What makes gravity apply force to objects? Why does gravity not get pulled into the center of gravity itself, or scatter? I know that every physical body has some amount of gravity, that attracts other bodies. I also know that in order to make one physical body move, another physical body must apply force to it, so what is that physical body that gravity is made out of?
Also electric fields of magnets and atoms. What keeps the electrons from flying away from the nucleus? Is it same as gravity?
Is gravity our spacetime “sinking” into itself due to high amount of matter in one spot? Then what physical thing is the spacetime made out of to be affectable by matter?
I know its a lot of speculation and questions on my part, but i am fascinated by how physics does its thing.
In: 109
Well concerning gravity, objects of high mass deform what is known as “spacetime”, which is the “fabric of the universe”. This deformation causes objects to fall inward to the gravitational center, or changes the path of photons travelling through warped spacetime, like gravitational lensing around a black hole.
The ELI5 version is to imagine a bowling ball on a tightly spun sheet fixed at all 4 corners. The ball curves the sheet more and more at the part of the sheet where the ball is closest to the sheet, decreasing over distance. A similar deformation caused by mass is presumed in gravity, but in 3 dimensions instead of the 2 in this example.
To answer your primary question at a basic level, gravitational fields aren’t made of anything, they are rather a distortion or *change in shape* of the something (spacetime) that was always there (relatively speaking). Think of the surface area of a balloon before and after being inflated as a concept.
Electric fields probably operate along similar principles due to the similarities of the formulas for these, but I will read the better answers about that here eagerly.
Hi /u/Zynthonite!
Your question concerns the [ontology](https://en.wikipedia.org/wiki/Ontology) of fields in physics. (Tl, dr: Ontology is a discipline in philosophy of science that asks what things exist *out there*.) And the answer is that we are really not sure what the answer to that question is.
To make things even more messy, we *do know* that the nature of the gravitational field is fundamentally different from the nature of quantum fields, such as the electromagnetic field.
To understand, why this is the case, let’s take a closer look at the question what gravity *is*:
#Gravity:
(EDIT: for a great video on this subject, see the link on the bottom)
According to the best theory of gravity we currently have – General Relativity – gravity is not a force at all. What we see as gravity is the effect of mass-energy curving spacetime.
To understand how a curvature of spacetime can lead to the effects we observe around us, we have to understand how curved surfaces change the behaviour of straight lines.
First things first: an object that has no force acting on it is force-free. Force-free objects do not accelerate and, therefore, move along straight lines.
In a flat geometry, two straight lines which are parallel at one point will remain parallel for all times. That is, two parallel straight lines will never cross on a flat surface.
So far so intuitive, right?
But what happens, if those straight lines do not move across a flat surface, but instead along a curved surface? We call such straight lines on curved surfaces [geodesics](https://en.wikipedia.org/wiki/Geodesic).
Imagine a [sphere](https://i.stack.imgur.com/x2xDm.jpg) with two lines perpendicular to the equator. As they are both perpendicular to the same line, they are parallel at that altitude.
Imagine two objects that are moving along the lines perpendicular to the equator. They start out parallel, and move in a straight line upwards. Despite the fact that neither of them is turning, the two objects that started out moving along parallel lines will meet at the north pole. Hence, despite the fact that both objects are force-free at all times, they experience relative acceleration.
Such trajectories, that lead across curved surfaces without turning are called geodesics and they can be thought of as straight lines on curved surfaces. Objects under the influence of gravity follow [geodesics](https://en.wikipedia.org/wiki/Geodesic).
As gravity curves spacetime, geodesics can experience relative acceleration despite the fact, that both objects following said geodesics are force-free. And this relative acceleration of force-free bodies is what Newton mistook for the gravitational force. According to GR, though, there is no force, only curvature which causes force-free objects to move along paths that seem accelerated to outside observers.
This explains why photons are affected by gravity despite having zero mass: photons travel through spacetime and spacetime is affected by gravity.
So that’s gravity. And why are quantum fields so different?
#Quantum Fields:
Quantum mechanics is a little bit peculiar, because we have a formalism that is very good at describing and predicting the behaviour of quantum-mechanical systems, but it is *very* bad at *explaining what happens*. That is, there is no good *interpretation* of the formalism.
In quantum mechanics, the state of objects is described by their [wave function](https://en.wikipedia.org/wiki/Wave_function). That is, the square of the absolute value of the wave function in any one place tells you, how likely you are to measure the particle at that point in space.
**However**, it it very unclear, whether the wave function itself is *real*, that is, it is unclear whether the wave function is nothing but a calculation device, or if it corresponds to some *real* object out there.
This problem is brought to the forefront during the measurement process: The most common interpretation of quantum mechanics – the so-called [Copenhagen Interpretation](https://en.wikipedia.org/wiki/Copenhagen_interpretation) tells us that the wave function instantly collapses in the moment the particle is measured. However, it cannot provide any causal explanation how or why this should happen, and this leads to serious problems for the interpretation of the measurement process (a problem that is compounded by things like entangled particles etc.).
This inability to explain the measurement process is – appropriately – called the [measurement problem](https://en.wikipedia.org/wiki/Measurement_problem), and it is one of the unsolved problems in physics.
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(The following portion is not necessarily ELI5-material, and it is not required in order to understand the larger point I am trying to make. Thus, this is simply supplementary material for the interested reader)
The question, whether quantum fields are real or not is further complicated by quantum field theory:
Fundamentally, we are left with two choices: either we make an ontological commitment to particles or to fields. Thus, either particles are real, physical objects, or fields and its associated invariants are real, physical objects.
The aforementioned [Copenhagen interpretation](https://en.wikipedia.org/wiki/Copenhagen_interpretation) denies any ontological relevance to the wave function, it is tempting to assign ontological relevance to particles.
Unfortunately, this interpretation is seriously undercut by the [Unruh effect](https://en.wikipedia.org/wiki/Unruh_effect). According to the Unruh effect, one observer in an inertial frame of reference will see a vacuum, while an accelerating observer will see a thermal bath of particles. Thus, the number of particles an observer measures is dependent on their frame of reference.
This suggests, that particles do not have ontic relevance, and the actually fundamental entity is the quantum field.
However, this notion of associating invariants with reality is undercut by Quantum Field Theory: According to representation theory, two representations are considered equivalent, if they can be unitarily transformed into each other. That is two representations π1 and π2 of an algebra of operators *A* in Hilbert spaces H1 and H2 are unitarily equivalent iff Uπ1(A)=π2(A)U for a certain unitary map U: H1 -> H2 where all A are elements of *A*.
Since QFT has infinite degrees of freedom, according to the Stone-von Neumann theorem, there are infinitely many unitary inequivalent representations.
Since inequivalent representations imply a *different structure* of the Hilbert Spaces, no invariants can exist that are covariant between those spaces.
—-
#Gravity vs. Quantum fields
I hope you can see now, that quantum fields and gravity are very different. This ontologically different nature contributes to our inability to reconcile quantum mechanics and general relativity, the two great theories in physics: While both general relativity and quantum mechanics work well in their own realms they break down in situations where both become relevant at the same time. These situations include extreme events like black holes or the big bang.
Our inability to reconcile these theories limits our understanding of such extreme events.
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For a great video on the basics of GR, check out [this](https://www.youtube.com/watch?v=NblR01hHK6U) video by PBS Spacetime.
We actually do not know what gravity is–we just know what it does.
Warps spacetime, you say?
Well, we actually do not know what spacetime is.
We don’t know much at all about this stuff, but just may be getting there.
Or we may never know, just as a dog will never be able to understand algebra.
The answer isn’t completely ELI5-able, but there are two things that might help you here:
First, part of the problem is that you’re operating on this flawed premise: “in order to make one physical body move, another physical body must apply force to it.”
Forces, like the electromagnetic force or gravity, **don’t require physical contact to induce** ***accelaration***. Acceleration is a change in an object’s motion (either starting to move from relative rest, increasing or decreasing speed, or changing direction). Gravity isn’t a physical body – it’s not a *particle*, like a proton or a quark or something.
A magnet, for example, doesn’t have to touch a nail to make it move (different force, same idea). In the case of gravity, the amount of gravitational force is proportional to both mass and distance. Something massive and close exerts a lot of force, while something distant or small exerts very little.
Second… we don’t 100% know why gravity does what it does. Best guess, *very* roughly speaking, is that mass warps spacetime in a way that makes massive objects flow toward each other down the spacetime gradient. I’m sure you’ve seen the [rubber sheet analogy](https://www.youtube.com/watch?v=MTY1Kje0yLg). That’s an analogy to help people visualize the concept, rather than an actual explanation, but it’s about as close as you can get for ELI5. Getting into the moving parts much more than that requires equations and stuff.
And even then, the equations just describe the behavor – theoretical physics has been hammering away at the *why* of fundamental forces since at least the 17th century (Newton). We’ve gotten pretty good at figuring out exactly how gravity, electromagnetism, and the strong and weak nuclear forces operate – but we still don’t know exactly what makes them happen.
I’ll take a crack.
The thing we call the electric field is a “field of influence”. It’s the English language expression of a mathematical concept that the closer you are to a an electric charge the more of an effect is has on you. Ditto with gravity.
They aren’t “made” or anything, they are the express a concept.
As our experiment get more and more sophisticated we (us, humans) found that you can’t have an electric field with a magnetic field. So we just call it the EM field. And when it’s really strong, sometimes, electrons and other particles will “appear” out of nowhere.
According to one of my favorite YouTuber, quantum physics is a word physicists use when they don’t know what’s happening; like his girlfriend could be asking why did you forget the bread during the grocery shopping and the physicist would say “quantum physics bitch”
What is gravity made of?
Short answer: space.
Longer answer:
Well this stuff is difficult, we don’t call it rocket science for nothing. I don’t think there is any explanation a 5yo will grasp, even my own 7yo kid won’t. But we can shoot for 10yo and hope for the best?
I’m going to talk only about gravity. And I’m not really going to talk about gravity, I’m going to talk about a toy model that will sort of illustrate the basic concepts. That’s because the human brain simply can’t directly comprehend it (some claim to, but who knows if they really do). The reason we can’t should become clear. So what we’re going to do is make a toy that has some of the same features and then show how they would have counterparts here in the “real world”. Yes?
Imagine a person that is flat, and they live in a flat universe. So instead of our three dimensions of width, height and depth, they only have the first two, width and height. We’ll call them flatties. Everything we see in our universe is in our 3D space, everything in theirs is in that 2D space. So if two siblings live in that universe, then they are the same plane – you don’t have flatties above each at different depths, they’re all on the same big piece of paper.
Since flatties evolved in a universe with only two dimensions, their brains can only understand two dimensions. It doesn’t make a difference how many dimensions there *actually* are, they just can’t see them. So they assume it really is two, because they never see more than that. You believe what you see, and you see what you believe.
Now here’s the thing, what happens if their universe isn’t actually 2D? Take that piece of paper they live on and push the two sides towards each other so it turns into an upside-down U-shape. From their perspective, nothing has changed, everything still looks like it’s 2D because they simply can’t see *that* direction. But their universe, as whole, is actually 3D.
Now what happens when the one flattie walks over to her brother? Well we know through the First Law that objects will not change its motion unless a force acts on it, or in other words, they want to travel in a shortest distance. To you, that shortest distance is clearly outside of the paper, but again, they can’t see that, to them, that shortest distance is along the surface of the paper, in the shape of a U.
So, that means **every step they take, the universe forces them to move in a non-obvious direction**. And how do they interpret that? Well if you’re not moving in a “straight line”, there must be a force acting on you – that’s what the First Law says, right? So what is this force that we cannot see but seems to be coming from the ground? Well, let’s call it “gravity”.
So what is the force of gravity in their universe? Nothing. It’s just geometry.
You can see where this is going. We do not live in a 3D universe, it’s actually 4D, and just as the flatties can’t see curved space in 3D we can’t see the curved space in 4D. You bent the paper with your hands, but here in our universe there is an effect where the local density of energy causes a sort of similar bending (but more complex). You could imagine this in flatland by drawing the flatties on paper using watercolors, the paper will crinkle up where you paint, so in their universe there is an effect where the local density of flatties results in more curvature. Of course, it’s much more complex, but you get the idea.
So when you try to walk from A to B here on Earth and the ground keeps hitting your feet, it feels a whole lot like something is pulling you down to the ground. But it’s not that, it’s more like the Earth and you are moving in different directions in 4D, and due to the local curvature those two paths intersect. And they always will, because of the form of that curvature.
Now this does leave one bit unexplained. You’ll note that you’re still getting pulled into your chair even if you’re *not* moving from A to B. Well that’s because there’s one odd thing about this 4th dimension, its always moving. Want to see it? Look at a clock. Notice it keeps changing? Well guess what, that’s your location along the 4th axis. So even though you seem to be “sitting still”, nope, sorry, you’re still moving. And since you’re moving one way, and the Earth another, boom, “gravity”.
So wait, *why* is this 4th dimension always moving? Because the universe exploded. The Big Bang, remember that thing? It’s not something that *happened*, it’s something that’s still going on, the Big Bang is name for when it *started*. You can call it “universal expansion” or lots of other things, but it’s all the same effect… we are getting bigger in 4D.
There is nothing pulling on you. There is no “force”. You’re going along a path that is the shortest distance between today and tomorrow, and the Earth is on a different one, and they intersect.
So, back to the question… What is gravity made out of? Nothing. Its just the shape of the universe.
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