Re: TomTom's stupidity (re: was always TomTom's stupidity)
- From: stevendaryl3016@xxxxxxxxx (Daryl McCullough)
- Date: 10 Oct 2005 09:51:03 -0700
schoenfeld1@xxxxxxxxx says...
>Daryl McCullough wrote:
>> schoenfeld1@xxxxxxxxx says...
>> >Now, you acknowledged yourself that this type of acceleration would
>> >cause the rod to stretch in it's own frame. It boggles the mind that
>> >you still don't see how this violates all the known physical laws.
>>
>> Why do you say that? Don't just say "it violates all the laws",
>> say *which* law. Give the law, and say how this violates it.
>
>It violates conservation of energy because the energy required to
>stretch the rod, as observed in the rod's frame, is not given to the
>rod by anything and is not expended by the rod in any way.
On the contrary, if a rod is accelerating, it is because it
has rockets attached to it. Any energy involved in stretching
or accelerating the rod comes from the rocket. It doesn't violate
conservation of energy.
>> 3. If a rod is travelling at speed v relative to inertial frame A,
>> and the rod has length L in frame A, then it has length gamma * L in
>> the rod's own rest frame.
>> If so, then doesn't 3 say that if a rod accelerates so as
>> to maintain a constant length L as measured in its initial
>> frame, then it will have to *grow* (to length gamma * L) in
>> its own rest frame?
>
>Yes it does, and hence the failure of SR.
This has nothing to do with GR versus SR. GR says
the *same* thing.
>> There is nothing mystical about the forces. To accelerate the two
>> ends of a rod simultaneously, you have to attach rockets to each
>> end.
>
>Correct.
>
>> If the front rocket is travelling faster than the rear rocket,
>> then the force of the *rocket* will stretch the rod.
>
>If it's travelling faster, which it isn't.
That depends on how the rockets fire. If the rockets
fire in such a way that the front rocket accelerates
in exactly the same way as the rear rocket (as measured
in the initial frame A), then the rod will remain a
constant length (as measured in the initial frame A)
but the rod's proper length will *grow*.
>> The forces aren't mystical.
>
>They cannot be accounted for by a physical mechanism.
You don't think that burning rocket fuel is a physical
mechanism?
>> You misunderstand what the equivalence principle says. One
>> way to state the equivalence principle is in terms of Special
>> Relativity: In a reference frame that is in freefall under
>> the influence of gravity, Special Relativity can be used to
>> describe physics locally. The EP is *about* SR; or rather,
>> it is about the *relationship* between gravity and SR. GR
>> doesn't need the equivalence principle; it was a bridge
>> principle that could be used to solve certain problems
>> (for example, gravitational time dilation, the bending
>> of starlight) *before* the full theory of GR was developed.
>
>The view that GR/strong EP is necessary only for gravitational motion
>may be the view you hold,
No, that's not my view. My view is that
(1) GR is only needed when spacetime is curved. It is not
necessary for flat spacetime.
(2) The equivalence principle is about the *relationship*
between certain situations in curved spacetime and certain
analogous situations in flat spacetime. In particular,
the physics of a local freefall frame in curved spacetime
is *approximately* the same as the physics of an inertial
frame in flat spacetime. The physics of a frame that is
accelerating in the presence of gravity is *approximately*
the same as the physics of an accelerating frame in flat
spacetime.
The signficance of the EP is that it allows us to transform
certain problems involving gravity into analogous problems
that *don't* involve gravity (and so can be solved using SR
alone). As I said, the significance of this is that in solving
problems involving gravity, SR is sufficient as long as you are
only dealing with physics *locally* (in a small region of spacetime).
GR reduces to SR in a small region in the same way that a sphere
"looks" like a plane if you only consider a tiny region of the
sphere. That's what the EP says.
>and the view your tutors hold. But it is not
>the view Einstein held
Yes, it is the view that Einstein held. You are misunderstanding
Einstein's position.
>> Lets just concentrate on those first two pulses.
>> We'll assume that v1 is still pretty small compared
>> with the speed of light,
>
>I can't let that pass.
Fine. Let's not make that approximation. It doesn't matter.
>> so the length of the rod
>> is still approximately 1 meter long after the first
>> pulse,
>>
>> which occurs at time t=0 in frame A. Now,
>> let e_r be the event at which the second pulse
>> happens at the rear rocket.
>
>Sure.
>
>> Let e_f be the event
>> at which the second pulse happens at the front
>> rocket.
>
>Sure.
>
>> In frame A, e_r and e_f happen at the same time.
>
>Wrong. The first pulse was simultaneous in frame A. Since the rod has
>now changed velocity relative to A, the events e_r and e_f will not be
>simultaneous relative to A anymore.
You are talking about a different situation. I stipulated
that the rockets fire in such a way that the pulses are
simultaneous in frame A. That is an assumption about the
behavior of the rockets. We are investigating the *consequences*
of that assumption.
>Remember, it is the actual rod which triggers the acceleration,
>not some clocks in A's frame.
Why do you say that? The rockets are programmed to fire pulses
once per second (as measured in frame A). Their program isn't
affected by the rod; the rod is just going along for the ride.
>The events e_r and e_f will be simultaneous relative to frame
>B - and frame B has velocity v1 relative to A.
You are talking about a different situation, in which the
program of the rockets is different.
You have to get it straight what, exactly, you are assuming
about the acceleration profile of the rod. If you assume that
the acceleration is such that the rod maintains a constant
proper length, then yes, the rod will maintain a constant
proper length. If you assume that the acceleration is such
that the front end and the rear end accelerate simultaneously
(as measured in frame A), then the rod will *not* maintain
a constant proper length.
You can program the rockets to do it either way. SR can handle
either case.
>> But in the frame of the rod, which is travelling
>> at speed v1, these events happen at *different*
>> times.
>
>No. As stated, since the rod is no longer in A the second pulse will
>not be simultaneous to A.
That depends on what the program governing the rockets is.
Assume that each rocket has an associated clock. Each rocket
has an associated program of the form of a list of times:
tau_1, tau_2, tau_3, etc. The rocket fires every time the
clock reaches a time on the list.
If the rockets have identical clocks, and identical lists
of times, then the rocket on the front end will always fire
at the same time as the rocket on the rear end (as measured
in frame A). If this happens, then the proper length of the
rod will increase.
In order to maintain a constant proper length of the rod,
the rocket in front must accelerate *less* than the rocket
in the rear. So the program tau_1, tau_2, tau_3, etc. for
the front rocket cannot be the same as the program for
the rear rocket.
--
Daryl McCullough
Ithaca, NY
.
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- From: schoenfeld1
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