On Tue, Dec 24, 2024 at 12:17 AM Alan Grayson <[email protected]> wrote:
> > > On Monday, December 23, 2024 at 10:04:41 PM UTC-7 Jesse Mazer wrote: > > On Mon, Dec 23, 2024 at 11:47 PM Alan Grayson <[email protected]> wrote: > > On Monday, December 23, 2024 at 9:06:31 PM UTC-7 Jesse Mazer wrote: > > On Mon, Dec 23, 2024 at 8:55 PM Alan Grayson <[email protected]> wrote: > > On Monday, December 23, 2024 at 4:09:38 PM UTC-7 Jesse Mazer wrote: > > On Mon, Dec 23, 2024 at 4:10 PM Alan Grayson <[email protected]> wrote: > > On Sunday, December 22, 2024 at 10:05:54 PM UTC-7 Jesse Mazer wrote: > > BTW, since you seem to be interested in a scenario where the car and > garage are exactly matched in length in the garage frame, something which > isn't true in Brent's scenario, here's a different scenario you could look > at, where I'm again using units where c=1, let's say nanoseconds for time > and light-nanoseconds (i.e. distance light travels in one nanosecond) for > distance. > > --Car's rest length is 25, garage's rest length is 20, car and garage have > a relative velocity of 0.6c, so gamma factor is 1/sqrt(1 - 0.6^2) = 1.25 > > > *OK. * > > > --In garage rest frame, garage has length 20 and car has length 25/1.25 = > 20. In the car rest frame, the garage has length 20/1.25 = 16 and the car > has length 25. > > > *OK, assuming car is moving, but I wouldn't call that "in the car rest > frame" since you have garage length as contracted. AG * > > > BTW I forgot to reply to this line since it was an overall "OK", but just > wanted to note that this is the standard meaning of "[object's] rest frame" > in physics--it refers to the inertial coordinate system where the object, > in this case the car, has position coordinates that don't change with > coordinate time, so the car is said to be "at rest" in this coordinate > system. It is the garage, not the car, that is moving in the car's rest > frame, since the garage's coordinate position does change with time in this > frame--this relative perspective on who is "moving" and who is "at rest" is > just as true in classical mechanics as in special relativity (though of > course there is no length contraction accompanying motion in classical > mechanics), see the discussion of Galilean relativity at > https://www.physicspace.com.ng/2018/09/galilean-relativity-2.html with > Galileo's own discussion of an observer below decks of a windowless ship > who has no way of knowing if the ship is at moving smoothly over the water > or at rest relative to it. If you don't understand this sort of basic > observation about classical mechanics in an inertial coordinate system > (along with other basic observations like the classical relation between > 'length' and coordinates of endpoints of an object, or classical relation > between 'velocity' and the way position coordinates of an object change > with coordinate time), that's something you really need to bone up on a > little before tackling relativity. > > Jesse > > > IMO, the rest frame is defined as the initial conditions in this problem > when the car isn't moving, and is longer than the garage. > > > This isn't really a matter of opinion, just standard terminology; in > physics books (in classical mechanics as well as relativity) you will only > ever see "rest frame" defined relative to specific objects, and you will > never see any reference to "the" rest frame without it being defined > relative to such an object, nor is the phrase "isn't moving" understood as > meaningful unless you add something like "isn't moving relative to [some > other frame or object]". Please don't make up your own terminology, > > > *I'm definitely NOT doing that. Rather, that's how the frame names have > been used throughout this discussion by members of this MB. AG* > > > I doubt you are remembering that correctly, especially if you are > referring to either Brent or myself who have been most active in talking > about the details of specific numerical examples. You can go back through > the threads at https://groups.google.com/g/everything-list , just looking > at Brent's first few posts on this thread, he seems to consistently talk > about what's happening in "the car's reference frame" or "the garage's > reference frame", no comments about "the rest frame" or saying anything > "isn't moving" except in relation to another object, and I'm sure I > wouldn't have done that either. > > > *That's what I've been saying. No one uses "rest frame" when describing > the results in either frame when the car is moving. You introduced that > terminology recently, claiming it is standard. AG* > No one uses the words "the rest frame" in isolation as you did (that was what I was calling non-standard terminology), but they certainly use phrases like "the car's rest frame" and "the garage's rest frame" to refer to the inertial frame where the object in question has a fixed position coordinate (so they have exactly the same meaning as 'car's reference frame' or 'garage's reference frame'). It's more common for examples in relativity to involve rockets, so if you do a search on http://books.google.com for the phrase "rocket's rest frame" (putting it in quotation marks so google understands you want the whole phrase), you will find plenty of textbook examples, and it seems there are even more examples if you use the slightly different wording "rest frame of the rocket". Meanwhile your phrase "the car is moving" is also non-standard terminology since it isn't in relation to any particular object or reference frame. In one frame the car is at rest and the garage is moving, in the other frame the garage is at rest and the car is moving, neither is more correct than the other. 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/CAPCWU3JROKp%3DwLYstbJr3khR27Utqk5r1XEBy%3DF8_h4CxYO4Dw%40mail.gmail.com.
