being orbited by a white dwarf star much like the moon orbits the earth. The pulsar and its white dwarf companion were in turn both orbiting another star, much like the way the earth and the moon both orbit the sun. By monitoring the light signals coming to earth from the spinning pulsar, and looking at how the arrival times of those light signals differed when the pulsar moved away from and towards Earth, the scientists were able to make extremely precise measurements of the motion of the stars. And what they found was that the equivalence principle seems to hold, even for gravitationally-complex bodies like neutron stars. The pulsar and the white dwarf star, despite having different masses and different compositions, appeared to both be accelerating towards the star they were orbiting at the same rate. Much like Galileo's different rocks falling towards the earth.

So for now it looks like our old pal Einstein was right yet again. Fortunately we didn't need to go lobbing rocks off of tall buildings to get to the bottom of it this time.


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