“Yang-Mills” is just the name for a class of theories which have a certain kind of symmetry and which include as a special case parts of the “standard model” which physicists use to predict the behaviour and interactions of elementary particles .
The “existence” problem here is that the various procedures used by physicists to make calculations in these theories have never been proved to actually always work. They involve making sequences of successive modifications from some initial guess according to patterns that are known (ie proven mathematically) to work in simpler theories for producing a sequence of numbers that actually converges to a well-defined result (that is independent of the starting point). But proofs of effectiveness have never been found for the theories that are actually used to describe elementary particles. What is therefore not yet known to exist is a set of well-defined final predictions (ie an actual theory defined by the proposed procedures).
The calculations can be done in various ways, and do seem to produce useful approximations to what we actually see in experiments, but we don’t know that the results will actually converge if we keep on going. So we don’t know for sure whether or not we have a well-defined theory. (This applies even to the case of Quantum Electrodynamics, but there is some hope that the more complicated symmetries of a Yang-Mills theory may help to guarantee convergence.)
As an analogy (not to the physics but to the state of our knowledge) imagine coming across a ladder standing up in the middle of a field. It reaches up so far that you cannot see if it is stabilized in any way at the top; but you want to get a better view of what is around you, so you climb up the first few rungs and can see over the nearest hedge (and what you see from the ladder does match what you can see by walking across the ground). But now you want to look over the nearby hill. Perhaps you could climb higher, but what if the ladder is only precariously balanced? If it is infinitely long then it may have enough inertia not to be disturbed by your climbing, but on the other hand it may have enough stretch and flexibility that if you get high enough the part you are on will fall down anyhow. And even if the ladder is infinitely long and stable, on a spherical Earth there is a limit to how far you will actually be able to see (and perhaps there is important stuff happening on the far side that will eventually affect you). So the ladder may never tell you everything you need to know, and if it swings about you may never be sure that your view is ever the “correct” one, so there is no actual final prediction that it tells you.
The “mass gap” issue has to do with whether or not, if we leave out ElectroMagnetism, it is possible to clearly distinguish the vacuum as having strictly less energy than other states, and is also related to having more rapid falloff of non-EM forces such as those between nucleons. (This is actually a much weaker condition than the strict “confinement” that we actually expect for the forces between quarks within nucleons and pions, but proving it might be a first step towards that.)
One reason for combining this more specific “mass gap” issue with the more general and abstract question of “existence” is because, in some simpler cases (of just one or two space dimensions) the techniques used to prove “existence” of a well-defined quantum field theory also prove (and to some extent make use of) the existence of a mass gap.
Source: (1000) Alan Cooper’s answer to What is the Yang–Mills existence and mass gap in layman’s terms? – Quora
The Pauli exclusion principle allows us to approximate the wave functions of valence electrons by treating the inner electrons and nucleus together as a single source of potential; and then by treating the ionic cores as fixed we can solve the Schrodinger equation for the valence electrons and calculate its lowest energy level as a function of the relative coordinates of the cores. Minimizing that function then allows us to determine the optimal bond lengths and their relative orientations.
Source: (1001) Alan Cooper’s answer to How does quantum mechanics treat atomic bonds, and what role does the Pauli exclusion principle play in this context, considering also that electrons are everywhere in space according to their wave function rather than confined to fixed orbits? – Quora
Quantum Mechanics is not a single theory. In the past there have been other attempts to describe the fundamental aspects of physics which used the word “quantum” in various different senses, but to most physicists nowadays it is a class of theories characterized by the property of having the “pure states” of an isolated system represented by rank one projectors (or equivalently rays or unit vectors) in a complex Hilbert space – and by a rule for predicting the probability distributions of outcomes for various possible experimental observations. Each such theory is internally consistent, but that doesn’t mean either that they are necessarily correct in their predictions or consistent with either one another or with other theories about the physical world.
Source: (1001) Alan Cooper’s answer to Why do people have different definitions of quantum? Is quantum mechanics a logically consistent, self-consistent theory? – Quora
What’s with the “if” in this question? And what do you mean by the word “co-moving” other than perhaps stationary with respect to one another?
The standard version of the Twin “Paradox” starts with two twins, who are obviously “co-moving” in that sense at birth, and a distant star which is also “co-moving” (ie stationary with respect to the twins). Then at some point one of the twins travels to the star and back (usually with unspecified periods of acceleration and mostly constant speed in both directions).
Any correct application of Special Relativity predicts that when they re-unite the traveller is younger. The age difference can be calculated in terms of any frame of reference and (for any specified acceleration history – including that of instantaneous speed jumps) the answer is always the same so there is no real paradox.
The alleged “paradox” arises only in the mind of someone who notices that the traveller perceives the homie to have been ageing more slowly during the constant-speed legs of the trip and then just ignores the fact that the traveller also perceives a sudden rapid ageing of the homie during the turn-around. (During that turn-around the traveller feels the force of acceleration and so is aware of being in a non-inertial frame, whereas the homie feels no such forces. So the situation is definitely NOT symmetrical.)
Source: (1001) Alan Cooper’s answer to How do you solve the Twin Paradox if everyone (including the waypoint) is co-moving at the get-go? – Quora
If we define both the observer and the “observed” as both being part of say an even bigger system, would the wave function still collapse in this system?
This conundrum is known as the Wigner’s Friend problem, though it is also often asked with reference to Schrodinger’s cat.
In my opinion its best resolution is in the understanding that the wave function or quantum state is not a property of the system itself but of its relationship to an observer, and I think this view is a better reading of what Hugh Everett was describing in his “Relative State” interpretation of quantum mechanics [which was re-presented later (mostly by others) as a “Many Worlds” interpretation where observations (and other interactions) continually cause the creation of new “branches” (in a way that Everett himself apparently once described as “bullshit” in a marginal note on someone else’s elaboration of the MWI)].
Source: (1001) Alan Cooper’s answer to Is the collapse of the wave function in Quantum Physics based on a system frame of reference? If we define both the observer and the ‘observed’ as both being part of say an even bigger system, would the wave function still collapse in this system? – Quora
Whether or not observation is the only way in which a wave function can collapse depends on what you mean by “collapse”, and that word is used by various people in reference to different aspects of the measurement and observation process – which can be considered as happening in two stages.
The setting involves a system in a pure quantum state which may have been prepared as an eigenstate of some observable (such as spin relative to a particular direction), and so is a nontrivial superposition of eigenstates of some other observable (such as spin relative to a different axis) which we now want to measure.
In the first stage, the system of interest interacts with a larger more complex system which is not fully known and so is in a statistical mixture of pure states (represented by a density matrix rather than a single state vector). If the larger system is suitably designed as a measuring apparatus, then the interaction leads to the state of the combined system approaching a statistical mixture of states in which the subsystem of interest is in an eigenstate of the observable and the measurement apparatus is in a related state which involves some macroscopic feature (such as a pointer, a readout panel, or a bright spot on a phosphor screen) which has a corresponding humanly visible value. Henceforth the system acts as if it is in just one eigenstate which is not yet known but is subject to classical probabilities. This process eliminates the possibility of future interference between the eigenstates that was possible while the state of the system was in a pure state (represented by a coherent wave function) and so is often called “decoherence”; and since it reduces the system to being effectively in just one eigenstate it is often identified with “collapse of the wave function”. It actually happens in almost any interaction with a complex system (even when there is no humanly visible related macroscopic property of the system). So, for those who identify decoherence as collapse, it is indeed possible for collapse to occur without observation.
But after this kind of “collapse” we still don’t know what the measured value actually is, even though we can think of it as having just one of several precise values – each with some known probability.
The second stage of the observation process is where the conscious observer notices which value is present. Some people think of this as where the “collapse” happens, but here it is not really collapse of the wave function but rather of the classical probability distribution (similar to the case of a coin toss which starts of in a stochastically mixed state and collapses to just one case when we see the result).
The difference from a coin toss is that in that case we assume that all along the system was really in whatever particular state we eventually observe, and that state could have been determined with certainty just by making more observations at the start; whereas in the quantum situation the uncertainty seems to be essential until we actually experience the result. This leads to a philosophical problem for those who think that the quantum state is a property of the system itself rather than of its relation to the observer as it seems to imply that the experience of a conscious observer has some physical effect on the universe and raises the problem of Wigner’s friend who watches an experiment before Wigner does and seems to collapse the wave function even though the friend is himself just a complex quantum system who Wigner sees with a wave function that does not collapse until the information reaches his (Wigner’s) own mind.
To my mind this is resolved by seeing the quantum state as a description not of the universe but of its relationship to the observer; and I think this view is a better reading of what Hugh Everett was describing in his “Relative State” interpretation of quantum mechanics which was re-presented later (mostly by others) as a “Many Worlds” interpretation where observations (and other interactions) continually cause the creation of new “branches” (in a way that Everett himself apparently once described as “bullshit” in a marginal note on someone else’s elaboration of the MWI).
Source: (1001) Alan Cooper’s answer to Is observation the only way in which a wave function can collapse? – Quora
What many people misunderstand is that in quantum theories the “state” of a system is not a property of the system itself but rather of how it appears to an observer.
There are actually at least two stages to the observation process. One is when the system of interest interacts with the much more complex system of a measurement apparatus whose precise quantum state is too complex for the observer to keep track of and so has to be expressed as a statistical mixture. This can have the effect of causing the combined system, in which the observed subsystem was initially in a pure “coherent” superposition state (with interference still being possible between different possible observed eigenvalues), to end up very close to a statistical mixture in which each possible measured value of the observed quantity has a well defined value with no interference between them. This “decoherence” process can be caused by interaction with any sufficiently complex system (even, as Viktor Toth notes, just a brick) and it does modify the observed (as does any interaction with anything – even just another simple quantum system). But it still leaves the actual value of the observation unspecified. The “collapse” process, which identifies which particular value has occurred, only happens in the mind of the observer whose conscious experience corresponds to just one of many possible histories of the universe. But this doesn’t modify the observed – at least no more than it modifies everything in the universe that is dependent on that observed value. (For example if we are in a room together and I see a red flash then the you that I see will also see a red flash, but if you see a blue flash then the I that you see will also have seen a blue flash.)
Source: (1001) Alan Cooper’s answer to In the quantum mechanical idea that the observer modifies the observed, can the observer be an insect? – Quora
The idea that the experience of an accelerated observer might be approximated by considering its worldline as comprising many small inertial pieces is a good one. And during each inertial step the speed of light seems to be constant everywhere. But at the velocity boosts or “frame jumps” between the steps, the apparent coordinates of all events (including those on the world line of a light signal) get shifted, so the light seems to jump ahead or back. Taking the limit of these approximations leads to the conclusion that the light signal does not seem to have constant velocity from the point of view of the accelerated observer. (Since the “frame jumps” lead to coordinate changes that are proportional to the distance of the event from the observer, this does not change the fact that every light signal seems to have the same speed when it reaches the observer, so there is no local change and it is just when the signal is far away from the observer that its velocity appears to vary.)
Source: (1001) Alan Cooper’s answer to A rotating frame can be divided into an infinite number of infinitesimal inertial frames. According to SR (special relativity) the light speed in each inertial frame is constant. Is light speed therefore constant in say the Sagnac experiment with SR? – Quora
A Quora question asks:
From your own perspective, frame of reference, if you are not accelerating, are you not moving at all?
Perhaps the reason people find this difficult to grasp is because of the ambiguous and unnecessary words “at all” at the end – which lend support to the idea that the word “moving” without mention of a context has any meaning at all. It does not.
And it never has. The claim that something is “moving” has never had any meaning without identification of relative to what, and it is perhaps unfortunate that by convention we often take the word without modification as meaning relative to the Earth without explaining that convention to small children (who then grow up thinking that there must be some kind of absolute property of “moving”).
From my own perspective, while sitting still in a bus I am not moving relative to the bus, but I know damn well that I am moving relative to the world outside. And it’s not just from my own perspective. Everyone else agrees with both of those relative motion statements.
So although it is just a tautology that from every perspective I am never moving relative to myself, it is false (or rather meaningless) that there exists any perspective from which I am ever not moving at all.
Source: (1000) Alan Cooper’s answer to From your own perspective, frame of reference, if you are not accelerating, are you not moving at all? – Quora