Luboš Motl

Czech Republic

motls.blogspot.com

Age: 41

Hi, I am a string theorist and a publicist.

14h
comment If the moon was rapid enough would it be able to orbit the earth from a close distance?
A good point if it's right.
Jan
23
comment If dark matter exists, could there be dark life?
In some viable models, the dark matter is comparably complex to ordinary matter of the Standard Model. In many others, it's much more elementary - less composite. We can't really be sure which answer is right at this point, not even estimate probabilities. It's about prejudices.
Jan
23
answered How do I solve this Gaussian path integral?
Jan
19
comment Why are Majorana fields usually used to introduce gravity in the Rarita-Schwinger Lagrangian?
By the "real one-half of the Dirac spinor", I mean one-half of the degrees of freedom that are present in the Dirac spinor, a spinor with 4 complex components transforming as a spin-1/2 representation of the Lorentz group, and the one-half is obtained by demanding a reality condition so that the 4 complex components become 4 real components. If you understand that the word "Majorana field" in that explanation may mean a field with any $j\in Z+1/2$ and a real condition, then it's just fine, but then I don't understand too well why you don't understand the original text.
Jan
19
answered How does QFT interpret the Negative probability problem of the real scalar fields' Klein-Gordon equation?
Jan
19
answered Why are Majorana fields usually used to introduce gravity in the Rarita-Schwinger Lagrangian?
Jan
19
reviewed Reject suggested edit on Why does maximal entropy imply equilibrium?
Jan
18
comment Atomic Physics: stimulated emission
Because the probability of emission is proportional to $N+1$ while the probability of absorption is $N$. Moreover, absorption needs the atoms to be in the unexcited state, for the energy $E_0+hf$ to be in the spectrum. So absorption appears when the atoms are unexcited, and emission when they're excited. Just to be sure, when the atoms are excited, $N+1$ goes to emission and $0$ goes to absorption - the difference is in no way small.
Jan
18
comment Boundary conditions / uniqueness of the propagators / Green's functions
Prahar, I would have to see exactly what calculations you want to confirm or do. In other words: "assumption for what?" The normal quadratic calculation assumes that the field has a vanishing vev everywhere in the spacetime.
Jan
17
comment Boundary conditions / uniqueness of the propagators / Green's functions
Apologies, I probably can't. This question may mean something very special and simple or something general and ill-defined. If a field simply has some vev, one defines another field by subtraction, and that field has vev=0 at infinity, so it's reduced to a normal calculation. If you want something more general, one must ask why the field isn't constant at infinity, and this may have different causes requiring different care etc.
Jan
16
awarded Nice Answer
Jan
15
answered What is the motivation behind the GSO projection in superstring theory?
Jan
14
comment Eigenvectors of a symmetric positive definite Toeplitz matrix
One reason why I am mixing them up is that I am actually inclined to think that the OP primarily cares about the practical, not theoretical, questions.
Jan
13
awarded Yearling
Jan
13
awarded Yearling
Jan
8
comment Intensity in rainfall and global warming
This is a physics questions-and-answer servers where people provide answers based on the accurate enough analysis of the science, not a computer modeling server where people are supposed to uncritically believe features of one computer program or another. By "textbook material", I meant objectively important basic information about a subject that someone should know. When the education system works, this physics-based classification is a sociological claim, too - people learn what's important. When the education system breaks down, the actual textbooks may deviate from the textbook material.
Jan
6
comment What's the symbol behind Einstein's head?
By the way, on the blackboard, the symbol for the speed is really $q$, not $v$, see comments about Geschwindigkeit $q'$ here: einsteinpapers.press.princeton.edu/vol4-doc/71
Jan
4
comment Classical black holes?
The size of the Universe was not constant in the past but the size of the visible Universe actually is converging to a finite constant, very close to the current size, because in the asymptotically far future, and it's almost the case now as well, the Universe will be (close to) a de Sitter space with a fixed spacetime curvature radius (given by dark energy, i.e. the cosmological constant), and the size of the visible universe up to the event horizon is a fixed multiple of the curvature radius. All these radii are close to "dozens of billions of light years".
Jan
4
comment Classical black holes?
Dear Timaeus, every quantum mechanical theory will imply that the long-distance effective theory contains not just the Einstein-Hilbert term $R$ but also the higher-derivative terms of the form I mentioned. One may see the explicit form of these terms in string theory which is really the only consistent theory of quantum gravity we know but the arguments that these terms arise may be presented independently of string theory, just with the knowledge of the renormalization group etc. So what you want to learn is either "renormalization group" or "string theory" or both.
Jan
3
comment Tesla's theory of gravity
Dear @TomAndersen, the difference is that alchemy was the state-of-the-art chemistry of Newton's times, and astrology was a broader science about the implications of celestial bodies – a science that was later made obsolete mostly by insights that followed from Newton's works. On the other hand, Tesla's musings about gravity and many other things were not state-of-the-art descriptions of anything. They were wrong even relatively to the ideas of his time.
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