Hot answers tagged theory
26
It's very hard to imagine that there is any sensible model consistent with OPERA's results. (Aside from models of unaccounted-for systematic uncertainties in the experiment.) We know that we live in a world described to very high precision by Lorentz-invariant quantum field theory, so the most sensible way to look for Lorentz violation is to start with such ...
23
I am afraid that one has to go to a "very unusual segment" of theoretical literature if he wants any papers about superluminal neutrinos. Guang-jiong Ni has been authoring many papers about superluminal neutrinos a decade ago:
http://arxiv.org/abs/hep-ph/0103051
http://arxiv.org/abs/hep-th/0201077
http://arxiv.org/abs/hep-ph/0203060
...
14
In the following arXiv article Landau's Theoretical minimum, the author details personal experiences on being one of 42 students ever who passed it, as well as some details of the examination. He also mentions some questions in passing, from which I gather most of them were similar to the exercises and examples of his famous books.
Quote:
Landau gave ...
13
A field theory is a physical description of reality in which the fundamental entities are fields, i.e. objects having no definite spatial location but a certain value or intensity at each place.
Examples of fields are the temperature in a room, for each location in the room, a temperature can be specified, although in most cases temperature will be pretty ...
13
Off the top of my head, the Cosmic Microwave Background radiation was hypothesized as a consequence of Big Bang Theory before it was observed by accident by Penzias and Wilson. Also the light element abundances, also a consequence of BBT, was theoretical and is still being refined today through observations that supported the initial theory.
I don't know ...
12
Neutron stars were predicted in 1934 by Baade and Zwicky, one year after the discovery of the neutron. They were not observationally confirmed until 1965 by Hewish and Okoye. It's hard to beat a prediction that sat around for 30 years before being confirmed.
11
This web page provides a good explanation: http://www.thebigview.com/spacetime/spacetime.html
To oversimplify the explanation, you have to understand the curvature of space time around a black hole. The basic principle is that because of the curvature of spacetime around a black hole, the amount of "distance" a beam of light has to cover is greater near a ...
10
I don't know if these rise to the level of "useful," but:
Yang-Mills theory with gauge group $G_2$ is interesting because $G_2$ has trivial center. So people simulate it on a lattice, try to understand in what sense it might be confining, how string tensions scale, if it has a deconfinement phase transition, and so on. The idea is that looking at a group ...
10
Update to address new questions.
The answer to this question is no. At least if you take the question purely formally. Only theories such as classical field theory, quantum field theory and continuum mechanics are field theories (you generally recognize them by having continuous degrees of freedom; also they usually have the word field in the title :-)). ...
9
Yes.
G2 shows up often, starting with atomic physics (perhaps Racah is the first; see R. E. Behrends, J. Dreitlein, C. Fronsdal, and B. W. Lee, “Simple groups and strong interaction symmetries,” Rev. Mod. Phys. 34, 1 (1962).). You will find some refences in my 1976 Phys rev paper on cns.physics.gatech.edu/GroupTheory/refs . I have whole folder of physics ...
9
Information is a purely mathematical concept, usually a a characteristic of uncertainty (of a probability distribution function), but can be interpreted in different ways. In the simplest form it is introduced in information theory as a difference between uncertainties of two distributions, with uncertainty being the logarithm of a number of possible ...
9
I'm a bit worried about getting a reputation for citing myself too much, but I'll go for it anyway. (In my defense, I always admit it when I'm doing it!)
John Baez's and my pedagogical paper The Meaning of Einstein's Equation aims to address exactly this question. We describe the meaning of spacetime curvature and the way that Einstein's equation connects ...
9
The list of topics can be found here (in Russian, of course). Nowadays students are examined by collaborators of Landau Institute for Theoretical Physics. Each exam, as it was before, consists of problems solving. For every exam there is one or several examiners with whom you are supposed to contact with to inform that you're willing to pass this particular ...
9
Why are the physical sciences described perfectly by mathematics?
They are not.
Here's a simple example: QED is arguably one of the most precisely predictive theories in physics, yet its underlying mathematics are unavoidably based on approximation methods, so you can never really calculate an ultimate answer using it. Feynman himself did not believe ...
8
Tesla was an engineering giant but it is true that in most cases, he was just a crackpot when it came to theoretical physics. This "theory of gravity" is one of the major ones.
It wasn't really connected with gravity - the attraction of objects to the Earth etc. with a universal acceleration - by anything else than a wishful thinking. As expected for the ...
8
Dear Chad, you misinterpret the statement that "the known sources of CP-violation are not enough to explain the matter-antimatter asymmetry in the Universe."
You seem to think that the statement means that the known CP-violating parameter (namely the CP-violating phase in the CKM matrix) and the processes based on it are qualitatively insufficient to ...
8
Instantaneous velocity can never be measured since there is no way in the real world to do anything instantaneously. All measurements take some amount of time to peform.
For example the comment to the question mentioned using the Doppler effect to measure instantaneous velocity. That is not possible since to measure the frequency of a wave you have to ...
8
There are thousands of such examples, it is basically all situations in condensed matter physics. You see a lot of regularities that have no explanation.
Here's one of the most annoying ones for me: Moseley's law--- you can knock out one of the two electrons most tightly bound to a heavy atom (in the K-shell). This leaves a hole orbiting the nucleus. The ...
8
Go out and discover those "other explanations" (and accumulate sufficient supporting evidence, of course) and you can laugh at the dark matter specialists.
Until then dark matter is the simplest hypothesis on offer that explains multiple observations in one go (galactic rotation curves, cluster dynamics, super cluster dynamics, the bullet cluster, the ...
8
The missing mass problems are several sets of observations that could be explained if there were some matter that has mass (interacts with other matter via gravity) but does not interact with light. The same distribution of this missing mass would explain all of them. All competitors that have been explored fail to explain at least one.
I only partially ...
8
No, physics is not rigorous in the sense of mathematics. There are standards of rigor for experiments, but that is a different kind of thing entirely. That is not to say that physicists just wave their hands in their arguments [only sometimes ;) ], but rather that it does not come even close to a formal axiomatized foundation like in mathematics.
Here's an ...
7
Insect flight is different than bird flight. With insects, the rapidly moving wings, which do a figure 8 sort of motion, generates a vortex tube over the wings. This vortex by Bernoulli principle has less pressure, which permits the larger air pressure underneath to lift the animal up. If one is trying to understand insect flight according to the mechanics ...
7
The problem is that Markov chains are inherently lossy--- so in physics as it is commonly understood today, the answer is no. A Markov chain will always lose information about the initial state, as it relaxes to a stable distribution, while a quantum mechanical system does not do this. The modern understanding of a physical system is as a quantum markov ...
7
The proposed partition of physics into Thermodynamics, Classical Mechanics, and Quantum Mechanics is quite arbitrary. To take just one conspicuous example, statistical mechanics does not fit, as it is the discipline that mediates between these three areas of physics.
The Physics and Astronomy Classification Scheme (PACS)
...
7
There are several points of evidence that the Oort Cloud exists, though it is indeed still a hypothesis and lacks direct observation.
The first is indirectly observational, as proposed by Ernst Öpik back in 1932 as the source of long-period comets. This was revised by Jan Oort in 1950. All you need to determine an orbit is three observations of the ...
7
In addition to the others, there are other famous theoretical predictions that were then seen in the sky:
Neptune!
Asteroids. (from the failure of Bode's law)
Lagrange point objects
Inspiralling neutron star binaries
Supernova neutrinos(Colgate and White 1966).
GZK cutoff
The recent cosmology revolution was a combination of theory and experiment. The ...
7
The binding energy curve for nucleons in nuclei shows which atoms can take part in fusion, releasing energy in the process.
Fusion happens as one goes from left to right, until reaching Fe, iron. From there to the right it is fission that will release extra energy
This is an example of a fusion reaction, the one that is actually being materialized in ...
7
Could this imply there is a formulation where that value comes naturally...
This sentence implicitly assumes that analytic continuation is "unnatural". But the truth is the other way around: analytic continuation is one of the most natural mathematical procedures in physics. On the contrary, it's functions – especially functions of momenta or energy – ...
6
Since there are already outstanding technical answers to this question, I think we should add some better philosophical underpinnings for you to explore that might help with gaining a better intuitive feel for what information is.
Warren Weaver provided an excellent discussion on information theory in 1949 in his paper entitled "Recent Contributions to The ...
Only top voted, non community-wiki answers of a minimum length are eligible
