





Q: How fast does the spot move?
A: It depends how far away the wall is.

Q: How fast can the spot possibly move?

Q: What is moving across the wall?


  • 2
    $\begingroup$ Great answer, or at least I would give the same one. ;-) +1 $\endgroup$ – Luboš Motl Mar 15 '11 at 18:37
  • $\begingroup$ You don't say so explicitly (and I don't have access to the PRL paper), but I assume they either didn't measure the signal velocity or measured it and found that it was less than c. A similar paper: arxiv.org/abs/quant-ph/0407155 $\endgroup$ – user4552 May 31 '13 at 20:55
  • $\begingroup$ I don't see what waving a torch around has got to do with waves propagating along their direction of travel to somewhere there hadn't previously been any waves at all. $\endgroup$ – Adrian May Jul 24 '15 at 15:40
  • $\begingroup$ Adrian May: it sounds like you have a question, but I'm not clear what yet. Maybe expand on your point a bit, and we can improve this answer together? $\endgroup$ – Andrew Jul 25 '15 at 18:59
  • $\begingroup$ @Andrew: Why there is no limit on "how fast can the spot possibly move" ? You can not move your torch with speed greater than speed of light. $\endgroup$ – atom May 4 '16 at 10:05

Let me make an analogy which may be helpful.




  • $\begingroup$ I do not understand this analogy quantitatively. I am not a coastal person and the few times I've been on a beach I've spent admiring anthropic forms ;) Anyhow, my problem is, that the shallow water phase velocity and the shallow water group velocity of the ocean waves is the same. So I do not understand your analogy. Here is a link oceanworld.tamu.edu/resources/ocng_textbook/chapter16/… $\endgroup$ – Approximist Mar 15 '11 at 18:21
  • $\begingroup$ It's certainly not intended to be any sort of literal analogy to water physics. I'm just noting that the point at which the waves breaks can move with speed faster than the waves coming up the shore. This is very obvious and has been observed by all at the beach. Hmmm. Maybe I should add in a photo of actual waves... $\endgroup$ – Carl Brannen Mar 15 '11 at 22:50
  • $\begingroup$ I will look into youtube and try to get this. It sounds interesting. And thanks a lot for answering. +1 $\endgroup$ – Approximist Mar 16 '11 at 6:48


  • $\begingroup$ Hi! Thanks for answering. I was having some trouble convincing myself that this propagating maximum carries no information content. See edit. $\endgroup$ – Approximist Mar 14 '11 at 22:24
