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Does a vibrating neutral object emit EM radiation caused by the vibration? If the object is a molecule, does it radiate? and if the object is a atom?

I am not refering to thermal radiation. It does radiate, whenever it moves or not. I mean the EM radiation caused by the vibration ifselft. And I am hoping that this post will not be put on hold, and will lead to future meaningful discussions

Qmechanic
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Cang Ye
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  • https://physics.stackexchange.com/q/72174/ –  Jul 05 '19 at 02:26
  • https://en.wikipedia.org/wiki/Thermal_radiation –  Jul 05 '19 at 02:40
  • Thermal radiation is a type of EM radiation, that can come from vibrations of charge neutral objects –  Jul 05 '19 at 02:40
  • Yes. Thermal radiation is a type of EM radiation. When a charged particle moves back and forth, it will radiate EM radiation caused by the acceleration beside thermal radiation. How about a uncharged ball? – Cang Ye Jul 05 '19 at 02:46
  • I think the faster you bounce a basketball it will start to melt from heat and emit more thermal radiation. –  Jul 05 '19 at 02:48
  • https://en.wikipedia.org/wiki/Black-body_radiation –  Jul 05 '19 at 02:50
  • There is an asymptote that goes to longer wavelengths so bits of radiation that aren't in the IR spectrum could probably be detected. And if something is hot enough you can see visible light. This is what happens when you vibrate stuff faster and faster, it gets hotter, and emits more higher frequency radiation. –  Jul 05 '19 at 02:52
  • Thermal radiation is different thing. it has a continous spectrum, see Planck's black body formula. The radiation caused by acceleration is not such thing. – Cang Ye Jul 05 '19 at 03:14
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    https://physics.stackexchange.com/questions/30547/do-composite-particles-that-are-electrically-neutral-but-have-charged-constituen –  Jul 05 '19 at 03:29
  • https://physics.stackexchange.com/questions/377342/why-does-an-accelerating-neutral-object-not-emit-bremsstrahlung-radiation?rq=1 –  Jul 05 '19 at 03:30
  • thermal radiation is caused by acceleration like any em wave, there are just a bunch of things being accelerated –  Jul 05 '19 at 04:31

1 Answers1

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There are a few things to clarify:

  1. Thermal radiation is EM radiation, generated by the thermal motion of particles in matter.

  2. All matter with temperature above absolute zero emits this kind of radiation, and the motion of particles will create:

    1. charge acceleration

    2. dipole oscillation

Even the CMB is thermal radiation.

It is very important to understand that thermal radiation is a form of converting kinetic energy into EM radiation.

Thermal energy consists of the kinetic energy of random movements of atoms and molecules in matter. All matter with a temperature by definition is composed of particles which have kinetic energy, and which interact with each other. These atoms and molecules are composed of charged particles, i.e., protons and electrons, and kinetic interactions among matter particles result in charge-acceleration and dipole-oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy away from the body through its surface boundary.

https://en.wikipedia.org/wiki/Thermal_radiation

Now you are saying that you still want to differentiate this from thermal radiation, and you are not talking about thermal radiation. You say that you are talking about EM neutral objects, you say like atoms and molecules, and if they vibrate, do they emit EM radiation or not.

Now you are assuming that you can have a atom or molecule isolated in free space, vibrating.

The definition of vibration is mechanical oscillation.

You are talking about composite objects, like atoms and molecules. So I will assume you are asking about EM neutral composite objects, that are isolated in space, vibrating (mechanically oscillating).

Now atoms are defined to be EM neutral, but there exists dipole and quadrupole, and higher order fields, and these fields can be attractive, and this is why we can talk about the QM phenomenon of covalent bonds, this is how atoms can combine into molecules, sharing valence electrons shells.

You are distinguishing atoms in these bonds in solids, liquids, and gases, having degrees of freedom of rotational, translational and vibrational motions (interpreted as temperature, or heat capacity), from the free neutral atom in your case, that vibrates in free space.

You are saying, that a neutral object, in free space, isolated, would vibrate, and thus create EM radiation. Now as per SR, an accelerated charge will emit EM radiation.

In your case, since we take the atom as EM neutral, though it is vibrating, it will not emit EM radiation, as long as:

  1. it is isolated, free in space, not in a molecule, or in a lattice, or in a liquid or gas

  2. its EM charge distribution is in equally distributed (dipole moment)

Árpád Szendrei
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  • A charged flywheel rotates (or vibrates) at high speed and emits electromagnetic radiation. It's easy to measure. A common flywheel without electricity will not emit electromagnetic radiation when it rotates at the same speed. As mentioned before, thermal radiation is not included here. Thermal radiation always exists, whether charged or not, whether rotating or not. In my opinion, electrically neutral objects do not emit radiation when they move at variable speeds. – Cang Ye Jul 06 '19 at 02:07
  • Objects consist of neutral molecules and atoms. Their dipole radiation, even if it exists, is a small amount of higher order, which is neglected here. It's not measurable. – Cang Ye Jul 06 '19 at 02:08
  • Electrons and proton radiate when moving at variable speeds, but the whole neutral molecule or atom does not (neglecting dipole radiation, high order small quantity). The question is, have the photons emitted by electrons and protons cancelled out or neutralized? Can two different photons neutralize each other? – Cang Ye Jul 06 '19 at 02:19
  • @CangYe the electrons and protons (and neutrons) are in a stable atomic system. You are trying to handle them separately. They are in a stable atomic system, that is EM neutral, and that is why it does not emit EM radiation (disregarding dipole) when accelerated. The electron and protons (and neutrons) are all bound, creating a composite quantum system. This composite quantum system is what is EM neutral, and that is why its components do not radiate. What you are talking about is destructive interference, when partial waves of photons cancel each other. In this case, they do not need to. – Árpád Szendrei Jul 06 '19 at 05:41
  • Thanks. OK, neutral atoms don't emit EM radiation when accelerated. I totally agree with you on this. But, according to mainstream, blackbody radiation is caused by kenitic movement of atoms and molecules. I strongly doubt this view of mainstream physics. – Cang Ye Jul 06 '19 at 08:24
  • @CangYe yes, but in that case, blackbody radiation is just thermal radiation. It is because the atoms in that case are not free, but are in a lattice or liquid or gas, and have some degrees of freedom of rotational, translational, vibrational energies. – Árpád Szendrei Jul 06 '19 at 09:08
  • I don't know how to AT you on my Samsung phone. Mainframe physics believe what you said. But atoms in gas are free, which still emit thermal radiation. For example, sparse hydrogen atom gas does so while emiting discrete visible light spectrum. Thermal radiation is a always existing thing for anything at anytime. In my opinion, even for the atoms in lattice, their movement don't emit EM radiation. The EM radiator is elections, rather than atoms or molecules. – Cang Ye Jul 06 '19 at 11:05
  • @CangYe in gases, the molecules collide, and excite each other. – Árpád Szendrei Jul 06 '19 at 11:16
  • Yes, excited atom gas needs lot of atoms. The discrete H spectrum are emitted by a huge number hydrogen atoms, not by one single atom. But the thermal radiation is not the case. Even if we test one single atom, we can still observe its thermal radiation. So I said that it's a always existing thing for anything at anytime. – Cang Ye Jul 06 '19 at 11:30
  • @Árpád Szendrei I was thinking the scenario expected vibrations without anything else around, but if something is in undisturbed free space, what is making it mechanically oscillate? –  Jul 08 '19 at 08:59
  • If a single atom is in free space without interaction with other things, it will not mechanically oscillate. But this is only an ideal scenario and not exist in universe. There is microwave background radiation anywhere. The electrons inside the atom will interact with the radiation. Electrons inside any object takes in radiation energy from surrounding and emits energy. Blackbody does so. – Cang Ye Jul 08 '19 at 11:27