Dynamic relationship of the subatomic particles

  • #1

Main Question or Discussion Point

Hello,
I'm quite new to the concepts of quantum theory. So as such I have an interesting quandary. Keep in mind that I don't have as much back ground in study of physics as the lot of you but I will try my best to explain my dilemma.

Given
1)Both the electron and nucleus have a charge and velocity at any given time.
2)That because of this both are emitting electromagnetic waves.
3)Also that the EM radiation being emitted by the nucleus is being absorbed by the electron/s in orbit around the nucleus.

This would be great in the Bohr's model, but since we know that the electron is extremely elusive, there is a small problem. How is it that the photons know exactly where to go to be absorbed?

Is it possible that nucleus knows something we don't (where the electron/s will be at any given point)?

That and the only way that I can see the electron absorbing all the EM radiation is for an electron to have an orbit dependent on the movement of the nucleus (which as I understand it is completely impossible), or for the space inside an atom be warped in such a way for the light to be absorbed by an electron no matter where it is in the atom (even inside the nucleus).

There is another alternative, that the photon emission by the nucleus is everywhere until it is observed and hence absorbed by the electron. This seems apt by what I understand of quantum theory, but is seems more like a cop out to me.
 

Answers and Replies

  • #2
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I don't think the energy has to be localized at the ''center'' of the photon,so your last sentence is probably close to the correct answer,however I don't think we will ever be able to view exactly what happens on such a small scale,that is why it's all calculated with mathematical functions.
 
  • #3
Vanadium 50
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Your statements #2 and #3 are not correct. That's probably the source of the confusion.
 
  • #4
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If there is relative, non-uniform motion between the electron and the nucleus, how does it avoid forming an accelerating dipole? If there is an accelerating dipole, won't it generate a time varying EM field?

I always thought the answer was that the electron was not "moving" (in a classic sense), it was "tunneling" (or some such).

Yet if there is no dipole, what generates hydrogen bonding in chemistry? If there is a dipole (at least on some molecules), where does the energy for the resulting EM field come from?
 
  • #5
Vanadium 50
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Classically, they would radiate. Quantum mechanically, they do not. It is not true that the electron radiates and the radiation is absorbed by the nucleus.
 
  • #6
my concern was not with the electron but with the proton / nucleus. However, you have brought up an interesting view. If the electron by itself can not emit em radiation then nor should the nucleus of an atom. This does bring up a different question, why doesn't a electron emit em radiation?
 
  • #7
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my concern was not with the electron but with the proton / nucleus. However, you have brought up an interesting view. If the electron by itself can not emit em radiation then nor should the nucleus of an atom. This does bring up a different question, why doesn't a electron emit em radiation?
You have a lot of faulty understanding of physics here. It started right away from your "given". In fact, all of them can be shown to be false under various conditions (what is the velocity of an electron in the s-orbital?).

Secondly, an electron in an atom does NOT emit EM radiation, unless it is undergoing a transition (read the FAQ thread in the General Physics forum). But this is not a correct view either. The change in state is not simply due to the electron, but it is due to the whole atom! We say that the excited ATOM undergoes the transition, and this is reflected by the change in state of an electron. But the electron, by itself, cannot undergo such transition. This is because it requires the whole atom to produce all those energy levels.

It would be helpful, in the process of learning, that you first of all establish the validity of your starting point, before jumping off it to go on to other things.

Zz.
 
  • #8
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Accelerating charge creates EM radiation. If electrons orbited the nucleus atoms would need to radiate continuously. Measurements show that they don't. Therefore electrons don't orbit.

They seem to move by applying the uncertainty principle to choose a new location at random. This is irrational from a classic point of view, since the uncertainty principle arises from our lack of measuring ability. Many would argue that something is going on that we can't see. But what we can't see and measure is not science, it's opinion.

Therefore scientists don't question (much) what is "really" happening. They assume the uncertainty principle is more fundamental than our simple inability to observe and suddenly everything makes sense, at least from a mathematical perspective.

This is the basis for the field of quantum mechanics. It is also implied by accepting the very concept of science as observation based. What's "really" going on isn't real unless it can be observed.
 
  • #9
alxm
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If electrons orbited the nucleus atoms would need to radiate continuously. Measurements show that they don't. Therefore electrons don't orbit.
Well, classical physics says that. And classically, a particle such as an electron would have a minimal kinetic energy of zero, so it could lose energy and stay stationary at the nucleus, where the potential energy is obviously lowest.

The basic assumption the Bohr made with his model, which was the start of quantum mechanics, was: What if the electron is only 'allowed' to occupy certain energy states? In that case, it can't radiate away energy if it's already at its minimum. The uncertainty principle, which came later, is one way you can rationalize this. Certainty in position leads to uncertainty in momentum. So if the electron is, to a high degree of certainty, located near the nucleus, it must have a large uncertainty in momentum. In other words, the less 'spread out' the electron is, the higher its kinetic energy (and vice-versa). So if the electron's location was extremely spread out, its potential energy would increase, since it's further from the oppositely-charged nucleus. But if it's very concentrated near the nucleus, its kinetic energy increases. The lowest energy level (the Ground State) is actually somewhere in-between.

Whether or not electrons 'orbit' is a question of semantics. I would say they orbit, but that they don't have orbits, in the sense of a well-defined trajectory around the atom. Quantum mechanical particles don't follow trajectories of any kind; that would imply that they had an exact position and momentum. But they do move, in some sense. No the classical sense of motion, but they do have kinetic energy and they do exhibit various dynamical effects of motion. (For instance, they're affected by special relativity) If 'moving around the atom' is to be considered 'orbiting', albeit not moving in any kind of classical sense, then they certainly do that.
the uncertainty principle arises from our lack of measuring ability
Well, no. It's a fundamental 'rule' of how quantum-mechanical particles act, whether or not we're measuring them. There have been very many threads on this board explaining all this, so I won't bother repeating it.
They assume the uncertainty principle is more fundamental than our simple inability to observe and suddenly everything makes sense, at least from a mathematical perspective.
It makes sense from a practical perspective as well. An electron in an atom or molecule doesn't in any significant way act as if it were a classical object with a definite momentum, location, trajectory, etc. So it doesn't really 'make sense' to assume they would have those properties, except for the fact that it'd be convenient for us if the same physics described everything. But we know now that that's not true.

I wouldn't say most physicists have simply accepted they cannot know a property such as the location of an electron in a molecule. I'd say they just don't view it as a meaningful concept. At least I don't.
 
  • #10
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Perhaps you didn't express yourself well. You seem to have said that the uncertainty principle arises as a rule from the opinion of a majority of scientists, not from measurement and observation.
 
  • #11
alxm
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You seem to have said that the uncertainty principle arises as a rule from the opinion of a majority of scientists, not from measurement and observation.
Well, the Uncertainty Principle doesn't come directly from measurement and observation. It's a derived, theoretical result from more basic postulates of quantum mechanics. You can debate the choice of those postulates (there's https://www.physicsforums.com/showthread.php?t=470380" that seems to be doing that right now), but the Uncertainty Principle isn't usually directly included among them. (It usually comes around from the Cauchy-Schwartz inequality after assuming the states form a Hilbert space, bla bla bla..)

The Uncertainty Principle isn't a matter of opinion. What is, though, is how you choose to interpret it. In that context, there's a clear majority (starting with the Copenhagen interpretation, but not limited to it) who don't view the idea of a particle having an exact momentum/position as meaningful.

On the other hand, you have adherents of 'hidden-variable theories' who do hold that there's an exact position/momentum/etc that we just don't know about. However, those interpretations don't change the uncertainty principle itself, because they still do not allow you, even in principle, to determine that exact value of the position and momentum. So in both cases you still have the uncertainty principle, and in both cases it's a hard "in principle" thing and not just an experimental "in practice" thing.
 
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  • #12
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Historically the uncertainty principal arose like other theories. Observations were made. An hypothesis was proposed. It failed some tests. The old hypothesis was rejected and a new one was proposed. Hence it seems appropriate to say "the uncertainty principle arises from our lack of measuring ability" because that was the problem that led Heisenberg to place all the known data in matrixes.

The idea of having basic postulates should be an anathema to scientists. They are useful for teaching engineers or teaching the current state of knowledge, thus deriving them serves a useful purpose. But it's not science; it's teaching or engineering. Philosophy and math are based in logic. Science is empirical. It is the application of the scientific method.

Yet quantum mechanics is a complicated field with lots of tough math. Thus a lot of mathematicians get involved. They normally think in axioms, so it's understandable for them to want to do that here as well. But getting locked into axioms doesn't lead to new discoveries (though trying to find axioms can point out flaws in old theories; as I said it's still useful).

Some of the posts on the postulate thread have subtly (too subtly, IMO) tried to point this out. It's something we were supposed to learn in grade school.
 
  • #13
alxm
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Historically the uncertainty principal arose like other theories. Observations were made. An hypothesis was proposed. It failed some tests. The old hypothesis was rejected and a new one was proposed. Hence it seems appropriate to say "the uncertainty principle arises from our lack of measuring ability" because that was the problem that led Heisenberg to place all the known data in matrixes.
Now you're just trying to pretend your words meant something other than they did.
Saying "the uncertainty principle arises from our lack of measuring ability" is not the same thing as saying
"the theory of the uncertainty principle arose from a lack of measuring ability". The uncertainty principle itself
does not arise from our lack of measuring ability; as I already said, that is a common misconception, one which
you were promulgating when you spouted the nonsensical:
This is irrational from a classic point of view, since the uncertainty principle arises from our lack of measuring ability.
The meaning you're ascribing to your words now make absolutely no sense in that context. You plainly stated that electrons "move" by choosing locations
at random using the uncertainty principle (which they do not), and that this is 'irrational' because the uncertainty principle comes about from human
measurements. Neither part of that rationale is correct.

Besides which, it would not add anything to a discussion to say a new scientific theory was based on apparent inconsistencies between previous theories and experiment. They all are.
The idea of having basic postulates should be an anathema to scientists.
The fundamental goal of all science is reduction: To describe as much as possible from theories relying on as few basic observations and assumptions as possible. I.e. postulates.
Science is empirical.
So are postulates in physics. You don't seem to understand the difference between a postulate in physics and an axiom in math. The two are not the same thing.
 
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  • #14
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So this guy walks on to a physics forum and asks a question that shows a lack of understanding of the field. I offer a way of thinking about the problem that follows the historical development of the field. You get on my case because one of my intermediate steps wasn't mathematically rigorous to your standards.

So why would the assumption of a Hilbert space not be the same as an assumption of the uncertainty principle? Or do you think Hilbert spaces were handed down from God (or possibly your professor).

It's my understanding that Heisenberg's major contribution was the assumption of an inner product space. Of course I could have said that, but it wouldn't have added any clarity for Servantes135 (plus I didn't know it at the time, I just knew the outline of how the field progressed).

Another way of expressing yourself might have been to point out to Servantes135 how important the dot product (inner product) was to the field and how the uncertainty principal arose from this fact. This would have informed Servantes135, gently chided me for my failure to point this out, and made you look wise.

You then could have then expounded on the various interpretations and what they made of electron motion, once again, calling me a moron without actually saying it.

Instead you're left calling me a pretender. Possibly not a good career choice. For all you know I'm some idiot politician who controls your funding (your in luck today, I'm not).
 
  • #15
Ok guys, I didn't post this thread to discuss views on how appropriate the uncertainty principal is or not. I posted this thread because when I asked my physics prof. these certain questions he responded by telling me he had no clue. So I figured that I should go some place where I could get the answers that I was looking for.
I do have to thank you for explaining certain aspects that I didn't understand, and for interpreting what I'm trying to ask so that every one else would stop patronizing me. As I said before I have a limited understanding of quantum theory. I know this, you all know this by the questions I'm asking.
So I shall try to rephrase my questions.

Does the nucleus of an atom change it's position in the atom?
If so, does it send out em radiation, because any charged particle that accelerates is supposed to do so?
 
  • #16
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I've waited for someone who knows more to answer. No luck.

Does the nucleus move? Yes, but it drags the electron with it since it outmasses it. Mostly the coordinate system is chosen with the nucleus at the origin and nucleus motion/tunneling is ignored. It would need to be considered for velocities near lightspeed I think.

Does this then send out an EM field? No, not generally. A point distant from the atom sees the positive field from the nucleus, but it also sees an equal and opposite field from the electron, assuming a uniform wavefunction (most are). Under moderately rare conditions the the wave function may be deformed, in which case a dipole field arises. Then accelerating the atom should cause EM radiation.
 
  • #17
for the case of this argument let us assume that the model of the atom is similar to the model of our solar system. (yes I know that it doesn't work like that especially since the electron doesn't orbit around the nucleus in a classical sense, but please follow me any way you might learn something from the analogy)

The sun isn't directly in the center of our little solar system because of the many gravitational forces that are applied to it. Basically it wobbles from on position to another as it orbits the milky way. I was wondering if the nucleus of the atom did the same, epically since gravity seems to be the weakest force inside the atom.
 
  • #18
ZapperZ
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for the case of this argument let us assume that the model of the atom is similar to the model of our solar system. (yes I know that it doesn't work like that especially since the electron doesn't orbit around the nucleus in a classical sense, but please follow me any way you might learn something from the analogy)

The sun isn't directly in the center of our little solar system because of the many gravitational forces that are applied to it. Basically it wobbles from on position to another as it orbits the milky way. I was wondering if the nucleus of the atom did the same, epically since gravity seems to be the weakest force inside the atom.
Y'know, you CAN do this by solving everything in the center of mass system. All you need to do is figure out where the center of mass is, and then compare that if you simply use the center of the nucleus (or the center of the attractive potential). Go ahead and do that. Once you figure out the difference between the two, try and come up with the consequences of using one versus the other, and see if that makes ANY difference.

Zz.
 
  • #19
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Y'know, you CAN do this by solving everything in the center of mass system. All you need to do is figure out where the center of mass is, and then compare that if you simply use the center of the nucleus (or the center of the attractive potential).
Note that this is where the slightly different energy levels for hydrogen, deuterium and tritium come from. The solution of the Schrödinger equation turns out to be the mathematically the same for all of these. Instead of the mass of the electron, it uses the reduced mass of the system, which is very nearly equal to the mass of the electron because the electron is much less massive than the proton, but is slightly different for the three isotopes because the nuclear masses are different.
 
  • #20
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In an isolated atom, the nucleus is at the center. But in an accelerated atom it's generally not. Most, if not all, of this is because acceleration is generally caused by EM forces. When you throw a baseball the electric fields in your hand press on the electric fields of the ball.

The induced dipole EM radiation will be a high order effect (third, fourth or more) and any field will be lost in the noise of the generating effect. BTW, the energy for the resulting EM radiation could come from the acceleration.

Perhaps if you found a convenient black hole and chucked a snowball (crystal water -- polar molecule) in it?
 

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