John Mayer sang Gravity on his 2005 live album Try! In the song, he regrets, “Gravity is working against me/And gravity wants to bring me down.” A later 2006 version has another talented singer Alicia Keyes performing background vocals.
I saw Mayer live in 2007 when he opened for the Dave Mathews Band at Lane Stadium in Virginia Tech. Dave Mathews had organized A Concert for Virginia Tech to help heal the significant recent trauma that the university had recently experienced.
It was a memorable evening that closed hopefully with the Dave Mathews Band covering Bob Marley’s Every Little Thing Is Gonna Be Alright. Indeed, I found later that if I tried hard enough, every little thing did eventually become alright. In a world where a tragedy can lead to a new stony-hearted norm, Mathews and Mayer showed great kindness and empathy.
My perception of gravity is however different from Mayer’s. He enriches us through metaphors, symbolism and rhythmic aesthetics whereas I am more inclined towards the rational and tangible.
Stripped of poetry, gravity is simply a force. It can be interpreted in our daily lives through Newton’s second law of motion, F = ma, which conveys that the force that we experience equals our mass times the acceleration that is imposed upon us.
On earth, let’s call the acceleration due to gravity as g. In other words, the force exerted on us by gravity equals our body’s mass multiplied by g. This force is our weight. It’s well known that on earth g is greater than it is on the moon. Since the force that an astronaut experiences on the moon is lower than her weight on earth, she should be able to skip and soar far more readily on the moon than on earth.
For any of us to soar, as we can on an airplane during its flight, the aircraft’s engines must exert a force that is strong enough to overcome the pull of our weight back towards the earth. If the force generated by the engine is large enough, the craft will reach an escape velocity, freeing itself from the earth’s gravitational pull. That’s what a spacecraft does.
A typical airplane requires oxygen for its engine to run. It cannot escape from the earth since the oxygen that the aircraft engine “breathes” diminishes in successive upper layers of our atmosphere. That’s why rockets have chemical engines with onboard oxygen and do not have to rely on ambient oxygen for propulsion, but that’s a story for another day. Back to gravity, or really, gravitational waves.
What is spacetime? We’re used to interpreting space and time separately in the form of the expanse around ourselves and the passages of our lives. This construction makes sense to us in our roles as individual observers of the goings on in our universe, as we do so through the events in our daily lives and the movements of celestial objects.
Spacetime is independent of the observer and that observer’s movements. Instead, it consists of events, for which spacetime does not separate the contexts of when and where.
When events are separated by a time period in spacetime so that a cause and effect relationship can be inferred between them, they are time-like. Other events are light-like when they propagate in the form of waves through spacetime at the speed of light. Space-like events that have insufficient time intervals separating them make it impossible to infer a preceding cause and a subsequent effect for them.
Gravity materializes when the mass or energy of an event, for example the motion of a celestial body, is able to curve spacetime. This curvature also changes the time felt by an observer, which Einstein described in his theory of relativity. If we come close enough to a very large mass that disturbs spacetime, we will age slower. The further away we are to that mass, the faster we age.
When two celestial objects accelerate, but there is no symmetry in their separate motions, they generate gravitational waves that propagate through spacetime. These waves decay rapidly and so their effects become smaller and smaller as they emanate outward from the accelerating masses.
It is these waves that were recently detected. By studying gravitational waves, it might become possible to understand the early universe.
After fourteen or so billion years since the inception of our universe, wouldn’t that be something?
(Many thanks to two noted scholars, the astrophysicist Doug Welch and the immunologist, Judah Denburg, for stimulating this post over a very pleasantly spent evening. Why are they noted scholars? Doug’s “Aha!” moment created a better explanation of supernova light echoes and Judah’s seminal work elucidated the relationship between stem cells, allergies and inflammation.)