I can’t believe that it has been more than a year since I’ve written anything here. I’d blame the lingering effects of the Great Pandemic, except that doesn’t make any sense at all.
Even today, all I have to offer you is something partial and inconclusive. It is just a question, really. But to ask it I need think about some other questions and so set the stage for my final, wild conjecture.
What is the nature of gravity? What is it that causes massive objects, with no physical interconnection, to exert forces upon each other over such vast distances?
I’m no expert, and as such I have no problem mixing current and viable theories together with ones that make no sense. So what might be the mechanism through which gravity exerts its pull? Is there a gravitational ether through which an object’s mass can influence other bodies. How about gravitons, cycling back and forth between (astronomically speaking) nearby entities? Are we simply missing something important in a unified field theory that would explain the nature of a gravitational field? Is there even any such thing as gravity in the first place?
Einstein’s General Relativity explains that gravity does not, in fact, exist. The acceleration that we feel, standing here on Earth every day, is not due to the Earth’s downward pull upon us, but rather due to the fact that we are standing in an accelerating frame of reference. His demonstration of the truth of this is if we were in, instead, an accelerating reference frame in empty space, we would experience exactly the same environment. This is called the “equivalence principle.”
To put it in simplified terms, the reality is that the earth and everything on it is accelerating upwards at (from where we sit) 9.8 m/s2. So then why isn’t the earth and everything on it exploding outward into a million pieces? If that’s what you are imagining, you are trying to think of that acceleration as only a spacial phenomenon. Einstein’s explanation requires considering the integrated space-time in which the universe exists.
The short (very short) answer is that our non-inertial frame is accelerating in space-time even as the spacial coordinates, as centered on the earth, remain fixed. In terms of the governing equation, the spacial acceleration term is exactly balanced out by the space-time curvature term. We are accelerating in space so as to PREVENT us from accelerating* in space-time.
This conveniently solves the problem of the mechanism by which gravity effects objects through vast distances of spacial vacuum. If there is no gravity, there is no “force” holding an object within its stable orbit… it’s simply an object in motion that shall remain as such, as Newton might have said. It does so, however, by creating a new and equally-perplexing problem.
If an orbit is simply a straight line in space-time, this suggests that the presence of a large mass has “distorted” space-time itself. In this understand, a large mass (or a small one for that matter) bends space all around it so as to create a “gravitational well.” But by what mechanism can a lump of matter distort the very nature of time and space and for vast distances all around it? Isn’t this an even harder “reality” to conceptualize than determining the physics of a gravitation force (which, absence of a unifying theory notwithstanding, doesn’t look all that different from an electromagnetic field)?
Which Came First?
My question, one that it has now taken me over 4,000 words to formulate, is quite simple. What if we have the cause and effect backward? Rather than matter distorting space and time, maybe it is the ripples in space and time, caused by the vast energies involved in the formation of the universe, that results in mass being exactly where and when it is.
To give an example, maybe our sun is at the exact shape, size, and position that it is because there happens to be, right there, a sun-sized distortion in the shape of the surrounding space. Sounds crazy, right? But how can you tell the difference? Did the gravitation hole that the sun sits in gather up the sun? Or did the sun make the hole? Chicken or egg? If it’s the former, though, no weird theories are needed to explain how the sun “bends” space… the space was already bent.
For myself, it is interesting that, just as I was trying to articulate this concept and get it written down, I read of exactly the sort of experiment that help might dis/prove my point. NASA recently launched a spacecraft so as to collide with an asteroid. The experiment, called the Double Asteroid Redirection Test (DART), was designed to help evaluate our ability to divert astral objects from their course. What I read about DART had to do with that double asteroid thing and the measurement of experimental results.
NASA’s spacecraft struck the smaller of a pair of asteroids; the smaller being in orbit around the larger. The mission’s success was measured through the deviation of the orbit of that smaller, struck asteroid around the larger of the pair. In the first couple weeks of October, calculations confirmed an orbital shift similar to what was expected.
The fun part, as far as this post is concerned, came near the end of the article. It mentioned that we earthlings have more in our arsenal beside diverting a trajectory through a collision. These include, said the DART team, “shooting asteroids with ion beams” or using “a so-called gravity tractor.” This last is defined as “a spacecraft that looms near an asteroid and exerts gravitational pull on the space rock for an extended time.”
If there is no gravity, how does this work? It must require an altering of the “shape” of space/time in a non-trivial way for what, frankly, is a fairly modest expenditure of energy. Is there a way to tease out some of the reality of an answer to my “which came first” question? Maybe I can come back here (in less than a year, this time) with some thoughts about thought experiments.
*The straight line, inertial frame in which we wish to remain is “free fall.” Or to put it another way, an orbit. Let’s save the implications of that for another post.