Why doesn't the electron fall into the nucleus?

[granpa]

The question I have is, has anyone ever purposed a therory of air or a gas like substance being bound inside the valence shell, and equally divided between the other shells, I see this like compressed air inside a basketball. Heat would have an effect and an expansion and contraction would take place.
Ron

that was my first thought too. but the electron is a single indivisible particle. if it seems odd to you that it can be a single indivisible particle and at the same time be spread out over an entire orbital then it shouldnt. it can even be spread out simultaneously over several orbitals (as when it is absorbing or emitting light). that's called 'superposition'.

 

[Count Iblis]

So, in your opinion, what is the definition of moving?

In quantum mechanics, it makes sense to define a particle to be moving if it is described by some wavepacket. It then has an approximate position and momentum. You can also argue that since a plane wave represents a particle with precisely determined momentum, such a state should also be considered to be "moving". However, in that case, the state is not evolving in time (apart from a phase factor).

Thing is that you can't separately specify momentum and position in quantum mechanics. Once you've specified all the mometum components of a state, the state is completely fixed; all the position components of the state thus fixed.

In classical physics you can consider momentum and position to be completely independent. Our notion of "motion" is grounded in classical physics, so it is not a well defined concept to start with.

It is similar to building a house out of bricks and then looking at the house from some distance so that you don't see the individual bricks anymore. You can make all sorts of objects with different shapes out of bricks. But if you look closely and you see the individual bricks you will be constrained in what shapes you can make. You can only make 90 degree turns, so smooth curves do not exist at this level.

Similarly, we can build a state that looks like a particle moving from one position to another by making a wavepacket. If you look at it from a large length scale and you cannot resolve the width of the wavepacket, you see what looks like a particle that seems to have a definite position which is moving at a definite velocity.

But what we see is an illusion. The laws of physics forbid that such a state could exist at all. Yet, we have defined the very concept of motion to refer to exactly such a state.

 

[RonL]

that was my first thought too. but the electron is a single indivisible particle. if it seems odd to you that it can be a single indivisible particle and at the same time be spread out over an entire orbital then it shouldnt. it can even be spread out simultaneously over several orbitals (as when it is absorbing or emitting light). that's called 'superposition'.

My thought stemed from the many times i have looked at something through a spinning prop, and knowing from experience that a dense object will not pass through a shop fan. The electron has small mass and due to it's velocity, is spread all around the shell area as though it were a solid wall. If any substance exist that fills spaces between shells down to the nucleus, it seems that the movements might be somewhat like our upper atmosphere, where layers move past each other yet do not mix.

I feel that a thought is the start of any learning process, for me, I'm sure I'll look back at this in the future and sink a little lower in my chair, or it might be the start of something special for me at least.

 

[granpa]

you are talking as though the electron shell were hollow. it isnt.

 

[RonL]

you are talking as though the electron shell were hollow. it isnt.

WELL, that sure changes things

 

[Born2bwire]

You have to define pecisely what you mean by "moving". So, you need to write down some observable, e.g. the momentum operator, and look at the expectation value. In case of a particle in a box in some energy eigenstate, the expectation value of the momentum is zero. The particle in an energy eigenstate is in a superposition of two states with opposite momenta.

It should be noted that any stationary state will have an expectation value of zero for the momentum. Eigenstates for the particle in a box, hydrogen atom, etc. derived from the time-independent Shroedinger equation will all have zero expectation value for the momentum.

 

[DrChinese]

You don't mean to say local hidden variable? Even unlocal hidden variable interpretations have been disproven? I was unaware of this.

As you say, the generally accepted view is that there can be no successful local hidden variable theories. I generally stick to the solid stuff (such as Bell). However...

I would say we are very close to the point where even non-local HV theories will be generally considered as "no-go". Now, if we want to debate the underlying validity of the science, that belongs in another thread. And I am not advocating this position anyway, as there is currently a lot of controversy in the area. But there is a LOT more that just a few papers on the subject. And there is a LOT more that just a few people who have already come to this conclusion. The conclusion being based on the following approaches: Kochen-Specker, GHZ, Hardy and Leggett. So my answer is based on my opinion of where the scientific winds are blowing. And I think that HV candidates will have a tough time against these.

Now for the Bohmians: I personally am very interested in finding the best answers to these and other questions about non-locality as an approach. Honestly, it is the most "logical" explanation. And I realize that Bohmians do not consider theirs a non-contextual theory (although I do because it claims to be deterministic). We are lucky to have several top-notch Bohmian theorists on this board, and I think their contributions are very exciting. So I certainly hope no one takes offense because I am not trying to stir up a hornet's nest.

But my opinion doesn't really matter anyway, what I am really saying is that the scale is in the process of tipping in the view of the scientific community at large - and I consider this a relatively new development. If this were a boxing match: I would be hoping I had bet against HV approaches. But the match is not yet over, and I expect there will be a lot of relevant interesting papers coming out in the few years.

 

[alxm]

This isn't suggesting that electrons "move" continuously, right?

They're in constant motion, yes. But they don't move continuously as in 'a continuous path', no. But that much has already been established when saying they don't have a definite location.

See, there's a very common pedagogical problem here. First people learn about the Bohr model and how that's wrong - the electrons don't have a definite location, and are instead described by a location-probability distribution (orbital).

It's then easy, and very common, to make the mistake of thinking that that's the whole picture - that the electron isn't moving, but is stationary - because the probability distribution is stationary. But that would only be half the picture. They still move. Just not in continuous paths. There's no accounting for purely dynamical effects such as correlation energy (the effect on kinetic energy from the correlated motions of electrons), unless you actually think of them as moving.

Is it conceptually difficult to reconcile the idea that the thing is moving, yet has a constant distribution of location-probabilities? Yes. But that's just quantum mechanics for you. Neither particle nor wave, you know.

 

[thoughtgaze]

Is it conceptually difficult to reconcile the idea that the thing is moving, yet has a constant distribution of location-probabilities? Yes. But that's just quantum mechanics for you. Neither particle nor wave, you know.

How is it conceptually difficult? You can take an analogy to be the motion of a particle oscillating on a pendulum. The probability of measuring the particle at some point at some random time will be constant yet the thing is moving. The probability density is greatest at the turning points of oscillation and is minimum at the equilibrium point.

 

[ZapperZ]

How is it conceptually difficult? You can take an analogy to be the motion of a particle oscillating on a pendulum. The probability of measuring the particle at some point at some random time will be constant yet the thing is moving. The probability density is greatest at the turning points of oscillation and is minimum at the equilibrium point.

These two statements that I highlighted appear to be contradictory to each other.

The reason why the scenario is conceptually difficult is that it makes OTHER issues difficult. A pendulum has its bob a specific location as a specific time, not spread out all over its trajectory, the latter of which is the conventional picture adopted by standard QM. And there ARE consequences of such a scenario, ranging from molecular bonds, to the existence of the coherent gap in SQUIDs experiments.

Besides, is the mathematics describing the two systems even equivalent?

Zz.

 

[thoughtgaze]

These two statements that I highlighted appear to be contradictory to each other.

The reason why the scenario is conceptually difficult is that it makes OTHER issues difficult. A pendulum has its bob a specific location as a specific time, not spread out all over its trajectory, the latter of which is the conventional picture adopted by standard QM. And there ARE consequences of such a scenario, ranging from molecular bonds, to the existence of the coherent gap in SQUIDs experiments.

Besides, is the mathematics describing the two systems even equivalent?

Zz.

I meant that the probability distribution is constant IN TIME yet the thing is moving. The probability distribution is clearly not constant with regard to position. And no the mathematics describing these two systems are not equivalent. I think that's because of how we perceive things though, perhaps they are equivalent with regard to some other dimension... anyway this is not the point. I was only showing how it's not conceptually difficult even in the classical sense to think that a probability distribution is constant yet have the particle moving.

 

[JayAaroBe]

I've read all the threads relating to this question but my mind is just having trouble with the concept. Let's take the simplest case: a hydrogen atom with one proton and one electron. They have opposite charges and attract. If I think of each as a particle, then my mind wonders why wouldn't the two particles attract each other and come together and collide.

The only way I can make sense of it is as follows, and tell me if this is an okay way to think of it: the electron is not a particle, but rather a spherical cloud of charge. In this case, I think of it the electron not as a single point but rather as the atmosphere around a planet and the planet itself as the proton (I know the sizes are not to scale). This is the only way I can think of it that makes any sense to me and answers the question of why an electron is not pulled into the nucleus (the proton).

 

[eaglelake]

Re: why doesn't the electron fall into the nucleus!?


This is what happens when we insist on using classical physics to describe a quantum system! Classically, an electron-proton system has no equilibrium state – the electron will spiral into the nucleus while radiating away its orbital energy. There is no such thing as a “classical atom”. There are only “quantum atoms”. And, quantum mechanics is about probabilities. It does not describe the motion of the particles involved. In fact, the electron and the proton are entangled in a way that has no classical analog. We must think of the atom as a single entity. The electron and proton are not separate objects that have independent identities . I know this is not what most of you want to hear, but there is no “electron moving around a nucleus”!

So, what does quantum mechanics tell us about the hydrogen atom? It tells us the possible values to expect IF WE MEASURE the energy, for example, and it tells us the probability of obtaining each energy value. Notice that we do not know the atom’s energy, but only the value we might get as a result of an energy measurement. This is because the atom is further entangled with the energy measuring device: the atom is non-separable from the rest of the experimental apparatus. The bottom line is this – we only know that we have an experimental apparatus involving hydrogen atoms that measures the energy.

It is very difficult to discuss such things because we are using the language of classical physics to describe non-classical events. This is an unavoidable dilemma that physicists are forced to live with. And it certainly generates a lot of controversy!
Best wishes

 

[A. Neumaier]

The only way I can make sense of it is as follows, and tell me if this is an okay way to think of it: the electron is not a particle, but rather a spherical cloud of charge. In this case, I think of it the electron not as a single point but rather as the atmosphere around a planet and the planet itself as the proton (I know the sizes are not to scale). This is the only way I can think of it that makes any sense to me and answers the question of why an electron is not pulled into the nucleus (the proton).

This is indeed the most sensible classical view of an atom among the various possibilities, but
it doesn't explain what you'd like it to explain. Indeed, a classical charged electron cloud would still be sucked in by the proton. So, no matter how you do it, classical pictures for quantum phenomena are somewhat limited. The stability of an atom cannot be explained classically, but requires an understanding in terms of quantum mechanics.

 

[Ikoro]

energy and momentum is conserved by the electron...In some sense the electron is in a non-inertial reference frame because of its constant motion or velocity.