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Bonds that are larger and looser tend to vibrate at lower frequencies than those that are small and tight. This rule of thumb applies both to classical mechanical systems and to quantum transitions, and so it “explains” why CO2 tends to respond to longer wavelengths than O2 and N2 (and much longer than those required to change the energy levels of electrons within those molecules).

The measured locations and widths of the corresponding absorption bands fit very closely with calculations based on quantum and statistical mechanics, so it can reasonably be said that we understand very well why they are where they are.

It is, however, just a fluke that the vibrational excitation modes of CO2 (and H2O and CH4) happen to fall near the peak intensity of thermal radiation at the Earth’s temperature of around 300K. (And they might be less effective than other choices as “greenhouse gases” on a planet that was either white hot or not illuminated by the Sun.)

Source: (1002) Alan Cooper’s answer to Why is CO2 transparent to incoming shorter infrared wavelengths of light, but absorbs outgoing longer infrared wavelengths from Earth’s surface? Are they certain these IR wavelengths that are more affected than others, and if so, why? – Quora

Quantum Mechanics “explains” the lack of radiation from electrons in the ground state of an atom by telling us that our idea of electrons (or anything else) as discrete particles with well-defined positions and momenta is wrong – and that the bound electron is not in fact accelerating on a curved path around the nucleus, but rather has a range of possible values for our attempts to measure its positions and momenta at different times (with the property that the probability distribution of these values is concentrated around the nucleus but invariant with respect to time).

Source: (1002) Alan Cooper’s answer to I still don’t understand why an accelerating electron doesn’t emit electromagnetic radiation as it’s quantized motion around the nucleus. How does quantum mechanics explain the prevention of this radiation? – Quora

The difference between a density matrix and a state vector is that the latter corresponds to having more complete information about the system. Let’s see how this plays out in a simple example.

A density matrix that is diagonal with entries of {1}/{2} means that we are talking of just a two dimensional state space (such as that of a single electron whose position we are ignoring), and that we have chosen a particular observable, such as spin component in the z direction, to determine the basis with respect to which you are representing the state.

In this context there are actually many different pure states for which the probability of having spin up or down in the z direction are equal. It could for example be certain to have a particular value in (say) the x direction, or the y, or any other direction perpendicular to the z axis. Each of these corresponds to a “ket” whose components (relative to the z eigenstates) are of the form {1}/{\sqrt{2}} multiplied by complex phase factors (and the different relative phases correspond to different directions of spin in the xy plane). On the other hand, representing the state by that density matrix means that we have prepared the electron in such a way that its spin has equal probability of being measured up and down in the z direction, but also that all ways of getting that result are equally likely so there is also equal probability of getting up or down in any other direction.

Source: (1002) Alan Cooper’s answer to Why can’t the density matrix with 1/2 in their diagonals be equal to a ket wave function with 1/2 in each entry. Do they not both describe how each 0 or 1 state has a 1/2 probability? – Quora

People often ask what it means for an electron to have spin 1/2.

Here is my attempt at an informal explanation.

It means that electrons (and most other elementary particles) are represented by wave functions or fields whose values are not given just by complex numbers (the “scalar” or “spin zero” case), but instead by complex vectors (of “internal” coordinates) which admit a finite dimensional representation of the rotation group. The action of a rotation on a state then corresponds to the usual change of position in space combined(*) with a reorientation of the “internal” coordinates.

It turns out (due to mathematics that I cannot usefully(*) insert here) that the possible results of measuring angular momentum corresponding to “internal” properties of a particle occur with increments of just half of those corresponding to measurements of the classical orbital angular momentum.

And the electron happens to be an example of the simplest kind of non-scalar field.

(*) – The crux of this can perhaps be inadequately explained by saying that the way the internal and external actions of the rotation group have to be related is such that one full rotation in the position space produces a sign reversal in the internal space and so to bring everything back to where it started actually requires two full rotations in the position space.

Source: (1001) Alan Cooper’s answer to Why is the spin of an electron equal to half? What does it mean by half? – Quora

Spacetime is like a movie reel, or better a stack of pictures on top of one another. Time is just a way of labelling the individual frames or pictures, and the word “move” just means to have different coordinates in space at different times ie to be at different places in different pictures. It doesn’t make sense to ask for the image of something in one picture to be in a different picture.

Source: (1001) Alan Cooper’s answer to If time is the 4th dimension, why can’t anything move in either direction like the other 3 axes? – Quora

For any motion with constant acceleration over a time interval, the product of acceleration times distance travelled over that interval is equal is equal to half the change in the square of the speed in the direction of acceleration. (This is just the junior high school version of the calculus identity \int{x’’ dx}=\int{x’’ x’ dt}=\int{x’ x’’ dt}=\int{x’ dx’}=\Delta(x’^2/2).)

So the quantity that is increased by applying force through a distance (to do “work” on a particle) is quadratic in its speed. But why is this important enough to give it a special name? That is because it allows us to define a quantity that is conserved throughout the evolution of any physical system.

As a result of Newton’s law of action and reaction (which is basically just a way of expressing conservation of momentum), in the motion of any system of particles the sum of mv^2/2 for all the particles plus the net work done against outside forces is a constant. We call this the “Energy” of the system and identify the part involving the speeds (that does not include work against the outside world) as the “kinetic” part of that energy – and the outside work (which includes an arbitrary constant depending on what we take as the starting point) is a called “potential” energy since it could in principle be returned to the system in future interactions.

Source: (1001) Alan Cooper’s answer to Why does kinetic energy increase quadratically, not linearly, with speed? – Quora

Spacetime with matter and gravity is not symmetrical. There is a particular singularity relative to which what we identify as our experience includes information (which we call memories) about events “closer” (not in spatial distance but in the spacetime metric) to the singularity (which we call the “past”), but not about those further away (which we call the “future”). Part of what is in our memory is experiences in which the scope of our memory was smaller. This gives us the feeling of becoming progressively further away from the singularity – ie of moving “forward” in time.

Source: (1001) Alan Cooper’s answer to Why do we experience the fourth dimension as constantly moving in one direction? – Quora

John Platts’s answer to Why does a mirror reverse things horizontally but not vertically? includes some nice illustrations and discussion but declares that the front-to-back reversal is not horizontal.

Interesting point. But I think we use the word “horizontal” to refer to any part of a line or plane that is perpendicular the line joining it to the Earth’s centre, regardless of whether or not our view of it actually appears parallel to the horizon. We often test for this property by looking at it from a position where it lines up (left-to-right) with the horizon but two people building a house will generally agree that a beam is horizontal even when they are not looking at it from such a position. (If that were not the case the word would be a lot less useful because the property of a beam being horizontal would depend on the observer and would be lost whenever she changes her orientation.)

So, in my understanding, horizontal lines aren’t necessarily parallel to the horizon in a perspective view, but (if the world was flat) their infinite extensions (would) all terminate on it (though due to the Earth’s curvature that actually happens a tiny bit above what we see as the visible horizon).

Naive students sometimes ask what would happen if electrons or other fermions were “forced” to violate the Pauli Exclusion Principle, but they never suggest any idea of what this would actually mean.

Whatever they are imagining probably involves thinking of those fermions as independent particles such that you can somehow keep track of which is which; but in quantum theory there is no way of doing that, and no-one has ever suggested an alternative theory that fits the facts and in which such separate identities make sense.

Source: (1000) Alan Cooper’s answer to What happens to fermions that are forced to violate the Pauli exclusion principle? – Quora