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.

# Month: January 2022

## (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).

## Zurek on decoherence

This is the 2002 revision of his 1991 review in Physics Today

## My Quora Answers on the Twin “Paradox”

Source: *(33) Search*

20191102 https://www.quora.com/How-is-the-Ehrenfest-paradox-resolved/answer/Alan-Cooper-5

20191208 How-does-special-relativity-explain-the-twin-paradox-since-acceleration-is-required?

20200929 How-do-you-explain-Einstein-s-twin-paradox-in-an-easy-to-understand-summary?

20210515 Why-does-time-dilation-slow-only-one-twin-in-the-twin-paradox?

20220109 How-does-the-twin-clock-paradox-in-SR-really-work?-Please-see-the-comment-for-the-specific-case

## (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.

## Are the electric and magnetic fields always perpendicular to each other?

Not always. The rate of change of either is perpendicular to the other, but there is no such general relation between the actual values. In fact for static fields it is easy to make them parallel – as, in the picture below, at the centre of a current carrying loop with positive and negative charges above and below:

## If magnetism is the relativistic effect on the electric field, how does someone explain magnetic fields generated by a particle’s spin? Also, how does someone explain light as both fields propagating perpendicularly if they are ‘the same thing’?

The only situation in which magnetism could be considered to be just the relativistic effect on the electric field is one in which there is an inertial frame in which all charges are stationary. In any situation with relatively moving charges there is a magnetic field in every frame, and it is only the way in which the splitting of the combined electromagnetic field varies between observers that is a relativistic effect. In particular, a current loop always produces a magnetic field in every reference frame and the situation for electron spin is analogous (though not quite the same because it’s a pure quantum effect and not really due to actually moving charges).

In the absence of any static fields, the electric and magnetic fields are perpendicular to one another but for light waves they both propagate in the same direction.