# How can we experimentally confirm that atoms/molecules in a solid actually "move"?

The atoms in a solid are so attracted to each other that they "vibrate" and don't move past each other.

How do scientists "measure" that atomic vibration in a solid (let's say at room temperature)?

As a raw, uneducated person it is easy for me to conclude that the solid is completely at rest and no part of it is "moving". So, what is the experimental evidence which shows that my conclusion is totally wrong and that the tiny invisible atoms are actually "jiggling"?

In the case of the Brownian motion, it is somehow easier (more intuitive and common sense) to assume that the invisible atoms are "moving" and thus "hitting" the colloidal particles. However, regarding a solid... I can't even imagine how I can detect that atomic "vibrations" because I can't see them or feel them.

• This group has taken images of a molecules normal mode of vibration. Hope this is useful. sciencedaily.com/releases/2019/04/190403135014.htm and this phys.org/news/… Jun 27 at 4:05
• You do feel the temperature of the solid. That is, roughly speaking, a measure of the degree of motion of the particles in the solid. Jun 27 at 6:30
• But the feeling of temperature does not give rise to the concept of motion (jiggling, vibrating and so on). My question is: what is the experimental evidence which confirms the motion of the particles? Jun 27 at 8:07
• Well, at first just think about resonant frequencies, due to which we can play on guitar. Exactly because guitar strings - solid objects - can vibrate due to lattice micro-deformations. IF pressure waves couldn't travel in strings material lattice, then no vibration and no sound would be produced. Jun 27 at 11:42
• Not an answer, but perhaps it will help shift your thinking, and shed some light: you say that the atoms are "so attracted to each other". But, if they only attract each other, then why they don't all collapse on top of each other? In a crystalline solid there are no struts that hold the atoms in place, like in the models used in the chemistry class - yet it's not at all easy to compress a solid. So, think about what holds the atoms away from each other, too. You'll see that there's basically no chance that they won't jiggle around. Jun 29 at 5:05

The motion of atoms can be studied using various techniques based on neutron scattering. Unlike X-ray scattering, where X-rays are reflected by the electronic clouds surrounding atoms, neutrons are scattered primarily by the nuclei. Time-resolved versions of neutron scattering (like spin echo) allow observing how displacement of atoms happens in time.

Collective motion of atoms, such as, e.g., vibrations of crystals (phonons), can be also studied using infrared spectroscopy or Brillouin scattering (which is similar to Raman scattering, but involving absorption/emission of phonons).

Finally, nowadays atoms can be viewed under electronic or atomic force microscope (although they "jiggle" too fast to actually see them moving in real time).

• "Unlike X-ray scattering..." The Debye–Waller factor is important for both X-ray and neutron diffraction, and I'd think that at least the inner shells of electrons with the highest electron density would track the motion of the nuclei fairly faithfully if we're talking about thermal velocities, so I'd naively assume the same time-resolved physics can be seen with X-rays as with neutrons. Am I wrong? (I'm always happy to be!)
– uhoh
Jun 27 at 19:10
• Unlike an x-ray photon with a similar wavelength, which interacts with the electron cloud surrounding the nucleus, neutrons interact primarily with the nucleus itself, as described by Fermi's pseudopotential. Neutron scattering and absorption cross sections vary widely from isotope to isotope. en.m.wikipedia.org/wiki/Neutron_scattering Jun 27 at 20:11
• There is nothing in your quote that says the inner shells don't move along with the nucleus so that's not really addressing my comment nor the issue. Also, please just use normal text not code block/monospace format to leave a comment. (discussion in meta) Simple quotation marks are sufficient to indicate a quote.
– uhoh
Jun 27 at 20:18
• I didn't say anything specifically about inner shells - my concern here was that the electron density might have configuration different from that of nuclei (which I took here for position of atoms). CDW or plasmon would scatter x-rays, but not indicative of the positions of nuclei. Jun 28 at 6:15
• Okay as long as you're not excluding X-rays and saying only neutrons can be used, that's fine. I thought you were suggesting that X-ray diffraction would not show the same motion that neutron diffraction can show.
– uhoh
Jun 28 at 8:24

We talk about “jiggling atoms” because the classical harmonic oscillator explains how solids store heat at high temperatures. Bolstering the argument, the quantum-mechanical oscillator explains why the heat capacity is reduced at low temperatures, as the energy between the ground and first excited states of the oscillator becomes large relative to the thermal energy available.

The pop-science statement that the atoms in a solid are “always jiggling” as related to the result that a quantum harmonic oscillator has nonzero energy $$\frac12\hbar\omega$$ in its ground state. This may not be very convincing evidence of atomic harmonic motion — after all, many things are usefully approximated as harmonic oscillators. However, a collective excitation of the oscillators in a material is known as a “phonon.” The name intentionally suggests that a phonon is a “quantum of sound” in a way analogous to the photon, the quantum of light. There are a number of connections between the “incoherent” phonons which describe heat transfer within a material and the “coherent” phonons which describe the propagation of sound. We have excellent reasons to think of sound as macroscopic vibrations in a material.

• you may be able to address this comment
– uhoh
Jun 27 at 19:13

For me, the most salient fact arising from molecular “jiggling” is simply thermal radiation. It has the advantage of being relatively easy to observe (using thermal imaging at room temperature, and just your eyes at red-hot temperatures and higher), but I suppose whether you consider it convincing evidence of molecular oscillation depends on how comfortable you are with the fact that accelerating charges produce radiation.

Another way is to look at the quantum efficiency of photoelectric sensors using indirect band gap materials like silicon. For such materials, a long wavelength photon needs the assistance of a phonon (lattice vibration) to produce an electron-hole pair. A consequence is that the sensor's quantum efficiency varies with temperature.