On Wednesday, January 29, 2025 at 1:30:28 AM UTC-7 Alan Grayson wrote:
On Wednesday, January 29, 2025 at 12:24:33 AM UTC-7 Jesse Mazer wrote: On Wed, Jan 29, 2025 at 1:24 AM Alan Grayson <agrays...@gmail.com> wrote: On Tuesday, January 28, 2025 at 9:01:14 PM UTC-7 Jesse Mazer wrote: On Tue, Jan 28, 2025 at 8:54 PM Alan Grayson <agrays...@gmail.com> wrote: On Tuesday, January 28, 2025 at 2:56:32 PM UTC-7 Brent Meeker wrote: On 1/28/2025 6:49 AM, Alan Grayson wrote: I figured you'd jump on my word "separation". You have no idea what I mean? Of course, events with different coordinates are separated in a physical sense. Otherwise they'd have the SAME coordinates! But separated wrt spacetime events means no causal connections; whereas timelike events DO have causal connections. Of course, you know this, so please stop splitting hairs to make an argument. As for relative velocity, if you don't know what I mean, then you don't know what the v means in the gamma function. Again, stop splitting hairs. Oh, about GPS, I will look up this issue, but I was informed of it from a Ph'D in physics from Brent's Ph'D alma mater, University of Texas at Austin. It's surely NOT a distraction if it establishes that results in SR are physically real, not just appearances. AG There's an unfortunate but common confusion. The un-intuitive aspects of special relativity are physically real, but not it the sense that they happen to the moving object. If SR predicts length contraction, is the object is really shorter? (1) It's really shorter in the reference frame where it's moving. (2) It's not shorter in it's own frame. And (3) it's a different degree of shorter in other reference frames where it is moving with different velocities. Just looking at (2) people assume that it means (1) and (3) are just appearances. What's true is that *the contraction, relative to things in some reference frame, with respect to which it's moving, is real. *Brent *It's a baffling result. The LT doesn't tell us what will be MEASURED in a moving target frame being observed from a rest frame wrt length contraction and time dilation, so the result is just an APPEARANCE from the pov of the rest frame; and yet, from the pov of GPS clocks, these effects are real and measureable. This was the conclusion I argued, which is why I referenced the GPS clocks. * Brent's comment wasn't saying there was any disagreement between what coordinates the LT predicts for a given frame and what is really true (or really measured) in that frame, just like I wasn't saying that (see my last response above). You're really deluding yourself by rushing to read every explanation people give you as confirmation of your pre-existing fixed opinions. Jesse IMO you're deluding yourself in one important respect; your insistence that the results of the LT from the pov of some rest frame predicting length contraction in a frame moving wrt to it, can be measured in that moving frame; This statement is hard to follow because you ignore the distinction I made between frames and objects-- *I can't help you if you refuse to use your imagination. A rod or any object moving wrt a fixed source frame using the LT, or an object in moving frame at rest in that frame when the LT is applied from a fixed source frame, will be predicted as contracted. Period. AG* if we have some object whose length we want to talk about, and we know the coordinates of the worldlines of the front and back of the object in the first (source) frame and then use the LT to predict its coordinates (giving us its length) in the second (target) frame, you can't make any general statement about whether the LT will be "predicting length contraction" of the object until you know the velocity of the object itself in each frame. If the object has a higher velocity v_rt in the target frame than its velocity v_rs in the source frame, the LT will predict the object will be contracted in the target frame; on the other hand, if the object has a lower velocity v_rt in the target frame (including the case I analyzed where v_rt = 0) than its velocity v_rs in the source frame, the LT will predict the object is EXPANDED in the target frame, not contracted, compared to its length in the source frame. In the past you disagreed with this, do you still disagree or have you changed your mind? Please give a clear answer on this, telling me whether you now AGREE or DISAGREE that when the rod has v_rt in the target frame lower than its v_rs in the source frame, the LT predicts the rod's length in the target frame is expanded, not contracted. And if you disagree, please address the questions I asked in my last reply to you (the one before my reply to your comment on Brent's post). *The source frame is always fixed if the LT is applied, so offhand I can't say I agree or disagree in this case. If the rod is moving, it is contracted from the pov of the source frame. If you want the source frame to be moving, then the only way to apply the LT is to consider relative motion, with one frame at rest. Offhand I can't say I agree or disagree, except to say that from the pov of whatever frame is fixed, to new target is contracted. AG* *The target frame is moving wrt the source frame. Objects in the target frame are at rest within that frame, and contracted according to relativity. One can also consider a moving rod as the frame AND the object under consideration. This is how to model and analyze a shortened trip to Andromeda. If you have a better way to model it, I am all ears. AG* So we're both correct from different points of view, but you were mistaken to ignore my comments about GPS. Also, to be candid, I don't appreciate your comment that I am rushing to accept an opinion that confirms my pre-existing fixed opinions. You like to focus on coordinates, but the fact is you were mistaken in claiming the LT makes a measurable prediction of what a source frame predicts. It does in the GPS case, but not in the case of what a target frame predicts internally. AG You never addressed my response to you about the GPS in my post at https://groups.google.com/g/everything-list/c/ykkIYDAL3mTg/m/ximYgKzKDAAJ <https://groups.google.com/g/everything-list/c/ykkIYDL3mTg/m/ximYgKzKDAAJ> -- any coordinate system covering a non-infinitesimal region of curved spacetime is non-inertial, and the LT isn't relevant to non-inertial coordinate systems. *An object in free fall is in inertial motion, called a geodesic in GR. The LT is probably applicable for infinitesmal motion notwithstanding that this is occurring in curved spacetime. But I'm NOT an expert on how or why SR is used in GPS to make clock corrections. What I do know is that it IS used, that consequently the LT is likely applied in some way, and I gave this example just to show that whereas the LT does NOT give predictions concerning what is predicted for objects moving wrt a fixed frame, one cannot categorically claim that it never does. IOW, for inertial motion, the LT sometimes gives APPEARANCES, and sometimes gives REALITY. AG* But looking into this a little more, it seems based on p. 2-3 of http://math.bme.hu/~matolcsi/gpsmegjelentejp.pdf that at some point in the GPS calculations they do use an approximation that treats the spacetime around the Earth as flat so an inertial coordinate system can be used, and then they add higher-order corrections to account for the fact that the spacetime is actually curved and this is relevant to gravitational time dilation. But even if there were no gravity and we were just trying to define a GPS-like system to adjust clocks with various states of motion so they were all synchronized in a single inertial frame (as in the 'Suppose for a moment there were no gravitational fields' comment in the second to last paragraph in 'the realization of coordinate time' section of the GPS paper at https://pmc.ncbi.nlm.nih.gov/articles/PMC5253894/#Sec4 ), say the frame where the center of the Earth is at rest, I still don't understand why you think this would indicate any conflict between what the LT predicts and what is measured--the whole point of a GPS system is that the ticking rate of the clocks is being artificially adjusted so it no longer matches the "proper time" of an un-adjusted clock following the same trajectory, but instead matches the coordinate time in some preferred coordinate system you've programmed the clocks to keep pace with. If you have a system of adjustments like this for clocks in flat spacetime where inertial frames can be used, then if you know the adjusted ticking rate of a clock in some source frame (along with the coordinates of its worldline in this frame), you can use the LT to correctly predict the adjusted ticking rate of that same clock in a different target frame. 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 everything-list+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/everything-list/1c5af075-1cba-4ab4-b1e0-75c0d3c6a992n%40googlegroups.com.
