Jesse, Not correct. My present moment does NOT say "that there is an objective common "present moment" for events that are *not* at the same point in spaceTIME (my emphasis)."
My theory says that there is a common universal present moment shared by all points in SPACE, not spaceTIME. Because clocktimes can obviously have different t values within that present moment. Second, thanks for the long explication following, which I more or less agree with. But my question remains: If coordinate time is just an alternate coordinate system then for the twins to be at the SAME place in that coordinate system there must be some actual t-value describing that point that both twins agree upon. What is that t value, and how does it relate to the t values of the clock times of the twins' two different clocks? What is the actual coordinate time t-value of that point in which the twins have different clock time t-values? If coordinate time is just a different choice of coordinate system you must be able to answer this question and provide a t value that is the same for both twins. And of course there simply is NO clock that displays that coordinate time t value is there? Doesn't that make it highly suspect and give my argument some merit? There seems to be NO way to actually measure coordinate time. So as I said, it's just a calculation, not an actual OBSERVABLE empirical FACT. That seems to imply it has no objective reality doesn't it? Thanks, Edgar On Saturday, February 1, 2014 1:21:41 PM UTC-5, jessem wrote: > > > > > On Sat, Feb 1, 2014 at 12:31 PM, Edgar L. Owen <[email protected]<javascript:> > > wrote: > >> Jesse, >> >> Yes, that "being at the same point in spacetime" is CALLED the present >> moment that I'm talking about. >> > > > But your present moment goes beyond that and says that there is an > objective common "present moment" for events that are *not* at the same > point in spacetime. My point is that you have no real argument for > generalizing "there is an objective truth about whether events coincide at > the same point in spacetime" to "there is an objective truth about whether > events occur at the same time, event if they are at different points in > spacetime"--the first does not in any way imply the second. > > >> >> You are probably repeating the claim that 'coordinate time' falsifies >> p-time. It doesn't. Coordinate time is an attempt to explain the obvious >> problems with clock time not actually explaining a common present moment >> that obviously exists. This is done by coordinate time saying OK we have to >> account for the twins being at the same point in spacetime when they >> compare clocks so let's just invent a coordinate system that acts as if >> clock time doesn't have any effect on something we will call coordinate >> time. >> > > No, coordinate time is not meant to "explain" how events can coincide in > spacetime--rather the basic starting assumption is that spacetime has an > objective geometry, different coordinate systems are just ways of labeling > that geometry. Think of a globe, with outlines of continents, rivers etc. > on it. It's certainly true that you can *describe* the shape of a river or > coastline or whatever using some coordinate system defined on the globe > (latitude and longitude for example), but the actual geometry of the > shapes--including the notion of the "length" along a particular path > between two points (like the length along a river between between two > branching points)--is assumed to be more fundamental, prior to any choice > of coordinate system. Physicists think of spacetime like that--it has an > objective geometry, defined in terms of the lengths of any possible path > (whether "timelike", "spacelike" or "lightlike"). Coordinate systems are > just ways of labeling this preexisting geometry, and all coordinate systems > must agree on these more basic "geometric" facts (like the "proper time" > along a timelike path between two events). In general relativity the basic > idea of the "metric" is to translate between coordinate intervals and > "real" geometric quantities like proper time--the equations of the metric > will look different when expressed in different coordinate systems, but in > each coordinate system you can integrate the metric to calculate proper > time along any timelike path, and you'll get the same answer in each case. > > Suppose instead of a globe we are talking about geometry on a flat plane, > which has some roads on it. The geometry of the shape of the roads, the > distance along each road between any two points, is again taken as > fundamental, but here it would be natural to define a Cartesian coordinate > system on the plane to label points, with an x and a y axis. But we have a > choice of how to orient these axes--depending on the angle of the axes > relative to the geometric features like roads, we may get different answers > to questions like "do these two points along the road have the same > y-coordinate or different y coordinates"? This is akin to how in flat > spacetime, we can choose different inertial coordinate systems which give > different answers to questions like "do these two events have the same > t-coordinate or different t coordinates?" > > But clearly for roads on a plane, there is an objective geometric truth > about questions like "do these two roads ever meet at the same point on the > plane?" or "if these two roads cross at points A and B, what is the length > along each road between A and B?" The answers to these questions don't > depend on your choice of cartesian coordinate system. Similarly there is an > objective answer, in terms of the geometry of paths through spacetime, to > questions like "do these two worldlines ever meet at the same point in > spacetime?" or "if these two worldlines cross at events A and B, what is > the proper time elapsed on each worldline between A and B"? > > In contrast, your argument seems to be that in order to make sense of > questions like "how much has each twin aged between the point where they > departed and the point where they reunited", we need an "objective" > t-coordinate which gives a single correct answer to whether two events > happened at the same t-coordinate or different t-coordinates. But in terms > of the analogy, this would be like if someone claimed there was no way to > talk about the distance along different roads between places where they > cross without having an "objective" cartesian coordinate system which gives > a single correct answer to whether two points in space share the same > y-coordinate. Presumably you understand why this is silly in the case of 2D > geometry, so why isn't it just as silly when it comes to the geometry of > paths in 4D spacetime? Can you name any relevant difference between the two > cases that would make an objective coordinate system necessary in one case > but not the other? > > > >> >> Coordinate time is half way to p-time but hasn't incorporated the whole >> insight... It basically says let's pretend clock time doesn't really happen >> so the twins can end up at the SAME point of spacetime because it's obvious >> they actually did. >> > > All coordinate systems are defined in terms of local readings on clocks > and rulers spread throughout space--in most cases these coordinate clocks > and rulers are imaginary, because we can use what's known about physics to > deduce what they *would* read in the neighborhood of any event, but it may > clarify your understanding of "coordinate time" to imagine that such a > network has actually been physically constructed. See > http://www.upscale.utoronto.ca/GeneralInterest/Harrison/SpecRel/SpecRel.html#Exploringfor > a diagram of what the network would look like to define position and > time in an inertial coordinate system, and section 2.1 on p.8 of > http://physics.mq.edu.au/~jcresser/Phys378/LectureNotes/VectorsTensorsSR.pdffor > a diagram of how this can be extended to arbitrary non-rectilinear > coordinate systems. > > So with that in mind, a statement like "the event of twin A turning 30 and > the event of twin B turning 40 both happened at the same spacetime > coordinates in my frame, x=5,y=10,z=0,t=10" just means that when twin A > turned 30 and Twin B turned 40, they were both next to the same coordinate > clock which read a time of 10 when those events happened next to it, and > that particular clock the one that's attached to the x=5 marking of an > x-axis ruler, the y=10 marking of a y-axis ruler, and the z=0 marking of a > z-axis ruler. So it's really just a statement about 3 different clock > readings coinciding at the same point in spacetime, rather than just the > original 2--which means the notion of "coinciding at the same point in > spacetime" has to be more basic than "happening at the same coordinate > time", and indeed once you grant this basic geometric notion, you can > understand "the event of twin A turning 30 coincided at the same point in > spacetime with the event of twin B turning 40" without any need to refer to > the fact that both events *also* coincided with the event of that > coordinate clock reading 10. Spacetime geometry is the fundamental thing, > not coordinate systems. > > 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]. 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