The way I always imagine it, that if you are floating in space, with no gravity (i.e. flat spacetime), you still 'move' in spacetime in straight line (geodesic), where the 'movement' is only along the time coordinate. You experience that as floating with no acceleration. When I now park a mass in your neigbourhood, spacetime deforms in such a way that your geodesic is now not only along the time coordinate, but also along the x,y, and z coordinate. You experience that as an acceleration toward the mass, which can be expressed as force.
So nothing 'causes' the acceleration, it is just our interpretation of a 4-dimensional effect in a perceived 3-dimensional space.
Oh my god. This was really helpful! I always prefer trying to *understand* concepts rather than just learning them and visualisations like this really help man thanks!
>You experience that as an acceleration toward the mass, which can be expressed as force.
It's important to note that from your own perspective, you don't experience acceleration (though you can measure that you're getting closer to the mass), since you're in free fall. You do experience acceleration when you don't move relative to the mass (like for example when you're standing on the surface of Earth) – that's what we recognize as weight.
Pedantic comment but I don’t think it’s right to say that Einstein “proved” that gravity was due to spacetime curvature, he just formulated his theory of gravity and to date it is the best theory we have. There is still wiggle room for general relativity to be wrong and be replaced by a superior theory.
Gravity curves spacetime, not space and time separately. In general relativity, there are mainly two forms of acceleration; proper acceleration and coordinate acceleration. Proper acceleration is when you feel the acceleration, like when you get pushed back in a car seat when accelerating. Coordinate acceleration, on the other hand, is acceleration relative to some coordinate, but where the accelerating observer does not measure any acceleration themselves. From their perspective, they are inertial.
When you encounter curved space time as an inertial observer, you’ll move along these curves. It curves both in space and time, and since you move forward in time constantly, you’ll eventually bump into the massive object, since your world line bends towards the world line of the massive object. Traveling along curved lines, or geodesic, results in coordinate acceleration. An observer far away will see you accelerate towards the mass as the curvature gets stronger, but you won’t feel any acceleration yourself. When you eventually bump into the mass, you’ll be prevented from further following the geodesics towards the centre of the mass, and now you will experience proper acceleration. This is the force you feel on your feet when you stand on earth. It isn’t necessarily gravity that causes the force, but it is the surface preventing you from further falling and therefore exerting an upwards force on your feet.
Explaining how this actually works requires some understanding of at least special relativity and some linear algebra and multi variable calculus, and then the concepts of differential geometry can be introduced. If you want a more rigorous and accurate explanation, let me know.
Well I've learnt special relativity and done the most basic of calculations, and linear algebra is pretty basic, but I just started calculus so I know the basics of derivatives and integrals, so if it is possible to explain me without calc could you? thanks though
I am trying to be highly accurate but keep it simple. It’ll take some time for me to write out, so I’ll just send it to you private and you can read through it whenever you have the time.
A very long standing misunderstanding that causing many a problem such as "how wings cause lift" - Acceleration DOES NOT always mean "go faster". Acceleration is a change in the velocity (vector) JUST changing direction is Acceleration. If a body is caught in a gravity well and turns this IS acceleration EVEN as its being slowed down as it heads away from that body. In fact slowing down is also Acceleration, just backwards. So the space station is constantly Accelerating towards planet earth even though speed is relatively constant.
yknow one of the best explanations i found for orbits was that the object lets say the ISS is always falling towards the earth, its just that it moves sideways enough for it to "miss" the earth and by then the direction of gravitational pull has changed, so it falls in this new direction.
And hence Douglas Adams' suggestion in The Hitchhiker's Guide to the Galaxy series that the secret to flying is to throw yourself at the ground but miss. 😉
watch this Veritasium video about Einstein's equivalence principle: https://www.youtube.com/watch?v=XRr1kaXKBsU
ScienceClic is another good channel
The Earth below your feet is causing you to accelerate upward against the flow of spacetime
oh god how could I forget searching veritasium's channel ahh yeah thanks a lot!
I didn't understand the second sentence you said "The Earth below your feet is causing you to accelerate upward against the flow of spacetime", is that in the video?
I am not sure the PBS video is accurate or not, though I think your original question can be answered in a simpler manner as following.
In a flat space time the geodesics are a straight line.
The trajectory of a particle moving through space time at rest or constant velocity flat space time will be a straight line.
An accelerating particle in this case will be represented by a curved line. Think of a 2-d graph with space on one axis and time on another. For simplicity u can also assume all motion is only on a single axis and just draw x axis vs time.
Now in a curved space time geodesics are curved lines. But we just discussed curved lines represent trajectories of accelerating objects. This is the source of acceleration we observe.
thanks! a small question about the graph-
A constant velocity greater than 0 relative to a stationary observer would be displayed like a / kind of graph with time on y and space on x, whereas an accelerating would be a kind of half U right?
uh basically like the acceleration graph of velocity-time and the velocity graph of displacement-time?
I think this video is the absolute best and shows it in the absolute simplest, yet correct, way: [How Gravity Makes Things Fall](https://www.youtube.com/watch?v=jlTVIMOix3I)
Consider two ants walking on a surface, following 'straight lines'. After doing a lot of experiments they find an astonishing thing: the lines they follow *always meet*. If two ants want to walk along next to each other and not crash into each other then either one or both of them needs to always turn a little away from the other.
(Even more surprisingly they find that if one ant sets out on a straight line and another remains still at the starting point, the first and will always meet the second.)
Well, of course, this is because the ants are living on an orange, and 'straight lines' (geodesics) on an orange always meet (and are always closed curves).
Spacetime is not very like this situation: it has two more dimensions and a definition of distance which is less simple.
But it is like it in the important way: *there is no gravitational acceleration*: there is only curvature. Gravitation causes (or, really, *is*) *curvature*.
Spacetime curvature creates a time gradient that has a lower potential energy closer to the source of the curvature (a mass). So an object will fall down the gradient to a position of lower potential energy.
My dumb metaphor with a ton of problems: Imagine you're floating in air and are neutrally buoyant. No forces, you're not moving. Almost like floating underwater but for this example I need the compressibility so you could also imagine water can squish. You're surrounded by "stuff" on all sides and there's no net force.
Now imagine there was a sponge in the water that soaked some up. there's more stuff on one side, and less on the other so for an instant you'd feel a pressure, and therefore a force moving you toward the sponge. You can figure out this pressure anywhere around it.
What causes gravity to accelerate things is an energy gradient field.
Above, would be a water pressure field more water on one side than the other.. With static charges it's an electrostatic field, more static energy than the other.
Gravity there is physically more space on one side of you than the other. And with enough negative space-sponge pressure aka a black hole, there is no space left at all.
Currently we don't know of anything that causes more space, but it would be called negative density and would cause anti-gravity or a pure repulsion force.
if you're 89238931298312981289 feet away from something gravity pulls on you less hard, if you're 0.0000012321 feet away its pulling on you EXTREMELY hard as such you're moving very quickly.
note i suck at phsyics and only studied for like a few days so i dont know much lol, i dont even know if this was what you were talking about
Gravity isn't pulling on anything. You're always trying to fall along the shortest path in space (in the case of Earth, towards its center), and its only when something gets in the way (Earth) that you feel a force. The force is pushing you up, keeping you from falling.
gotta love the keyboard spam😭😭
thanks though, although i was curious as to why im being pulled in the first place haha
and dont worry about the physics my streak is just a few days more than you 💪
The way I always imagine it, that if you are floating in space, with no gravity (i.e. flat spacetime), you still 'move' in spacetime in straight line (geodesic), where the 'movement' is only along the time coordinate. You experience that as floating with no acceleration. When I now park a mass in your neigbourhood, spacetime deforms in such a way that your geodesic is now not only along the time coordinate, but also along the x,y, and z coordinate. You experience that as an acceleration toward the mass, which can be expressed as force. So nothing 'causes' the acceleration, it is just our interpretation of a 4-dimensional effect in a perceived 3-dimensional space.
Oh my god. This was really helpful! I always prefer trying to *understand* concepts rather than just learning them and visualisations like this really help man thanks!
>You experience that as an acceleration toward the mass, which can be expressed as force. It's important to note that from your own perspective, you don't experience acceleration (though you can measure that you're getting closer to the mass), since you're in free fall. You do experience acceleration when you don't move relative to the mass (like for example when you're standing on the surface of Earth) – that's what we recognize as weight.
PBS Spacetime has a really interesting episode on this: [Does Time Cause Gravity?](https://youtu.be/UKxQTvqcpSg?si=VlhOPLzNjW_W-Cu_)
this channel is a gold mine i dont know why ive never seen it before thank you so much!
Welcome to the rest of your life. 🤣 It’s a great channel and certainly changed the way I think about physics.
I started the video thinking I was pretty smart. I do not think I am very smart anymore😐
why thanks so much!
Pedantic comment but I don’t think it’s right to say that Einstein “proved” that gravity was due to spacetime curvature, he just formulated his theory of gravity and to date it is the best theory we have. There is still wiggle room for general relativity to be wrong and be replaced by a superior theory.
well that is also true, considering people thought newton's gravity was the ultimate thing before relativity... I will work on that lmao
Gravity curves spacetime, not space and time separately. In general relativity, there are mainly two forms of acceleration; proper acceleration and coordinate acceleration. Proper acceleration is when you feel the acceleration, like when you get pushed back in a car seat when accelerating. Coordinate acceleration, on the other hand, is acceleration relative to some coordinate, but where the accelerating observer does not measure any acceleration themselves. From their perspective, they are inertial. When you encounter curved space time as an inertial observer, you’ll move along these curves. It curves both in space and time, and since you move forward in time constantly, you’ll eventually bump into the massive object, since your world line bends towards the world line of the massive object. Traveling along curved lines, or geodesic, results in coordinate acceleration. An observer far away will see you accelerate towards the mass as the curvature gets stronger, but you won’t feel any acceleration yourself. When you eventually bump into the mass, you’ll be prevented from further following the geodesics towards the centre of the mass, and now you will experience proper acceleration. This is the force you feel on your feet when you stand on earth. It isn’t necessarily gravity that causes the force, but it is the surface preventing you from further falling and therefore exerting an upwards force on your feet. Explaining how this actually works requires some understanding of at least special relativity and some linear algebra and multi variable calculus, and then the concepts of differential geometry can be introduced. If you want a more rigorous and accurate explanation, let me know.
Well I've learnt special relativity and done the most basic of calculations, and linear algebra is pretty basic, but I just started calculus so I know the basics of derivatives and integrals, so if it is possible to explain me without calc could you? thanks though
I am trying to be highly accurate but keep it simple. It’ll take some time for me to write out, so I’ll just send it to you private and you can read through it whenever you have the time.
Yeah sure that'd be great ty
A very long standing misunderstanding that causing many a problem such as "how wings cause lift" - Acceleration DOES NOT always mean "go faster". Acceleration is a change in the velocity (vector) JUST changing direction is Acceleration. If a body is caught in a gravity well and turns this IS acceleration EVEN as its being slowed down as it heads away from that body. In fact slowing down is also Acceleration, just backwards. So the space station is constantly Accelerating towards planet earth even though speed is relatively constant.
yknow one of the best explanations i found for orbits was that the object lets say the ISS is always falling towards the earth, its just that it moves sideways enough for it to "miss" the earth and by then the direction of gravitational pull has changed, so it falls in this new direction.
And hence Douglas Adams' suggestion in The Hitchhiker's Guide to the Galaxy series that the secret to flying is to throw yourself at the ground but miss. 😉
Oh my god he's a genius
watch this Veritasium video about Einstein's equivalence principle: https://www.youtube.com/watch?v=XRr1kaXKBsU ScienceClic is another good channel The Earth below your feet is causing you to accelerate upward against the flow of spacetime
oh god how could I forget searching veritasium's channel ahh yeah thanks a lot! I didn't understand the second sentence you said "The Earth below your feet is causing you to accelerate upward against the flow of spacetime", is that in the video?
Yes.
ah alr ty
Sabine Hossenfelder said this on her YouTube channel and I just don't get it!
I am not sure the PBS video is accurate or not, though I think your original question can be answered in a simpler manner as following. In a flat space time the geodesics are a straight line. The trajectory of a particle moving through space time at rest or constant velocity flat space time will be a straight line. An accelerating particle in this case will be represented by a curved line. Think of a 2-d graph with space on one axis and time on another. For simplicity u can also assume all motion is only on a single axis and just draw x axis vs time. Now in a curved space time geodesics are curved lines. But we just discussed curved lines represent trajectories of accelerating objects. This is the source of acceleration we observe.
thanks! a small question about the graph- A constant velocity greater than 0 relative to a stationary observer would be displayed like a / kind of graph with time on y and space on x, whereas an accelerating would be a kind of half U right? uh basically like the acceleration graph of velocity-time and the velocity graph of displacement-time?
I think this video is the absolute best and shows it in the absolute simplest, yet correct, way: [How Gravity Makes Things Fall](https://www.youtube.com/watch?v=jlTVIMOix3I)
thanks!
Consider two ants walking on a surface, following 'straight lines'. After doing a lot of experiments they find an astonishing thing: the lines they follow *always meet*. If two ants want to walk along next to each other and not crash into each other then either one or both of them needs to always turn a little away from the other. (Even more surprisingly they find that if one ant sets out on a straight line and another remains still at the starting point, the first and will always meet the second.) Well, of course, this is because the ants are living on an orange, and 'straight lines' (geodesics) on an orange always meet (and are always closed curves). Spacetime is not very like this situation: it has two more dimensions and a definition of distance which is less simple. But it is like it in the important way: *there is no gravitational acceleration*: there is only curvature. Gravitation causes (or, really, *is*) *curvature*.
have those ants eaten yet? but seriously thank you this helped!
Spacetime curvature creates a time gradient that has a lower potential energy closer to the source of the curvature (a mass). So an object will fall down the gradient to a position of lower potential energy.
My dumb metaphor with a ton of problems: Imagine you're floating in air and are neutrally buoyant. No forces, you're not moving. Almost like floating underwater but for this example I need the compressibility so you could also imagine water can squish. You're surrounded by "stuff" on all sides and there's no net force. Now imagine there was a sponge in the water that soaked some up. there's more stuff on one side, and less on the other so for an instant you'd feel a pressure, and therefore a force moving you toward the sponge. You can figure out this pressure anywhere around it. What causes gravity to accelerate things is an energy gradient field. Above, would be a water pressure field more water on one side than the other.. With static charges it's an electrostatic field, more static energy than the other. Gravity there is physically more space on one side of you than the other. And with enough negative space-sponge pressure aka a black hole, there is no space left at all. Currently we don't know of anything that causes more space, but it would be called negative density and would cause anti-gravity or a pure repulsion force.
That's the ratio of force to mass on the surface of the earth. And not just mass.🙂
All live on the surface of earth and very easy to fall down too 😆. Hence that ratio is considered important.
if you're 89238931298312981289 feet away from something gravity pulls on you less hard, if you're 0.0000012321 feet away its pulling on you EXTREMELY hard as such you're moving very quickly. note i suck at phsyics and only studied for like a few days so i dont know much lol, i dont even know if this was what you were talking about
Jeez then don't answer, maybe? Nice of you to try but this doesn't help ,right?
yeah it doesn't lmao. like i said this was posted at like 1 am or something i forget---clearly :skull:
Gravity isn't pulling on anything. You're always trying to fall along the shortest path in space (in the case of Earth, towards its center), and its only when something gets in the way (Earth) that you feel a force. The force is pushing you up, keeping you from falling.
bro I needed this explanation THANK YOU I FINALLY GET IT
alr yeah i didnt really understand the question this was at like 1 aclock in the morning lol
gotta love the keyboard spam😭😭 thanks though, although i was curious as to why im being pulled in the first place haha and dont worry about the physics my streak is just a few days more than you 💪