Thanks for taking the trouble to reply, and thanks also for the analogy of parsecs. I did a cosmology module as well as quantum mechanics, and yes the parsec only really made sense in astronomy when absolute distance scales were uncertain.

At that time, all astronomers could do for reporting absolute measurements was to measure the angle of parallex. If you measure the angle of parallex to a star, i.e., the difference in apparent angle (relative to far more distant stars in the sky) for two times in the year 6 months apart (when the Earth is at different sides of the sun), this parallex or variation in apparent angular position can be measured in seconds of arc, i.e. 1 parsec is 1 second of arc in the sky, which is 1 part in 3600 of 1 degree of angle of the sky.

Hence 1 parsec is 1 second of arc difference in star location seen from opposite sides of the sun. Since it has been determined accurately that the radius of the Earth’s orbit is about 150 million km, it follows from the trigonometry of a right-angled triangle that a star with a parallax of 1 parsec would be at a distance of (1.5*10^8)/sin(1/3600) = 3.1*10^13 km.

What’s surprising is here is that this kind of conventionalism is the cause of a major failure by Hubble. Instead of thinking deeply about his recession law v/R = H, he expressed H conventionally in units of km/s/Mparsec. If he had thought about it, spacetime implies that you can represent a distance as a time. If he had written the Hubble law that way, he would have v/t = Hc, which is interesting since it naturally has units of acceleration.

Even if you just take the regular mainstream Hubble law v = HR, you can see that it implies acceleration: a = dv/dt = d(HR)/dt = (H*dR/dt) + (R*dH/dt) = Hv + 0 = HHR. So the Hubble law itself predicts that the universe is accelerating at the small rate of about 6*10^{-10} ms^{-2}. This is such a tiny acceleration that it was first observed only in 1998 by Saul Perlmutter’s clever automatic supernova-signature detecting software which was directly run with live digital input from CCD telescopes.

Mainstream cosmology is completely half-baked because it doesn’t bother to analyse the few solid facts it has at it’s disposal. Everytime I try to pointed out that it’s possible to prove the universe was accelerating (and I published it in 1996, years before the discovery), and the allied facts that the outward acceleration implies an outward force which leads to quantitative predictions in quantum gravity, I was just censored out for dozens of reasons. People don’t listen because they either (1) assume that the mainstream orthodoxy is gospel truth, or because they (2) completely reject the big bang and recession discovery factual evidence and want to preach about false “tired light” nonsense (against the facts) for pseudoscientific, metaphysical personal reasons . It’s very weird how orthodoxy is so helpful in experimental particle physics, but is unhelpful in other areas.

the centimeter is, I am afraid, a very convenient unit for particle physicists. We measure in centimeters everything from detector components to beam sizes. Who cares about SI ? Luminosity is universally used in units of cm-2 s-1 in collider physics (I am not so sure about neutrino beams though). GeV are also very convenient because a proton’s mass is roughly a GeV, and because momenta of the order of a GeV are what you usually measure in the detectors.

I am of course not saying that international units are not useful. All I am saying is it is much more important that a system is used everywhere in a given branch of science, than that it is used across disciplines in, say, 80% of the world. If astronomers prefer parsecs (another quite odd unit, you’ll concede) to SI units they have their reasons. We have ours… I have never seen an example of a calculation requiring luminosity and distances on a parsec scale in the same sheet of paper, so we are ok.

Cheers,
T.

Is it just pride that particle physicists at the forefront of everything use obsolete units prove they are above the pettiness of regulations? (In school when doing elementary physics twenty years ago in England, using CGS units was just as criminal as not rounding calculations results. Writing 10^2 cm in place of the correct result 1 m would result in no zero for the question. A bit difficult to master, since most of the old books I learned physics from were in CGS units.)

It used to make sense to use electron volts for the kinetic energy a particle gains in an accelerator before a collision. If an electron was accelerated by an electric field potential of 1,000,000 volts, it would have an energy of 1 MeV.

But it’s not directly measuring the energy of a particle, it’s like describing how fast your car is going by stating the number of litres of petrol (gasoline) required to accelerate the car to a given speed.

E.g., if it takes 1 litre of fuel to get up to 100 miles per hour, we could refer to speed in units of litres of petrol, which is just as logical as referring to the kinetic energy of a particle in terms of the number of volts of the field which was responsible for accelerating that particle up to a particular speed (and energy). Maybe instead of referring to 30 GeV jets, people should logically refer to 4.8 nJ jets?

I think July is still optimistic… But it is mostly a gut feeling. In any case, July is indeed the second half of 2008.
As for 10^34, it is a mistake on my part. Indeed, it will probably take three years to get there. However, I doubt that we would keep running at 10^33 during the first three years if we ever could go up with luminosity safely, just to “study the SM backgrounds”, though…

Cheers,
T.

indeed, the bunches are thin, thin needles – a few tens of microns across, for a longitudinal length (along the beam direction, z-axis) of 10-20 cm. I have the actual numbers somewhere, will try to dig them out for you. In the meantime I can add that yes, we can only distinguish separate collisions along the z direction. The silicon tracker has a resolution comparable to the beam transverse size, a few tens of micrometers, so multiple interactions – which are spread out several centimeters across – can be separated quite easily.

Cheers,
T.