Why does the electron orbit the nucleus?

[SLD]

I know the short answer is the electromagnetic force, but that of course just begs the question of how the electromagnetic force actually accomplishes this feat. Are electrons and protons in a simple hydrogen atom throwing off a bunch of photons to each other?

I suppose the same thing could be asked of the sun and earth with respect to gravity.

 

[Shadowy Man]

The answer will depend on how deep down the physics rabbit holes you want to go.

The Bohr model has the electron “orbit” like a planet around the star due to the electric potential, but this isn’t actually a good model.

The QM model fixed Bohr’s problems by having the electron have a wave function with quantized states within the nucleus’ electronic potential. The word “orbit” doesn’t really make much sense anymore.

I don’t know too much about quantum field theory so all the virtual particle mediation of forces is a bit beyond my understanding.

 

[bigfield]

I know the short answer is the electromagnetic force, but that of course just begs the question of how the electromagnetic force actually accomplishes this feat. Are electrons and protons in a simple hydrogen atom throwing off a bunch of photons to each other?

I suppose the same thing could be asked of the sun and earth with respect to gravity.

In quantum mechanics, electromagnetic interactions, including the attraction of electrons to photons, is modelled by the ongoing exchange of virtual photons. These are different than real photons in that they don't actually have any energy. Plus, electrons don't orbit the nucleus like a planet orbits a star. Instead, electrons move around the nucleus in an unpredictable manner.

The motion of the Sun and Earth is best modelled by relativity. Massive objects cause spacetime to curve, and the motion of objects follows that curvature.

 

[SLD]

I know the short answer is the electromagnetic force, but that of course just begs the question of how the electromagnetic force actually accomplishes this feat. Are electrons and protons in a simple hydrogen atom throwing off a bunch of photons to each other?

I suppose the same thing could be asked of the sun and earth with respect to gravity.

In quantum mechanics, electromagnetic interactions, including the attraction of electrons to photons, is modelled by the ongoing exchange of virtual photons. These are different than real photons in that they don't actually have any energy. Plus, electrons don't orbit the nucleus like a planet orbits a star. Instead, electrons move around the nucleus in an unpredictable manner.

The motion of the Sun and Earth is best modelled by relativity. Massive objects cause spacetime to curve, and the motion of objects follows that curvature.

The problem is, as Shadow points out, is that each of these explanations are just question begging. And one must delve into the intricacies of QED. Which requires a graduate level education to understand.

But another question arises as well. How does the electron orbit the nucleus? Not in the sense of the exchange of virtual particles, but what kinds of paths does it actually take, and what determines that path? Orbit is a poor word of course. Obviously it’s random and is the result of quantum fluctuations, but it never just stops in mid space and reverse itself, right? So it’s not 100% random. There are physical limits like conservation of momentum, and other constraints. Are we even able to determine the paths?

 

[Shadowy Man]

In QM “path” is not a meaningful word. The electron has a probability distribution. You only know where it could be. To say it follows a path is extra formalism that doesn’t add predictive value to the theory.


I’ll admit it’s been a while since my last Quantum class, so maybe I’m wrong.

 

[thebeave]

As I recall from my college physics courses 35 years ago, the electron orbit was described as a "probability cloud".

 

[bigfield]

The problem is, as Shadow points out, is that each of these explanations are just question begging. And one must delve into the intricacies of QED. Which requires a graduate level education to understand.

Science will never arrive at a complete answer. Every time scientists answer a question, it just provokes another one. Scientists will be delving forever.

But another question arises as well. How does the electron orbit the nucleus? Not in the sense of the exchange of virtual particles, but what kinds of paths does it actually take, and what determines that path? Orbit is a poor word of course. Obviously it’s random and is the result of quantum fluctuations, but it never just stops in mid space and reverse itself, right? So it’s not 100% random. There are physical limits like conservation of momentum, and other constraints. Are we even able to determine the paths?

Quantum mechanics says we can't predict either the exact location or momentum of an electron, which means that we can't predict which path an electron follows, let alone figure out what determines that path, or whether it even follows a continuous path in the first place. On top of that, the double slit experiment shows that electrons exhibit wave-like properties, which means that electrons are not simply little specks zipping around outside the nucleus.

 

[bilby]

It doesn't.

Electrons have a higher probability of being found in an area close to their associated atomic nucleus than they do of being found anywhere else; But their probability of being found vastly further away is non-zero. Their exact position cannot be known at the same time as their exact momentum, so any kind of orbital motion is impossible - or at least, impossible to ever observe, which is a distinction best left for philosophers to argue over.

 

[steve_bank]

I thought the current model has electrons as a swarm around the nucleus, not discrete orbits like planets.

I'd say we can't answer why, but we have models that work in predicting higher level experiments, like electric current.

In an interview Carver Meade said 'I don't know if electrons exists, but I can do useful things with the concepts'.

I used to have a copy of Millikan's book The Electron.

He described his oil drop experimental set up and it had all his measurement data with oil drops.

In terms of experiment it appears to be a discrete particle in that energy can only change in discrete increments.


Models are an interpretation of observation.

 

[James Brown]

Why don't the electrons and the protons click together like all all things with opposite charges do?

 

[skepticalbip]

Why don't the electrons and the protons click together like all all things with opposite charges do?

They would if they were little balls like is popularly imagined. The problem is that they are nothing like anything we would commonly be familiar with. There is no description of what they are as far as I am aware. There is only some understanding of the effects of their behavior under given conditions.

 

[Shadowy Man]

Why don't the electrons and the protons click together like all all things with opposite charges do?

The electron can only exist in quantized energy states within the potential well generated by the. nucleus. No zero energy state is allowed.

In classical EM, an orbiting electron would emit radiation and it would spiral into the nucleus. That was one clue that modern physics needed to be invented to explain the structure of the atom.

 

[Treedbear]

I don't feel so bad anymore about getting a C in high school chemistry after failing to make a model of an atom out of styrofoam balls.

 

[Loren Pechtel]

The answer will depend on how deep down the physics rabbit holes you want to go.

That's what tends to happen when you ask why about most anything in physics. It's rabbit holes inside rabbit holes until you either reach your limits, or his the limits of human knowledge.

 

[bilby]

Electrons and photons really aren't like anything in human experience at the macroscopic scale. People talk about 'particles' as though they were miniature billiard balls; Or 'waves', as though they were vibrations in a guitar string. But they're really neither of those, and it's not particularly better in most cases to say that they have characteristics of both. They are like neither; They are a thing unlike anything with which we are familiar, but they do obey very clear and (nowadays) well understood rules of behaviour. Some of those rules are probabilistic, but that's OK because for most applications, we deal with large numbers of these entities, and the probabilities cancel out into a single macroscopic behaviour which we call classical physics. If you are dealing with trillions of them, you usually don't need to know or understand how each individual is behaving in order to understand the behaviour of the collection.

If you do need to deal with small numbers of subatomic particles, then it's easier not to try to make analogies with familiar macroscopic objects; Just "shut up and calculate", as David Mermin put it.

Sadly, this approach is not considered viable for teaching schoolchildren, so they are given models that everyone knows are wrong, and then asked to unlearn those models (often more than once) as their education advances. Couple this with the authoritarian nature of most schooling, and you get students with little understanding of reality, and even less faith in the honesty of those who do have an understanding of it.

I do wonder whether it might be better to start by saying that it's futile to attempt any analogy, and just give students the facts, and show them the experimental approaches that allowed us to determine what those facts are.

 

[steve_bank]

http://web.physics.ucsb.edu/~phys128/experiments/electron/Millikan-Oil-Drop-Manual-AP-8210A.pdf

An updated version of the oil drop experiment.

Milikan's apparatus was more primitive and took him years to gather all his data working nights. Not as well known, he was up there wit Einstein and others.

There were competing theories to explain electric current. His won out.

 

[Bomb#20]

Why don't the electrons and the protons click together like all all things with opposite charges do?

As long as you don't want to go very deep down the physics rabbit hole, the simple answer is that they have "angular momentum". The electron's angular momentum is the component of its velocity at right angles to the direction to the proton, multiplied by its mass, multiplied by its distance from the proton. Angular momentum is "conserved", i.e. it's a law of physics that you can't ever create or destroy angular momentum. Whenever you add some angular momentum to one thing you have to subtract an equal amount from something else, just like the conservation of energy. Because of this, whenever a spinning object shrinks, provided it doesn't bump into something else that it can transfer some angular momentum to, it has to spin faster, in order for the angular velocity times the distance to the center to stay the same. The standard example is a spinning figure skater -- when she pulls her arms in she spins faster. The point being, if an electron clicked together with a proton, the distance between it and the proton would go to zero, so its angular velocity would have to go to infinity. Since the electron can't speed up to infinity, it can't ever land on the proton.

 

[barbos]

Why don't the electrons and the protons click together like all all things with opposite charges do?

As long as you don't want to go very deep down the physics rabbit hole, the simple answer is that they have "angular momentum". The electron's angular momentum is the component of its velocity at right angles to the direction to the proton, multiplied by its mass, multiplied by its distance from the proton. Angular momentum is "conserved", i.e. it's a law of physics that you can't ever create or destroy angular momentum. Whenever you add some angular momentum to one thing you have to subtract an equal amount from something else, just like the conservation of energy. Because of this, whenever a spinning object shrinks, provided it doesn't bump into something else that it can transfer some angular momentum to, it has to spin faster, in order for the angular velocity times the distance to the center to stay the same. The standard example is a spinning figure skater -- when she pulls her arms in she spins faster. The point being, if an electron clicked together with a proton, the distance between it and the proton would go to zero, so its angular velocity would have to go to infinity. Since the electron can't speed up to infinity, it can't ever land on the proton.

Well, it's a completely wrong theory as to why electron can't fall on nucleus.
Electron change angular momentum when it emits radiation. Photons have angular momentum. So total angular momentum conserves.

 

[Bomb#20]

As long as you don't want to go very deep down the physics rabbit hole, the simple answer is ...

Well, it's a completely wrong theory as to why electron can't fall on nucleus.
Electron change angular momentum when it emits radiation. Photons have angular momentum. So total angular momentum conserves.

I.e., you can always go deeper down the rabbit hole; but the point is to provide an intuitive picture. Suppose light weren't quantized and the "ultraviolet catastrophe" were real -- how fast would an electron be going when it hit the nucleus?

 

[barbos]

As long as you don't want to go very deep down the physics rabbit hole, the simple answer is ...

Well, it's a completely wrong theory as to why electron can't fall on nucleus.
Electron change angular momentum when it emits radiation. Photons have angular momentum. So total angular momentum conserves.

I.e., you can always go deeper down the rabbit hole; but the point is to provide an intuitive picture.

There is no intuitive picture. And THAT's the point.

Suppose light weren't quantized and the "ultraviolet catastrophe" were real -- how fast would an electron be going when it hit the nucleus?

Does not matter, classical EM radiation still carry angular momentum. Also, ultraviolet catastrophe is a separate problem.