HN Theater @HNTheaterMonth

⬐ PaulHoule

I'd like to take a cat to a space station like they did in some Heinlein books.

One would think they'd learn to deal with weightlessness better if they had a few weeks to get used to it rather than a few minutes on a plane.

Don't know what you'd do for the litter box though.

⬐ qbasic_forever

Cats can be trained to sit on and use a human toilet seat. A space toilet with a vacuum would be more challenging but I bet a dedicated animal behaviorist with enough time and training could get a cat used to using a space toilet.

⬐ nicwolff

Not if it sounds anything like a terrestrial vacuum cleaner – cats do not get used to those.

⬐ FlyMoreRockets

It may just be easier to design a zero G litter box. A screen bottom with a coarse granular material and sufficient airflow through the bed could work. No idea how a cat would dig in zero G though.

⬐ throwanem

Sturdy screening to give claws a grip would probably serve as the equivalent of human handholds just generally. I don't know how comfortable or feasible walking on it would be, but I bet a cat could learn pretty quick to catch on to it and then push off along a desired vector, and they're already better at gyroscopic pointing than we are.

⬐ Jaruzel

However, Heinlein cats also walk through walls - not good when space is on the other side.

⬐ qbasic_forever

Judging by how my cat can disappear and reappear at will all around my place... I wonder if household cats already have that ability.

⬐ jimmygrapes

Pixel walks through walls because nobody explained that it can't be done, so I imagine the same would apply for breathing in and staying pressurized in a vacuum

⬐ robbedpeter

Diapers, maybe, or the litter / pan would have to be under a constant slight suction? A big enough hamster wheel to give the cat gravity?

Someone's gonna have to answer this question in the next decade when rich space tourists want to bring their pets.

Heck, a space cat live stream could probably fund a significant part of a new station.

⬐ jxy

You can probably put a few mice in a centrifuge of 1 meter and they would be happy. Make the centrifuge 10 times larger, we can fit a litter box.

⬐ gnu8

This sounds like a worthy experiment. Why don't we already have a mouse artificial gravity habitat on the ISS?

⬐ MisterTea

> Don't know what you'd do for the litter box though.

dear god... the image of a regular litter box in zero G made me wince. I had a cat years ago who was an excavator and make a big mess whenever she went on archeological digs.

Another thought, cats typically bolt from the litter box after pooping (one guy I have does an amazing 90 degree wall kick-walk ninja move to run downstairs.) Not happening in 0G :-)

⬐ pkdpic

> Don't know what you'd do for the litter box though.

pray

⬐ PaulHoule

I thought about picking up the turds with gloves, wondered what I'd do about the urine, or what I'd do if the stools were loose and then I thought "are cat diapers a thing?" and it is available COTS

https://barkertime.com/designer-cat-diapers/

⬐ Buttons840

Has an animal without "grabbing appendages" ever been in space? What did it do?

⬐ Symmetry

Tadpoles have been:

https://www.newscientist.com/article/mg14519690-700-space-fr...

⬐ leshow

Yep, from fruit flies, dogs, mice to fish: https://en.wikipedia.org/wiki/Animals_in_space

⬐ throwanem

Who says cats don't have grabbing appendages? Ever tried to deal with a frightened cat who wanted to put you between them and whatever they were scared of? Like if Velcro were made out of fish hooks - they can grab like a hot damn, and in my experience "damn" is the very least of what you'll say when they do.

More to the point, cats climb trees with their claws, so making ISS interior surfaces amenable to feline use is just a matter of finding a material that's enough like bark for them to get a grab, and durable enough to hold up under long use.

⬐ cecilpl2

> Don't know what you'd do for the litter box though.

Cat diaper?

⬐ rdl

My plan is to go to Mars with the first domestic pet cat; it implies a level of civilization/comfort/safety (to be able to have pets) which is consistent with what I'd want myself. Maybe 10k people? 50k? Hopefully I live that long.

⬐ roywiggins

Shackleton's ill-fated antarctic expedition had a cat, so maybe not as much of a guarantee as you'd think...

https://en.wikipedia.org/wiki/Mrs_Chippy?wprov=sfla1

⬐ PaulHoule

Ocean ships have rats so they would tend to have cats even if people didn't encourage them.

With the COTS technology of cat diapers I think they could have a pet cat on the ISS but they don't want the risk that it goes wrong and they'd have to put it down.

⬐ charlieyu1

I think the bigger problem is for a cat to survive g-force on launch

⬐ rdl

I doubt cats have a substantially harder time surviving g-forces than humans. Smaller animal should be less affected; plus, commercial manned launch g-forces are going to be pretty low peak anyway. Maybe 4G?

⬐ rtkwe

There's also the smell, by all accounts the ISS already smells a little bad from decades of BO and grime, I would not want to add the smell of cat shit to that.

⬐ popcube

do not worry, we will discard whole it soon...

⬐ rtkwe

The decommission date gets pushed back any time it gets close I doubt it will actually be considered until there's major structural issues. The time always seems to be "when we have a commercial alternative" which is economically a pretty long ways off from the looks of it.

⬐ jarofgreen

And loving pet owners will want to note it did not end well for the cat

⬐ qbasic_forever

Every major sea voyage of the past had a cat or two, they were necessary to keep rats from eating all the food supplies.

Apparently the cat on board the expedition that discovered New Zealand immediately upon reaching land jumped out and grabbed a small flightless bird (then a brand new species discovery) and dragged it on board to eat.

⬐ aspenmayer

Cats have been a menace to the native kiwi birds ever since.

⬐ progre

What? To me it looks like they try to turn their bodies but in freefall they can't make sense of where down is. No reflexes lost.

⬐ ordu

Yeah, those guys kept kicking or pushing cats preventing them to orient themselves. Cats were surprised by the lack of gravity, but they were not given a chance to adapt to new circumstances.

⬐ itslennysfault

I was thinking the same thing. They're still rotating the way they normally would. Toward the end of the video the orange cat floats "up" from the floor toward the ceiling and turns it self around to "land" on the ceiling feet first.

⬐ tyingq

These parabolic flights give you ~25 seconds of a close to zero-g environment, but bookended by some pretty uncomfortable ~+-2g before and after periods. It would be interesting to see the cats in a more stable and extended zero-g environment to see how they adapt over time.

⬐ pwdisswordfish9

A buttered piece of bread would be cheaper

⬐ kazinator

When a cat is in free fall, that is weightlessness. At least initially, until air resistance starts limiting the velocity.

In zero-g, you do not directly sense in which way gravity is pointing.

Probably, under the conventional free fall situation, the cat is relying on visual clues, and the sensation of air moving through its fur, to establish which way it is falling, as the basis for the righting reflex: which way to aim the paws. Those clues are absent in the simulated zero-g environment, which feels like free fall, but the cat doesn't see any relative movement to anything, or feel any air movement.

⬐ aidenn0

So maybe repeat the experiment, but with a giant fan?

⬐ kazinator

Then, separately, experiment with LCD or projector screens simulating motion, and then the two in combination.

(Remembering to reset the cats to initial values of the feline parameters before each attempt, or else using freshly allocated cats.)

⬐ someweirdperson

Don't forget to shave the cats before conducting the experiment.

⬐ geenew

Could you accomplish the same thing in a ground based lab with a fan blowing air up at ~80 degrees? If airflow is used for orientation, then the test subject should orient to be parallel to the direction of airflow as it is falling.

All for Science, of course.

(The 80 degrees thing is there so they don’t hurt themselves when they reach the ground - hopefully being only 10 degrees off vertical will be recoverable).

⬐ floxy

>the cat is relying on visual clues

That should be pretty easy to test. Anyone know if blind cats land on their feet? If blind cats are in general too geriatric to test on, what about a blind fold, and dropping your cat upside-down on a bed?

⬐ emerged

It wouldn’t be a super ethical test.. but I wonder if a cat would adapt and start doing some really crazy ninja moves all over the place, if you left him in zero-g for a few years.

⬐ Epiphany21

Unethical? Have you seen the inside of the ISS? It's cat heaven with all the cables and stuff hanging around, and nothing is too high to jump on when you're almost weightless :)

⬐ secondaryacct

You misunderstand cat adaptation !!! These little bastards would learn to swim lazily before spending any effort on ninja moves :D Have you seen a well-fed pampered cat, the thing will meow for his food before even turning his head towards its human slave.

Those auto righting reflexes are just there to ensure their eternal survival when presented to (rarer and rarer) danger.

They're Gods making us do their bidding while they pretend we're the masters :D

⬐ Steltek

I feel like there was an indirect experiment here: how quickly defenseless humans can be mauled by a struggling cat in an enclosed space.

⬐ bagels

Humans can reason about the physics, might have an advantage of efficient escape.

⬐ balls187

Footage here: https://www.youtube.com/watch?v=okZW3_5Gr4s

⬐ technick

Buttered Cat Paradox

https://en.wikipedia.org/wiki/Buttered_cat_paradox

⬐ Simplicitas

In a priori, isn't this kinda obvious ... LMAO

⬐ xwdv

I’m not sure what they expected? Which way is a cat supposed to auto-right to in a weightless environment?

⬐ jstanley

The cat might use visual cues to right itself.

It might have been interesting to re-run the test with a more obvious "horizon" and see if the cats react to that. Then if the cat still doesn't right itself, it seems like it is using acceleration rather than vision to determine which way is down.

⬐ nealabq

Do we know it's visual? If you drop a cat in darkness (onto a pillow of course), will it right itself? What if you put it in an elevator that accelerated downwards at 1G so the cat fell to the ceiling? How does it know which way is down? Is it the horizon, or the bright sky, or does the cat remember the scene before it was dropped? Or is it the air rushing by?

⬐ cecilpl2

> Then if the cat still doesn't right itself, it seems like it is using acceleration rather than vision to determine which way is down.

This is not possible because objects (including cats) in freefall do not experience the sensation of acceleration.

⬐ smaudet

I think that's the point, they do not experience acceleration in zero g, which is the point of testing visual/aerial cues, which DO remain in free fall (electric fans, horizons).

I'm fairly sure you just read this backwards, thought I'd explain better.

This is interesting because, it provides information on how the cat brain might function - if they have accelerometers embedded in their brains, that is quite interesting, but somewhat unlikely (birds have magnetic sensors, not impossible or unprecedented), if, perhaps more likely, they have incredibly sensitive hearing or sensation on their fur, which they interpret as acceleration, that is also interesting, in a different way.

⬐ haneul

This is possible because in free fall on earth, there are directionality signals when blinded - for example, air resistance.

⬐ true_religion

Before a cat (or any object) experiences free fall, they experience acceleration so they know which way gravity is pulling them. Surely, a cat can remember what happened to it in the last few seconds and instinctually put its feet in that direction?

⬐ throwawayben

You don't feel any acceleration from gravity - it can only be determined from other senses like vision or the air flowing over your skin/fur

⬐ true_religion

What about inner ear sensations? Or the pressure of your organs pushing against one another?

⬐ cecilpl2

None of the above. This is because every atom in your body is accelerating at the same rate, so there's no pressure involved.

⬐ moron4hire

Any fall is a weightless environment, whether it's in a plane or off a building.

But in a plane, you don't have the air rushing past your whiskers.

⬐ ce4

Probably the same reason as discussed here recently: A naked skydive inspired a way to keep pilots oriented in flight (military.com) https://news.ycombinator.com/item?id=30959126

Edit: Probably that submission led to this one.

⬐ teraflop

But a cat free-falling from a height is also weightless, and yet they manage to right themselves using perceptual cues. The point of the experiment is to see how the behavior changes when the sensation of weightlessness is presented for an extended period of time, without the "falling" motion (relative to the perceived local environment) that normally accompanies it.

⬐ ars

This is what most of the people replying here are missing - all falls are "free fall" (zero g).

Yes the cat manages, but not in this situation.

⬐ NovemberWhiskey

The cat's system has memory. The transition into free-fall from a state where it was previously experiencing ground reaction force provides a cue for the self-righting reflex on the direction of "up/down". Apparently even blind cats can self-right based on their vestibular system.

⬐ unfocussed_mike

Dear scientists, for the love of all that is good, don't repeat this test.

Cats aren't ever going to forgive you for engineering a situation where they are seen to land in an ungraceful way, and there's a tiny chance that they will make us all pay when they evolve opposable thumbs.

⬐ frederikvs

Cats can't be bothered to evolve opposable thumbs. They have human staff to take care of anything that requires an opposable thumb, why would they bother doing it themselves?

⬐ cpsns

I have 3 cats with “thumbs”. Believe me they’re working on evolving them.

⬐ tenebrisalietum

Cats are crafty enough to both survive and plan for a post-human landscape, such as climate change, World War III, or supply chain issues driving up cat food prices.

⬐ Zobat

As suggested by some I guess cats would (eventually) learn to move in zero-g. I also guess that these cats didn't get enough time to "learn" what to expect and their disorientation kept them from turning to land on the oncoming wall. Their eyes tells them that the "human" is sittning on the floor and for some reason they're floating towards the ceiling.

It's not like cats evolved in a place where the direction of up changed that often.

I wonder if "zero-g cats" would learn to use their claws to hold on to "walls" (provided suitable surfaces were provided).

⬐ arwhatever

Am not surprised that zero G severely disorients them, already knowing how a much a simple piece of tape disorients them. :-)

https://youtu.be/mdBegLNE6OU

⬐ alex_young

What was the zero-g cat dropkick at the end about? Seems pretty unethical. https://youtu.be/O9XtK6R1QAk?t=29

⬐ syllospri

That's not what a dropkick is. In a similar way to someone else pushed a cat, the man used his foot to move the cat away from the floor, but because of the zero-g, it caused the cat to move to the ceiling. Hardly unethical.

⬐ engineer_22

->Cats in zero-g lose their auto-righting reflex

Same for humans.

⬐ Melatonic

We probably should keep cats out of space - can't have them spontaneously evolving into Kzinti or something :-D

⬐ hosh

Poor cats. It looks like they are completely outside the parameters of their instinctual movements.

⬐ JumpCrisscross

SmarterEveryDay did a great video series exploring cats' self-righting mechanisms [1].

[1] https://www.youtube.com/watch?v=RtWbpyjJqrU

⬐ c22

Do cats in zero-g fall?

⬐ yupper32

It's relative.

⬐ Rebelgecko

I think this post is using a technically-wrong-but-still-more-or-less-gets-the-point-across definition of "zero-g"

⬐ glouwbug

Considering cats fall feet first and buttered toast butter side down you can power your spaceship by taping buttered toast to your cat's back and attaching the cat toast contraption to the rotor of a generator

⬐ FabHK

That experiment has been performed and indeed produces surplus energy, according to reliable sources :-)

https://www.youtube.com/watch?v=Z8yW5cyXXRc

⬐ ISL

To a reasonable approximation, cats only fall in zero-g.

⬐ ramesh31

It unironically makes me very happy to know that probably hundreds of thousands of our defense dollars were diverted to studying cats.

⬐ KarlKemp

This is the sort of experiment the biologists I know come up with towards the end of a long night.

⬐ williamtrask

This seems perfectly evolved. In zero-g they have no need to auto-right. They’re not falling!

⬐ eterevsky

Can a cat barf from disorientation?

⬐ progre

Yes.

Source: Have traveled with carsick cat.

⬐ rossdavidh

We know humans in zero-g often barf a lot, and cats often barf a lot, so it seems like cats in zero-g was a high-risk experiment.

⬐ carabiner

At the 28 sec mark, in the upper right one of the cats appears to be walking on the ceiling.

⬐ gene-h

the cat righting maneuver is a zero-angular momentum maneuver, so if the cat has a slight amount of spin, it is not possible for the cat to correct it. That may be what happens in the video

⬐ djmips

I believe that cats, like humans would adapt to weightlessness.

⬐ digitalsanctum

You had me at "cats in zero-g"

⬐ daneel_w

Since the ability depends on motion sensing, is this really surprising to anyone? In other news, kettlebells lose their "heavy" in zero G.

⬐ daveloyall

Off topic: Kettlebells might lose their "heavy" in micro-gravity[1], but they don't lose their "massy". I first read the term "massy" in some sci-fi book, by Heinlein, I believe.

It takes more effort to overcome the inertia of an object with a lot of mass than it does to overcome the inertia of an object with less mass. So, you can still use massy objects to work out in micro-gravity! Or so I've been led to believe by convincing fiction. :)

1: I've also been led to believe that micro-gravity is a better term than zero-g for the conditions experienced during free fall.

⬐ ghostly_s

(~1965), not (2011).

⬐ xallarap

None

⬐ draw_down

None

⬐ hjek

This is animal abuse!

⬐ madacol

http://paulgraham.com/heresy.html

⬐ pdonis

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.

⬐ tomxor

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

⬐ kadoban

> who could refuse

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

⬐ tomxor

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.

⬐ sophacles

I'm pretty sure the enemy gate is down.

⬐ Urgo

Yes, this!

⬐ fullstop

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.

⬐ lgessler

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.

⬐ pdonis

> 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.

⬐ throwawayben

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

⬐ Rygian

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

⬐ sdeframond

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

⬐ nemothekid

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.

⬐ ndr

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

⬐ mrexroad

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.”

⬐ jameshart

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.

⬐ sacrosancty

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.

⬐ pdonis

> 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.

⬐ sacrosancty

Fair enough.

Regarding your second point, I think everywhere in the universe has some direction of the gravity field, however weak it is, and that would be the downward direction. But yea in some ideal place with exactly zero gravity, there's that special case.

⬐ e_y_

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.

⬐ pdonis

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.

⬐ dfxm12

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.

⬐ pdonis

> 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.

⬐ miked85

Never underestimate cats.

⬐ mrtweetyhack

None

⬐ lisper

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.

⬐ btilly

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.

⬐ state_less

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.

⬐ pdonis

Yes, this makes sense.

⬐ JumpCrisscross

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-...

⬐ pdonis

> 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.

⬐ sandworm101

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.

⬐ can16358p

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.

⬐ screye

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.

⬐ lisper

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.

⬐ bhedgeoser

Do they?

⬐ tailspin2019

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

⬐ throwanem

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

⬐ vageli

> 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.

I don't understand, how can you be falling if you are in zero g? Doesn't that mean you are experiencing gravity? What forces would be acting to make the cat fall if not gravity?

⬐ MarkMarine

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

⬐ dotopotoro

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

⬐ MrPatan

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

⬐ metrognome

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

⬐ IncRnd

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

⬐ metrognome

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.

⬐ IncRnd

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.

⬐ metrognome

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.

⬐ IncRnd

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?

⬐ sly010

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?

⬐ None

None

⬐ thunderbong

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

⬐ onion2k

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...

⬐ sandworm101

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.

⬐ noizejoy

> They see it coming and understand what that means... Unlike sperm whales.

… you may have confused cats with petunias :-)

——

For those not familiar with the reference: [0] https://www.goodreads.com/quotes/198068-another-thing-that-g...

⬐ dwiel

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

⬐ sandworm101

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

⬐ geph2021

  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.

⬐ sandworm101

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.

⬐ gricardo99

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".

⬐ sandworm101

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

⬐ chrischen

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?

⬐ klysm

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

⬐ sandworm101

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.

⬐ IncRnd

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.

⬐ vba616

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

⬐ sandworm101

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

⬐ sacrosancty

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.

⬐ chewbacha

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.

⬐ ehnto

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.

⬐ Teknoman117

> Unlike sperm whales.

Or pots of petunias.

⬐ noizejoy

Actually the petunias *did* know [0]

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

> It will go upwards, yes, but only because of an upwards velocity.

In this picture, what imparted the upwards velocity to it, given its starting point on the ground?

>> a falling ball [...] in reverse

> it will still be accelerating downward.

It seems strange to ask this of someone with your choice of username, but for the sake of anyone else who is reading along in this thread :

Which way does a freely-falling accelerometer point? [1]

(Ignoring the atmosphere.)

- --

(But if we don't ignore the atmosphere, in the limit where a smartwatch inside a dropped wall reaches terminal velocity, which way does the accelerometer point? If the smartwatch can move freely within the ball, where does one expect it at terminal velocity, and where does one expect it far from terminal velocity? cf. "vomit comet" [5]).

- --

[Now make the ball transparent to a broadcast of the accelerometer's reading. Who is "more right" about the interpretation broadcasted value: you standing below and observing it moving ever faster towards terminal velocity, or a skydiver who manages to tuck into a shape that allows her to remain alongside the ball-and-accelerometer for some time at the start of their descent? What do they make of your on-the-ground accelerometer as they are (a) far from terminal velocity, (b) as aerodynamics separates one from the other?]

- --

I agree that the value of the accelerometer on the ground with you does not change if we run a film of this experiment backwards; it's the only one that should remain effectively constant throughout.

As we take the anti-falling ball from resting on the ground -- can we agree its accelerometer will be at the ground-facing end of the ball, and that it will have the same reading as your always-on-the-ground accelerometer? -- to a drop balloon hovering kilometres overhead, what do you expect the direction of the accelerometer to be? How does the magnitude evolve, approximately? (In the human-time direction, experimenter lifts the ball off the floor of the gondola of the balloon, carries it to the door, extends her arm, and drops the ball. The ball falls and eventually crashes onto the ground right beside you. Let's pretend we absorb bounces and avoid bursts and breaks of the ball resp. accelerometer inside the ball.)

- --

> It is velocity that switches signs

(with the caveat that we are supposing that we are working with a family of systems of coordinates, and coordinate velocity, and a way for all parties to measure that; or that everything is moving slowly with respect to light and aren't aiming for maximal precision, so don't engage Pound-Rebka and similar results if we want to measure spectroscopically rather than kinematically (or astrometrically, per IAU [2]) [3]. More generally, velocity can get really complicated especially when we are dealing with a system in which matter's coupling to gravitation is what's under study.)

Taking a different view, Baez says, "In general relativity, we cannot even talk about relative velocities, except for two particles at the same point of spacetime -- that is, at the same place at the same instant. The reason is that in general relativity, we take very seriously the notion that a vector is a little arrow sitting at a particular point in spacetime. To compare vectors at different points of spacetime, we must carry one over to the other. The process of carrying a vector along a path without turning or stretching it is called `parallel transport'. When spacetime is curved, the result of parallel transport from one point to another depends on the path taken! In fact, this is the very definition of what it means for spacetime to be curved. Thus it is ambiguous to ask whether two particles have the same velocity vector unless they are at the same point of spacetime."[4] and I find it hard to disagree.

- --

[1] If you have an iPhone or iPad, I highly recommend https://phyphox.org/ and a case like https://griffintechnology.com/survivor and a visit to a grassy or snowy park where you can give your device a good hard throw (or e.g. climb a tall tree or form a human pyramid and drop it). Don't rely solely on gedanken experiments or search-engine results. Go measure! What a pity Einstein didn't have these tools for his jumping-off-a-roof, and that Newton didn't for his apple tree.

You'll want to consider https://phyphox.org/experiment/acceleration-with-g/#more-680

[2] Soffel et al. The IAU 2000 resolutions for astrometry, celestial mechanics and metrology in the relativistic framework: explanatory supplement. Astron. J. 126(2003), 2687–2706 ( https://arxiv.org/abs/astro-ph/0303376 ).

[3] Lindegren, Dravins. The fundamental definition of ‘radial velocity’. Astron. Astrophys. 401(2003), 1185–1202 ( https://arxiv.org/abs/astro-ph/0302522 ).

[4] http://math.ucr.edu/home/baez/einstein/node2.html

[5] For example https://www.youtube.com/watch?v=i2KrEH-8lJw https://www.youtube.com/watch?v=O9XtK6R1QAk

On point 3, compare how well a cat's algorithm for landing on its feet works in its evolutionary adapted environment to how well it works in 0-g: https://www.youtube.com/watch?v=O9XtK6R1QAk&feature=youtu.be

On point 2, I feel you should caveat that's only if group selection is a thing, which many don't think it is.