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bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 10 months
seen 6 hours ago

Hi, I am a string theorist and a publicist.


Nov
24
comment What kind of invariants are proper time and proper length?
You may also talk about the proper distance between the two world lines of the two end points of the rod (the maximum - because of the Lorentzian signature - distance between any selected pair of points on these two lines). If you do so, this proper distance is again an invariant, and it's equal to the length of the rod at rest. But just the "length of the rod" isn't the common "width of the strip" in the spacetime. In non-rest frames, the "length of the rod" is measured along a slanted direction across the strip, so it's shorter $\gamma$ times, @Frank
Nov
24
comment What kind of invariants are proper time and proper length?
Dear Frank, the length of a rod is not called a proper length. The proper length has an extra adjective and it always refers to the scalar length between two particular events (points) in the spacetime. The length of a rod is frame-dependent (i.e. it is not invariant under Lorentz transformations) because in different frames, the length of a rod is interpreted as the proper length of different line intervals, due to the relativity of the simultaneity.
Nov
21
comment About the Ether Theory acceptance
Even the claim that one needs quantum fields is debatable. One may describe the whole QFT Hilbert space in terms of particles only, so that the vacuum is really unique and empty, and one doesn't "need" to talk about quantum fields at all.
Nov
21
comment About the Ether Theory acceptance
Aether may sound as anything that fills the vacuum, @WetSavannaAnimalakaRodVance. After all, the classical notion of "aether" - which used to be synonymous with "quintessence" - was some sort of a spirit. But since Newton, "aether" was used as a hypothetical material composed of particles. Newton was actually trying to explain gravitation via aethers of different densities etc. Those things culminated in the 19th century when the aether had very playful models with gears and wheels, by FitzGerald and others...
Nov
16
comment The physical definition of work seems paradoxical
Dear @bobie, I am OK enough with your answer not to compete, especially because I know that I would get much fewer votes because almost no one reads old questions... The quadratic relationship is natural but people considered other dependences as well, and were actually confusing the momentum with energy for quite some time, and before Newton, the right questions weren't even formulated to be able to decide what is right and what is wrong. Moreover, in special relativity, the simply quadratic relationship becomes inaccurate.
Nov
15
comment The physical definition of work seems paradoxical
@Bobie: Coriolis was also the first man who coined the term "work" for the force integrated over distance, en.wikipedia.org/wiki/Gaspard-Gustave_Coriolis - The knowledge behind the concept of "kinetic energy" isn't just some vague guess about a power attributed to some mysterious "living force". What matters is the right interpretation or the well-defined framework that allows to decide whether something is right or wrong, and Leibniz had nothing of the sort. It's more wrong (but similar) to cite him as the originator of KE than to quote Lorentz as the father of special relativity.
Nov
15
comment The physical definition of work seems paradoxical
@Bobie: I didn't mean just "your" personally – I meant "your" in the plural, counting everyone who prefers historical speculations over the obvious physics content. The relevant physics knowledge equivalent to kinetic energy should be assigned to Newton, and the particular formula $mv^2/2$ may only be attributed to the first person who was actually using the correct laws - i.e. post-Newton - and it happened to be Gaspard-Gustave Coriolis. Lord Kelvin and Thomas Young coined the name "kinetic energy" later. But everything before Coriolis were fairy-tales about "living forces".
Nov
15
comment The physical definition of work seems paradoxical
Dear Bobie, it interests me but I totally disagree with your interpretation. Leibniz said mathematically wrong things about all these energy-related questions so I just won't count him as a "discoverer" of the concept. Any agreement with the reality - the right power of the velocity etc. - is a coincidence.
Nov
14
comment Why does smashing a TV remote load its batteries?
If the lithium ions cause an insignificant or tiny redistribution of the locations of lithium vs the electrolyte, then the capacity of the battery (energy or charge per unit mass) is also insignificant and tiny! You can't really separate these things. In lithium-ion batteries, there are no electrons flowing through the electrolyte - it is an electron insulator - and it's only the ions that make the current and ions are heavy because they include the nuclei. You can't make this motion negligible for efficient batteries - it's how they work!
Nov
14
comment The physical definition of work seems paradoxical
An even better idea, @Bobie - it is indeed -a in some other words. Lots of mess. ... Otherwise there's nothing inherently "Newtonian" about energy and work. The detailed formulae change but there are still quantities under these names that are related but not synonymous - both in relativity and quantum mechanics. The existence of these quantities, and their changes, is due to the time-translational symmetry of the laws of physics (Noether's theorem) which doesn't need to make any "classical physics like" assumptions.
Nov
13
comment The physical definition of work seems paradoxical
You're right, @Bobie! I thought it was international because -a is the right declension in plural in Latin. But the 1st case in Latin is synonymūs, I guess, not synonyma! ;-) So it's indeed just some Czech idiosyncrasy based on declension of "město" (town). You can still find "are not synonyma" via Google on 92 pages, including Wikipedia main pages. ;-)
Nov
8
comment Is the best data about Mercury's perihelion shift really 60 years old?
I believe that e.g. this 1972 paper journals.aps.org/prl/abstract/10.1103/PhysRevLett.28.1594 is more accurate, isn't it?
Nov
3
comment The gravitational constant $G$ theoretically?
Right, if you set a constant equal to one, then it is obvious that the work needed to "calculate it" is just to say "one". No work. But there are real things that may be calculated in physics, like positions of celestial bodies and their shape and millions of other real things. Universal constants are not supposed to be the product of calculations (especially not the dimensionful ones), they're input, and the dimensionful ones are input that only links the calculation of something real to social conventions.
Nov
3
comment Can action be unbounded from below?
Dear @Ruslan, it doesn't matter that the endpoints are fixed. If the potential is sufficiently positive/negative, it will dominate the action. Take $V=K/r^2$. Trajectories with fixed endpoints that visit the region $r=+\epsilon$ will clearly be dominated by the $V$ term which may be arbitrarily positive or arbitrarily negative for various $K$, and extreme for small enough $\epsilon$, and the kinetic energy term will be negligible in comparison.
Nov
2
comment Can action be unbounded from below?
Of course, there exist. What about mechanics with $S=\int dt\,L$ and $L=E_{kin}-E_{pot}$ with some general potential energy unbounded from either side?
Nov
2
comment The gravitational constant $G$ theoretically?
Dear Ben, it is no "boosterism". It is a very important result of the physics research (much like the roundness of the Earth and other things) that string theory is the only known - and probably only mathematically possible - framework to describe Nature in which all dimensionless parameters are in principle calculable. In other words, QFT and string theory are the only known frameworks incorporating relativity and quantum mechanics, and QFTs without adjustable parameters (like pure QCD) are clearly insufficient to account for the diversity of particles we know in Nature.
Oct
29
comment How does a plunger (toilet plunger) work?
I think so. But we haven't really heard what his complaint was so it's hard to say that he was wrong.
Oct
29
comment How does a plunger (toilet plunger) work?
If you press it down, or pull it up later, you make the pressure "above the dirt" greater, or smaller, than the pressure "below the dirt", which means that there is an overall force from above, or from below (later), which has a chance to move the dirt up or down. It doesn't really matter in which way you move the dirt, the goal is to make it move.
Oct
28
comment Generalization of canonical commutation relation
ACuriousmind: by diagonalizing the operator and applying the function to each eigenvalue, right? It can easily be non-smooth.
Oct
28
comment Poisson brackets of the Kepler Problem
Much like the commutators in quantum mechanics, the Poisson bracket of the angular momentum [or its z-component] with something, XY, tells you what XY gradually transforms into under rotations [under the z-axis, or another axis, depending on the component]. The things you wrote down are scalars, so they don't transform under rotations which is why the Poisson bracket is zero.