Re: Spaceship Question
- From: "PD" <TheDraperFamily@xxxxxxxxx>
- Date: 24 Jan 2006 13:32:21 -0800
Henri Wilson wrote:
> On 24 Jan 2006 05:50:01 -0800, "PD" <TheDraperFamily@xxxxxxxxx> wrote:
>
> >
> >Henri Wilson wrote:
> >> On 23 Jan 2006 18:55:20 -0800, "Randy Poe" <poespam-trap@xxxxxxxxx> wrote:
> >>
>
> >>
> >> 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.
>
> So it follows that nothing actually happens to the ship or anything on it.
No physical agent is doing anything to change the ship or anything in
it. If that's what you meant, that's right.
>
> >> ....this is most difficult to comprehend.
> >
> >Why? Length is not an intrinsic property of the spaceship.
>
> Why do you think that? The ship occupies a length of space. ...as does a metre
> rod.
> For convenience, we can define the ship's length as ONE length unit and use it
> as a universal length standard.
That standard would only apply in a particular reference frame (one
where the ship is at rest). Length is the result of a *procedure* that
is inherently observer-dependent. There is nothing about the ship that
you can call spatial length that is independent of the observer.
>
> I think you will now agree that this standard remains the same no matter how
> the ship negotiates space.
No, I quite disagree. See the above.
>
> >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.
>
> You seem obsessed with observers and their measurements. Observers don't change
> physical properties of observed objects, you know.
That's right, but length is not a physical property of the ship.
>
> >
> >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.
>
> The ship's length is defined as ONE SPACE UNIT.
That definition would not work for all reference frames. (Or if you
mandated that it does, then the length of everything else would change
in space units.)
>
> It doesn't alter with velocity changes.
Sure it does. Length is not a physical property of the spaceship that
is observer-independent. Length is the result of a *procedure* that is
inherently observer-dependent.
>
> >> >
> >> >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.
>
> Theoretically, the distance between the two can be measured by connecting a
> large number of standard rods between them.
> That is certainly not going to change just because a moving observer goes past.
The moving observer would not agree that those standard rods are the
same length as the standard rod he is carrying with him.
> Why would you think it would?
>
> A_________rod_________B
>
> ->O O1<-
>
> You seem to be under the delusion that the rod changes length because of the
> presence of differently moving observers. How strange...
Yes, that's right. Note that nothing is happening to the rod per se,
though something is happening to its length. That's because length is
not a physical property of the rod.
>
> >> >> 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.
>
> Hooray!
The guy on the ship is not going to measure any difference in the
period of the ship's clock, no matter how the ship moves.
>
> >
> >>
> >> >> 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.
>
> I don't consider such observations at all.
> If the ship's rods and clocks don't change after a firing of the engine then
> they must be exacrtly as they were beforehand.
That depends on what "they" stands for. If you mean the rods and the
clocks, then I agree. If you mean the rod's length or the clock's rate,
as measured on the ship, then I agree. If you mean the rod's length or
the clock's rate, as measured by any other observer, then I don't
agree.
> Therefore they tick at the same
> rates as they did on the asteroid.
>
> >> >> 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.
>
> Time, like length cannot be affected by remote observer movement.
Any quantity that is the result of a procedure that is inherently
observer-dependent will naturally be affected by remote observer
movement. This isn't true for quantities that are inherent properties
of the object, but length of the object and the rate of the process in
it are not inherent properties of the object.
>
> .....maybe you should take up spoon bending.
Maybe you should ask yourself what "length" means.
PD
.
- References:
- Re: Spaceship Question
- From: Randy Poe
- Re: Spaceship Question
- From: PD
- Re: Spaceship Question
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