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If an atom flies long enough into deep space, without colliding any other matter or EM waves (purely thoughtful experiment), what does happen?

  1. Because velocity vector is unchanged, no EM waves are generated, so no energy is lost and atom continues the travel eternally.
  2. Electrons spin around nucleus and thus generate EM field and somehow emit photons. This somehow (how?) causes kinetic energy decline and atom eventually stops (absolute zero).
  3. Electrons spin around nucleus and atom loses potential energy with photons, but this doesn't affect kinetic energy. So atom continues to fly eternally, but internally reaches zero potential energy and electrons collapse with nucleus.

So far the most plausible is (1), but I can't understand why Wikipedia says that all matter loses energy with radiation.

All normal (baryonic) matter emits electromagnetic radiation when it has a temperature above absolute zero. The radiation represents a conversion of a body's internal energy into electromagnetic energy, and is therefore called thermal radiation. It is a spontaneous process of radiative distribution of entropy.

I'm trying to understand why this radiation is produces in matter.

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    $\begingroup$ Electrons don't 'spin' around the nucleus and don't emit photons. Only excited atoms emit photons. $\endgroup$ – Gert Jul 25 at 19:48
  • $\begingroup$ @Gert yes, I now understand that part about "excited"/"ground" states. $\endgroup$ – danbst Jul 25 at 20:11
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If the atom is in the ground state it will not radiate and eternally move with the same energy and momentum, provided no nuclear reaction happens. If it is in an excited state it will decay first, which will alter its energy and momentum. After that it will continue with its final energy and momentum.

There no such thing as the temperature of a single atom and any radiation it emits cannot be considered to be thermal.

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  • $\begingroup$ so, "thermal radiation" occurs only during moments when atom changes it's direction (by interacting with other atoms or oscillating)? $\endgroup$ – danbst Jul 25 at 19:05
  • $\begingroup$ Quoting myself: "any radiation it emits cannot be considered to be thermal". $\endgroup$ – my2cts Jul 25 at 22:47

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