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In the case there is no air and your eye are closed,

then does falling from the sky under gravity have the same feeling as floating in space? Can our body feel that we are accelerating without the air hitting us.

If not how are they different?

Also are free fall and zero g the same thing cause when we are falling freely we are accelerating at g towards earth then why would it be called "zero g"?

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    $\begingroup$ Note that even in orbit, you are not floating in space but falling under the Earth's gravity. So, few, if any, people have really experienced floating in space. The men who went to the Moon would have had a brief period when the gravity from the Earth and the Moon balanced but, even then, they were still subject to the Sun's gravity. No one has escaped that. $\endgroup$ – badjohn May 26 at 12:46
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    $\begingroup$ Achievement awarded: Discovered General Relativity. $\endgroup$ – Aron May 27 at 3:07
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    $\begingroup$ @Ali They are the same thing. The fact that they are the same underpins the entirety of General Relativity. If you can prove they are different, then you my friend can collect your Nobel Prize. $\endgroup$ – Aron May 28 at 8:43
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    $\begingroup$ @Aron refer to this Wikipedia article- en.m.wikipedia.org/wiki/Weightlessness . It just says what I am saying and I am no physicist. If you can kindly elaborate on your point I shall remove my comment as it may misdirect some. $\endgroup$ – Ali May 28 at 12:04
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    $\begingroup$ @Ali, my point is that being supremely pedantic about the various definitions of weightlessness is meaningless, because you're always under something's gravitational influence. As long as you're not under any acceleration (including the 9.8m/s/s acceleration away from the planet's surface that you're feeling right now just sitting at your computer), there is no difference between free fall, weightlessness, or a complete lack of gravitational influence whatsoever. $\endgroup$ – Ghedipunk May 29 at 15:38
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Yes, they feel the same, and this observation is fundamental to how we think of gravity. Einstein said that not only do they feel the same, they are the same: movement under gravity alone is the same thing as movement under no force at all. The name for this assumption is the equivalence principle, and it underlies General Relativity: because we know that things experiencing no force at all move in straight lines through spacetime, we also know that things moving under gravity alone move in straight lines through spacetime, and this works because what gravity does is to curve spacetime, so that 'straight lines', which are now called geodesics, have properties which straight lines in a flat spacetime do not have, such as intersecting more than once.

To be slightly more precise about this: there is (in GR) no local distinction between movement under gravity alone and movement under no force at all: because gravity distorts (curves) spacetime, there are experiments you can do which are not local which will tell you whether you are moving under gravity or under no force. Geometrically, these experiments consist of establishing whether straight lines have the properties you would expect in a flat spacetime or whether they have properties you would expect in a curved spacetime; physically the experiments consist of detecting 'tidal forces' which are forces which cause two separated objects (the being separated is what makes the experiment non-local), initially at rest relative to each other, to want to move away or towards each other over time.

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  • $\begingroup$ I don't know if it's accurate to say there is no local distinction. There is no pointwise distinction, but there is a local distinction. A point particle cannot tell the difference between floating in space and falling under gravity, but a human can. Ask a human near a black hole whether falling toward a black hole feels any different from floating through space. He'll have a definite answer, which will consist mostly of screaming. Tidal forces are local invariants of space-time. $\endgroup$ – Charles Hudgins May 29 at 18:20
  • $\begingroup$ Another way of putting this: as the astronaut is screaming in pain from being spagettified by the black hole, you will not be able to convince him to stop screaming by changing your coordinate system. Pain is diffeomorphism invariant. $\endgroup$ – Charles Hudgins May 29 at 18:47
  • $\begingroup$ @CharlesHudgins: there is a well-defined sense in which all manifolds look like $\mathbb{R}^n$ over small enough scales: that's what 'local' means. Those scales can be fairly small if the curvature is large, but they are never a single point. $\endgroup$ – tfb May 29 at 22:56
  • $\begingroup$ I misspoke. I should have said "isometry invariant." It is not the case that all pseudo-riemannian manifolds are locally isometric to $\mathbb{R}^n$, but, as you said, it is (by definition) the case that all pseudo-riemannian manifolds are locally diffeomorphic to $\mathbb{R}^n$. It is the failure of the space-time manifold to be locally isometric to flat space near a black hole that causes the astronaut to cry out in pain. $\endgroup$ – Charles Hudgins May 30 at 4:06
  • $\begingroup$ @CharlesHudgins I think it is the case that you can always pick coordinates at a point so the metric is $\mathrm{diag}(1,\ldots,-1,\ldots)$ & its first derivatives vanish but the second ones don't: that's what I meant by 'locally like': it is only extended objects which feel tidal forces. $\endgroup$ – tfb May 31 at 9:30
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In essence, yes. Being on a space station in orbit basically IS falling due to gravity, it's just that the astronaut and the space station keep missing the Earth due to constantly moving sideways so they never hit the/fall on the Earth. But they basically ARE falling.

Our bodies can't tell the difference, because all your body parts are accelerating and moving at the same rate, they're not in any tension in relation to each other so it's like there's no force, none that you, the person, can feel anyway.

There are some minor differences, tidal forces, but these effects are minor unless you're orbiting near a black hole etc. Tidal forces: slightly stronger gravity near the gravity source, so your feet, for example, are pulled sightly stronger, but these effects are minor usually. Astronauts on the ISS certainly don't feel it.

The term "zero-g" just means you don't feel any gravity, not that there isn't any. Of course, if you were in the void, far far far away from any gravity source, you would still be in "zero-g" because you wouldn't feel any... because there is none.

"g" here refers to a thing called "gravitational acceleration on Earth" btw, which is $g=9.81\:\rm m/s^2$. Fighter pilots go through 5g and more because they accelerate a lot... gravitation itself being irrelevant here, it's all about the felt acceleration itself. Emphasis on felt. Astronauts accelerate too, as I've said, but they, the persons, don't feel it, because they aren't squished onto anything, like the fighter pilots are squished onto their jet engines.

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  • $\begingroup$ And note that fighter pilots only feel Gs when doing maneuvers or changing speed. The SR-71, even though it may be going faster than speed of sound, do not really feel a whole lot of Gs when cruising. If they kick in the after burners to evade enemy fire, then yeah, a lot until their bodies finish accelerating. $\endgroup$ – Nelson May 28 at 6:34
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    $\begingroup$ Here is the reference to the first paragraph: “(…) there is an art to flying (…): how to throw yourself at the ground and miss.” ― Douglas Adams, "Life, the Universe and Everything" $\endgroup$ – Hermann May 28 at 9:13
  • $\begingroup$ Need to mention that if you were falling into back hole - then at some point tidal forces will tear you apart into small peaces. So in a very stong gravitational fields - there is a real difference between falling under gravity and floating. $\endgroup$ – Agnius Vasiliauskas May 28 at 10:04
  • $\begingroup$ @guest apart from tidal effects are there any "natural" free falls ? What I mean by that is does Nature display free fall in any respect? $\endgroup$ – gansub May 28 at 13:01
  • $\begingroup$ @AgniusVasiliauskas With a large enough black hole, the tidal forces at the event horizon are small enough for a person to survive. $\endgroup$ – Acccumulation May 28 at 15:01
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This answer mainly expands on earlier ones as I think a little more can be said on tidal forces.

Floating in space and falling under a uniform gravity are indistinguishable if you don't have any external reference points to observe. However, if you are falling feet first (for example) towards Earth, or any other planet, then gravity is not uniform for a couple of reasons.

Firstly, your feet are slightly closer to the centre of the Earth than your head so your feet experience slightly stronger gravity than your head. This is experienced as a (very small) force trying to stretch you from head to foot.

Secondly, because the attraction is effectively towards a single point at the centre of the Earth, the direction of gravity is very slightly different for your left shoulder and your right shoulder. This leads to a very small net force compressing you from each side of your body and front to back as well for the same reason.

In practice, with something as small as a human and such a comparatively weak gravity, you won't be able to detect the differences but these are the same forces which generate tides when you get to the scale of the Earth & Moon. Going further, Stephen Hawking came up with the word spaghettification in "A Brief History of Time" to describe the effect of an object getting too close to a black hole and experiencing these forces. The name says it all, really.

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  • $\begingroup$ The top voted answer didn't miss this. It specifically mentions tidal forces, and that they can essentially be omitted when orbiting objects like the Earth, because the effects aren't strong enough to notice. $\endgroup$ – JMac May 29 at 14:23
  • $\begingroup$ @JMac Indeed. Teach me not to read all the other answers thoroughly enough first. I'll leave the answer up, however as it expands on the top voted information. I will edit the first sentence though, so apologies if your comment doesn't make sense to later readers. $\endgroup$ – Alchymist May 29 at 14:31
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Yes, they are both same (with at least one exception given below), because their state (of motion or rest) is only being influenced by "curvature of space" alone. There is no other external force at work. Because they are freely moving/floating under influenced by "curvature of space", they do not feel that curvature. That state is referred to as weightlessness. They both feel weightless.

There is one exception though - near the black hole, the spaghettification becomes noticeable/observable/painful.

So, someone falling freely near a black hole, will have different feeling as compared to someone floating freely into far space, or falling freely around an ordinary planet.

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  • $\begingroup$ Please look at Scott Seidman answer about the biological aspects. $\endgroup$ – Lelouche Lamperouge May 29 at 15:26
  • $\begingroup$ @LeloucheLamperouge I don't think his biological aspect answer is a good one. The study he cites wasn't looking at freefall. He seems to be relating the perception to the frequency of the force; but for some reason he's assuming freefall has some frequency while floating doesn't. Neither have a frequency as far as I'm aware, and he hasn't clarified what frequency he expects from freefall. $\endgroup$ – JMac May 29 at 15:50
  • $\begingroup$ @JMac: I agree, even if related (which I did not check), that answer would be very round about way to arrive at spaghettification effect which I think has nothig to do with frequencies. Gravity has not been quantized yet successfully. $\endgroup$ – kpv May 29 at 16:10
  • $\begingroup$ @kpv I don't even think they're referring to the spaghettification effect. Even that wouldn't have frequency components AFAIK. There would be different forces acting on different parts, causing internal forces that you could feel; but none of those would be cyclic, and thus frequency is still irrelevant. $\endgroup$ – JMac May 29 at 16:22
  • $\begingroup$ @LeloucheLamperouge: Do you still think the answer you referred to, and spaghettification effect that I mentioned, are related? $\endgroup$ – kpv May 29 at 18:22
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While the physics is equivalent, the two sensations might well be perceived of as different. The system sensing accelerations tends to interpret higher frequencies as translational motion, and lower frequencies as a reorientation with respect to normal gravity. (See Seidman, S., Telford, L. & Paige, G. Exp Brain Res (1998) 119: 307. https://doi.org/10.1007/s002210050346), for example). People rarely fall forever. Sometimes we fall for a long time, though. I would imagine that the sensory experience of space might approach that of a parachute jump, for example, which would have very low frequency components.

Also, our sensory systems know that we live in a 1-g environment. There are a number or famous illusions that occur when this is violated (Cohen, Malcolm M. "Elevator illusion: Influences of otolith organ activity and neck proprioception." Perception & Psychophysics 14.3 (1973): 401-406, for example)

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  • $\begingroup$ It's not clear to me how the linked article relates to the situation being described. The wording on the article is a bit hard to follow for me, but the article is about "dynamic linear acceleration", whereas falling under gravity seems like it would be closer to "static linear acceleration". The article is a bit too bio-medical focused for me to understand exactly what they are concluding though. $\endgroup$ – JMac May 28 at 16:55
  • $\begingroup$ In a parachute jump, you quickly approach terminal velocity, at which point your internal organs feel the normal 1g inside your chest, just like if you were lying on a table. You're not in free fall because of air resistance. (I've gone skydiving once, and yes you get that falling sensation only right at the start for a few seconds.) $\endgroup$ – Peter Cordes May 28 at 20:42
  • $\begingroup$ @JMac The question is does floating in space have the same feeling as falling from the sky. The physics answer is "the accelerations are the same", and the psychophysical answer is "no" for most circumstances involving real falls. You can't answer that without referencing the physiology and psychophysics. It probably makes the question off topic here, and better suited for the space exploration stack. $\endgroup$ – Scott Seidman May 28 at 21:07
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    $\begingroup$ @ScottSeidman What frequency difference are you expecting between freefall without air hitting us, and floating in space? Neither should have any frequency components. $\endgroup$ – JMac May 29 at 11:44
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    $\begingroup$ @ScottSeidman I think you are interpreting the question different than everyone else, including OP. OP is not calling freefall/orbit "floating in space". When they say floating in space, the question highly implies they are talking about no acceleration acting on them, i.e. away from gravitational influence floating in interstellar space. They want to compare that with the feeling of freefall, where you experience a constant acceleration due to gravity, but no other forces. You seem to be implying one of those scenarios has a frequency component, but it's not clear how. $\endgroup$ – JMac May 30 at 12:39

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