What are Field Quanta?
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If all of nature is made of field quanta - or excitations of the quantum vacuum, what is the nature of these quanta, and why do they often seem to behave as particles?
A quantum of a field is the smallest possible excitation of a field. It lives and dies as a single entity. If we're referring to a photon, it starts "life" as an emission from something like a molecular oscillator in the blackbody experiment. It "dies" when it eventually is absorbed somewhere.
That may sound like a particle. However, unlike a particle, it is not localized in space along the way. Just like any other wave in nature, it tends to spread as it travels - like a ripple in a pond. The quanta are traveling waves. This singular entity, if left uninterrupted will literally be heading everywhere at once. Even if it has a short time to travel, it spreads out over vast, vast regions as compared to what we would think possible for a particle. The strange things that "particles" do are some of the apparent paradoxes in physics to which QFT has better answers.
One part about these spread-out quanta that is perhaps a bit hard to swallow is how they collapse. While this quantum or field excitation might be a wave spanning a vast region of space, upon absorption by an atom (in the case of a photon), something called field collapse takes place. In an instantaneous process, the quantum delivers all its energy and momentum to one place at one time and nothing remains. It is like touching part of a spreading ripple on a pond and finding that you suddenly absorb all its energy and the pond everywhere becomes still - not just at the spot you touched.
I understand if this sounds strange. But let me mention two facts. We know for certain that nature is quantized. If these field quanta are the smallest possible units of excitation, then how could we possibly expect only part of it to be absorbed while the rest remains for even a moment? Further, while much or all of this sounds strange and perhaps fictitious, it is born out by experiments of which we'll speak in due time. In light of such experiments, this picture becomes the most sensible picture I have ever seen.