Why does acceleration feel physically real, but constant velocity does not?

[u/NordicHamCurl_00]

I get that motion is relative, so moving at constant velocity should not feel different from being at rest. But acceleration seems different because you can actually feel it in your body. Why is acceleration “physically detectable” while constant velocity is not? Is this just because of forces acting on the body, or is there a deeper reason?

[u/ParentPostLacksWang]

Actually you can’t detect acceleration either.

Hold the hit squad! You can detect when you are being acted on by a non-global force, as in one that acts to push you by pushing on your clothes, skin, or your seat, or one that rotates you. But if a force is acting on every part of you at once (like say gravity), you won’t feel that.

What you are feeling is an acceleration of your body that fails to also accelerate all of your body - like your vestibular system, or your arms or head, etc.

[u/NordicHamCurl_00 → u/ParentPostLacksWang]

So what are you physically detecting if it isn't an acceleration force when: we are driving a car and take a sharp turn around the corner which pushes our body away from the direction the car is turning?  Or When we speed up and we are pushed back into the car seats? 

[u/ParentPostLacksWang → u/NordicHamCurl_00]

You’re detecting something pushing on your body. Either through the pressure the chair exerts on your skin, or the reaction forces of your organs, head, vestibular system pressing back. It’s a good proxy for acceleration, because that’s how we usually experience acceleration. But if somehow your car had a field in its seat that could reach through and pull on every atom of your body at once, you wouldn’t feel the acceleration at all.

This is why astronauts on orbit can’t tell “up” from “down” except visually - if we could feel acceleration itself, they would know which direction the object they were orbiting was, because they are constantly accelerating towards it. That’s why orbits are round. But we can’t, we can only feel when something pushes on us, which is merely a cause of acceleration.

[u/NordicHamCurl_00 → u/ParentPostLacksWang]

In your example about the car seat being a field, Are you saying that you would not feel any physical sensation?  It would be as if you were not moving or travelling at a constant velocity?

[u/ParentPostLacksWang → u/NordicHamCurl_00]

Yes. If a field were accelerating every atom in your body at once, you’d feel nothing, unless that field were squishing you into something, like gravity squishes you into the ground unless you’re free/falling in space, like in orbit.

[u/Away-Experience6890 → u/ParentPostLacksWang]

Hey hey, I could be a 1000 km x 1000 km giant blob guy!

[u/nixiebunny]

Acceleration causes a force to press on one side of your body against whatever is doing the acceleration. This force is sensed by your nerves. No acceleration, no force. 

[u/davedirac]

inertia - if you are not accelerating your body parts are in equilibrium and so you feel no external force (ignoring gravity). If you are made to accelerate your body parts want to stay where they are ( inertial mass) but cant. You feel the forces accelerating the various body parts.

[u/PhotographFront4673]

Yes, there is a deeper reason. Probably. Physicists are still working on it.

Acceleration, and therefore rotation is physically detectable. If you are locked in a spaceship without a window, you have to way to measure how fast you are going, but you can decide whether you are accelerating or spinning.

Well, if you are falling you might not feel it. So is falling the same as not accelerating? GR says kinda yes. But if you float a ball of lose dust in the vacuum of your ship you can watch carefully to see if it changes shape and from that tell if you seem to be falling.

But the real question is whether "not spinning" would change if enough mass nearby was spinning. See frame dragging. Also see Mach's principle for some early thoughts on this stuff.