I have been recently reading quite a little about Kaluza Klein theories. I am still far from mastering this but I am curious if any experiment that may disprove or give hints of the existence of extra spatial dimensions is to be expected to be performed relatively soon. , for example, says that the LHC might give some clues, but it doesn't really dive into much detail. So, in some detail, which experiments might say something about this, and when might they be expected?
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I have read and heard in a number of places that extra dimension might be as big as $x$ mm. What I'm wondering is the following: How is length assigned to these extra dimensions?   I mean you can probably not get your ruler out and compare with the extent of an extra-dimension directly, can you? So if not how can you compare one dimension with the other? Does one have some sort of canonical metric? Could one also assign a length (in meters) to time in this way?
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This is really a follow up to the  question.  In the first answer to the question, Ted Bunn says:     However, the best available data seem   to indicate that the Universe is very   nearly spatially flat. This means   that, if we do live in a 3-sphere   Universe, the radius of the sphere is   very large, and the distance you'd   have to travel is much larger than the   size of the observable Universe.   My question is, can the available data be used to set a lower bound to the size of the Universe?   In other words, suppose the Universe was actually shaped like a 3 sphere. Then based on our best data, what would the "radius" of that sphere be? How much larger is that than the observable universe?
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Regarding the cosmological selection hypothesis and testable predictions, Lee Smolin asserted the following:     "Smolin: I did make two predictions which were eminently checkable by   astrophysical and cosmological observations, and both of them could   easily have been falsified by observations over the last 20 years, and   both have been confirmed by observations so far.      One of them concerns the masses of neutron stars and the prediction is   there can't be a neutron star heavier than about twice the mass of the   sun. This continues to be confirmed by the best measurements of the   masses of neutron stars."   What is he referring to? As far as I know, the neutron star mass limit is a prediction of GR, and doesn't suggest that any fine-tuning is involved in it. Is this correct?
89255
I was browsing through Foundations of Space and Time, a compilation of essays on various theories of quantum gravity. The following passage in the introduction intrigued me:     Each compactification leads to a different vacuum state.... at least one state should describe our Universe in its entirety.... the enormous number (~10^500 at last count) of solutions, with no perturbative mechanism to select mechanism to select among them, leads some critics to question the predictive power of the theory..Even more worrying is that, while the theory is perturbatively finite order by order, the perturbation series does not seem to converge.   I don't know anything about string theory and so I could not make head or tails this. All I know is that ~$10^{500}$    is a very large number.    What exactly is a 'solution' in string theory? Is it a spacetime metric of some sort or the terms of a S-matrix of some sort?   Why are there so many 'solutions'?  I thought string theory was supposed to be finite, why do perturbative series still diverge? Is there any experimental technique to limit the number of 'solutions'?   Will experimental techniques be able to pinpoint a solution within present day string theorists' lifetimes too? If not, how long will it take before we can experimentally probe these things?  Are string theorists completely relaxed about these issues? Or are they in anguish?
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seems like a really hot topic and I'd like to start reading about. I am looking for the best introduction at my level, i.e. I have a background in QFT, CFT and general relativity at the level of a master student in theoretical physics. What would you recommend me to start tackling the subject? I have been looking for resources and so far I have noticed:  -this synthetic introductory lectures by Horatiu Nastase:   -the videos of lectures done by P. Vieira at Perimeter for Perimeter Scholar International students:   -the subject starts entering the most recent textbooks on string theory. We have the Schwartz and Becker (  ) and also the Kiritsis (  )  I probably missed a lot of resources, as the literature on the subject is already quite huge. I would really appreciate some advice on that, as I already had the frustration of losing my time on not-so-good books when started to learn something new, so if I could (to the best) avoid that this time...
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I would like to know if there are some open mathematical problems in General Relativity, that are important from the point of view of Physics. I mean is there something that still needs to be justified mathematically in order to have solid foundations? Please tell me.  Thanks!  Edit: The same question was asked (and answered from a mathematical perspective) on .
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String theory gives physicists reason to believe that particles are 1-dimensional strings because the theory has a purpose - unifying gravity with the gauge theories.   So why is it that it's popular belief that particles are 0-dimensional points? Was there ever a proposed theory of them being like this? And why?   What reason do physicists have to believe that particles are 0-dimensional points as opposed to 1-dimensional strings?
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I was thinking of the noted 1980 paper by Sidney Coleman and Frank de Luccia--"Gravitational effects of and on vacuum decay"-- about metastable vacuum states that could tunnel to a lower energy "true vacuum" with catastrophic results. I suspect the answer is that if our vacuum is a false one, we wouldn't "see" the true one til it hit us.
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It is usually said that existence of discrete spacetime violates Lorentz symmetry. What quantity is used to quantify such violation? I mean could someone points a reference for a derivation that shows such analysis.  My other question is: if Lorentz symmetry is violated, does that imply space-time is discrete? or not necessarily?)
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One way that I've seen to sort-of motivate string theory is to 'generalize' the relativistic point particle action, resulting in the .  However, once you see how to make this 'generalization', it becomes obvious how to write down the action, not just for a string, but for manifolds of higher dimension as well.  In fact, Becker-Becker-Schwarz (the main source I happen to be learning from) actually do this.  But (as far as I have read), they merely write down the action and do nothing further with it.  My question is:  what happens when we proceed along the same lines as string theory, but when replacing a string with a 2-manifold, the simplest example of which would be the 2-sphere, a "shell/membrane"?  Assuming 3 spatial dimensions, this is the highest dimensional manifold we can consider (because we don't allow for non-compact manifolds).  Furthermore, there is only one compact manifold of dimension 1; however, there are infinitely many compact manifolds of dimension 2, which could potentially make the theory much richer (and probably much more difficult).  For example, in string theory, we are stuck with $S^1$ (if you insist upon having no boundaries), but if you allow for 2 dimensions, we could consider the sphere, the torus, etc.  Because it seems that this path is not ever presented (in fact, I've never even heard of it), I would presume that something goes wrong.  So then, what exactly does go wrong?
9160
From what I understand, information is communicated instantly between two quantum-entangled particles regardless of the spatial distance between them. However, does this necessarily imply superluminal data transfer? If the "distance" between the particles is simply assumed to be measured in our common 3D euclidean space then that would seem to imply superluminal communication. However, could there be other paths between the two particles that lie beyond our observable 3D euclidean space, other paths that involve additional dimensions in which the particles remain very close or some sort of folding of space on other dimensions that provide an information pathway? Anybody aware of any such research/findings? Thnx.
120991
Baryon asymmetry refers to the observation that apparently there is matter in the Universe but not much antimatter. We don't see galaxies made of antimatter or observe gamma rays that would be produced if large chunks of antimatter would annihilate with matter. Hence at early times, when both were present, there must have been a little bit more matter than antimatter. This is quantified using the asymmetry parameter  $\eta = \frac{n_{baryon} - n_{antibaryon}}{n_{photon}}$  From cosmological measurements such as WMAP,  $\eta \approx (6 \pm 0.25) \times 10^{-10}$  However, the source of baryon asymmetry is said to be one of the Big Problems of Physics.  What is currently the state of the art regarding this puzzle? What's the best fit we can get from the Standard Model? What do we get from lattice simulations?
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I'm not a physicist; I'm just curious about what would happen.  According to this video "" (at 2:50):     "Somebody decided to leave the detectors on but just not take any data. [...] So if it was also going to a magnetic tape - there was no magnetic tape loaded. [...] And what do you think happed? They got this. They got the diffraction pattern."   So the act of measuring (beeing concious of the data) destroys wave-form pattern?  If so lets consider these cases:   What if I'd throw a dice after the experiment to decide if I should look at the data or to destroy it? (e.g. look at the data if the number on dice is even) What if the information about measurement would need to travel longer period of time than the particle would need to hit the wall?    To above cases: Would it mean that information travelled instantly - faster than light or that "the particle can look into the future"?
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Recently, I have heard of extensions of the double slit experiment that propose to use a bacterium as the particle. I've also heard that the double slit experiment, in principle, would still show an interference pattern if it were done with a human or a car, as long as there is no observer.  This made me wonder: if this is true, and I exist in a superposition, do I have a mass which exists in several locations at the same time and with an associated weight of 200lbs in each of my superposition locations?  If that were true and a fact, wouldn't that account for the missing mass which we attribute to dark matter? Also, even if on some other level this is true about particles only, wouldn't this be where we would find a link between quantum mechanics and gravity, linking it with relativity?  Forgive me if this is a silly question as I am almost 63 years old and have been almost 50 years out of school, but I just love to read and think about science and learn as much as I can.
14987
Is  (i.e. repulsive gravity) possible in string theory?  I have read some articles about scientists making assumptions about the existence of anti-gravity, but is it possible in string theory?
117035
Let's assume that the existence of gravitons is theoretically proven or they are detected by LHC could one interpretaion be proven correct?
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I know that there's big controversy between two groups of physicists:   those who support string theory (most of them, I think) and those who oppose it.   One of the arguments of the second group is that there's no way to disprove the correctness of the string theory.  So my question is if there's any defined experiment that would disprove string theory?