# All Questions

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### Strings and QFT: particles moving backward in time?

New question: In string theory and QFT, do particles travel back in time? Not related to antimatter: Do they travel back and forth in time in reality or are these just interpretations of mathematical ...
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### Interacting particles

We are familiar with the grand partition function for the grand canonical ensemble. This makes me wonder: what kinds of modifications would be required if the particles interacted? Thanks.
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### Why work to change velocity from 0 to 20 km/h is less then from 20 to 40? [duplicate]

Imagine spaceship in vacuum with mass = 1. At beginning, it has velocity 0, and kinetic energy 0. $$W_1 = 0$$ Then, it turns on its engine, and changes velocity from 0 to 20 (delta v = 20). It's ...
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### Newton's Third Law of Motion

I want to start off by saying that I've looked around for other explanations, but I've not really found any satisfying ones. My question is basically the whole "Why can anything move at all?" ...
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### Information bearing degrees of freedom of a quantum simple harmonic oscillator

I am trying to make sense of arXiv:physics/0210005. I am confused with the concept of information bearing degrees of freedom of a system mentioned at the very beginning. To verify the arguments of the ...
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### Why is the equation for Entropy of an ideal gas that undergoes reversible change in T at constant Pressure like this?

Why is the equation for change in Entropy for a reversible change in $T$ at constant $P$ described as $$\Delta S = n C_p \ln\frac{T_f}{T_i}$$
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### Frictional force on a rope wrapped around a drum

So there's a rope wrapped around a circular drum, subtending an angle $\theta$. A large force, $T_A$, pulls in in one direction, and a much smaller force $T_B$ pulls in in another direction. The ...
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### Conservation of Angular momentum in the dipole selection rules

If the total angular momentum J of an atom is not changing during a dipole transition, where does the angular momentum for the photon come from?
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### Pendulum axes confusion

Suppose I have a simple pendulum and I want to calculate its acceleration when the bob reaches the maximal angle. I usually choose my axes such that the y-axis will be parallel to the rope. Then the ...
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### Hamiltonians, density of state, BECs

When working with Bose-Einstein condensates trapped in potentials, how can one tell what the density of state of a system of identical bosons given the Hamiltonian, $H$? (I have been told that it is ...
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### microcanonical distribution

We know that in an isolated system, the density matrix is the microcanonical distribution matrix. That this the possibility for all the states with energy in a certain interval is a constant? But how ...
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### Do particles travel backward and forward in time? [duplicate]

All these classical ideas are pointless and obsolete today, because in quantum mechanics, the particles are completely different objects, defined by quantum motion of fields, not by the location of ...
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### Multiple vacua vs. vev's in qft

Take a (possibly supersymmetric) relativistic quantum field theory: when we construct it, we suppose that there is a unique vacuum state $|0\rangle$ which is Lorentz invariant, vector of some Hilbert ...
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### metric tensor of expanding universe

Why is the metric tensor of a expanding universe a function of time? Why is it not a function of distance between the galaxies? I heard this from a lecture. Can anyone help me understand?
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### Motion of mercury [duplicate]

I studied that mercury motion around the sun slightly displace by a certain value in each year. But, this is not predicted by kepler until general theory of relativity. What does general theory does ...
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### Conversion of a star

I need to know what would happen to a star that has size 2 times the solar mass. I guess it would either be a neutron or red giant. Is that right?
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### What exactly does this news story mean by “magnetic charge”?

I recently saw this article in my Google News page, whose headline and first paragraph are: Antimatter's Magnetic Charge Revealed Scientists say they've made the most precise measurements to ...
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### Why should multiple versions of a weak measurement preclude it from being a measurement of intrinsic properties of some system

I've been trying to understand Stephen Parrott's criticisms of weak measurement, outlined most concisely here: http://arxiv.org/abs/0909.0295 One of his major criticisms is that weak measurement is ...
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### Landauer's principle vs Wien's displacement law

Can we argue based on Landauer's principle that if one bit information is changed inside a blackbody, the total radiated energy should be at least or in order of kTln2? If it is so, can we also argue ...
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