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Comment by pdonis | Hacker News Reader

In zero-g there is no such thing as "landing on your feet" because there is no such thing as "landing". There is no "down" direction so of course cats can't detect something that doesn't exist.

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The problem with that theory is that while a cat is falling it is in zero G.

It actually makes an interesting puzzle to figure out what a cat is actually responding to when it lands on its feet after a fall.

That is only true in a vacuum though. The cat is feeling the air resistance locally on the parts that face forward while G is applied uniformly.

That's one possibility but I'm pretty sure that's not it. If it were, then if you dropped a cat in a stiff breeze it would land sideways.

Do they?

There's only one way to find out...

I'll bring the cat if you'll bring the wind tunnel!

> The problem with that theory is that while a cat is falling it is in zero G.

Ah, yes, I see your point. Air resistance would eventually kick in and provide positive G, but not in a short enough fall. And cats are heavy enough that "a short enough fall" probably includes most falls in which they are observed to land on their feet.

A cat's whiskers can detect very minute air movements. Even without whiskers, a human face can feel slight puffs of air at tiny speeds, speeds that falling objects hit within inches. Wind alone should give the cat enough directional information within milliseconds. Whether it can adjust its body position in time is a different issue. It certainly knows in which direction it is falling practically instantly.

Cats also have memory. They remember in which direction gravity was pulling them before the fall. Absent every other sensation that memory should be enough to get the process going even absent new sensations.

About the memory part, it can be interesting to see in a perfectly black spherical chamber to first lose sensation of any direction, then to try the experiment to rule out the possible effect of memory.

Yet, I hope no one tries this as it might perhaps be traumatic to cats.

I'm sure that it keeps track of which direction is up with the same inner ear mechanism that we use for our balance. Given that it starts properly oriented thanks to gravity, and doesn't spend long falling, this gives it a good idea which direction is up when it lands.

Spend long enough out of gravity, and it will get confused. As do we.

Yes, this makes sense.

Could also be the sensation of air rushing past [1]. Curious to test the competing hypotheses--dead reckoning from initial alignment versus air movement--in a wind tunnel.

[1] https://www.military.com/history/how-naked-skydive-inspired-...

That’s what I thought too. The cat loses its plumb vector if subject to zero g for too long. Under normal circumstances, the cat quickly reorients along the previous plumb vector.

Could be wrong, but that’s my guess.

Nice joy ride BTW.

The cat is being accelerated at 9.8m/s2 towards the earth while falling. What do you mean by this?

Isn't the cat also being accelerated at 9.8m/s2 towards the earth while in "zero g"?

While falling, the cat is in an inertial reference frame, so it is not accelerating. The ground is actually accelerating upward at 9.8 m/s2, counteracting the flow of spacetime.

This Veritasium video gives an intuitive explanation: https://youtu.be/XRr1kaXKBsU

A cat is not a rigid body and is absolutely accelerating towards Earth, experiencing the physical effects of gravity.

There's a difference between accelerating compared to a coordinate system, and accelerating compared to an inertial rest frame. If you set a fixed coordinate system relative to the ground, then yes, the cat is accelerating towards the ground.

Consider this: if you hold an accelerometer while stationary on the ground, it will read 1g (accelerating). If you read an accelerometer while in free-fall, it will read 0g (ignoring wind resistance). In the first scenario, you are accelerating compared to your rest frame, even if you are standing still.

This is not an arbitrary distinction either. The 1g of acceleration while stationary on the ground produces measurable relativistic effects.

An accelerometer doesn't measure what you think. It measures the difference between the acceleration of the casing and the internals, for an example. It is designed to show 0 in free-fall. Yet, it still is under the force of 1G towards the center of the Earth.

There's more nuance to it than that, so let me try to clarify again. Like you said, the accelerometer shows the difference between the casing's reference frame and the internal's reference frame. The internals ideally maintain an inertial reference frame. It's only "designed" to show 0g in free-fall because the difference between the reference frames of the casing and internals _is_ 0 when you are falling. Because, again, when you are falling, you are in an inertial reference frame - you are not accelerating against the flow of spacetime.

The only time the accelerometer reads something other than 0 is when something pushes it away from an inertial reference frame. This is true when you're on the ground: spacetime is curved, and "flows" towards the center of the Earth. The ground pushes you against the flow of spacetime, and the difference between these two frames of reference is 1g. Note that this 1g is not "caused" by gravity, it's caused by the electromagnetic force. Forces push matter away from an inertial reference frame. Gravity is different. It decides where an inertial reference frame goes by curving spacetime - it is not a force.

Hypothetically, if you were are the center of the Earth, the accelerometer would read 0g. There would be nothing pushing you in any particular direction - you would be in an inertial reference frame (ignoring the minor detail of being crushed by all of the Earth's mass).

Again, I'm trying to show the subtle difference between accelerating compared to a fixed coordinate system, and accelerating compared to an inertial reference frame. The fixed coordinate system does not take into account the curvature of spacetime. If it did, the coordinates would be "accelerating" towards the center of the Earth at 1g - congratulations, you've defined an inertial reference frame.

I hope that makes sense. It blew my mind when this idea clicked: that space and time are not two distinct things, they are two parts of the same thing.

Well, yes, but I never disputed any of that. I wrote, "A cat is not a rigid body and is absolutely accelerating towards Earth, experiencing the physical effects of gravity." Are you saying that is wrong?

In video cat is also falling with same acceleration towards the earth (just surrounding airplane does so as well)

Whiskers. Every cat is perfectly aware of the rapidly accelerating wind rushing past its head. Then they have ears to hear that wind. Lastly they have eyes and some experience with life on solid ground. They see it coming and understand what that means... Unlike sperm whales.

> Unlike sperm whales.

Or pots of petunias.

Actually the petunias *did* know [0]

[0] https://www.goodreads.com/quotes/198068-another-thing-that-g...

All of those would still happen in zero-g though right?

Yes. Toss a cat off a roof and the cat experiences "zero-g" as it accelerates towards terminal velocity.

I don’t think you experience zero g, you feel acceleration right?

Having done "zero-g" in aircraft many times, you do feel it. Your blood shifts. Your spine decompresses. Your head gets lighter. You very much know that you are falling.

That's not true zero-g. In a plane you are experiencing the force of acceleration towards Earth, which is closer to 1G than to 0G.

I can assure you that while being trained in basic aerobatics we regularly went well beyond the zero-to-one G range.

A lot of confusion here probably comes from the fact that g-force is not the same as acceleration. It's apparent acceleration. So 0g in an aircraft is entirely possible. So is 0 acceleration but that's not 0 g, that's 1 g.

When do people mean anything other than "free fall" by "zero g"?

  the cat experiences "zero-g" as it accelerates towards terminal velocity

I think you meant the cat experiences 'zero-g' AFTER it's done accelerating and has reached terminal velocity.

I'm guessing in most cat falling situations the cat does not reach terminal velocity, so it is accelerating the whole time and can adjust based on the direction of acceleration.

No. Terminal velocity is one-g, the same as sitting in a chair. A skydiver at terminal velocity feels exactly as much force/acceleration from the wind as they would feel force from lying down on the ground.

That's right, the forces cancel each other out, so that's "zero-g".

If you're accelerating, you're experience some "g" force, not "zero-g".

Except acceleration due to gravity. Freefall is acceleration without feeling any apparent force.

I’m sitting on the toilet right now, and the forces are canceling each other out so that I’m not sitting in the toilet. Is this 0g?

Yea, but since the air in the plane is also in free fall it’s moving with the cat and thus doesn’t provide appropriate signal.

If you have a cat in a cylinder of air in space, and push it toward one end of the cylinder, I think it would get those signals since the cat is being pushed through the air.

Maybe "down" is buffered and the buffer is read at the moment of release. But the buffer is only a few hundred millisecond wide.

If the "wind" theory is true, would a cat would have it's paws pointing upwards while flying up?

I don't get it. How can it 'fall' if it is zero gravity?

while a cat is falling it is in zero G

Are you saying that objects that are falling aren't impacted by gravity? Because that's not true...

Pretty sure this was a low effort excuse to play with cats in zero-g... and who could refuse :D

> who could refuse

Anyone who has "played" with cats in unfamiliar circumstances, like a bathtub. I hope they wore their chainmail.

Good point, although as long as they are not in contact with any surfaces it seems pretty safe... Until they figure out how to propel themselves, which would be gross.

I'm pretty sure the enemy gate is down.

Yes, this!

In The Expanse books they made a point of showing that the inners (earth / mars) had difficulty with zero-g combat because they would try to lean back into corners and inadvertently push themselves out into the open.

That's certainly the intuition most people would have about this, but it's still interesting to see it borne out when in principle there could have been other, surprising signals that cats respond to in order to right themselves.

> That's certainly the intuition most people would have about this

Actually, though, it's wrong. See my responses to others elsewhere in this thread.

zero-g is the same as falling though, just with no apparent acceleration.

I've never thought critically about this - but in free-fall on earth, you are falling through the air which could be used to measure the direction of the fall.

Is it? What about the resistance of air that was not going at your own speed?

Acceleration is a vector. A vector is magnitude and direction. "Down" is a direction. If there's no acceleration, there's no "down". That there is no acceleration in "zero g" is a critical difference between "falling" and "zero g" in this context. Therefore, they aren't the same.

> Acceleration is a vector.

True, but which kind of acceleration are we talking about?

A cat in the "zero g" in the experiment described in the video has no coordinate acceleration relative to the Earth. Whereas a cat falling off a ledge to the floor does have coordinate acceleration relative to the Earth.

But both cats have zero proper acceleration--they are both weightless. (Air resistance will become significant at some point during a fall from a height to the floor, but cats are heavy enough that I don't think that would be significant in most falls where cats are observed to land on their feet.) And "zero g" means zero proper acceleration, not zero coordinate acceleration. So the GP is correct and my original comment was in error: cats in both situations are in "zero g" so that can't be what is causing the different behavior in the two situations.

I think most people would consider “falling” to be going “down” a gravity well. Stable orbits around a gravity well, or at sufficient distance to not be influenced by it, are not what most would consider to be “falling.”

But that’s a misunderstanding. Stable orbits are free fall.

There is no such thing as a stable orbit “at sufficient distance to not be influenced by” gravity.

There’s no such thing as a sufficient distance.

I think more precisely, the traditional definition of an orbit (stable or not) is that it's influenced by gravity. You could be in a situation where the influence of gravity was negligible (say, far beyond any galaxy) but it wouldn't be considered an orbit at that point.

I guess certain multi-body situations like Lagrange points might make it debatable about which "direction" you're falling though.

I agree with mrexroad. Falling implies downward velocity. Free-fall is a different term that implies downward acceleration. For example, a ballistic projectile fired up that then falls back down first climbs, then falls, but it is in free-fall the whole time.

> Falling implies downward velocity. Free-fall is a different term that implies downward acceleration.

The GGP didn't say zero-g "is" falling. They said it's "the same as" falling. Which, for purposes of this discussion, it is, for the reason I gave--the key common property is being weightless, i.e., free falling.

Note, btw, that "free-fall" does not necessarily imply "downward acceleration". It just means "weightless". You could be weightless, in free fall, far out in deep space well away from all gravitating bodies, so that there is no well-defined notion of "downward acceleration" in your vicinity.

In the sense that matters for this discussion, zero g is the same as falling--both are weightless conditions. So the GP is correct and my original comment was in error; "zero g" can't be what is making the difference.

If there's no rushing wind tingling your hairs, then it's not the same.

Next : dropping a cat in 0g in a wind tunnel

Never underestimate cats.

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