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bio website motls.blogspot.com
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visits member for 3 years, 9 months
seen 16 hours ago

Hi, I am a string theorist and a publicist.


21h
comment In QED, why is the $e^- + e^+\leftrightarrow\gamma$ process forbidden on-shell?
No, Love Learning, one can't. Furry's theorem is only about the expectation of the product of currents - and one needs both creation of a current (the electron and the positron Dirac operator) as well as a photon. The "dynamical part" of the amplitude behind this forbidden process doesn't really vanish; it is purely the kinematical part - one that only depends on the momenta - that vanishes for reasons that Valter described.
1d
comment Few particle fermion system wavefuction
Yup, sorry. The number of states -- the dimension of the HIlbert space - is "k choose N" then.
1d
comment Few particle fermion system wavefuction
No. Maybe imagining that it's "many terms" - namely $3!=6$ here and $n!$, n factorial, more generally – is a bit misleading way to describe the situation. The vector I wrote down, a superposition of 6 other terms, may be viewed as "six terms", but it's still "one vector", and what is variable is the coefficient in front of this vector (the complex probability amplitude), so the vector describes "one" possibility for 3 fermions in 3 states. For $N$ fermions in $k$ (fewer) states, you get more than one possible states but if the number of "boxes" is the same as the number of f., there's 1 option
Oct
17
comment How did Newton discover his second law?
I agree with David. The usage of an apple as an example of the effects of terrestrial gravity came from Newton himself, not some other storytellers, and the basic point behind Newton's gravitational breakthrough was the unification of the motion of terrestrial and celestial bodies, and an apple and the Moon were representatives of these two that Newton actually had in mind!
Oct
17
comment What happens to waves when they hit smaller apertures than their wavelenghts?
Huygens' principle is a more specific result going well beyond the dimensional analysis and I haven't used it. We haven't even used the dispersion relations (e.g. the constancy of the speed of waves as a function of the frequency). This simple interference of the points is similar to but simpler than Hyugens' principle, and it works both in odd and even dimensions.
Oct
17
comment What happens to waves when they hit smaller apertures than their wavelenghts?
Dear Ben, the factors of $g,\rho,\lambda$ and their powers, whatever is needed for a given type of wave, may always be added uniquely by dimensional analysis. There is some energy flux for $d\sim\lambda$ and relatively to this energy flux $E_0$, if $d\lt \lambda$, the energy flux for a smaller hole is comparable to $E_0\cdot (d/\lambda)^4$ where $E_0/\lambda^4$ was simply inserted to get the correct result (boundary conditions) for $d=\lambda$. It's only the $d^4$ that is "nontrivial" in the result.
Oct
17
comment Can the High beta fusion reactor work?
Yup, that's exactly what I meant by a "basic thing" – that it's an omnipresent fundamental problem that every researcher in the field has had to deal with much of the time. ... I obviously don't claim that this project has to work, it probably won't. But I think it would be unfortunate to dismiss the work of a trained professional (plus colleagues) in a big corporation tasked with a similar task just because they're not finished yet or because their approach differs from some other, hugely funded approaches, without an actual physics argument why their approach is less promising.
Oct
16
comment What happens to waves when they hit smaller apertures than their wavelenghts?
It's trivial to see why it scales like $(d/\lambda)^4$. One gets constructive interference (nearly the same phases) from $O(d^2)$ points (area) in the hole, so the amplitude goes like $d^2$ which is why the intensity goes like $d^4$. Note that this is for propagation in 3+1 dimensions. For 2+1 dim, like water waves, one only gets $d$ for the amplitude and $d^2$ for the intensity.
Oct
16
comment Can the High beta fusion reactor work?
Could you guys try to expand it to a full answer that actually tries to argue that the whole compact design violates some physics argument? I have doubts about these comment-sized answers of yours. It's hard for me to believe that the guy with a fusion-related NASA-led PhD from MIT doesn't know the basic things about the plasma confinement. I guess that he knows what "magnetic mirror" is as well and if the problem was a lack of funding, it may have been overcome in Lockheed Martin, right?
Oct
15
comment How did Einstein derive general relativity?
It wasn't a big deal. Just in some semi-popular context, he said that string theory predicted general relativity in the sense that GR on Earth was only discovered before ST due to historical coincidences. It's possible that there are other civilizations where ST was discovered first and GR with all of its usual features derived as the string theory's consequence.
Oct
13
comment How did Einstein derive general relativity?
Doubts about this self-evident observation are usually just a misinterpretation of someone's dislike for the treatment of the gravitational field as a quantum field theory for spin-two quanta. But this is a totally legitimate description, one that is almost mandatory if one wants to use the language of the QFT, and of course that string theory naturally produces this description/decomposition, too. QFT theorists often use it. But all other valid ways to interpret or visualize the curved spacetime are there in string theory, too.
Oct
13
comment How did Einstein derive general relativity?
Yes, of course that they did recognize and would (soon or later) recognize the geometric interpretation. It's really the point of this Witten's thought experiment about the world where string theory is discovered first, when GR is unknown. One can see that adding a vertex operator of the graviton to the world sheet action is equivalent to curving the metric and this point is manifest.
Oct
13
comment How did Einstein derive general relativity?
I don't know how to say it more clearly and generally but all the suggestions that string theory is "missing something important" relatively to GR are completely wrong or irrational or nonsensical verbal games with words that don't mean anything or demagogic misinterpretations of ways to write something instead of their physical content. Everything that has a physical sense about physics of GR or ST at long distances is encoded in some observables and those are exactly the same, so the physics-legitimate parts of these theories are exactly equivalent in the limit.
Oct
13
comment How did Einstein derive general relativity?
String theory doesn't imply that a "flat background" is any special relatively to other solutions. It implies that the configurations-without-excitations are exactly those Ricci-flat or similar backgrounds that follow from GR. At long distances, string theory is exactly equivalent to GR coupled to a few matter fields including all conceptual and "philosophical" points that have any sense. It is a fact in GR as well that GR may be expanded around a background, e.g. the flat background, and treated as spin-2 fields. It's an extremely smart, particle-physics-friendly treatment of GR.
Oct
12
comment The rebound height of a mass on a trampoline
Yes, I think you may want to ask a new question because the comment above is a combination of requests to explain words you may find by a single simple internet search with some explanations of something I wrote a long time ago which may or may not be relevant to what you really need etc., and the comments are probably not a good place for those....
Oct
12
comment The rebound height of a mass on a trampoline
The tilde means "is proportional to", $y\sim x$ means $y=kx$ for a constant $k$. I am not sure whether I am able to say whether the picture is "right" or "wrong" without further captions.
Oct
12
comment Jarlskog Invariant and its mathematical origin
But the standard parameterization isn't the only parameterization. If the matrix is complex in the standard parameterization, it doesn't mean that it can't be brought to a real form.
Oct
12
comment Jarlskog Invariant and its mathematical origin
I don't understand your comment, user. Your comment is exactly equivalent to the original question and my answer was written in order to answer this question, so why do you ask again? Obviously, CP-violation is present if and only if the matrix cannot be brought into a real form. So even if $s_{13}=0$ but some of the other sines and cosines entering $J$ are zero so that $J=0$, then it is possible to bring the matrix into a real form. The transformation needed to do so is different than those you probably have in mind but it exists.
Oct
11
comment Group theoretical reason that Gluons carry charge and anticharge
Just a correction. The 2-dimensional rep of $SU(2)$ is not real. You need complex numbers but it is equivalent to its complex conjugate - we say that it is pseudoreal or quaternionic.
Oct
9
comment Group theoretical reason that Gluons carry charge and anticharge
Dear Jakob, I think that these are basic questions about group theory and group theory in physics. You find them on first pages of every introductory text about these matters. Take e.g. Howard Georgi, Lie algebras in particle physics, or anything simpler. It doesn't really make sense to answer your questions because you're effectively asking about all the basics of group theory and to answer, one would have to effectively reproduce a whole textbook on these matters because you seem to be starting from scratch.