Recent content by GRQC

Those are all layman overviews of special relativity, not advanced modern treatments on the subject. I wouldn't take them as absolute gospel. Take it from a physicist who specializes in relativity: there is no such thing as "relativistic mass" nor is there such a thing as "rest mass". There...

I have a Ph.D. and am a physics professor.

Sol2, your posts have nothing to do with large extra dimensions. I don't understand the point you're trying to make. There are lots of theories which require extra dimensions. That's not new. Large extra dimensions are a very specific area. Are you confusing LED and LQG?

This is true -- the notion of extra dimensions isn't new, of course, but the idea that they could be larger than Planck scale was a novelty with some interesting observational consequences (deviations from Newton's law, anomalous cooling in supernovae, black hole pair production at TeV...

The papers I was referencing are: http://www.arxiv.org/abs/hep-ph/0109287 ("Cosmic Rays as Probes of Large Extra Dimensions and TeV Gravity") http://www.arxiv.org/abs/hep-ph/0109247 ("Cosmic Rays and Large Extra Dimensions") It's been a while since I read them, but I believe the first...

The Randall-Sundrum LED theory is distinct from ADD. The former does not require compactified (KK) dimensions. Instead, they place severe constraints ("warp factors") on how much gravitational fields can leak into the EDs.

Careful how you interpret the article. We don't *know* that these fundamental constants (alpha, c, hbar, G, etc...) were different in the past. It's just a theory which is being used to try and explain strange observations (for example, the accelerated expansion of the universe). This isn't a...

No, LED effects manifest themselves at much smaller energies (in the case of ADD, the electroweak unification scale, i.e. TeV eneries). Extra dimensions were used as an explanation for the 'knee' in the CR energy spectrum, which conveniently occurs at about a TeV.

Since the compactification scale was considered to be o(Planck) before, even a femtometer would be considered "large". Not exactly true. The ADD paper placed an *upper limit* of a millimetre scale for the radius of the dimensions. Specifically, their paper had two "free parameters" --...

Actually, measuring density in mass/area isn't that uncommon, especially in radiation and particle physics. Usually this is referred to as "mass thickness", and e.g. with radiation shielding is a method of determining the amount of matter needed to attenuate gamma rays (independent of the...

This was my basic question. It's easy enough to correct the reported pressure to the absolute pressure, but why do I care if the corrected pressure is 101 kPa if I'm experiencing 88 kPa of pressure?

You're missing a factor of g in your formula. It should read: P(z) = Pa exp(-zg/RT) So, in this case you would find P/Pa = 0.796, or about 80%. If you want to introduce temperature variations, you would include the temperature lapse function, T(z) = T0 - Lz, in the original differential...

So are they adjusted according to the Standard Atmosphere? That would make sense (sort of). For example, the conditions I cited yesterday in Mexico City would correspond to a density altitude of about 200m (not 2000 m, as it should be). Actually, do you have an official reference for this...

Actually, most cities are well below about 4 km, let alone 5 miles. The point remains: Mexico City's air density is about 80% that of sea level (an elevation of 2.2 km). A barometer should read a lower pressure. So why does a weather forecast show a pressure of about 100 kPa? This doesn't...

No, I'm aware of the "correct" interpretation. If you look at weather forecasts, for example, the pressure is *not* as low as you would expect. For example, currently in Mexico City it is 100.76 KPa at 17 C (77% rel. humidity). If you calculate the density altitude which corresponds to those...