54,258 reputation
234123
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location Greece
age 74
visits member for 3 years, 10 months
seen 50 mins ago

Retired experimental particle physicist.

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


50m
comment How do two electrical charged particles know to repel or attract each other?
@Sofia I am an experimental physicist. For me , exists means "can be measured by some proxy" , not that a mathematical formula exists. As there are other ways of measuring and fitting the crossections etc not needing the mathematics of virtual particles, I do not apply the verb "exists" to a virtual electron, as I apply it to the on mass shell electron that can leave a track in my chamber :).
1d
comment Why does iron sink in molten iron instead of floating?
you are missing a concluding sentence to the effect that iron in solid form is denser than in liquid, to complete the answer.
1d
comment Pre-Higgs Energy of electron
Without the higgs mechanism all masses are zero. Energy is a variable depending on context. Have a look for essentials en.wikipedia.org/wiki/Higgs_mechanism
1d
comment Decoherence, collpse, and WHEN does the collapse occur?
what is your time zone? It is almost seven in the morning here . I will be able to chat after 15:00, my zone
1d
comment Why does the blackbody radiate even at thermal equilibrium?
The cavity is an idealized model where only photons exist in equilibrium exchanging energy with walls of fixed temperature, . It is an analogue model for the observed black body radiation from matter. As @garyp says there is not equilibrium with the environment for the black body radiation that all matter emits at a given temperature. en.wikipedia.org/wiki/Black-body_radiation
1d
comment Why did “tickling the dragons tail” by Louis Slotin not cause an explosion?
The crucial point here is that the mass available was subcritical. Criticallity was induced by hand with the reflectors, not with an increase in mass. The time constants of the system "subcritical +reflectors" by the unfortunate experiment itself show that supercriticallity was not attained, since there was time for the reaction of the physicist. Supercriticallity goes off into explosion in miliseconds.
1d
comment Why did “tickling the dragons tail” by Louis Slotin not cause an explosion?
@AlanRominger Ifyour read the link about the accident, the plutonium lump was subcritical , the reflections just turned it to critical, i.e the beginning of the chain reaction, not supercritical, the explosive state. It is not a molotof bomb, where it is enough to have a spark. the three stages of k are crucial as well as the geometry.
2d
comment Decoherence, collpse, and WHEN does the collapse occur?
Coherence means that the phases are known. If you have two particles you need a different hamiltonian from one particle, you need a different one for each particle under considerations, but coherence means that the phases are known. The density matrix formalism disposes of hamiltonians , but shows up the coherence in the off diagonal elements for as many particles as the problem needs. The part of the dm that belongs to macroscopic apparatus is diagonal only, and it is due to the above mentioned dimensional difference
2d
comment Decoherence, collpse, and WHEN does the collapse occur?
well quantum mechanics exists because h_bar is different than zero. Commutation relations, if h_bar is zero, the whole apparatus of quantum mechanics stops having a meaning. It is because h_bar is so small that classical mechanics works macroscopically. Only the classical hamiltonian would make measurement sense. if you substitute numbers in the picture in the other question the HUP is always obeyed because the measurement errors are large and there is no constraint. The density matrix formalism gives a handle on how this can happen, since in principle the underlying frame is QM.
2d
comment How does Gravity behave at high energy?
have a look at the big bang cosmological model en.wikipedia.org/wiki/Big_Bang and TOE en.wikipedia.org/wiki/Theory_of_everything
2d
comment Why are neutrons present in an atom?
It is the strong force that attracts the neutron to the proton.en.wikipedia.org/wiki/Table_of_nuclides . It is much stronger than the electric repulsion if close enough. Two protons cannot get close enough for the strong force to appear because of the electric repulsion . in hydrogen one proton and one electron form an atom.
2d
comment How is the electostatic field propagated by the vacuum? How does a charge feel the presence of another charge when there is NOTHING between them?
the number of photons is not a good quantum number because they are bosons, not because they are massless. At one point neutrinos were thought to be massless, but we counted the lepton numbers that were conserved.
Nov
23
comment Decoherence, collpse, and WHEN does the collapse occur?
I think you are confusing entanglement with decoherence. Decoherence loses phases, entanglement keeps them, maybe distorted but phases are there. In my opinion decoherence happens when h_bar no longer characterizes the interactions, distances are too large or energies, and h_bar can be considered zero. Then the phases no longer have a measurable effect and the system decoheres. ( with exceptions like lasing and crystals etc)
Nov
23
comment Decoherence, collpse, and WHEN does the collapse occur?
I do not see why you say "The single particle wave-function (1) was decohered, but no collapse occurred.". Inmy books as long as you have kets the phases are there. If it is a many body system ( a lasing laser for example) they may be modified in the ensemble , but still there if you think density matrix off diagonal elements.
Nov
23
comment Why does the earth suck both positive and negative charges?
note that positive charges in solids do not move. It is the lack of electrons that creates a positive chanrge.
Nov
22
comment What events lead to quantum decoherence?
@Sofia almost bedtime for me too. tomorrow then,,maybe we could chat if we could find a common time.
Nov
22
comment Differences between astronomy, astrophysics and cosmology?
The last question is not a physics question.Could be asked in chat once you get enough points
Nov
22
comment What events lead to quantum decoherence?
@Sofia as for the mixed states in a density matrix formulation, the expression is a quote from the wiki article. After all a mixed state is an ensemble of pure states, and if you only have one state no need for statistical formulations, I think.
Nov
22
comment What events lead to quantum decoherence?
@Sofia I think they do not really vanish, they are zero within our measurement possibilities. It is the transition between the h a number comparable with the elementary and atomic dimensions and h set to zero for all intents in classical physics. It is not zero, but the uncertainty is so small that the classical mechanics formulations are accurate for macroscopic purposes.
Nov
21
comment Is it more difficult to create higher energy photons, given there is excess available energy?
This to be really answered needs somebody working on this or similar. The general answer is that in the quantum mechanical frame in which these observations belong it is not only energy that decides the probability of transitions from one energy level to the other. Quantum numbers play a definitive role in the wave function , spin particularly in this case. Suppression in the probability of emitting or absorbing a photon comes from the probability given by the square of the wave function. So it is not only just energetically favorable, dropping from higher to lower energy levels.