Re: Spaceship Question
- From: "PD" <TheDraperFamily@xxxxxxxxx>
- Date: 24 Jan 2006 05:50:01 -0800
Henri Wilson wrote:
> On 23 Jan 2006 18:55:20 -0800, "Randy Poe" <poespam-trap@xxxxxxxxx> wrote:
>
> >
> >Henri Wilson wrote:
> >> Is has been established that absolutely no changes occur inside a spaceship as
> >> a result of its engine being fired for a short period. Its length and mass
> >> remain as they were and its clocks all tick at the original rates.
> >
> >As measured internally. However, outside observers observe
> >that the Lorentz contraction equations hold.
>
> Geez you SRians sure are confused!
>
> I asked you if a spaceship increases or decreases length after firing its
> engine for a period.
> None of you gave an intelligible answer.
>
> Now you say it depends on the observers.
> Since the firing of the engine causes the spaceship to increase speed wrt half
> the observer in the universe and slow down wrt the other half, the gamma term
> 1-(v/c)^2 also both increases and decreases simultaneously.
>
> I gather you are also saying that the spaceship both lengthens and shortens
> simultaneously, when it fires its engine.
Rather, it lengthens for one observer, and it shortens for another
observer, yes.
>
> ....this is most difficult to comprehend.
Why? Length is not an intrinsic property of the spaceship. There is no
reason why the same measurement cannot result in different values for
two observers looking at the *same* spaceship at the *same* time.
If, however, you cannot free yourself from insisting that length is an
inherent property of the spaceship, then indeed I can understand why
this would be most difficult to comprehend.
>
> >
> >Observers inside the spaceship observe that outside lengths,
> >for instance the distance to a remote star, are Lorentz
> >contracted.
>
> Are you claiming that the distance between the asteroid and the star
> miraculously changes when the ship takes off?
>
> ....this is most difficult to comprehend.
Again, you have the preconception that the distance between the
asteroid and the star is an inherent physical property of the pair. It
is not.
>
> >
> >> It follows that the length of time during which the acceleration takes place
> >> cannot affect this principle.
> >
> >What is the principle? Are you trying to pretend that a principle
> >about how things are perceived in their own rest frame is
> >a principle about how they are perceived in a moving
> >frame?
>
> The principle is that nothing actually happens to the ship or its clocks when
> its engine is fired for a period. Obviously the length of the period doesn't
> matter if nothing happens anyway.
> Wouldn't you agree?
Yes.
>
> >> Consider then, a spaceship resting on a remote asteroid. It is fitted with a
> >> new super engine capable of accelerating the ship for long periods at up to
> >> 0.001 c/sec^2, as measured with onboard accelerometers.
> >
> >c is a velocity. c/sec has units of acceleration. Perhaps you meant
> >0.001 c/sec.
>
> Yes, sorry, you know what I meant.
>
> >> The ship heads for a star that is calculated to be 1 LY away. It fires its
> >> engine continuously for 100000 seconds, reaching a speed of 100 c wrt the
> >> asteroid.
> >
> >Nope. The accelerometers do indeed measure 0.001 c/sec
> >(300,000 m/sec^2, or about 30,000 g), but this does not
> >translate into an increase of velocity by 0.001c every second
> >as measured by how fast things outside are going by.
>
> But outside things are both increasing and decreasing speed wrt the spaceship
> while its engine is firing. Is that important?
>
> >
> >> The ship reaches the star in less than 5 days.
> >
> >It is possible that the ship achieves a gamma large enough
> >that it can traverse this distance in 5 days ship time (though
> >not with the numbers you gave, as the relationship between
> >thrust and velocity is more complicated than that).
>
> Sorry, the spaceship experiences thrust. It doesn't have a 'speed'. It has a
> speed wrt the asteroid, if that's what you mean.
> Since those onboard know their acceleration and since it has been established
> that nothing actually happens to the ship's mass during the acceleration
> period, they can calculate that velocity.
>
> >However, if
> >they were receiving signals from the star, they would have noticed
> >that those signals were blue shifted along with the Lorentz
> >transformation of everything else outside. A year's worth of
> >signals are received from the star while the ship travels
> >5 days of ship time.
>
> Indeed, a year's worth of signals is received by the ship during the five days,
> heavily blue-shifted according to the integral of (c+v)/c.
>
> >> When it returns to the asteroid, its clocks, which had not physically changed
> >> in any way, must read the same as the ones that were left there.
> >
> >They have not physically changed, but their relationship to the
> >ship clocks changed. While the ship was ticking off 10 days
> >round trip, the asteroid's clocks were ticking off 2 years.
>
> Ah! That's the source of your error.
> You have assumed the ships clocks slow down....but we have established that
> nothing happens to them physically due to the acceleration.
Your conflict comes from assuming that the observation that the ship's
clocks slow down must be because of a physical process affecting the
clocks. Not so.
>
> >> So why all this crap about fictitious 'contractions', time dilations, twins
> >> paradoxes and presumed time problems associated with traveling to distant
> >> stars?
> >>
> >> All we need is a super-efficient rocket engine (fusion?) that will provide
> >> adequate thrust and we can fly to places like Andromeda in quite short times.
> >
> >Arbitrarily short ship time. Time still passes for people on earth
> >and Andromeda.
>
> Yes, the same five days pass throughout the universe.
Not so.
PD
.
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- Re: Spaceship Question
- From: Randy Poe
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