space related Q`s.
Rob Doedee
Could someone please help me with two questions.First, would a gun fire a
projectile in zero atmosphere just as effectively as it would on earth.A
sorkgroup discussion we had on this matter recently had two trains of
thought.A. that lack of oxygen would stop projectile from having any force
upon firing, and B. that a bullet being a sealed unit it would still have
the same capacity to kill as it would on earth. So that is Q 1.
Now the second Q relates to light v gravity and i had this one posed to
me by a friend.
If the sun were to suddenly go out, and giving that the light rays take
about 9 minutes to arrive here,would the gravitational effect of there
being no sun be felt before the light rays stopped?? there fore gravity
being quicker or faster than light??? Sends ya thinking doesnt it!! look
forward to any replies that could clear these up for me,Thanks
Sheldon Townsend
> Could someone please help me with two questions.First, would a gun fire a
> projectile in zero atmosphere just as effectively as it would on earth. .
>. . So that is Q 1.
The propellants in a cartridge are self oxidizing, therefore, the gun would
fire in a vacuum the same as otherwise. It would just be real quiet.
> Now the second Q relates to light v gravity and i had this one posed to
> me by a friend.
> If the sun were to suddenly go out, and giving that the light rays take
> about 9 minutes to arrive here,would the gravitational effect of there
> being no sun be felt before the light rays stopped?? there fore gravity
> being quicker or faster than light??? Sends ya thinking doesnt it!!
If the sun "were to suddenly go out" the mass would still be there and there
would be no gravitational effects. But that's not what you're asking. If
the sun mysteriously disappeared, the "information" carried by the photons
that the sun was gone would arrive at the same time as the gravitational
information (carried by gravitons? no one know, yet). Anyway, gravitational
effects travel the same rate as light.
Shel
Respond to s...@pacifier.com
J. Scott Miller
Rob Doedee wrote:
> [first question deleted for lack of interest]
> Now the second Q relates to light v gravity and i had this one posed to
> me by a friend.
> If the sun were to suddenly go out, and giving that the light rays take
> about 9 minutes to arrive here,would the gravitational effect of there
> being no sun be felt before the light rays stopped?? there fore gravity
> being quicker or faster than light??? Sends ya thinking doesnt it!! look
> forward to any replies that could clear these up for me,Thanks
The following comes from the sci.astro FAQ file. There is probably a similar answer in
the sci.physics FAQ file, and perhaps one which answers your first question.
--
J. Scott Miller, Program Coordinator jsmi...@homer.louisville.edu
Rauch Memorial Planetarium http://www.louisville.edu/planetarium
University of Louisville
D.04 Does gravity travel at the speed of
light?
Author: Steve Carlip <car...@dirac.ucdavis.edu>,
Matthew P Wiener <wee...@sagi.wistar.upenn.edu>
To begin with, the speed of gravity has not been measured directly in
the laboratory---the gravitational interaction is too weak, and such an
experiment is beyond present technological capabilities. The "speed of
gravity" must therefore be deduced from astronomical observations, and
the answer depends on what model of gravity one uses to describe those
observations.
In the simple Newtonian model, gravity propagates instantaneously: the
force exerted by a massive object points directly toward that object's
present position. For example, even though the Sun is 500 light seconds
from the Earth, Newtonian gravity describes a force on Earth directed
towards the Sun's position "now," not its position 500 seconds ago.
Putting a "light travel delay" (technically called "retardation") into
Newtonian gravity would make orbits unstable, leading to predictions
that clearly contradict Solar System observations.
In general relativity, on the other hand, gravity propagates at the
speed of light; that is, the motion of a massive object creates a
distortion in the curvature of spacetime that moves outward at light
speed. This might seem to contradict the Solar System observations
described above, but remember that general relativity is conceptually
very different from Newtonian gravity, so a direct comparison is not
so simple. Strictly speaking, gravity is not a "force" in general
relativity, and a description in terms of speed and direction can be
tricky. For weak fields, though, one can describe the theory in a
sort of Newtonian language. In that case, one finds that the "force"
in GR is not quite central---it does not point directly towards the
source of the gravitational field---and that it depends on velocity as
well as position. The net result is that the effect of propagation
delay is almost exactly cancelled, and general relativity very nearly
reproduces the Newtonian result.
This cancellation may seem less strange if one notes that a similar
effect occurs in electromagnetism. If a charged particle is moving at
a constant velocity, it exerts a force that points toward its present
position, not its retarded position, even though electromagnetic
interactions certainly move at the speed of light. Here, as in
general relativity, subtleties in the nature of the interaction
"conspire" to disguise the effect of propagation delay. It should be
emphasized that in both electromagnetism and general relativity, this
effect is not put _ad hoc_ but comes out of the equations. Also, the
cancellation is nearly exact only for _constant_ velocities. If a
charged particle or a gravitating mass suddenly accelerates, the
_change_ in the electric or gravitational field propagates outward at
the speed of light.
Since this point can be confusing, it's worth exploring a little
further, in a slightly more technical manner. Consider two
bodies---call them A and B---held in orbit by either electrical or
gravitational attraction. As long as the force on A points directly
towards B and vice versa, a stable orbit is possible. If the force on
A points instead towards the retarded (propagation-time-delayed)
position of B, on the other hand, the effect is to add a new component
of force in the direction of A's motion, causing instability of the
orbit. This instability, in turn, leads to a change in the mechanical
angular momentum of the A-B system. But *total* angular momentum is
conserved, so this change can only occur if some of the angular
momentum of the A-B system is carried away by electromagnetic or
gravitational radiation.
Now, in electrodynamics, a charge moving at a constant velocity does not
radiate. (Technically, the lowest order radiation is dipole radiation,
which depends on the acceleration.) So to the extent that that A's
motion can be approximated as motion at a constant velocity, A cannot
lose angular momentum. For the theory to be consistent, there *must*
therefore be compensating terms that partially cancel the instability of
the orbit caused by retardation. This is exactly what happens; a
calculation shows that the force on A points not towards B's retarded
position, but towards B's "linearly extrapolated" retarded position.
Similarly, in general relativity, a mass moving at a constant
acceleration does not radiate (the lowest order radiation is
quadrupole), so for consistency, an even more complete cancellation of
the effect of retardation must occur. This is exactly what one finds
when one solves the equations of motion in general relativity.
Are there future prospects for a direct measurement of the speed of
gravity? One possibility would involve detection of gravitational
waves from a supernova. The detection of gravitational radiation in
the same time frame as a neutrino burst, followed by a later visual
identification of a supernova, would be considered strong experimental
evidence for the speed of gravity being equal to the speed of light.
However, unless a very nearby supernova occurs soon, it will be some
time before gravitational wave detectors are expected to be sensitive
enough to perform such a test.
References:
There seems to be no nontechnical reference on this subject. For a
technical reference, see
T. Damour, in _Three Hundred Years of Gravitation_, S.W. Hawking and
W. Israel, editors (Cambridge Univ. Press, 1987)
For a good reference to the electromagnetic case, see
R.P. Feynman, R.B. Leighton, and M. Sands, _The Feynman Lectures on
Physics_, chapter II-21 (Addison-Wesley, 1989)
Parent document is top of "[sci.astro] Astrophysics (Astronomy Frequently Asked
Questions)
(4/8)"
Carl Warnell
In article: <01bcc0f5$83a0b840$2257...@suzydoo.pipeline.com.au> "Rob
>
> Could someone please help me with two questions.First, would a gun fire a
> sorkgroup discussion we had on this matter recently had two trains of
> thought.A. that lack of oxygen would stop projectile from having any
force
> upon firing, and B. that a bullet being a sealed unit it would still have
I'm not sure what you mean by 'lack of oxygen stopping the projectile
having any force upon firing' but the propellant in a bullet contains its
own oxidant, so it should work equally well in a vacuum. Actually, it
should travel further as there's no air resistance.
> Now the second Q relates to light v gravity and i had this one posed to
> me by a friend.
> If the sun were to suddenly go out, and giving that the light rays take
> about 9 minutes to arrive here,would the gravitational effect of there
> being no sun be felt before the light rays stopped?? there fore gravity
> being quicker or faster than light???
It would take the regulation 8 minutes for us to feel it. Until then, the
event is outside our past light cone.
--
_____ . . <|
Carl Warnell ' o/ FORE! . . |
b...@bogart.demon.co.uk | . . . . |
/| . . ...o
Russ Smith
Sheldon Townsend wrote:
>
> > Could someone please help me with two questions.First, would a gun fire a
> >. . So that is Q 1.
>
> The propellants in a cartridge are self oxidizing, therefore, the gun would
> fire in a vacuum the same as otherwise. It would just be real quiet.
>
There would be no friction (windage losses) so the projectile would
travel
farther and straighter in a vacuum. Therefore, it would be more
accurate
and effective than in an atmosphere...
>
> > Now the second Q relates to light v gravity and i had this one posed to
> > me by a friend.
> > If the sun were to suddenly go out, and giving that the light rays take
> > about 9 minutes to arrive here,would the gravitational effect of there
> > being no sun be felt before the light rays stopped?? there fore gravity
> > being quicker or faster than light??? Sends ya thinking doesnt it!!
>
> If the sun "were to suddenly go out" the mass would still be there and there
> would be no gravitational effects. But that's not what you're asking. If
> the sun mysteriously disappeared, the "information" carried by the photons
> that the sun was gone would arrive at the same time as the gravitational
> information (carried by gravitons? no one know, yet). Anyway, gravitational
> effects travel the same rate as light.
>
> Shel
>
> Respond to s...@pacifier.com
Russ
Henry Spencer
In article <3439d...@news.pacifier.com>,
Sheldon Townsend <set@p_acifier.com> wrote:
>If the sun "were to suddenly go out" the mass would still be there and there
>would be no gravitational effects. But that's not what you're asking. If
>the sun mysteriously disappeared, the "information" carried by the photons
>that the sun was gone would arrive at the same time as the gravitational
>information (carried by gravitons? no one know, yet). Anyway, gravitational
>effects travel the same rate as light.
Yes and no. *Changes* in a gravitational field travel at the speed of
light... probably. (This has not been confirmed experimentally yet, but
the speed of light is what the theory predicts.)
It's important to understand that nothing has to travel between Earth and
Sun for Earth to remain in its normal orbit. Earth orbits the Sun because
the Sun's mass warps space-time into a curved shape; the Earth is simply
following the straightest path it can in this curved environment. There
is no exchange of any sort of radiation between Earth and Sun in this
process, and hence it's not meaningful to talk about the speed at which
such an exchange happens.
--
If NT is the answer, you didn't | Henry Spencer
understand the question. -- Peter Blake | he...@zoo.toronto.edu
Brian Trosko
Rob Doedee <suz...@pipeline.com.au> wrote:
: If the sun were to suddenly go out, and giving that the light rays take
: about 9 minutes to arrive here,would the gravitational effect of there
: being no sun be felt before the light rays stopped?? there fore gravity
: forward to any replies that could clear these up for me,Thanks
Well, if by 'the sun goes out' you mean that fusion just mysteriously
stops right this instant, we wouldn't even notice anything special for a
few thousand years at the minimum, except perhaps for a sudden neutrino
shortfall. Later, things would get interesting, but the *mass* of the sun
wouldn't change, and it's far enough away to be a point source, so the
Earth's orbit wouldn't change.
If by 'the sun goes out,' you mean it just disappears, then the answer is
"We don't know." There's nothing in any theory of how things work that
would allow for the sun to just disappear, so it shouldn't be surprising
that our theories of how things work can't really predict what would
happen if it did. Maybe it would take 8 minutes for the spacetime
distortion to reach us. Maybe it would happen right away. Maybe the
World Turtle would shake us off its back and swim away. Who knows? Who
cares? Can't happen. Or, if it could, any answer anyone could give is
pretty damn well meaningless.
Brian Trosko
Russ Smith <russ-...@kalamazoo.net> wrote:
: There would be no friction (windage losses) so the projectile would
: travel
: farther and straighter in a vacuum. Therefore, it would be more
: accurate
: and effective than in an atmosphere...
Unless, say, the projectile vacuum-welds itself to the barrel of the gun.
Or the moving parts of the gun get vacuum-welded to each other. In either
case, hold it well away from your face when you fire it. And wear gloves.
Manfred Bartz
Henry Spencer <he...@zoo.toronto.edu> writes:
> It's important to understand that nothing has to travel between Earth and
> Sun for Earth to remain in its normal orbit. Earth orbits the Sun because
> the Sun's mass warps space-time into a curved shape; the Earth is simply
> following the straightest path it can in this curved environment. There
> is no exchange of any sort of radiation between Earth and Sun in this
> process, and hence it's not meaningful to talk about the speed at which
> such an exchange happens.
The original question "What would happen if the sun suddenly
vanished?" asks about something that is not possible. However, it is
a scenario which can be used to try and understand gravitational
effects.
What occurred to me is that in this scenario, the warping of space
coused by our sun would suddenly disappear, setting off a
gravitational wave which travels at the speed of light (as far as I
know). Since the earth's diameter is about 12000km, the wavefront
would take 25msec to travel through Earth and during that time our
planet would be exposed to differential forces. E.g. part of the
planet follows the original path in time/space, the other part the new
path. I suppose, it would not be too difficult to calculate the
actual forces, but my math is a bit rusty so I'll leave that to
someone else. :-)
--
Manfred Bartz <mba...@werple.net.au>
-------------------------------------------------------------------------------
Unsolicited commercial E-Mail will be billed at $500 per message for proof-
reading services. This rate is not negotiable, there are no quantity discounts.
-------------------------------------------------------------------------------
Sheldon Townsend
> Unless, say, the projectile vacuum-welds itself to the barrel of the gun.
> Or the moving parts of the gun get vacuum-welded to each other. In
either
> case, hold it well away from your face when you fire it. And wear
gloves.
(Somebody will probably berate me for over-simplification again, but I'll
plunge in anyway.)
A few definitions: bullet: lead (or lead covered with a harder metal)
projectile.
casing: brass cylinder, closed at one end with a small hole and open at the
other (may also be tapered toward the open end).
cartridge: What a lot of people call a bullet. The bullet is crimped into
the open end of the casing, with propellent in the casing, and a primer on
the closed end of the casing over the small hole. This combination is
called a cartridge.
Okay: I don't think the projectile would have time to vacuum weld (a
phenomena I admit knowing almost nothing about other than that it happens)
to the barrel. Bullets travel at something like 1500 feet per second (much,
much variation between pistols, rifles, calibers, powder loads, bullet mass,
pahse of the moon, etc.) so in a six inch barrel of a pistol the bullet
would travel it in approximately 0.0002 seconds, an 18-inch rifle barrel,
0.0006 seconds. And the chamber (where the bullet sits before being fired)
doesn't fit the bullet as snugly as in the barrel (otherwise, you couldn't
put the cartridge in the chamber).
The bullet if pretty tightly held by the casing (a hand loader could tell
you how much). You can remove the bullet with a pair of pliers (I don't
recommend this). If there's going to be vacuum welding I would guess it
would be between the bullet and the casing. I don't think this is a
problem. The ignition of the propellent would probably be sufficient to
break the weld. And the force of the propellent combustion would be
sufficient to keep it from vacuum welding.
If the casing vacuum welded to the chamber in a revolver you wouldn't be
able to reload after firing all the cylinders (ranging from 10 to 5
depending on caliber and size of gun). In a auto-loader such as a ".45
auto" the spent casing would not extract and the next round would not load
into the chamber (this has happened to me, not due to vacuum welding but
poorly made ammunition, it's not a safety hazard, just annoying and ruins
the smashed cartridge). Same for a semi-auto or rifle or full auto rifle
(a/k/a machine gun). For a bolt action rifle I don't know if the bolt would
not pull back or the bolt wouldn't extract the spent casing. Probably the
latter.
The brass of the cartridge is pretty weak. In fact, it needs the chamber of
the gun to enclose the explosion. If you have fired a revolver and noticed
the cartridges slip right into the chambers but after firing you have to use
the ejector rod to push them out: they got tight. If you but a cartridge in
a vise and tapped the primer with a nail (I DO NOT recommend this) the
bullet would not fly straight and fast and true, the cartridge would
explode. This expansion would more than likely cause vacuum welding than
the bullet flying down the barrel at 1000mph.
If the gun parts vacuum welded (or jammed due to freezing lubricant) the gun
would not function but it wouldn't be dangerous, even if it did so after
firing the first cartridge. The only dangerous situation I can see is the
bullet stopping the the barrel due to vacuum welding (again, I think this
highly unlikely) and then where do all those expanding gases go? You gun
blows up.
And I think that if you were in vacuum, you would be wearing gloves. At
least I'd hope so!
Shel
respond to to s...@pacifier.com
Christopher P. Winter
I don't know a whole lot about vacuum welding myself. But I do recall
that the experiments on LDEF found very little evidence for it.
Henry Spencer
In article <3447d34d...@nntp.ix.netcom.com>,
Christopher P. Winter <cpwi...@ix.netcom.com> wrote:
>I don't know a whole lot about vacuum welding myself. But I do recall
>that the experiments on LDEF found very little evidence for it.
The incidence of cold welding, aka vacuum welding, is greatly exaggerated.
It afflicts only some metals and rarely occurs unless other problems are
also present (e.g., inadequate lubrication for sliding surfaces). The
Lockheed Space Materials Handbook (1969) says that as of its publication
date, there were no known cases of *unintentional* cold welding in space.