Hello friends,i have heard that electrons are called fermions. Because they have half integral spin.What do they mean by this? Does it mean that they actually spin? Then what is the exact meaning of half integral spin. And also i have a doubt. Electrons revolve around nucleus. Is it because they revolve they rotate?? That means electron have angular momentum?
Something with spin ½ needs two full rotations (2 x 360°=720°) to get it back to the same state. Have a look at what is spin? by Markus Ehrenfried for that. But note that where he says "There is nothing in our macroscopic world which has a symmetry like that", he isn't quite right. A moebius strip has a symmetry like that, in that you have to trace around it twice to get back where you started. Follow the link on Markus's website to The discovery of the electron spin by S.A. Goudsmit if you're interested. I like that kind of thing myself. But you have to be a little cautious about things like this: 'When the day came I had to tell Uhlenbeck about the Pauli principle - of course using my own quantum numbers - then he said to me: "But don't you see what this implies? It means that there is a fourth degree of freedom for the electron. It means that the electron has a spin, that it rotates"'. See below as to why. Yes and no. If you think of a tornado, the air is spinning, but the tornado isn't. People might say "Of course a tornado is spinning", but think about it: if you stopped it spinning it wouldn't be a tornado. It would be a straight gust of wind, just moving air. In this sense tornado spin is intrinsic to what it is. No spin, no tornado. It's something similar for an electron. Rotation is present, and angular momentum. Otherwise the electron wouldn't have a magnetic moment: "A loop of electric current, a bar magnet, an electron, a molecule, and a planet all have magnetic moments." This is backed up by the Einstein-de Haas effect which "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics." It's important to note though the electron isn't some little billiard-ball thing spinning like a planet. It has a wave nature, we can diffract electrons. Yes and no. Electrons have their spin angular momentum and exhibit orbital angular momentum in an atom, see spin (physics). And whilst in a sense they do revolve because they rotate, see atomic orbitals and note this bit: "The electrons do not orbit the nucleus in the sense of a planet orbiting the sun, but instead exist as standing waves." The best mental picture I can offer is that the electron is something like a moebius electromagnetic standing-wave going round and round, and when it's in a hydrogen s1 orbital it's like you're playing hula-hoops with it. It isn't quite as simple as that, because it's no ring, it's more spherical, hence spherical harmonics, and there's things like the g-factor. But think fields and waves rather than billiard-balls and planets and it's a good first step.
Photons and helium atoms obey Bose-Einstein statistics, which means that nothing prevents two bosons from sharing the same quantum numbers of angular momentum, position and momentum. Thus bosons have those weird low-temperature effects like lasers and super-fluidity. Electrons and nucleons obey Fermi statistics, which means that something prevents two fermions from sharing the same quantum numbers of angular momentum, position and momentum. This leads to Chemistry as the periodic table is what you get when electrons try to crowd around a nucleus but they all can't be in the lowest energy state. This is what exerts pressure to keep white dwarfs and neutron stars from total gravitational collapse. This, ultimately, is what gives matter it's classic property of something that takes up space. The basic physical theory that explains photons and electrons in a common framework is called quantum field theory, and in this framework of assumptions there is a theorem called the spin-statistics theorem which says in four-dimensions of space-time any particle defined as having an intrinsic angular momentum equal to an integral multiple of Planck's (reduced) constant behaves like a boson. And anything with half-intergral spin behaves like a fermion. Electrons, whenever measured, always have an instrisic angular momentum of either \(-\frac{\hbar}{2}\) or \(+\frac{\hbar}{2}\) in the direction the experiment was designed to measure electron angular momentum. No. Models where an electron is a sphere or a ring that actually spins don't make sense in light of experiment, relativity and quantum mechanics. The best model is that the electron is a point-like quantum object which just happens to also carry angular momentum. In experiments to measure spin, the value measures is always: \(\frac{\hbar}{2}(2k + 1)\) where k is an integer. Free electrons, electrons which are not part of atoms, continue to have intrinsic angular momentum, so there is no connection between revolution about a nucleus and intrinsic angular momentum. In addition, the quantum mechanics of electron orbitals says the angular momentum of any eigenstate is an integral multiple of \(\hbar\), so this cannot explain half-integral spin. Because the universe has physical laws that ignore absolute angular position and only relative angular position matters, angular momentum is preserved and intrinsic angular momentum happens to be a possible quantum statistic of particles. As it happens, our universe is filled with particles of various masses and intrinsic angular momenta. Higgs boson, helium-4: \(0\) electron, quark, nucleons: \(\frac{\hbar}{2}\) photons, gluons, Z-bosons, W-bosons: \(\hbar\) Omega-minus: \(\frac{3\hbar}{2}\) gravitons: \(2 \hbar\)
This is theoretical, not observed. http://www.markusehrenfried.de/science/physics/hermes/whatisspin.html
The objection is similar in flavor to anti-scientific claims that GR's modeling of space-time as a four-dimensional manifold is not evidence that space-time has the same structure as a manifold. The same observations which support classical GR also support a quantum model of gravity where a spin-2 quantum field mediates gravity. Because it is universally attractive, spin-1 is not viable. The details of observation heavily favor spin-2 over spin-0. And since the predictions are equivalent, and science there is strong empirical support for the existence of gravitational waves carrying amounts of energy and momentum, and since no physical principle is stated why action should be quantized for every phenomena except gravitation, then it follows that the existence of quantized packets of gravitational radiation (gravitons) are theoretically supported to about the same degree as "the sun will rise tomorrow." This evaluation is supported by textbooks and review articles, including: http://pdg.lbl.gov/2012/reviews/rpp2012-rev-gravity-tests.pdf which describes standard GR as "Einstein’s pure spin-2 theory" http://relativity.livingreviews.org/Articles/lrr-2006-3/ (Especially section 6.4) As these references are stronger indications of the state of knowledge and level of debate in the scientific community than your general education resource, I submit that your distinction between theoretical entities and observed entities is weakly supported and argued to the point of unworthiness as a forum contribution. Besides, anyone who knows anything about particle colliders and gravitational wave sources and detectors would realize that direct physical confirmation of the spin of individual gravitons is a physical experiment that (barring new physics) you basically need Dr. Manhattan or similar weakly-god-like entity to run for you. It is probably not an experiment one would choose to do on the surface of an inhabited planet. The experimental design of a similar experiment with light should suggest many of the difficulties: http://prola.aps.org/abstract/PR/v50/i2/p115_1
There's no actual support for gravitons, rpenner. They aren't supported like the sun will rise tomorrow. And there is a very important physical principle concerning action and gravitation. But let's not disrupt ash's thread with an unseemly argument, please start a new thread and let's talk about it there.
So it is this spin property that gives protons and other quantum particles it's properties.. In the link that is given proton has a positive charge because of their quarks..All adding the spins gives a charge... Then why electron have a negative charge?
Not quite. Spin is a part of it. For example a positron has the same spin ½ as a proton, and the same charge, but not the same mass. It's best to compare electrons and positrons. These have the same mass, opposite charge, and they both have spin ½. When you look at say the Standard Model you will find references to chirality. Chirality is "handedness". The electron and the positron are similar but different, something like your two hands. For an analogy you can make two moebius strips, one which twists clockwise, the other which twists clockwise. The both have a spin ½ feature in that you have to go round twice to get back where you started. But they have the opposite chirality. That's what distinguishes the electron from the positron.
"For an analogy you can make two moebius strips, one which twists clockwise, the other which twists clockwise" Other twists Anticlockwise right?? And clockwise and anticlockwise is used to denote different charge right??
As far as physicists know, there are no known substructures for the electron. Thus electron is essentially point like
There's various answers to this. You could say electromagnetic field, or electron field, or stress-energy. The important thing to remember is that modern physics concerns quantum field theory and the wave nature of matter, wherein the electron isn't some little billiard-ball thing that "has" a field. Instead everything is just fields and waves, and a wave is usually a field-variation that propagates through space. Unless it's a standing wave, when it's just a field. For example the evanescent wave is also call the near field. I'm afraid that's something of a non-sequiteur, Secret. I know that you can find fairly authorititaive sites that say this, but it's at odds with quantum field theory. See this bit from the wikipedia article: "In summary, the classical visualisation of "everything is particles and fields", in quantum field theory, resolves into "everything is particles", which then resolves into "everything is fields". Fields just aren't little pointlike things. For an analogy of the non-sequiteur, imagine you're dangling from a helicopter probing a whirlpool with a bargepole. When you can't feel anything hard down there, you then announce that whatever it is down there causing this whirlpool must be really really small. But "everything is fields" means there isn't anything small down there.
To give an example for Farsight's explanation, when the electron wave function is in a bound state (attracted by an opposite charge) it becomes a standing wave function. Thus, there will only be certain discrete standing wave patterns that can develop, which depend on the total energy of the electron wave. When the electron wave changes from one standing wave function to another, it will absorb or emit an electromagnetic disturbance of that same discrete energy (a 'photon'). Its 'pointlike' nature is simply saying that there is no 'internal structure', i.e. it is not made from more fundamental particles (unlike protons which are made from quarks). There is no simple answer as to 'how' the 'spin' of an electron develops, as there is nothing that is spinning like a top.
I think there is, Walter, where you view the electron as a standing-wave function in a "bound" state even when it isn't attracted by an opposite charge. Standing waves are interesting in that there's no apparent motion, see for example the optical cavity. However there is motion hiding in there, it's just that you have no clear way of seeing it. I don't know if you've seen any wave-tank experiments, but when you send two waves towards each other so that they're out of phase when they meet in the middle, the water there is flat and motionless. However the waves don't destroy one another, they carry on right through the flat region and keep on going. An optical cavity is something like a very short wave-tank with only one wave and only the flat bit. But if you whip away one of the sides, the wave comes out at c from a "standing" start. So it couldn't have been truly standing, it has to have been moving at c in both directions all the while. And there is a c in the Dirac equation.
"The best mental picture I can offer is that the electron is something like a moebius electromagnetic standing-wave going round and round" An interesting analogy . . . perhaps the electron standing wave 'flips' 180 deg during each of it's paths, requiring that it make two passes to arrive at it's original configuration . . . . just OOB here . . .
There does not appear to be any 'substance' that is spinning like a top to create the spin-1/2; however there does appear to be 'something' that is rotating, and perhaps the rotation of a bound standing wave along the lines of a moebius strip is a good analogy. I haven't seen anything more in-depth as of yet, and this is an area of interesting research currently ongoing. This in turn could then be 'bound' in a standing wave pattern generating the well-described probability functions of electrons in bound states around nuclei (electron 'shell' structure). Perhaps rpenner has more insight as to what is actually 'spinning' to create the electron 'spin'.
A better mathematical treatment of this question can be found here: http://en.wikipedia.org/wiki/Dirac_equation
SO ELECTRONS ARE NOT POINT PARTICLES.THEY ARE JUST ELECTROMAGNETIC FIELDS... I HAVE A DOUBT. DOES MOTION OF PROTONS GENERATE A FIELD?WHAT IS IT?? SIMILAR TO ELECTROMAGNETIC DISTURBANCE(light)?
What are you trying to explain from this explanation Farsight? Are you saying that there is apparent motion but because it is moving at c,we cannot see it's motion?[I ACTUALLY DIDN'T MEANT THAT OBSERVATION as "see"] And also in optical cavity,those mirrors help in creating resonance... how protons play exactly the role(just like mirror's role) of making the electron field to have only discrete standing wave pattern? And how protons make electron waves to act like standing waves?Is it because just that protons attracts it? I have heard in double slit experiment that electron behave like particles when we "Observe". Since Electrons are just electron field,What happens to field when we observe? Does field becomes so small that we observe particle like interference?
Firstly, farsight should be ignored because he doesn't know what he's talking about. Electrons are not made up of the electromagnetic field. They are made up of the electron field. This may seem hopelessly circular, but these are just labels that we put on physical objects. You can think of it this way - an electron particle is an excitation of the electron field which is the more fundamental object. In the same way, all particles are excitations of their respective fields so photons are excitations of the photon field etc. ad infinitum. At this point I suspect you are wondering where the electromagnetic field has gone as there is no such thing as the electromagnetic particle. The electromagnetic field is really just another name for the photon field. Photons don't have a charge because the electromagnetic field doesn't interact with itself (you could say this the other way around too...). Don't feel bad if you don't understand this at the first attempt. There are many PhD students that don't understand this at the second and third attempt - it's a very confusing subject that confounds a lot of people. It's complexity is one reason why there are people around that want to do away with it and come up with their own crackpot BS, but there is no reason why nature should be simple and the experiments agree with QFT. To answer your question, when we measure something as a particle it behaves as a point particle. In that respect, electrons are point particles. There are some problems with this philosophically, for example if a point particle has mass then presumably it's density is infinite and it is really a black hole. I emphasise, these are philosophical problems and all experiments done confirm that particles are points and quantum field theory is the correct description at the energy level we perform the experiment at, right up to the very highest energies we can currently acheive.
