As @Andy_L already noted, this is not correct. The very fact that you are observing a velocity implies a different reference frame. We (as the two moving objects) may have the same coordinate system, but not the same reference frame. In any case, I’m not sure I understand what this had to do with my point, since the relativity of all velocities (and the equivalence of all inertial frames) and the paradoxical constant of the speed of light lead directly to special relativity.
I think you’re confusing everyday perceptions and irrelevant mechanical effects like wind and noise with the fundamental relativity of all velocities. You see things moving past you because they are in a different frame of reference than you, from which you gauge your velocity. You feel the pressure of the wind because it, too, is in a different frame of reference. You could fairly easily rig up a simulator to produce all those effects, thus “confirming” that you’re moving while in fact you’re perfectly stationary with respect to the ground. In fact, today’s sophisticated aircraft simulators produce all those visual and sensory effects (well, not wind!) including the noise and acceleration as you throttle up and watch the runway moving faster and faster underneath you, yet you’re standing perfectly still inside a big box in a large room. The acceleration of course is produced by tilting the whole apparatus.
Think in terms of being in a spaceship, floating weightless in interplanetary space. It has windows but the stars and other planets are too far away to detect movement. Or maybe it has no windows at all – better still. Now figure out how “fast” you’re going. The first question is, how can you possibly tell? The second question is, relative to what? Relative to the distant earth, relative to the planet that is your destination which is moving at a different speed in a different direction, or relative to something else? There is no absolute frame of reference.
Yes and no… Our vision is “designed to detect change”, in the sense that it’s easier to pick a moving object out of an image than it is to pick out a stationary object… but we can still pick out the stationary object if we put in the effort. And one could have a vision system (whether evolved or designed) that’s better at picking out still objects than we are. And one can say that “acceleration is changing velocity, and therefore we detect it”, but one could just as easily (but incorrectly) say that “velocity is changing position, and therefore we detect it”.
It really isn’t about “the biology of our senses”, or “the way we’re designed”. It’s about the fundamental physics of the way the Universe works.
To answer the OP, I believe you need to consider both gravity and biology. Linear and rotational acceleration are sensed by evaluating the inertia on receptor cells in the vestibular system (which processes information from proprioceptive, somatosensory, and visual systems). Absence of gravity offloads the vestibular system. Cause/effect. Gravity causes the effect on the vestibular system which results in feeling acceleration.
That’s a classic phenomenon on trains. Two trains sitting side-by-side at the platform, you notice the windows start gliding by and could swear you’re moving, then the end of the other train comes and you’re still sitting at the platform.
You can “feel” as though you are going fast through visual stimulation alone. If you are playing a car racing game, there are no forces on your body (other than the static pull of gravity as you sit in your chair[or to be more precise, the deflection from a geodesic you get from being held in your chair rather than freely falling to the center of the Earth]), but you can “feel” as though you are going really fast.
What’s the right way to describe what’s happening when you’re falling (in a vacuum), orbiting, or just hanging out in deep space – they will all feel the same, but in the first two, you’re accelerating and in the last one, you’re not.
I mean, we say we feel acceleration, which is true, but free fall seems to be something different.
It’s always freefall. There is nowhere in this universe you can go that is not being affected by gravity.
And relativity says that you are only accelerating if you feel the acceleration. If you are falling freely or in orbit, (neglecting air friction or other minor adjustments), you are not accelerating, you are following a straight line through space, if you are in orbit that space happens to be curved.
When we sit on our chairs, that’s when we are accelerating. A straight line would be to fall to the center of the Earth, and we are being deflected from that straight line by our interaction with its impenetrable surface.
In relativity, you would be considered to be in an inertial reference frame, if you are not feeling any acceleration.
This was discussed above, but in fact Einstein stated it quite explicitly:
E.g., the free-falling guy in an elevator is not being accelerated in the sense he does not feel any accelerating force, his path follows a geodesic in any case, and in this sense there is no “absolute acceleration”
Those orbiting satellites that detect the intensity of Earth’s gravitational field do not do it using a built-in accelerometer— they tell whether they are speeding up or slowing down with respect to other satellites. Or, if you are on the ground, you can essentially drop a weight in a vacuum and measure how fast it speeds up.
There’s a related deep and slightly weird issue that isn’t really settled, the question of why rotation (about an axis, not a free-fall orbit) appears to be absolute. If I spin a bucket of water, the water is pushed to the outside of the bucket by the apparent centrifugal force. This implies that rotation is absolute, not relative. But why, and is it really, and where does the apparent force ultimately come from? The crucial question, not very amenable to experimentation: if, instead, I were to rotate the entire universe around a notionally non-rotating bucket, what would happen?
@Tibby is right - The biological answer to the OP’s question is that our internal accelerometers are optimized to sense acceleration through the physical principle of inertia. Specifically the inertia acting on tiny aragonite stones attached to cellular hairs in a gel matrix. Constant velocity isn’t going to move the stones relative to the hairs, acceleration is.
The force comes from the bucket. “Centrifugal force” is actually inertia: the water wants to go in a straight path (geodesic), but the walls of the bucket exert an inward pressure (centripetal force) that prevent linear motion. Similarly, the centripetal force on the classic spinning ice skater is the molecular cohesion of the skater’s mass: if the skater spins fast enough (as in really, really fast), centrifugal inertia will cause their body to explode and fly off smithereenally in straight-ish lines.
I understand the conventional account of centrifugal force. That’s not the issue. The question is, why is rotation apparently absolute, discernable without outside reference by observing the presence of a centrifugal force. Why is there a privileged non-rotating frame? I suggest reading the Wikipedia articles that I linked to if you’re not familiar with the problem.
I guess I didn’t make my point clear enough. There is only one kind of velocity (relative velocity) and our bodies can feel it. Those senses are limited, just like all of our senses. The fact that our bodies cannot feel imaginary things (like absolute velocity) is not surprising.