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seen Mar 13 at 0:58

Diego Mazón


Mar
21
awarded  Good Question
Mar
13
comment What fundamental principles or theories are required by modern physics?
S-matrix is connected with change in time, not space!!
Mar
13
comment What fundamental principles or theories are required by modern physics?
"except to the extent that you place boundary conditions" That's not enough? 1)Asymptotic states are defined with respect to time, not space. 2)Amplitudes (computed from path integrals as you like) are causal. Causal is a space-time property, what's the space-space equivalent? 3) Time plays a role in quantum mechanics in connection with measurement even when there is no evolution (H=0). I agree that both are exact as far as we know, we aren't discussing that.
Mar
12
comment What fundamental principles or theories are required by modern physics?
@RonMaimon Note that I'm not claiming they aren't equivalent; only that it's not that clear.
Mar
12
comment What fundamental principles or theories are required by modern physics?
@RonMaimon I think you are at least oversimplifying the equivalence between boost and spatial rotations. The rotational group is compact, unlike the Lorentz group. Also, the different roles that time plays in a quantum theory might make boost different. The equivalence btw spatial rotations and boost is not as obvious as the 4d approach to Special Relativity apparently suggests. Causality could be another reason. In addition, rotational inv is more fundamental in that breaking rotational inv. implies Lorentz violation, whereas the reverse is false.
Feb
8
comment Why not using Lagrangian, instead of Hamiltonian, in non relativistic QM?
@StanShunpike In order for a theory to be Poincare invariant, the Lagrangian needs to be a Poincare scalar, what it is easy to see. The equivalent condition in the Hamiltonian formalism is that there is a Poincare algebra with the Hamiltonian as the zero component of the 4-momentum. This condition needs to be checked, as it is not elemental to see.
Jan
27
comment If a star were to suddenly dissapear, would it still have gravity?
@WetSavannaAnimalakaRodVance Yes. classical = non-quantum in my vocabulary.
Sep
29
reviewed Approve Lagrangian mechanics is different form Newtonian?
Sep
24
awarded  Autobiographer
Jul
25
awarded  Nice Answer
Jul
19
awarded  Necromancer
Jul
12
awarded  Yearling
Jul
2
awarded  Curious
May
30
awarded  Nice Question
Mar
7
comment Why am I wrong about how to view gauge theory?
In this context a redundant transformations means that it does not change the physical state of the system but only the field related to it (and that creates spurious d.o.f).
Mar
7
comment Why am I wrong about how to view gauge theory?
1) Are you saying that non-large gauge transformations (those that are continuously connected with the identity) change the physical state of a system? 2) The gauge transformations you say that they are not redundancies, do they tend to the identity in the space-time boundary? Are they the so-called "large gauge transformations"? Thanks.
Feb
20
awarded  Nice Answer
Feb
17
awarded  quantum-field-theory
Feb
10
comment When is quasiparticle same as elementary excitation, and when is not?
@huotuichang The Poincare algebra has two quadratic Casimir invariant: the square of the cuadri-momentum (the particle's mass) and the square of the Pauli- Lubanski (related to the spin or helicity of the particle), which self-commute. Therefore, the eigenvalues of these operators are good label. This classification is originally due to Wigner. Elementary particles are also classified according to the way they transform under the gauge group of the standard model $U_y(1)\times SU_l(2)\times SU_c(3)$
Feb
7
comment When is quasiparticle same as elementary excitation, and when is not?
Hello @IsidoreSeville Then, can some of you provide the definition of "elementary excitation" in condensed matter physics? I only see one sense in which an "elementary excitation" can be elementary. If this answer is correct, then, in my opinion, this condensed matter terminology should be abandoned because it is confusing and useless.