Re: Beam me up - trying to get a basic understanding of GR
- From: "N:dlzc D:aol T:com \(dlzc\)" <N: dlzc1 D:cox T:net@xxxxxxxxxx>
- Date: Tue, 3 May 2005 06:22:52 -0700
Dear Curious:
"Curious" <anthonyroseuk-curious@xxxxxxxxxxx> wrote in message
news:e803aff.0505030401.418177e7@xxxxxxxxxxxxxxxxxxxxx
> Trying to understand SR and GR at a basic level,
> I've read around on the web and now have
> questions I can't find answers for. Being new to
> this all, and having only physics 1, I may be
> inadvertently resurrecting dumb questions, but
> if someone has the time and inclination to work
> with me on this I'd appreciate some answers to
> these questions.
>
> What I seem to have found so far:
>
> 1. SR is invalid. Inertia, motion, aging etc.
> depend on the locations of masses around us
> in the universe.
SR is valid, just not over a Universal scale. Curvature (which
SR can't handle) does preclude the use of SR, for all except
gross calculations.
> There is an absolute frame of reference,
> the universe.
There *may be* an absolute frame of reference. It has not been
shown that all the visible light sources in the Universe don't
have a net velocity wrt this "absolute frame of reference". And
there is no magical property that being in that frame of
reference provides you, since all frames are relative...
> (Whether it is infinite or not we don't know.)
"Know" is pretty loose. What we see is consistent with "infinite
by being closed". Think gerbil ball.
> 2. GR replaces SR.
GR is more General, by definition. GR is not required for all
problems. GR itself fails at the quantum level. Models all have
a domain of applicability.
> Some say that it works because it takes
> gravity into account, but some say it is
> also invalid, either logically inconsistent
> or they point to a math flaw.
Some people say that the world is flat, even to this day.
Consider your source.
> Some questions I have now are:
>
> a. If you agree that SR is dead, can
> you explain how GR avoids the bullet that
> killed SR?
SR is alive. GR is more General, since it describes gravity, and
is predictive.
> I thought that GR ADDs the space-curving
> effect of gravity to SR?
No. SR requires flat space. GR does not.
> b1. GR also states, like SR, that time
> dilates. How can time dilation explain
> the measurement of light speed as c in
> both the cases of a spaceship
> approaching and departing from the
> light source at near light speed? Surely
> in one of the cases time dilation is an
> exacerbating rather than mitigating
> factor?
Avoid emotional labels. Relativistic Doppler shift has "two"
components, yes. One is from the source being closer/further
with each "event". The other includes the dilation term. They
are seperable.
> b2. Imagine two space-stations ST1,ST2
> exactly one light-minute apart in their frame
> of reference, and two space-ships SP1,
> SP2 travelling at near-light-speed also the
> same distance apart,
In who's frame? ST1+ST2, or SP1+SP2?
> approaching the two stations along the
> same axis on which they lie:
> ST1. . . .ST2 <-SP1. . . .<-SP2
> When spaceship SP1 is alongside ST1,
> and SP2 alongside ST2, spaceship SP2
> flashes a beam of light towards ST1.
> The light reaches ST1 in one minute
> ('static'-time).
OK, so the two ship's separation is in ST1+ST2's frame. They see
the separation as "greater than" in their frame.
> Spaceship SP2 reaches ST2 a second
> or two later than the light.
> b3.1 Does SP2 perceive the distance
> between ST1 and ST2 to be as far as
> we do?
Less than. Length contracted. Illusions are like that.
> b3.2 How many of SP2's seconds
> does it take the light to reach ST2?
length / c. Since SP2 is co-located with ST2 at the time of
emission, this should be "zero".
> c. Is it possible that we can use SR
> as a useful approximation, like Newtonian
> physis, under limited circumstances?
Used all the time. You can use SR when curvature is minimal, or
when your can't measure precisely enough to require GR. Just as
you can use Newton when speeds are much less than c, or you can't
measure very precisely.
> d. Are there situations in which GR is
> known to be experimentally dead wrong,
> and if so, do we use GR as a useful
> approximation, under limited
> circumstances?
Yes. It doesn't apply at the quantum level. No theory based on
a continuum of space and time would apply.
As a "maybe", GR requires Dark Matter and Dark Energy to describe
what we see in the Universe. The quantity of this "otherwise
unobservable" stuff is more than 95%. So GR does a good job for
about 5% of the Universe. Not bad for a race bound to the
surface of a planet.
But GR isn't keeping us here. We are.
David A. Smith
.
- References:
- Beam me up - trying to get a basic understanding of GR
- From: Curious
- Beam me up - trying to get a basic understanding of GR
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