## (3) Alan Cooper’s answer to How can a moving/faster observer see a ‘stationary’ clock running slower, when it doesn’t, e/specially when his really does? All the YouTube videos about the Twin Paradox say so. In what sort of universe is that possible? – Quora

The relativity principle does NOT say that all frames are equivalent, just the inertial (unaccelerated) ones. In the twin “paradox” situation, the traveler and stay-at-home both figure that the other is ageing more slowly while their relative velocity is constant but when the traveler turns around he or she figures that the stay-at-home suddenly ages much more rapidly. The two may have different explanations in mind for the effect, but when they get back together they agree on the result.

## (3) Alan Cooper’s answer to How does the twin (clock) paradox (in SR) really work? Please see the comment for the specific case. – Quora

According to the comment, what this question is really asking us to address is something completely different from “How does the twin (clock) paradox (in SR) really work?”

What the comment asks us to explain is as follows: “A clock flies around the equator eastwards, it ages slightly less. A clock flies around the equator westwards, it ages slightly more. Than a clock which stayed, at home.”

The explanation for this (in the context of either Special OR General Relativity) is that the alleged effect exists only for motion relative to the Earth’s surface (which is already rotating). So if I stand still on the Earth’s surface I will age more slowly than a twin who stands still relative to the Earth’s centre (which entails flying Westwards at a rate of about 1000mph) and more quickly than a twin who flies Eastwards and so is actually moving more quickly relative to the Earth’s centre.

Note: As with the usual out-and-back twin “paradox” what makes the travelers different from the stay-at-homes is that they are not in fixed inertial frames but are accelerated. And in the case of this scenario the twin flying West at 1000mph is basically staying stationary wrt the Earth’s axis which (if we neglect the curvature of the Earth’s orbit) is pretty close to being in an inertial frame. But the one standing still is actually rotating with the Earth and so is accelerating (centripetally) towards the axis. And the one flying East is actually accelerating even more. According to SR, an observer who is accelerated figures that stationary clocks towards which she is accelerating are speeded up by an amount that more than counterbalances the fact that if the speed was fixed she would figure that they were slowed down. And if you do all the calculations it turns out that when they get back together they all agree on their relative ages (with Eastflier younger than Standstill and Standstill younger than poor old Westflier(who is actually the only one who is really standing still)).

## (3) Alan Cooper’s answer to What is the Calabi-Yau manifold? Are we ‘inside’ it right now or is it just a mathematical concept? – Quora

Calabi-Yau manifolds (there’s not just one) are a type of mathematical concept, but they’re not “just” that as they do have applications in certain attempts to describe physics. The role they usually play in physics is to help us formalize the relationships that we postulate between the various internal variables that describe what particles are likely to show up at a point in space time. As such the theory often combines the six dimensions of a Calabi-Yau manifold with the four dimensions of the space-time that we are ‘inside’ to get a total of ten dimensions. But the extra dimensions are often either considered to be very small in some sense, or to have the part that contributes to the physics we see be just a slice through the whole thing. In the first case it makes more sense to say (as in James Bridgeman’s answer) that there’s a C-Y manifold at every point inside us (rather than vice versa), and in the second case that the entire space-time we live in is just a (4d) slice through the extended C-Y manifold (with other slices or “branes” perhaps corresponding to “alternate universes” of some kind). But neither of these cases is in any sense known to be true. So far it’s all just speculative construction of mathematical models that might eventually be shown to describe our actual physics.

## (3) Alan Cooper’s answer to Why is the Raman effect’s classical interpretation not adequate? – Quora

The classical theory of the Raman effect is not adequate for determining the actual spectrum because it allows the molecule to have arbitrary amounts of vibrational (or rotational) energy and in fact the possible energy levels are quantized (just like everything else in physics).

## (3) Alan Cooper’s answer to Does any physicist truly understand wave function collapse? – Quora

There are certainly many physicists who use the term “wave function collapse” to refer to something that they understand well enough for their own purposes. Whether or not that means they “truly understand” it depends on what you mean by that (in my opinion rather silly) expression.

## (3) Alan Cooper’s answer to Will the unresolved philosophical issues behind quantum theory ever be fully resolved? – Quora

There are no “unresolved philosophical issues behind quantum theory” that have been identified in this question. So it’s kind of like asking when will all the dinosaurs on the moon be dead.

## (3) Alan Cooper’s answer to How come quantum mechanics hasn’t been fully replaced by quantum field theory in the physics community? – Quora

On the one hand, Quantum Field Theory is just a special case of quantum mechanics. It’s just the quantum mechanics of fields (corresponding to situations whose classical analogues involve an infinite number of degrees of freedom). So replacing quantum mechanics with QFT is like replacing dogs with dobermans. Yes, we could replace all other dogs with dobermans, but they’d still be dogs (and for some purposes less useful than the ones they replaced). On the other hand, in a situation with only a limited number of degrees of freedom (such as where there are only low energy interactions between a fixed number of particles – in the analysis of a chemical bond formation for example), the use of quantum field theory would be like keeping track of the motions of all the engine components in a car when all we are interested in is the effect of a collision on a crash test dummy (or replacing a dachshund with a doberman for flushing rabbits out of their burrows).

## (9) Alan Cooper’s answer to What is the Calabi-Yau manifold? Are we ‘inside’ it right now or is it just a mathematical concept? – Quora

Calabi-Yau manifolds (there’s not just one) are a type of mathematical concept, but they’re not “just” that as they do have applications in certain attempts to describe physics.

The role they usually play in physics is to help us formalize the relationships that we postulate between the various internal variables that describe what particles are likely to show up at a point in space time. As such the theory often combines the six dimensions of a Calabi-Yau manifold with the four dimensions of the space-time that we are ‘inside’ to get a total of ten dimensions. But the extra dimensions are often either considered to be very small in some sense, or to have the part that contributes to the physics we see be just a slice through the whole thing. In the first case it makes more sense to say (as in James Bridgeman’s answer) that there’s a C-Y manifold at every point inside us (rather than vice versa), and in the second case that the entire space-time we live in is just a (4d) slice through the extended C-Y manifold (with other slices or “branes” perhaps corresponding to “alternate universes” of some kind). But neither of these cases is in any sense known to be true. So far it’s all just speculative construction of mathematical models that might eventually be shown to describe our actual physics.