If I understood the accepted answer here correctly,then matter can spontaneously form. But has this ever been observed?
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$\begingroup$ There was a search for positrons possibly resulting of the annihilation of dark matter , without a noticeable success. Not so spontaneous, but the stock of our classical matter increases in this scenario $\endgroup$– user46925Commented Jan 29, 2016 at 16:20
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The answer you cite is somewhat simplified to suit the level of the question. The vacuum does not consist of pairs of particles and anti-particles popping into existence and then disappearing again. When calculating the properties of the vacuum it's true that we use Feynman diagrams showing the creation of particle/antiparticle pairs, but these are virtual (anti)particles that are a computational device. They do not mean particles are literally appearing from nothing.
Virtual particles cannot be observed but their effects can. For the vacuum the obvious evidence is the Casimir effect. Since this has been experimentally measured you could regard it as an observation of virtual particles appearing and disappearing, however I must emphasise that the virtual particles are not really particles in the common sense of the word.
answered Jan 29, 2016 at 13:01
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$\begingroup$ Thanks, is there anything to be found about actual particles appearing? Having no education in physics to speak of, I would expect that since the causes cited in that answer ("existence of quantum fields and the laws that govern them") have been in place for ages, if matter can form spontaneously, it would have been observed, or otherwise a reason why it hasn't. $\endgroup$– G. BachCommented Jan 29, 2016 at 13:41
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$\begingroup$ As far as I know we have never observed real (not virtual) particles appearing from the vacuum. An example would be Hawking radiation, but this remains a theoretical prediction and has never been observed - though optical analogues have. $\endgroup$ Commented Jan 29, 2016 at 14:21
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$\begingroup$ It seems that the question follows the paradigm of creation of matter from the dark energy, populated in some magazines , lectures and publications. ie on arxiv : "Particle creation has been considered as a possible justification for the accelerated expansion of the universe, obeying the second law of thermodynamics, together with the possible existence of Dark Energy." Accelerated Expansion of the universe based on Particle Creation-Destruction Processes and Dark Energy in FLRW universes by Alberto C. Balfagon $\endgroup$– user46925Commented Jan 29, 2016 at 14:38
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$\begingroup$ I find too much emphasis on phrases like "virtual (anti)particles that are a computational device" ends up creating the opposite misconception that too literal interpretation of the Feynman diagram method does: one of under appreciating the physical meaning of those terms. Individual terms can be selected and made physical under the right circumstance in when we "knock a pair on-shell" in processes like highly forward pion production in $p(e,e')$ scattering. Yes, the terms of the series are math, but it's math that has physical meaning and can be accessed in the laboratory. $\endgroup$ Commented Jan 29, 2016 at 14:54