Imagine that you are an astronomer with interesting ideas about the secret laws of the cosmos. Like any good scientist, you plan an experiment to test your hypothesis. And then suddenly the bad news: there is no way to test it, except perhaps a computer simulation. Cosmic objects are too large and uncomfortable to grow in a Petri dish or collide as subatomic particles.
Fortunately, there are rare places in space where nature conducts its own experiments - like PSR J0337 + 1715. This triple system was first observed in 2012, and in 2014 scientists officially announced its discovery. It is located 4200 light years away in the constellation Taurus.
Three dead star cores spin in a dance that could confirm - or lead to revision - Einstein's idea of spacetime. The stakes are high. In the 1970s, a system of two dead stars provided strong, albeit circumstantial, evidence supporting Einstein's theory of general relativity, and that the gravitational waves that LIGO eventually found did exist. For this work, scientists received the Nobel Prize.
To understand PSR J0337 + 1715 as part of the experiment, Joshua Sokol with New Scientist proposes to represent it as a physical location. At about the same distance from the center of the system, at which the Earth revolves around the Sun, lies a cold white dwarf, the remnants of the solidified core of a star like ours. A little further off there is another hotter white dwarf. It should "scream brightly" in the sky, says Scott Ransome of the National Radio Astronomy Observatory in Virginia, who oversees the system.
Every 1.6 days, this inner white dwarf orbits a companion that is invisible to the naked eye. But in X-ray or gamma-ray vision, the two white dwarfs are relatively dim compared to their companion, a spherical 24-kilometer-long object whose mass is one and a half times the mass of the Sun.
It is a pulsar, the remnant of a much larger star. It rotates once every 2.73 milliseconds, like a cosmic dust demon. Each rotation releases a beam of radio waves into the sky that reaches the Earth with each rotation - we use its ultra-precise signals as a cosmic clock. And since these bodies have intense, tangled gravitational fields, and we have clocks tied to them, it would be extremely convenient to test Einstein.
Ransom's team is tracking the ticking of the pulsar, measuring how the orbits of the three bodies change, and comparing the results with the predictions of Einstein's theory. They focus on one idea especially seriously.
Think of the apocryphal story of Galileo on the Leaning Tower of Pisa, who threw objects on the ground to show that different masses take the same amount of time to travel the same distance. Astronaut David Scott did the same experiment on the moon with a feather and hammer.
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The principle of so-called strong equivalence in general relativity continues this idea. He argues that even objects with their own gravitational fields should react to gravity in the same way as others.
As with feathers and a hammer, the inner white dwarf and the much heavier pulsar should behave the same under the gravitational pull of the outer white dwarf. If not, the orbit of the inner pair will become more elongated than expected - and the principle of equivalence will be violated, and general relativity is wrong.
And then there will be shock and awe. But such a shock could sooner or later be expected, since general relativity is notorious for not wanting to be friends with other theories of nature.
“Any theory of gravity other than general relativity basically predicts that a strong principle of equivalence will fail at some level,” says Ransome.
At the September Pulsar Conference in the UK, Ransom's team hopes to announce new results, starting with the work of Anna Archibald, who will test the equivalence principle 50 to 100 times better than ever before. They haven't done so yet, says Ransom, because there are some data patterns that seem to violate the principle of equivalence that need to be explored more closely.
“Obviously this is going to be powerful, so we want to make sure we understand the data correctly,” says Ransom. At the moment, computers are still doing analysis.
What are the chances that when the work comes out, people will get excited?
“Most people believe that a strong principle of equivalence cannot fail at this level. This is one of the reasons why we constantly bang our heads against the wall."
Perhaps PSR J0337 + 1715 is the perfect space experiment: an experiment in which general relativity will definitely break, not on paper, but for sure. Or we'll wait a little longer.
Ilya Khel