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How gravity really works

  1. Nov 3, 2011 #1
    How does gravity work? This seems to be one of the most often asked questions and one that has consumed many people. It appears no one really knows and no one is saying. Einstein, Newton and many other ingénues minds have offered formulas and analogies that beautifully calculate or predict what gravity will do but none of them describes how it works. I don’t know if I’m more surprised of that or that no one is still offering a description.
    Relativity says gravity is the curving of space-time by any object that has mass. The analogy of a rubber sheet being stretched by a heavy object is a nice one-dimensional visual of what gravity may look like but it does not explain how it works. The analogy is even confusing to some non scientific people who believe we know how gravity works. Gravity is always described as a pulling force. One of the newer hypotheses has something called gravitons which are really the same thing but again not explaining how gravity works just putting the question off another step. For more than eighty years we have collected data and asked the same questions and still no answers. Maybe were asking the wrong questions or putting too much faith in the same old models.
    How gravity works should be one of the most important questions. When we solve that mystery it will answer so many questions in science, engineering, biology and especially cosmology. I’m sure life as we know it will change dramatically after that. Changes like the way we travel on earth or in space will lead to even more discoveries. Knowing how gravity really works should go a long way in answering many of the questions we have in cosmology.
    I am new to Physics Forum and cosmology for that matter. Like many of you I too am totally fascinated with cosmology. I enjoy reading the intelligent and well thought postings. It’s nice to be able to ask questions and right away get many answers, descriptions and great discussions.
    My background is in mechanical engineering but in the last two years I have read a lot on cosmology and the history behind it. I am a fast learner and can usually reason my way through things at least with the information I have. That being said I still can’t get past this issue with gravity. I few weeks ago we had a thread going where the subject of kinetic gravity came up. I was a little surprised at the negative reaction to it and if no one wants to discuss it I promise I will not bring the K word up again. At the time I did not know that the subject of kinetic gravity was banned from this forum.
    I’m not sure why there was such a quick and almost dogmatic rejection of the subject. It being the only theory in the last 265 years that has attempted to explain how gravity works physically. The standard theories which also have many problems hardly even try to physically explain gravity and when they do there’s always some mysterious element, force or set of circumstances that needs to be injected.
    Kinetic gravity does not have half the problems that standard gravity has. If nothing else kinetic gravity is great for modeling ideas and allows you to see things in reverse. It’s also great for brainstorming and I was wondering if there could be some place in the forum to discuss different or new ideas. I understand that you need to keep a handle on the nontraditional science theories but any discussions on “how gravity really works” I think would be important even if some don’t agree. Especially when it's the only idea being proposed. I love reading this forum and do not wish to be banned for bringing up good subjects like this. Maybe there could be some closet area you could hide us in. At least until the conversation has finished. Thanks
  2. jcsd
  3. Nov 3, 2011 #2
    Kinetic gravity, for those who are wondering, is the theory that the gravitational "pull" experienced by an object is actually due to a push, resulting from many impacts on the opposite side. There is supposed to be a flux of small particles pervading space, the presence and movement of large objects like the sun and the earth creates regions of different density and hence different pressure in the flux, and (e.g.) our attraction to the earth's surface is because of such a pressure gradient.

    It's an idea which can be developed mathematically (in quantum form as well as classical form), and its empirical merits can thereby be assessed. But I want to comment on one thing that Bill says, that it's "the only theory in the last 265 years that has attempted to explain how gravity works physically" (my emphasis). This must refer to the fact that the microscopic picture of physics in kinetic gravity is one of collisions between hard objects. This is a form of physical interaction with which we are all familiar on a gut level, thanks to having spent our lives in the macroscopic domain.

    Bill, here's the problem. Do you have any way of "physically" explaining why hard objects rebound when they collide? Or is this just an axiom with you? I am not criticizing kinetic gravity as a theory; what I'm criticizing is your view that an explanation is only physical if it reduces to certain familiar concepts. I would also add that there's no reason to focus only on gravity in this regard. What about the other forces, like electromagnetism?

    But let's focus on this topic of colliding objects. According to current physics, they are made of atoms, they repel upon collision because of electromagnetic repulsion, and they maintain their shape despite the impact because of rigidity arising from a combination of electromagnetic forces and the Pauli exclusion principle (which keeps electrons out of the same energy level). So the rebounding of one thing from another thing is explained in terms of momentum conservation, the exchange of virtual photons, and quantum mechanics.

    If you are going to maintain that kinetic gravity is favored because of conceptual simplicity, then you will have to overlook the whole modern understanding of what an atom is and how it works. Kinetic gravity in its classic atomistic form - involving an endless cosmic blizzard of colliding rigid particles - knows nothing about the quantum model of the atom.

    So you have a choice here. You can broaden your inquiry to include version of kinetic gravity in which the push-force comes from quantum particles and quantum interactions; but then you can't claim conceptual simplicity, at least not in the sense of the explanation resembling everyday experience. Or, you can make your focus on classic kinetic gravity, but then you are at odds with the modern quantum-based physics, and you need to be simultaneously seeking an alternative picture of electromagnetism and nuclear physics as well.
  4. Nov 3, 2011 #3
    Thanks for responding

  5. Nov 3, 2011 #4
    Newton had a field theory of gravity, Einstein had a geometric theory of gravity, string theorists have a quantum theory of gravity (and each of these builds on the one before). All these theories have some sort of cause and effect picture of the world. In Newton, mass creates a force field, and motion responds to forces. In Einstein, mass warps the geometry, and motion responds to geometry. String theory is closer to the particle view of gravity - there are graviton strings - although because it's a quantum theory you have a wave/particle duality. Why don't these count as physical explanations?

    So far, you allow for momentum transfer through impact, and drag caused by a field, as physical explanations. But I just don't see a difference of principle between these, and the theories of gravity that I listed above. Either way you need mathematics, so there are equations, but there is also a physical concept in all cases.
    OK, so you have the beginning of a physical hypothesis. But all the standard models that you call un-physical also have qualitative ideas like these at their roots. I don't mean the exact same ideas - just that they have a set of simple concepts that add up to a theory of the world.
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