Jesse, OK, what I don't understand in this clearer example near the end of your post is you say "The coordinate time of an event *is* just clock time on the local coordinate clock that was at the same point in spacetime as the event".
This clock, call it C, on the grid that was at the same point in spacetime as the meeting event (which takes place on earth) is also a clock on earth, at earth's location on the grid. Twin B's clock also stayed at that exact same x,y,z point on the coordinate grid during the trip, and there was no relative motion between B and C. So why does B's clock read 40 years and clock C, which you claim gives the t-value of the meeting event, read 50 years when they were both at the same location during the trip? Aren't you mistaken here since clocks B and C are comoving throughout the duration of the trip and thus must remain synchronized? If that is true you seem to be saying that we must preferentially take the stay at home twin's clock time as the correct t-value of the same point in spacetime that the meeting occurs, the clock time of the observer that didn't move from the start to end point. Is that correct? If so, again it's just a definition, and a strange one at that, because no matter if the traveling twin resets his clock to that t-value you claim is the correct/natural? t value of the meeting event, his age still remains just 30. Edgar On Thursday, February 6, 2014 6:12:10 PM UTC-5, jessem wrote: > > > > On Thu, Feb 6, 2014 at 5:30 PM, Edgar L. Owen <[email protected]<javascript:> > > wrote: > >> Jesse, >> >> You are misunderstanding most of my points here! >> >> By standard I just mean any usual analysis that computes the correct >> answer of the twins' clock time differences when they meet. It seems to me, >> correct me if I'm wrong, that your coordinate time analysis just comes up >> with the exact same clock time differences using a different coordinate >> system. Is that not so? >> > > Different than what, exactly? Did I mention any other coordinate system? > > >> >> Of course you need some coordinate system to do relativity calculations. >> I never claimed you didn't. >> > > You said "I fail to see what we get out of this approach that standard > relativity calculations don't give us", where "this approach" referred to > my coordinate-based analysis. So that suggested you were drawing a contrast > between "this approach" and some distinct "standard relativity > calculations". What did you mean by that latter phrase? > > > >> >> It wasn't I that said a coordinate time analysis wouldn't give the >> correct answer. I said it doesn't give any calculation of what the present >> moment IS in which its calculated results occur which they must have to to >> make sense. >> > > By "what the present moment IS" do you mean your own p-time, as opposed to > just clock time simultaneity? If so, I simply don't see why you "must have" > such a p-time in order for these results to "make sense", the notions of > "same point in spacetime" and "coordinate time" which I have been making > use of seem perfectly adequate to me. I thought you were trying to make an > *argument* as to why they are inadequate on their own, one that goes beyond > "it's too counterintuitive" or "it doesn't match our qualitative conscious > experience of time". If you have such an argument, please present it, > making no reference to conscious experience or intuitions! > > > >> I thought you said, contrary to my thinking, that coordinate spacetime >> would do that >> > > Do what? Establish an absolute definition of "present"? If so, of course > not, I never suggested such a thing...if something you mean something else > by "do that", please elaborate. > > > >> but I don't see it doing it and you agreed that is an independent >> definition, so I don't see the sense of your diversion into coordinate time >> > > Because you kept asking me questions about it! > > >> Re your last paragraph: First what do YOU mean by proper time? Do you >> simply mean their clock times on their clocks or some other time? >> > > Yes, that's what "proper time" means in relativity, the proper time for > any observer (or other objects) is just the time that they would measure on > a clock carried with them. Usually physicists talk about the proper time > interval between a pair of events on a given object's worldline, to avoid > ambiguity about when the clock was set to read "0". > > > > >> >> And you say in the last paragraph "then the event of twin A turning 30 >> is assigned the same t-coordinate as the event of twin B turning 40." Who >> does this assigning? And what time is the "then" in which the assigning >> takes place? >> > > The coordinate system I have been discussing. > > > >> >> Is this just some arbitrary assignment after the meeting, >> > > No, it's the reading on the coordinate clock that was at the same point in > spacetime as the meeting. > > > > >> in the same sense that you said that the same point in spacetime had to >> be independently defined? What is "the same t-coordinate in which A turns >> 30 and B turns 40"? What's the value of that t-coordinate that is not the >> same as the different clock times? >> > > > Again, the coordinate clock. I already explained this in detail several > times, this was the point of my introduction of the third clock besides the > clocks of the two twins, the coordinate clock which I labeled "clock C" > that I discussed in > https://groups.google.com/d/msg/everything-list/jFX-wTm_E_Q/m91rxoG5LvkJand > https://groups.google.com/d/msg/everything-list/jFX-wTm_E_Q/JJKlMk7vDNYJ > > > >> And what type of time coordinate is it? Clock time, coordinate time, >> proper time? >> > > > The coordinate time of an event *is* just clock time on the local > coordinate clock that was at the same point in spacetime as the event. And > of course all clock times are also proper times for the clocks themselves. > > >> And finally you say "for example both events might be assigned a time of >> t=50 in some coordinate system". That just seems you are saying that it's >> possible for the twins to reset and synchronize their clocks after they >> meet which is obvious. >> > > No, t=50 is the time on the coordinate clock. The twins don't do anything > to their clocks. > > > >> >> But even if they do that, one twin still is REALLY younger than the >> other. That real actual time disparity can NOT be reset. There is a real >> absolute time and age difference that relativity can CALCULATE but >> relativity CANNOT explain why that time and age difference exists in the >> same present moment the twins share. >> > > I don't know what counts as an "explanation" for you--presumably any > explanation that didn't make use of p-time would *by definition* be > unsatisfactory from your perspective--but conceptually I am satisfied with > the geometric explanation. If I lay out two tape measures on the floor that > both have their 0-mark at the same point A, then the two tape measures take > different paths and meet again at a second point B, the marking on each > tape measure at B may be different because one tape measure's path between > A and B may be straighter (and thus shorter) than the other's. It's pretty > much the same for paths between a pair of events in spacetime, although > because of the way spacetime geometry works, the "straighter" path through > spacetime is actually the one with the *longer* proper time interval. > > > >> >> So again I don't see the coordinate time approach adding anything to the >> discussion. It still, correct me if I'm wrong, does NOT calculate the fact >> that the twins meet up with different clock times in a SAME present moment.. >> > > It doesn't include an *absolute* present moment. But it does calculate > that they meet up with different clock times at the same coordinate time > and position, and thus at the same point in spacetime as defined by the > light signal method. Again, are you simply saying in advance that nothing > can count as an "explanation" if it isn't based on your own preferred > metaphysics of presentism? > > Jesse > -- 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 [email protected]. To post to this group, send email to [email protected]. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.
