113,714 reputation
7170337
bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 6 months
seen 11 hours ago

Hi, I am a string theorist and a publicist.


Mar
29
answered What is the interaction with Higgs field(s) that give the quarks so much different masses?
Mar
29
answered Why are WW gg ττ branching ratios so similar for a 115 GeV SM Higgs?
Mar
29
answered How can a Higgs decay to heavier products than its mass?
Mar
29
revised can one introduce magnetic monopoles without Dirac strings?
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Mar
29
answered can one introduce magnetic monopoles without Dirac strings?
Mar
27
comment Does a photon exert a gravitational pull?
Well, almost right except that the photons also carry a nonzero pressure, the spatial components $T_{ii}$, which also influence the shape of the metric tensor i.e. the gravitational field. In general relativity, not only the total mass or mass density but also the rest of the stress-energy tensor (density of momentum, flux of momentum etc.) affects the resulting gravitational field. Whenever a component of the stress-energy tensor changes, the gravitational field changes as well. That's what Einstein's equations clearly say. At infinity away from the photons, only the total mass matters.
Mar
27
comment Bell's theorem and why nonlocality is problematic
The comment above is echoed by physicists from many cultures. For example, Hawking in his "changed my mind" paper why information is preserved arxiv.org/abs/hep-th/0507171 shows that the path integral in the far future is still dominated by the trivial topology spacetime that doesn't see any causally separated interior. That's why the trivial-topology-calculation's qualitative result - the information may get out - is ultimately right. At any rate, locality is surely not "abolished" everywhere because of those insights.
Mar
27
comment Bell's theorem and why nonlocality is problematic
Dear Ron, it's not really right to say the "export of the information from the black hole" proves that locality is absent in gravitating theories in general. Locality is still 100% perfectly valid in gravitating theories around a Minkowski background. The information may get out of the black hole because the causal separation of the black hole interior is just an approximation; analogously, in quantum tunneling, information can tunnel through a finite barrier although it looks "robust" classically. However, there are still "infinite barriers" in Minkowski space that protect exact locality.
Mar
27
comment Bell's theorem and why nonlocality is problematic
Dear Ron, locality isn't dead. It's completely exact in the non-gravitational description of any theory, including the CFT in AdS/CFT, and even in the gravitating case, nonlocal effects are exponentially tiny, very restricted and careful, and have surely nothing to do with the explanation of entanglement experiments whether it's in AdS/CFT or anywhere else. Nathaniel: indeed, Einstein was among the people who was confused by basics of quantum mechanics in the 1920s but also 1930s and later. What makes the confused people in 2012 worse is that they had 50+ more years to understand; they failed.
Mar
27
comment Is all angular momentum quantized?
That's surely right and excitations in superconductors may have quantized spin. Still, it is hard to find that the background superconducting "medium" would be an angular momentum eigenstate. This won't really happen easily.
Mar
26
revised What's the classical limit of the pressure of the ultrarelativistic Bose gas
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Mar
26
answered What's the classical limit of the pressure of the ultrarelativistic Bose gas
Mar
26
comment Bell's theorem and why nonlocality is problematic
Nathaniel, there may be a "debate" among popular books and armchair physicists – and confused non-expert physicists themselves – but in physics as a scientific discipline, these issues have been settled since the 1920s. User1247, nonlocal effects are indeed not measurable in this world and the simple reason why they're not measurable in this world is that they don't exist in any physical sense at all. When one says that they violate relativity, one talks about a hypothetical world in which they would exist. This world ain't ours.
Mar
26
revised Entanglement and relativity
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Mar
26
answered Bell's theorem and why nonlocality is problematic
Mar
26
comment Entanglement and relativity
"There are thousands of papers on nonrelativistic measurement theory (in a preferred frame) and only few in a covariant setting." - As I explained, it's because there is absolutely no conceptually new mystery associated with the "switch" to relativistic quantum theories. Relativistic quantum theories are just a special case of quantum theories that also preserve some extra symmetry (whose nature has nothing to do with the foundational issues of quantum mechanics); relativistic theories are not qualitatively different from other quantum theories when it comes to the quantum issues.
Mar
26
answered Is all angular momentum quantized?
Mar
26
answered Does the second law of thermodynamics tell me how the entropy changes?
Mar
26
comment Why does large curvature of spacetime imply high temperature?
Even if someone sits on a surface of a neutron star, he may not be ripped apart and the situation may be stable. However, the phonons and other virtual particles inside him may undergo similar dynamics as the atmosphere and exhibit the adiabatic lapse rate, something that makes these quasiparticles inevitably warmer in the deeper levels of the gravitational field - the temperature gradient is again linked to the spacetime curvature. There are various effects and they may have different temperatures and there could possibly exist loopholes in which they're absent... Someone should answer fully.
Mar
26
comment Why does large curvature of spacetime imply high temperature?
Dr Bollinger's answer is on the right track but I believe that because he rejects the key concept of tidal forces, someone who is confident that the right buzzword is the "tidal forces" should post a full-fledged answer here (David?). The canonical example is the observer approaching a black hole singularity. He's torn apart by the tidal forces which are really the nonrelativistic equivalent of the spacetime curvature. Tidal forces rip him apart, big force, big temperature.