Severian

It is significantly harder to find a correct statement...

Light is massless because it is the gauge boson of a local U(1) symmetry. In other words, the universe has a symmetry, called the electro-magnetic gauge symmetry, which ensures that the photon is massless.

This was actually the original explanation of FTL in Star Trek. They fire gravitons out the front of the ship, so that space is warped and 'shrunk', then pass over the shrunk space and let it expand behind. The idea was that since one is passing over less space, one can effectively travel faster than c. They forgot that the gravitons can't travel faster than c though, so you can't warp the space fast enough to allow FTL travel. (in princple though, you could use it to make a ship faster (<c), although it would be very inefficient.)

[math]\gamma \gamma \to e^+e^-[/math] is the creation of matter from energy. All it requires is for the photons to have total energy in the centre of mass frame greater or equal to twice the mass of the electron (about 1 MeV). So this is very easy to do.

I find it rather amusing that in the first episode he claims that Einstein made no significant contributions late in life because he became disconnected from experimental reality, and then in subsequent episodes went on to say that String Theory is the way forward. Am I the only one to see the irony of that?

I really need to walk across the corridor and ask some questions, because I am aware that the people in the office opposite mine are working on this very subject....

Well, presumably one could curve spacetime 'by hand' in such a way that it appears that one body is gravitationally repelling another, but this is not possible within the framework of GR.

that is not what he meant - he meant that you should not change the sizes you make things in, just change what you call them.

Well, that is a very good question. In fact, this is what the COBE satelites answered. You are correct - if the pre-galaxy plasma was completely homogenous and isotropic, there would be no structure formation. Everything would get gravitational pulsl from everything else, which would all cancel out, and there would be no movement. What the COBE satelite showed (and WMAP later) was that the universe was not completely homogenous after all - there were very tiny amounts of structure. As soon as you have some structure, it will start to magnify (because the forces no longer balance) and eventually one gets galaxies. What caused the original (very small) structure is a different question...

No it doesn't. All it requires is that the objects are stuck to the sheet (which is reasonable). The moving objects don't bend toward the gravitational attractors because they are lower down, but because the space is curved. If they travel in a straight line, they will still bend towards the large masses because a 'straight line' does. Edit: So the previous poster's idea of having a big ball under the sheet pushing up wouldn't create antigravity - it would produce normal gravity, becaues the important thing is not whether parts of the space are 'higher' than others, it is whether the space is curved or not.

Yes. The galaxies themselves are not expanding.

Why do you say that? What does quantising gravity have to do with creating anti-gravity? Anti-particle have positive masses, not negative ones.

A graviton is Majarana, so it is its own antiparticle (this is true for the photon too).

I have not.

I think the categories need clarification. We often see posts on the same topic in different categories, just because the poster hasn't checked every category before posting and isn't sure which category the post belongs to. Otherwise, it is great as it is.

Virtual particles don't travel faster than light. The link to which you refer discusses the collapse of the particle's wavefunction, and explains that even though that collapse is faster than light (instantaneous) there is no information flow, so there is no contradiction.

Why don't homosexual people find the idea of a 'homosexual gene' offensive? Doesn't it imply that they are not masters of their own desires but somehow dictated to by their genetic makeup? Isn't it more attractive to consider one's sexuality as a statement of our personalities rather than the function of a single gene?

It is a lot more complicated than that once you include dark energy or a cosmological constant. These sites might help: http://superstringtheory.com/cosmo/cosmo21.html http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec21.html and on

Questions of geometry are sometimes confusing. It is best to think in terms of 'what happens if I go in a straight line'. If two people start walking (or better, travelling in a space ship!) parallel to each other at the same speed and in a straight line, what happens to the distance between them. In a flat universe it stays the same, in an open one it becomes more, while in a closed one it becomes less. That is really all that the different geometries say: it is really just telling you how to measure distance. The reason that you are having difficulty is because the 2d representations that one always sees are embedded in a 3d space, but one cannot in one's mind embed a 3d space into a 4d one. In fact, one really shouldn't even view it this way, because our (3+1)d universe is (presumably) not embeded in a (4+1)d universe in this way (there may be extra dimensions, but they would be compactified). Similarly I think it is dangerous to think of general relativity purely as a spacte-time geometry effect, because of the associated preconceptions we carry about in our minds. It is better to think of the metric as a rule for measuring distance, rather then describing some curvature of space-time. (This is of course the definition of 'curvature', but the word itself carried too much baggage imho.)

Yes - that is true. (Although this assumes isotropy - and anisotropous universe could I suppose be flat in some directions and curved in others, like a cylinder.) Maybe I expressed myself badly above. Edited for clarification...

WMAP has shown shown that the universe is (to a good accuracy) flat: http://map.gsfc.nasa.gov/m_mm/mr_content.html Edit: one could of course agrue that it could still be closed, because the measured value of Omega is 1.02+-0.02, but if the universe IS flat we will never be able to prove Omega=1 with 100% certainty (since we will always have some experimental error).

I suspect JaKiri isnow being deliberately misleading.... Jaques statement that the universe is expanding away from every point is correct, and since a 'certain point' is a subset of 'every point' then it must be expanding away from a certain point too. But I don't think Jacques is talking about a 'certain point' in this sense. What (I think) he was complaining about is that there should be no special reference frame, and this is correct.

No they won't. It is parabolic: y=x^2 so the ends would never meet.

I am finding lately that when I am using math mode to display formulae via latex that they are not showing up properly. I just get a 'missing image' red cross, whith the latex commend itself. Is anyone else finding this, or is it just me?