110,985 reputation
7163324
bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 3 months
seen 4 hours ago

Hi, I am a string theorist and a publicist.


4h
comment Quantum Entanglement - What's the big deal?
Entanglement is just the most general correlation described in terms of this more accurate and more compatible-with-experience theory, quantum mechanics. The precise way to calculate the correlations and probability distributions are given by the laws of QM. The laws of classical physics - any classical theory - would be wrong. But it's not "weird" for some theories to be wrong. Most theories people can invent are wrong. This whole ritual of saying that QM or entanglement is "strange" is just an obsessive religious exercise of people who refuse to accept modern physics.
4h
comment Quantum Entanglement - What's the big deal?
But my point is that none of these aspects of quantum mechanics is "weird" in the sense that it would contradict some experience we have really had in the previous centuries. The only thing that these features and predictions of quantum mechanics disagree with is classical physics - an approximate theory that was invented to describe people's observations up to 1925. But classical physics doesn't directly follow from our experience in any way, of course. Quantum mechanics is more, and not less, compatible with our everyday experience than classical physics.
4h
comment Quantum Entanglement - What's the big deal?
Dear @TerryBollinger, after 2+ years :-), let me answer your question. Be sure that I am well aware of Bell's theorem - I have taught this theorem and all these matters at Harvard, too. There is nothing shocking about Bell's theorem. It's just a rudimentary application of quantum mechanics to a very simple problem of 2 spins. What would be "weird" would be if the most elementary properties of Nature such as spins had "hidden variables" behind them. The world just doesn't work like that; it works according to the laws of quantum mechanics.
16h
comment Is there a spacelike curve connection two events in Minkowski space?
Dear Richard, you have only proved that it is a timelike curve if $v\lt c$ at every moment $t$. If $v\gt c$, the curve is spacelike. ... If two points in the Minkowski space are time-like-separated - if the straight line between them is timelike - then you won't find any spacelike trajectory between them.
1d
comment Is there a spacelike curve connection two events in Minkowski space?
Dear @Richard, what is exactly the "problem" you are referring to? What you described, with the $ct$ as the time component, is the most widespread description of a timelike curve. That's exactly what a timelike curve is - a trajectory $\vec x(t)$ which is moving at a speed $v\lt c$ at each moment. Spacelike curves indeed correspond to propagation by $v\gt c$ in your parameterization, which is prohibited for massive objects.
1d
revised Is there a spacelike curve connection two events in Minkowski space?
added 217 characters in body
1d
answered Is there a spacelike curve connection two events in Minkowski space?
1d
comment Did NASA mistakenly create a warp field?
Dear Kyle, I am not sure. This article was posted on Sputnik News, a rather major Russian information source that was previously running the news of RIA (try en.ria.ru), and it cites posts on a NASA forum. This is in fact a rather typical example of the flow of information through the media - similar sources are being used, and similar news outlets are key in spreading them. The result in no way guarantees that the "hot news" inside is right. There are both tons of false positives and false negatives, big events that don't get reported and bogus events that do get reported.
2d
comment Why is there zero point energy at absolute zero temperature?
Because energy is something else than entropy. The lowest (zero) entropy occurs if there is the minimum number of microstates that have approximately the same macroscopic appearance, namely one such state. So it's an isolated state of the lowest entropy. But such a state, by the laws of quantum mechanics, especially the uncertainty principle, has to balance the kinetic and potential energy, so both of them (and their sum) are nonzero. To have an exactly zero energy for a harmonic oscillator (and similar other systems) would mean to have $x=0,p=0$ exactly which is forbidden.
2d
answered Are electrons just incompletely evaporated black holes?
2d
comment Did NASA mistakenly create a warp field?
Zero. Warp drive is prohibited by relativity, energy conditions, and other laws of Nature, see e.g. quora.com/… Sorry, Kyle and his upvoter, but the laws of Nature are a vastly more accurate guide than what the media write or not write.
2d
comment The integral is zero! $\int \frac{\mathrm{d}^d k}{(2\pi)^d} = 0$
Dear Marcel, when doing physics and related forms of calculus, the sum isn't interpreted as a limit of partial sums but in a more general, natural, and physical way, and it is often literally written to be equal to the constant for a very good reason. Divergent sums and integrals appear everywhere in quantum field theory (physics) but that doesn't mean that one can assign them with any value he wants. They have to be regularized, renormalized, and the final result may depend on some parameters. In $\sum n$ the dependence goes away and the finite part of the sum is always equal to $-1/12$.
Apr
23
comment Does the path integral measure have dimension?
In principle, one can imagine the logarithm of dimensionful things as well - but they produce things like bizarre additive log(meter) terms. But in quantum mechanics, there is a reason why you are right: one computes the path integral over a spacetime with a thermal circle. And the Euler character of $A\times B$ is $\chi_A\times \chi_B$, and because $\chi$ of a (thermal or other) circle $S^1$ is zero, $\chi=0$. In classical statistical physics, one must be more careful about the normalization factors in front of $Z$ when going in the continuum limit.
Apr
23
answered Does the path integral measure have dimension?
Apr
18
comment Double slit experiment in the Heisenberg picture
Try to read > motls.blogspot.com/2015/04/… for my treatment of the double slit experiment in the Heisenberg picture.
Apr
18
comment 273 + degree Celsius = Kelvin. Why 273?
Particle physicists do, and they usually express energy - and therefore also temperature - in electronvolts and its power-of-ten multiples. Room temperature is about 0.025 electronvolts: wolframalpha.com/input/…
Apr
16
comment Making a bright beam of artificial white light
@fffred, fair enough. But you're asking people to solve a very technical engineering problem. Physics is really about the individual mechanisms that may make something work. One may discuss the useful components - small holes, strong sources, lenses, mirrors, reduction of size of the source of light etc. It's pretty clear that there is no miraculous way to make ordinary light laser-like without combining the things above. Why don't you buy a laser? They are cheap. You may combine several colors of laser beam, too.
Apr
16
comment Making a bright beam of artificial white light
Dear @fffred, you mentioned my name but I think that you have ignored 100% of my comment. I was talking about a parabolic mirror in the flashlight, not a pair of holes.
Apr
16
comment Light clocks measure conformal time - detailed argument
Sorry, this equation is completely wrong. If you have $r$ as a function of $\tau$, the correct formula is still $r(\tau)=r(\tau(t))$, i.e. $r=r$. There is no $dt/d\tau$ here. The latter factor only appears if you express the derivative $r'$ - $dr/dt$ and $dr/d\tau$ differ by this factor.
Apr
16
comment Ratio of electrons and protons in Universe
Quantum field theory shows that particles may be born out of nothing as long as a few general conservation laws - energy, momentum, electric charge, angular momentum - are conserved. So yes, they did arise from "nothing" corpuscular (at the end of inflation).