if all motion is relative, how do we know the Earth isn’t stationary with everything else in the Universe rotating around us, albeit in a right weird and in uniform way.
In: Physics
*ununiform way… Darn Amazon fire auto correct & me not catching it
So there are two thoughts here depending on your goal.
One: The understanding of the nature of the universe:
“All motion” is not relative. Relativity only holds for something called “inertial frames” which is a fancy way of saying that your reference need to obey Newtons first law: Objects at rest stay at rest, and objects moving will move in a straight line, unless acted on by another force.
In the simplest (ELI5) terms: velocity is relative but acceleration is not.
Something “Rotating around us in a right weird and [non-]uniform way” would require objects to be under weird and non-uniform acceleration. Since acceleration is not relative, we could detect that acceleration if it existed.
Two: The “usefulness” of a theory as a model. The current theory lets us correctly predict a lot of useful things. As far as I can tell the “weird and non-uniform way” motion would allow us to predict less. So the value of the current theory as a model would be higher than the value of the new theory as a model.
All motion is relative means just that. Things move relative to each other. You ask if the Earth might be stationary. Stationary relative to what?
You could, for purposes of whatever problem you are trying to solve, define the Earth as your frame of reference. We often do this when trying to solve a local problem, such as understanding the path a ball takes when we throw it.
If you define the Earth as your frame of reference for calculating how to get to the moon, it turns out the math is really complicated. As others have noted, at that scale the earth is an accelerating frame of reference. It is much easier to use a frame of reference that isn’t accelerating. So for going to the moon you probably want to define the Sun as stationary and use that frame. But if you are trying to go to another star, then you want to define our galaxy as our frame of reference.
Motion is relative, but acceleration is not. At least not in the same way.
You know how in a spinning carnival ride, you get pushed to the side? Or how, when you’re in a car that’s speeding up, you get pushed back into the seat?
In cases like those, it’s possible to construct a reference frame in which you are stationary, but those reference frames necessarily contain [fictitious forces](https://en.wikipedia.org/wiki/Fictitious_force).
Don’t let the name fool you. Those forces are real. You can still feel and measure them. But they’re not produced by any physical object, they’re just a consequence of the accelerating reference frame. Only non-accelerating reference frames don’t have any fictitious forces.
The Earth is rotating about it’s axis, and revolving around the sun, which is revolving around the galaxy. All of those are accelerations. It’s possible to construct a reference frame in which it is stationary, but such a reference frame will have these fictitious forces.
The patterns and relative motions of these things were noticed and measured, early on scientists and theologians did think the universe revolved around the Earth, but the measurements only made sense when we revolved around the Sun (heliocentric) later measurements then were able to show that the Sun was also in motion.
You *can* indeed assume a reference frame where the Earth is stationary. However, the earth interacts with other celestial bodies (most importantly with the sun via gravity), and would thus immediately go into motion in that reference frame. So you cannot *maintain* the stationariness of the Earth in any reference frame.
Because the same theories that tell us all motion is relative also tell us that you cannot be stationary. In order to not move, you’d have to be stationary relative to some special fixed point in the Universe. No such point exists, entirely because if it *did* exist, then the speed of light would be fixed relative to that point, and thus anything moving would see a change in the speed of light relative to them.
In reality, it’s been repeatedly shown via experiments that it does not matter where you are, what direction you’re traveling in, or what speed you’re going; light will *always* move at **exactly** C (in vacuum) relative to you.
Geocentrism is different from relative lack of motion. You can mathematically describe a geocentric system, it would be complicated AF.
All motion is **not** relative, only motion at constant velocity, called an inertial frame of reference. Velocity includes direction, so if you are rotating or orbiting another object, your velocity is constantly changing and are not in an inertial frame of reference.
We know, because “everything in the universe” would need a reason to rotate around the Earth. Some sort of force that’s created by a planet like the Earth that would keep everything rotating around it, and this force must not be created by any other planet that’s similar to Earth, because the universe rotates only around us and nothing else.
So if this force is created by the Earth, we’d feel it, most strongly, at its origin, no? And we haven’t discovered anything other than the [4 fundamental forces](https://www.space.com/four-fundamental-forces.html). So what we’re observing doesn’t match this theory that the Earth is the center of the universe.
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