If gravity is not a force, why do things move?

Factual Questions
1

I asked a question here some time ago about the source for motion due to gravity and wasn’t completely satisfied that I understood the answer. I think I may have resolved it for myself, but could use some feedback on whether my understanding is on the right track, if simplistic.

I’m familiar with the visual aid of a bowling ball depressing the center of a rubber membrane and how a marble moving through this space follows a curve in space-time created by the masses. What has puzzled me is that if gravity is not a mysterious force, why the marble moves in the first place. If the marble is stationary relative to the bowling ball because I’m holding it, what causes it to move when I let go?

So I’ve been doing some reading and my current understanding is that even if the marble is stationary relative to the bowling ball with regard to the three spatial dimensions, it is still moving in the dimension of time and that the natural state of things is to move, not rest. That if it is not moving in a spatial dimension it is only because there is some force holding it in place.

Am I on the right track?

Thanks.

2

I think the issue here is everything is moving. There is no such thing as “motionless.” You are only motionless relative to something else (relativity). That does not mean you (or whatever) is not moving. Indeed, you are. You are (mostly) motionless compared to your desk you are at. But you are certainly moving…quite fast. Rotation of the planet, orbit around the sun, however our galaxy is moving and so on.

So, anything in motion should follow the lowest energy path to get from A → B (assuming you aren’t firing your rocket engines…just coasting). That is the marble rolling around the bowling ball on a trampoline example you used.

ETA: Maybe a better way of answering the OP is when the universe poofed into existence all things created had momentum. That can never go away. It can change and be altered but it is always there.

3

In our everyday lives, we’re used to things not moving. That’s usually because of frictional forces. Put a marble on a hill and eliminate the friction and it will move toward the bowling ball. It can’t not move, because that would take energy, a consequence of what WaM said about the least energy path.

All of spacetime is curved. All things move toward valleys created by masses.* Local valleys can be points in larger valleys. The moon orbits the earth which orbits the sun which orbits the center of the galaxy which orbits the center of mass of the local group of galaxies, which is moving towards a region called the Great Attractor, the gravitational center of the Virgo Supercluster, and on and on. Nothing is still because everything is curved.

* Some things appear to be still or moving apart. Orbits are mostly stable because they are merely things moving at an outward speed which balances the inward pull. Outside forces can overcome the local valley. The moon moves slowly away from the earth because of tidal forces and angular momentum. Planets can get kicked out of systems because of imbalanced gravitation effects. An exploding star provides the energy to move objects violently. Energy makes everything possible short term, but the curvature of spacetime affects everything in the long run.

4

Not only is everything moving, but everything is moving at c. It’s just that, for most things (so far as we can tell, everything but a photon or graviton), at least some of that motion is in the time direction. When we say that something is at rest (in some reference frame), that means that all of its motion is timewards (in that reference frame). When we say that something is moving slowly, that means that most of its motion is timewards. When we say that something is moving at close to c, we mean that almost none of its motion is timewards.

5

Mind blown.

6

I do not think anyone is arguing that gravity is not a force. In terms of theory, you may have heard of “gravitons” alongside known particles like photons and pions. What you are talking about is the geometry of space-time as a result of this force— a freely moving particle will travel along a geodesic (“straight” line) as might be expected.

7

That’s one of the key points of General Relativity. Gravity is not a force (or at least, not a “real” force); something moving under the effect of gravity and nothing else is moving in a geodesic, so it’s not accelerating, so there’s no force acting on it.

8

I think I can semi-hijack now the main question has been answered.
I recall it being said that gravity is not actually considered a force by physicists, but the Higgs field interaction is. But google is telling me I’m wrong.

Can anyone help me with what am I misunderstanding / misremembering?

9

I have never heard of the Higgs field interaction, itself, being referred to as a force. It’s governed by the Weak Force, but there’s a heck of a lot going on with the Weak Force that doesn’t involve the Higgs at all.

10

Gravity is routinely considered a force in some contexts, and routinely not in others. No harm either way if the context is suitable.

If we want to consider gravity as not-a-force, then we are considering that “motion in a straight line” is a more complicated concept in general relativity. To wit –

In freshman physics you learn that an object in motion (through 3D space) will stay in motion unless acted upon by a force. We call that inertia. The object just chugs along in a straight line.

In special relativity, we cannot freely talk about space and time separately, as they are intimately linked into a single 4D spacetime. But the same inertia idea holds: an object moving through 4D spacetime continues on a steady, straight path if you don’t interfere with it. As @Chronos notes above, that motion can be purely in the time direction (for a spatially stationary object), or it can be in a mixed direction with some steady spatial and temporal progression. The idea of a “straight line” starts to get less visual now, and the technical term “geodesic” is used to mean the most direct path between two points in a more complicated geometry like this (here, 4D spacetime).

In general relativity, 4D spacetime can be warped. Now a geodesic is further from the idea of a “straight line”, and in fact if we try to insist that an object’s motion stays seemingly “3D straight”, we must apply a force since a 3D straight path in this case is not along the geodesic. Conversely, if we let an object move freely near a gravitational source without additional influence, it follows the appropriate geodesic which appears like a curved path when referenced to some rectilinear coordinate system.

You may be thinking of how the Higgs field relates to particle masses. This is often discussed in popular media as stemming from the Higgs field “dragging” on other particles or some such. This metaphor is terrible and provides no usable intuition (even if it provides comfort), but all the same you might find within it a statement that the Higgs field provides a “force” on the particles to imbue them with mass.

Alternatively, you may be thinking of how the Higgs field (or particle) can be involved in particle interactions in the same way that, well, any particle can. The idea of forces is relevant here, but the Higgs is not special in its involvement (even compared to, say, an electron.)

Alternatively alternatively, you may be thinking of how the Higgs field is part of the process by which the electromagnetic and weak forces are unified, although this is now quite far from the original statement.

I suspect it’s just the first item, since the other two are rather obscure while the first shows up in popular science writing.

11

In virtually all popular science accounts, four fundamental forces are given: gravity, electromagnetism, the weak interaction, and the strong interaction. Gravity may or may not be a force in various advanced contexts, but I’ve seen it stated as one about a million times. That may be misstated but to not call it a force in any nonexpert setting will lead to confusion unless the distinction being made is super clear.

12

Ugh, yeah, that’s probably what @Mijin is thinking of. I have no idea who came up with that analogy, but I really wish they hadn’t, because it’s wrong on so many levels (for one thing, inertial mass behaves nothing at all like movement through a viscous material, and for another, that explanation usually asserts that the Higgs field is the source of mass, while most of the mass that we’re familiar with actually comes from phenomena unrelated to the Higgs process).