“The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.” -Hermann Minkowski
When it comes to gravity, you probably think you understand it pretty well.
Everything with mass (or energy) attracts everything else with mass-or-energy, explaining everything from falling terrestrial objects to the orbits of the planets to the formation of the largest structures in the cosmos.
And yet, this picture is only an approximation of what we know to be a more fundamental truth. The idea that objects feel gravitational forces and accelerate in response to them falls well within the realm of our common experience, and it’s very tempting to describe all gravitational interactions in those terms. This is what our best understanding of reality was for centuries, thanks to the work of Isaac Newton.
But if we did that, we’d miss out on some very, very important subtleties of Einstein’s relativity. In particular, the biggest revolution that came along with Einstein’s work was the idea that instead of space and time being independent, fixed entities, they were actually an inseparable combination — spacetime — whose shape itself determines the trajectory of all objects, both massive and massless, that lie within it.
In addition, the shape (or curvature) of spacetime is determined by the presence and distribution of all the matter and energy that exist in that spacetime itself! When we have an idealized system, like a very heavy mass that’s orbited at a large distance by a much smaller mass, Newton’s gravity — and Newton’s picture of forces and acceleration — are an excellent approximation.
But even excellent approximations have their limits.
One of the remarkable conclusions you arrive at in Newtonian gravity is that any tiny mass that orbits a much larger one will revolve around it in a perfect ellipse, returning along the same exact path each and every revolution. When Kepler discovered that the planets did, in fact, make ellipses around the Sun, this was an unexplained phenomenon until Newton’s law of gravity came along. But, like I said, even though it’s a very good one for our Solar System, this is only an approximation.
In reality, all of the planets fail to make a closed ellipse in their orbit around the Sun, missing by just a tiny amount. Interestingly enough, the closer you are to the largest mass, the more you miss your last orbit by. This is because spacetime is actually curved more severely the closer you are to a larger mass, and where the spacetime curvature is more severe, that’s where the most interesting, non-Newtonian predictions of General Relativity come into play.
One of the more spectacular predictions of General Relativity — in sharp contrast to Newtonian gravity — is that not only do these orbits fail to close, but over long enough timescales, they actually decay. That’s right, if you wait long enough, the planets will all eventually spiral inwards towards the center of our Solar System, where they’ll be gobbled up (or, for a less scary phrasing, where they’ll merge) with the mass at our center.
Don’t be alarmed by this; it’ll take some 10150 years for this to happen, far longer than the lifetime of any star in the Universe. But that’s only because all the planets are so far away from the Sun, relative to its paltry mass. But this means if we can find a system where a mass orbits much, much closer to the dominant mass in its system, we should be able to test this relativistic prediction, and see whether, in fact, the orbit does decay, and whether it decays at the rate predicted by Einstein’s theory or not.
To practically test this, a Sun-like star will simply not do, for the simple reason that it’s too big! But if we had an object that was as massive as the Sun, but maybe only the physical size of a mountain, we’d be in business. Luckily for us, not only the Universe but our own galactic neighborhood is littered with these objects: neutron stars!
These objects are the leftover cores of supermassive stars that have exploded in a Type II supernova, but are not quite massive enough to collapse down into black holes. One of the most massive neutron stars known is PSR J0348+0432, which weighs in at about twice the mass of the Sun, but is only maybe 10 kilometers (6 miles) in radius. For a neutron star, it’s remarkable for three reasons:
- It’s a pulsar, which means that, as it rotates, it sends out radio emissions in two beams. While it’s conceivable that all neutron stars are pulsars, we’re fortunate enough to have one of these beams point directly at us, which is very rare. 25-times-per-second, we receive a very regular pulse from this neutron star, which is observable with a good radio telescope.
- It’s in a binary system, which means that there’s another mass orbiting it. This is a very special case for Einstein’s relativity, as we’ll not only have precessing elliptical orbits, but also orbital decay and — if we can someday detect it — gravitational radiation.
- And finally, that other mass is a white dwarf star, a very small object about the mass of the Sun but the physical size of Earth, that’s so close to the neutron star that it completes an orbit every 2.5 hours, meaning that the entire orbit of the white dwarf around this neutron star would fit inside of our Sun!
Here’s an artist’s impression of what this would look like.
But there’s never been a system found where gravitational decay was occurring this fast, or where we’ve been able to study spacetime that’s curved this strongly. In other words, this is new territory for relativity, and one of the strongest tests ever performed! Want to know what we found?
The orbital period of the binary changes by a cumulative eight microseconds-per-year, in exact agreement with Einstein’s predictions! This is really remarkable, because many of the serious competing alternatives to Einstein’s relativity have much larger predictions in regions of strongly curved spacetime; this observation rules them out!
So if you’ve been wondering what Einstein and General Relativity have done for you lately, here’s your answer: in the most extreme conditions ever tested, where the curvature of spacetime is stronger than any system we’ve ever tested before, General Relativity’s predictions exactly matched the effects we painstakingly observed.