I don't understand gravity. The popsci explanation is that it's created by the curvature of spacetime and objects aren't actually attracted to one another but are only following that curvature. But that only explains why objects that are already moving fall down. Why would a stationary object follow that curvature and fall? If I drop a pen, where is that force coming from to push it down? Is it because they're moving through time? Please help
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Like the cage fairground ride but inverted, an effect of mass.
>Is it because they're moving through time?
As you already said, it's curvature of spacetime, not just curvature of space.
Instead of time maybe it has something to do with the planet's speed? Earth's still moving over 100,000km/h through space so objects here aren't truely stationary. When they move through space they move through Earth's spacetime curvature, therefore falling down?
No, movement is all relative, the earth is moving that speed relative to the sun, but a different speed relative to the galaxy, or relative to other galaxies.
The whole point of the spacetime explanation of gravity is to show that it isn't really a force. It might seem odd but an object under the influence of only gravity is at rest, the same as an object under no apparent forces. The force we feel as gravity is really the normal force resisting our downward trajectory.
If you were in a closed box in the middle of space, and suddenly a planet appeared next to you, you wouldnt be able to tell anything had changed until you hit the atmosphere.
Your hypothesis is disproved by gravitational waves, if it was not a physical effect they would not propagate
Gravitational waves are propagating spacetime curvature. They fit excellently into my explanation. Gravity is a physical thing, it's just not a force the way EM is, it's a consequence of curvature.
>Gravitational waves are propagating spacetime curvature
spacetime curvature is a mathematical tensor field to quantize the effects of gravitational influence, gravity is not "spacetime curvature", its effect is the influence on the parameters of space and time
>Gravity is a physical thing, it's just not a force the way EM is
They are both forces propagating through a field of materia, like soundwaves, literally the exact same mechanism
Spacetime is just a mathematical analogue because there is no verified quantization of gravity
this is word salad,
>Why would a stationary object follow that curvature and fall?
When you put a ball on a slope, it falls down. When you put an object on a spacetime curve, it falls down.
Sounds like circular logic to me. Why does it fall down? Because gravity. But what is gravity? The object falling down.
>Sounds like circular logic to me.
It is. In reality, all matter is a manifestation of electrical activity and what we call gravity comes from electromagnetism.
https://vixra.org/pdf/1608.0426v6.pdf
based knower
https://iopscience.iop.org/article/10.1088/1742-6596/1956/1/012017
Well yes, gravity is an observed fact that things fall down. What we discovered with general relativity is that they follow spacetime curvature when falling.
Yeah, that's because "gravity" is a descriptive term. It's like how 'heat' is a descriptive term, when in reality it is something more like the transfer of thermal energy between particles.
>When you put a ball on a slope, it falls down. When you put an object on a spacetime curve, it falls down.
you said the same thing twice
The point was an object at rest has no reason to keep following the curve (fall down), only already moving objects would actually follow the curve. If you stop your car while you're turning left on a road your car won't continue to turn left. It's now stationary and the curve is irrelevant.
The question was why objects keep following the curve of spacetime even at rest
>The point was an object at rest has no reason to keep following the curve (fall down)
Except for, you know, gravity?
gravity isn't a product of spacetime being curved? what do you mean
Gravity is the force, spacetime is the mathematical model
Its the other way around, everything in space moves and to stop it there is force required. Here on earth its a bit unnatural because of friction.
>The question was why objects keep following the curve of spacetime even at rest
You've no doubt seen a 2D graph of x vs t in a high school class. If something sits at a constant x for all t this still corresponds to an entire vertical line in the 2D "spacetime" graph.
In 4D spacetime similarly something at rest in some spatial coordinate system still involves an entire trajectory in spacetime. The claim of general relativity is that a trajectory of an object freely falling due to gravity is "straighter" than the trajectory of the object at rest. Speaking more precisely the trajectory of the freely falling object is a geodesic in spacetime, whereas the trajectory at rest is not.
It is because they are moving through time. A stationary object on a spacetime diagram looks like a straight vertical line. When space is curved, the line on the diagram also curves, seeming to move in space relative to everything around it despite the fact that it is actually stationary.
Is this the reason that time moves differently in proximity of "heavy objects"? Like a planet or black hole
Its popsci. GR does not say what causes gravity. It just stages if gravity is, what are corrections if speeds are high or masses are great.
>It just stages if gravity is, what are corrections if speeds are high or masses are great.
Why are so many posts utter rubbish like this now? Is it an increasing number of chat bots? Not even ESLs and schizo posters make such gibberish.
You are really stupid, aren't you? Special relativity is a correction when speeds are near the speed of light. Similarly, you correct physics in general relativity. But it takes more general situations in account, including accelerations. Gravity causes accelerations. Thus gravity is a prime example for general relativity. General relativity can be used for electromagnetic forces too. General relativity does not know what causes gravity.
>I don't understand gravity
You don't need to
>aren't actually attracted to one another but are only following that curvature
It's all geometry
Nobody actually knows what gravity is, per se, we just know how it works.
Imagine if we discovered that dark matter wasn't real and was simply explained by galaxies moving at different absolute speeds
The pen is not stationary, it is stationary to your point of reference. It is a moving object to a point of reference outside of the gravitational field of the Earth. If there wouldn't be a "floor" stopping the pen it would fall towards the core of the planet. As the curvature of Spacetime on that point is greater than the escape velocity of the pen. So yes, gravity is curvature of spacetime. Where it comes from, besides from Mass and the interaction with the Higgs Field, is unknown.
>be me, 2nd grade
>Ask class and teacher why we stick to the earth
>entire class yells at me
>uuuh anon, because le GRAVITY, are you a moron??
>no shit, but WHY is there gravity, I ask
>blank stares
>tfw physicists still can't answer a question I had when I was 7
heh, whose a moron now, Ms. Jacobs?
>cover a spherical object with water
>spin it at high speed
>all the water flings off
>cover a planet with water
>spin it at high speed
>gravity pulls the water in
Why is this?
>more mass has higher gravitational influence than less mass
A very alien concept, I know
Why do astronauts not stick to the outside of a space shuttle? Or even to the floor on the inside? The space shuttle has more mass and they're in an environment where Earth's gravity isn't influencing them, so shouldn't the smaller astronaut mass be attracted to the larger space shuttle mass? Why do they float around freely?
>Why do astronauts not stick to the outside of a space shuttle? Or even to the floor on the inside?
1. gravity is extremely weak, you require great masses for great influence
2. the gravitational potential is not great enough and the gravitational field will stabilize
If you and a screwdriver were placed in some space 5 feet away where there were no other forces acting on you the screwdriver would never come to you, because although your mass causes an influence on the gravitational field, the energy would eventually stabilize and the "pull" of the gravitational field would stop
>The space shuttle has more mass
It's not enough
>they're in an environment where Earth's gravity isn't influencing them
Bruh, Earth's gravity is influencing the moon, why in goddamn wouldn't it influence something barely above the atmospehere
>and they're in an environment where Earth's gravity isn't influencing them
They are still in the gravitational field of the earth.
>Why do astronauts not stick to the outside of a space shuttle?
Because gravity forces are tiny and trumped by orders of magnitude by other forces, circulating atmosphere drag been most powerful. If you drop object in space ship they end up primarily sucked on the grills of air intakes of life support system, or in the pockets of turbulence.
also the atmosphere is a fluid which encapsulated the earth like a shell
Because the earth is massive enough to overcome the force propelling the water with its gravitational force. The oceans are constantly moving on the surface of the earth.
Here's a crash course on gravity:
>Force
Gravitational force = (G*m1*m2)/r^2, where G is the gravitational constant, m1/m2 are the masses of the two objects in the system, and r is the distance between their centres of mass.
>Potential
The integral of (G*m1*m2)/r^2 gives you -(G*m1*m2)/r, which can be equated to mgh with some algebraic substitution.
>Acceleration
Since gravitational force fields follow an inverse square radial distribution, an object under the influence of gravity appears to accelerate over time.
>Escape velocity
In order to escape from a gravitational field, you need to travel at sqrt(2G*m1/r) metres per second. You can derive this by equating the kinetic energy of the moving object to the potential energy equation.
>Tl;dr
Revise calculus and it'll make sense.
I can't grasp this whole thing about space becoming stretched as one approaches a spherical object like a planet or star. So you get more space the more you become cramped? No matter how I try it doesn't make any sense.
Imagine a normal 3d grid lattice and at the central point within the grid the cube is bigger than those cubes further out. Just how does that work?
If you want to re-conceptualize imagine the following. An object wants to move through spacetime for point A to B. In an "empty" space that object would move on a real-time basis to point B on a straight line. However when a Massive object is near its path it will pull that object towards it.
This would mean that 1) The object will not be moving the same distance from A to B but rather a longer, curved path around the Massive object as the Massive object keeps on pulling on the object until the object is out of its gravitational influence. 2) It will take the object longer to reach point B as it is being slowed down by the pull of the Massive object.
This seems that spacetime is stretched around this massive object. This is essentially the Spacetime curvature/stretch you would see when observing this interaction outside of the gravitational field (given light is less influenced by this field).
As for spherical objects as of the uni-directionality of gravitation towards the center. Your cube will also turn into a sphere given if its mass is large enough.
Honest question. Why do you waste your time typing out such utter shit like that? Are you really that ignorant or are you just having a giggle? Perhaps you are just practicing your English, or maybe you are just a chat bot.
OK but if time and space are the same thing how come we cant move backwards in time
>if time and space are the same thing
They aren't
>how come we cant move backwards in time
Because
1. Time isn't a force, but a measurement of physical interactions
2. It would require you to reverse the entropy of the entirety of reality, aka undo the actions of everything in existence up to the point you wish to go
That's because the thermodynamic laws of the Universe state that it will always flow towards a maximum state of entropy; not in reverse.
It is possible to revert information on a Quantum scale but that doesn't negate the general flow of time of the Universe itself.
https://news.mit.edu/2022/quantum-time-reversal-physics-0714
They aren't, but sci-fi is way cooler if we pretend they are.
pisses me off that nobody's given you the right answer in this whole thread of midwits. you're honestly on to the right idea, and your question is reasonable: why is a stationary object still affected?
the answer is that it's spacetime that's curved, not just space, and the fuller answer is that there are no stationary objects in spacetime. every object is moving at the speed of light in spacetime. "stationary" objects are moving at the speed of light fully in the direction of time. light itself moves at the speed of light fully in the direction of space. everything else is somewhere in between.
>why is a stationary object still affected?
BECAUSE GRAVITY
>there are no stationary objects in spacetime. every object is moving at the speed of light in spacetime
Nothing moves at the speed of light, the speed of light is the maximum theoretical speed of information propagation through matter, if your matter is heavy, requires more energy to move and is spaced apart (like air) the information propagation is slower (like sound) if your matter is light and densely packed together (like the electromagnetic field) then your information propagation is faster (electromagnetic waves), everything in reality is just the transfer of work from one system to another
Just shut up you illiterate ignorant fricking idiot.
If you don't like the answer then don't ask the question, homosexual
>the frickhead thinks his monkey brain gibberish is an "answer"
You fricking dumbshit fricks up every discussion with your moronic blithering
Everything is monkey brain gibberish to a mentally moronic monkey like (You)
moron
Can you show me a timestamped photo of you measuring the curvature of spacetime?
Bro it's literally in the room with us right now you're telling me you don't see it?
You want a timestamp of a measurement of time? Pick one homosexual. I'm not time stamping a picture of my clock.
I said spacetime.
Well I'd need a picture of a clock to measure the time half of it.
No, I want to see a physical measurement of the singular material called spacetime which supposedly curves in measurable ways.
if you perceive no time when moving at c, does that mean the universe would speed up and instantly run its entire course from your perspective if you became absolutely stationary?
No, things would merely appear to you as passing by faster because you're taking in more information due to moving so fast, but it's not like you would perceive anything because your atomic reactions would stop working and it's not really possible for you to travel at lightspeed due to electromagnetic drag, unless you were made of protons, in which case you would get very close to c, but not quite, due to the same effect, regular matter can't travel at c
>things would merely appear to you as passing by faster because you're taking in more information due to moving so fast
On the contrary, the entire universe would appear to be frozen in time, because relative to (you), you're at rest and the rest of the universe is moving at the speed of light.
Inertial time dilation is symmetric. Only in the presence of "gravitational" potential (including that caused by acceleration) is time dilation asymmetric. (This is the solution to the twin paradox.)
Wrong again. Keep quiet if you dont know and let your betters speak.
Go back to your trailer, trash.
Shut the frick up, babbling moron.
>cant understand basic science
>resorts to childish name calling
Subhumans like you should be banned from participating in any intelligent discussion.
>resorts to childish name calling
WTF AIbros, I thought spambots were supposed to be self-aware by now.
Nice, just keep derailing the thread with your bullshit posts. Arsehole.
>posts nothing of value in thread
>attacks others unprovoked
>shoots the messenger for noticing
No
The solution to the twin paradox is that one twin isn't in an inertial reference frame and has a shorter world line through spacetime. It has absolutely nothing to do with gravity.
Yes, in theory it does.
Shut the frick up your wiener sucking halfwit.
Gravity is an emergent property of multiple particles congregating together. Think of it as a dance until the floor sinks.
It's electrons all the way down, things with static charge affect small shit like paper trash too, why are scientists unable to generalize this?
One thing that popsci midwits often miss is that the "curvature of spacetime" is just an EQUIVALENCE. The force of gravity accelerates all things uniformly, which is EQUIVALENT to spacetime curving, but that doesn't mean there's a literal fabric of spacetime being stretched and pulled around. Spacetime is just a convenient mathematical model, and not a real physical substance.
The relativistic approach is purely geometric. Time is treated as any other direction in space and not like a position index like in most mechanics. The relativistic derivation doesn't discriminate time thus you don't find time derivatives treated differently. If any, they are always accompanied with space derivatives of the same order.
Think of kinetic energy, and how, in everyday uses, is expressed as 1/2 mv^2 or p^2/2m, such as v is the time derivative of the position vector. This expression is not independent of reference frame as it turns out in, yet is sufficient approximation for most uses from engineering to molecular chemistry to even non-relativistic quantum physics. In relativistic aspect it expressed as m ugu such as u is a tangent vector to position in a 4-dimensional space, and g is the metric given by diag(-1,1,1,1).
This is quite an abstraction, and turns out that classical mechanical laws derive from this at low speed. The difference is that the above expression is purely geometric and doesn't include any special derivatives. A position 4-vector given by x, will have its tangent 4-vector expressed as dx/ds with s being an arbitrary position index.
Now the premise here is that it is possible to express gravity in a purely geometric sense. Think of 2d surface given by x^2+y^2-z^2=0. We will think of this surface as a silly embedding of the actual 4-space into a 2-d surface parameterized with x and y and time parameterized by z. Now you draw a circle on that surface around the origin with radius equal to 1. As the circle is confined within the surface, if you need to move your circle you will have to change its radius. Its points will either get further away or closer as z tends to 0. The mouvement is natural and doesn't require any "force" acting on the circle. It just follows the curvature of the surface on which it sits.
You did not bring relativity so you can do this trick to classical poisson equation of gravity
> In relativistic aspect it expressed as m ugu such as u is a tangent vector to position in a 4-dimensional space, and g is the metric given by diag(-1,1,1,1).
No it isn't. Study basic relativity theory more before you write long rambling incorrect posts like this.
"ugu" is a non-dynamical constant if u means the derivative of the position 4 vector with respect to proper time, and it is something that depends on frame if you use some other parametrization.
Completely wrong.
Your grandfather must have really fricked your ass hard to damage your brain so much.
>a norm dependant on the space metric is not invariant under change of coordinates.
Did you study basic geometry?
What I am talking about goes way over the head of your elementary school education. Try graduating high school before posting your drivel dickwad.
>littering the thread with his autistic screeches
Yup. Time to sage.
Congratulations pigfricker, you have managed to spam this fine thread with your lingering mediocrity. Here's your "special" Big Frickhead medal.
>pigfricker
>fine thread
>lingering mediocrity
>Big Frickhead medal
Can you talk like a normal human being instead of with this edgy redditor "quip" speak, like you are delivering some epic stinger line in Rick & Morty?
That shit rots your brain, son.
It's the same shit every thread. You offer a veritasium tier explaination, people correct you, you fly into an autistic rage, then start regressing into quip speak.
And we all know it's you. I would be shocked if numerous people are adopting this dumb persona.
If you aren't willing to be corrected, you are going to go through life with this same, surface level understanding of physics.
He's obviously not me (the guy who called you out on your post saying the kinetic energy was given by the norm of the tangent 4-vector). Learn how to use an anonymous message board
>Oh no! I am not the same person! Not at all!
moron gets called out and tries to deflect while back tracking like a b***h.
I guess I'm not surprised, but you failed to understand my post. The tangent vector of a curve parametrized by the coordinate time is of course not invariant under changes of coordinates.
If you instead parametrize the curve by proper time (or arc length in a Euclidean signature) then the tangent vector is invariant under changes of coordinates. But the norm of the tangent vector is just a constant (1 in appropriate units) and has nothing to do with the kinetic energy.
I haven't seen a single actually good explanation of GR in this thread so here it is:
If you're near a gravity well, the parts of you that are closer to it are passing through time faster than the parts of you that are farther away from it. This produces a time gradient, which is an unstable low entropy state that the particles in your body try to fix by sliding toward that part that's passing through time faster, kind of like a chronological venturi effect.
That's assuming GR is real, which it probably isn't, but that's another topic.
>That's assuming GR is real, which it probably isn't
Relative measurements aren't real?
wrong and moronic
why do we consider GR still relevant,
if it explains 5% of galaxies rotation curve,
without the dark matter hypotesis ?
isn't GR a fascinating model,
unfortunately not fitting all data very well ?
isn't it time to move past it ?
why are we still banging our head against the wall with the dark matter thing ?
Because despite 100 years of people seething at relativity, nobody's come up with a better theory.
There isn't a better theory dumbass.
That's what I said, Tyrone. Work on your reading comprehension.
Frickboi, there is no better theory because it doesn't exist. That's different from pretending there MIGHT be a better theory, which is what you imply.
homie, you're literally arguing with voices in your head. Get that noggin' checked out.
Just work things by taking care of finite speed of em radiation. Remove any instantious forces. See what happens in stationary lab. Accept the fact where space ship is not equal to earth but has their clock delayed.
Somebody actually has come up with a better theory. QI accurately predicts galaxy rotation and universe expansion without any dark stuff.
criticizing st. einstein the infallible soience god of the atheists is antisemitic, thats why it isn't permitted
>why do we consider GR still relevant
Because relative measurements are a thing?
the problem is time isnt real and people just cannot wrap their heads around the concept.
time has got nothing to do with it frickhead.
Mass distorts Higgs field and this creates what we call gravity. Next question.
>just like unfold space bro
>Why would a stationary object follow that curvature and fall?
I also dont understand this. The curvature is 4-dimensional so the idea is that objects follow geodesics in this 4-dimensional space. However i dont know why it is the case that objects follow geodesics.
It could just be like newtons laws where an object is said to just keep traveling in a straight line in space. A straight line is a geodesic in flat space.
I dont think this is something that is proven, it must be an axiom of general relativity. Objects follow geodesics, everything else follows from there.
if you roll a ball down a slope, guess what? IT GOES DOWN! Shouldn't be hard for even your peanut sized brain to understand.
Because as others have said. Mass curves spaceTIME, not just space. So mass is also causing a part of your velocity through time to be oriented towards that mass (which is why you move toward the mass). The closer to the mass you are, the greater the curvature, the more of your velocity through time is oriented toward the mass.
The extreme case is when you reach an event horizon of a black hole, where your velocity through time is completely oriented toward the blackhole.
>why it is the case that objects follow geodesics
Its just the principle of least action.
Mass does not curve space at all. Its only spacetime that can be trannified
There's one extra mass part to all mass that is lateral. That's the vacuum.
All mass is a mass in itself. Due to vacuum.
The universe is 20* it's observable size because of additional vac.
It's a perfect vacuum full of Higgs bosons. The current state of modern physics.
Spacetime is just a model. Higgs boson is pushed by electrons and this creates a gravity well.
>Its just the principle of least action.
Call it whatever you want. Its just an axiom. The principle of least action is meaningless since the action is undefined from any basic principles, you either make it up or find one that matches experiments.
>Because as others have said. Mass curves spaceTIME
Cool but i dont remember asking you
I cant believe you actually posted this absolute nonsense. Did you get a primitive chat bot to write it? Instead of posting gibberish try learning some real physics.
It's what "mass curves spacetime" is mathematically, you colossal moron.
>Cool but i don't remember asking you
>"Why would a stationary object follow that curvature and fall?"
You literally did ask.
>You literally did ask.
I said it was an axiom, you went NOO ITS THE ACTION.
Its the same. An axiom. Something has to describe the motion, a force, a hamiltonian/lagrangian/action/force. Its not something you can prove, its an axiomatic starting point.
General relativity states that orbits are geodesics in curved space. Why? They just are.
This can be expressed very graphycally: When you find what these geodesics are for weak gravitational fields you can draw their projection in 3D space, since its impossible to draw them for 4D. Well the drawing is just that of a normal orbit, an ellipse or in general a conical section (the flat shape you get by cutting a cone with a plane, at some angle), so general relativity reproduces a result of classical gravity for weak fields.
>Why would a stationary object follow that curvature and fall?
There are no stationary objects. Everything is moving all the time. The only reason things appear stationary around you is becauae the stick to a bigger object that is not stationary.
>There are no stationary objects
the manifold of reality is stationary
you don't see it bc photons mostly dodge it, but imagine they didn't. What would you see? Where would "straight ahead" go, then? We see photons but our bodies are atoms.
Absolute state of this thread. No you are not smart at any level if you spend time arguing like this.
who said I'm smart?
Mass warp.
glowies attempting to derail this thread because it must be hitting close to something they don't want you to know....
>I don't understand gravity.
No one from this species does.
There is no need to mystify it.
No.
One based scientist made an article about sound clocks, technically it was relativity of sound, not light. Sound is waves of air. Technically one could make general relativity of vacuum cleaner. Like gravity sucks mass, so does it.
If someone made a vacuum cleaner that would suck faster than 343 m/s. It would be a black hole in airspace time continuum. Now laugh about it. Then, after you have laughed, think about it. And realise the true nature again.
The earth is flat and everything gets pulled downard. Ez
Popsci exaplanation of gravity is just a circular argument
Think of the classic rubber sheet analogy. Massive objects warp the rubber sheet, which in turn makes other massive objects roll down the rubber sheet incline. Why are the massive objects warping the rubber sheet and rolling down its incline in the first place? Because of gravity pulling them down. What is gravity? I'm glad you asked. Think of the classic rubber sheet analogy. Massive objects warp the rubber sheet…
It's rubber sheets all the way down.
(Real answer: it's because energy warps time much more so than space.)
How bullshit is this?
>So, understanding relativity and gravity is mainly a matter of overcoming our resistance to a peculiar, embarrassingly implausible idea: that while you read this, the space that contains our planet — including every molecule in our bodies, everything we can see out to the horizon, and every ruler on its surface — will double in size.
>So this relativistic idea of “space expanding” is not about growing distances between galaxies. It’s much more disruptive and even disturbing. It’s about the size of the atoms, rulers and objects all around us that we consider fixed in scale — including our bodies and our planet — actually continuously growing. And one result is everyday gravity.
https://davidlevitt.medium.com/space-itself-is-expanding-gravity-and-general-relativity-explained-6395aa2e4d69
I often see explanations of general relativity that interpret the components of the metric tensor as space getting stretched or shrunk. I don't like these explanations because the components of the metric tensor depend on which coordinate system you're using, and which coordinate system you're using is an artificial choice. The simple way to explain the problem: If space is shrinking or stretching, what is it shrinking or stretching relative to? The answer is that all objects are shrinking or stretching relative to a particular coordinate system. But the coordinate system was an arbitrary choice, and in another coordinate system, the object you said was stretching might be shrinking instead.
This page seems to be inspired by this sort of interpretation of the metric tensor, although it's not particularly clear how or if the notion of "space expanding" he uses is related to the metric tensor. At one point the page says
>when space and rulers are expanding, the distances between things — as measured by those rulers — are shrinking.
The fact that the distance between things falling towards each other as measured by rulers is shrinking at an accelerating rate is the real phenomenon here. I would suggest ignoring everything this guy says about the rulers themselves expanding. If whenever he says something like that, you perform the substitution "things expanding -> distances shrinking" then the page seems to be accurate. In particular, if you flip the sign in the equation he writes and instead write
[eqn]frac{d^2V}{dt^2} = - 4 pi G M[/eqn]
you will have an accurate mathematical description of what happens to the volume enclosed by a surface drawn through a bunch of free-falling objects.
Also, if you perform this substitution you can avoid reaching a very wrong conclusion from the page. This guy keeps using "expanding space" to indicate things getting closer together as measured by rulers. But when astronomers say the universe is expanding, they mean the opposite: Things are getting farther apart as measured by rulers. I could imagine someone reading this page and wrongly reinterpreting what the astronomers are saying.
>Gravity is exactly the opposite of what we usually think — that it’s NOT a pull downward, toward the surface of the earth — it’s a push outward
Anyone who says this should be shot
This part is accurate. The ground has a proper acceleration upwards. Proper acceleration is, of course, measured relative to what a free-falling body would do.
you should "reach" lightspeed in around one year, at 1G.
You don't reach lightspeed ever at constant *proper* acceleration. And reach lightspeed relative to what?
well clearly. I was just going long with the analogy. we accelerate at 1G yet go nowhere.
We do exactly that. If you somehow took a strip of spacetime from along my path through spacetime, cut it out of curved spacetime and pasted it onto flat spacetime, then in one year I'd be going 0.77 times lightspeed (not lightspeed as you said). Of course that's an imaginary situation, and since the spacetime is actually curved, I don't go anywhere.
That's in one year of proper time, to be precise so you can reproduce the calculation. One year of time in the reference frame in which I would initially be at rest would get me to a smaller speed.
One year proper time gets me to 0.77c, two years to 0.968c, three years to 0.9959c. That's important because I can use that pasted strip of spacetime as a perfectly good mathematical model for things happening inside the thin strip, so I can't be doing silly things like breaking the lightspeed barrier.
>Gravity is exactly the opposite of what we usually think — that it’s NOT a pull downward, toward the surface of the earth — it’s a push outward
Anyone who says this should be shot
>Proper acceleration is, of course, measured relative to what a free-falling body would do.
How is the body falling if gravity is pushing it away?
Whoops, post window glitched lel
His precise wording is bad here. Obviously it's not gravity pushing me outward, but the normal force of my chair on my buttocks. But the immediate local cause of falling objects getting closer to the ground at an accelerating rate is the upward proper acceleration of the ground, caused by the forces from the rock below.
Now if we replaced the earth with a black hole of the same mass so there was no ground, those falling objects would still be getting closer to the black hole at an accelerating rate, just as they are in reality getting closer to the earth's center at an accelerating rate. This phenomenon does not have anything to do with the ground. But it is not a local phenomenon; it is caused by the curved spacetime between the falling object and the center of the earth.
>But the immediate local cause of falling objects getting closer to the ground at an accelerating rate is the upward proper acceleration of the ground, caused by the forces from the rock below.
>But it is not a local phenomenon; it is caused by the curved spacetime between the falling object and the center of the earth.
Then which one is it, these are two very conflicting statements.
Read what is being said more carefully.
It's been said before, but the reason things fall is because spacetime is curved, not just space. For a simple analogy that's better than the rubber sheet, take the surface of a globe and imagine that north is forward in time. Starting at two points on the equator, draw two "lines" (actually great circles) to the north. Each point on the line represents the location of an object at a particular time. At the equator, the lines are parallel to each other, meaning the rate of change of the distance between the lines as you move north starts at zero. But as you go further north, the lines begin to get closer together at an accelerating rate, and they eventually meet at the north pole.
Of course, this is just a two-dimensional slice of what's going on. I've posted some techniques for visualizing curved spacetime in its full four-dimensional glory a few times, which you can find in the archive at:
https://warosu.org/sci/thread/14766902#p14772501
If you want to go into more depth, a good starting point would be Taylor and Wheeler's introductory-level and free book "Spacetime Physics". It's mostly about special relativity (spacetime without curvature), but you'll need to grok that before you can understand general relativity (curved spacetime as a theory of gravitation). It does talk about general relativity a little; I got the attached picture from Chapter 9.
https://www.eftaylor.com/spacetimephysics/
You should also start learning about tensors, which are an important mathematical tool used throughout general relativity. With special relativity and tensors under your belt, you'll have no trouble learning general relativity.
To explain it a little better, suppose in the pic in B doesn't want to meet A but instead stay the same distance. Then he has to constantly turn to the right. If he were turning at the same rate on a flat surface, he would start moving away from A. But he doesn't, because of the curvature between him and A.
>If I drop a pen, where is that force coming from to push it down?
it's coming from a massive object, every object with mass exerts a gravitational pull on every other massive object in the universe, they're pulled together like magnets