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bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 7 months
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Hi, I am a string theorist and a publicist.


Aug
20
answered Why does a moving fan seem transparent?
Aug
20
comment Spherical and chromatic aberration correction
Carl, feel free to write an answer. There aren't any "aspheric lenses" that will eliminate the basic wave properties of light. There is no geometry that will perfectly compensate these phenomena. By blocking the edges, I meant that one ignores the region with the edge effect - covers a part of retina, if you wish. I didn't mean a change of the geometry at the locus of the lens which is clearly no useful in eliminating wave phenomena.
Aug
20
comment Can I move an oil tanker?
It moves out of the way "automatically" if the ship is moving from one plane to another, as long as there's space for that. Especially at low velocities, there is no obstruction or net force. The only net force that is added would be drag, en.wikipedia.org/wiki/Drag_(physics) , but that scales like the second power of the speed and is totally negligible here.
Aug
20
comment Spherical and chromatic aberration correction
The edges will always be out of focus, it results from the wave character of light. You may cover the edges not to see this problem. Or you may use a different wavelength of the light or electromagnetic waves to change the thickness of the problematic edge.
Aug
20
answered Euclidean geometry in non-inertial frame
Aug
20
comment Is the wave-particle duality a real duality?
It's been known since the beginning that quantum mechanics eliminates the possibility to describe the world by any objectively determined "pictures". But it's been also clear since the 1920s what the new theory actually is, what it needs to know, what it can predict, how it predicts it, and that this new framework is both internally consistent and complete as well as compatible with all the experiments we know. Any assertion contradicting these facts is just an example of pseudoscience, sometimes sold by popular books.
Aug
20
answered Two definitions of Green's function
Aug
20
comment Can I move an oil tanker?
DJohnM: very important warning! Dear @CarlWitthoft - do you think that the water adds another factor of two because one needs to pump some momentum to the ship, and the same (?) momentum with opposite sign to the water it replaces? I actually doubt this can be right. The most direct thinking is that the pressure on both sides of the ship stays constant, so the force from the water from both directions cancels and it is right to calculate it just like if the ship were in the vacuum (on perfect wheels).
Aug
19
awarded  Guru
Aug
19
comment Can I move an oil tanker?
Yes, Voitcus, one may stop it by pushing from the other side. ;-)
Aug
19
comment Kinetic energy as $\pi k_B T$
General physical quantities are almost always irrational - and almost never rational (or rational multiples of pi, for that matter). The rationality, like the value, depends on units. None of these things are an issue. The issue is that energy, momentum, as well as temperature are quantities that are well-defined including the numerical factors. There aren't any ambiguities in the values. There aren't any ambiguities about the value of the Boltzmann constant, as long as one keeps its definition, and so on and so on. You're just trying to cover a wrong numerical factor by layers of fog.
Aug
19
comment Kinetic energy as $\pi k_B T$
Sorry, no one else has used $k^*_B$ before, so this is clearly a new invention that can in no way fix the errors in anything that was written before. This frantic redefinition only reinforces the point that the original authors were simply sloppy about all coefficients of order one. One can write formulae that are fine up to numbers of order one. But if one writes actual numerical constants everywhere, they should be correct and only one numerical constant in each equation is correct.
Aug
19
comment Derivation of eq 6.17, Peskin and Schroeder
Sorry, there are just so many errors in the way how you "copied" it. First of all, the disk should be a simple dot, the 3D inner product. Second, the $E-\vec p$ should be simply $E-p$, it is nonsense to subtract a vector from a scalar. Third, second powers are heavily missing above $E$ and $p$ in your formulae, much like factors of 2. Also, the first $=$ should actually be just $\approx$, approximately equal. I don't recommend anyone going through this question before at least 80% of these bugs are fixed. It's a mess.
Aug
19
comment Can I move an oil tanker?
Some numbers: $F$ may be 1,000 newtons. An oil tanker is 50,000 tons or 50 million kg. So F/2m is 1,000 over 100 million = 1 / 100,000. This time t^2 should be 0.1 so t^2 is 10,000, so t is indeed just 100 seconds. A few minutes of intense pressure should be enough to move the oil tanker by 10 cm. Beware: the oil tanker won't stop.
Aug
19
comment Kinetic energy as $\pi k_B T$
Apologies, after going through your answer, the factor looks as wrong as it did before. There is no freedom in the definition of the momentum, energy, or absolute temperature. The purely numerical factors must be determined and everything I see indicates that de Broglie has simply determined them incorrectly. An average spherically symmetric particle of an ideal gas simply carries $E=3kT/2$ in average and the fact that it may also be described as a wave can't contradict this fact.
Aug
19
comment How relevant is the Heisenberg Uncertainty Principle?
I meant the same and HUP may be an axiom and the rest of QM follows from that. [x,p]=i*hbar really "follows" from the delta x times delta p is greater than hbar/2. A nonzero commutator between observables is the only conceivable "mathematical" reason why there could be a limit on the product of uncertainties. Within "classical plus quantum physics", it's easy to prove this rigorously.
Aug
18
comment How relevant is the Heisenberg Uncertainty Principle?
Dear Anna, I know that I and many others phrase the organization of QM so that HUP is a derived fact. However, I disagree with the flat context-free statement that "HUP is not a postulate" because it implicitly says that it cannot be considered a postulate in any organization of the assumptions.
Aug
18
comment Why can't photons have a mass?
Hi @XiaoyiJing - most things are related in some way. The mass gap of Yang-Mills fields is just a different statement of the fact that there exist no massless - or arbitrarily light - allowed (color-neutral) bound states in QCD and similar theories. For QCD, the conclusion is the opposite one - a massless particle does not exist.
Aug
17
comment Where is the Hydrogen the Sun consumes?
Right. And the Sun not only "doesn't need" to suck material from space. It would have no effective way to do so, either. The Sun holds 99.8% of the mass of the Solar System - outside the Sun, this large volume of the Universe is really really empty.
Aug
17
comment How relevant is the Heisenberg Uncertainty Principle?
You know, the reason why I found this seemingly terminological exchange worth responding to is that I feel that your ultimate reason of not respecting HUP as a postulate is your real opinion that it could be avoided or it is not important. It is very important. It expresses what QM is new about - and the mathematical details of QM may be extracted from it.