Recent content by Diploria

Thanks Avodyne, and everyone - much clearer now :)

Thanks all, that's helpful. I also belatedly found a related thread where this seems to make sense to me: Combined with some info on 'Fermi velocity', I get this as a summary-to-myself: given QM, there's no thinking in terms of velocity of a only-particle-like individual electron within the...

Thanks all for taking the time to reply. However, I don't think my main question has been answered yet. It's not so much about the value of the fine-structure constant, it's about the meaning of velocity in this situation. Bill, I realize that the Bohr model is outdated. But the question is...

Hello, 1. I read here and here that in the hydrogen atom, electrons move at approximately ~1/137c. In the first link they speak of "zipping around the nucleus", presumably figuratively, because it is often stressed that QM has superseded the earlier model of electrons flying around. Instead we...

This may be getting too off-topic... but this struck me as bewildering: That sounds like an 'unmoved mover' -- not something I'd think has a place in modern physics. I can more or less see the sense in which it would require zero force. On the other hand: how would something accelerate...

Right... so I was imagining a true vacuum both inside and outside the cylinder. Then there is nothing to stop the piston once it's set in motion (I'm assuming there is no friction either). But to get it moving, and again to make it stop after a little while, you'd have to apply a (possibly...

Thanks again, Peter. Right. But that statement ("If a piston capping a cylinder of vacuum is pulled out, producing more vacuum, the vacuum within the cylinder then has more energy which must have been supplied by a force pulling on the piston. ") seems odd too. Why should it be so? Sure...

Thanks all for your input! Let me try to summarize and add a few more notes... ◈ So the ridge-building mechanism seems known: ◈ On the evolutionary advantage (protection from water forces, predators?) we have just speculation. It seems clear that the calcium skeleton of any coral colony is...

Thanks everyone, you really help me understand (a bit more) and your efforts are much appreciated! Right, I get it... a bit better. Baez's article was enlightening. I found Wright's a little harder for my current understanding. I'll keep the "Observational Limits" details for some possible...

Hello, Brain corals, like my avatar's Diploria, get their moniker for visually obvious reasons. I wonder whether there are informed hypotheses how come they look that way. The answer to such a question would probably be partly a morphology/pattern formation story, and partly an adaptive story...

Thank you, both -- those are very helpful answers! :) Right... I didn't realize that 'negative pressure' is special in that sense -- it's not expanded upon much in the book -- "meaning that the pressure sucks inward instead of pushing outward" -- that sounds like an everyday occurrence, but...

Hello, A bit of a long post - still trying to get clear for myself what I really want to ask. You can perhaps skip the background info below and go straight to 'My question is'. Background On p300 of my Penguin edition of "The Fabric of the Cosmos", Brian Greene writes: The recession speed...

Right, thanks... ... and I notice I made a mistake in my previous post: the slope is just a number so all my statements about slopes with units are wrong. Still can't quite pinpoint what about this is throwing me off. Something about the slope expressing ##\Delta t/\Delta x## for both t' and...

Thanks - again both enlightening answers. I think I understand everything that is being said. And yet: my original brain-block remains. It may have to do with understanding slopes more than relativity. :confused: I'm still trying to put into words what I'm seeing, and my intuition (which may be...

Many thanks, all. Those are helpful answers. Please do, if you can. I had a quick stab at it just now, but not quite getting there. But suppose that in the picture mentioned, the t' axis was drawn at a 90° angle to the x' axis. Would the result not suggest that, from the point of view of the...