116,192 reputation
7180347
bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 7 months
seen 16 hours ago

Hi, I am a string theorist and a publicist.


Sep
1
awarded  Good Answer
Aug
31
answered Physically, what causes joint pain?
Aug
30
comment Why Does The Moon Apear White/Grayish and The Sun Yellow?
Dear @MarcvanLeeuwen , if you have learned to read the digit "6", you would be able to notice that according to the evidence, my answer is the one which has a value while your remarks are those that don't. 1) Not being yellow and being more blue are exactly synonymous because yellow and blue are colors that are inverse to one another. 2) The Purkyně effect surely is an effect of perceived colors changing according to the brightness, strikingly contradicting your invalid claims. 3) You haven't said anything about the effect because you have said nothing relevant for the question.
Aug
30
comment Why Does The Moon Apear White/Grayish and The Sun Yellow?
Dear @House, during lunar eclipse, the Moon appears red because it indeed primarily sends us reflected red radiation. Google search e.g. for "Why a totally eclipsed moon looks red". The Moon should be black in the shadow if things were perfect. But the atmosphere allows the radiation to bend and the red radiation bends exactly mildly enough in the atmosphere to get from the Sun to the Moon around the Earth, while the blue and other light is basically filtered out.
Aug
30
comment Why Does The Moon Apear White/Grayish and The Sun Yellow?
Dear @MarcvanLeeuwen - not sure why you think so. First of all, grass is green, not white (or blue). Second of all, if grass is just weakly illuminated, be sure that it will be subject of the Purkyně effect - appear bluer - just like everything else. Why do you think that grass gets an exemption? - Nathaniel: you completely reverse this discussion. The fact that the Moon is white in daylight (because there's enough light around for cones) is my and Purkyně's point while Marc's point is to spread doubts about this fact!
Aug
30
awarded  Nice Answer
Aug
29
comment Why Does The Moon Apear White/Grayish and The Sun Yellow?
Dear Marc, the intensity of light that we receive from the Moon - if you look directly at the Moon, or if it is the only object illuminating the Earth - is many many orders of magnitude weaker than the intensity of the light from the Sun. The ratio (full moon vs noon of the Sun) is 200,000, if you need to know. Most children know that the night - even when the Moon is out there - is darker than the day.
Aug
29
answered Why Does The Moon Apear White/Grayish and The Sun Yellow?
Aug
28
answered How can I estimate CO2 emissions for a vehicle travel distance
Aug
28
answered Difference between symmetry and invariance
Aug
28
awarded  Enlightened
Aug
28
awarded  Nice Answer
Aug
28
comment is there an operator which measures the mass of particles?
Please get well and don't erase questions. Maybe someone will understand both of us. ;-)
Aug
28
comment Why don't photons split up into multiple lower energy versions of themselves?
@annav - hmm, Anna, but it's still true that for a general massless particle, the splitting is possible in the collinear case and is pretty important. $(E,E,0,0)$ splits to $(kE,kE,0,0)+((1-k)E,(1-k)E,0,0)$. The amplitude of that for non-Abelian gauge bosons is the structure constant of the group.
Aug
28
comment Why don't photons split up into multiple lower energy versions of themselves?
@ACuriousMind - your point is definitely important and the collinear splitting of a massless particle into two is not zero in general. For example, it's important for 1 gluon going to 2. For photons, this amplitude is 0 due to the Abelian gauge symmetry and other symmetries. While Anna's argument is OK to settle similar things for "electron emitting a photon" which can't happen, I would say that it is ultimately an unusable argument simply because a photon (massless particle) doesn't have a rest frame, so one can't deduce anything from this non-existent frame.
Aug
28
comment Why don't photons split up into multiple lower energy versions of themselves?
Dear Anna, you're surely exaggerating when you say that the momentum and energy conservation laws must be violated "both", right? It's enough to violate the energy law, the momentum may be OK. With the initial photon momentum $\vec P$, you can surely find two momenta of final photons $\vec K,\vec L$ so that $\vec K+\vec L = \vec P$, can't you? For both of these final photons, you may set $E = pc$, so the violation of energy conservation is the only problem then.
Aug
28
comment is there an operator which measures the mass of particles?
When we describe the observations - e.g. a quantum system by a Hilbert space and operators - the whole formalism implicitly assumes that all such things we haven't observed yet and we have no reason to believe to be observed in the future (like the variability of the electron mass) - are indeed absent. So the formalism doesn't allow $m_e$ to be anything else than a fixed parameter whose value is simply inserted. It's a universal constant like $\pi$ except that the value is less unique and it's not mathematically calculable out of nothing (so far).
Aug
28
comment is there an operator which measures the mass of particles?
The numerical value of the electron mass depends on the units, obviously. But once you fix your units by a well-defined definition, the numerical value is completely universal. Nature guarantees that. One may derive that assuming that our Standard Model etc. is right. And one may also observe the constancy of the electron masses from observations. It's in principle plausible that the mass will be observed to be non-constant in the future, and a more accurate theory will have to be modified, but this is a pure speculation unbacked by anything.
Aug
28
comment is there an operator which measures the mass of particles?
Apologies, @AcidJazz, I probably don't understand the question. The constancy of electron's mass and the attractiveness of gravity are 2 laws of physics. They're different laws but their being laws means the "same thing", like for all other laws of physics. I don't understand what the other thing that a law of physics could mean, "set at that for experimental work", could mean. All laws of physics are reversely extracted from the experiments. We're never sure that they're the 100% final answer. They rarely are. But they're still laws of physics.
Aug
28
comment is there an operator which measures the mass of particles?
Well, yes and no. The operator $P_\mu P^\mu$ is an operator but relativistic laws mean that in a free theory, it's equal to the $c$-number $m^2$ for each particle. And when we do include the interactions, $P_\mu P^\mu$ becomes a rather hard to interpret operator because in the interacting theory, single- and multi-particle states gets mixed into superpositions. There are different values of masses we insert or measure - bare, pole mass, one in this or that renormalization scheme - but the difference between them has nothing to do with their being $c$-numbers.