Physics
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Energy

# If it is common to have a positive and negative with an energy such as electricity or magnetism could dark energy be the other pole for gravity?

## "color: #000;" id="Positive/negative_energy">Positive/negativeenergy

### "color: #000;" id="Energy_or_charge?">Energy 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

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.

### "color: #000;" id="Measuring_energy">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.

### "color: #000;" id="Why_the_conventions?">Why theconventions?

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

## "A.">A.

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

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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