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I used to check every question here but not any more. Physics Overflow, which was built from the archives of a defunct "Theoretical Physics Stack Exchange", seems more promising as a place for advanced physics Q&A.


Jan
10
comment Approach of the Andromeda Galaxy
"at what point will the galaxy belonging to the weaker SBH enter the event horizon of the stronger" The event horizon of a black hole is the point of no return, where even light can't escape, and even for a supermassive black hole it's still a small region of space, about the size of a solar system. Objects outside a black hole's event horizon are still attracted by the black hole, but they will just orbit it without falling in and becoming part of it.
Jan
9
comment Approaches to Quantum gravity
Well, string theory is where you could learn the most. Ideally, anything you do should be complementary to string theory, so you're at least not filling your mind with false ideas. You could work on "Vasiliev gravity" or "higher spin gauge theory", which is another theory, including gravity, for which gauge/gravity dualities exist. Twistors are also potentially relevant. arxiv.org/abs/0808.3907 is a paper which tries to apply twistors, in the contemporary way, directly to general relativity.
Jan
6
comment Bohmian loophole in PBR-like theorems
In the usual formulation of Bohmian mechanics, the equations of motion for the particles have some dependence on a wavefunction, which acts as a "pilot wave". But I'm saying you can just substitute a given wavefunction into those equations of motion, so you're left with nothing but particles that interact according to a nonlocal potential. So in this altered formulation, the quantum potential is not part of the physical state, it's part of the equations of motion.
Jan
6
asked Bohmian loophole in PBR-like theorems
Jan
5
comment Hidden observers in Double Slit experiments - Do they matter?
In dx.doi.org/10.1007/BF00734319 they say that "transparency of the mirror" or "visibility of the interference pattern" can serve as the third observable, but I do not understand what concrete measurement is involved.
Jan
5
comment Hidden observers in Double Slit experiments - Do they matter?
In those works, they are measuring neither which way the particle went, nor what the interference pattern on the screen was, but some third observable which is correlated with both of those properties, but so weakly that some quantum coherence is preserved. They call it an "unsharp observable" and it has some similarity to the idea of a "weak-valued measurement", a pointer variable which has the right expectation value to track the property it represents, but whose variance is enormous...
Jan
3
comment If the ground states of interacting QFTs are so complicated, how did Nature find them?
@Hugh Allen: Do you also believe that you cannot fall completely asleep, because you cannot observe yourself do this?
Dec
25
comment Does quantum Zeno effect play role in astrophysics?
Not unless the first galaxy is projecting an intense focused beam of neutrinos at the second galaxy, or something like that.
Dec
22
comment Does quantum Zeno effect play role in astrophysics?
It doesn't mention observers, but it looks like it was motivated by the idea that it's specifically observers that cause the QZE. Otherwise I don't know why you would imagine e.g. that one galaxy reduces decay rates in its neighbor.
Dec
22
comment Does quantum Zeno effect play role in astrophysics?
So the answer to your question is that, probably there is an astrophysical environment somewhere in which there are interactions occurring with a similar net effect. But the subtext of your question seems to be that the QZE has to do with the presence of observers, and that's not what it's about.
Dec
22
comment Does quantum Zeno effect play role in astrophysics?
The quantum Zeno effect occurs when the effect of some interaction is to produce a slower version of the wavefunction evolution that would have occurred in the absence of the interaction. It is not specifically about "observers", and it requires interactions which specifically have a "freezing" or "restoring" effect on the quantum state. For example, en.wikipedia.org/wiki/… mentions repeated applications of ultraviolet pulses to a collection of ions, which reduced the amplitude for the ions to enter an excited state...
Dec
10
awarded  Announcer
Dec
5
awarded  Caucus
Dec
4
comment How quarks converted into leptons
It's the detailed interaction of the particles with the electromagnetic field which suggest fractionally charged constituents, e.g. physics.ohio-state.edu/~kass/P780_L8_sp03.ppt slides 13-14 (decay of vector mesons) or arxiv.org/abs/hep-ph/0210054 (various other processes).
Dec
4
comment Do unoriented strings possess asymptotic states?
... a baryon, a collection of N strings terminating on a compact brane. A non-gauge-invariant object like a diquark might be part of a baryon, e.g. you would think of a brane with 3 strings attached, and two of the strings very short, so that the third, long string can then be conceived as connecting a quark and a "diquark object". But all this would be just one way that gauge theory embeds in string theory. A truly comprehensive account might list half a dozen ways to get one from the other.
Dec
4
comment Do unoriented strings possess asymptotic states?
The right way to visualize perturbative string theory is as defined by Riemann surfaces (possibly with boundaries) with marked points corresponding to the asymptotic states. (A conformal rescaling of the worldsheet is used to map the infinite string history onto the finite Riemann surface.) An unoriented string could be conceived as an equal superposition of oppositely directed oriented strings... Concerning the relation with gauge theory: an individual "quark" would be a string between a color brane and a flavor brane; a meson, a string between two flavor branes...
Dec
2
comment How quarks converted into leptons
Since this is the direction of your thinking I will point out arxiv.org/abs/hep-ph/0408305 which is a paper trying to unify "iquarks" (author's name for quarks with integer charge) with leptons. That is a paper with no citations and it's probably making a mathematically wrong argument somewhere. If you want to understand what physicists already think, you need to understand fractionally charged quarks, and the SU(5) theory might help too. But people follow their own ideas right or wrong, and maybe you can learn something else from this professional paper even if it is wrong.
Dec
2
comment How quarks converted into leptons
Normally by "lepton" we mean particles that don't interact via QCD. You seem to be thinking of "integer charged quarks"? That is an old idea, but there are some (difficult) experiments which do seem to falsify it directly. Also, the fractional charges start to make sense in a unified theory like SU(5) grand unified theory.
Dec
2
comment How quarks converted into leptons
Quarks aren't observed because of en.wikipedia.org/wiki/Color_confinement ... someone will post about this. Like anna said, in your diagram, the quark and antiquark confined in the pion are annihilated, but that is a different thing, it is not the reason why you don't get single free quarks (except at very high temperatures).
Nov
24
answered Help an aspiring physicists what to self-study