Take a photon and a graviton. We know they must interact because gravity affects light. But how is this happening on the quantum level? What iow would be the medium of their interaction other than mass?
How the interaction of gravity and light actually happens on a quantum level is a good and interesting question, for which there is no commonly accepted answer at the moment. In order to fully describe this interaction one would need not just a model of quantum gravity, but also a model for a unification of quantum gravity with QED. We don't have that yet; all we can do at present is geometrically describe the trajectory of light in the presence of gravitational fields. It should be noted though that gravitons, if they exist, would mediate only changes in the gravitational field, not the field itself.
I'm not advocating gravitons, and in fact elsewhere think I gave a convincing indirect example that discredits the notion of spin 2 graviton. But in say superstring theory, doesn't the static gravitational field correspond to exchange of closed-loop strings as virtual gravitons?
I cannot answer this with any authority, as I am honestly not certain. My understanding is that yes, closed strings do correspond to gravitons, but these are only exchanged to mediate changes in gravitation. The point is that the String field is only mathematically consistent in a curved space-time background, which is where the static fields come in. I do concede that my understand on this might by flawed, though. I know, it was not my intention to imply that you were. Sorry if I came across that way.
Well my understanding will be far less no doubt. Just that I only recall claims that the 'fundamental' lowest level excited closed loop string corresponded in weak gravity at least to the g field, and thought there was a two-fold correspondence, with both virtual and real, as paralleled in static vs radiation fields in EM. No need and no offense taken at all.
Well done. And just like the proverbial cat, q-reeosity has gotten me into some nasty situations, but unlike cat, still breathing, touch wood.Please Register or Log in to view the hidden image!
It isn't happening on the quantum level because the graviton doesn't even really exist. If it did we would be able to see changes in gravity visably. For example, if you are on a rollercoaster and then feel weightless, you do not see streaks and flashes of light like they do when they hit the hyperdrive in Star Wars.
I can't believe no one has mentioned the gluon. The gluon is massless and mediates the strong force and they can interact with one another. Photons don't interact with one another as they possess a particular kind of symmetry property. In terms of the formalism which describes them in quantum field theory photons are a U(1) gauge field, where U(1) is the set of complex rotations \(e^{i\theta}\). It is 'abelian' which means \(e^{i\theta}e^{i\phi} = e^{i\phi}e^{i\theta}\) and as a result no interaction terms arise when you crunch the algebra. Gluons are described by the SU(3) gauge field and they do not commute, so an interaction term appears in the QCD model. Gravitons are less clear as their symmetries are of a subtly different kind and there is currently no known version of their construction via the quantum field theory method just mentioned. No, they mediate the field itself, just as photons mediate the electromagnetic field, not just changes. The background field is just a 'fuzz' of virtual particles, while the interactions between say two electrons might be done via 'real' photons, though the distinction is somewhat artificial in many instances. The string field is consistent in any space which satisfies particular conditions. How those conditions arise is, unsurprisingly, somewhat technical (vanishing beta function in the renormalisation group flow of irreducible representations of string tensor states...) but when you do crunch the algebra you find that in the series of expressions you generate the first expression amounts to the Einstein Field Equations, which is what you'd expect of a gravitational model's non-quantum contribution, as that's general relativity. The series then adds 'corrections' to this which are parametrised by the string length scale. If you 'zoom out' and only consider length scales which are so big the strings are effectively of zero length, which is what the scales of standard GR are, then these corrections vanish. This is a good thing in two ways, pertaining to experimental issues with string theory. Firstly if string theory predicted a first contribution different from the Einstein Field Equations it would be demonstrably wrong and thus falsified. Thus string theory has made and passed a falsification proposition. The second is that it also makes prediction about the tiny deviations we would expect to see in gravitational systems on ultra small or super massive scales. Flawed reasoning, as usual from you. We do see changes in gravitational fields visually, it is just that it is only in extreme cases and only using equipment. The fact our senses are not precise enough to detect picosecond variations in things doesn't mean they aren't there. When you walk up stairs the change in the gravitational field means time passes faster for you and while we don't visually observe it atomic clocks can easily detect such things. We've observed the impact gravitational fields have on light, it was one of the first demonstrations of general relativity back in 1919. We've measured the red and blue shift that gravity can have on light, though the shift caused by shining a laser up a flight of stairs is too small for our eyes to detect. No, you don't. But why should the effect be that pronounced? Why should the effect be perceptible to human senses? I know you struggle to grasp this fact but your experience of day to day life doesn't prepare you for or inform you about 99.99999+% of phenomena in the universe. You are not the yardstick by which the universe is measured. Get over it. Not knowing we have almost a century of data about how light and gravity interplay with one another just shows how your knowledge base would be embarrassing to a 15 year old high school student who didn't sleep through physics class.
