Severian

I have a paper copy of the Phys.Rev.D Particle Data Book sitting in front of me. You can also get the same stuff at http://pdg.lbl.gov/

Sorry, I was trying to answer your question. Maybe I misunderstood? I was trying to say that one can at least see that alpha is dimensionless using the formulae which you wrote down earlier. To answer your equstion more specifically: Q: 'Is it obvious to you that in Planck units the value of the Coulomb constant is alpha?' A: No it is not, because alpha is a dimensionless number and depends solely on the details of the theory. One could imagine having a 'super-electromagnetism' which was ten times as strong as standard electromag and would have an 'alpha' which was approx 1/14. Is this what you were meaning?

Since I can't post new threads this seems an appropriate place to comment.... When I look at my 'posting rules' I have: You may not post new threads You may post replies You may post attachments You may not edit your posts I have tried logging out and in again, and no change. I seem to remember something about needing to reply to an email which I would be sent on registering. If this is the problem, I never got the email, and I can think of 2 possible reasons: 1. I mistyped my email address. 2. My spam-filter ate it. Could a mod please send my a PM confirming my email address and then resend it? Cheers

You may not post new threads You may post replies You may post attachments You may not edit your posts Hmmm....

You can't derive the dimensionless constants this way but you should be able to see consistancy with units (or in other words you should be able to derive the unit dimensions of alpha. Planck units are [math]c = \hbar = 1[/math], and since you know c and hbar in SI units you can write 1 metre in terms of c etc... Looking at your earlier formulae, your problem could be that for gravity you are using masses in the numerator in one case, which are dimensional, whereas for electromag you are using charges which are not?

No there isn't - I only have 'Quote' and 'Quick reply'....?

Yeah - it looks a bit crap, but I can't figure out how to edit these posts.....

Quantum mechanics is intrinsically non-local because, as you say, you are determining the outcome of a measurement 300 million light years away. But it is not a problem for relativity because you can't pass information using this mechanism.

Cross-posted there. It is only appropriate to use string theory at energies where gravity is strong and therefore important (because otherwise you could use the Standard Model SU(3)xSU(2)xU(1) gauge theories much more easily). Gravity becomes stringer and stronger at high energies and is expected to become of a similar strenght to the other forces at around [math]10^{19}[/math] GeV, or in other words 100000000000000 times the energy of our biggest particle accelerators.

If you are really really good at maths you should in principle be able to explain low energy dynamics in terms of string theory too, but it is not the way people usually think about it. It is like asking someone what they put in the sauce they served with dinner, and getting the reply 'strings'.

No - a particle in a parabolic potential well will oscillate backwards and forwards indefinitely. It was just that my analogy to explain it doesn't. My point was that it is not the motion itself which is the problem - it is the energy leak.

I would define perpetual motion as something which moves in perpetuity, but you are right that most people would define it as you say. I was maybe being a little facetious. As for parabolic potential potential well: imagine a parabola, like the valley between two hills, and imagine letting a ball roll down one of the hills. It will speed up in its way down and roll up the other side. Now in my analogy, the ball will lose energy due to friction, air-restistance etc, so it will not move indefinitely, but one could imagine the idealised situalion where it would roll back and forth indefinitely.

As jaKiri said, in string theory everything is made out of strings - so if string theory is correct then yes, dark matter are strings. But since string theory is only appropriate to use at very very high energies one has to ask what is the effective dark matter particle at low enrgies (ie what is the string theory low energy manifestation which provides so much of the mass in the universe). Most physicists seem to think dark matter is the lightest neutralino, and that would be my guess too.

[math]E=mc^2[/math] only applies to a particle at rest. If it is moving it should by [math]E^2=m^2 c^4+p^2c^2[/math] where the 'p' is momentum. (Setting p=0 gives [math]E=mc^2[/math] again.)

[math]alpha = \frac{e^2}{4 pi epsilon_0 hbar c}[/math] (sorry, I don't know how to do hbar in html...) It is only 1/137 at macroscopic distances, and then it is really 1/137.03599976, so it isn't realy exactly 1/137. It also changes with energy - at energies of the W-mass for example it is approximately 1/128.

There is no problem in physics with 'perpetual motion'. Light from distant galaxies has been in 'perpetual motion' for billions of years to get to us. A particle in a parabolic potential well will oscillate backwards and forwards forever, exchanging kinetic and potential energy back and forth. The problem is conservation of energy. Since energy is conserved, as soon as your system has any energy leak, it will lose energy and eventually run out of energy and stop. Since it is almost impossible to not lose energy, practical perpetual motion seems impossible. Even if it were possible it would not be useful for what people now think of when they say 'pertepual motion' - it would not provide a source of energy since if you start removing the energy the 'perpetual' motion will soon stop.

There are plans to build a photon collider as an offshoot of a electron-positron linear collider. The important thing is not how fast they can go, but how much energy they have, so their mass (and the fact that they travel at c) doesn't come into it - after all, on the scale of a TeV, electrons are effectively massless too.

No (to both questions). I was just wondering.....

Why are you posting this? Are you Martin Bojowald?

Hi guys - sorry about the post above (I wanted to check if I could post, thinking I would edit it, but can't find the edit button.....(I can be a bit thick sometimes)) Anyway, I wanted to comment that 150 is a little unfair. First of all, the extra particles are not particles in the Standard Model - they are particles introduced by Supersymmetry. Supersymmetry introduces a new particle for every particle we have in the Standard Model: for every fermion we get an extra boson and for every boson we get an extra fermion. If I remember correctly the Minimal Supersymmetric Standard Model then has 123 particles (or thereabouts). But that is not very fair either because supersymmetry has also introduced a new fermionic 'dimension' into the game, and the new particles are really just the component of the old particles in the new femionic dimension. So they are really just different facets of the old particles and not new ones at all!