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234128
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location Greece
age 74
visits member for 3 years, 11 months
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Retired experimental particle physicist.

The picture is a fayum . It looks like aunts and cousins of mine :).


1d
comment What was the entropy of the universe at the time of the Big Bang?
@MBN en.wikipedia.org/wiki/… " 1>a situation where matter is forced to be compressed to a point (a space-like singularity)"
1d
comment What was the entropy of the universe at the time of the Big Bang?
@MBN hypersurfaces are not part of the microstates defining temperature, one needs "particles" in space and time. As I said it is my comfort blanket in this area.
1d
comment The status of the BICEP2 'discovery' after Planck 2014
@CuriousOne I agree to that last, though I do wonder how irreducible can a background drawn by rough strokes can be.
1d
comment Is there a material that could “convert” near infrared frequency to infrared?
You realize of course that any absorptive material will do that through black body radiation but it will be a dispersed spectrum? You want a coherent beam out of this, or at least a beam? rp-photonics.com/frequency_doubling.html for doubling frequency exists, not for halving though .
1d
comment The status of the BICEP2 'discovery' after Planck 2014
@CuriousOne they were sitting on the data for some time. If you look at the Planck analysis it is even worse, it fits polarization on large temperature maps whereas if you look at the experimental setup of BICEP2 it is very elegant measuring tiny temperature differences in small detectors.
1d
comment The status of the BICEP2 'discovery' after Planck 2014
I check for news every now and then : bicepkeck.org/#papers
1d
comment The status of the BICEP2 'discovery' after Planck 2014
@CuriousOne I read it up at the time. It is a different experimental method than the Planck. At the time Planck was giving limits with large errors. Their experiment did measure polarization with five sigma from zero, not limits. The lensing they avoided by choosing the a one degree window and dust was corrected by using Planck published data at the time. After the new results they were supposed to come out with a common paper with Planck by the end of November but no sign of it yet. Disagreement?
2d
comment The status of the BICEP2 'discovery' after Planck 2014
In their publication already part of the signal had been subtracted to account for the dust.
2d
comment Is nature quantized?
in fact it is "because" the black bodies studied experimentally were composed of particles(molecules) in rotational and vibrational levels in the lattice that the model of quantized harmonic oscillators works so well.
2d
comment Can open, unsafe nuclear fusion reaction burn the atmosphere?
in the answer by physicsphyle Bethe is mentioned, and more calculations. Considering the difficulty of getting fusion going for peaceful purposes because of the very high temperatures and confinement needed for fusion plasma to be self sustained, it is not impossible that Bethe calculated that and assured them.
Dec
19
comment Why is every particle a sphere?
@Gowtham Yes, but that is all we can perceive with our instruments, thus the shape, the probability distributions for the loci of charges around the atom.
Dec
19
comment Why is every particle a sphere?
atoms have complicated shapes en.wikipedia.org/wiki/Atomic_orbital
Dec
18
comment Elastic scaterring of neutrinos - Why does the electron neutrino dominate this channel
handwaving: extra Feynman diagrams means extra probability of interaction kvi.nl/~loehner/saf_seminar/2010/… figs 5.1.3a
Dec
18
comment Why do physics students find vectors so difficult to deal with?
I would start the year by tabulating which student had planear geometry with its axioms , trigonometry and solved problems. Nothing like it to give a feeling about the difference between one dimension and two dimensions. If they had not had any then they should be introduced to it, before entering all the complications of higher dimensions. After all we all walk in two dimensions, and every body knows that a diagonal line through a field is faster than going around the two sides.
Dec
18
comment Why do physics students find vectors so difficult to deal with?
Well, I got trigonometry and Pythagoras, planar geometry axioms and all in third grade in highschool back in 1954 . Nothing like drawing circles and computing angles to show the difference between one dimension and two dimensions. Had no difficulty when meeting vectors later.
Dec
18
comment Why do physics students find vectors so difficult to deal with?
I wonder whether they are getting any geometry in their math curriculum in high school? Pythagoras theorem, trigonometry etc?
Dec
17
comment can we detect the photons in the interaction of two charged bodies?
We use the word virtual to differentiate from the real input and output lines ; because Feynman diagrams simplified logically the very complicated integrals one had to solve for scattering theory using the scattering matrix, and it helps keep track of the quantum numbers, an electron always is spin 1/2 , lepton number 1 etc, so the calculations do not get confused.
Dec
17
comment can we detect the photons in the interaction of two charged bodies?
An accelerating particle is a different story, it needs different Feynman diagrams : the electron interacts with an off mass shell photon from the field that gives it the accelerating impulse and a real photon radiates off. There is still a virtual photon from the electric or magnetic field interacting with the electron.
Dec
17
comment Why can't dark matter be baryonic?
Have a look at pdg.lbl.gov/2014/reviews/rpp2014-rev-dark-matter.pdf .
Dec
17
comment Why can't dark matter be baryonic?
I am curious, my first impulse would be that baryonic matter would contain charged particles and charged particles interact electromagnetically. Neutral ones are either in atoms, and they also interact and give spectral lines, or unstable. One hypothesizes exotics for dark matter just because in contrast to baryons, electromagnetic interactions are higher order and less probable.