Known physics defeated by simple puzzle?
in [Relativity]
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From: Ste on 6 Apr 2010 21:02 On 6 Apr, 23:25, Edward Green <spamspamsp...(a)netzero.com> wrote: > On Apr 5, 7:41 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > > > > > > On 5 Apr, 22:10, PD <thedraperfam...(a)gmail.com> wrote: > > > It contracts to fit the barn in the rest frame of the barn, which is > > > why the ladder makes no marks on the barn doors when the doors are > > > shut at the same time. In the rest frame of the ladder, the ladder > > > does not contract and indeed does not fit inside the barn at all. In > > > the rest frame of the ladder, the reason why there are no marks on the > > > barn doors when they are shut is that they were not shut at the same > > > time in this frame. > > > This logic is easily defeated Paul, because if we contracted the > > ladder's length *just* enough so that it marked the door in the barn > > frame (in other words, the ladder has contracted just enough to manage > > an interference fit with both doors shut), then this cannot be > > accounted for in the ladder frame (because, in the ladder frame, if > > the ladder is *even larger* relative to the barn than when it started, > > then the ladder could not possibly mark the doors in the same way). > > Sure it could. The "interference fit" corresponds to opening the > leading door just as the ladder impacts it, and closing the trailing > door just in time to impact the ladder. But in the time of the > ladder's frame, these events are no longer simultaneous. Well, yes. But there is still the question of how the doors failed to shut simultaneously, when we specifically set them up so they would.
From: Inertial on 6 Apr 2010 21:15 "Ste" <ste_rose0(a)hotmail.com> wrote in message news:5879f68f-13b1-46e1-a00b-51cc900297f8(a)s9g2000yqa.googlegroups.com... > On 6 Apr, 23:25, Edward Green <spamspamsp...(a)netzero.com> wrote: >> On Apr 5, 7:41 pm, Ste <ste_ro...(a)hotmail.com> wrote: >> >> >> >> >> >> > On 5 Apr, 22:10, PD <thedraperfam...(a)gmail.com> wrote: >> > > It contracts to fit the barn in the rest frame of the barn, which is >> > > why the ladder makes no marks on the barn doors when the doors are >> > > shut at the same time. In the rest frame of the ladder, the ladder >> > > does not contract and indeed does not fit inside the barn at all. In >> > > the rest frame of the ladder, the reason why there are no marks on >> > > the >> > > barn doors when they are shut is that they were not shut at the same >> > > time in this frame. >> >> > This logic is easily defeated Paul, because if we contracted the >> > ladder's length *just* enough so that it marked the door in the barn >> > frame (in other words, the ladder has contracted just enough to manage >> > an interference fit with both doors shut), then this cannot be >> > accounted for in the ladder frame (because, in the ladder frame, if >> > the ladder is *even larger* relative to the barn than when it started, >> > then the ladder could not possibly mark the doors in the same way). >> >> Sure it could. The "interference fit" corresponds to opening the >> leading door just as the ladder impacts it, and closing the trailing >> door just in time to impact the ladder. But in the time of the >> ladder's frame, these events are no longer simultaneous. > > Well, yes. But there is still the question of how the doors failed to > shut simultaneously, when we specifically set them up so they would. Simultaneity is frame-dependent. The doors shut simultaneously according to any observer at rest wrt the barn. Ie if you attach synchronized (in the barn frame) clocks to the two doorways, every observer at rest wrt the barn will agree that the doors shut at the same time. Other appropriately relatively moving observers would agree that the clocks showed the same time when the doors shut, but would disagree about those clocks being synchronized. A pole frame observer in particular would disagree about the clocks on the barn door being synchronized, and would measure that the front door (furthest away from the pole) closed (and reopened) before the other door.
From: Ste on 6 Apr 2010 21:17 On 7 Apr, 01:34, "Inertial" <relativ...(a)rest.com> wrote: > "Ste" <ste_ro...(a)hotmail.com> wrote in message > > > >> This business about a thing fitting and not fitting being a > >> contradiction... > > >> A penny on the armrest of a plane is at rest in one reference frame > >> and not at rest in another reference frame. Is it not obvious to you > >> that there is a logical contradiction in those claims? > > > No, because *everyone* agrees about how the penny is moving with > > respect to any other object. > > >> How does the > >> penny change from being at rest in one frame to not being at rest in > >> another frame, without there being some physical interaction with the > >> penny to cause that change? > > > Because it *doesn't* change from being "at rest" to "not at rest". > > There is no "change" at all. > > YES !!!! > > > Both the man on the plane, and the man on > > the ground, agree that the penny is stationary relative to the flyer, > > and moving relative to the man on the ground. The penny does not > > "change" from being stationary to moving. What changes is the object > > to which you are making reference. > > You are fine with the notion of different observers measuring different > velocities for a coin, but all agreeing that it is at rest wrt some given > observer (the one on the plane). They *don't* measure different velocities Inertial! That's what I'm saying to you. They both measure the *same* velocity between any two objects. *Both* measure that the coin is not moving relative to the man on the plane. *Both* measure that the coin is moving relative to the man on the ground. They are not measuring "different velocities" between the same objects. They are actually measuring the velocities between *different objects*. > Yet you seem to have problems with the similar situation about the pole > 'fitting in a barn'. It only 'fits in the barn' wrt the farm-based observer > (for whom the barn is at rest) (just like the coin is at rest only for the > plane observer). All observers agree that the farm-based observer will > measure the pole as 'fitting in the barn' (just as the ground observer will > agree that the coin is at rest wrt the plan observer). But we are not talking about what another observer *would* measure. We are talking about what each observer *actually* measures. As I said, with the coin on the plane scenario, both the man on the ground, and the man on the plane, can both measure the speed of the coin relative to themselves and their counterpart, and they will *both* agree on each other's measurements. That is, both will agree that the coin is not moving relative to the man on the plane, and both will agree that the coin *is* moving relative to the man on the ground. In this ladder and barn paradox, neither observer actually agrees about the timing of the doors, or the length of the ladder (even though they can both speculate about what the other observer *would* measure). One says the ladder got bigger, the other says it got smaller. One says the doors shut simultaneously, the other says they didn't. One says the ladder marked the doors simultaneously, the other says it didn't.
From: Sue... on 6 Apr 2010 21:31 On Apr 6, 9:02 pm, Ste <ste_ro...(a)hotmail.com> wrote: > > Well, yes. But there is still the question of how the doors failed to > shut simultaneously, when we specifically set them up so they would. Thus far I count three *different* defences to pole and barn in this thread so you can't possibly win. "Your Honor, I will show first, that my client never borrowed the Ming vase from the plaintiff; second, that he returned the vase in perfect condition; and third, that the crack was already present when he borrowed it." But that does not mean you are wrong. That sort of condition usually mean you are right. Sue....
From: Peter Webb on 6 Apr 2010 21:39
<SNIP> > > Any change in velocity involves acceleration, because that is what > acceleration is. > >>> >> > Indeed, a bullet that flies past >>> >> > your ear undergoes a dramatic change of relative velocity to you as >>> >> > it >>> >> > passes (it approaches and then recedes), >>> >>> >> No .. it doesn't >>> >>> > I would argue that it does, >>> >>> And you'd lose. It has the same velocity the whole time (unless you >>> include >>> slowing due to air-resistance, and acceleration due to gravity .. but I >>> don't think that is what you meant) >> >> I don't see how this can be true. > > Then thingk > >> The bullet changes angle, distance, > > You get that because it is moving .. not because of acceleration > >> and speed, and direction, relative to the head as it passes. > > No .. it doesn't. It has the same velocity and direction the whole time > in the head's frame of reference. it is moving at a constant speed in a > straigh line (we are ignoring air reistance and gravity here) > >> Are you sure we aren't arguing over the difference between "velocity >> relative to a coordinate system" (which I think is what you mean), and >> "velocity relative to each other, with no coordinate system in >> play" (which is what I mean - if indeed I'm using the right words to >> describe it)? > > No .. we are arguing over your completely misunderstanding the motion of a > bullet. > No, the problem is that Ste does not understand basic concepts like acceleration, and hence basic Newtonian physics. A pre-requisite, I would have thought, for understanding SR. |