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229100
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location Greece
age 74
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Retired experimental particle physicist.

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


Jul
4
awarded  Nice Answer
Jul
3
awarded  Nice Answer
Jul
3
comment Black and white matters. But why and how?
@JasonC I tend to answer to the level of the question and quetioner ( in this case a 15 year old programmer from India). I did say turns "light into heat" without drawing in the kinetic theory .
Jul
3
comment Black and white matters. But why and how?
@DavidHammen thanks, I have edited ( I had the link to the spectrum , I just did not do the eye integral :( .)
Jul
3
revised Black and white matters. But why and how?
correction after comment
Jul
3
comment Gravitational collapse of a photon cloud
The Heisenberg Uncertainty principle has to be taken into account, the volume cannot go to zero for photons, so the energy density has to be high. See my comment to the question
Jul
3
comment Gravitational collapse of a photon cloud
Well, my answer was deleted so: 1) photons do not form a cloud, they each run away in a straight line with velocity c and do not interact with each other. 2) the heisenber uncertainty principle has to be taken into account when thinking of overlapping photons in a small volume. 3) there exist fusion reactors using lasers in a sphere concentrating at the center in order to create the plasma energy for fusion. No danger of gravitational collapse has been envisaged as far as a I know.en.wikipedia.org/wiki/Inertial_confinement_fusion
Jul
3
comment Non-additivity of energy (thermodynamics)
"An adiabatic process is a process that occurs without the transfer of heat or matter between a system and its surroundings.[1][2] A key concept in thermodynamics, adiabatic transfer provides a rigorous conceptual basis for the theory used to expound the first law of thermodynamics." en.wikipedia.org/wiki/Adiabatic_process .
Jul
3
comment Non-additivity of energy (thermodynamics)
The first law is conservation of energy thermodynamically. But thermodynamics does not know about chemical energy, or potential energy, it assumes particles with no special energy releasing or absorbing content. So the thermodynamic internal energy U does not take into account these putative energy transformations. Bringing two isolated systems into contact where radiation and mass can be transfered can introduce chemical reactions , or transitions from one energy state to another which will destroy the sum balance. All forms of energy must be taken into account for energy conservation.
Jul
3
answered Will a blanket warm you if you are underwater?
Jul
3
revised Black and white matters. But why and how?
added 1 character in body
Jul
3
revised Black and white matters. But why and how?
added 493 characters in body
Jul
3
answered Black and white matters. But why and how?
Jul
3
answered Non-additivity of energy (thermodynamics)
Jul
3
comment Would half of electromagnetic wave be absorbed on first sensor?
well, it does not work that way. Sensors have a given area that responds either to individual photons ( as in photomultipliers) or the the summed up energy of individual photons. As I said , you cannot turn the beam around in the way you imagine, half wavelength, it is not physically possible because of the tiny, nanometer wavelengths and the very high velocity of light. Suppose you could, which is impossible , the photons making up the front wavelength would hit the frontal detectors and the ones turned would hit the 180 deg one. 1/2 1/2 by construction ( which is impossible really)
Jul
3
revised Would half of electromagnetic wave be absorbed on first sensor?
clarification and link added
Jul
3
comment Would half of electromagnetic wave be absorbed on first sensor?
@user22180 Have a look at my answer to this question physics.stackexchange.com/questions/105417/…
Jul
2
answered Would half of electromagnetic wave be absorbed on first sensor?
Jul
2
comment The speed of light during the inflationary period
please give a link or a reference for us to find a link for this assertion "cosmologists...speed of light different" . In my books the inflationary period was hypothesized because in the old Big Bang model one could not explain the isotropy in CMB because in the old BB the regions at the time of decoupling of the photons would not thermodynamically come into equilibrium because there were inaccessible regions due to the velocity of light being necessary for thermodynamics. The inflationary period introduced quantum mechanics and has an inflaton field which homogenizes everything then.
Jul
2
answered For what rare events was the LHC built?