# Why do our bodies feel acceleration but not velocity **URL:** \ **Category:** Factual Questions\ **Created:** [November 18, 2021, 5:05pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568 "2021-11-18T17:05:44Z")\ **Posts on this page:** 15\ **Page:** 4
**Author:** ![wolfpup](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/wolfpup/32/10618_2.png) [@wolfpup](https://boards.straightdope.com/u/wolfpup)\ **Post date:** [November 21, 2021, 1:26pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/61 "2021-11-21T13:26:41Z")
> [@eschereal](#): > > Special relativity only applies to non-linear motion, though. If I am moving toward you in a straight line at an unvarying .5c and we cannot clearly resolve any other frame of reference, then you are moving toward me at an unvarying .5c and we are in consonant frames of reference. 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. > [@Pleonast](#): > > When I’m walking, my body can sense it. I see things moving past me. I feel the pressure of the air resistance. > > Do not forget that relative velocity is a kind of velocity. As we all know, it’s the only kind of velocity that is detectable. And our bodies do detect it. 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 21, 2021, 1:36pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/62 "2021-11-21T13:36:14Z")
> [@Hampshire](#): > > Exactly this. The biology of our senses are designed to detect change. Sight senses change in light waves, hearing senses changes in sound waves, etc. etc. 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. ---
**Author:** ![Tibby](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tibby/32/17030_2.png) [@Tibby](https://boards.straightdope.com/u/Tibby)\ **Post date:** [November 21, 2021, 3:18pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/63 "2021-11-21T15:18:34Z")
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. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [November 21, 2021, 4:22pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/64 "2021-11-21T16:22:31Z")
> [@Andy\_L](#): > > And when you’re sitting in your parked car, and the cars on both sides of you happen to start backing out at the same time, you will get the sense that you’re moving forward 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. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [November 21, 2021, 6:02pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/65 "2021-11-21T18:02:26Z")
It also depends on what you mean by “feel”. 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. ---
**Author:** ![RitterSport](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rittersport/32/6326_2.png) [@RitterSport](https://boards.straightdope.com/u/RitterSport)\ **Post date:** [November 21, 2021, 6:08pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/66 "2021-11-21T18:08:10Z")
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. ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [November 21, 2021, 6:21pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/67 "2021-11-21T18:21:57Z")
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. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 21, 2021, 6:52pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/68 "2021-11-21T18:52:42Z")
This was discussed above, but in fact Einstein stated it quite explicitly: > [@](#): > > We have therefore no reason to suppose in the present state of our experience that the systems [an accelerated one, and one “at rest” in a homogeneous gravitational field]… differ in any way, and will therefore assume in what follows the complete physical equivalence of the gravitational field and the corresponding acceleration of the reference system. > > This assumption extends the principle of relativity to the case of uniformly accelerated translational motion… The heuristic value of the assumption lies therein that it makes possible the replacement of a homogeneous gravitational field by a uniformly accelerated reference system, the latter case being amenable to theoretical treatment to a certain degree. * * * > [@](#): > > This assumption of exact physical equivalence makes it impossible for us to speak of the absolute acceleration of the system of reference, just as the usual theory of relativity forbids us to talk of the absolute velocity of a system; and it makes the equal falling of all bodies in a gravitational field seem a matter of course. 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. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [November 22, 2021, 7:24am UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/69 "2021-11-22T07:24:13Z")
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? > **[Absolute rotation](https://en.wikipedia.org/wiki/Absolute_rotation)** > > Pages for logged out editors learn more > > In physics, the concept of absolute rotation—rotation independent of any external reference—is a topic of debate about relativity, cosmology, and the nature of physical laws. > For the concept of absolute rotation to be scientifically meaningful, it must be measurable. In other words, can an observer distinguish between the rotation of an observed object and their own rotation? Newton suggested two experiments to resolve this problem. One is the ef... > **[Mach's principle](https://en.wikipedia.org/wiki/Mach%27s_principle)** > > Pages for logged out editors learn more > > In theoretical physics, particularly in discussions of gravitation theories, Mach's principle (or Mach's conjecture) is the name given by Einstein to an imprecise hypothesis often credited to the physicist and philosopher Ernst Mach. The hypothesis attempted to explain how rotating objects, such as gyroscopes and spinning celestial bodies, maintain a frame of reference. > The proposition is that the existence of absolute rotation (the distinction... ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [November 22, 2021, 7:44am UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/70 "2021-11-22T07:44:51Z")
@Tibby is right - The _biological_ answer to the OP’s question is that our [internal accelerometers](https://en.wikipedia.org/wiki/Otolith) 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. ---
**Author:** ![eschereal](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eschereal/32/18939_2.png) [@eschereal](https://boards.straightdope.com/u/eschereal)\ **Post date:** [November 22, 2021, 2:00pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/71 "2021-11-22T14:00:24Z")
> [@Riemann](#): > > 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 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. ---
**Author:** ![Riemann](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/riemann/32/3133_2.png) [@Riemann](https://boards.straightdope.com/u/Riemann)\ **Post date:** [November 22, 2021, 9:07pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/72 "2021-11-22T21:07:54Z")
> [@eschereal](#): > > The force comes from the bucket. 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. ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [November 22, 2021, 9:10pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/73 "2021-11-22T21:10:48Z")
> [@wolfpup](#): > > > [@Pleonast](#): > > > > When I’m walking, my body can sense it. I see things moving past me. I feel the pressure of the air resistance. > > > > Do not forget that relative velocity is a kind of velocity. As we all know, it’s the only kind of velocity that is detectable. And our bodies do detect it. > > 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. 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. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [November 27, 2021, 6:19pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/74 "2021-11-27T18:19:44Z")
> [@eschereal](#): > > 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. Bringing an abrupt end to _[Whatever] on Ice._ ---
**Author:** ![k9bfriender](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/k9bfriender/32/3283_2.png) [@k9bfriender](https://boards.straightdope.com/u/k9bfriender)\ **Post date:** [November 27, 2021, 6:43pm UTC](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568/75 "2021-11-27T18:43:54Z")
> [@DesertDog](#): > > _[Whatever] on Ice._ Relativistic Centrifuges on Ice. [Previous page](https://boards.straightdope.com/t/why-do-our-bodies-feel-acceleration-but-not-velocity/954568.md?page=3)