On Tue, Feb 11, 2014 at 9:23 PM, Edgar L. Owen <edgaro...@att.net> wrote:
> Jesse, > > Let me clarify my response since I see it's slightly ambiguous. > > First every observer in the universe is ALWAYS at the same point in p-time > ALL the time with all other observers. No exceptions. > > The question is what clock times of various observers correspond to a same > point of p-time? > > The answer is that to find out what t of any observer in any relativistic > frame corresponds to any t' of any other relativistic frame you just pause > the experiment so that all relativistic effects freeze at that instant. > Are you just going to completely ignore my point that "at that instant" is ambiguous unless you already know which event on B's worldline occurs at the same "instant" in p-time as an event on A's worldline? Again: if you want to pause B "at the same instant" that A turns 60, but one frame says that at the instant A turns 60, B is 48, while another frame says at the instant A turns 60, B is 75, what PHYSICAL PROCEDURE would you suggest to determine when to "pause" B? (unless of course you acknowledge that p-time simultaneity can't be determined by any physical procedure, and is just an unknowable metaphysical truth) Please don't answer "pause B at the same instant A turns 60" because that's not a physical procedure, just a statement of faith that there is some objective frame-independent truth about B's age "at that instant". > > > If all there is is just non-accelerated, non-gravitational relative > motion, you don't even have to pause the experiment. All you have to do is > note that A's clock in his frame will be the same as B's clock in his > frame, for all t and t' values > Are you saying that even if A and B are *not* at rest relative to one another, as long as they are moving inertially free from gravity you still assume that at a single point in p-time their clocks have the same reading? Presumably this would only be if they had synchronized their clocks at some point in the past, like the moment they passed next to each other at the same point in spacetime? But then see the questions in my other recent post about whether you even agree that clock readings that happen at the same point in spacetime must happen at the same p-time... Jesse On Tuesday, February 11, 2014 7:46:30 PM UTC-5, jessem wrote: > > > > On Tue, Feb 11, 2014 at 7:08 PM, Edgar L. Owen <edga...@att.net> wrote: > > Jesse, > > Your example does NOT establish any inconsistency. I NEVER said "I'm > pretty sure you've said before that you agree that if SR predicts two > clocks meet at a single point in spacetime, their two readings at that > point must be simultaneous in p-time)." That is NOT true. Only if there is > no relative motion or acceleration is it true. I really wish you could just > get the basics of the theory straight. > > > I thought you agreed on my operational definition of "same point in > spacetime", and that events that satisfied this definition would also occur > at the same point in p-time. I wonder if you actually are correctly > understanding what I say in the quoted sentence, because I find it hard to > believe you would deny it if you understood it correctly. > > Let's say we have two twins moving towards each other at some nonzero > velocity, and they pass right next to each other without either one > accelerating. Relativity can be used to predict their respective ages at > the moment they pass (if we idealize them as pointlike observers, the > "moment they pass" can refer to their worldlines passing through precisely > the same position and time coordinates). To use my usual numbers, > relativity might say that twin A is turning 30 and twin B is turning 40 at > the moment they pass. In terms of my operational definition, if A was > sending a continual stream of light signals to B and seeing how long it > took to receive the reflected signal, the time interval on A's clock > between sending a signal and receiving the reflection would approach zero > as his own age clock approached 30, and the age he would see on B's age > clock in the reflected light would approach 40 as he approached 30. > Likewise, if there was a camera at the point in space they passed, and it > took a photo just as they passed, the photo would show A's age clock > reading 30 and B's age clock reading 40. And if A had a bomb that would > destroy anything in his immediate local vicinity but would leave anything > at a distance from him unharmed, then if A set it to go off when he turned > 30, B would be killed at age 40, but if A set it to go off at any other > age, B would survive unharmed. > > Given that relativity would predict all these things, are you saying these > predictions could all be correct, but that A turning 30 and B turning 40 > would *not* be simultaneous in p-time, not even approximately so? Or are > you actually saying relativity would be *wrong* in the predictions above > when it predicts the event of A turning 30 will have the same x,y,z,t > coordinates as the event of B turning 40? Or did you just misunderstand > what I meant when I said "two clocks meet at a single point in spacetime, > their two readings at that point [A turning 30 and B turning 40 in this > example] must be simultaneous in p-time"? Or would you say "none of the > above"? Please give a clear answer to this question. > > > > > The method is trivially simple. I'll give two approaches: > > > 1. Instantaneously pause all relativistic effects at any time t on A's > clock and read the time t' on B's clock. These clock times are a point when > A and B were/are in the same p-time current moment. > > > > "Instantaneously pause" has no frame-independent meaning in relativity, do > you disagree? If A and B are in relative motion, and unlike my example > above, B is *not* at the same point in spacetime as A when A turns some age > (say 60), then different frames disagree on what age B is "at the same > instant" that B turns 60. So if one frame said B was 48 at the same instant > A turned 50, and another frame said B was 75 at the same instant A turned > 50, then at what age should B's motion relative to A be "paused"? We don't > have an "objective instantaneous pause machine" that can settle the > question empirically, it has to be *our choice* when to subject B to a > sudden acceleration to instantaneously bring him to rest relative to A. > Again, do you disagree? > > Since the whole rest of your explanation depends on this notion of an > "instantaneous pause", I'll await a response to this question before > dealing with the rest of your discussion of your "method". > > Jesse > > > > > 2. Do the same thing for any t you wish. The t' that corresponds will be > the clock time in the same present moment of p-time as the t you paused at. > > 3. In general if you want to know what clock time t' of B occurred in the > same p-time as any time t on A's clock, all you have to do is pause the > experiment at t so that all relative motion ceases and just read t' on B's > clock. > > Because this can be done at any point t on A's clock we can always > determine what t' on B's clock occurred in the same p-time as that t simply > by reading B's clock. > > Note this is exactly what happens when the twins meet up in the same > p-time present moment and read each other's clocks to determine what clock > times occurred at the same p-time, in that same common present moment. > > > You can also do this with a calculation as well as by pausing the > experiment. > > 1. Note there are two classes of relativistic effects in the general case: > a. Reciprocal temporary effects of relative motion in which A and B each > see the other's clock slow by the same amount. These effects vanish when > relative motion ceases and A and B do NOT agree on these effects because > they are equal and opposite. No permanent actual age differences are > produced by this type of effect. > b. Persistent and agreed effects of acceleration and gravitation in > which one clock slows permanently relative to the other and both A and B > agree on the amount of slowing. These effects persist after the > relativistic differences vanish. They are permanent. And both A and B agree > on these effects. These effects manifest as real permanent age differences. > > 2. At any desired time t on A's clock, identify, calculate and discard the > effects of relative motion of type a. so that the only effects between A > and B left are of type b., the actual real actual age differences up to > point t on A's clock. We keep only the effects that would be/are permanent > (type b. above) and disregard those that are not (type a. above). > > This is effectively the same as pausing the experiment at any t, because > that is just a simpler method of eliminating effects of type a. > <div style="font-size > > ... -- 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. -- 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. 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