On Sat, Jan 25, 2025 at 9:34 PM Alan Grayson <[email protected]> wrote:
> > > On Saturday, January 25, 2025 at 6:47:22 PM UTC-7 Jesse Mazer wrote: > > On Sat, Jan 25, 2025 at 8:07 PM Alan Grayson <[email protected]> wrote: > > On Monday, December 9, 2024 at 2:01:28 PM UTC-7 Brent Meeker wrote: > > > > > Nothing odd about dilation and contraction when you know its cause. > > But what is odd is the fact that each frame sees the result > > differently -- that the car fits in one frame, but not in the other -- > > and you see nothing odd about that, that there's no objective reality > > despite the symmetry. AG > > The facts are events in spacetime. There's an event F at which the > front of the car is even with the exit of the garage and there's an > event R at which the rear of the car is even with the entrance to the > garage. If R is before F we say the car fitted in the garage. If R is > after F we say the car did not fit. But if F and R are spacelike, then > there is no fact of the matter about their time order. The time order > will depend on the state of motion. > > Brent > > > Jesse; it's the last two of Brent's sentences that I find ambiguous. What > does he mean? > > > What about them do you find ambiguous? > > > > > He's just saying that if there's a spacelike separation between the events > F and R (as there was in his numerical example), then you can find a frame > where R happens after F (as is true in the car frame where the car doesn't > fit), and another frame where F happens after R (as is true in the garage > frame where the car does fit). > > > *What does he mean by "But if F and R are spacelike, then there is no > fact of the matter about their time order."? (What you wrote above?) More > important I just realized that in the frame of car fitting, the events F > and R aren't simultaneous, so how does one apply disagreement on > simultaneity when one starts with two events which are NOT simultaneous? AG* > I didn't use the word "simultaneity" in my previous comment, and Brent didn't either in the comment of his you quoted. Disagreement about the order of non-simultaneous events is different from disagreement about simultaneity, but they are connected: one way to think about the connection is that if all frames agreed about simultaneity (as they do in the Galilei transformation of classical mechanics), they would also all necessarily agree on the order of any pair of events. So disagreement about simultaneity is a necessary condition for disagreement about order of events which aren't simultaneous in either frame, though to figure out the details of disagreement about order (for example to get the conclusion that different frames can disagree on order when events are spacelike separated, but not when they are timelike separated) you have to look at the specific t --> t' equation that's used in the Lorentz transformation. > > > I also wonder what happens when we transform in the > reverse direction from the pov of simultaneity, from the car frame to the > garage frame? TY, AG > > > Brent didn't mention a direction in which the transformation is being > taken, but regardless of whether you start with the coordinates of F and R > in the garage frame and transform to the car frame, or start with the > coordinates of F and R in the car frame and transform to the garage frame, > you get the same answers about what the coordinates of these F and R are in > each frame. For instance if you start with the coordinates x,t of F in the > garage frame and apply the LT > > > *But don't you have to start with two events which are simultaneous in one > frame, to get a disagreement in simultaneity in a second frame, but F and R > are not simultaneous in car fitting frame? AG* > F and R are not simultaneous in either frame in Brent's example, but disagreements about order can be seen by applying the LT to the coordinates of non-simultaneous events in much the same way that disagreements about simultaneity can be seen by applying the LT to the coordinates of events which are simultaneous in the frame you start from. 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/CAPCWU3Lv9ZJ0o2f%3DPieeWp65FGLiZDJNkh3aq6XqXVD8MEU5sg%40mail.gmail.com.
