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Quantum Mechanics
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from https://www.newscientist.com/definition/quantum-field-theory/
Quantum field theory marries the ideas of other quantum theories to depict all particles as “excitations” that arise in underlying fields. The British physicist Paul Dirac started the ball rolling in the late 1920s with his equation describing how relativistic electrons – and with it most other matter particles – behave.
Standard quantum theory as developed by the likes of Niels Bohr and Werner Heisenberg in the 1920s is fine for describing the workings of individual particles in isolation and at slow speeds. But to explain their interactions in the real world, you need something more.
In particular, you need to marry quantum theory with special relativity, Einstein’s theory of how space and time warp for things travelling at high speeds. Special relativity says mass and energy are interchangeable, as embodied by the equation E=mc2. Heisenberg’s quantum uncertainty principle, meanwhile, says particles can borrow energy from the vacuum for a certain amount of time.
from https://plato.stanford.edu/entries/quantum-field-theory/
In contrast to many other physical theories there is no canonical definition of what [quantum field theory] QFT is. Instead one can formulate a number of totally different explications, all of which have their merits and limits. One reason for this diversity is the fact that QFT has grown successively in a very complex way. Another reason is that the interpretation of QFT is particularly obscure, so that even the spectrum of options is not clear. Possibly the best and most comprehensive understanding of QFT is gained by dwelling on its relation to other physical theories, foremost with respect to QM, but also with respect to classical electrodynamics, Special Relativity Theory (SRT) and Solid State Physics or more generally Statistical Physics. However, the connection between QFT and these theories is also complex and cannot be neatly described step by step.
If one thinks of QM as the modern theory of one particle (or, perhaps, very few particles), one can then think of QFT as an extension of QM for the analysis of systems with many particles—and therefore with a large number of degrees of freedom. In this respect going from QM to QFT is not inevitable but rather beneficial for pragmatic reasons. However, a general threshold is crossed when it comes to fields, like the electromagnetic field, which are not merely difficult but impossible to deal with in the frame of QM. Thus the transition from QM to QFT allows treatment of both particles and fields within a uniform theoretical framework.
from https://www.forbes.com/sites/startswithabang/2020/10/09/ask-ethan-when-did-the-universe-get-its-first-quantum-fields/?sh=6bb6ab7e29c9
When Did The Universe Get Its First Quantum Fields?
No matter how we look at the Universe — at low temperatures or ultra-high energies, from our own backyard to the most distant recesses of the observable cosmos — we find that the same laws of physics apply. The fundamental constants remain the same; gravitation appears to behave the same; the quantum transitions and relativistic effects are identical. At all points in time, at least for the parts of the Universe we can observe, General Relativity (governing gravity) and Quantum Field Theory (governing the other known forces) appear to apply in the exact same form we find them appearing here on Earth….
“When did the first quantum fields form in the universe? Have they been there since the Big Bang or even from the inflationary period before?”
Perhaps surprisingly, quantum fields are there even under conditions where you might not expect them. Here’s what we know so far.
When we think about fields, most of us conceive of them the same way scientists did back in the 1800s: when you have some type of source — like an electric charge or a permanent magnet — it creates a field around it at every point in space. That field exists whether or not there are other particles there to be affected by it, but you can detect the presence of that field (as well as what it affects and how) by observing what happens to charges of various types that interact with that field.
Iron filings, which themselves can get magnetized, respond to magnetic fields by aligning along the direction of a field. Electric charges, in the presence of an electric field (or in motion in the presence of a magnetic field), will experience a force that accelerates them dependent on the strength of the field.
Even gravitation, whether in Einstein’s or Newton’s conception, can be visualized as a field: where matter or energy of any form will respond to the cumulative gravitational effects at its location in space, determining its future trajectory.
This visualization, as useful and common as it might be, only works in non-quantum settings, however. It’s an excellent illustration of how classical fields work, but we live in a fundamentally quantum reality. What we conceive of in the classical world — that fields are smooth, continuous, and that its properties can exist anywhere along a spectrum from a theoretical minimum to a theoretical maximum — no longer applies in a quantum Universe.
Instead, a quantum field isn’t only present where you have a source (like a mass or a charge), but rather is omnipresent: everywhere. If you have charges present, such as:
masses (for gravity),
electric charges (for electromagnetism),
a particle with a nonzero weak hypercharge (for the weak nuclear force),
or a color charge (for the strong nuclear force),
they behave like an excited state of the field, but the field is present regardless of the presence or absence of charges. What’s more: the field is quantized, and its zero-point energy, or the lowest energy level it can occupy, is allowed to have non-zero values.
In other words, “empty space” as we understand it, with no charges, masses, or other sources of the field in it, isn’t exactly empty, but still has these quantum fields present within it. That means that quantum fluctuations, which arise as a consequence of the quantum nature of these fields combined with Heisenberg’s uncertainty principle, exist throughout all of space as well…
For as long as spacetime has existed, some version of quantum fields must have existed as well. But whatever occurred in our Universe prior to the final tiny fraction-of-a-second of inflation can never be observed or accessed from within our observable Universe…
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