115,949 reputation
7178345
bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 7 months
seen 23 mins ago

Hi, I am a string theorist and a publicist.


21m
comment is there an operator which measures the mass of particles?
The rest mass is fixed because it's a law of physics. For all states that are allowed in Nature, the rest mass of an electron is always 511 keV. So the mass $m$ is an operator on the Hilbert space that is proportional to the unit operator/matrix. We say that it is a $c$-number, a classical number. The equally big value is the only eigenvalue of the operator. Every state of the Hilbert space is an eigenstate of this operator with the same eigenvalue. Almost just like $\pi$, is a fixed parameter of the laws of physics, not a dynamical variable.
4h
comment Do apparent event horizons have Hawking radiation?
Yes, it's directional. The "apparent source" at infinity has no preferred direction but we're observing it from a direction. Yes, it's from behind. Because of the equivalence principle, one may see that the direction is the same in the black hole case or Unruh case. The direction where the "source" sits is the side on which the stomach presses the interior of your belly. That's true for a black hole above which you try to keep your distance, so it must be true for any acceleration. The source is a distant point from which you accelerate away.
14h
comment Suntan: UV absorption vs daytime
Good for reminding me about this question, Rick, I missed anderstood's comment before - even though I was interested about this question every summer including this one. ;-) The drop in the morning and afternoon seems almost linear - I sort of expected it to be more brutal, exponential, like exp(-t) in the evening, because of the exponentially decreasing transmission through the absorbing layer, but it apparently ain't so.
1d
comment How does work function transform in Einstein's special theory of relativity?
Plain and old special relativity holds anywhere and everywhere and always (at least if GR is not needed - if the gravity is weak or otherwise easily incorporated). The laws have the simple form in all reference frames. Just the laws describing the properties of materials, including ionization, are simpler in the rest frame of the material. But in each frame, you may simply say that the law is "whatever you get by Lorentz-transforming the laws in the rest frame into our frame".
Aug
25
comment How does work function transform in Einstein's special theory of relativity?
In the movie frame, the metal is moving, and the minimum energy of the photon to knock off an electron is then obviously different, so that the condition is equivalent in all frames. The minimum energy to knock off an electron is normally specified in the metal's rest frame.
Aug
21
comment A classical universe from a quantum one?
Our Universe never becomes exactly classical. Clas. ph. holds approximately only. The history we believe going back to the Big Bang is a retroactive interpretation of the results of measurements we have (or a given observer has) made. It allows us to be "almost" sure about the value of many quantities (quantitative or qualitative descriptions of the events in the distant past) because those values or assumed facts are very likely to evolve to the future outcomes we have actually observed. But if we can't determine these answers about the past, then they are as undetermined as everything in QM.
Aug
21
comment Can pure-bosonic string theories exist in curved spacetime?
Interesting question. For some examples of non-SUSY AdS/CFT examples that rely on the validity of the BF bound, try e.g. arxiv.org/abs/hep-th/0105047 - a problem is that the paper is in "SUGRA" only. The critical dimensions aren't imposed in general and the CFTs are only described rather vaguely by some operators and properties.
Aug
21
comment Could we be on the inside of a concave hollow universe?
Thanks, user, there is obviously a pragmatic perspective from which it's bunk and I would support such a perspective in most practical contexts - when someone would try to use the hollow Earth perspective to advocate... pretty much anything. ;-) But if one really asks only whether it's compatible with the empirical evidence in physics etc., well, it is.
Aug
21
comment Diameter of manifold with negative curvature
Apologies, it seems too mathematically hard for me. Under the normal metric, the unitary group SU manifold is positively curved, isn't it?
Aug
21
comment Why does a moving fan seem transparent?
Dear @DigitalTrauma - I am sorry for that but it's not my fault. Someone simply vetoed your edit probably because it's problematic to change the content of other users' answers if there seems to be nothing "obviously" in need of a change. I would have okayed your edit, of course! ;-) But if you do another edit, I can't guarantee that no one else will look at it before me so my opinion is irrelevant. The editing is a collective activity of the users with a certain reputation or more who get there at the right time.
Aug
21
comment Diameter of manifold with negative curvature
I think it would be more sensible on the math forums. Typically, negative curvature does imply "noncompact" shape i.e. infinite diameter - the hyperbolic space is noncompact, after all. However, you may have things like orbifolds of the hyperbolic planes/spaces which probably may have a finite diameter, unless I misunderstand something.
Aug
21
comment Global symmetries in type IIB string theory vs type IIB supergravity
Good question. I think that the limit is a bit harder to see on the gauge theory side. Try to see e.g. arxiv.org/abs/hep-th/9811047
Aug
21
comment Why does a moving fan seem transparent?
Dear Mazura, I used the word "leafs" - which sounded strange (especially because the green ones are "leaves", I thought) - because the OP did so before me. The question was later edited while "leaf" stayed in my answer. But if you care, coincidentally, the Czech language does use the word leaf ("list") for the blades on a fan or a turbine. A synonym of this leaf/list is "lopatka" which is a little shovel.
Aug
20
comment Euclidean geometry in non-inertial frame
Alejandro, it's clearly not quite the same because the planet on the orbit may rather easily shrink in the direction of motion and no one cares whether it occupies a smaller fraction of the circumference than it did at rest. The problem with the disk is that the proper length of the circumference seems to be "prescribed" by the character of the object, and when this should get Lorentz-contracted at the same moment, a paradox follows. The resolution of the paradox is that no perfectly rigid objects are allowed by relativity.
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
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
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
comment Can I move an oil tanker?
Yes, Voitcus, one may stop it by pushing from the other side. ;-)