Hi, I have a question:
How does people keep balance when they riding a bicycle or skating? Moving seems to be essential, when riding a bicycle, we can argue that the wheel is rotating, so it's angular momentum keeps the balance. But in the case of skate, nothing is rotating, but people seem to...
Hi,
I don't know whether the conserved quantities in QM are conserved in every conditions. For example, a state can be a non-eigenstate of energy, and when we measure the energy of this state, it falls into an eigenstate of energy. The former state does not have a definite energy, but the...
Hi,
In Weinberg's QFT book(section 2.2), he said after proved the generator of the symmetry group is Hermitian and can be a candidate for an observable:
My questions:
1. Does the observable here mean at least in principle, we can measure it in experment?
2. Does generator of any symmetry...
Hi,
I'm just starting to read Polchinski's book on string theory. I have the follow questions:
1. In the intructionary part, he said different kinds of string theory arise from different ground states(vacuum), but I don't understand why the difference on dimension of spacetime or open/closed...
I think this is not a QM question, but rather a question of classic EM theory. Envolving QM into it does not make any difference. But as the two always appear together, and we can not create a single charged particle from vacuum and watch it creating fields, although I take fields as a effect...
Thank you! I think now I am clear. The article you suggested is of great help. In that article, the author does not simply define all symmetry as linear unitary operator, but treat mathematical and physical symmetry differently. A symmetry of any physical sense need to preserve Hamiltonian.
I hear Lattice QCD can do some non perturbative calculations, for example, it can give hadron mass up to a few percent in most sence. In lattice QCD, we can begin with the Euclidean but entire action, take the action as a probable denisty of all field configurations, place space on lattice and...
Hi, i am confused by the following question:
When an observer is going towards the horizon of a BH from outside, he finds the light from far away so blue shifted that the far future of the far away world presents in front of him. Does that mean when he is crossing the horizon, he has seen...
Hi,
We often say, an observer near the horizon of a BH finds the light traveling from far outside the horizon blue shifted, or an observer away from the BH finds the red shift of light from near the horizon. We get a conclusion that a clock near the horizon goes faster than a clocker far...
As far as i know, QFT does a good job in some problems concerning bound states, e.g. the Lamb shift in light atoms, which has been calculated using QFT in Weinberg's book. But in bound states with strong interaction, as a bound state is a low energy state(~MeV), the strong interaction becomes so...
Commute means the two operators concerned have so many common eigenstates that these states forms a complete set. This physically mean the two measurment makes no difference when one performs before or after the other.
i hope it can be of help:)
Hi, i am confused on this question: Wigner proved that a symmetry is represented by either a linear unitary operator or an anti-linear anti-unitary operator. But does it's inverse right? i.e.
Does any linear unitary operator stand for a symmetry transformation?
It seems to be right, as a...
Perhaps it is a stupid question, but really confuses me.
In QFT, virtual particles can be produced from vacuum, say, (M,0,0,0) and (-M,0,0,0)
In Einstein's theory, if we consider the (-M,0,0,0) virtual particle, make it's energy-momentum tensor and solve the Einstein equation, and take the...