What is a charge? Really, what is it?

[SpaceNerdz]

I was wondering if anyone can give a deeper answer to what a charge is, other than what is already given in Wikipedia. We know that it is positive and negative etc, but all these things just "describe" a charge. But what is it ?? It seems like a lot of the answers I found just dances around this question by describing the properties that it has, but what is it fundamentally ? Can anyone answer this ?

 

[Danger]

I honestly don't know how that can be answered. Fundamentally, it's an accumulation of quark properties, but that sure doesn't clear up anything. Somebody here might have some way to explain it more clearly than Wikipedia does, but it won't be easy and I certainly can't do it. Sorry.

 

[jtbell]

Check out the "Similar Threads" listed at the bottom of this page. You're far from the first person to ask this question. :D

 

[Dale]

a lot of the answers I found just dances around this question by describing the properties that it has

I don't think that there is any dancing around involved. Charge IS a property so of course the answers describe the properties! That seems to be the only possible kind of answer you could get.

 

[russ_watters]

More broadly, what anything "actually is" is a list of its properties (and actions, which are basically the same thing).

 

[SpaceNerdz]

The reason I am asking this here is because when I ask my professor at Columbia University ( he's a particle physicist ), he stared at the wall about it for 15 seconds, looked at me, and said he really doesn't know. To Dale and russ: charge is a property of fundamental particles ( there's no denying that ), but to just say that that would be sufficient is not enough, I think. It's a bit like saying the sky is blue, but what is blue ? Of course, for a blue sky, we can go into EM radiation etc etc. "Blue" as a property itself doesn't mean anything .. ... by way of analogy. I was just wondering if anyone could explain to me something more fundamental about what a charge is , rather than just it's properties... ...

 

[russ_watters]

The reason I am asking this here is because when I ask my professor at Columbia University ( he's a particle physicist ), he stared at the wall about it for 15 seconds, looked at me, and said he really doesn't know.

He does know, he's just not used to the question being worded that way. We, on the other hand, get this question about once a week.

...but to just say that that would be sufficient is not enough, I think. It's a bit like saying the sky is blue, but what is blue ? Of course, for a blue sky, we can go into EM radiation etc etc. "Blue" as a property itself doesn't mean anything ...

Sure it does. "Blue" is a range of EM light wavelengths (centered around 420 nm) that stimulate a particular type of photoreceptor cells in the human retina:

That is a complete explanation of what "blue" is. There just plain isn't any need for anything "more".

http://en.wikipedia.org/wiki/Trichromacy#Mechanism_of_trichromatic_color_vision

I'm really not trying to be condescending, but these types of questions are often thought to be "outside" of science and needing some sort of philosophical realm to explain them (which is why your professor got so uncomfortable), when in reality, science does just fine.

 

[SpaceNerdz]

Sure it does. "Blue" is a range of EM light wavelengths (centered around 420 nm) that stimulate a particular type of photoreceptor cells in the human retina:

But that's just what I said by analogy. To say that the sky is "blue" without going on to say anything about EM waves doesn't actually say anything much about the sky, or "blue" for that matter. It's like saying that "blue" is just not red, not green, not orange, not violet etc -- > Without using EM to explain it, " blue" has no meaning ( physically anyway ).

Similarly, I'm trying to understand what a charge is physically ( not philosophically -- I can't stand philosophy ), and to say that a charge is a fundamental property of matter, or that it has a positive and a negative, doesn't actually explain what a charge is.

 

[Drakkith]

Similarly, I'm trying to understand what a charge is physically ( not philosophically -- I can't stand philosophy ), and to say that a charge is a fundamental property of matter, or that it has a positive and a negative, doesn't actually explain what a charge is.

My two cents:

Consider that two particles will either attract or repel each other, with the strength of this attraction of repulsion varying with the distance between the charges. What causes this to happen? The simplest answer is that each particle possesses a property we have labeled "charge". But what is charge? We define it to be the cause of the interaction between two "charged" particles.

Note that what we've done is we've taken an observation, that two particles will attract or repel each other, and come up with an explanation for it. We say, "The particles interact because they are charged". This isn't a philosophical answer, this is how science works and how we've figure out how the universe works so far, by creating explanations for our observations that enable us to make testable predictions. There is no "what is charge really". The full extent of what charge is lies in a simple definition combined with the mathematical rules governing how it works.

You can get as detailed as you'd like in describing how it works, you can bring in the electromagnetic field, photons, virtual particles, and anything else you want, you can even write books and articles translating the math into a language that the layman can understand, but in the end it comes down to a simple definition: Electric charge is the property of matter that causes it to experience a force in the presence of an electric field.

It's really as simple as that. ;)

 

[Dale]

to just say that that would be sufficient is not enough, I think.

Luckily we don't just say that.

We also say all of the different ways that charge behaves, every way that it influences matter and energy. We also say all of the ways that it is related to other concepts, such as Noether's theorem relating the conservation of charge to the gauge invariance of fthe Lagrangian.

With all of that we have a complete picture of charge, every behavior and influence and relationship. If that doesn't satisfy the "what is it really" question then nothing scientific will satisfy it.

 

[SpaceNerdz]

With all of that we have a complete picture of charge, every behavior and influence and relationship.

That's great ! ... what's the complete picture of charge then ?

 

[Drakkith]

That's great ! ... what's the complete picture of charge then ?

It's the sum of knowledge explaining how charged particles interact with themselves and with various fields.

 

[Matterwave]

That's great ! ... what's the complete picture of charge then ?

If you wanted to know the latter half of Dale's sentence regarding the Lagrangian, it would be "charge is the spatial volume-integral of the 0th component of the Noether current associated with the global abelian gauge symmetry of the underlying particle fields e.g. $\psi\rightarrow e^{i\alpha}\psi$". But I don't think this tells you any better what charge "really" is than the other descriptions given above.

 

[SpaceNerdz]

It's the sum of knowledge explaining how charged particles interact with themselves and with various fields.

That's not very insightful statement, because it's a statement you could have made without knowing much about charge... ...

 

[Drakkith]

That's not very insightful statement, because it's a statement you could have made without knowing much about charge... ...

And yet it's the answer to your question.

 

[SpaceNerdz]

And yet it's the answer to your question.

No, it really isn't. My question is about what a complete picture of charge is ( physically ) . The statement you gave merely re-states my question.

 

[Drakkith]

Your question has been answered more than once. Despite your insistence that you want some kind of a "physical" answer to what charge really is (whatever a "physical answer" even means here), you seem to be wanting some philosophical answer that science can't answer. It's like asking what a dog really is, and then handwaving the answer away when you're given the complete description of what dogs generally look like, how they behave, and where they came from. Therefore, thread locked.

 

[Dale]

Sorry Drakkith, I thought that I should at least point the OP to QED

That's great ! ... what's the complete picture of charge then ?

The complete picture is called Quantum Electrodynamics, or QED: http://quantummechanics.ucsd.edu/ph130a/130_notes/node508.html

It is summarized in this one key equation:

In the classical limit QED reduces to Maxwell's equations and the Lorentz force law. To my knowledge, there is no EM experiment whose results cannot be described with QED, so it forms a complete picture of all known behaviors of charge.