----- Original Message ----- From: "Brad DeLong" <[EMAIL PROTECTED]> To: "Killer Bs Discussion" <[EMAIL PROTECTED]> Sent: Sunday, April 25, 2004 2:08 PM Subject: Re: does time exist
> >Brad said: > > > >> But isn't our intuition wrong--or perhaps it would be better to say > >> that our intuition does not prepare us to study quantum mechanics > >> and relativity? It's true that brains that have our intuitions of > >> space and time tend to help the selfish genes that program them > >> replicate themselves. But "fitness" is not the same thing as > >> "truth"... > > > >Indeed not. But, so far as I can tell, Dan isn't saying that we have > >intuitions about time that may or may not be correct, but that time > >*exists* because we have intuitions. I presume he means that something > >in our brain organises sense impressions into a spatial and temporal > >structure. I don't doubt that this occurs, but like you I think that > >this structuring only gives us an approximation to what's out there in > >nature. Furthermore, I think that it presupposes at least some kind of > >temporal structure (although perhaps I could be convinced that what > >looks like a temporal structure is in fact a constraint on spatial > >patterns in some kind of universe without time, or without temporal > >flow [as, indeed, the universes in some theories of quantum gravity > >might be, what with the vanishing of the Hamiltonian and all]). > > > >Rich > > > >_______________________________________________ > >http://www.mccmedia.com/mailman/listinfo/brin-l > > > When I turn over the CD I am holding in my hands, and look at the > rainbow pattern of light coming from the overhead lamp, hitting the > CD, and being reradiated back into my eyes, isn't the rainbow > diffraction pattern pretty strong evidence that the photons spread > out over all space and all time (with some amplitude) on their > journey from the CD to my eye? In a word, no. Let us consider light that originates from a very small light bulb, and is radiated in all directions. Some of the light hits the disk and is reflected into your eye. The reflection angle is dependant on the frequency of the light (and on the nature of the CD surface, of course). Let us now consider just a short burst of light from that lamp. It is connected to a switch that can turn in on and then off very quickly. Let us replace your eye with an instrument that can record such a short burst, and is also able to determine the wavelength of the light. Given that the total distance from the lamp to the CD to the instrument is about 3 meters, we have a transit time of no more than about 10^-8 sec. If we have an equally short pulse of light, we would have a span of little more than 2x10^-8 seconds to consider. Let us consider two events: the start of the pulse at the lightbulb, and the end of the recording of light at the instrument. These are two positions A, and B in spacetime. With reflection, the two events can be seen to be spacelike, and not timelike. Thus, A (which is the start of the pulse) is always before B, (which is the end of the recording of light.), no matter which reference system one is standing in while considering the two events. Further, nothing before A or after B can influence the events. We can also discuss spacial limitations. We see that only that space that is is the after half of the light cone for A and the before half of the light cone for B need be considered. Thus, we have a relatively restricted volume of spacetime that needs to be considered. Dan M. >Isn't that difficult to reconcile with > a temporal structure in which some things "were," other things "are," > and still other things "will be," and in which the "are" becomes > "were" and the "will be" becomes "are"? Given what I've said, not really. Causality is unidirectional. Jokes about preliminary exams in physics included proposed questions like: "discuss time reversal in physics. Be sure to include the influence of Einstein on Newton and Galileo." We need to remember that science describes what we observe. It is neutral about the inherent reliability of observations vs. ultimate reality. That's a philosophical, not a scientific question. Dan M. Dan M. _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
