In grade school we learned that gravity is an always-attractive force that acts between particles of matter. Later on, we learn that it has an infinite range through space, weakens as the inverse-square of the distance between bodies, and travels exactly at the speed of light.

But wait….there’s more!

It doesn’t take a rocket scientist to remind you that humans have always known about gravity! Its first mathematical description as a ‘universal’ force was by Sir Isaac Newton in 1666. Newton’s description remained unchanged until Albert Einstein published his General Theory of Relativity in 1915. Ninety years later, physicists, such as Edward Witten, Steven Hawkings, Brian Greene and Lee Smolin among others, are finding ways to improve our description of ‘GR’ to accommodate the strange rules of quantum mechanics. Ironically, although gravity is produced by matter, General Relativity does not really describe matter in any detail – certainly not with the detail of the modern quantum theory of atomic structure. In the mathematics, all of the details of a planet or a star are hidden in a single variable, m, representing its total mass.

The most amazing thing about gravity is that is a force like no other known in Nature. It is a property of the curvature of space-time and how particles react to this distorted space. Even more bizarrely, space and time are described by the mathematics of GR as qualities of the gravitational field of the cosmos that have no independent existence. Gravity does not exist like the frosting on a cake, embedded in some larger arena of space and time. Instead, the ‘frosting’ is everything, and matter is embedded and intimately and indivisibly connected to it. If you could turn off gravity, it is mathematically predicted that space and time would also vanish! You can turn off electromagnetic forces by neutralizing the charges on material particles, but you cannot neutralize gravity without eliminating spacetime itself. Its geometric relationship to space and time is the single most challenging aspect of gravity that has prevented generations of physicists from mathematically describing it in the same way we do the other three forces in the Standard Model.

Einstein’s General Relativity, published in 1915, is our most detailed mathematical theory for how gravity works. With it, astronomers and physicists have explored the origin and evolution of the universe, its future destiny, and the mysterious landscape of black holes and neutron stars. General Relativity has survived many different tests, and it has made many predictions that have been confirmed. So far, after 90 years of detailed study, no error has yet been discovered in Einstein’s original, simple theory.

Currently, physicists have explored two of its most fundamental and exotic predictions: The first is that gravity waves exist and behave as the theory predicts. The second is that a phenomenon called ‘frame-dragging’ exists around rotating massive objects.

Theoretically, gravity waves must exist in order for Einstein’s theory to be correct. They are distortions in the curvature of spacetime caused by accelerating matter, just as electromagnetic waves are distortions in the electromagnetic field of a charged particle produced by its acceleration. Gravity waves carry energy and travel at light-speed. At first they were detected indirectly. By 2004, astronomical bodies such as the Hulse-Taylor orbiting pulsars were found to be losing energy by gravity waves emission at exactly the predicted rates. Then in 2016, the twin LIGO gravity wave detectors detected the unmistakable and nearly simultaneous pulses of geometry distortion created by colliding black holes billions of light years away.

Astronomers also detected by 1997 the ‘frame-dragging’ phenomenon in X-ray studies of distant black holes. As a black hole (or any other body) rotates, it actually ‘drags’ space around with it. This means that you cannot have stable orbits around a rotating body, which is something totally unexpected in Newton’s theory of gravity. The Gravity Probe-B satellite orbiting Earth also confirmed in 2011 this exotic spacetime effect at precisely the magnitude expected by the theory for the rotating Earth.

Gravity also doesn’t care if you have matter or anti-matter; both will behave identically as they fall and move under gravity’s influence. This quantum-scale phenomenon was searched for at the Large Hadron Collider ALPHA experiment, and in 2013 researchers placed the first limits on how matter and antimatter ‘fall’ in Earth’s gravity. Future experiments will place even more stringent limits on just how gravitationally similar matter and antimatter are. Well, at least we know that antimatter doesn’t ‘fall up’!

There is only one possible problem with our understanding of gravity known at this time.

Applying general relativity, and even Newton’s Universal Gravitation, to large systems like galaxies and the universe leads to the discovery of a new ingredient called Dark Matter. There do not seem to be any verifiable elementary particles that account for this gravitating substance. Lacking a particle, some physicists have proposed modifying Newtonian gravity and general relativity themselves to account for this phenomenon without introducing a new form of matter. But none of the proposed theories leave the other verified predictions of general relativity experimentally intact. So is Dark Matter a figment of an incomplete theory of gravity, or is it a here-to-fore undiscovered fundamental particle of nature? It took 50 years for physicists to discover the lynchpin particle called the Higgs boson. This is definitely a story we will hear more about in the decades to come!

There is much that we now know about gravity, yet as we strive to unify it with the other elementary forces and particles in nature, it still remains an enigma. But then, even the briefest glance across the landscape of the quantum world fills you with a sense of awe and wonderment at the improbability of it all. At its root, our physical world is filled with improbable and logic-twisting phenomena and it simply amazing that they have lent themselves to human logic to the extent that they have!

**Return here on Monday, March 13 for my next blog!**

One puzzle for me is whether spacetime is “something”, i.e. is gravity caused by distorting spacetime (and if so, how is it distorted just like that?) or are spacetime distortions the most convenient means of describing gravity?

Three other interesting points are that gravity does not depend (as far as I know) in any way on the composition of the generating mass; it does not get screened or modified in anyway if it goes through any mass; and it is so weak compared with the other fundamental forces. Of course it has to be, or we would not exist, but that does not explain why it is so weak.

Hi Ian! Always look forward to your engaging comments!

I have to say that after several decades reading what is going on in theoretical physics in this area I still have a tough time wrapping my brain around the nature of space. I ‘get’ all of the theoretical issues that swarm around this phenomenon and know very well the mathematics of general relativity. I have also read carefully Einstein’s original thinking about spacetime, and the thinking of the folks that work in this area today. There is currently no explanation for how matter ‘generates’ gravity ,given that the field of gravity is spacetime itself. Moreover, spacetime is not a fixed thing whose geometry determines the shape of worldline geodesics. Instead it is the worldlines themselves that are fundamental and they determine the geometry of spacetime. THAT is a very hard thing to consider given that we all have been trained to think in terms of fields being primary and particle trajectories are derived from the a priori shapes of these fields. But in general relativity, and for modern physicists working in quantum gravity, worldlines come first and spacetime is generated from the geometry of manifold created from these worldpoints (ie spacetime events) that are organized through simple local relationships involving the lightcones at each point which decide causal connections. I get a real headache thinking about this because my classical field perspective keeps getting in the way. At age 64 I don’t think I have too many more productive years to resolve all of this for myself. sigh!

As for age, I sympathise. I am very nearly 75, and there will be a lot I won’t know when I cash out.