On Thu, Dec 19, 2024 at 5:44 AM Alan Grayson <[email protected]> wrote:
> > > On Thursday, December 19, 2024 at 3:05:17 AM UTC-7 Alan Grayson wrote: > > On Thursday, December 19, 2024 at 1:21:05 AM UTC-7 Jesse Mazer wrote: > > On Thu, Dec 19, 2024 at 1:05 AM Alan Grayson <[email protected]> wrote: > > On Wednesday, December 18, 2024 at 4:04:43 PM UTC-7 Jesse Mazer wrote: > > On Wed, Dec 18, 2024 at 4:58 PM Alan Grayson <[email protected]> wrote: > > On Wednesday, December 18, 2024 at 2:42:39 PM UTC-7 Brent Meeker wrote: > > On 12/17/2024 11:21 PM, Alan Grayson wrote: > > On Tuesday, December 17, 2024 at 10:16:51 PM UTC-7 Brent Meeker > wrote: > > On 12/17/2024 7:52 PM, Alan Grayson wrote: > > On Tuesday, December 17, 2024 at 6:57:28 PM UTC-7 > Alan Grayson wrote: > > On Tuesday, December 17, 2024 at 2:33:46 PM > UTC-7 Brent Meeker wrote: > > On 12/17/2024 9:25 AM, Alan > Grayson wrote: > > Yes, you look at it just in terms of lengths, which is what I did in the > first pair of diagrams. But the relativity of simultaneity is another > way to look at the same problem, which is what I showed in my last posting. > > *Another way, but not the only way. AG * > > We seem to be on the same page concerning use of length contraction to > explain the > differing results in the frames under consideration. But I remain unclear > how the > disagreement of simultaneity can also give the same results. For example, > suppose > from the pov of the garage frame, the car fits in the garage for > sufficient v, with room > to spare, but the front and rear end EVENTS do not Lorentz transform into > simultaneous > events in the car frame. Can't there be other ways for the car to fit, > using another set > of events which* are* simultaneous in the car frame? AG > > Sure. If the car's speed was just right, it would be the same length as > the garage. Then in the diagram A and B would be at the same time in the > garage frame the car would be just the right length such that the rear of > the car entered the garage just as the front exited the garage. Since we > know the car is 12 long and the garage is 10 long we can calculate the > required speed from 10/12 =sqrt{1-v^2} which yields v=0.553 if I did the > arithmetic right. > > > That would be 0.553c. So, if the front and back events in the garage frame > are simultaneous in the car frame AND in the garage frame, > > Nobody said that the events were simultaneous in the car frame. The car > is contracted in the car frame. You keep throwing shit in problem just to > keep it going. I'm starting to suspect you're just a troll. > > Brent > > > > *My question for you is this; when will you learn to read English? You act > like an uneducated prick who can't read basic English. The consensus view > in the physics community is that the solution to this problem involves > disagreement about simultaneity. I don't see this as correct. For example, > that's what Quentin wrote several times, mocking me, and that's what a link > claimed, without proof, which someone posted. And even Jesse, if I read him > correctly, claims that the result in one frame must be false if there's no > simultaneity.* > > > What do you mean by "if there's no simultaneity"? What I said was that the > prediction of the two frames would disagree about local events (a genuine > physical contradiction) in an imaginary universe where both inertial frames > *did* agree about simultaneity (i.e. there is no relativity of simultaneity > like in the real-world theory of relativity) but where they still each > predicted objects in the other frame would experience length contraction. > > Anyway, it'd be helpful if you'd go back to that last comment of mine and > answer my questions about whether you understand how classical space/time > plots work, and also whether you understand that in relativity you have to > use the Lorentz transformation on the coordinates of an event labeled in > one frame to find the "same event" in a different frame, with the result > that any *specific* pair of events on the front & back of the car that are > simultaneous in the car frame are non-simultaneous in the garage frame > (although in the garage frame you can find a *different* pair of events on > the front & back of the car which are simultaneous in the garage frame but > not the car frame, which is what Brent was talking about). > > Jesse > > > *Yeah, I understand that we must use the LT to transform between inertial > frames. AG* > > > I wasn't just asking about transforming between frames in general, I was > asking if you understand that coordinates in a given frame are used to > identify individual physical events, and the LT are then used to identify > the coordinates of the "same event" in a different frame (this implies that > a pair of events at the front and back of the car which are simultaneous in > some frame *cannot* be simultaneous in any other frame that's moving > relative to that one, at least not in a problem with only one spatial > dimension). > > Also, you didn't address my question about if you understand how > *classical* plots of position vs. time work, which I asked because I was > trying to figure out what aspect of my comment about worldlines in > relativity you couldn't follow. Here again was my question, in a post you > didn't reply to: > > 'Can you try to be specific about what aspect of it you find hard to > follow? First of all, do you feel you have a good grasp of plots of > position vs. time in classical physics, where there is no disagreement > about simultaneity or time or distance intervals, or do you need a > refresher on the classical graphs before trying to follow the relativistic > ones? Do you understand for example why if we had a classical graph with > various lines or curves representing the worldlines of objects, with time > on the vertical axis and position on the horizontal axis, then if we wanted > to know the position of all the objects at a particular time, that would > involve drawing a horizontal line of fixed time coordinate (a classical > line of simultaneity, which doesn't change from one frame to another) and > seeing where the horizontal line intersects with the worldlines of the > objects?' > > > * but if the events of measuring front and back in garage frame, for a > perfect fit, why do we care how they transform in the car frame, since the > problem is completely solved using length contraction?* > > > I have already explained the point here is pedagogical (in several other > posts that you never responded to--I think it would help the discussion if > you would respond to every post where I ask you a question, instead of > taking a sporadic approach). Here was what I said in the first post where I > made this point: > > 'The reason physicists bother to talk about a hypothetical scenario like > this is pedagogical, they want to get students to think about situations > where the perspective of different frames might *seem* to lead to real > physical contradictions, and then looking at it more closely they'll > understand how the "real" physical predictions in relativity are always > about local events, and that by considering different definitions of > simultaneity we can show the two frames do agree about all local events on > rulers and clocks. > > > *"Pedagogical" means what? * > Relating to how the subject is taught, in this case specifically which concepts any teacher would see as important for students to understand. If a student doesn't understand that different frames agree on all local events, then they basically don't understand the first thing about how relativity works. > *If car fits in one frame and not in another, isn't that what we would > expect, and yet in my prior post I wrote that this seems contradictory? Why > do you expect the frames must agree about this kind of local event? To > avoid a contradiction? AG* > As long as the laws of physics are Lorentz-invariant, that guarantees that when different inertial frames apply the same equations (including length contraction) they will get locally identical predictions, assuming they both are using initial conditions which are equivalent under the Lorentz transformation. But are you asking a different question about what is the motive for demanding that any claims about how things work in different frames needs to pass the test of giving identical local predictions, in order to qualify as good physics? If so just consider that there are all sorts of local interactions in physics, like collisions, that cause changes that different frames couldn't disagree about without being obviously inconsistent. For example, say you have a clock that's wired to a small bomb that will cause a localized explosion, which will be triggered when it reads 100 seconds. And say you have another object in motion relative to the clock/bomb, say a glass of water, which is going in the opposite direction so they will cross paths. Imagine different frames could disagree in their prediction about whether the event of the clock/bomb crossing paths with the glass of water coincided was at the same local point in space and time as the clock reaching 100 seconds--like, one frame predicts the clock reads 90 seconds when they cross paths, a second frame predicts the clock reads 100 seconds when it crosses paths with the glass of water. In this case, the second frame would predict the glass of water was right next to the bomb when it exploded, and so predicts that the glass will be broken up after the encounter. Meanwhile the first frame would predict the glass of water has already put some distance between it and the bomb by the time the bomb exploded, so the glass would be intact after the explosion. This is a clear physical contradiction, no? They can't both be right, and you could easily falsify one frame's prediction just by looking at the glass afterwards. On the other hand, if all frames agree in all their predictions about local events as in relativity (assuming Lorentz-invariant laws of nature), then you don't get any contradictory predictions about such localized physical interactions which affect the state of objects later. You may find it counter-intuitive that they still differ in some kind of non-local bird's-eye account of what happened, but you can't point to any differences they will see on any measuring-instruments (since instrument readings are also local events), like what a clock mounted on the back of the car reads as it passes by the front of the garage. > > > Do you disagree with my point that if different frames *didn't* have > differing definitions of simultaneity, it would be impossible for the two > frames to disagree about whether the car or garage was shorter without this > leading to conflicting predictions about local events, like what the clocks > mounted to front and back of the car will read at the instant they pass > clocks attached to the front and back of the garage?' > > > *I don't see how simultaneity or not helps in this situation. It seems > impossible for the car to fit when in motion. AG * > It helps by showing how the car can fit in the garage's frame without leading the garage frame and the car frame to disagree in a single prediction about local events. Does your "seems impossible" just mean you find it counter-intuitive, not that you have a concrete argument about why you think it *would* lead to disagreements in predictions about local events? > > And in a later post, I elaborated on why differences in simultaneity are > critical to avoiding contradictory predictions about localized physical > events: > > 'In an imaginary alternative physics where different frames had no > disagreement about simultaneity but different observers still all believed > the length contraction equation should apply in their frame, then this > would be a genuine paradox/physical contradiction, because different frames > would end up making different predictions about local events. Think about > it this way--if there were no disagreement about simultaneity, there could > be no disagreement about the *order* of any two events (this would be the > case even if observers predicted moving clocks run slow like in > relativity). But if observer #1 thinks the car is shorter than the garage, > he will predict the event A (the back of the car passing the front of the > garage) happens before event B (the front of the car reaches the back of > the garage), and if observer #2 thinks the car is longer than the garage, > he will predict B happens before A. If there were no disagreement about > simultaneity this would lead them to different predictions about readings > on synchronized clocks at the front and back of the car/garage at the > moment of those events, specifically whether the clock at A would show a > greater or lesser time than the clock at B.' > > Jesse > > > *Jesse; in the near future I will try to address each of the issues you've > raised,* > > OK, please prioritize answering the question about whether you understand the basics of how position vs. time plots work in classical mechanics, because that really is a crucial prerequisite if you want to hope to understand anything about spacetime diagrams in relativity. If you don't understand it I'm sure I could find a site that lays out the essentials. And as a follow-up, did you ever study the basics of algebraic geometry? Like if you had to plot a function like y = 4x + 5 on a graph with x and y axes would you know how to do it? Likewise would you know the algebra needed to figure out where that function intercepts with another one like y = 2x +10? > * but for now let me just say I don't understand how to resolve this > issue, and my tentative pov is that relativity just isn't correct. Listen; > we start in a rest frame of a car which is longer than a garage. and have > no problem asserting that it won't fit. And that's how things seem from > both entities with physical observers. So far so good. Now we imagine the > car in motion and apply length contraction in both frames and we get > opposite results; namely, that in the car's frame, it won't fit in the > garage, but in the garage frame it does fit, and the fits gets easier as > the car's velocity increases. If I imagine a real car and a real garage, > from one frame it doesn't fit, the car's frame, and from the other frame, > the garage, it does fit. So, if intially the car doesn't fit, from the pov > of both physical entities should I expect contrary results when the car is > in motion? Maybe so. But I still can't wrap my head around the alleged > claim, that the observed reality will be frame dependent. I mean, how can > two observers in different frames, looking at a real car, disagree on what > they see?* > > What do you mean "see"? Are you talking about what they see visually, in terms of when light from different events reaches their eyes? If so, do you understand that when we talk about "simultaneous" events in any frame, we are *not* talking about events that are seen simultaneously in a visual sense by an observer at rest in that frame, unless the observer happens to be positioned equidistant from both events? This was another point I made in an earlier post (at https://www.mail-archive.com/[email protected]/msg97741.html ) which you didn't respond to: 'Note that when we talk about what happens in a given frame this is not what any observer sees with their eyes, it's about when they judge various events to have happened once they factor out delays due to light transit time, or what times they assign events using local readings on synchronized clocks that were at the same position as the events when they occurred. For example, if in 2025 I see light from an event 5 light years away, and then on the same day and time in 2030 I see light from an event 10 light years away, I will say that in my frame both events happened simultaneously in 2020, even though I did not see them simultaneously in a visual sense. And if I had a set of clocks throughout space that were synchronized in my frame, when looking through my telescope I'd see that the clocks next to both events showed the same date and time in 2000 when the events happened.' > * Incidentally, I just noticed that in one of Brent's recent posts with > two diagrams, he says there is a disagreement about simultanaeity, but I am > not sure if he's referring to comparing the two frames, and when I > interpreted this as his comparison, he got angry, denying my > interpretation. My bias is that the frames should agree (on what a bird's > eye observer would see?), but does that require disagreement about > simultaneity? AG* > > What does "bird's eye observer" mean, if it's supposed to be something more than just the sum total of all local events? Jesse > -- You received this message because you are subscribed to the Google Groups "Everything List" group. 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