104,888 reputation
7145297
bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 9 months
seen 17 hours ago

Hi, I am a string theorist and a publicist.


Apr
10
comment If energy is only defined up to a constant, can we really claim that ground state energy has an absolute value?
Right, @Marek, $E_0-V_{\rm min}$ is independent of conventions. However, one may always imagine that $V(x)$ was different by $\hbar\omega/2$ than we thought and we will produce the same energy levels. Of course, then we must ask whether $E_0$ and $V_{\rm min}$ may be measured independently. It depends what tools we have to measure them. You have to assume that we can - $V_{\rm min}$ may be measured by localizing the electron, except that then it has a huge kinetic energy.
Apr
10
answered If energy is only defined up to a constant, can we really claim that ground state energy has an absolute value?
Apr
10
comment Are the basic postulates of QM the only set of postulates that can give rise to a sensible semi-probabilistic physical theory?
if you allow the collapse to be a "real event that changes something" and if you allow genuine non-locality associated with such a collapse, then you may design lots of non-QM theories that satisfy your conditions. All of them will be pathological at a theoretical level and inconsistent with observations. There are lots of attempts to "model" the quantum collapse by easily "visualizable", spiritually classical processes. All of these things are wrong. QM is the unique solution to a set of sensible conditions but you haven't described those conditions accurately.
Apr
10
revised Did the researchers at Fermilab find a fifth force?
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Apr
10
comment Mathematical justification of Hartle-Hawking “no boundary” proposal
Otherwise, approximating the HH state by a minisuperspace approximation etc. is always a problem and can lead to wrongness by itself. It's surely true. However, it may also be a good approximation for many problems. At any rate, this problem has nothing to do with the Euclideanization: the approximations done in the Euclidean signature are likely to be more accurate, much like in other cases where the Wick rotation is used. It is being used for a good reason - it makes the maths more convergent etc. E.g. path integrals only "rigorously" exist as a measure in the Euclidean signature.
Apr
10
comment Mathematical justification of Hartle-Hawking “no boundary” proposal
What I want to say is that HH were formulating a hypothesis about a state, and one can use any mathematical machinery to formulate a hypothesis. They used what they considered the most natural one, one which solved the WDW equation etc., and I tend to agree. Testing against evidence is what decides about the validity of a scientific hypothesis. I don't understand your problem. In the second part of the question, you assume that the expression in the Minkowski space are always "more true" than those in the Euclidean space. You have no evidence for that and in fact, I believe you're wrong.
Apr
10
comment Mathematical justification of Hartle-Hawking “no boundary” proposal
I find your question tendentious and the wording misleading. The HH wave function is just a mathematical expression to determine a preferred state of the Universe on a three-sphere. It uses maths of a Euclidean spacetime because it is natural and the Ansätze are allowed to use any maths: you may still view the Euclideaniation as a math trick. Moreover, contrary to your implicit assertions, quantum field theory in a Euclidean spacetime is more mathematically well-behaved than quantum field theory (or QG) in the Lorentzian space, so the Euclideanization makes things better, not worse.
Apr
10
revised Mathematical justification of Hartle-Hawking “no boundary” proposal
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Apr
10
comment Does throwing a watch into the air cause it to gain or lose time?
Your "identical in theory" comment is absolutely isomorphic to someone's comment that the Evening Star and the Morning Star are the same object "only in theory". One needs a "theory" to argue that there is an object called Venus whose position is changing during the day. The person doesn't have to believe that "theories can be trusted". However, as you can see, the person is completely stupid: theories are just necessary tools to learn about the reality. Your situation is unfortunately isomorphic although one talks about "slightly more complicated" questions.
Apr
10
comment Does throwing a watch into the air cause it to gain or lose time?
You also write: "In addition, acceleration and gravitational fields are only theoretically identical. One can imagine gravitational theories where they are different. Consequently the physical situation is not identical." - Nope. You are completely wrong. First of all, gravitation appeared in both question: the two questions are identical. Moreover, the effect of gravitation and acceleration is identical in practice, not just theory. The fact that you don't believe general relativity has nothing to do with it: GR has been proved by science while your-like alternatives have been refuted.
Apr
10
comment Does throwing a watch into the air cause it to gain or lose time?
Carl, it's not just answers that are equal - after all, there's not any special answer in this question yet so we can't compare. The only difference I see between the questions is that Feynman/Einstein were "blasting" a rocket with clock off while you are "throwing" clock up. Does this change really justify another entry? Is that what you really believe?
Apr
10
comment Does throwing a watch into the air cause it to gain or lose time?
I agree with @Marek, too. Sorry, @Carl. Moreover, I don't think that an Internet user who doesn't think about these matters physically will find it via Google, anyway. There are many ways (wordings) to formulate the same problem. A female extraterrestrial alien is in love but she's older than her sweetheart, and is offered to be repeatedly shot by a cannon by the Jovian Hegaxon Department of Defense. Will she accept? I write it to Google and it doesn't give me your question (later, it will give, when my comment is added to the index haha).
Apr
9
revised Are these radioactive particle matter and air emmissions dangerous, 2000KM from Fukushima?
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Apr
9
revised How is formula for converting pressure from mmHg to Pa derived?
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Apr
9
revised How is formula for converting pressure from mmHg to Pa derived?
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Apr
9
answered How is formula for converting pressure from mmHg to Pa derived?
Apr
9
comment projectile power with air resistance
I think that your equations are not correct. Shouldn't $V_x,V_y$ be derivatives of $X,Y$? They surely aren't in your case. They're related by transformations that resemble the deletion of random characters. ;-) Moreover, $\theta$ seems to be dimensionless from some equations and frequency from other formulae.
Apr
9
comment Perturbation of a Schwarzschild Black Hole
Moreover, the main reason why my and our analytic calculation became more famous than it deserved was that it proved the appearance of $ln(3)$ in the frequencies, in agreement with a heuristic guess based on numerical calculations, and this constant has been used to defend "discrete models of black holes", including loop quantum gravity, before it became very clear that the result doesn't generalize to other black holes and the "discrete interpretation" is misguided for many other reasons, too. See also motls.blogspot.com/2005/03/…
Apr
9
comment Perturbation of a Schwarzschild Black Hole
Thanks a lot, @Daniel, but it is very undeserved. I and we with Andy just calculated highly damped quasinormal modes (with high $Im(\omega)$ of the frequency) of the Schwarzschild in 3+1 and then any dimension and the Reissner-Nordstrom one, with some incomplete results on others. The long-time behavior is dominated by the low-lying quasinormal modes (those that are not highly damped) with the lowest Im(omega), those that are hard to calculate. In the Schwarzschild time, $\delta g_{\mu\nu}$ then goes like $\exp(-Im(\omega)_{\rm min} t)$.
Apr
9
comment Perturbation of a Schwarzschild Black Hole
Thanks, @Deepak, your generosity is appreciated but your compliment is undeserved. ;-) I didn't discover the concept of quasinormal modes or the fact that the ringing modes describing the exponential approach to the static black hole solution at later times. I (and, later, we with Andy Neitzke) just analytically calculated the quasinormal frequencies (of the highly damped modes) for some common black holes. However, the long-term behavior of the deviation from the sphericity etc. is governed by the low-lying quasinormal modes with a small Im(omega) - I haven't found anything new on them.