How do we know there are only two types of electric charge? [duplicate]

Question

What I know about this topic

From books I know that in ancient times the Greeks did experiments such as rubbing two glass rods with wool to make them repel each other, rubbing two plastic rods with fur to make them repel, and bringing a glass rod and plastic rod (already rubbed) to make them attract each other. Based on these experiments, the books conclude that there are two types of charges, which we arbitrarily call positive and negative.

This idea of two types of charge seems like just an assumption, but there are also many theories based on it such as the explanation of Rutherford's $\alpha$-ray experiment; also our understanding of chemistry and many parts of physics are dependent on this idea.

I've researched this topic for several hours but I found nothing satisfying. I read posts such as How did physicists know that there are two kind of charges?

Problems bothering me

How do we know that, just because two possible things happen, i.e. attraction and repulsion, there are therefore two types of charges? Couldn't there also be a third, neutral charge?

Also, what was the original reasoning for there being charges with similar properties, before we knew about protons and electrons?

Answer 1

There's only one type of electric charge, but its value can be either positive or negative.

Couldn't there also be a third, neutral charge?

There is no such thing as a neutral kind of electric charge; that would be self-contradictory.

However, there are multiple kinds of charge in nature, one of which is electric charge. Gluons, for instance, carry colour charge but are electrically neutral, whereas electrons carry electric charge and are colour-neutral. Quarks carry both electric and colour charge. Neutrinos carry neither electric nor colour charge.

How do we know that, just because two possible things happen, i.e. attraction and repulsion, there are therefore two types of charges?

It's not just that.

Imagine if there were really two kinds of electric charge; call them Q and R. That would mean that two bodies with opposite Q would attract each other, two bodies with like Q would repel each other, two bodies with opposite R would attract each other, and two bodies with like R would repel each other. But Q charges wouldn't care about R charges and vice versa.

So for example, you could have a bunch of objects that all have a nonzero charge under Q, and any two of those objects would either attract or repel each other. But then you could introduce an object that has zero Q but nonzero R. That object would act neutral toward the Q charges. But take that object away and place it near other objects with R charges, and it wouldn't act neutral anymore; it would experience attraction and repulsion.

But in nature, no such thing happens. When an object is found to be electrically neutral—neither attracted to nor repelled by an electrically charged object—it really is electrically neutral. There is no other kind of electrically charged object you can place it near, such that it'll experience an electrostatic force.

The fact that we don't observe such a thing implies that all of our observations of electrostatic interaction can be explained by just one kind of electric charge, Q. However, as mentioned above, there are other kinds of charge beside electric charge. Those have different kinds of force law, so you couldn't ever confuse them with electric charges.

Answer 2

There are many types of charge. It's simply a matter of language: we have other names for the other types. We don't call them "electric".

Answer 3

I don't know much about history, but presumable people observed attraction and repulsion and declared there were 2 types of charges, which were associated with electricity.

Today we know there is only one kind of electric charge, which we know is a property of particles. Neutral particles have no charge, which is not a type of charge. Charged particles come in the arbitrarily assigned positive and negative varieties.

The charged can be deduced from observations of bending direction in a magnetic field, which is how the positron was discovered.

There could be a new type of charge, a magnetic charge, aka the magnetic monopole. Searches for such a particle have been negative.

With the development of gauge theories, it's become clear that conserved charges are their associated fields are attributable to local gauge invariance of the theory.

For electromagnetism, gauge invariance requires 1 charge and 1 associate field (comprising electricty and magnetism).

The generalization of EM gauge theories, called Yang-Mills theories, have a gauge symmetry group: $SU(N)$, which is the symmetry of $N\times N$ unitary matrices. Such a theory requires $N$ types of charges, and indeed, quantum chromodynamics is such a theory associated with $SU(3)$.

Thus, there are three types of charges, which are called color-charges: red, green, and blue. The "negative" version of these are anti-red, anti-green and anti-blue, or, thanks to a coincidence with human's color vision: cyan, magenta and yellow, respectively.

There's also the Weak force, which is related to $SU(2)$, which necessitates 2 new types of "charge", but it's all mixed up. Literally.

Answer 4

This is a very interesting question but mostly connected to history and mathematical logic.

There weren't that many people in Ancient Greeks, and though they did many amazing things, it's a period when witches were making significant advances in nature medical science, and, again, there weren't many people, and so they needed something useful. Even of today, three (color) charges weren't that kind of useful unless in some very special colliders, and no one had observed them naked.

Also, it might noted that there wasn't zero in roman numerals.

That one might thought, there was $+1$ and there was $-1$, why not there be $0$? Unfortunately, there literately wasn't a $0$ in the ancient Greek. Greeks couldn't even write it out.

Being the Greeks in the ancient Greek, the conclusion needed to be something very useful, so one could see the set of rods to be in the equivalent class and the set of wool being in another equivalent class, and see what conclusion can be made out of those division.

This came in the group theory in the form of binary operation, the multiplications. One could argue that $+$ and $\cdot$ were just the same representation. However, history let the Chinese Reminder Theorem to be found in China because the tools the ancient Chinese mathematics relayed on was primarily for the additions and subtractions, and it made sense that the theorem were discovered in the frequent usage. Similar story happened when European invented half of the thermal statics mechanics, and American invented the other half(The $dE$ thing).

Going back to the group multiplication, other than the equivalence class, the mathematical logic of the transitive relation were also very useful. This, in group theory, assuming commutativity of the person's action(Alice hold a rod while Bob hold a wool were not much different than Alice hold a wool and Bob hold a rod, notice that this had nothing to do with the charges itself since they were in different equivalence classes), placed constraints. Basically, the "conclusion" of the two type of charges.

And so, yes, there could be more than two charges if a civilization's history and environment were different. For example, if there were beings living close to the burst of high energy cosmic rays shower at the habitat, they might use three charges.