# Do Elementary Particles Jiggle Like Atoms?

Do Elementary Particles "Jiggle" Like Atoms?

Like the question says, if atoms jiggle about because their constituent sub-atomic particles are moving within the atom, is this movement simple orbital/linear displacement or is there also a "jiggling" to it?

If there is a "jiggle" or vibration or whatever that is independent of an elementary particle's translational movement, any such movement involves repeated cycles of acceleration and deceleration, so what is 'powering' it?

IH

## Answers and Replies

I don't know too much about elementary particles but if I had to guess I'd say that the elementary particles don't 'jiggle' about in the sense you mean because it seems to me that the cause for your notion of 'jiggling' is the result of the interaction of those particles inside the atom. Since the elementary particles aren't made of anything smaller (as far as we know), then this jiggling motion shouldn't be observed. I would say that they are probably in constant motion because they are exchanging bosons like crazy, and when we get that small, we have to talk about conservation of energy and momentum. So I'd imagine they are constantly colliding with each of their respective force carriers, and in that sense they would be in motion.

But again, I'm not too knowledgable on the subject. Could anyone else verify this?

mfb
Mentor
if atoms jiggle about because their constituent sub-atomic particles are moving within the atom
I don't think that is a useful way to visualize atoms. The particles are not moving in the sense that they would change their position in the atom.

You always have position(/momentum) uncertainty, but that has nothing to do with "jiggling".

I don't think that is a useful way to visualize atoms. The particles are not moving in the sense that they would change their position in the atom.

You always have position(/momentum) uncertainty, but that has nothing to do with "jiggling".

What would they change in the atom then? As for the atomic jiggling description, it is one of the basic introductory points in Feynmann's Six Easy Pieces. It is supposed to denote heat energy.

IH

mfb
Mentor
What would they change in the atom then?
There is no change, the atom is the same all the time (unless it decays to another atom, or electrons change orbitals, or similar transformations happen).

As for the atomic jiggling description, it is one of the basic introductory points in Feynmann's Six Easy Pieces. It is supposed to denote heat energy.
Heat energy? Then it is probably the motion of the whole atom, and I guess it refers to solid objects (but it is similar for liquids and gases). The atoms there are not at rest, they are constantly moving a bit, and the acceleration comes from forces of other (moving) atoms nearby.

This question actually touches the very deep question of the interpretation of quantum mechanics.
It is tempting but problematic to interpret quantum states as describing particles moving in potentials. Tempting because local realism is intuitive. Problematic because of Bell's inequality which invalidates local realism.
To say that atoms "jiggle" is either a classical statement or an expression of local realism.

If there is a "jiggle" or vibration or whatever that is independent of an elementary particle's translational movement, any such movement involves repeated cycles of acceleration and deceleration, so what is 'powering' it?

Even if there was totally classical motion of particles in atoms, there would still be no need for any energy input to power this motion, any more than the Earth needs energy input to keep whirling about the sun, or Halley's comet if you prefer an example with larger speed changes. It is a lossless system (assuming the atom is in the ground state)

But to add to the "jiggling" discussion, although it is true that atoms are usually thought of as sitting in time-independent states, so that nothing is changing, nevertheless the constituent particles do have energy and momentum, and in fact their probability current swirls about in all sorts of cool ways. Maybe one could say it is a bit like the stationary waves you get in rivers (e.g. ), except it is like the river loops back on itself, and, although the water is swishing along, the state of the river is time-independent (macroscopically at least... the analogy only goes so far...).

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Well, if you will take the motion of a dirac particle and evaluate it's velocity by dx/dt=(i/h-)[H,xk],you will find it equal to cαk.This α is an operator in dirac eqn. with which you might be aware of.The relation holds even in presence of EM field(Aμ).If now you look at the time derivative of αk,You will find that velocity operator is not a constant of motion even if the particle is free.Now if you solve it for the coordinate operator,you will find an extra term whose expectation value will not give the trajectory of wave packet according to classical law.This extra term implies very rapid oscillation in addition to rectilinear motion.This oscillatory motion is known as 'Zitterbewegung'.

Bill_K
.This extra term implies very rapid oscillation in addition to rectilinear motion.This oscillatory motion is known as 'Zitterbewegung'.
One should not imagine that zitterbewegung is a real phenomenon. It's a symptom of the failed interpretation of the Dirac equation as a single-particle wavefunction, and does not occur in QFT.

One should not imagine that zitterbewegung is a real phenomenon. It's a symptom of the failed interpretation of the Dirac equation as a single-particle wavefunction, and does not occur in QFT.

Dumb question but just to clarify Bill comment: are *massive* elementary particles expected to display oscillations in their translational trajectories like photons or not. Can, say, an electron have a 100% linear trajectory?

IH

mfb
Mentor
Which oscillations in a trajectory of a photon do you mean?
Photons are often drawn with wave-like lines, but this has nothing to do with the direction of the photons.

Which oscillations in a trajectory of a photon do you mean?
Photons are often drawn with wave-like lines, but this has nothing to do with the direction of the photons.

Then I misunderstood the nature of a photon's trajectory.

Simply put, can massive elementary particles travel in a 100% straight line or is their trajectory wave-like?

IH

mfb
Mentor
If there is no force on the particle, they travel in a straight line - as good as you can define this with position uncertainties. There is clearly no oscillation involved, however.

Thanx mfb

As for the atomic jiggling description, it is one of the basic introductory points in Feynmann's Six Easy Pieces. It is supposed to denote heat energy.

I don't think so......I have Feynman's SIX NOT SO EASY PIECES...but no mention there of particles nor heat energy nor 'jiggling'.

Anway, I think mfb has a good suggestion regarding 'jiggling':

I don't think that is a useful way to visualize atoms.

That's because you are asking a classical question in a quantum forum.....

Hassan:
[QUOTE...is this movement simple orbital/linear displacement or is there also a "jiggling" to it?][/QUOTE]

sounds like you are using the old Bohr model:....planets orbiting a sun, electrons orbiting a nucleus model....that's an awful model to use in a quantum context. Neither describes the standing waves of modern quantum mechanics. Instead consider atoms bundles of energy waves.

Particles have localized mass/energy called quanta, the source of which are quantum fields [waves]....There are 'electron waves' in a cloud around a nucleus, not point like particles...or at least we have no evidence of any.
Illustrations here: http://en.wikipedia.org/wiki/Atomic_orbital#Orbitals_table
and be sure to note the first sentence in that section....

hydrogen-like wave functions

from another discussion:
In general, in quantum mechanics, a thing that is time-independent like a particle is described by a time-independent wave function, and does not in any sense "move". An electron does not orbit the nucleus. A particle in the ground state of a harmonic oscillator does not slosh back and forth. And elementary particles with spin do not rotate.

So a hydrogen atom has a 'spin', the angular momentum in its rest frame. This is the angular momentum contribution to the kinetic energy of a hydrogen atom, but nobody thinks the atom is 'spinning'...nor jiggling, whatever 'jiggling' means.

Also, a particle is a wave [function] until it is detected as a local quanta; as far as we can tell a detected particle is point size...without measurable dimension. That's the Standard model of particle physics: wave descriptions of particles, point like [zero size] interactions. The wave function description of a particle is of the particle itself, not a trajectory. However if you know the double slit experiment you know that the wave like nature of a particle and superposition accounts for the paths and the point like displays.

When you put a bunch of atoms together, say in a solid, their collective behavior is different from individual atoms...electrons, for example, behave differently and appear to be a different 'size' than in a free atom...photons may be absorbed by lattice vibrations, quanta/particles called phonons....and such structure vibrations reflect heat energy. This is because the structure has different degrees of freedom and so it's energy characteristics display somewhat differentl;y than the individual constituent particles.

I don't think so......I have Feynman's SIX NOT SO EASY PIECES...but no mention there of particles nor heat energy nor 'jiggling'.

You are correct Naty, it is not in SIX *NOT SO* EASY PIECES, it's in the first tome, Six Easy Pieces as I mentioned...easy to mistake one for the other on the spur of the moment I admit...

Thanx for your input, much appreciated.

IH

One should not imagine that zitterbewegung is a real phenomenon. It's a symptom of the failed interpretation of the Dirac equation as a single-particle wavefunction, and does not occur in QFT.

Well,the darwin term can be qulitatively explained using zitterbewegung.of course,we have other fancier way of obtaining this term.

One should not imagine that zitterbewegung is a real phenomenon. It's a symptom of the failed interpretation of the Dirac equation as a single-particle wavefunction, and does not occur in QFT.

I agree that zitterbewegung cannot be a real phenomenon. If it were it would be described by some equation of motion.
The ad hoc assumption of zitterbewegung is the result of a failed interpretation of the Dirac equation as an Euler-Lagrange equation.

Which oscillations in a trajectory of a photon do you mean?
Photons are often drawn with wave-like lines, but this has nothing to do with the direction of the photons.

This is another deep topic. The discussion whether photons are particles or waves is 300 years old and is not over. See http://www-3.unipv.it/fis/tamq/Anti-photon.pdf

mfb
Mentor
This is another deep topic. The discussion whether photons are particles or waves is 300 years old and is not over. See http://www-3.unipv.it/fis/tamq/Anti-photon.pdf
Photons are quantum-mechanical objects. In terms of physical predictions, there is nothing to discuss. The discussion is just about the most useful models to describe physics with words.

it's in the first tome, Six Easy Pieces as I mentioned...easy to mistake one for the other on the spur of the moment I admit...

I did not even know he wrote the two....

Photons are quantum-mechanical objects. In terms of physical predictions, there is nothing to discuss. The discussion is just about the most useful models to describe physics with words.

The predictions are undisputed, but not the interpretation.