Now this raises an interesting question, then. You are suggesting that we can “feel” acceleration but cannot “feel” constant velocity simply because we have the sensory mechanism to feel acceleration but we don’t have the sensory mechanism to feel velocity. So maybe we just didn’t happen to evolve that?
So, the question: Would it be even possible to have a sensory mechanism to sense constant velocity? We do have some senses that can kinda-sorta tell us that: When we move (say, for example, riding a motorcycle) we can feel the wind against our face, and judge velocity from that. But that “feels the same” as standing still on a windy day. So it’s not a perfectly reliable sense. ETA: And, of course, it doesn’t work if you are traveling in an enclosed vehicle.
It seems like, from the laws of special relativity, that it would not even be possible for us to have a sense of velocity.
Special relativity is not even at play. More like garden-variety relativity. Imagine you get on a plane at Kennedy with a ticket to Heathrow. An hour into the trip, you try to figure out how fast you are going. Some six or seven miles down there, you can see a mostly featureless expanse of water. You are not sure how far down it is exactly, but that is kind of irrelevant. There is no point of reference you can use to assess your progress. The plane could be going 550mph or it could be going 200mph, and there is no way for you to tell. Even those clouds over there are of little use, as you cannot determine their scale. The only sense of velocity you can get is that one guy who reacted badly to the meal and is moving rapidly toward the back of the plane.
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, and had better hit the brakes before you run into something in front of you (or at least that’s happened to me) - because our vision detects relative velocity.
No, it’s totally a physics question. If there’s no physical effect, then there can’t be any biological effect, because all biology is, ultimately, physical. Not just human bodies, but no device at all, can detect velocity itself.
Everything is ultimately detected via sensors, isn’t it? One could say that those gravimetric satellites cannot detect acceleration itself [again reminding us of the thought experiment with a man in a free-falling elevator], but they can feel how far they are from the other satellites. Which in turn necessitates microwave detection, atomic clocks, etc.
Why would it not be valid to say that the human body can detect velocity by looking out the window of your automobile? Or that the cops can detect how fast you are going using radar, or a ship use a log line, or any other gadgets you can imagine.
I have no idea about the feasibility of a mechanism to sense constant velocity. But I can imagine a number of reasons biologically not to do so. Remember all of the different velocities/accelerations that you are experiencing right now. First, the constant rotation of the earth that you are attached to presently. Add to that the motion we have around the sun, and the movement of our solar system within our galaxy which itself is going somewhere pretty fast. Lots of things to sense and with no benefits.
Evolution pressures have selected what works in the everyday world, and features that do not enhance our survival in some way are not generally chosen…(except by stuff like random mutations that don’t have any selective pressure one way or another, and genetic drift and so forth, etc, etc)ss
We can imagine lots of cool things we would like to have, but that is not how evolution works.
Perfectly valid, but that’s not what the non-detectability of velocity and the “equivalence of all inertial reference frames” assertion is about. The assertion is that velocity cannot be inferred entirely from within any frame of reference, but only with reference to something outside. Your car quite readily always informs you of your speed, but that’s because it measures it with reference to the ground.
I’m sure you know all this, but I’m just addressing your specific point. The relativity of velocity – both in classical physics and in special relativity, is fundamental. If you then add the fact that the speed of light is in fact absolute regardless of reference frame, then you have a paradox whose resolution from those two principles leads directly to the equations of special relativity.
Interesting, as I’ve never thought of it that way.
So no device all by itself can detect velocity, but the same is not true for acceleration, correct? In other words, you can buy accelerometer chips, but not velocity chips.
The accelerometer chips measure (for example) deflection of a flexible beam [is that “acceleration itself”??]. The device is presumably self-contained. If you integrate those measurements, you can still calculate (some relative) velocity, though; that is how those inertial navigation units work. That should count at least as far as your velocity is well-defined.
Theoretically speaking, anyway. Obviously those things drift, which is why navigational devices also look at the ground, stars, GPS, or whatever.
This is a popular statement to respond to. I like how half of people assume that I don’t know that Astronauts are floating around and the other half assume that I don’t know that there is still gravity up by the ISS.
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. Without a change we don’t “sense” anything. It’s what a sensory deprivation chamber tries to do. Eliminate any change that can be detected by our senses.
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. Of course, in the real universe, that kind of situation is all but impossible. Nothing moves in a straight line at an unvarying speed.
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 believe you are mistaken about special relativity. As I understand it, two people with a constant relative velocity are by definition in different reference frames; in fact, although special relativity can be used to understand situations involving varying relative velocities, almost all examples that are provided when explaining special relativity involve constant relative velocities.
We detect and process second order results of velocity, many of which are external to us. But that doesn’t change the fundamental - there is no mechanism to determine velocity as an absolute velocity. Mostly because there is no such thing.
We can process external input as movement but it is unreliable. Air movement is more likely to be wind than you moving. And depends on an external input - the air. Visual input relies on the external objects, and can still only tell you a relative velocity to that object. If there are a lot of objects moving around you at different velocities you won’t process the input as your own velocity but as lots of things buzzing around you. The moving train illusion further exemplifies the problem.
Humans are hopeless at integrating inputs. Get in an elevator in a tall building. Apart from the feeling of acceleration and deceleration at the start and end of the journey you have no intrinsic sense of speed. You forget the acceleration as soon as it passes.
Mechanisms to integrate acceleration can get you a velocity relative to the point they start operation. But you define that initial velocity arbitrarily. Plus if you have no external knowledge of your surroundings you can’t tell acceleration from gravity. So again you need external information that isn’t possible to determine from your measurements.
Eventually - the bottom line is that our bodies don’t feel velocity because we as an individual are not moving. Velocity doesn’t involve exchange of energy and so there is no information to be had. We do feel acceleration, because we do accelerate, and there is energy to be found as organs within us strain and thus information is to be had.
If you are accelerating, something is pushing on you to make you go faster. You feel the push.
If you are not accelerating, nothing is pushing on you. If you and the plane are going the same speed, nothing is pushing on you to keep you at that speed, so you don’t feel anything.
One might also note that acceleration is a vector quantity: if you are decelerating, you usually feel that as well (e.g., your seatbelt is pulling on you).