cantab

Why are the charges on the electron and proton the same?

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I got to thinking about this just now. Why are the charges on the electron and proton the same? (Well, same magnitude, opposite sign). One is a fundamental particle, the other is a seemingly unrelated composite particle, so is there a strong theoretical ground for them being equal?

For that matter, are they the same? It seems like all methods to measure the elementary charge are measuring it on electrons. Have their been any direct measurements of the proton charge? We know from chemistry that they have to be very close, but would chemists pick up say a one part per million difference in electron and proton charges?

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Refer to the 'charge' in this table, considering that a proton is composed from 2 Up and one Down quark:

File:Standard_Model_of_Elementary_Partichttps://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg

A longer explanation: https://en.wikipedia.org/wiki/Quark#Electric_charge

 

Edited by swjr-swis
image not working...

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Doesn't really answer his question. Science is pretty good at explaining the 'how'. The 'why' takes a bit more work. We'll get there eventually. We might already be there, but that probably takes several years of grad school to properly understand, QED and QCD and such.

Or, you can use Pratchett's catch-all explanation for stuff like this, which is "because quantum".

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Quick question. Do the numbers beside "spin" actually represent angular momentum?

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Did some reading up on this one. It gets pretty deep into some really nasty areas of physics.

I started out on the premise that there's a "smallest possible" quantum of electrical charge. That apparently is the case, but the electron and proton aren't it. The smallest possible unit of electrical charge seems to be a quark, with a charge 1/3 that of an electron--except that the only stable groups of these quarks are in threes, resulting in particles with +1e or -1e; anything else is unstable and goes kablooie, then reforms into stable stuff. So the answer is that the units we see are the only ones that are stable.

Caveat: this is only the smallest possible charge we know of--it's entirely possible quarks are themselves made up of smaller particles with smaller units of charge.

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1 hour ago, WedgeAntilles said:

The smallest possible unit of electrical charge seems to be a quark, with a charge 1/3 that of an electron

You either got confused with weak hypercharge, or got led astray all together. This is not how quanta work in QM. The quantum of something is the smallest amount of change, and the quantum of electromagnetic charge is 1e. No interaction can change charge by less than 1e. A good example is a weak interaction between electron and an up quark, which can produce an electron neutrino and a down quark. The lepton's charge went up by 1e, and quark's dropped by 1e, despite the fact that quark's charge is fractional. There is still no interaction that will change a charge of something by 1/3 e.

The problem with electrical charge is that we observe it as a single quantity, but it's really a composite. It goes back to the whole mixing of U(1) and SU(2) symmetries that Higgs Boson is involved in. The quantity corresponding to the U(1) is the weak hypercharge, YW, which works the way you'd expect charges to work. The other quantity, corresponding to SU(2), is the weak isospin, T. And like any isospin, it has three components, giving us, finally, the formula for total electric charge, Q = T3 + YW / 2.

This is where things get really interesting. I'm not really aware of any constraints on YW. I have no idea why it's +1/3 e for all quarks, and +1 e for all leptons. But the actual unit of charge at 1e comes from the weak isospin part. The weak interaction flips it between +1/2 e and -1/2 e, exactly like the electron's spin. Which means that the total change of charge is always 1e.

 

Edit: Yeah, looking a bit more into literature, there is no reason in Standard Model for weak hypercharge to be quantized, since U(1) has a trivial generator. Still if values for leptons and quarks were anything else, we simply wouldn't have chemistry, and that'd be kind of bad. I suppose, that's the underlying appeal of Supersymmetry.

Edited by K^2

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I'm just repeating in here what I read on other sites. They were all written by other people, so blame them, not me! :lol:

I did find stuff about quarks having a quantum of +/- 1/3 e, but that none of those are stable by themselves.

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If the charges were different would atoms still exist in the same way, and if not would we be here to ask the question?

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8 hours ago, WedgeAntilles said:

I'm just repeating in here what I read on other sites. They were all written by other people, so blame them, not me! :lol:

I did find stuff about quarks having a quantum of +/- 1/3 e, but that none of those are stable by themselves.

It's not a quantum of charge. Whoever called it a quantum was wrong. The quantum is still 1e. It's the same as with spin. Electron's spin is ±1/2ћ, but the quantum of angular momentum is always 1ћ. So the spin can only change by 1ћ.

Example, spin flips from +1/2ћ to -1/2ћ. As it does that, it will emit a photon (possibly a virtual one), that will carry angular momentum of +1ћ. Which you can picture as EM vacuum going from 0ћ to +1ћ as that photon is created. You should note similarities with the process I described for a weak interaction in my previous post. The only weird thing about quarks is an offset of +1/6 e, due to the weak hypercharge of +1/3. So instead of -1/2 e and +1/2 e, you end up with -1/3 e and +2/3 e. Likewise, offset for leptons is +1/2 e, due to weak hypercharge of +1.

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I like to see the Feynman-Stueckelberg-interpretation https://en.wikipedia.org/wiki/Antiparticle#Feynman.E2.80.93Stueckelberg_interpretation as reason: Positrons are electrons moving backwards in time. Hence their interaction is time-reversed, but otherwise the same. It is disputed in how far this relates to reality and in how far it is just a mathematical feasable description, but if it is so, it is a quite good reason.

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I say "tomato", you say "tomato".........

Errr.....gee, that joke doesn't work when it's typed instead of spoken, does it?? :lol:

I'm fine with my use of the word "quantum" the way I used it.

 

2 hours ago, K^2 said:

It's not a quantum of charge. Whoever called it a quantum was wrong.

Actually, we can't ever know that for sure. A while back (i.e. a couple thousand years) it was thought matter could be divided infinitely and still retain the same properties. Back then, no such thing as a "quantum" was known to "modern" science.

"1e" is merely the smallest possible unit of electrical charge that we know of.......

Keep that in mind.

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7 minutes ago, WedgeAntilles said:

Actually, we can't ever know that for sure. A while back (i.e. a couple thousand years) it was thought matter could be divided infinitely and still retain the same properties. Back then, no such thing as a "quantum" was known to "modern" science.

"1e" is merely the smallest possible unit of electrical charge that we know of.......

Keep that in mind.

The only interaction that can change the charge of a particle is the weak interaction via the weak isospin flip. That is restricted to 1e by the SU(2) symmetry. Change by another increment would require a new fundamental force to be discovered. And not just any force, but one that fits a list of criteria that will make a profound impact on a host of known phenomena, making it incredibly unlikely to have been undetected to this day. I won't say impossible, of course, but this is not "Oh, we just don't know any better," type of thing. It's something with a known cause, well-understood mechanism, and very little in terms of possible loopholes. I am objectively confident in saying that 1e is the quantum of electric charge.

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27 minutes ago, WedgeAntilles said:

Then we disagree.

You disagree with Field Theory? Bold. I'm sure it's based on decades spent in the field and results of colossal research effort on your part, and not on total ignorance of the theory. Because, you know, that would be kind of dumb.

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I don't disagree with field theory, I disagree with you.

But if you're going to be rude about it, then we're done.

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Loads of this quantum physics is way complicated ...

But it sounds like from what K^2 is saying, the quantisation of charge is connected to these mathematical symmetries, which in turn come from the symmetrical nature of space? In other words charge is quantised because all directions and positions are equivalent? Elegant, if I'm understanding this right.

And you say that SUSY might complete the picture by constraining this "weak hypercharge" element?

I'll probably be better able to understand the other side of my question: How might we go about measuring the charge on the proton, without making the assumption it's the same elementary charge as on the electron? Has this measurement been done and how precise is it?

So far the best I've turned up is that you might be able to measure the mass of a known quantity of hydrogen atoms, then take off the electron masses and binding energies to get the proton mass. Measuring the proton's mass-to-charge ratio very precisely is well-established. I've no idea of the details, or if this is even a practical method.

Edited by cantab

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7 hours ago, cantab said:

Loads of this quantum physics is way complicated ...

"You don't really understand something unless you can explain it to your grandmother."

I forget where I heard that, but I took it to heart. Physics degree or not, everybody should try and keep it to words that don't need people to be looking stuff up on dictionary.com.

So here ya go: as far as we know, the Universe had to start out electrically neutral. Zero charge, total. So after the Big Kaboom, whenever a positive charge popped up somewhere (the "how" is not important here) an equal negative had to pop up somewhere. And pretty much everything in the universe has a "smallest possible amount". There's a shortest possible distance (nothing can be smaller or shorter), a minimum possible amount of mass, a minimum possible amount of electricity. That's why the electron and the proton have the same amount of electric charge: they're both the minimum possible that we can observe in our universe.

Whether they're the smallest possible amount of electricity, PERIOD, seems to be a point of contention..... :lol: Anyway, there ya have it: an answer that doesn't need a dictionary. If you're interested in what it is that gives the electron and proton their electric charge......well, that's all theoretical.

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On Thursday, January 28, 2016 at 0:36 AM, Kaos said:

I like to see the Feynman-Stueckelberg-interpretation https://en.wikipedia.org/wiki/Antiparticle#Feynman.E2.80.93Stueckelberg_interpretation as reason: Positrons are electrons moving backwards in time. Hence their interaction is time-reversed, but otherwise the same. It is disputed in how far this relates to reality and in how far it is just a mathematical feasable description, but if it is so, it is a quite good reason.

I like that interpretation since it, as I understand it, neatly answer the question of where all the antimatter that should have come from the big bang is. It all simply went on racing the other way in time. Of course this is probably false. After all I don't qualify even as a theoretical physics hobbyist.

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17 hours ago, cantab said:

How might we go about measuring the charge on the proton, without making the assumption it's the same elementary charge as on the electron?

There are tons and tons of indirect measurements that would have revealed tiniest of discrepancies. The transition energies can be measured extremely precisely. You can use EPR to correct these and get many decimal places on your charge estimate. You could get q/M ratio directly by shooting charged particles past a magnet. You can look for excess electrons in conductive materials to prove that the proton/electron ratio in neutral materials is 1:1. I could probably go on.

Direct measurements are hard, but we very rarely need them.

10 hours ago, WedgeAntilles said:

"You don't really understand something unless you can explain it to your grandmother."

My grandmother is perfectly happy with "God made it that way." Fortunately, this wasn't an acceptable standard of understanding for people who actually advanced our knowledge and gave us medicine and technology.

People are perfectly happy to accept that they aren't as strong as someone else. Or not as rich. But the moment the subject touches intelligence, every fool seems to take a position that if they can't understand something, than neither can anybody else. So if you can't explain it to them, it's the fault of the person who does the explaining. Obviously, it can't possibly be insufficient education or insufficient mental capacity. That's the reason we still have people trying to ban evolution being taught in schools. That is not a good company to be in.

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58 minutes ago, K^2 said:

My grandmother is perfectly happy with "God made it that way."

And you understood that explanation, even if you disagreed with it, right? That's what I call keepin' it simple!

 

58 minutes ago, K^2 said:

People are perfectly happy to accept that they aren't as strong as someone else. Or not as rich. But the moment the subject touches intelligence, every fool seems to take a position that if they can't understand something, than neither can anybody else. So if you can't explain it to them, it's the fault of the person who does the explaining.

Yup. That's why you keep it in layman's terms so they can't blame you for using big fancy words that few people can spell. ^_^

(and also to maximize the chance that someone without a degree in particle theory, but who wants to understand it, can understand it!)

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16 minutes ago, WedgeAntilles said:

And you understood that explanation, even if you disagreed with it, right? That's what I call keepin' it simple!

 

Yup. That's why you keep it in layman's terms so they can't blame you for using big fancy words that few people can spell. ^_^

(and also to maximize the chance that someone without a degree in particle theory, but who wants to understand it, can understand it!)

Quantum Mechanics isn't simple, therein lies the problem. English isn't adequate to explain it, only math can do that. Unfortunately, the universe doesn't care whether or not we can explain it in our own language.

Edited by SargeRho

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3 hours ago, WedgeAntilles said:

(and also to maximize the chance that someone without a degree in particle theory, but who wants to understand it, can understand it!)

They should also make violins simple enough that anyone can play them. That's not how the real world works. It's not enough that you want something. There is also a factor of training and even natural capability. You can't understand Quantum Mechanics, let alone Quantum Field Theory. You lack training for it. Why do you insist that things you can't understand get dumbed down until you do?

This kind of attitude is precisely the problem we are constantly facing in trying to prevent education system from completely collapsing. Everyone demands to make things understandable to them, which leads to entire education system devolving to a common denominator. This is bad. It's bad for you, because you simply need to get over it, it's bad for people who are actually trying to learn something and are willing to put in necessary effort, and it's really bad for society, because we simply end up lacking sufficient quantity of skilled individuals. And I honestly don't give a crap about your problems, but you are exhibiting the attitude that's destructive and you should at least have the common sense to stop.

If you don't understand something, and you are actually willing to learn it, I can point you to a course on linear algebra that will get you started. Keep working at it, and in just a year or two, you'll have a pretty good grasp on basic QM. In a decade, you might start understanding basics of field theory. If you aren't willing to put in that sort of effort, not understanding is your fault and nobody else's. If you can't understand even that, then you really have a problem.

Edited by K^2

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Calm down K^2.

Wedge, part of the problem with quantum mechanics is that 1, nobody *really* understands it. One can know the mechanisms and the math, but there's always that veneer of quantum weirdness that eludes explanation. Anything you've heard about probabilities and uncertainty* is just a result from one interpretation of quantum mechanics, the Copenhagen Interpretation, but there are other entire viewpoints on what's going on down there.

As a result of this, we have problem B, that it can't adequately be explained to laypeople in a way that really is accurate. You might think you know some aspects of quantum physics, but, as K^2 said, you aren't trained in it and only know a rough, simplified explanation. In third grade I was taught that there were no numbers lower than zero (I called BS on this, but my teacher was insistent), and in 8th I was told it was impossible and pointless to square root a negative number. For what I needed to know these explanations were enough, but they weren't *right*. K^2 or another particle physicist could try to dumb-down their field enough for you or another layman to think they understand, but at the end of the day they'd likely have to make too many sacrifices for the explanation to be accurate or really anything but unnecessary false knowledge.

 

*DISCLAIMER: I am not a physicist, and any statement I make is made out of ignorance and the illusion of knowledge I will be elucidating later.

Edited by NFUN

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