6 ms·
the "problem" here is that the graviton "has to" exist. why? Forces have to be conveyed by something. If you want to push someone, you have to physically touch
by wayn3 9y ago
the "problem" here is that the graviton "has to" exist. why?
Forces have to be conveyed by something. If you want to push someone, you have to physically touch them. you have to REACH them. you cant just push the air, across the room, and expect them to feel the push.
this is true on the microscopic level as well. when particles exert gravity on each other, they do so at a distance. but something has to cross this distance. it is not ordinarily obvious how the sun traps our planet in its gravitational field - at a distance. there has to be something that exchanges the gravitational force between sun and earth. we call this thing the graviton. it is important to notice that something that does the "job" of the graviton HAS TO exist. maybe its not a particle. but something causes the exchange of gravitation and thats what we're looking for.
you know that "in space, no one can hear you scream". thats because there is no air that could make the sound waves travel.
in a similar way, without the graviton, there would be no "medium" that conveys the gravitational force.
the problem with detecting the graviton is that it is very weak. we would have to build really expensive machines to "observe" it.
explaining this theoretically is not difficult. we have plenty of "theories". the problem is confirming them with experiments.
- AstralStorm 9y agoNothing in physics "has" to be something or other. It is well known that QM is incomplete, so forcing forces to be quantum might not be the right way forward at all. Strings are not quanta to begin with. They behave somewhat similarly at lower energies at most. Forces are essentially a model too, and are compatible with both wave, quantum and string physics. Each substantially different.
- wayn3 9y agohence the quotation marks. heres your "well, actually" cookie.
- justin_vanw 9y agoI'm not an expert, but I simply don't understand your reasoning here. For example, lets suppose some mass is moving at a constant inertial velocity through empty space. To me it seems that your reasoning would require gravitons to communicate to the mass that there are not other masses nearby and so to 'tell it' that 'straight line' for it means to go in a euclidian straight line. Otherwise, how does it know? I think baked into your logic is that there is something special about geodesics in the presence of masses and so you need to tell the moving mass to 'curve', but to the mass it is just going straight, even if to an external observer it appears that it is curving or even orbiting.
- wayn3 9y ago- its not my reasoning, i'm just trying to explain quantum field theory. - whether you call it "gravitational force" or "curvature of spacetime along whose geodesics massive objects slide" - the effect has to be mediated by a "particle". for popular media, "particle" is too big a word, because people tend to think of protons or atoms. subatomic particles are just excitations of quantum fields. little blips of localized energy, of which we are only able to see the top layer. ^ this has nothing to do with general relativity. general relativity describes the macroscopic world pretty well. it generally breaks down on very small scales. how planets move is described very well by general relativity. how they mediate the involved forces is not described at all. edit: i just thought about that straight line statement. there seems to be a misconception that a geodesic is a "generalized straight line". That is not remotely true. Geodesics, in mathematics, are "shortest paths". While that happens to coincide with what a straight line does in a plane, generalizing that meaning in the other direction doesn't work. In general relativity, we talk about geodesics when we mean "out of all the possible paths we can take, we are choosing the one that minimizes energy loss". That is, then, a geodesic. But a geodesic is far from a straight line in terms of movement. Its the path of least resistance in the energy picture. If you ask "whats the difference?" - the difference is that a straight line in energy space is not a straight line in regular space. Earth, for example, is travelling along a geodesic. But it is clearly accelerated towards the sun. There is nothing "straight line" about it. When you fall into a black hole, you travel along a geodesic. But it wont feel like a straight line to you at all. That you happen to be travelling along a straight line in the absence of forces is just a tautological truth. Applying differential geometry to that statement just makes it way more complicated to state the obvious.
- cygx 9y agothere seems to be a misconception that a geodesic is a "generalized straight line". That is not remotely true. Geodesics, in mathematics, are "shortest paths". Geodesics being generalized straight lines is exactly true. Also note that they are not necessarily shortest paths: In the framework of affine connections, they are defined as autoparallels. Earth, for example, is travelling along a geodesic. But it is clearly accelerated towards the sun. Earth is in free fall around the sun, so accelerometers will read 0. That's the whole point of General Relativity: Geodesic motion is not a consequence of Newton's second law, but the first one.
- wruza 9y agoSorry for interrupting your discussion with maybe dumb question, but isn't gravitation defined by spacetime geometry rather than some sort of particle exchange? Do not particles always fly forward, with forward changing its meaning with time?
- wayn3 9y ago"spacetime geometry" is a mathematical formulation of the physics that happens to correctly describe WHAT is happening. it makes no prediction at all about WHY its happening. einsteins theory does not explain, and does not try to explain, why the gravitational field exists. it just tells us the effects of the gravitational field being around.
- wruza 9y agoThat's "why" thing is new to me. I read before that there is no why, only how. "Why do particles exist? Why is something fundamental like it is?" Are these questions to be answered in ST framework?
- wayn3 9y agoThere can be relative WHYs. We will probably not be able to answer why particles exist. But we can tell you why gravity works the way it works. I dont know what ST refers to.
- auggierose 9y agoThe whole point of general relativity is that there doesn't need to be a particle that does something, but that gravity is just an emergent property of space. So no, we don't know that a gravity particle has to exist.
- ylem 9y agoThe problem is that we have two formulations of physics. Basically, gravity and everything else. Theories such as QCD, QED, etc give rise to quantum mechanics, electricity and magnetism, etc. In these theories + and - charges for example feel an attraction to each other due to an exchange of "virtual" particles. General relativity is based on geometry as you say. Physicist would like to have one framework to describe everything--that's why it would be nice to find a graviton.