Severian

Have you ever read any of Ian Bank's 'culture' novels? That is a future society which is truely communist. They have AIs as an integral part of their society and are technologically advanced enough to be able to have whatever goods they want. The AIs do all the administrative work. In such a society capitalism doesn't work, because you can have whatever you want and there is no need for money. Their only problem is boredom (which is actually quite central to the books). (And with regards to sex, their society is very open and sex is something that they do at the drop of a hat - even often changing their sex for pleasure.) So, in some ways, communism is the ultimate goal of society.

That line of reasoning does not lead to that conclusion. Since the equivalence principle does indeed tell us that you can't tell the difference between gravity and any other acceleration, then any acceleration really is gravity when viewed in the correct frame.

That is not true, it is not the same thing at all. In a QM system you can have interference between the different states which gives you a measurably different result from that which would be obtained by no interference. It is the presence or lack of this interference which is indicitave of whether the wavefunction has collapsed. Classical physics does not have this effect. (Notice that unobservability of interference does not mean that you have none.)

What a wonderfully ignominious end for such a breathtakingly dreadful site.

For my first comment (Sisyphus' question) I was meaning that we should challenge this as a philosophy. It has become a taboo in recent years for criticising anything anyone does which "doesn't effect anyone else" (my point being that there is no such thing). As for the second, I meant polygamy.

And where are you going to get 0.5g of antideuteron from? You are not Dan Brown are you?

How about the idea that you should be allowed to do anything you want as long as it "doesn't effect anyone else". Alternatively polygamy is a good one.

The interesting thing about this thread is that the possible existance of a 'cure' for homosexuality is regarded by many as a bad thing in and of itself. I would have thought that the existance of the option to change your preferences would be a good thing. In fact, I would contend that it is never a bad thing unless you force people to take the 'cure'. Look at it this way, imagine a pill (or perhaps a CD that you play at night or something) which would allow you to change your sexual preference one day to the next. Wouldn't that be really handy? Can't get a girl, take the pill and go look for a boy instead! You discover your wife is into bondage but you don't get it - play the CD that night and you will have great kinky bondage sessions tomorrow! After all, if there is (in principle) a cure for homosexuality, then there is also (in principle) a cure for hetrosexuality. If you are going to legislate that people don't have the right to make that decision themselves, aren't you restricting their rights?

The founder of Google just married the daughter of a particle physicist I know.

Let me try and answer a few of the things in this thread. 1. If you collapse the top-quark wavefunction to a point, then it is indeed infinitely dense (much more than a gold nuclei). 2. Since you cannot measure position infinitely accurately, you can never collapse it to a point, so the density is finite but very large (still larger than a gold nuclei). 3. The question 'what mediates the Higgs field' is ill-posed, since what is being mediated? One normally uses the word 'mediate' in the context of a force, and even though the Higgs boson does transfer momentum, it is usually not thought of as mediating a force. Indeed, the 'Higgs field' and the 'Higgs boson' are not the same thing. The Higgs boson is a quantum of oscillation of the Higgs field about the vacuum state.

But then you need to explain why our brains cannot process information in more than 3 dimensions. If we truely existed in more spatial dimensions and there was not some physics reason we couldn't access them (such as compacification or restriction to a brane etc) then one would expect us to evolve to be aware of them.

I asked this question in an oral exam once. The other examiner called me cruel afterwards.

This is bogus reasoning. You only classify objects with less than 3 spatial dimensions as 'impossible' because you have never seen one. But the reason you have never seen one is because you live in 3 spacial dimensions. It is technically possible for extra dimensions to predict the right amount of Dark Matter, but to do this, the extra dimensions have to be of the scale of an inverse TeV and you have to invent some new parity to keep the Kaluza-Klein excitations stable. (And the reason we have not detected the extra dimensions is not because we have 3d vision - it would be either that photons are not allowed to travel in the extra dimension, or more likely, that the extra dimensions are curled up really small.) Incidentally, the scientific community does take extra dimensions very seriously, as you can see from the number of papers on the subject: http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+title+extra+dimensions&FORMAT=www&SEQUENCE=

I challenge you to make an observation without involving a person.

I also can't imagine that race was an issue. It is much much more likely to have been a personality clash. I have certainly overlooked people in job applications who are really great physicists simply because I don't like them and would not like working with them. I think that seems a much more likely explanation. Also, in what sense is he 'prominent' if he doesn't have tenure yet?

I am actually sitting in CERN at the moment (I was down the ATLAS pit just this afternoon) and there is certainly no-one talking about this rumour here. The rumour from John Conway a few months ago just seems to have been all hot air (and I believe John had his fingers rapped by the management for it), and I imagine this is the same story. The problem is that the Tevatron just doesn't have the luminosity to see such a heavy Higgs boson, and won't have for some time. So if it were real, it would almost certainly NOT be the Higgs boson of the Standard Model (or the supersymmetric version) because the rate must be too high. If the bump were at about 115GeV then maybe, but not 180GeV. (The Higgs boson gets harder to create the heavier it gets.) As for immortal's idea of a Z boson, this won't wash because the Z boson mass is about 91.2GeV, so the resonance bump would be much lower. It could possibly be a Z', ie the gauge boson of an as yet unknown new force, but then one would have to think of some reason why it wouldn't show up in precision tests (which rule out Z's up to about 800GeV if I remember correctly). I suspect the bump will mysteriously go away with more data. Edit: Also, the latest top mass measurement is lower than previously. the latest number is 172.5 +/- 2.3 GeV.

Well alpha definitely does change with time to some extent. Its value is dependent on energy, so when the universe was very hot, it would have a different value. I presume this is taken into account for the dark energy measurements(?).

That's because it is relativity! Maxwell's equations are relativisticly invariant (which is not bad for 1861, being 44 years before Einstein's Special relativity in 1905).

We actually have sorfware that scans reports to check for plagiarism or inconsistant writing. That is why we sometimes ask for electronic versions of reports.

I have been completely unable to get mathematica to work on my laptop

Yes, you could do this in principle on the Earth (eg. a space elevator) but you couldn't on a Black Hole because the energy to overcome the gravitation would be infinite. Yes, you are right. My description was a little simplistic, although it suffices for the question that was asked. I am not sure what you are asking. I think what I said before was slightly wrong. Your legs would not be able to communicate with your body in any way, so there could be no passing of momentum, so no force. In other words, the gauge bosons which mediate the other forces are as trapped in the BH as everything else is. So your legs would be 'chopped of'' after all. The point I was making is that you would have lost your legs long before the EH because the effective force carriers are travelling slower than c, so their 'EH' is further out. No. the nerves signals don't move anywhere near c.

What does having no charge have to do with it? That is like saying, the bus is not red so how can it carry passengers?

The event horizon is just the point where you cannot return in principle, even if you really really try very hard. All gravitational objects have what is called an escape velocity. This is a speed that you have to acheive to be able to leave the object behind (get out of its gravitational potential). For example, for the Earth it is around 40,000 km/h (so pretty fast!). A black hole is not really any different, and in my opinion is really far too glamourised in science. A black hole is just when the body' escape velocity is greater than the speed of light, so nothing can obtain the escape velocity, not even light (which is why it is 'black'). But everything else is the same. If you could hypothetically build a spaceship (and crew) which was able to withstand the tidal forces, you could pass over the event horizon like normal and wouldn't really notice. You would only notice when you tried to get out again. The downward force on your legs is perfectly continuous as you pass the event horizon. It doesn't suddenly become huge and chop them off. However, it is already huge, so you have probably not felt your legs for quite some time, because the energy required to pass the blood or nerve signal up your legs was probably far too much already 10 miles up! You would only stop feeling your legs as you pass the horizon if you are some sort of light-based creature whose legs communicate with the rest of you by sending light signals.

Gluons and (presumably) gravitons also have no mass. Photons are not just 'pure energy'. They have other characteristics too. They carry energy, but they are not 'just' energy.

I think (in my opinion) this is easier to understand in terms of virtual particles. 'Real' particles obey Einstein's famous formula: [math]E^2=p^2c^2+m^2c^4[/math] where E is energy, p is momentum and m is mass. All well and good. When a particle obeys this equation we say that it is 'real' or 'on mass-shell' (the latter terminology can be understood by realising that m is an invariant length in 4-momentum space, so the equation defines a spherical surface in 4-d momentum space, or a 'shell'). But particles can be 'off-shell' too. Then the above equation is violated - the energy is not equal to the summed squares of the momentum and mass. This is analagous to the HUP, but here we are redefining what we mean by energy such that it is still conserved (so the E here isn't the E in the HUP, the [math]\sqrt{p^2c^2 +m^2c^4}[/math] is). We instead violate the mass-shell relation and that allows us to tunnel through the barriers. However, how far off-shell (i.e. how much the relation is violated) is inversely related to the lifetime of the particle. To be exact, the particle's lifetime is proportional to [math] \left| \frac{1}{E^2-p^2c^2-m^2c^4} \right|^2[/math] (neglecting the natural decay width for the moment), so a virtual particle cannot live very long. This is the equivalence of the HUP only allowing the borrowing of energy for a limited time, because the particle must decay very quickly if it is far off shell.