A couple of videos ago, I spoke about the speed of gravity. In it, I said something about the force
of gravity. And that unleashed a firestorm in the comment section, with lots of people admonishing
me, saying that gravity isn’t a force. That turns out to be an interesting question,
with lots of subtle philosophical, tautological, and linguistic implications. What better way to
spend the next ten minutes or so?
(intro music)
Is gravity a force? Betteridge’s law could apply here. The law was coined by British
journalist Ian Betteridge, and it states that “Any headline that ends in a question mark can be
answered by the word no.” So, if that particular aphorism is true, we have our answer. This is my
shortest video ever. I’m giving Ian a break. Please share. Physics is everything. Done.
On the other hand,
perhaps that’s a bit hasty. Maybe we need to dig into it a bit deeper. To do this right,
we first need to ask ourselves “what is a force?”
According to Merriam-Webster, the relevant definition of force is “an agency or influence
that if applied to a free body, results chiefly in an acceleration of the body and sometimes in
elastic deformation and other effects.” Other dictionaries say similar things.
And because physics definitions are
often different than colloquial ones, I dug up my Freshman introductory physics textbook – Serway,
if you want to look it up – says that a force is that which causes a body to accelerate, but it
also notes that not all forces causes motion, if there are forces that counteract one another, like
how gravity pulls me down, but the floor pushes me up, and the net result is that I don’t move.
So, there are a couple
of different levels of understanding gravity. The first complicated one is from Newton. He related
force and acceleration. Force was mass times acceleration. And, of course, force made things
move. So, at least as far as Newton is concerned, gravity is most certainly a force. He didn’t
really know how it worked, but it was a force. One mass somehow reached across the distance and
tugged another mass, causing it to move.
That “as far as Newton is concerned” is super important. What it really means is “according
to Newton’s theoretical framework.”
The problem arises in 1915, when this troublemaker came up with a different theory of gravity.
Combined with some insights from one of his professors, a guy by the name Hermann Minkowski,
we now treat gravity very differently.
In Einstein and Minkowski’s formulation of Einstein’s theory of general relativity,
what was really going on was mass distorted space, which is a very weird concept.
This picture shows you how it is often portrayed.
Far from any mass, space is what is called flat, meaning you can draw it in a three
dimensional grid of straight lines. This is called Cartesian space. In Cartesian space,
an object will travel in a straight line, essentially it follows the direction of space.
This is the equivalent of Newton’s first law, which says that an object in motion will continue
in motion unless acted on by an outside force.
However, when mass is introduced to the picture, it bends space. If you then introduce another
mass, that second mass still follows the lines of space, but these lines of space are bent. Thus,
in the Einstein and Minkowski formulation of gravity, gravity isn’t a force like Newton
claimed. Objects still move in the direction space is bent, but from the perspective of a
person thinking space is flat, they will say that the direction the object moves is curved.
So the object looks like it's experiencing a force, but that’s because the shape of space
isn’t obvious.
And, again, the phrase “in the Einstein and Minkowski formulation of gravity” is important
here. It translates to the more formal “with in the theoretical construct of general relativity,
gravity is not a force, but rather a distortion of space.” And, of course, that is the origin of
those somewhat stuffy admonitions.
Since Einstein’s theory makes better predictions than Newton’s, then this settles things, right?
Gravity isn’t a force.
Well, here is where we have gotten hasty. For one thing, we know that Einstein’s theory of gravity
is incomplete. And, because of that, we need a replacement theory, or at least an extension,
which we generically call quantum gravity. I made a video about quantum gravity, and I put a link in
the description below.
The problem is that none of the theories of quantum gravity have been validated.
That means that there are lots different ways to imagine gravity at this deeper and most
fundamental level. Now, I can’t talk in detail about every possible alternate theory of gravity.
Really, each idea should get a video of its own. I’ve made a few which you can watch if you want,
but here I will instead just sketch out some ideas – just realize that each topic will
be only mentioned and not receive a deep dive.
So one very simple and very common idea is that quantum gravity is transmitted by the exchange
of particles called gravitons. One particle emits a graviton, which is absorbed by another and this
results in a gravitational attraction. This is very similar to the way electromagnetism works,
in which one charged particle emits a photon, which is absorbed by another, and the result
is a force. Under this theoretical paradigm, gravity then becomes a force again…after all,
if electromagnetism is a force and gravity is similar to electromagnetism, then it wouldn’t be
unreasonable to call gravity a force too.
Of course, the nature of the graviton differs in the many theories of quantum gravity. In some
superstring theories, in which the smallest building blocks of matter aren’t particles,
but rather tiny vibrating strings, the graviton is one particular vibrational mode of a string shaped
kind of like a hula hoop.
At high energy, gravitons interact with particles and generate a gravitational force.
In order for superstring theory to work mathematically, at the energies that are found
in space, the curvature of spacetime described by Einstein arises from gravitons vibrating in a
coherent state, which sort of means synchronized vibrations. This looks like the curved space
thing, but has a different cause.
Then there’s the idea of loop quantum gravity. In this theory, spacetime is
quantized, which means there is a shortest distance and a briefest duration of time.
In this theory, these little bits of space are attached together. I kind of imagine it
like chain mail, with the individual loops held together to make a piece of armor.
This paradigm is more like Einstein’s model, in that it is geometrically motivated, but it suffers
from the fact that a path from loop quantum gravity to general relativity still eludes us.
Another quantum idea is
that gravity isn’t fundamental, but rather emerges from deeper principles, including string theory,
black hole physics, and quantum information theory. Entropic gravity acts differently than
accepted gravitational theory and it is slower to weaken at cosmic distances. This is one possible
explanation for why galaxies rotate quicker than they should. Basically, entropic gravity tries to
explain the same things that dark matter does.
In any event, entropic gravity invents ordinary gravity starting not from forces or geometry,
but rather entropy and the statistical behavior of microscopic degrees of freedom.
And there are other ideas
about quantum gravity. Maybe one day I’ll do a deep dive into all of them. But let’s get back
to the “is gravity a force” thing.
The key thing is that understanding any physical phenomenon requires a theoretical framework.
In Newtonian thinking, gravity is a force. In the Einstein formulation, it isn’t.
And we don’t know at the quantum level. If gravitons are right, then gravity is a force at
high energy, but at lower energy, the theories must look like the geometrical explanation.
But looking like something is different
than being something – after all, Newtonian gravity usually looks like Einsteinian gravity.
The bottom line is that it’s hasty to make any
claims about the nature of gravity. It’s a force, or it’s geometry, or it’s caused by a buzzing of
quantum particles. The answer you get depends on the theoretical framework you’re using. However,
stripped of theory, the true answer to the question of the nature of gravity is that
it’s exactly the same as when someone asks what’s going on with you and your ex….it’s complicated.
(phasing sound)
Okay- that was a tricky video, highlighting that our understanding of a physics thing is
necessarily guided by our theories, which are both incomplete and only apply under specific
conditions. Because of that, you need to be careful when you make claims about what is
going on, because you’ll probably be only partly right and then only some of the time. I hope
you’ll like, subscribe and share and be sure to come back to learn more about the underpinnings
of physics theories. And it’s totally worth it, because, after all- physics is everything.
(outro music)