On Wednesday, December 25, 2024, Alan Grayson <[email protected]> wrote:
> > > On Wednesday, December 25, 2024 at 5:14:21 PM UTC-7 Jesse Mazer wrote: > > > > On Wednesday, December 25, 2024, Alan Grayson <[email protected]> wrote: > > *Why do refer to transformations that don't preserve time ordering? IIUC, > such transformations only occur when assuming motion faster than light. * > > > No, that’s not correct. Motion faster than light would be required if > there was a claim of causal influence between events with a spacelike > separation; but there’s no such claim here; in both Brent’s example and > mine, if we consider the event A of the back of the car passing the front > of the garage and the event B of the front of the car reaching the back of > the garage, there is a spacelike separation between those events, and > neither event has a causal influence on the other. > > > *I'm asking a general question. Why do you refer to failure of time > ordering? What was the point you thought you were making? AG* > > > Because as you previously agreed, the question of whether the car fits > reduces to the question of whether the event A = back of car passes front > of garage happens before, after, or simultaneously with the event B = front > of car reaches back of garage. Since these events have a spacelike > separation in both Brent’s and my numerical examples, in relativity > different frames can disagree on their order, that’s the whole reason we > say frames disagree on whether the car fits. > > > *As I recall, you were writing about the failure of TIME ordering, and > this would mean violation of causality, not what we're discussing on this > thread. AG * > You either recall incorrectly or misunderstood at the time, but disagreement about the time ordering of two events A and B does NOT imply any violation of causality; it just implies the spacetime interval between A and B is spacelike, but normally this is combined with the assumption that there are no causal influences between events with a spacelike separation. Do you understand what the spacetime interval is? If I gave you the difference in time coordinates T = tB - tA for the two events along with the difference in position coordinates X = xB - xA, would you know how to calculate the spacetime interval and judge whether it is timelike, spacelike or lightlike? > > *But if so, you're not within the postulates of SR, which is what this > discussion is about. So what point do you think you're making? AG* > > *Re: paradox: Assume there's an observer located in the garage. This > observer is in the garage frame. This observer sees the car easily fit in > the garage. Imagine another observer riding in the car. This observer is in > the car frame and observes being in the garage but never fitting in the > garage. What are the observations when the two observers pass each other, > in juxtaposed positions?* > > > I’ve asked this before, but by “see” do you mean in terms of when the > light from different events reaches their eyes, or something more abstract > like a computer animation they create of when events occur in their frame, > once they have measured the time and position coordinates of all events > using local readings on rulers and clocks at rest relative to themselves? > > > *Nothing more abstract. One observer sees the car sticking outside the > back of garage, the other sees it inside, when both are juxtaposed. * > > > You didn’t quite answer my question—you are just talking about what they > see with their eyes, right? > > > *I used the word "see". Is this not clear enough? AG* > Not entirely, since it’s routine in relativity problems to use words differently from everyday speech, for example in ordinary speech when you talk about “observing” some event we are usually talking about visual sight, but in relativity talking about what someone “observes” always refers to how things happen in the coordinates of their frame, not to visual sight. > > > If so, there is no disagreement between observers passing through the same > point in spacetime about whether the car fits in a visual sense. > > > *Really? So if the garage is 10' long in rest frame, * > > Do you mean 10’ in the garage’s rest frame? As I said before, just using “rest frame” without specifying a particular object is unclear. > > *and car is .00001' long in garage frame when car is moving, and car is, > say, in center of garage, the observer in car frame, residing inside car, > won't observe his car just won't fit in garage because of huge contraction > of garage in car frame, when both observers are juxtaposed, presumably at > the same point in spacetime?* > You would have to specify more details, like the rest length of the car and the relative velocity of car and garage and the location of the observers, in order to determine whether both observers at that point see it fit or both observers see it not fit. But suffice to say *if* an observer at rest relative to the garage is visually seeing the car fit when the observer is passing through a given point in spacetime, then an observer at rest relative to the car who is passing through that same point in spacetime is also visually seeing the car fit (even if the car does not fit in terms of local position and time measurements in his frame), this is a straightforward consequence of all frames agreeing about local configurations of photons at a single location in spacetime. I could give a numerical example at some point to illustrate this, but if you couldn’t follow my earlier numerical example I doubt this would be clear to you either, which is why I suggest it would be a good idea to return to my last response to one of your “?” responses on that example and continue from there. Jesse > > The question of which photons from which events on the past light cone of > that point are arriving there at that moment is a question about the local > configuration of particles (photons) in that region, i.e. a question about > local physical facts. If the photons from each end of the car arriving at > that point were emitted from points where each respective end of the car > was in the garage, both observers see it fitting in the visual sense of > both ends appearing to be inside the garage. > > But as I pointed out to you earlier, this is not what physicists generally > mean by fitting, since even in classical physics with no length contraction > and no disagreements over simultaneity, as long as light travels at a > finite speed you can have a situation where some observer *sees* both ends > of the car inside the garage even though they are never simultaneously > inside in any inertial frame’s coordinates. > > > For example, if the observer is located at the front of the garage, they > will see the back end of the car pass the front of the garage as soon as it > happens, but they will be getting a delayed image of the front of the car, > so they may be seeing an image of when it was still in the garage even > though according to the definition of simultaneity that is shared by all > classical frames, it has really passed through the back of the garage by > that moment (because the car is longer than the garage). It’s likewise > possible to construct a classical example where the observer is located > closer to the back of the garage and due to light delays they never see the > car fit in a visual sense even though it does fit in terms of simultaneity > of all classical frames (because the car is shorter than the garage). > > 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 view this discussion visit https://groups.google.com/d/ > msgid/everything-list/a63520c8-3828-4194-b5c3- > 97b12505ec97n%40googlegroups.com > <https://groups.google.com/d/msgid/everything-list/a63520c8-3828-4194-b5c3-97b12505ec97n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "Everything List" group. 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