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bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 3 years, 11 months
seen 17 hours ago

Hi, I am a string theorist and a publicist.


Jan
12
answered If humans were able to catch all sun energy reaching the earth for their use, will the climate change?
Jan
11
answered Will Gauss's law still hold in case of deviation from inverse square law?
Jan
11
comment What was missing in Dirac's argument to come up with the modern interpretation of the positron?
Well, @Ron, it would be an electron-positron annihilation, wouldn't it? The process itself violates energy conservation so the extra energy above $2m_e$ would have to be emitted, e.g. into photons. Both you and Whelp seem to mix the two pictures in inconsistent ways. I either use both electrons and positrons, in which case there are no "holes" and all physical states of the particles have a positive energy, or I only use one of the particles, e.g. electrons, in which case there are no positrons and even holes shouldn't be talked about because negative- and positive-energy states are on par.
Jan
11
comment Would the north poles of two magnets repel each other if a weaker south pole was inserted between them?
The geometry behind the "inserting" isn't quite clear. Also, the electrostatic and magnetostatic interactions are pairwise. The force acting on A is the sum of forces from all other objects, so the force between A and B (a contribution to the force acting on A; or B) isn't affected by any additional C. C itself may exert forces on A,B but these are different forces. These are reasons why your questions sounds incomprehensible to me.
Jan
11
comment What was missing in Dirac's argument to come up with the modern interpretation of the positron?
Instead, one may consider a process by which positive-energy electrons "fall" to the states where they have negative-energy: they fall to the empty region of negative-energy states, gradually filling it. This may be imagined to be one of infinitely many steps in which the Dirac sea is being formed. But this step clearly cannot be interpreted as one electron's becoming a hole. Quite on the contrary, it is an annihilation of an electron with a hole. When the previously empty negative-energy state is filled, the hole disappears; it is not being created. You have it upside down.
Jan
11
comment What was missing in Dirac's argument to come up with the modern interpretation of the positron?
Otherwise Dirac never thought that an electron should become a positron. This is just a totally flawed "application" of the hole theory. Note that a positron is a hole in the sea, so the claim means, using the original unphysical vacuum which has empty negative-energy states, that an electron should become a hole. But an electron can't become a hole: this would violate the charge conservation law regardless of the vacuum you consider primary.
Jan
11
comment What was missing in Dirac's argument to come up with the modern interpretation of the positron?
Dear @Whelp, well, maybe you read the original Schrödinger paper about the Zitterbewegung or some other sources that just uncritically reproduced it. However, if you read serious sources, like mostly correct textbooks on QFT, you will never encounter Zitterbewegung because it only belongs to the history of physics and this paper by Schrödinger was wrong for the reasons I explained. Maybe the authors of texts you've read were simply not competent.
Jan
11
revised What was missing in Dirac's argument to come up with the modern interpretation of the positron?
added 1082 characters in body
Jan
11
answered What was missing in Dirac's argument to come up with the modern interpretation of the positron?
Jan
11
comment Do magnets lose their magnetism?
I personally added my own downvote, your second one, because you impliticly say that the reason why the magnetization stays is the energy of the magnetic field itself, like $\int B^2/2$. However, the latter is positive, so Nature would surely love to spontaneously reduce it if it could. This was really the (valid) point of the OP. The actual reason why it doesn't happen is that there is a negative interaction energy from the elementary magnets' being oriented in the same direction.
Jan
11
comment Do magnets lose their magnetism?
If magnets fail to be closed systems, the violation is so small that it doesn't affect this question. In the very same sense, it is nonsensical to talk about domains because the question is actually about magnets which means that the whole bulk of the material is one large domain. And the essence of the right answer is that this macroscopic magnetic field won't spontaneously disappear so smaller domains etc. will not form. So your complaint, Ron, boils down to the fact that you don't know the right answer to the question by the OP.
Jan
11
comment Why is it concluded that the cosmos is expanding when in fact the observations are for events further back in time?
Dear @DDD, my answer (its last paragraph) answers even the question that you didn't ask. The sentence "expansion is decelerating not contracting" is completely nonsensical, by the way. Expansion can't be contracting (because contraction is the opposite of expansion), and even if we decided that this pair "contracting acceleration" means something, it would mean the same thing as "decelerating expansion", so the sentence would be untrue. I know that David won't like it but I guess that someone who doesn't understand the words "expansion" and "acceleration" in general shouldn't study TBBT.
Jan
10
comment Why doesn't phase space contain acceleration/forces?
Forces are functions of positions and velocities, so in each point of the phase space, one may calculate the forces that act on any object. In this sense, the phase space contains both forces and accelerations.
Jan
9
revised Energy momentum tensor from Noether's theorem
added 540 characters in body
Jan
9
revised Energy momentum tensor from Noether's theorem
added 540 characters in body
Jan
9
answered Energy momentum tensor from Noether's theorem
Jan
8
comment Why is spacetime near a quantum black hole approximately AdS?
By the way, you will have to give me over 15,000 fraudulent downvotes similar to this one to catch up with me, good luck.
Jan
8
comment Why is spacetime near a quantum black hole approximately AdS?
No, sorry, this is not a possible interpretation of your flawed answer. The question you unsuccessfully tried to answer was one about decoherence – an important process in quantum mechanics – and the sentence made it very clear that you thought that the AdS near-horizon geometry depended on the discussion of quantum mechanics (or even decoherence) which it doesn't. As you can see, twistor understood it in the same way which led to his confusing question with the word "quantum" which has nothing whatsoever to do with the statement that AdS is the near-horizon geometry.
Jan
8
comment Physical meaning of the energy density of an electrostatic field
Otherwise this discussion is increasingly suggesting that it's a complete waste of time. The correct title of your question may have been "Try to convince me that the electric field contributes to energy but I assure you that I will always dogmatically oppose what you say for totally irrational reasons because I have already decided that electrodynamics has to be nonsense so you will waste your time." I got caught, I often do.
Jan
8
comment Physical meaning of the energy density of an electrostatic field
Dear Marco, whether charges are ultimately moving while work is being done is completely irrelevant for the fact that the electromagnetic fields contribute to energy. The situation is fully symmetrical. You could have also said: "I can't figure out how charges may ever accelerate each other without changing the electromagnetic fields in between." It's just your irrational prejudice to think that the mechanical energy is the only right one or the primary one, to try to reduce everything to charges and mechanics. Treat the formulae mathematically! The electric and mechanical terms are on par.