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[McTraveller]

I was just pointing out that at least some of the high velocity changes our planet undergoes are imperceptible to us, because of our being part of earth's reference frame.

Human acceleration detection threshold by sensation is roughly 0.2g (2 m/s²); anything below that and you're not likely to notice if you don't have a visual frame of reference. 

This is why some elevators you don't feel, and others you do - it's all about how quickly they accelerate.

The acceleration required to hold us to the spinning earth's surface at the equator is only 0.03m/s² - well below the detection threshold of your senses and well below the acceleration due to Earth's mass.  Note also this is not a "high velocity change" - it's a small change in a very fast velocity.

Same with Earth around Sol: While our linear speed around the sun is quite fast, the angular velocity is only about 2 x 10^-7 rad/s.  1 AU is 1.5 x 10¹¹ m.  So even though that radius is huge, we only need an acceleration of about 0.006 m/s².  The dependence on angular velocity squared dominates things.

Put another way:  the scales (orders of magnitude) involved with space travel are just so far out of human everyday experience they seem non-intuitive.  Most people think we would have a huge acceleration toward the sun.. but it's really quite small.

[Il Palazzo]

Even if the centripetal acceleration were high, well above that 2 m/s^2 (I'll take your word for it being the limit), you wouldn't feel it, because the only kind of gravity you can feel is tidal gravity. I.e. the deviations from uniformity of the gravitational field.

[McTraveller]

Even if the centripetal acceleration were high, well above that 2 m/s^2 (I'll take your word for it being the limit), you wouldn't feel it, because the only kind of gravity you can feel is tidal gravity. I.e. the deviations from uniformity of the gravitational field.

I don't think that's quite correct - maybe I'm misunderstanding you?  What we feel isn't non-uniformity of a gravitational field. What we feel is internal stress fields.

When you stand on the ground, there is a stress field set up in your body which results in your acceleration with respect to the ground to equal zero (assuming your'e standing).  This is in an essentially uniform gravitational field; there is no meaningful change in the field over the distance of a human body.  When in free fall - also essentially in a uniform gravitational field - you feel weightless because there is no internal stress field any more.

I'm not even sure we would "feel" a strongly changing gravitational field; I can't remember enough of general relativity to know if large gravitational gradients actually produce stress - I think that they don't, because isn't any path through a gravitational field the same as free-fall, meaning you can't feel it, regardless of the change in field strength/direction?

[Dorsidwarf]

Im pretty sure a rapidly-changing gravitational field would be noticable, because the direction you were being pulled in would change constantly and you'd puke and fall over because the balance thingies in your ear have a fit.

if you're driving along in a car at a steady 10 m/s, you arent going to notice any acceleration, but if the car rapidly changes direction drastically, you'll sure feel that change in velocity.

[Il Palazzo]

Even if the centripetal acceleration were high, well above that 2 m/s^2 (I'll take your word for it being the limit), you wouldn't feel it, because the only kind of gravity you can feel is tidal gravity. I.e. the deviations from uniformity of the gravitational field.

I don't think that's quite correct - maybe I'm misunderstanding you?  What we feel isn't non-uniformity of a gravitational field. What we feel is internal stress fields.

When you stand on the ground, there is a stress field set up in your body which results in your acceleration with respect to the ground to equal zero (assuming your'e standing).  This is in an essentially uniform gravitational field; there is no meaningful change in the field over the distance of a human body.  When in free fall - also essentially in a uniform gravitational field - you feel weightless because there is no internal stress field any more.

That's all correct, but not what I meant. In what you describe, it's not gravity that we feel, but the stresses in our body induce by the surface acting to push us away from the free-fall geodesic - there's an agreement, but that's also kinda the point.  That's not gravity. Gravity, where the field is uniform, accelerates all parts of the body equally, so there are no internal stresses to produce sensory (prioproceptive) response. With the absence of such stresses, i.e. in free fall, gravity is undetectable.
Where you have a non-uniform field (I mean spatially, not changing with time), and the system under analysis is not a point mass, then the system will be subject to tidal forces, proportional to the size of the system and the mass of the gravity source, and inversely proportional to the cube of the distance from that source.
This doesn't require GR, btw. If you place an extended body in a central gravitational field, then the parts along the radial direction will undergo stretching, while the parts along the tangent direction will be squished, due to the differences in magnitude (radially) and direction (tangentially) of acceleration vectors acting on different parts of the system.
That's nothing more fancy than how tides work.

[Il Palazzo]

Im pretty sure a rapidly-changing gravitational field would be noticable, because the direction you were being pulled in would change constantly and you'd puke and fall over because the balance thingies in your ear have a fit.

if you're driving along in a car at a steady 10 m/s, you arent going to notice any acceleration, but if the car rapidly changes direction drastically, you'll sure feel that change in velocity.

That's only because what accelerates you is the car seat/seatbelt/floor, the whole assembly, acting on parts of your body (bum, legs, back, etc.).
If you imagine an uniform gravitational field, accelerating every molecule of your body equally, that then begins to change in a way that preserves the uniformity, then your inner ear has no way of knowing that anything has happened. There would be no reason for the hairs that detect acceleration to be bent out of their initial position, because both the hairs and the inner ear in which they sit, and the entirety of your body would change acceleration at the same rate.

[Max™]

Bonus: if you were able to feel shit like local variations in pretty much any of the tensors in GR... you're fucked, I don't think any environment in the universe besides maybe the surface of a neutron star spinning fast enough to almost tear apart or an actual event horizon is going to give curvatures that dramatic over such small scales as a human body can span.

The planet you sit on is curved in such a way that it is deeper towards the center than you'd expect from measuring the circumference.

The entire planet gives an excess radius measured in millimeters last I checked.

[Egan_BW]

how about you go skydiving and tell us how undetectable the presence of gravity is

[Il Palazzo]

Missing the point much?

[wierd]

Just for clarity; how big of a fluctuation in local gravity are we talking here?

While your inner ear might not notice, your muscles sure might.  That would include your heart.  If you bounce local gravity up and down 10%, you will lose coordination, as the energy needed to lift limbs will be constantly changing 10%. Likewise, you might notice strange flutters in your heart rhythm, as your blood gets easier to pump, then harder to pump again.

Or am I thinking about this wrongly?

[Il Palazzo]

While your inner ear might not notice, your muscles sure might.  That would include your heart.  If you bounce local gravity up and down 10%, you will lose coordination, as the energy needed to lift limbs will be constantly changing 10%. Likewise, you might notice strange flutters in your heart rhythm, as your blood gets easier to pump, then harder to pump again.

That's only if you're standing on some kind of ground. I.e. when you're not in free fall. So what you feel is due to gravity, but it's not gravity.
The distinction is not cosmetic.

For our purposes we can reduce the human body to two unconnected, stationary points floating in empty space at a set distance. Asking for whether this human-lite can sense anything due to any sort of acceleration means asking if the distance between the two points changes. This is a justifiable simplification, because whether we're talking about structures in the inner ear, or muscle tension, or blood pressure - all of these rely on one part of the body being accelerated w/r to some other part.

In a uniform gravitational field the nett acceleration between the two points is zero - so it doesn't sense anything. In a uniform field which changes with time identically at all points - there's also no sensation, because the nett acceleration stays 0.
If you add a surface that pushes with an extra force on one of the points but not on the other - the human senses it, but this is the result of that extra force we added in.
The only way a gravitational field can make the human perceive something is if the field is non-uniform, so that one point has different acceleration than the other, resulting in them coming together or apart.

If we then go back to the original discussion, of whether we can feel acceleration in orbit, then we should see that comparing human sensory thresholds with orbital acceleration is a red herring. Because it's not acceleration of the entire system towards the Sun or the planet that matters, but the differences in acceleration within the system. And on the scale of a human, those are way below even the already small numbers in McTraveller's post (I'm getting a difference of ~0.0000006 g for tides raised on a human by Earth, which are going to be the largest).

[Reelya]

Hey I don't think anyone else in the thread picked up on this one:

http://astronomy.com/news/2018/11/astronomers-find-a-solar-twin--a-star-that-looks-almost-exactly-like-our-sun

Astronomers have found a star that was likely born in the same stellar nursery as our Sun. The newfound sibling is only the second ever to be identified.

They've found a "nearby" star that's a dead ringer for our own sun in terms of age, composition, and trajectory through the galaxy. It's possible that it was born from the same gas cloud that our own sun formed out of.

[Kagus]

Hey I don't think anyone else in the thread picked up on this one:

http://astronomy.com/news/2018/11/astronomers-find-a-solar-twin--a-star-that-looks-almost-exactly-like-our-sun

Astronomers have found a star that was likely born in the same stellar nursery as our Sun. The newfound sibling is only the second ever to be identified.

They've found a "nearby" star that's a dead ringer for our own sun in terms of age, composition, and trajectory through the galaxy. It's possible that it was born from the same gas cloud that our own sun formed out of.

"Sun, you have a brother".

[wierd]

While your inner ear might not notice, your muscles sure might.  That would include your heart.  If you bounce local gravity up and down 10%, you will lose coordination, as the energy needed to lift limbs will be constantly changing 10%. Likewise, you might notice strange flutters in your heart rhythm, as your blood gets easier to pump, then harder to pump again.

That's only if you're standing on some kind of ground. I.e. when you're not in free fall. So what you feel is due to gravity, but it's not gravity.
The distinction is not cosmetic.

For our purposes we can reduce the human body to two unconnected, stationary points floating in empty space at a set distance. Asking for whether this human-lite can sense anything due to any sort of acceleration means asking if the distance between the two points changes. This is a justifiable simplification, because whether we're talking about structures in the inner ear, or muscle tension, or blood pressure - all of these rely on one part of the body being accelerated w/r to some other part.

In a uniform gravitational field the nett acceleration between the two points is zero - so it doesn't sense anything. In a uniform field which changes with time identically at all points - there's also no sensation, because the nett acceleration stays 0.
If you add a surface that pushes with an extra force on one of the points but not on the other - the human senses it, but this is the result of that extra force we added in.
The only way a gravitational field can make the human perceive something is if the field is non-uniform, so that one point has different acceleration than the other, resulting in them coming together or apart.

If we then go back to the original discussion, of whether we can feel acceleration in orbit, then we should see that comparing human sensory thresholds with orbital acceleration is a red herring. Because it's not acceleration of the entire system towards the Sun or the planet that matters, but the differences in acceleration within the system. And on the scale of a human, those are way below even the already small numbers in McTraveller's post (I'm getting a difference of ~0.0000006 g for tides raised on a human by Earth, which are going to be the largest).

Doesn't gravitational energy propagate at the speed of light in vacuum?  This is not a rhetorical question, because if there is a speed of propagation, it defacto prevents "Instant" changes in local intensity.  That spike might be so short lived that you cannot reasonably feel it, but it would still be a local incongruity between what any two points would experience relative to each other and the source of the fluctuation. 

The sensitivity of the system would be enhanced by increased distances between the two points of measurement.  (EG, if we simplify the system to two points, like you suggest, and have two point masses travelling perfectly parallel to each other on identical vectors, but have them say-- 1 lightyear apart -- Then introduce a gravity wave--- the gravity wave has an epicenter, and falls off proportionally to the inverse square of distance, per newton.  The wave will intersect the first point mass 1 year before it overtakes the other. The gravitational influence of the wave will be stronger when it intersects the first point, than when it intersects with the second. (For completeness, there is an edge case where the distances between the two points and the epicenter of the wave are identical, and the wave would intersect both points simultaneously, and equally, but this would be pretty rare in nature.)  After interaction, the two points will no longer be on parallel vectors of travel, by very measurable degrees.  With point masses that are very close together, the degree of difference will be much smaller to measure, but not zero.)

[Il Palazzo]

Sure, if you make the system large enough, all sorts of previously negligible effects can no longer be ignored.
The point was, though, that on the scale of a human being, they are negligible, and have nothing to do with the magnitude of centripetal acceleration.