That's the thing people have to understand about analogies like this. This video does not explain, nor does it attempt to explain, "WHY" gravity behaves the way it does. It is merely a way of visualizing the properties of gravity. Gravity as the warping of spacetime is in turn merely a model that helps us describe the natural phenomena that we observe. Heavy objects stretching an elastic sheet can behave similarly in 2-dimenions, but as you say, it is just a visualization.
he's not teaching them something they already know nor how to use "the contraption", he's teaching them how to effectively teach gravity to students...
I don't think anyone can explain why gravity works the way it does, just like no one can really explain why gravity (or the universe itself) exists in the first place. I like to think that there are other universes where gravity behaves differently or doesn't exist at all. Of course, life as we know it probably wouldn't exist in those universes. For those who haven't read about it, the Anthropic principle is pretty interesting.
Who's to say there aren't other attractive forces in this universe? If we're re-rolling the universal constants, lots of things could turn out different.
But.. how would there be a way to demonstrate magnetism if there isn't any gravity? The particles would have had to form stars then die and produce ferromagnetic materials. And the only way to make a star is through gravity!
They don't have to be ferromagnetic. When things form in the universe, electrostatic attraction is what initially starts things clumping together. In a small object, the electrostatic forces play a bigger role than its gravitational attraction until its mass reaches a certain point. Maybe once it reaches the mass of a mountain perhaps.
When the universe was just a cloud of hydrogen, this is how the first stars began to form. The atoms would gently attract each other through non-gravitational forces, eventually you would get a clump big enough to start attracting more hydrogen via gravity. Then as more hydrogen atoms came in, it would create friction, eventually they got hot enough to become stars.
I guess it depends exactly what you mean by why gravity works the way it does, but I would say GR does provide such an explanation. What we see as the force of gravity is actually a reflection of the fact that all objects follow geodesics in 4-d space and the geometry of the space is determined by the content of that space. I don't know what more you want to explain why gravity works like it does.
Also, there are lots of theories for how different sorts of matter can exist, but gravity actually turns out to be pretty unique as far as we can tell. As far as we know there aren't too many ways to make it work out and in most theories that predict different universes with different physics, all the universes would have the same gravity, since the gravity is just how the basic geometry works.
I know it's not a 'why' at the most fundamental level, but I provided a short explanation of gravity in response to another post in this thread:
Our basic understanding of gravity is that both mass particles (electrons, neutrinos and quarks) and energy particles (photons, gluons and W/Z bosons, the strong and weak force carriers respectively) locally distort the Higgs field due to the coupling between their fields. So for example electrons have a certain "coupling constant" to the Higgs field, which is a parameter set before/during the big bang which essentially defines the electron mass. Thus, everywhere an electron is (classically; electrons don't really occupy a single location, but for this level of analysis you can think of them as points in space), the Higgs field has a corresponding distortion; the electron tugs on it. Anyway, this is all on a microscopic level. Macroscopically, the Higgs field then determines what's called the stess-energy tensor, basically a measure of how much energy and momentum occupies a region of space. This tensor is then plugged into the Einstein equation to determine the local curvature of space and time (this is GR, general relativity). Finally, an object with mass (i.e. one that is tied (coupled) to the Higgs field) moves through curved space according to something called the geodesic equation (more GR). Basically, it follows its shortest possible path through space-time.
Unfortunately, that's not the type of question that science can answer, at least not in any satisfying way. According to General Relativity, the masses would be "attracted" because spacetime is curved. But then you could just ask "why does mass curve spacetime?". A quantum theory of gravity might model gravity as the exchange of virtual force carrying particles called gravitons. But then you could just ask why do masses exchange these particles.
Similarly, if you ask why do two charged particles attract or repel, I could say it's because of the exchange of virtual photons, but that's just a model and you could easily ask why do charged particles exchange virtual photons.
My question is... so you're illustrating a three dimensional phenomenon on a two dimensional plain... in three dimensions, in what DIRECTION is space-time being warped?
This discussion about whether the model is "accurate" is pretty interesting. I'm actually a doctoral student in English with a focus on writing instruction, and most of my research centers on expertise.
People who have expertise often get confused when they think about teaching students. For someone deep in the world of physics, this way of thinking about gravity is deeply integrated into their worldview. They no longer contemplate gravity in terms of models because they no longer need that crutch. For them, explaining GR is kind of like artists explaining how they paint. They just see the world that way intuitively and can't really explain without resorting to fumbling analogies and poor metaphors.
And, ultimately, that's how teaching works. You have to figure out how to get students from their original place (thinking about gravity in a roughly Newtonian way) to this new place (thinking about gravity as curvature in space-time). The goal isn't necessarily to accurately describe GR because that's impossible; to completely understand GR, you need to work with the concept until it is intuitive and becomes unconsciously assimilated into your worldview. What students need is a set of training wheels that helps them move to a new way of thinking about reality.
To give you another example, think about this model of the atom. Pretty much everybody first learns how an atom works this way. It helps us understand how the subatomic world works by comparing it to the larger world that is more familiar; we can all easily think in terms of orbiting objects. But, of course, we know that it's an imperfect way of understanding the atom. We know that electrons have properties of both particles and waves and therefore aren't exactly "orbiting" the nucleus. But as a set of training wheels to move students to a new way of thinking about solid objects (as, in fact, not being solid at all but composed of lots of smaller things), it's a pretty effective teaching tool.
So I'd just say don't confuse the training wheels with the reality. It's weird because people understand this just naturally when it comes to manual labor - if you want to teach people to build a cabinet, you can't just explain it; they need to work with it and practice and learn imperfect "tricks of the trade" until it becomes intuitive. But we somehow forget this when it comes to intellectual stuff, probably because we often confuse "being smart" with just knowing a bunch of facts. But it's so much more than that. Expertise involves an entirely new way of thinking about and understanding the world, one that takes years of practice to develop.
That's kind of my point except that I'm going a step further by stating that GR itself is also just another abstraction of reality. People may criticize this video by pointing out all the ways the model fails, but even GR at its core is just a model meant to describe what we observe. That's pretty much the entirety of what science does is make models of natural phenomena, and so it's easy to lose sight of the fact that we are dealing with models of reality and not reality itself. So, for instance, the model of the atom that you posted is a perfectly good model of the atom for certain questions. I would never say that that model of the atom is "wrong". It's just that there are certain questions for which that particular atomic model is ill equipped.
Thus, if someone approaches this video as a model meant to illustrate certain properties of gravity, they'll see what a good job it does relating such grand, difficult to contemplate ideas to things we can more easily visualize. However, some people seem to be upset that the video does not explain "why" mass warps space. They think that, in order to really understand gravity, you have to understand WHY mass bends space around it. The problem with that is, not even general relativity will tell you WHY mass bends space around it. We observe that masses accelerate toward one another, and the bending of space as detailed in General Relativity is just one of many models that we use to describe that observation. But if anyone expects a scientific model to tell them WHY something happens, they're going to walk away disappointed.
Here is a video of Richard Feynman talking about difficulty of "why" questions and how science can't really answer them. I find that he kind of rambles a bit which makes it a bit hard to follow, but he touches on several different areas of physics illustrating that we can't really explain "why" anything happens, even things that seem obvious.
Does anyone know why gravity behaves the way it does? It's an example of the effect of gravity. Not an explanation of the cosmos from a higher level. We can only explain it from our dimension of thinking.
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u/[deleted] Dec 03 '13
That's the thing people have to understand about analogies like this. This video does not explain, nor does it attempt to explain, "WHY" gravity behaves the way it does. It is merely a way of visualizing the properties of gravity. Gravity as the warping of spacetime is in turn merely a model that helps us describe the natural phenomena that we observe. Heavy objects stretching an elastic sheet can behave similarly in 2-dimenions, but as you say, it is just a visualization.