Ignorance on a massive scale: Is gravity a force mediated by force carriers or a deformation of spacetime caused by a large concentration of mass (or both)?
This seems like an intuitive way to visualize gravity: you have a massive object deforming spacetime, like a ball placed on a sheet, and objects in orbit feel this curvature, take curved paths around the massive object, and that’s called gravity. Here gravity is a consequence of a curved spacetime. While this visualization is easy to relate to, how can I visualize gravity on more local scales? An apple falls from a tree, because so close to the earth, spacetime is very distorted, but I cannot visualize it, and have never seen an image that tries to. Or is gravity instead a force mediated by some force carrier, which predominates on local scales, while spacetime curvature predominates on larger scales?
Apologies for a rambling question. TIA for informed answers.
So far as we can tell, it’s both. The best theory we have for gravity is general relativity, which describes gravity in terms of distortions of spacetime. But it must be quantizable somehow, which would presumably mean that, like all the other forces, it’s mediated by virtual particles. Certainly, we’ve detected gravitational waves, which would be streams of a great many such real particles. The catch is that, while we’ve fully developed theories to describe electromagnetism and the weak force via virtual particles, and we’ve got most of the framework for doing so with the strong force, nobody has any clue how to construct such a theory for gravity.
Well, first of all, in the vicinity of the Earth, spacetime is only very slightly distorted. You need things like black holes, or at least neutron stars, for “very distorted”. But that aside…
The key is that it’s not just space that’s distorted, but spacetime, and that’s very difficult to draw intuitive diagrams for. Every object is always tracing out a path through spacetime, even if (in some reference frame) it’s staying in one spot in space. And absent any non-gravitational forces, those paths are “straight”, or the closest thing to straight possible in a distorted spacetime. The distortions are such that if an apple starts off at a constant position in a tree, the “straight line” path will bring it closer to the center of the Earth.
I don’t quite see how the ball on a sheet analogy confuses you for the apple and the Earth. The Earth deforms space time, so the apple is sitting on a slanty “space-time”. If it’s not held up by some force, like the pull of the branch, it will “roll” down until the force of the surface holds it up.
Of course, as Chronos points out, it’s more complicated than that, but I’m not seeing how it breaks down for the apple and the Earth.
Right, as usual Chronos has done a good job explaining it. The rubber sheet analogy has often been criticized as inaccurate, but it does help lend intuitive insights to some important basic concepts. If you have a bowling ball forming a depression in a rubber sheet, and you send a marble in its general direction but not on a collision course, then depending on the mass of the bowling ball and the speed of the marble (assuming the marble has no rolling friction at all) the marble will either swerve into and crash into the bowling ball, or else be deflected by it and go on its way in a different direction, or else go into orbit around it. If the latter, the orbit may be an eccentric one, in which case the marble will be moving fastest at its closest approach and slowest when its furthest away as it continuously exchanges kinetic for gravitational potential energy. All of this mimics orbital mechanics pretty well.
As for the apple falling from the tree, relative distortion of spacetime in that tiny distance is pretty much nil, but what matters is the overall spacetime distortion created by the bowling ball, and specifically the gravity well surrounding it. A microscopic little tiny marble very close to the bowling ball, simulating the apple, will roll into the bowling ball quite fast because it’s on a steep gradient of the gravity well. An identical marble much farther away will start accelerating toward it more gradually because it’s on a shallower gradient (weaker gravity).
And here we see the major flaw of the rubber sheet analogy: It’s often interpreted in a way that itself requires gravity. Explaining something in terms of itself is no good.
The actual interpretation of the rubber sheet model doesn’t depend on some other source of gravity, and works equally well for dimples that are pushed “up” or “down”. Take the shape of that rubber sheet, and put a strip of tape along the surface. Put the tape down gradually, and keep it as smooth as you can. Start off far from the bowling ball, and aim the initial direction of the tape close to, but not quite directly at, the ball. The path of the tape will be deflected, because of the curvature of the sheet. And it’ll be deflected in exactly the same way, no matter whether you run the tape on the top of the sheet or on the bottom.
It’s compartively easy to broadly understand the relationship between gravitons and curved spacetime.
A central feature of quantum field theory is a procedure to take a relativistic classical field and get its quantum equivalent. The first major stumbling block to doing this for gravity though is that its relativistic description (general relativity) is not as a physical field, but as the curvature of spacetime. The curvature of spacetime is described by a field though g, but a physical field is defined on a space that has its geometry pre-defined, whereas the space that g is defined on lacks geometry, because the geometry is described by g. The question is then, can we instead find an equivalent description of relativsitc gravity as a physical field? The answer is yes you can: by pre-defining the geometry as being described by a field g’, we can say g = g’ + h, where we take h to be a physical field. You can now quantitize h and you get a quantum theory of gravity which has massless spin-2 gravitons as the force carriers and it is even possible to model certain situations with this approach.
However there’s both theoretical and practical problems to this approach, which is why the question of quantum gravity is still very much open. That said, gravtions are seen by many as a generic prediction of quantum gravity, but how fundametal they would be in a description that is more theoretically sound and/or practical isn’t known.
Tto add to this becuase the speed of light, which is 3 x 10[sup]8[/sup] ms[sup]-1[/sup] represents the scaling between time and space in spaccetime, 1 second is is 3x10[sup]8[/sup] “spacetime metres”, so its not too difficult to see how the paths of the Earth and the apple can coverge in one second if they start 5 metres apart with only a small curvature.