Quantum Theory

how is it that photons, to have mass, are bound to constant movement, and if they stop completely, they have no mass. isn't mass a property of everything that exists, whether it is in the form of energy or matter. if something has no mass then it doesn't exist. someone help me out here please lol

Rather a nice article by Paul Davies in this week's New Scientist http://www.newscientist.com/ch.....ysics.html Possibly the most extreme version of an anthropic theory I've encountered, but a lot of fun. It revolves around a quantum effect such that a particle may have a "fuzzy" future, and by extension a "fuzzy" past. It extends that to the entire universe and then argues that because we are here to observe it, we are fixing the laws of the universe acquired at the beginning such that a universe suitable for life, and us, is the result. Doubtless there are those of you out there far more able than I to comment intelligently on this. Weird but in…

When a single particle is fired at the double-slit wall, does it always go through both slits at the same time, or does it sometimes just hit the wall? I'm wondering because I'm trying to wrap my head around what counts as an interaction. If the particle goes through both slits all the time then it doesn't seem to be interacting with the wall, but if it goes through both slits only sometimes, then it must be interacting with the wall, in which case the wave function collapses into either "hit the wall" or "going through both slits".

Sorry in advance for the length of this question. Can someone show me the solution to the integral [math] \int\limits_{ - \infty }^\infty {\prod\limits_r {d\xi _r } } \exp ( - 1/2\sum\limits_{r,s} {K_{rs} } \xi _r \xi _s + V(\xi )) [/math] Where [math] K_{rs} [/math] is positive definite, non-singular, and symmetric and [math] V(\xi ) [/math] is analytic? The reason I ask is because in quantum field theory you encounter integrals like these in the Feynman path integral formalism. I want to show through a path integral integration for the photon propagator you get [math] - i\Delta '_{\mu \tau } (x,y) = \left\langle {T\{ A_\mu (x)A_\tau (y)\} } \right\ra…

Everybody says that the experimental violation of Bell's inequalities validates quantum mechanics and denies local realism. But Bell's theorem is based on the standard formulas for correlations and expected values, which are very intuitive but still arbitrary. Has anybody ever mentioned that local realism and violation of Bell's inequalities can be compatible if we change some rules for calculating proabilities and correlations?

Warm greetings I have recently been introduced to QED by reading a book on the subject, and was wondering if you would be kind enough to help me clarify my understanding thereof….. When ‘light’ is shone upon a potentially reflective surface, in simple terms, the majority of photons reflect at an angle equal to the angle with which they originally shone upon the surface. My query concerns the photons that reflect with an angle other than that with which they approached. When the parts of the surface that prevent those photons from been detected are ‘removed’ and the photons can be detected, is it because those removed angles of reflection were interferin…

I just starting to get into the stage of university physics that deals with quantum mechanics and i was curious about what types of mathematics are most prevalent in an introdutory course. I supose i know the math already due to the fact that i have the prerequisites. It would help me a great deal though if somepersons could tell me if there is any other math that would also be helpful, what math is mostly used so i don't get caught off guard, or a website where i can start early on quantum mechanics itself. thank you A Fool

http://www.slate.com/id/2167563/nav/tap3/ Apparently there's a "rumor" going around that the Tevatron found the Higgs Boson

Hi, I have a question on neutron interferometry. If I have a setup like in this figure , I would expect to measure the same signal at both detectors. However, this actually does not seem to be the case, as claimed for example here. My analysis of the situation would be: for the upper detector: interference of a two beams, one is reflected only at the mirror, which adds an addition pi to the phase accumulated during propagation. The other beam is reflected three times and picks up the phase change phi induced by the sample, so the phase difference between the two beams should be 2*pi + phi = phi for the lower detector: upper beam is reflected twice and pi…

I was just wondering, what sort of speeds do electrons move around at while they are in probability clouds?

I tried not to make the title of this thread too long so it might not be clear what I'm asking. Let me elaborate. I've always wondered how the pivotal idea that started the field of quantum physics lead to all the strange discoveries that showed up as the field matured (like superposition, random collapsing, quantum entanglement, etc.). The pivotal idea I'm talking about is the idea that energy can be quantized into indivisable units akin to material particles (like the photon is to the electron). That makes sense out of the "quantum" part of quantum physics, but I don't see anything "quantum" about the odd anomolies that were discovered later (like superposition, random …

Is quantum tunnelling the phenomenon I've heard of that requires particles to "borrow" energy from the universe? I mean, if a particle is to penetrate a barrier, one would think it needs a large amount of energy to do so. So is this where the concept of "borrowing" energy comes from?

The standard theory of wavefunction collapse describes particles existing in states of superposition, and then when they are measured they collapse into a classical state. But I think this is a little misleading. I could be wrong, so I pose this as a question. Isn't measurement never perfectly precise? Which means that whatever you're measuring (position, momentum, energy, time, etc.) is going to have some margin of error. The uncertainty principle tells us that this is not a shortcoming of our measuring instruments but with the nature of the superpositions states themselves. So if you want to measure position, you're necessarily going to have to accept the uncertainty in…

Hi everybody. I know that there were 3 versions of QM developed in the 1920s, Schrodinger's, Heisenberg's and Dirac's? I am under the impression that Dirac's is supposed to be the most mathematically elegant and shows the equivalance of the other 2, is this correct? Also what do you think are the main diferences between them and which do you personally prefer? Also as a first year student I have only been introduced to Schrodinger's version in my lecture course. Is it generally perceived to be the easiest version to understand or use?

As I understand it quantum cryptography presently depends on polarization. More specifically: However, recently, a method for snooping on the key without disturbing the polarization has been devised: http://www.emergentchaos.com/archives/2007/04/quantum_cryptography_crac.html My question is could something like a Bell test experiment be used to generate a shared random pad. Specifically, what if you had a central generator of entangled photons equidistant from two receiving stations, such that when either side measured a property, the property would be the same on both sides but random. Wouldn't this be a more secure approach? In effect, wouldn't t…

i.e., the hypothetical "particle responsible for gravity." Beyond the fact that everything else seems to be particles/waves, is there any reason to expect that such a thing exists? Given the ridiculously small effect/probability of such a thing, do we ever expect to actually detect such a thing? Wouldn't its effect fall within the range of quantum uncertainty? If we can't ever detect one, is there any point in talking about them? Further, if it is impossible to detect one, shouldn't we be saying that it doesn't exist? Talk down to me. I'm way out of my league with this stuff.

can someone explain the smaller particles, such as the quark, that make up the neutrons, electrons, and protons. (explain how they work)

Why are Quantum & Relativity theories incompatible? I don't see why they can't both be true, and I would like to have a discussion. Can anyone provide a (relatively) simple example where the two are at odds? Thanks.

Does anybody know of any good websites that give a quick run down of quantum mechanics in an easy-to-understand manner. Something like the following: http://particleadventure.org/ except geared solely towards quantum mechanics.

How does the behaviour of semiconductors vary with different densities of electrons or electron holes?

In what materials/states of matter do energy bands form? What determines whether an energy band will exist? For example, do energy bands ever form in gases? Cheers!

In terms of semiconductors, and electrons moving from the valence to conduction bands, how do the remaining electron holes contribute to electrical conductivity? Also, can anyone provide a short qualitative description of the properties of an electron hole as I am not totally familiar with the concept?

What is the difference between an energy band and an energy level (how, if at all, are they related)? (In summary) my current understanding is that an energy level is "any of the possible discrete energies of an atom, molecule, or nucleus." and that energy bands have something to do with semiconductors. Hopefully very soon my understand will be much greater.

Greetings to all May I please enquire about how the wave-function predicts the position of electrons? What I am most concerned about is how it deals with the defined states that electrons occupy. I am sure you are aware of the simplified explanations describing an ‘electron cloud’ around the nucleus, so how does this take into account the distinct states they occupy? Many thanks

http://www.nature.com/news/2007/070416/full/070416-9.html Would this include Bohmian mechanics?