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Jan
4
awarded  Announcer
Dec
2
awarded  Nice Question
Nov
27
awarded  Tumbleweed
Nov
20
asked Distinction between time-local, time-homogeneous, and Markov
Oct
18
comment Are vacuum fluctuations really happening all the time?
So yes, in this sense vacuum fluctuations are constantly happening. Such fluctuation are contingent on amplification processes, but are not contingent on humans.
Oct
18
comment Are vacuum fluctuations really happening all the time?
I'd like to clarify the statement you've quoted. As I said in the blog post, "real-life processes amplify microscopic phenomena to macroscopic scales all the time, thereby effectively performing a quantum measurement". Measurements-by-humans are not special or privileged. They are just another example of the sort of amplification processes that occur naturally, i.e., when a certain degree of freedom is copied onto other degrees of freedom. Vacuum fluctuation are contingent on such amplification processes, but they are not contingent on humans.
Sep
21
asked Canonical treatment of thermalization of two gases at different temperatures
Sep
14
comment Why are rockets launched vertically?
Sorry if I used "elliptic trajectory" where "elliptic instantaneous orbit" would have been more precise, but I thought the meaning was clear. I am well aware that the rocket does not follow a fully elliptic trajectory. Rather, the highly elliptical orbit is clearly the instantaneous orbit the rocket has a moment after it leaves the launch pad, i.e., the orbit it would follow if it cut it's engines and the Earth were compressed. And this is clearly the relevant initial orbit for orbital transfer. If you want to discuss this further, we'll need to move to chat.
Sep
14
comment Why are rockets launched vertically?
The highly elliptic trajectory is the starting orbit of a rocket on the launch pad, and the problem of getting from this orbit to a LEO circular orbit is exactly the form of the idealized problem where the Earth and atmosphere have been removed.
Sep
14
comment Why are rockets launched vertically?
Imagine the Earth is compressed to a point and that the rocket is given an infinitesimal amount of horizontal motion. Now the rocket is on a highly elliptic trajectory with apogee at the Earth's former radius and perigee near the center. What is the most efficient way to quickly raise the rocket's orbit to a circular orbit at LEO altitude? I believe it's by burning horizontally at first. Even if I'm wrong, it's definitely not obvious that vertical is the way to go based on the argument you gave ("connect these two potential levels ...by the shortest possible path...clearly vertical").
Sep
14
comment Why are rockets launched vertically?
But the goal is not just to get to that height, it's to get to that height going a certain horizontal speed. In particular, if the Earth and the atmosphere weren't in the way, it would be better to launch at an angle, which is exactly what is done when a craft wants to get from a low-Earth orbit to a higher one. (Hohmann transfers and that sort of thing.)
Sep
14
comment Why are rockets launched vertically?
This really doesn't answer the question well since rockets just going to low-earth orbit (a mere 250 km) aren't trying to escape the Earth's gravitational field, but they also launch vertically. (Criesto doesn't mention which sort of launch he's talking about.) Any satisfying answer is going to have to discuss the effects of drag from the atmosphere.
Sep
4
asked In a perturbative FRW cosmology, why do constant-density hypersurfaces define a good gauge?
Jun
19
comment What is discrete phase space?
The continuous Wigner function is just the Fourier transform of the position-space density matrix after changing coordinates to ($\bar{x}$,$\Delta x$). [Equivalently, the momentum-space density matrix in coordinates ($\bar{p}$,$\Delta p$).] Therefore, I think there should be a semi-discrete Wigner function when the configuration variable $x$ is continuous but bounded (so $p$ is discrete and unbounded) and fully discrete Wigner function when $x$ is discrete and bounded (and $p$ is also discrete and bounded).
Jun
19
comment What is discrete phase space?
Thanks very much for going to the trouble to write this down. I'm still trying to understand it. I wonder if maybe you've been too quick to dismiss the possibility that (some) discrete Wigner functions correspond to a dynamical system with discrete quantum state?
Jun
4
comment Question about canonical transformation
Are there good reasons to understand and use transformations that are locally canonical but not canonical?
May
12
comment How to calculate material depth at which penetrating radiation direction becomes randomized?
Theory of course. Using numerical codes for this would be like using an N body simulation for a single orbit.
May
12
revised How to calculate material depth at which penetrating radiation direction becomes randomized?
added 207 characters in body
May
12
asked How to calculate material depth at which penetrating radiation direction becomes randomized?
May
5
awarded  Enlightened