On Sat, Feb 1, 2014 at 9:00 AM, Edgar L. Owen <edgaro...@att.net> wrote:
> > > And of course it is OBVIOUS that the twins share a common present moment > when they compare clocks. Otherwise they couldn't compare clocks now could > they? > The fact that they can compare clocks, and agree for example that "twin A's turning 30 coincides with twin B's turning 40", is because they are making the comparison at the same point in spacetime (assuming ideal point-like observers*), and in relativity, all observers agree on which events coincide at the same point in spacetime (I've asked you several times whether you agree this is always true in relativity but you have refused so far to answer). Another way of putting it is that in any spacetime coordinate system for labeling the space and time coordinates of different events, the event "twin A turns 30" would have to be labeled with the same coordinate time (and coordinate position) as the event "twin B turns 40". In no way does any of this imply the notion of an objective "common present" for events which do NOT coincide at the same point in spacetime. *Of course real observers aren't point-like, but if you think of extended observers who compare clocks a few feet apart, there could be disagreements over which event happened first in precise terms (whether we're talking about visual observations of the events, or which event happened first in some inertial frame), but we can say that everyone would agree the two events happened within a few tiny fractions of a nanosecond from each other, so for all practical purposes there is no disagreement about the fact that twin A turning 30 coincided with twin B turning 40. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
