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Oct
8
comment Is classical electromagnetism a dead research field?
Isn't this so much a problem as an indication that the theory is incomplete and has to be replaced with quantum electrodynamics, which solves both problems? The self-reaction is subsumed into corrections to the electron propagator and the "reaction force" can be handled in the background field mechanism.
Apr
11
comment Are the $10^{500}$ different string theories being whittled down?
10^500 is a conservative estimate, the number might be much larger. But even 10^500 is beyond anything imaginable, the whole universe does not contain enough atoms to store 10^500 integers for example, let alone the parameters for 10^500 string models. Also each of those backgrounds potentially has a number of continuous parameters.
Apr
10
comment Does the mass point move?
Alfred Centauri, yes but it is equally impossible to exactly generate any kind of force, so to me this is as unrealistic as a sudden violent onset of a force at t=0, modelled by a step function. Realistically you will only be able to approximate it by forces that have some non zero time derivative at t=0.
Mar
29
comment Is space unending?
It is possible to describe a sphere intrinsically, without thinking of it as embedded in three dimensional space. Perhaps a better example would be the torus: The game world of some arcade video games like snake has that shape: If you leave the screen on one side, you reappear on the other side. This gives the example of a finite but unbounded two dimensional space.
Sep
6
comment Why is the $\langle v_{x}^{2} \rangle=\frac{1}{3} \langle v^2 \rangle$?
Generally $\langle A + B \rangle = \langle A \rangle + \langle B \rangle$, this is more or less build into the setup. After all expectation values are computed by evaluating certain sums or integrals.
Jul
27
comment Can physics get rid of the continuum?
@RonMaimon You are right, I am not terribly satisfied with my answer myself. The idea I tried to convey was that numerical analysis has been very successful in simulating classical physics, because they deal with differentiable functions, those functions look boring locally and differential equations can therefore be discretized successfully. The "therefore" sweeps the whole field of numerical analysis under the rug and you are right that there are many subtle problems, energy conservation for example.
Jul
26
comment Can physics get rid of the continuum?
@NickKidman I modified the last sentence, hopefully that is not bad form.
Jul
6
comment Why is ${\partial^i}{\partial_i\phi}$ = ${\partial^i {\phi}}{\partial_i{\phi}}$?
Where did you see that? One thing you can do is partial integration, but then you are missing at least one $\phi$. In deriving the Euler-Lagrange equations, you use partial integration.
Jul
1
comment What is Supersymmetry (SuSy)?
Each of those terms have a rather technical meaning. To explain them usually takes several lectures each, at least. That you ask about them in one question indicates that you have no, or little prior knowledge. I think you should at least indicate why you are interested in those questions.
Jun
27
comment Snooker/billiards cueing
@CaptainGiraffe Sommerfeld treats side queing in his Lectures on Mechanics, if I remember correctly. He certainly treats unver/over cueing.
Jun
17
comment Where do I start with Non-Euclidean Geometry?
@RonMaimon: I agree with you that unlike QFT and String theory the fundamental facts about general relativity are explained well in some text books, but that does not mean that there are no difficult questions left. To just give one example: Although one knows special solutions to the Einstein equations, there are no general existence theorems. Even a priori estimates are extremely hard.
May
27
comment Derivation of the supergravity action in 11D
@LuboŇ°Motl : Is the procedure of beginning from the free spectrum and adding interactions trial and error, or does it follow some algorithm? I imagine that you begin by postulating some supersymmetry transformation between the fields, compute and find what terms do not cancel, add those and so on. At some point you find that the remainder can be written as a total derivative and you are done. I think that is more or less what was done in the original paper. Given that there are many of those miraculous actions I feel that there should be a systematic way.
May
27
comment Physical Explanation of Being Able to “Think”
Physics alone will probably never give an explanation of why we are able to think. There is the subdiscipline of neurophysics (i.e. the study/modelling of neurons/neural networks in the brain). There are already semi-realistic models for the behaviour of individual neurons and there are methods of simulating networks of those. This is however a far way from understanding how cognitive functions arise from "similiar" networks in the brain. In other words up to now there is some model of the "hardware" but almost no understanding how those lead to cognitive functions.
May
24
comment How can/does calculus describe the movement of a particle?
@OllyPrice I think Khan Academy is overly verbose, there are many good books on calculus. More important than any resources is that you try to learn actively: I.e. as soon as you think you understood something try it out on lots of examples.
May
22
comment Derivation of the supergravity action in 11D
A counting argument can only give a neccessary condition not a sufficient one. I would be satisfied if there were a theorem that stated, that there is a (unique?) supersymmetric action, without exhibiting it explicitly. Ideally the theorem should give an algorithm to compute the action. This would be the situation one is in the case of General Relativity or Yang-Mills.
May
21
comment Give a description of M-theory your grandmother can understand
Explain M-Theory to a grandmother, who was a graduate student in theoretical physics after 1980 :).
May
18
comment Where do I start with Non-Euclidean Geometry?
I disagree that a high school student can understand GR in any deep way. Sure you can understand computations for the schwarzschild metric and similiar concrete examples, after being taught how christoffel symbols are to be computed and how one determines geodesics. But being able to perform computations is a long way from actually understanding what they signify.
May
18
comment Where do I start with Non-Euclidean Geometry?
It depends on why you want to learn general relativity. Their book contains a lot of insights, that you won't find in any other book. It would probably help to read it concurrently to a lecture or lecture notes. To get an idea what the bare essentials are.
May
18
comment Is anti-matter matter going backwards in time?
@RonMaimon Do you know of a good source, where the analytic structure of the amplitudes is discussed?
May
16
comment Why not using Lagrangian, instead of Hamiltonian, in non relativistic QM?
@RonMaimon There is a path integral formulation of the Hamiltonian formalism. It is equivalent to the usual formulation for theories quadratic in the momenta.