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Answered 2013-02-28 07:44:53
Positive/negative energyEnergy or charge?

When you talk about positive and negative electric/magnetic energy, I presume you're talking about positive and negative charge. This is a fallacy. It's like kinetic energy: you wouldn't say you had negative kinetic energy because you were moving backwards. Your momentum would be negative (assuming you had chosen the convention that forwards is positive) but not your energy. Kinetic energy is never negative.

Kinetic energy is not the same as momentum; likewise electrostatic energy is not the same as charge. When you separate electric charges (for example by rubbing a balloon against your hair) you create electrostatic energy. But there is no sense in which the positively charged part has more energy than the negatively charged part. Insofar as it makes sense to ask where the energy is located, the positively charged part may have more or less energy, depending on how the charges are distributed. And similarly with magnetism: there is no imbalance in the energy distribution between the two ends of a magnet.

Measuring energy

In fact, in some sense the concept of positive and negative energy isn't very meaningful. When we do calculations involving bodies on earth moving under gravity, we often assume a body at ground level has zero potential energy. But we could just as well assume a body at sea level has zero potential energy. It wouldn't affect the answer to questions such as "What is the final velocity of the tennis ball?". This is because what matters is really the change in energy.

The same is true of some other kinds of energy, such as chemical energy: We need a convention to decide when a system has zero energy. There are tables telling you the chemical energy of various compounds. These can be used to calculate the energy change (heat generated, light absorbed, etc) when a reaction occurs. Some tables use the convention that an element at room temperature and pressure has zero energy; other have other conventions; but all will give the same answer for the energy involved in the reaction.

Why the conventions?

In principle, we could in many cases avoid the choice of artificial conventions and use a more natural convention. For instance, for potential gravitational energy (ignoring everything in the universe except the earth and everything on it) we could assume that an object an infinite distance from the earth has zero potential energy (or, more precisely, the potential energy of an object tends to 0 as its distance from earth tends to infinity). But this would create practical problems in the calculation: We'd have to take into account the fact that the acceleration due to gravity decreases as you move away; and the energy would be a very large negative number. A 1kg mass at the surface of the earth would have a potential energy of -63 megajoules. The calculation would say something like: "...Initial potential energy: -63157839.1 joules. Final potential energy: -63157842.6 joules. Decrease in gravitational potential energy: 3.5 joules. ..."

As the most extreme example, we could use E=mc^2 to calculate the energy of everything. The energies involved would be immense. We'd have to abandon Newtonian physics and use relativity instead.


I think your proposal is very interesting. Dark energy is the term used to explain why the expanding universe is accelerating. If all there was in the universe was matter with attractive gravity then you would expect that gravity to slow down the expanding universe, after getting its initial burst of energy from the big bang. If there existed some matter with repulsive gravity, as you suggest, that might indeed be the source of the dark energy. BTW, matter which repels all other matter, even our ordinary matter, was first proposed by English physicist Hermann Bondi about 40 yrs ago. I don't think he seriously thought it existed but he used it to find an exact solution to Einstein's General Relativity Eqs. He imagined this "negative" mass, which repelled everything being next to ordinary mass ,which attracted everything. So the two masses sped off into space, one being attracted the other being repelled. This configuration allowed Bondi to find a solution to a dynamical problem using Einsteins Eqs. Anyway, maybe particle physicists have some objection to another gravity pole but I like your idea.

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