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this post was submitted on 25 Jun 2026
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We naturally move through spacetime on geodesics, which could be defined as “the trajectory along which you don’t feel the effects of gravity or acceleration”.
Could you elaborate?
I'm not an expert, but from my understanding it's kind of something like this:
So a geodesic is a line of shortest length between two points on a curved surface, like two points on a ball, the geodesic would be the shortest line that connects the two.
If you look at flight paths compared to a flat representation of Earth, you'll see that aircraft seem to follow a very strange arc, that's because they're actually flying the geodesic of their route, which looks funny when presented flat.
If you imagine Earth as the aircraft and spacetime as the globe, that's essentially what is meant. However do keep in mind that aircraft follow a two-dimensional geodesic, whereas we would follow a three-dimensional one due to spacetime being four-dimensional; due to how reductions work. (3D object casts 2D shadow, 4D object casts 3D shadow).
My knowledge is rather vague past this point, but essentially orbits are geodesics against 4D spacetime, and gravity affects that temporal component.
Very nice answer. Thanks. I get the geodesic part. It's the "don't feel gravity" bit I'm missing. Do we mean don't feel because it's not a force acting on an object it's a curvature in space-time making the geodesic?
That turn of phrase sounds off to me either way, because gravity defines it, so how x Can it be not felt? Do you just mean balanced between the geodesic gravitational "pull" and "push" of velocity's vector? (Orbital velocity)
Your first part was exactly correct by my understanding, gravity is not a force, it doesn't act on matter, so matter doesn't feel it, because in a over-simplification it essentially doesn't exist as far as matter is concerned.
Matter, the Earth, you and I, we occupy the three spatial dimensions of our universe where gravity does not act; we occupy the space part of spacetime. Space itself doesn't occupy time, but it is intertwined with it, and thus consequence is shared bidirectionally.
In my mind I treat it somewhat like a train, space is the carriage with things inside, and time is the track and locomotive. They are separate, but one can't be without the other (technically however in this analogy yes they can be, so it's not perfect).
Gravity can bend the track, which will result in space going in a different way, because it has to follow the locomotive (time).
Now, relative to inside the carriage (space), things will move, anything that is loose will likely drift around, especially if it's a tight bend; that's why gravity does manifest spatial forces such as weight, inertia, G-force, etc.
Orbits, say our moon orbiting us, are actually straight lines relative to the orbiting body; Earth doesn't "know" to curve around the sun, it has no "lateral" force, it is propagating through space in a straight line with velocity given to it billions of years ago. However, again, Earth is matter, which only occupies space, so while Earth is going along a straight line in space, inside that train carriage, spacetime, the locomotive and track, is bent into an ellipse due to gravity. Because matter travels in a straight line relative to itself that intrinsically resolves to a spacetime geodesic (again, to go back to the aircraft, they follow a straight line relative to themselves).
Again, not an expert, just a young adult with a fascination about space and time. :)
This kind of makes sense, i guess. We don't feel it much because a) earth is smoll and b) we mostly stay on its surface and keep the distance. Is that about right?
When we’re on the surface of the earth we do feel the effect of gravity, because the surface is preventing us from moving on a natural geodesic (which would be a free-fall/orbit).