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I am a total layman in physics, but I've been trying to understand the various existing theories and after reading/watching lectures on QFT for months I still can't find an answer to a few very basic questions.

1) Are quantum fields a property/function of spacetime like color is a property of an object? Meaning they could not exist on their own and are "caused" by existence of spacetime? Or is it the other way around and quantum fields were created first (during the Big Bang) with spacetime emerging from their interactions?

2) In QFT particles are more or less treated as excitations of the underlying quantum fields, but do those excitations happen because something external is causing them (e.g. interaction with another field) or is it something intrinsic (that perhaps can be considered a sort-of property of the field)? + Edit from comments: I am not really asking where particles come from, but rather "how"; i.e. in QFT are they treated as resulting naturally from the existence of quantum fields, or does the theory ignore this point altogether, or does it specify an external source for the field becoming excited?

Thanks a lot in advance!

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    $\begingroup$ A quantum field is a function of spacetime (this should be evident from any source about QFT), so your first question doesn't make a lot of sense to me - are function "properties" of their domains (most would say no)? I'm not sure what you're asking in your second, either - the world we see is full of particle - if you're asking "where they come from" you're less asking about QFT and more about what happened at the Big Bang. $\endgroup$ – ACuriousMind Jul 19 '15 at 19:47
  • $\begingroup$ @ACuriousMind "Function" is indeed a better word, thanks. And I am sorry that I wasn't bright enough to get that from "any" source about QFT. Regarding the second question - I am not really asking where particles come from, but rather "how"; i.e. in QFT are they treated as resulting naturally from the existence of quantum fields, or does the theory ignore this point altogether, or does it specify an external source for field becoming excited? $\endgroup$ – korkor Jul 19 '15 at 19:57
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1) Quantum fields are properties of the universe. They are functions of space-time whose values close to a particular space-time point $(t,\mathbf{x})$ determine all properties of the universe (gravitation, electromagnetic properties, colors, mass, density, currents, forces, elasticity, etc.) close to this time and position.

2) Particles are elementary excitations of the quantum fields in roughly the same sense as frequencies are elementary excitations of the pressure field in a flute, or of the stress field of a guitar string. In both cases, these excitations are properties of the fields themselves. In both cases, these excitations are also caused, namely by the past values of the fields, their self-interactions, and the forces at their boundary (if one is postulated, e.g., for quantum electrodynamics in a cavity).

However, in QFT applied to the whole universe there is no boundary, so everything is caused by the past values of the fields and their self-interactions alone.

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